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This paper provides a synopsis of discussions related to biomedical engineering core curricula that occurred at the Fourth BME Education Summit held at Case Western Reserve University in Cleveland, Ohio in May 2019. This summit was organized by the Council of Chairs of Bioengineering and Biomedical Engineering, and participants included over 300 faculty members from 100+ accredited undergraduate programs. This discussion focused on six key questions: QI: Is there a core curriculum, and if so, what are its components? QII: How does our purported core curriculum prepare students for careers, particularly in industry? QIII: How does design distinguish BME/BIOE graduates from other engineers? QIV: What is the state of engineering analysis and systems-level modeling in BME/BIOE curricula? QV: What is the role of data science in BME/BIOE undergraduate education? QVI: What core experimental skills are required for BME/BIOE undergrads? s. Indeed, BME/BIOI core curricula exists and has matured to emphasize interdisciplinary topics such as physiology, instrumentation, mechanics, computer programming, and mathematical modeling. Departments demonstrate their own identities by highlighting discipline-specific sub-specialties. In addition to technical competence, Industry partners most highly value our students' capacity for problem solving and communication. As such, BME/BIOE curricula includes open-ended projects that address unmet patient and clinician needs as primary methods to prepare graduates for careers in industry. Culminating senior design experiences distinguish BME/BIOE graduates through their development of client-centered engineering solutions to healthcare problems. Finally, the overall BME/BIOE curriculum is not stagnant-it is clear that data science will become an ever-important element of our students' training and that new methods to enhance student engagement will be of pedagogical importance as we embark on the next decade.

Objective There are currently no definitive disease-modifying therapies for traumatic brain injury (TBI). In this study, we present a strong therapeutic candidate for TBI, immunomodulatory nanoparticles (IMPs), which ablate a specific subset of hematogenous monocytes (hMos). We hypothesized that prevention of infiltration of these cells into brain acutely after TBI would attenuate secondary damage and preserve anatomic and neurologic function. Methods IMPs, composed of US Food and Drug Administration-approved 500nm carboxylated-poly(lactic-co-glycolic) acid, were infused intravenously into wild-type C57BL/6 mice following 2 different models of experimental TBI, controlled cortical impact (CCI), and closed head injury (CHI). Results IMP administration resulted in remarkable preservation of both tissue and neurological function in both CCI and CHI TBI models in mice. After acute treatment, there was a reduction in the number of immune cells infiltrating into the brain, mitigation of the inflammatory status of the infiltrating cells, improved electrophysiologic visual function, improved long-term motor behavior, reduced edema formation as assessed by magnetic resonance imaging, and reduced lesion volumes on anatomic examination. Interpretation Our findings suggest that IMPs are a clinically translatable acute intervention for TBI with a well-defined mechanism of action and beneficial anatomic and physiologic preservation and recovery. Ann Neurol 2020;87:442-455.

Purpose This study evaluated the macular microvascular changes in eyes with proliferative diabetic retinopathy (PDR) following panretinal photocoagulation (PRP). Design Using optical coherence tomographic angiography (OCTA), we prospectively studied 10 eyes of 10 subjects with high-risk PDR immediately before, at 1 month, and at 3-6 months following PRP, using a 3- × 3-mm OCTA scan at each visit. Methods The following parameters were calculated for the superficial (SCP), middle (MCP), and deep capillary plexuses (DCP): parafoveal vessel density (VD), adjusted flow index (AFI), and percent area of nonperfusion (PAN). Parafoveal SCP vessel-length density (VLD) was also evaluated. We performed univariate and multivariable statistics, adjusting for age and signal strength. To model the hemodynamic effect of PRP, we also present a mathematical model based on electrical circuits. Results We found no significant difference for the vascular density parameters following PRP, except for decreased density at the MCP at the latest timepoint in the adjusted multivariable model. PAN, a metric of nonperfusion adjusted for noise, and AFI, a surrogate metric of blood flow, showed significant increases at all capillary levels in the adjusted model. Our mathematical model explained how PRP would increase macular blood flow. Conclusions Using OCTA, we found an overall increase in the flow metrics of all capillary layers in the macula following PRP, unrelated to macular edema or thickening, in line with the mathematical model. Our results suggest an overall redistribution of blood flow to the posterior pole following PRP, adding a new dimension to our understanding of the complex biologic effects of PRP in PDR. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Purpose The purpose of this study was to determine whether the ability of the rat retina to control its pH is affected by diabetes. Methods Double-barreled H+-selective microelectrodes were used to measure extracellular [H+] in the dark-adapted retina of intact control and diabetic Long-Evans rats 1 to 6 months after intraperitoneal injection of vehicle or streptozotocin, respectively. Two manipulations-increasing of blood glucose and intravenous injection of the carbonic anhydrase blocker dorzolamide (DZM)-were used to examine their effects on retinal pH regulation. Results An increase of retinal acidity was correlated with the diabetes-related increase in blood glucose, but only between 1 and 3 months of diabetes, not earlier or later. Adding intravenous glucose had no noticeable effect on the retinal acidity of control animals. In contrast, similar injections of glucose in diabetic rats significantly increased the acidity of the retina. Again, the largest increase of retinal acidity due to artificially elevated blood glucose was observed at 1 to 3 months of diabetes. Suppression of carbonic anhydrase by DZM dramatically increased the retinal acidity in both control and diabetic retinas to a similar degree. However, in controls, the strongest effect of DZM was recorded within 10 minutes after the injection, but in diabetics, the effect tended to increase with time and after 2 hours could be two to three times larger than at the beginning. Conclusions During development of diabetes in rats, the control over retinal pH is partly compromised so that conditions that perturb retinal pH lead to larger and/or more sustained changes than in control animals.

Ketamine/xylazine anesthesia has been used primarily for short term procedures in animals, but two prior reports used intravenous ketamine/xylazine for experiments taking many hours. However, there is a discrepancy about the appropriate dose, which is resolved here. Adult Long-Evans rats were used for recording from the retina. Doses of Ketamine/xylazine were adjusted to minimize anesthetic in terminal experiments lasting 10 h. An allometric relation was fitted to the resulting data on doses as a function of body weight, and compared to prior work. The allometric relationship between the continuously infused specific dose and weight was: dose = 9.13 (weight)−1.213 (r2 = 0.73), where dose is in mg-kg−1-hr−1 and rat weight is in kg. The dose of xylazine was 3.3% of the ketamine dose. No attempt was made to explore different relative doses of xylazine and ketamine. Prior work is consistent with this relationship, showing that the earlier discrepancy resulted from using rats of different sizes. Ketamine at the doses used here still depressed the electroretinogram relative to historical controls using urethane. We conclude that intravenous ketamine dosing in rats should not use the same mg-kg−1-hr−1 dose for all rats, but take into account the strong allometric relationship between dose and rat weight. There is an advantage in using smaller doses in order to prevent unnecessary depression of neural responses., Biological sciences; Neuroscience; Physiology; Ophthalmology; Laboratory medicine; Rat; Anesthesia; Ketamine; Xylazine; Electroretinogram; ERG; Retina

Inhaled general anesthetics are used commonly in adults and children, and a growing body of literature from animals and humans suggests that exposure to anesthesia at an early age can impact brain development. While the origin of these effects is not well understood, it is known that anesthesia can disrupt oxygen regulation in the brain, which is critically important for maintaining healthy brain function. Here we investigated how anesthesia affected brain tissue oxygen regulation in neonatal rabbits by comparing brain tissue oxygen and single unit activity in the awake and anesthetized states. We tested two common general anesthetics, isoflurane and sevoflurane, delivered in both air and 80% oxygen. Our findings show that general anesthetics can greatly increase brain tissue PO(2) in neonates, especially when combined with supplemental oxygen. Although isoflurane and sevoflurane belong to the same class of anesthetics, notable differences were observed in their effects upon neuronal activity and spontaneous respiration. Our findings point to the need to consider the potential effects of hyperoxia when supplemental oxygen is utilized, particularly in children and neonates.

