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Purpose: Microvascular cell death is a prominent pathologic feature of the retinopathy associated with insulin-deficient diabetes. The aim of this study was to test the hypothesis that reduced insulin action may contribute to microvascular damage in the diabetic retina., Methods: Microvascular complexes were isolated from retinas of healthy rats and those made insulin deficient by streptozotocin. As a model of ischemia, freshly isolated microvessels were maintained in a glucose-free, low-oxygen solution. Cell viability in pericyte-containing retinal microvessels was assayed by trypan blue dye exclusion. Cleaved caspase-3 immunoreactivity and nuclear morphology were used to detect apoptotic cells., Results: Ischemia significantly increased apoptotic cell death in pericyte-containing microvessels of the normal rat retina. In a dose-dependent (IC(50) = 600 pM) manner, physiological concentrations of insulin markedly decreased ischemic cell death in the retinal microvasculature. This insulin-mediated vasoprotection was prevented by inhibitors of phosphatidylinositol-3-OH kinase and extracellular signal-regulated kinase. Soon after the onset of diabetes, insulin failed to significantly diminish cell death in ischemic retinal microvessels, in contrast to the prosurvival effect of this hormone on nondiabetic microvessels., Conclusions: Results of this study support the hypothesis that a physiological concentration of insulin exerts a prosurvival effect on ischemic retinal microvessels and that diabetes impairs the ability of insulin to protect against ischemia-induced microvascular cell death. The authors propose that the loss of insulin-mediated vasoprotection may be a previously unappreciated mechanism by which diabetic retinopathy progresses.

Evidence is accumulating that pericyte-containing microvessels, which constitute the largest component of the circulatory system, actively regulate capillary perfusion. Because the retinal vasculature is highly specialized for the local control of blood flow, experimental study of its microvessels is proving useful in the quest to elucidate the mechanisms by which local perfusion is regulated. The microcirculation of the retina is also a focus of considerable attention due to its vulnerability to diabetes, which is a leading cause of vision loss. Based on the premise that the transmembrane movement of ions plays a critical role in regulating the function of pericytes, investigators are using the patch-clamp technique to study these contractile mural cells. This review highlights recent progress made in understanding how ion channels and transporters mediate responses of the retinal microvasculature to vasoactive signals.

Purpose: It was recently proposed that activation of P2X(7) purinoceptors may play a role in causing cell death in the pericyte-containing microvasculature of the diabetic retina. This hypothesis is supported by the observation that diabetes enhances lethal pore formation in retinal microvessels exposed to synthetic P2X(7) agonists. The goal of this study was to determine whether purinergic vasotoxicity can be triggered by the endogenous molecule nicotinamide adenosine dinucleotide (NAD(+)), which is a substrate for ecto-ribosylation reactions known to activate P2X(7) receptor/channels in other cell types., Methods: Pericyte-containing retinal microvessels were isolated from normal and streptozotocin-injected rats. Trypan blue dye exclusion was used to assess cell viability, YO-PRO-1 uptake was used to identify cells with P2X(7)-induced pores, and ethenoadenosine antibodies were used to detect ecto-adenosine diphosphate (ADP)-ribosyltransferase (ART) activity., Results: In freshly isolated retinal microvessels, it was found that extracellular NAD(+), but not its catabolites, caused cell death (half-maximal effective concentration [EC(50)] = 2 nM) by a mechanism involving the activation of P2X(7) purinoceptors and the formation of transmembrane pores. A series of experiments provided evidence that NAD(+), which is not a direct purinergic agonist, serves as a substrate for ecto-ribosylation reactions that subsequently trigger P2X(7)-dependent cell death in the retinal microvasculature. Soon after the onset of diabetes, the sensitivity of retinal microvessels to the vasotoxic effect of extracellular NAD(+) increased by approximately 100-fold., Conclusions: Purinergic vasotoxicity triggered by extracellular NAD(+) is a newly recognized mechanism that may contribute to the cell death observed in the pericyte-containing microvascular of the diabetic retina.

Objective: Little is known about the electrotonic architecture of the pericyte-containing retinal microvasculature. Here, the authors focus on the cell-to-cell transmission of hyperpolarization, which can induce abluminal pericytes to relax and lumens to dilate., Methods: With perforated-patch pipettes, the authors monitored the membrane potentials and ionic currents of pairs of pericytes located on freshly isolated rat retinal microvessels. Voltage changes were induced by administering electrical stimuli into pericytes, miniperfusing the KATP channel opener pinacidil, or using oxotremorine to activate chloride channels., Results: Suggestive of extensive cell-to-cell communication, spontaneous voltage changes were strikingly similar in widely separated pericytes. In addition, injection of current into one of a pair of sampled pericytes always elicited a voltage response in the other sampled pericyte; the gap junction uncoupler, heptanol, blocked this transmission. In the dual recordings, hyperpolarization spreading from a current-injected pericyte decayed approximately 40% within 100 microm. In contrast, pinacidil-induced hyperpolarizations diminished by only approximately 2% in 100 microm. Depolarizations also appeared to spread with similar transmission efficacies., Conclusions: Based on the experiments, the authors propose that key features of the electrotonic architecture of retinal microvessels include highly efficient cell-to-cell communication within the endothelium and relatively inefficient transmission at pericyte/endothelial junctions. Thus, the endothelium is likely to provide an efficient pathway that functionally links contractile pericytes and thereby, serves to coordinate the vasomotor response of a retinal capillary.

Although inwardly rectifying potassium (K(IR)) channels are known to have important functional roles in arteries and arterioles, knowledge of these channels in pericyte-containing microvessels is limited. A working hypothesis is that K(IR) channel activity affects the membrane potential and thereby the contractile tone of abluminal pericytes whose contractions and relaxations may regulate capillary perfusion. Because pericyte function is thought to be particularly important in the retina, we used the perforated-patch technique to monitor the ionic currents of pericytes located on microvessels freshly isolated from the rat retina. In addition, because changes in ion channel function may contribute to microvascular dysfunction in the diabetic retina, we also recorded from pericyte-containing microvessels of streptozotocin-injected rats. Using barium to identify K(IR) currents, we found that there is a topographical heterogeneity of these currents in the pericyte-containing microvasculature of the normal retina. Specifically, the K(IR) current detected at distal locations is strongly rectifying, but the proximal K(IR) current is weakly rectifying and has a smaller inward conductance. However, soon after the onset of diabetes, these differences diminish as the rectification and inward conductance of the proximal K(IR) current increase. These diabetes-induced changes were reversed by an inhibitor of polyamine synthesis and could be mimicked by spermine, whose concentration is elevated in the diabetic eye. Hence, spermine is a candidate for mediating the effect of diabetes on the function of microvascular K(IR) channels. In addition, our findings raise the possibility that functional changes in K(IR) channels contribute to blood flow dysregulation in the diabetic retina.

We tested the hypothesis that extracellular lactate regulates the function of pericyte-containing retinal microvessels. Although abluminally positioned pericytes appear to adjust capillary perfusion by contracting and relaxing, knowledge of the molecular signals that regulate the contractility of these mural cells is limited. Here, we focused on lactate because this metabolic product is in the retinal extracellular space under both physiological and pathophysiological conditions. In microvessels freshly isolated from the adult rat retina, we used perforated-patch pipettes to monitor ionic currents, fura-2 to measure calcium levels, and time-lapse photography to visualize changes in mural cell contractility and lumen diameter. During lactate exposure, pericyte calcium rose; these cells contracted, and lumens constricted. This contractile response appears to involve a cascade of events resulting in the inhibition of Na+/Ca2+ exchangers (NCXs), the decreased of which function causes pericyte calcium to increase and contraction to be triggered. On the basis of our observation that gap junction uncouplers minimized the lactate-induced rise in pericyte calcium, we propose that the NCXs inhibited by lactate are predominately located in the endothelium. Indicative of the importance of endothelial/pericyte gap junctions, uncouplers of these junctions switched the pericyte response to lactate from contraction to relaxation. In addition, we observed that hypoxia, which closes microvascular gap junctions, also switched lactate's effect from vasocontraction to vasorelaxation. Thus the response of pericyte-containing retinal microvessels to extracellular lactate is metabolically modulated. The ability of lactate to serve as a vasoconstrictor when energy supplies are ample and a vasodilator under hypoxic conditions may be an efficient mechanism to link capillary function with local metabolic need.

The purpose if this study was to elucidate how extracellular ATP causes cell death in the retinal microvasculature. Although ATP appears to serve as a vasoactive signal acting via P2X(7) and P2Y(4) purinoceptors, this nucleotide can kill microvascular cells of the retina. Because P2X(7) receptor activation causes transmembrane pores to form and microvascular cells to die, we initially surmised that pore formation accounted for ATP's lethality. To test this hypothesis, we isolated pericyte-containing microvessels from rat retinas, assessed cell viability using Trypan blue dye exclusion, detected pores by determining the uptake of the fluorescent dye YO-PRO-1, measured intracellular Ca(2+) with the use of fura-2, and monitored ionic currents via perforated patch pipettes. As predicted, ATP-induced cell death required P2X(7) receptor activation. However, we found that pore formation was minimal because ATP's activation of P2Y(4) receptors prevented P2X(7) pores from forming. Rather than opening lethal pores, ATP kills via a mechanism involving voltage-dependent Ca(2+) channels (VDCC). Our experiments suggest that when high concentrations of ATP caused nearly all microvascular P2X(7) receptor channels to open, the resulting profound depolarization opened VDCC. Consistent with lethal Ca(2+) influx via VDCC, ATP-induced cell death was markedly diminished by the VDCC blocker nifedipine or a nitric oxide (NO) donor that inhibited microvascular VDCC. We propose that purinergic vasotoxicity is normally prevented in the retina by NO-mediated inhibition of VDCC and P2Y(4)-mediated inhibition of P2X(7) pore formation. Conversely, dysfunction of these protective mechanisms may be a previously unrecognized cause of cell death within the retinal microvasculature.