Purpose In the dark, photoreceptor outer segments contain high levels of cyclic guanosine 3'-5' monophosphate (cGMP), which binds to ion channels, holding them open and allowing an influx of cations. Ion pumping activity, which balances cation influx, uses considerable amounts of adenosine triphosphate (ATP) and oxygen. Light reduces cation influx and thereby lowers metabolic demand. Blood vessels are compromised in the diabetic retina and may not be able to meet the higher metabolic demand in darkness. Emixustat is a visual cycle modulator (VCM) that reduces chromophore levels and, therefore, may mimic light conditions. We evaluated the effect of emixustat on oxygen consumption and cation influx in dark conditions. Methods Cation influx was measured in rats using Mn2+-magnetic resonance imaging (MEMRI). Retinal oxygen profiles were recorded to evaluate oxygen consumption. In the MEMRI protocol, animals were treated with either emixustat or vehicle. In the oxygen protocol, animals were untreated or treated with emixustat. Results In vehicle-treated animals, cation channel activity increased in the dark. Emixustat treatment reduced cation channel activity; activity was comparable to vehicle-treated controls in light conditions. In vehicle-treated animals, minimum retinal oxygen tension decreased as the retina recovered from a photobleach, indicating that more oxygen was being consumed. Emixustat treatment prevented the decrease in oxygen pressure after photobleach. Conclusions Emixustat reduced the cation influx and retinal oxygen consumption associated with dark conditions. VCMs are a promising potential treatment for ischemic retinal neovascularization, such as that in diabetic retinopathy.

Using adaptive optics scanning laser ophthalmoscopy (AOSLO), we measured retinal blood velocity and flow in healthy control eyes and eyes of diabetic patients with or without retinopathy. This cross-sectional study included 39 eyes of 30 patients with diabetes (DM) with mild non-proliferative diabetic retinopathy (NPDR) or without retinopathy (DM no DR) and 21 eyes of 17 healthy age-matched controls. Participants were imaged with a commercial optical coherence tomography angiography (OCTA) device (RTVue-XR Avanti) and AOSLO device (Apaeros Retinal Imaging System, Boston Micromachines). We analyzed AOSLO-based retinal blood velocity and flow, and OCTA-based vessel density of the superficial (SCP), deep retinal capillary plexus (DCP), and full retina. Retinal blood velocity was significantly higher in eyes with DM no DR and lower in NPDR across all vessel diameters compared to controls. Retinal blood flow was significantly higher in DM no DR and lower in NPDR in vessel diameters up to 60 &mu, m compared to controls. When comparing flow outliers (low-flow DM no DR eyes and high-flow NPDR eyes), we found they had a significantly different retinal vessel density compared to the remaining eyes in the respective groups. Retinal blood velocity and flow is increased in eyes with DM no DR, while these parameters are decreased in eyes with mild NPDR compared to healthy age-matched controls. The similarity of OCTA vessel density among outliers in the two diabetic groups suggests an initial increase followed by progressive decline in blood flow and OCTA vessel density with progression to clinical retinopathy, which warrants further investigation.

Double-barreled H(+)-selective microelectrodes were used to measure local extracellular concentration of H(+) ([H(+)]o) in the retina of dark-adapted anesthetized Long-Evans rats. The microelectrode advanced in steps of 30 μm throughout the retina from the vitreal surface to retinal pigment epithelium and then to the choroid, recording changes in [H(+)]o evoked by light stimulation. Recordings were performed in diabetic rats 1-3 months after intraperitoneal injection of streptozotocin and the results were compared with data obtained in age-matched control animals. Brief light stimulation (2.5 s) evoked changes of [H(+)]o with amplitudes of a few nM. Throughout the retina, there was a transient initial acidification for ∼200 ms followed by steady alkalinization, although amplitudes and kinetics of these components were slightly variable in different retinal layers. No significant difference was found when the light-induced [H(+)]o changes recorded in various retinal layers of early diabetic rats were compared with the [H(+)]o changes from corresponding layers of control animals. Also, when H(+)-selective microelectrodes were located in the retinal pigment epithelium (RPE) layer, an increase in H(+) was recorded, whose time course and amplitude were similar in control and diabetic rats. However, a striking difference between light-induced [H(+)]o changes in controls and diabetics was observed in the choriocapillaris, in the thin layer (10-20 μm) distal to the basal membrane of the RPE. In control rats, choroidal [H(+)]o decreased in a few cases, but much more often practically did not change. In contrast, diabetic rats demonstrated either an increase (in half of the cases) or no change in choroidal [H(+)]o. The data suggest that the active participation of the choroidal blood supply in stabilization of [H(+)]o could be partially compromised already at early stages of diabetes in rats. Interestingly, it appeared that the acid removal by the choroidal circulation was compromised most after 1 month of diabetes and tended to improve later.

PURPOSE: Changes in retinal pH may contribute to a variety of eye diseases. To study the effect of acidosis alone, we induced systemic metabolic acidosis and hypothesized that the retina would respond with altered expression of genes involved in acid/base regulation. METHODS: Systemic metabolic acidosis was induced in Long-Evans rats for up to two weeks by adding NH(4)Cl to the drinking water. After two weeks, venous pH was 7.25±0.08 (SD) and [HCO(3)(−)] was 21.4±4.6 mM in acidotic animals; pH was 7.41±0.03 and [HCO(3)(−)] was 30.5±1.0 mM in controls. Retinal mRNAs were quantified by qRT-PCR. Protein was quantified with Western blots and localized by confocal microscopy. Retinal [H+](o) was measured in vivo with pH microelectrodes in animals subject to metabolic acidosis and in controls. RESULTS: NH(4)Cl in drinking water or given IV was effective in acidifying the retina. Cariporide, a blocker of Na(+)/H(+) exchange, further acidified the retina. Metabolic acidosis for two weeks led to increases of 40 to 100% in mRNA for carbonic anhydrase isoforms II (CA-II) and XIV (CA-XIV) and acid sensing ion channels 1 and 4 (ASIC1 and ASIC4) (all p

In diabetic retinopathy, neovascularization is hypothesized to develop due to hypoxia in the retina. However, evidence for retinal hypoxia is limited, and the progressive changes in oxygenation are unknown. The objective of this study was to determine if retinal hypoxia occurs early in the development of diabetes. Intraretinal oxygen (PO2) profiles were recorded with oxygen-sensitive microelectrodes in control and diabetic Long-Evans rats at 4 and 12 weeks after induction of diabetes. Diabetes did not affect oxygen consumption in the photoreceptors in either dark or light adaptation. Oxygenation of the inner retina was not affected after 4 weeks of diabetes, although vascular endothelial growth factor levels increased. At 12 weeks, average inner retinal PO2, normalized to choriocapillaris PO2, was higher in diabetic rats than in age-matched controls, which was opposite to what was expected. Thus retinal hypoxia is not a condition of early diabetes in rat retina. Increased inner retinal PO2 may occur because oxygen consumption decreases in the inner retina.

Neuronal activity is associated with transmembrane ionic redistribution, which can lead to an osmotic imbalance. Accordingly, activity-dependent changes of the membrane potential are sometimes accompanied by changes in intracellular and/or extracellular volume. Experimental data that include distributions of ions and volume during neuronal activity are rare and rather inconsistent partly due to the technical difficulty of performing such measurements. However, progress in understanding the interrelations among ions, voltage and volume has been achieved recently by computational modelling, particularly “charge-difference” modelling. In this work a charge-difference computational model was used for further understanding of the specific roles for cations and anions. Our simulations show that without anion conductances the transmembrane movements of cations are always osmotically balanced, regardless of the stoichiometry of the pump or the ratio of Na+ and K+ conductances. Yet any changes in cation conductance or pump activity are associated with changes of the membrane potential, even when a hypothetically electroneutral pump is used in calculations and K+ and Na+ conductances are equal. On the other hand, when a Cl- conductance is present, the only way to keep the Cl-equilibrium potential in accordance with the changed membrane potential is to adjust cell volume. Importantly, this voltage-evoked Cl--dependent volume change does not affect intracellular cation concentrations or the amount of energy that is necessary to support the system. Taking other factors into consideration (i.e. the presence of internal impermeant poly-anions, the activity of cation-Cl- cotransporters, and the buildup of intra- and extracellular osmolytes, both charged and electroneutral) adds complexity, but does not change the main principles., Author summary We have developed software that calculates membrane potential and cell volume that result from redistribution of principal ions (K+, Na+, and Cl-) during normal cellular activity and experimental manipulations. Calculations in the model are done by an iterative charge-difference method that makes few assumptions about governing equations. Most of the features that were considered to be important for volume and voltage regulation were incorporated in the model, including the unique capability to perform calculations with different values of transmembrane water permeability. We have used the program to reexamine interactions between ionic fluxes, membrane potential, and cell volume and found that there was a previously unappreciated difference in the way that the distribution of cations and anions affect the cell. Na+ and K+, which are distributed unevenly across the membrane by the Na+/K+-ATPase, are primarily responsible for the membrane potential, but, contrary to popular belief, do not directly participate in volume regulation. On the other hand, the Cl- conductance determines the extent of volume changes, because Cl- has to follow the changes of membrane potential, which inevitably leads to changes in cell volume. The software is available to download and use for other investigations.

We hypothesized that the retina of diabetic animals would be unusually acidic due to increased glycolytic metabolism. Acidosis in tumors and isolated retina has been shown to lead to increased VEGF. To test the hypothesis we have measured the transretinal distribution of extracellular H(+) concentration (H(+)-profiles) in retinae of control and diabetic dark-adapted intact Long-Evans rats with ion-selective electrodes. Diabetes was induced by intraperitoneal injection of streptozotocin. Intact rat retinae are normally more acidic than blood with a peak of [H(+)]o in the outer nuclear layer (ONL) that averages 30 nM higher than H(+) in the choroid. Profiles in diabetic animals were similar in shape, but diabetic retinae began to be considerably more acidic after 5 weeks of diabetes. In retinae of 1-3 month diabetics the difference between the ONL and choroid was almost twice as great as in controls. At later times, up to 6 months, some diabetics still demonstrated abnormally high levels of [H(+)]o, but others were even less acidic than controls, so that the average level of acidosis was not different. Greater variability in H(+)-profiles (both between animals and between profiles recorded in one animal) distinguished the diabetic retinae from controls. Within animals, this variability was not random, but exhibited regions of higher and lower H(+). We conclude that retinal acidosis begins to develop at an early stage of diabetes (1-3 months) in rats. However, it does not progress, and the acidity of diabetic rat retina was diminished at later stages (3-6 months). Also the diabetes-induced acidosis has a strongly expressed local character. As result, the diabetic retinas show much wider variability in [H(+)] distribution than controls. pH influences metabolic and neural processes, and these results suggest that local acidosis could play a role in the pathogenesis of diabetic retinopathy.

Purpose: Transient changes in intraretinal oxygen tension (PO 2 ) in response to light stimuli were studied in order to understand the dynamics of light-evoked changes in photoreceptor oxidative metabolism. Methods: PO 2 changes during illumination were recorded by double-barreled microelectrodes in the outer part of the perifoveal retina in five macaques (Rhesus and Cynomolgus) and were fitted to a single exponential equation to obtain the time constant (τ) and maximum PO 2 change. Results: At the onset of light, PO 2 increased at all illuminations in all animals. The magnitude of the light-evoked PO 2 change increased with increasing illumination over 3–4 log units but decreased in all animals at the maximum illumination. The median time constant of the PO 2 change (τ ) was 26 sec and was not correlated with illumination. The time constant for the return to darkness was similar for illuminations below rod saturation. Since O 2 diffusion is fast over the short distance from the choroid to the inner segments, τ reflects the time course of the underlying change in oxidative metabolism. Conclusions: Previous results suggested that two competing processes influence the change in photoreceptor oxidative metabolism with light, Na + /K + pumping and cyclic guanosine monophosphate (cGMP) turnover. Because a single exponential fitted the PO 2 data, it appears that these processes have time constants that differ by no more than a few seconds in primate. In monkeys, τ is longer than previously reported values for cat and rat. Longer time constants are related to larger photoreceptor volume, possibly because metabolic rate is controlled by intracellular Na + , and a change in intracellular Na + after the onset of illumination occurs more slowly in larger photoreceptors. The “metabolic threshold” illumination that reduced oxygen consumption by about 10% is approximately the same as the illumination that closes 10% of the light-dependent cation channels that are open in the dark. keyw ORDS: Non-human primate; Monkey; Retinal metabolism; Photoreceptor; Retina; Oxygen; Macaque

Fluorescein angiography (FA) is the current clinical imaging standard for vascular related retinal diseases such as macular degeneration and diabetic retinopathy. However, FA is considered invasive and can provide only two-dimensional imaging. In comparison, optical coherence tomography angiography (OCTA) is noninvasive and can generate three-dimensional imaging; investigations of OCTA already demonstrated great promise in retinal vascular imaging. Yet, to further develop and apply OCTA, strengths and weaknesses between OCTA and FA need to be thoroughly compared. To avoid complications in image registration, an ideal comparison requires co-registered and simultaneous imaging by both FA and OCTA. In this Letter, we developed a system with integrated laser-scanning ophthalmoscope FA (SLO-FA) and OCTA, and conducted simultaneous dual-modality retinal vascular imaging in rodents. In imaging healthy rodent eyes, OCTA can resolve retinal capillaries better than SLO-FA does, particularly deep capillaries. In imaging rodent eyes with laser-induced choroidal neovascularization (CNV), OCTA can identify CNV that eludes SLO-FA detection.

Adequate function of the retina is dependent on proper oxygen supply. In humans, the inner retina is oxygenated via the retinal circulation. We present a method to calculate total retinal oxygen extraction based on measurement of total retinal blood flow using dual-beam bidirectional Doppler optical coherence tomography and measurement of oxygen saturation by spectrophotometry. These measurements were done on 8 healthy subjects while breathing ambient room air and 100% oxygen. Total retinal blood flow was 44.3 ± 9.0 μl/min during baseline and decreased to 18.7 ± 4.2 μl/min during 100% oxygen breathing (P

Purpose To investigate inner retinal oxygen metabolic rate (IRMRO2) during early stages of type 1 diabetes in a transgenic mouse model. Methods In current study, we involved seven diabetic mice (Akita/+, TSP1-/-) and seven control mice (TSP1-/-), and applied visible-light optical coherence tomography (vis-OCT) to image functional parameters including retinal blood flow rate, oxygen saturation (sO2) and the IRMRO2 value longitudinally from 5 weeks of age to 13 weeks of age. After imaging at 13 weeks of age, we analyzed the imaging results, and examined histology of mouse retina. Results Between diabetic mice and the control group, we observed significant differences in venous sO2 from 9 weeks of age (P = 0.006), and significant increment in IRMRO2 from 11 weeks of age (P = 0.001) in diabetic mice compared with control group. We did not find significant differences in retinal blood flow rate as well as arterial sO2 during imaging between diabetic and control mice. Histologic examination of diabetic and control mice at 13 weeks of age also revealed no anatomical retinal alternations. Conclusions In diabetic retinopathy, complications in retinal oxygen metabolism may occur before changes of retinal anatomical structure.