The aim of this study was to identify the mechanisms by which angiotensin II alters the physiology of the pericyte-containing microvasculature of the retina. Despite evidence that this vasoactive signal regulates capillary perfusion by inducing abluminal pericytes to contract and thereby microvascular lumens to constrict, little is known about the events linking angiotensin exposure with pericyte contraction. Here, using microvessels freshly isolated from the adult rat retina, we monitored pericyte currents via perforated-patch pipettes, measured pericyte calcium levels with fura-2 and visualized pericyte contractions and lumen constrictions by time-lapse photography. We found that angiotensin activates nonspecific cation (NSC) and calcium-activated chloride channels; the opening of these channels induces a depolarization that is sufficient to activate the voltage-dependent calcium channels (VDCCs) expressed in the retinal microvasculature. Associated with these changes in ion channel activity, intracellular calcium levels rise, pericytes contract and microvascular lumens narrow. Our experiments revealed that an influx of calcium through the NSC channels is an essential step linking the activation of AT(1) angiotensin receptors with pericyte contraction. Although not required in order for angiotensin to induce pericytes to contract, calcium entry via VDCCs serves to enhance the contractile response of these cells. In addition to activating nonspecific cation, calcium-activated chloride and voltage-dependent calcium channels, angiotensin II also causes the functional uncoupling of pericytes from their microvascular neighbours. This inhibition of gap junction-mediated intercellular communication suggests a previously unappreciated complexity in the spatiotemporal dynamics of the microvascular response to angiotensin II.

In this study we tested the hypothesis that extracellular ATP regulates the function of the pericyte-containing retinal microvessels. Pericytes, which are more numerous in the retina than in any other tissue, are abluminally located cells that may adjust capillary perfusion by contracting and relaxing. At present, knowledge of the vasoactive molecules that regulate pericyte function is limited. Here, we focused on the actions of extracellular ATP because this nucleotide is a putative glial-to-vascular signal, as well as being a substance released by activated platelets and injured cells. In microvessels freshly isolated from the adult rat retina, we monitored ionic currents via perforated-patch pipettes, measured intracellular calcium levels with the use of fura-2, and visualized microvascular contractions with the aid of time-lapse photography. We found that ATP induced depolarizing changes in the ionic currents, increased calcium levels and caused pericytes to contract. P2X7 receptors and UTP-activated receptors mediated these effects. Consistent with ATP serving as a vasoconstrictor for the pericyte-containing microvasculature of the retina, the microvascular lumen narrowed when an adjacent pericyte contracted. In addition, the sustained activation of P2X7 receptors inhibited cell-to-cell electrotonic transmission within the microvascular networks. Thus, ATP not only affects the contractility of individual pericytes, but also appears to regulate the spatial and temporal dynamics of the vasomotor response.

Purpose: Pericytes are positioned on the abluminal wall of capillaries and are thought to play a role in regulating retinal blood flow. Although endothelin (ET)-1 is a putative endothelium-pericyte signal, the mechanisms by which this molecule regulates pericyte function remain unclear. Because ion channels play a vital role in the response of pericytes to extracellular signals, this study was undertaken to assess the effects of ET-1 on ionic currents., Methods: The perforated-patch configuration of the patch-clamp technique was used to monitor whole-cell currents of pericytes located on microvessels freshly isolated from the rat retina. To assay cell-to-cell coupling within retinal microvessels, a gap junction--permeant tracer was loaded through patch pipettes into pericytes and the spreading of the tracer detected by immunohistochemistry., Results: ET-1 acting through ET(A) receptors altered pericyte currents and caused depolarization of the membrane potential. The effects on pericyte currents were dynamic over time. Initially, the nonspecific cation (NSC) and calcium-activated chloride (Cl(Ca)) currents were activated and the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) current inhibited. Subsequently, by a mechanism sensitive to a protein kinase C (PKC) inhibitor, the NSC, Cl(Ca), and voltage-dependent potassium currents diminished as gap junction pathways closed within the microvessels., Conclusions: ET-1 regulates pericyte conductances by multiple mechanisms. One process involves a PKC-dependent closure of gap junction pathways resulting in loss of electrotonic input from neighboring cells. Thus, ET-1 not only affects individual microvascular cells, but also regulates the effective size of the multicellular functional units that may serve to control capillary blood flow. This regulation of intercellular communication within pericyte-containing microvessels may be an important, previously unrecognized, action of ET-1.