Quantitatively determining physiological parameters at a microscopic level in the retina furthers the understanding of the molecular pathways of blinding diseases, such as diabetic retinopathy and glaucoma. An essential parameter, which has yet to be quantified noninvasively, is the retinal oxygen metabolic rate (rMRO2). Quantifying rMRO2 is challenging because two parameters, the blood flow rate and hemoglobin oxygen saturation (sO2), must be measured together. We combined photoacoustic ophthalmoscopy (PAOM) with spectral domain-optical coherence tomography (SD-OCT) to tackle this challenge, in which PAOM measured the sO2 and SD-OCT mapped the blood flow rate. We tested the integrated system on normal wild-type rats, in which the measured rMRO2 was 297.86 ± 70.23 nl/minute. This quantitative method may shed new light on both fundamental research and clinical care in ophthalmology in the future.

Tissue [Formula: see text] was measured in the primary visual cortex of anesthetized, artificially ventilated normovolemic cats to examine tissue oxygenation with respect to depth. The method utilized 1) a chamber designed to maintain cerebrospinal fluid pressure and prevent ambient[Formula: see text] from influencing the brain, 2) a microelectrode capable of recording electrical activity as well as local[Formula: see text], and 3) recordings primarily during electrode withdrawal from the cortex rather than during penetrations. Local peaks in the [Formula: see text] profiles were consistent with the presence of numerous vessels. Excluding the superficial 200 μm of the cortex, in which the ambient[Formula: see text] may have influenced tissue[Formula: see text], there was a slight decrease (4.9 Torr/mm cortex) in [Formula: see text] as a function of depth. After all depths and cats were weighted equally, the average [Formula: see text] in six cats was 12.8 Torr, with approximately one-half of the values being ≤10 Torr. The kurtosis of the [Formula: see text] histogram, with all depths and cats weighted equally, was 3.61, and the skewness was 1.70.

Tissue[Formula: see text] was measured in the primary visual cortex of anesthetized, artificially ventilated, normovolemic cats to evaluate the effect of small doses [1 g perfluorocarbon (PFC)/kg] of a PFC emulsion (1 g PFC/1.1 ml emulsion; Alliance Pharmaceutical, San Diego, CA) on brain oxygenation. The change in tissue [Formula: see text]([Formula: see text]), resulting from briefly changing the respiratory gas from room air to 100% oxygen, was measured before and after intravenous infusion of the emulsion. Before emulsion, [Formula: see text] was 51.1 ± 45.6 Torr ( n = 8 cats). Increases in [Formula: see text] of 34.0 ± 26.1 (SD) % ( n = 8) and 16.3 ± 8.4% ( n = 6) were observed after the first and second emulsion infusions, respectively. The further increase in [Formula: see text] after the third dose (7.9 ± 10.5%; n = 7) was not statistically significant. The observed increases in tissue oxygenation as a result of the PFC infusions appear to be the result of enhanced oxygen transport to the tissue.

In 2011, the American Institute for Medical and Biological Engineering (AIMBE) (www.aimbe.org) celebrated its 20th anniversary by undertaking to identify major societal challenges to which medical and biological engineers can contribute solutions in the next 20 years. This report is a summary of the six major challenges that were identified. The report also discusses some specific areas within these high-level challenges that can form the basis for policy action, provides a brief rationale for pursuing those areas, and discusses roadblocks to progress. The six overarching challenges are: 1) engineering safe and sustainable water and food supply, 2) engineering personalized health care, 3) engineering solutions to injury and chronic diseases, 4) engineering global health through infectious disease prevention and therapy, 5) engineering sustainable bioenergy production, and 6) engineering the 21st century US economy. While arrived at independently by AIMBE, many of the elements overlap with similar challenges identified by other bodies. The similarities highlight the central mission of medical and biological engineers, working with other experts, which is to solve important problems central to human health and welfare.

Retinal O2 utilization was studied to identify the O2 consuming processes in the retina and the spatial distribution of those processes. Neural retina, retinal pigment epithelium and choroid were dissected from the toad eye and superfused with an oxygenated Ringer's solution. Double-barreled microelectrodes were used to measure O2 and local voltage simultaneously within the retina in both light and dark adaptation. The profile of PO2 was measured during a withdrawal of the electrode tip across the retinal pigment epithelium and through the neural retina. The PO2 decreased through the distal retina, reaching a minimum in the inner segment or outer nuclear layer, and then increased steadily through the proximal retina. From fitting PO2 profiles measured in the dark-adapted retina to a three-layer diffusion model, O2 consumption was found to be 1.0 +/- 0.4 and 0.4 +/- 0.3 ml O2 (100 g min)-1 in the outer and inner halves of the retina, respectively. Light decreased consumption in both halves of the retina. In steady illumination (500 nm) that saturated the ERG b- and c-waves, O2 utilization decreased significantly to 48% and 68% of the dark values in the outer and inner retina, respectively. When Na+ was removed from the superfusate to inhibit the photoreceptor Na+/K+ pump, O2 consumption in the outer retina decreased by about the same amount as in light, but O2 consumption in the inner retina was not significantly affected.

The purpose of this study was to investigate the oxygen distribution and consumption in the pigmented Long-Evans rat retina in vivo during dark and light adaptation, and to compare these results to previous work on cat and albino rat. Double-barreled microelectrodes recorded both intraretinal PO 2 depth profiles and the electroretinogram (ERG), which was used to identify the boundaries of the retina. Light adaptation decreased photoreceptor oxygen consumption per unit volume ( Q av ) from 3.0 ± 0.4 ml·100 g −1 min −1 (mean ± SEM) in darkness to 1.8 ± 0.2 ml·100 g −1 min −1 and increased minimum outer retinal PO 2 at the inner segments ( P min ) from 17.4 ± 3.0 to 29.9 ± 5.3 mmHg. The effects of light on outer retinal PO 2 and Q av were similar to those previously observed in cat, monkey, and albino rats; however, dark-adapted P min was higher in rat than cat. The parameters derived from fitting the oxygen diffusion model to the rat data were compared to those from cat. Oxygen consumption of the inner segments ( Q 2 ) and choroidal PO 2 ( P C ) in rat and cat were similar. P min was higher in rat than in cat for two reasons: first, rat photoreceptors have a shorter oxygen consuming region; and second, the retinal circulation supplied a greater fraction of consumed oxygen to rat photoreceptors. The average PO 2 across the inner retina ( P IR ) was not different in dark adaptation (25.4 ± 4.8 mmHg) and light adaptation (28.8 ± 5.4 mmHg) when measured from PO 2 profiles. However, with the microelectrode stationary at 9–18% retinal depth, a small consistent decrease in PO 2 occurred during illumination. Flickering light at 6 Hz decreased inner retinal PO 2 significantly more than an equivalent steady illumination, suggesting that changes in blood flow did not completely compensate for increased metabolism. This study comprehensively characterized rat retinal oxygenation in both light and dark, and determined the similarities and differences between rat and cat retinas.

Maintenance of core temperature is a major component of 24-h energy expenditure, and its dysregulation could contribute to the pathophysiology of obesity. The relationship among temperature, sex, and BMI, however, has not been fully elucidated in humans. This study investigated core temperature in obese and lean individuals at rest, during 20-min exercise, during sleep, and after food consumption. Twelve lean (18.5-24.9 kg/m(2)) and twelve obese (30.0-39.9 kg/m(2)) healthy participants, ages 25-40 years old, were admitted overnight in a clinical research unit. Females were measured in the follicular menstrual phase. Core temperature was measured every minute for 24 h using the CorTemp system, a pill-sized sensor that measures core temperature while in the gastrointestinal tract and delivers the measurement via a radio signal to an external recorder. Core temperature did not differ significantly between the obese and lean individuals at rest, postmeals, during exercise, or during sleep (P > 0.5), but core temperature averaged over the entire study was significantly higher (0.1-0.2 °C) in the obese (P = 0.023). Each individual's temperature varied considerably during the study, but at all times, and across the entire study, women were ~0.4 °C warmer than men (P < 0.0001). These data indicate that obesity is not associated with a lower core temperature but that women have a higher core temperature than men at rest, during sleep, during exercise, and after meals.

Oxygen tension (PO2) was measured with microelectrodes within the retina of anesthetized cats during normoxia and hypoxemia (i.e., systemic hypoxia), and photoreceptor oxygen consumption was determined by fitting PO2 measurements to a model of steady-state oxygen diffusion and consumption. Choroidal PO2 fell linearly during hypoxemia, about 0.64 mmHg/mmHg decrease in arterial PO2 (PaO2). The choroidal circulation provided approximately 91% of the photoreceptors' oxygen supply under dark-adapted conditions during both normoxia and hypoxemia. In light adaptation the choroid supplied all of the oxygen during normoxia, but at PaO2's less than 60 mmHg the retinal circulation supplied approximately 10% of the oxygen. In the dark-adapted retina the decrease in choroidal PO2 caused a large decrease in photoreceptor oxygen consumption, from approximately 5.1 ml O2/100 g.min during normoxia to 2.6 ml O2/100 g.min at a PaO2 of 50 mmHg. When the retina was adapted to a rod saturating background, normoxic oxygen consumption was approximately 33% of the dark-adapted value, and hypoxemia caused almost no change in oxygen consumption. This difference in metabolic effects of hypoxemia in light and dark explains why the standing potential of the eye and retinal extracellular potassium concentration were previously found to be more affected by hypoxemia in darkness. Frequency histograms of intraretinal PO2 were used to characterize the oxygenation of the vascularized inner half of the retina, where the oxygen distribution is heterogeneous and simple diffusion models cannot be used. Inner retinal PO2 during normoxia was relatively low: 18 +/- 12 mmHg (mean and SD; n = 8,328 values from 36 profiles) in dark adaptation, and significantly lower, 13 +/- 6 mmHg (n = 4,349 values from 19 profiles) in light adaptation. Even in the dark-adapted retina, 30% of the values were less than 10 mmHg. The mean PO2 in the inner (i.e., proximal) half of the retina was well regulated during hypoxemia. In dark adaptation it was significantly reduced only at PaO2's less than 45 mmHg, and it was reduced less at these PaO2's in light adaptation.

The oxygen distribution in the retina of six anesthetized macaques was investigated as a model for retinal oxygenation in the human retina in and adjacent to the fovea. Po2 was measured as a function of retinal depth under normal physiological conditions in light and dark adaptation with O2 microelectrodes. Oxygen consumption (Qo2) of the photoreceptors was extracted by fitting a steady-state diffusion model to Po2 measurements. In the perifovea, the Po2 was 48 ± 13 mmHg (mean and SD) at the choroid and fell to a minimum of 3.8 ± 1.9 mmHg around the photoreceptor inner segments in dark adaptation, rising again toward the inner retina. The Po2 in the inner half of the retina in darkness was 17.9 ± 7.8 mmHg. When averaged over the outer retina, photoreceptor Qo2 (called Qav) was 4.6 ± 2.3 ml O2·100 g−1·min−1 under dark-adapted conditions. Illumination sufficient to saturate the rods reduced Qav to 72 ± 11% of the dark-adapted value. Both perifoveal and foveal photoreceptors received most of their O2 from the choroidal circulation. While foveal photoreceptors have more mitochondria, the Qo2 of photoreceptors in the fovea was 68% of that in the perifovea. Oxygenation in macaque retina was similar to that previously found in cats and other mammals, reinforcing the relevance of nonprimate animal models for the study of retinal oxygenation, but there was a smaller reduction in Qo2 with light than observed in cats, which may have implications for understanding the influence of light under some clinical conditions.

The lack of capability to quantify oxygen metabolism noninvasively impedes both fundamental investigation and clinical diagnosis of a wide spectrum of diseases including all the major blinding diseases such as age-related macular degeneration, diabetic retinopathy, and glaucoma. Using visible light optical coherence tomography (vis-OCT), we demonstrated accurate and robust measurement of retinal oxygen metabolic rate (rMRO2) noninvasively in rat eyes. We continuously monitored the regulatory response of oxygen consumption to a progressive hypoxic challenge. We found that both oxygen delivery, and rMRO2 increased from the highly regulated retinal circulation (RC) under hypoxia, by 0.28 ± 0.08 μL min−1 (p < 0.001), and 0.20 ± 0.04 μL min−1 (p < 0.001) per 100 mmHg systemic pO2 reduction, respectively. The increased oxygen extraction compensated for the deficient oxygen supply from the poorly regulated choroidal circulation. Results from an oxygen diffusion model based on previous oxygen electrode measurements corroborated our in vivo observations. We believe that vis-OCT has the potential to reveal the fundamental role of oxygen metabolism in various retinal diseases.

Importance Diabetic macular nonperfusion leads to decreased perifoveal capillary blood flow, which in turn causes chronic ischemia of the retinal tissue. Using point-to-point correlation between spectral-domain optical coherence tomography (SD-OCT) and nonperfusion on fluorescein angiography, we observed that retinal capillary nonperfusion is associated with photoreceptor compromise on OCT. This study highlights a new concept of a possible contribution of the retinal deep capillary plexus to photoreceptor compromise in diabetic retinopathy in the absence of diabetic macular edema. Objective To report outer retinal structural changes associated with enlargement of the foveal avascular zone and/or capillary nonperfusion in the macular area of diabetic patients. Design, Setting, and Participants Retrospective observational cross-sectional study in 9 patients who were diagnosed as having diabetic retinopathy without diabetic macular edema and underwent fluorescein angiography and SD-OCT for diabetic retinopathy from July 8, 2014, to December 1, 2014, at a tertiary academic referral center. This analysis was conducted between December 2, 2014, and January 31, 2015. Main Outcomes and Measures Outer retinal changes on SD-OCT in areas of macular ischemia. Results The study included 13 eyes of 9 diabetic patients (4 men and 5 women aged 34-58 years) with a mean duration of diabetes mellitus of 14.5 years. Nine eyes showed outer retinal disruption revealed by SD-OCT that colocalized to areas of enlargement of the foveal avascular zone and macular capillary nonperfusion. Four fellow eyes with normal foveal avascular zones did not show any retinal changes on SD-OCT. Conclusions and Relevance Macular ischemia in diabetic patients can be associated with photoreceptor compromise. The presence of disruption of the photoreceptors on OCT in diabetic patients can be a manifestation of underlying capillary nonperfusion in eyes without diabetic macular edema. Ischemia at the deep capillary plexus may play an important role in these outer retinal changes.

Purpose To investigate the effects of a hereditary retinal degeneration on retinal oxygenation and determine whether it is responsible for the severe attenuation of retinal circulation in hereditary photoreceptor degenerations. Methods Seven adult Abyssinian cats affected by hereditary retinal degeneration were studied. Oxygen microelectrodes were used to collect spatial profiles of retinal oxygenation in anesthetized animals. A one-dimensional model of oxygen diffusion was fitted to the data to quantify photoreceptor oxygen utilization (Qo(2)). Results Photoreceptor Qo(2) progressively decreased until it reached zero in the end stage of the disease. Average inner retinal oxygen tension remained within normal limits at all disease stages, despite the observed progressive retinal vessel attenuation. Light affected photoreceptors normally, decreasing Qo(2) by approximately 50% at all stages of the disease. Conclusions Loss of photoreceptor metabolism allows choroidal oxygen to reach the inner retina, attenuating the retinal circulation in this animal model of retinitis pigmentosa (RP) and probably also in human RP. As the degeneration progresses, there is a strong relationship between changes in the a-wave of the ERG and changes in rod oxidative metabolism, indicating that these two functional measures change together.

We reviewed research on retinal oxygen (O2) distribution and use, focusing on O2 microelectrode studies in animals with circulatory patterns similar to those of humans. The inner and outer halves of the retina are different domains in terms of O2. Understanding their properties can suggest mechanisms of and therapies for retinal diseases. Inner retinal PO2 averages about 20 mm Hg. Effective O2 autoregulation of the retinal circulation ensures that inner retinal PO2 is relatively uninfluenced by systemic hypoxia and hyperoxia and increased intraocular pressure in healthy animals. Failures of the retinal circulation lead to tissue hypoxia that underlies the vasoproliferation in diabetic retinopathy and retinopathy of prematurity. Choroidal blood flow is not regulated metabolically, so systemic hypoxia and elevated intraocular pressure lead to decreases in choroidal PO2 and photoreceptor O2 consumption. The same lack of regulation allows choroidal PO2 to increase dramatically during hyperoxia, offering the potential for O2 to be used therapeutically in retinal vascular occlusive diseases and retinal detachment.

Transient changes in retinal oxygen in response to light stimuli were studied to further understand the light-evoked change in oxygen consumption. Double-barreled microelectrodes, which measured oxygen and local voltage simultaneously, were positioned near the photoreceptor inner segments of the toad neural retina-retinal pigment epithelium-choroid preparation. Light-evoked oxygen responses were measured in a normal [Na + ] solution, and in a test solution with lowered extracellular [Na + ] to inhibit Na + /K + pumping. Under the normal [Na + ] condition, retinal oxygen tension increased in response to light indicating that oxygen utilization had decreased. When the Na + concentration was lowered in the retina, the oxygen tension decreased in response to light, indicating an increase in oxygen utilization which was smaller than the Na + /K + pump effect and therefore masked under normal conditions. The increase in oxygen utilization in lowered [Na + ] was suppressed by adding 0·7 m m 3-isobutyl-1-methyl-xanthine, a phosphodiesterase inhibitor, suggesting that the response was largely due to hydrolysis and subsequent resynthesis of cyclic GMP. Results of fitting the light-evoked responses to exponential functions suggested that the decrease in oxygen consumption caused by slowing of the photoreceptor Na + /K + ATPase had a time constant between 130 and 180 sec and that the increase in oxygen utilization from increased cyclic GMP synthesis was faster.

Proliferative diabetic retinopathy is found in about half of type 1 diabetics and in approximately 10% of type 2 diabetics who have had the disease for 15 years.1 Panretinal photocoagulation (PRP) has been the most effective treatment,2,3 but the disease may continue or worsen in a substantial fraction of patients,4,5 and in one study, retreatment was effective in only approximately half the patients.5 Although PRP damages the retinal pigment epithelium and photoreceptor cell layer, it leaves the inner retina relatively unaffected6–9 and helps preserve vision. The leading hypothesis for the effectiveness of PRP is that the destruction of photoreceptors reduces the oxygen consumption of the lesioned area, allowing an increased rate of choroidal oxygen delivery to the inner retina.10 There is considerable indirect evidence supporting this hypothesis,11–13 as well as direct evidence from microelectrode measurements that show increased vitreal Po26–7,13–16 and intraretinal Po217 after photocoagulation in the retinas of animals. Choroidal Po2 is normally higher than inner retinal Po2.18–22 In order for the choroidal circulation to provide oxygen to the inner retina, the Po2 at the choroid must remain high after photocoagulation. However, we found that photocoagulation reduces choroidal Po2,17 and we hypothesized that the choroidal circulation is damaged by the lesion. Several investigators have provided histologic evidence that the choriocapillaris is damaged and does not heal completely after photocoagulation in humans,9,23,24 but the functional implications of this are rarely discussed. There have been only two studies of choroidal blood flow after photocoagulation. Chandra et al.25 measured blood flow with radioactive microspheres after photocoagulation. The flow in the retina and choroid of rabbits was reduced at times ranging from a few hours to several weeks after photocoagulation, and in monkeys, flow was reduced immediately, the only time point measured. However, their technique did not separate retinal and choroidal blood flow, and they did not determine whether the changes were local or global. In contrast, a recent study with laser Doppler flowmetry reported increased foveal choroidal blood flow 1 month after photocoagulation for proliferative diabetic retinopathy.26 We therefore reinvestigated the hypothesis that the choroid is damaged by photocoagulation, after both the large lesions used in our earlier study of retinal Po2,17 and smaller lesions more similar to those used clinically. We used in vivo scanning laser ophthalmoscopy (SLO) and microsphere counts in choroidal flat mounts in cats, techniques that could localize changes in choroidal blood flow.

PURPOSE. To test the hypothesis that intraretinal PO2 increases after photocoagulation. METHODS. Anesthetized cats underwent retinal argon laser photocoagulation. At least 4 weeks after treatment, PO2-sensitive microelectrodes were used to record intraretinal PO2 profiles from healed photocoagulation lesions in anesthetized cats breathing air. Histopathologic examination of the retinas was used to confirm that the photoreceptors were destroyed and that the inner retinal layers were preserved, though somewhat disorganized, as in human panretinal photocoagulation (PRP). RESULTS. The retina and tapetum were thinner in the lesioned retina than in the nonphotocoagulated retina. Average PO2 across the inner 50% of the retina was higher (22 10 mm Hg) in photocoagulated retina than in untreated retina (14 7m m Hg; P 0.01; n 13 cats). The minimum PO2 was also significantly higher, whereas choroidal PO2 was significantly lower in the photocoagulated retina than in untreated retina. No significant difference was found in the preretinal vitreous. After lesions, inner retinal PO2 could also be maintained above zero, even in the absence of retinal circulation. CONCLUSIONS. Previous measurements showed increased PO2 in the preretinal vitreous of rabbits and pigs (but not cats) after photocoagulation of the outer retina. These intraretinal measurements in cats provide further evidence for a chronic increase in inner retinal PO2 in lesioned areas during air breathing. (Invest Ophthalmol Vis Sci. 2008;49:380‐389) DOI: 10.1167/iovs.07-0065

Purpose To investigate the effects of hyperoxia on retinal oxygenation and oxygen consumption in the detached feline retina. Methods Retinal detachment was created in nine intact anesthetized cats by injecting 0.25% sodium hyaluronate in balanced salt solution into the subretinal space. Oxygen microelectrodes were used to collect spatial profiles of retinal Po(2) in both the attached and detached retina. A diffusion model was fitted to quantify photoreceptor oxygen consumption (Q(av)). Results In the detached retina, the Po(2) at the border between the retina and the fluid layer under the retina decreased; hyperoxia increased it to a level that was not significantly different from the control (attached retina, air breathing). Detachment did not change the Po(2) at the border between the avascular and vascularized retina; hyperoxia significantly increased the level. Oxygen consumption decreased to 47% +/- 18% of the control value in the detached retina during normoxia; hyperoxia increased Q(av) to 68% +/- 17% of control. Hyperoxia increased the average inner retinal Po(2) (P(IR)) in the detached retina to a level higher than that during normoxia. Detachment did not change P(IR) during normoxia. Conclusions Hyperoxia has been shown to improve photoreceptor survival in the detached retina. The present work suggests that hyperoxia is protective because it allowed increased photoreceptor oxygen consumption. Whereas normal Po(2)s were maintained at the inner and outer border of the avascular region during hyperoxia, Q(av) was not restored to normal, suggesting that other factors are involved in photoreceptor dysfunction during detachment in addition to insufficient oxygen delivery.

Objective: To examine the effects of acute hypoxemia and hyperglycemia on retinal pH to understand hyperglycemia-induced changes in the normal intact cat retina. Methods: Spatial profiles of extracellular hydrogen ion (H) concentration were obtained from the cat retina, in vivo, using pH-sensitive microelectrodes during normoxia ( arterial p artial p ressure o f o xygen [PaO2]=114.5±7.9 mm Hg), normoglycemia (plasma glucose concentration, 117±19 mg/dL), acute hypoxemia (PaO2= 29.5 ±2 .2 mm Hg), and acute hyperglycemia (plasma glucose concentration, 303±67 mg/dL). An H diffusion model was fitted to the outer retinal data to quantify photoreceptor H production. The inner retinal pH was also examined. Results: Hypoxemia induced a mean acute panretinal acidification of 0.16 pH units that originated from a 2.55fold increase in net photoreceptor H production. Hyperglycemia induced an acute panretinal acidification of 0.12 pH units; however, photoreceptor H production levels remained unchanged. Retinal pH changes followed the course of arterial PaO2 and blood glucose changes. Conclusions: The increase in photoreceptor H production during hypoxemia confirms the importance of glycolysis in the retina. Hyperglycemia-induced pH changes resulted from either increased inner retinal H production or decreased H clearance/neutralization. Clinical Relevance: The hyperglcemia-induced acidification that originates in the inner retina suggests that retinal acidosis may contribute to the development of diabetic retinal disease. Arch Ophthalmol. 2005;123:1684-1690

PURPOSE This work assessed the hypotheses that (1) hyperoxia is preferable to air breathing during retinal arterial occlusion, (2) hyperoxia during occlusion is beneficial in promoting recovery from arterial occlusion, and (3) hyperoxia has value even if it is delayed relative to the onset of the occlusion. METHODS Reversible branch retinal artery occlusion was produced by pressing with a glass probe onto an artery emerging from the superior part of the optic disc in the retina of anesthetized cats. During 2-hour occlusion episodes, the cats breathed 100% O(2), 1 hour of air and 1 hour of 100% O(2), 1 hour of air and 1 hour of 70% O(2), or air. Intraretinal ERGs were recorded before, during, and after the occlusion. RESULTS Hyperoxia during occlusion preserved intraretinal b-wave amplitude at 86% +/- 12% of normal; longer durations of increased oxygenation maintained the b-wave at higher levels during occlusion and increased the probability of b-wave recovery after occlusion; higher O(2) content in the breathing gas increased b-wave amplitude during recovery; and hyperoxia during occlusion decreased the time it took for the b-wave to recover after the occlusion. CONCLUSIONS Hyperoxia is preferable to air breathing during retinal arterial occlusion not only for maintaining b-wave amplitude during occlusion, but also for providing a shorter recovery time and better percentage recovery after the end of the occlusion. Even if it is not possible to begin hyperoxia at the onset of occlusion, it may still be valuable.

PURPOSE The Crabtree effect is the phenomenon of inhibition of respiration by glycolysis, as a result of elevated glucose levels. It is not certain whether the Crabtree effect occurs in the retina, which has a high glycolytic capacity. In the current study, in vivo photoreceptor oxygen consumption was examined during the normo- and hyperglycemic states in the dark-adapted cat retina to determine whether the Crabtree effect occurs in the outer retina. METHODS Spatial profiles of oxygen tension were obtained in the cat retina, in vivo, with the use of oxygen microelectrodes during control conditions and acute (5.19 +/- 0.83 hour) episodes of hyperglycemia (blood glucose, >350 mg/dL). The outer retinal portions of the profiles were fitted to a model of oxygen diffusion to quantify photoreceptor oxygen consumption. RESULTS Photoreceptor oxygen consumption did not significantly change during hyperglycemia compared with control conditions. Choroidal PO(2) decreased during hyperglycemia by an average of 5.8 +/- 7.4 mm Hg. This led to an increase in the fraction of O(2) used by the photoreceptors that was derived from the inner retina. Choroidal PO(2) did not recover when blood glucose levels were returned to normal. Average inner retinal PO(2) was not affected by the episodes of hyperglycemia. CONCLUSIONS The Crabtree effect does not occur to any significant degree in the outer retina, because hyperglycemia did not affect photoreceptor oxygen consumption. Choroidal PO(2) decreased during hyperglycemia, and the oxygen deficit was made up by the retinal circulation.

1. We studied the effect of locally applied 2-amino-4-phosphonobutyric acid (APB) on the response of X-type cat retinal ganglion cells to stimulation by sinusoidal gratings of photopic mean luminance. 2. Application of APB produced a decrease in the mean firing rate of on-centre X cells, but an increase in the mean firing rate of off-centre X cells. 3. The response and responsivity of both on- and off-centre X cells to drifting sinusoidal gratings were suppressed by APB. Under control conditions, the response amplitude was linear with grating contrast for contrasts less than 10%. During APB application, the slope of the contrast response curve was reduced, and linearity was maintained for a larger range of contrast. 4. Assuming that APB acts selectively on the cone to depolarizing bipolar cell synapse, the suppression of the response of on- and off-centre X cells suggests a functional link between depolarizing bipolar cells and both types of X cell. 5. Because APB reduces the centre response of both on- and off-centre X cells, caution should be observed in interpreting the effect of APB on higher visual centres in terms of the blockage of 'on' signals leaving the retina. 6. By analysing the spatial frequency tuning curve before and during APB application in terms of the difference-of-Gaussians model of receptive field structure, it was found that the balance between the integrated strengths of the centre and surround was not modified by APB. APB, however, had a differential effect on the summating area of the centre and surround. While the centre radius was unaffected by APB, the surround radius was reduced. This suggests that the peak sensitivity of the surround mechanism was reduced less severely than the peak sensitivity of the centre mechanism. 7. Strychnine also suppressed the response of the centre mechanism of both on- and off-centre X cells to drifting gratings.

1. Changes in different components of the cone-driven centre responses of cat retinal ganglion cells were studied before and during local application of 2-amino-4-phosphonobutyric acid (APB). Responses were elicited with bar stimuli whose luminance was above ('brighter') or below ('dimmer') the photopic background luminance. The bars were centrally located, and were similar in width to the receptive field centre. 2. APB acted differently on the on- and off-centre cells. For on-centre X and Y cells, all components of the responses to bright and dim bars were diminished by APB. For the off-centre X and Y cells. APB reduced all components except the transient increase in firing rate when the bright bar was turned off or when the dim bar was turned on. 3. These results suggest that the centre response mechanism of off-centre X and Y cells comprises APB-sensitive and APB-resistant components. The APB-sensitive component is more sustained and responds to both brightening and dimming stimuli. The APB-resistant component is more transient and responds primarily to dimming stimuli. For on-centre X and Y cells only APB-sensitive components could be demonstrated. 4. Experiments with stationary sinusoidal gratings modulated at 0.5-10 Hz showed that responses of off-centre cells were more affected by APB at low than at high temporal frequencies, confirming that the APB-sensitive pathway is responsible for more of the low temporal frequency responses. As expected from the responses to bar stimuli, APB had a uniform effect at all temporal frequencies in on-centre cells. 5. For off-centre cells, the APB-sensitive component is probably derived from input from depolarizing bipolar cells, and the APB-resistant component is derived from hyperpolarizing bipolar input, although one or both pathways could also involve amacrine cells. The combination of these pathways increases the range of temporal frequencies to which the cell can respond and also increases the range of response amplitudes. 6. The lack of differential effects on on-centre cells may have several explanations. The most likely explanations are that only depolarizing bipolars contribute significantly to the centre responses of these cells under the conditions of these experiments, or that there is an APB-sensitive synapse somewhere in the retina besides the one from cones to depolarizing bipolars.

The flash visual evoked potential (F-VEP), elicited by a 100 ms diffuse light flash presented at 2 Hz, was examined in the cat primary visual cortex (Area 17). Intracortical F-VEP depth profiles were recorded to characterize waveform changes with electrode depth. A positive surface component, with a latency of 200 ms, was the dominant waveform feature within the cortex, reversing in polarity and increasing in magnitude as the cortex was penetrated. Other prominent components with latencies of 30, 50, 100, and 125 ms were also observed. Changes in the waveform with stimulus duration and illumination were examined and revealed the sensitivity of prominent components to stimulus parameters.

1. The light peak is a large light-induced change in the DC potential across the eye (standing potential) that reaches its maximum in 5-13 min in mammals. The light peak of the intact cat eye was studied in order to define its cellular origin and stimulus-response characteristics. Direct-coupled recordings were made with a vitreal electrode and also with intraretinal and intracellular micro-electrodes. Light peaks were generally evoked with 300 sec periods of diffuse white illumination.2. Micro-electrode recordings made in the subretinal space just outside the apical membrane of the retinal pigment epithelium (r.p.e.) showed that the light peak was a change in trans-epithelial potential. No component was generated in the neural retina.3. Intracellular recordings from r.p.e. cells showed that the change in trans-epithelial potential resulted from a depolarization of the basal membrane (facing the choroid). This depolarization came after the hyperpolarization of the apical membrane that gave rise to the r.p.e. component of the c-wave of the e.r.g.4. The light peak amplitude at a constant retinal illumination was nearly linear with stimulus duration over the range 15-180 sec, and saturated at about 300 sec. The time-to-peak remained nearly constant at about 300 sec over this range. Large light peaks could be evoked with flashes as short as 10 sec if the retinal illumination was several log units above rod saturation.5. When stimulus duration was held constant at 300 sec, light peak amplitude was graded with illumination over a wide range, from 3 log units below to 2 log units above rod saturation. The threshold of the light peak was below that of the e.r.g. and only about 1.5-2.5 log units above the absolute threshold of the most sensitive ganglion cells. The increase of light peak amplitude above rod saturation was not due primarily to cones.6. The trans-epithelial light peak had an unusual dependence on stimulus area, being at least twice as large in response to diffuse light as it was in response to a large spot (10 deg diameter) of the same retinal illumination.7. These findings indicate that the light peak represents a normal physiological interaction between the retina and the r.p.e. They also suggest that the interaction involves a change in the concentration of a diffusible substance in the retina, which then either enters the r.p.e. itself, or triggers an internal messenger to cause the basal depolarization.

Previous work has shown that the cat retinal pigment epithelium (RPE) is the source of two potential changes that follow the absorption of light by photoreceptors: a hyperpolarization of the apical membrane, peaking in 2-4 s, which leads to the RPE component of the electroretinogram (ERG) c-wave, and a depolarization of the basal membrane, peaking in 5 min, which leads to the light peak. This paper describes a new basal membrane response of intermediate time course, called the delayed basal hyperpolarization. Isolation of this response from other RPE potentials showed that with maintained illumination the hyperpolarization begins approximately 2 s after light onset, peaks in 20 s, and slowly ends as the membrane repolarizes over the next 60 s. The delayed basal hyperpolarization is very small for stimuli less than 4 s in duration and grows with duration, becoming approximately 15% as large as the preceding apical hyperpolarization with stimuli longer than 20 s. Extracellularly, this response contributes to the transepithelial potential (TEP) across the RPE. In response to light the TEP first rises to a peak, the c-wave, as the apical membrane hyperpolarizes. For stimuli longer than approximately 4 s, the decline of the TEP from the peak of the c-wave results partly from the recovery of apical membrane potential and partly from the delayed basal hyperpolarization. For long periods of illumination (300 s) the delayed basal hyperpolarization leads to a trough in the TEP between the c-wave and light peak. This trough is largely responsible for a corresponding trough in vitreal recordings, which has been called the "fast oscillation." The term "fast oscillation" has also been used to denote the sequence of potential changes resulting from repeated stimuli approximately 1 min in duration. In addition to the delayed basal hyperpolarization, such responses also contain a basal off-response, a delayed depolarization.

These experiments were done to investigate the effects of light and darkness on the oxygenation of the retina in anesthetized cats. Measurements were made with double-barreled oxygen microelectrodes capable of recording both oxygen tension (PO2) and local voltages. Diffuse white illumination presented to a dark-adapted retina led to an increase in PO2 of up to 30 mmHg in the outer half of the retina. Changes were maximal at approximately 75% depth, corresponding to the outer nuclear layer. No change or decrease in PO2 was observed in the inner retina. Light-evoked increases in outer retinal PO2 were graded with the duration and strength of illumination, and were maximal in response to 60 s of illumination at rod saturation. For these stimuli, the increase at the onset of illumination was slower (average half-time, 12.2 s) than the recovery at the end of illumination (average half-time, 5.9 s), but for stimuli above rod saturation, PO2 recovered much more slowly. The profile of PO2 was measured during electrode penetration and withdrawal and during light and dark adaptation. Dark-adapted profiles were characterized by a minimum PO2 of nearly 0 mmHg at depths of 65-85%, and a steep gradient from the minimum to the choroid. During light adaptation at rod saturation, PO2 was elevated in the outer half of the retina and the minimum was eliminated. Fits of the profiles to a one-dimensional model of oxygen diffusion indicated that light reduced the oxygen consumption of the outer retina to approximately 50% of its dark-adapted value.

1. These experiments were done to investigate the effect of various degrees of hypoxia on the function of retinal ganglion cells (recorded in the optic tract) and on retinal oxygen tension. 2. The contrast sensitivity of the centre of X and Y cells, the surround of X cells and the non-linear subunits of Y cells were measured separately by choosing appropriate spatial and temporal parameters of a sinusoidal grating pattern. 3. Retinal oxygen tension was measured with a bipolar polarographic oxygen electrode positioned in the vitreous humor close to the retina. 4. The time course of changes in ganglion cell sensitivity and retinal oxygen tension was similar. However, oxygen tension frequently overshot the prehypoxic value at the end of hypoxia, while sensitivity did not. 5. The cat retina was rather resistant to hypoxia. Contrast sensitivity and mean firing rate did not change provided the arterial oxygen tension was above about 35 mmHg, but usually dropped precipitously at lower arterial values. 6. The apparent reason for this resistance is that retinal oxygen tension was well regulated, falling only 0.14 mmHg per mmHg of arterial oxygen tension for arterial values above about 35 mmHg, which corresponds to a retinal oxygen tension of about 10 mmHg. Retinal oxygen tension decreased more sharply (0.62 mmHg per mmHg) at lower values of arterial oxygen tension, where sensitivity also decreased. 7. The centre, surround and subunits reacted similarly to hypoxia. This suggests that lateral pathways (i.e. surround) and pathways which might be expected to use more synapses than the centre (i.e. surround and subunits) are not more susceptible to hypoxia.