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AimEpac1-/- mice, but not Epac2-/- mice have elevated baseline permeability to albumin. This study extends the investigations of how Epac-dependent pathways modulate transvascular exchange in response to the classical inflammatory agent histamine. It also evaluates the limitations of models of blood-to-tissue exchange in transgenic mice in DCE-MRI measurements.MethodsWe measured DCE-MRI signal intensity in masseter muscle of wt and Epac1-/- mice with established approaches from capillary physiology to determine how changes in blood flow and vascular permeability contribute to overall changes of microvascular flux. We used two tracers, the high molecular weight tracer (Gadomer-17, MW 17 kDa, apparent MW 30-35 kDa) is expected to be primarily limited by diffusion and therefore less dependent on changes in blood flow and the low molecular weight tracer (Dotarem (MW 0.56 kDa) whose transvascular exchange is determined by both blood flow and permeability. Paired experiments in each animal combined with analytical methods provided an internally consistent description of microvascular transport.ResultsEpac1-/- mice had elevated baseline permeability relative to wt control mice for Dotarem and Gadomer-17. In contrast to wt mice, Epac1-/- mice failed to increase transvascular permeability in response to histamine. Dotarem underestimated blood flow and vascular volume and Gadomer-17 has limited sensitivity in extravascular accumulation.ConclusionThe study suggests that the normal barrier loosening effect of histamine in venular microvessels do not function when the normal barrier tightening effect of Epac1 is already compromised. The study also demonstrated that the numerical analysis of DCE-MRI data with tracers of different molecular weight has significant limitations.

Temperature-sensitive liposomal formulations of chemotherapeutics, such as doxorubicin, can achieve locally high drug concentrations within a tumor and tumor vasculature while maintaining low systemic toxicity. Further, doxorubicin delivery by temperature-sensitive liposomes can reliably cure local cancer in mouse models. Histological sections of treated tumors have detected red blood cell extravasation within tumors treated with temperature-sensitive doxorubicin and ultrasound hyperthermia. We hypothesize that the local release of drug into the tumor vasculature and resulting high drug concentration can alter vascular transport rate constants along with having direct tumoricidal effects. Dynamic contrast enhanced MRI (DCE-MRI) coupled with a pharmacokinetic model can detect and quantify changes in such vascular transport rate constants. Here, we set out to determine whether changes in rate constants resulting from intravascular drug release were detectable by MRI. We found that the accumulation of gadoteridol was enhanced in tumors treated with temperature-sensitive liposomal doxorubicin and ultrasound hyperthermia. While the initial uptake rate of the small molecule tracer was slower (k1=0.0478±0.011s-1 versus 0.116±0.047s-1) in treated compared to untreated tumors, the tracer was retained after treatment due to a larger reduction in the rate of clearance (k2=0.291±0.030s-1 versus 0.747±0.24s-1). While DCE-MRI assesses a combination of blood flow and permeability, ultrasound imaging of microvascular flow rate is sensitive only to changes in vascular flow rate; based on this technique, blood flow was not significantly altered 30min after treatment. In summary, DCE-MRI provides a means to detect changes that are associated with treatment by thermally-activated particles and such changes can be exploited to enhance local delivery.

We tested the hypothesis that the anti-inflammatory actions of atrial natriuretic peptide (ANP) result from the modulation of leukocyte adhesion to inflamed endothelium and not solely ANP ligation of endothelial receptors to stabilize endothelial barrier function. We measured vascular permeability to albumin and accumulation of fluorescent neutrophils in a full-thickness skin wound on the flank of LysM-EGFP mice 24 h after formation. Vascular permeability in individually perfused rat mesenteric microvessels was also measured after leukocytes were washed out of the vessel lumen. Thrombin increased albumin permeability and increased the accumulation of neutrophils. The thrombin-induced inflammatory responses were attenuated by pretreating the wound with ANP (30 min). During pretreatment ANP did not lower permeability, but transiently increased baseline albumin permeability concomitant with the reduction in neutrophil accumulation. ANP did not attenuate acute increases in permeability to histamine and bradykinin in individually perfused rat microvessels. The hypothesis that anti-inflammatory actions of ANP depend solely on endothelial responses that stabilize the endothelial barrier is not supported by our results in either individually perfused microvessels in the absence of circulating leukocytes or the more chronic skin wound model. Our results conform to the alternate hypothesis that ANP modulates the interaction of leukocytes with the inflamed microvascular wall of the 24 h wound. Taken together with our previous observations that ANP reduces deformability of neutrophils and their strength of attachment, rolling, and transvascular migration, these observations provide the basis for additional investigations of ANP as an anti-inflammatory agent to modulate leukocyte-endothelial cell interactions.

Transport of macromolecules and transmigration of leukocytes across vascular endothelium are regulated by a tight molecular junction, but the mechanisms by which these two inflammatory events are differentially controlled in time and magnitude during aseptic cutaneous wounding remain elusive. A real-time fluorescence imaging technique was developed to simultaneously track influx of Alexa 680-labeled albumin and genetically tagged enhanced green fluorescent protein-neutrophils [polymorphonuclear neutrophils (PMN)] within the wound bed. Vascular permeability increased approximately threefold more rapidly than the rate of PMN influx, reaching a maximum at 12 h, on the order of ~0.15% per minute versus ~0.05% per minute for PMN influx, which peaked at 18 h. Systemic depletion of PMN with antibody blocked their extravasation to the wound but did not alter the increase in vascular permeability. In contrast, pretreatment with antiplatelet GPIb decreased permeability by 25% and PMN influx by 50%. Hyperpermeability stimulated by the endothelium-specific agonists VEGF or thrombin at 24 h postwounding was completely inhibited by blocking Rho-kinase-dependent signaling, whereas less inhibition was observed at 1 h and neutrophil influx was not perturbed. These data suggest that in aseptic wounds, the endothelium maintains a tight junctional barrier to protein leakage that is independent of neutrophil transmigration, partially dependent on circulating platelets, and associated with Rho-kinase-dependent signaling. enhanced green fluorescent protein-polymorphonuclear neutrophils; Rho-kinase; vascular endothelial growth factor; platelets

The recent experiments in Hu et al. (Am J Physiol Heart Circ Physiol 279: H1724-H1736, 2000) and Adamson et al. (J Physiol 557: 889-907, 2004) in frog and rat mesentery microvessels have provided strong evidence supporting the Michel-Weinbaum hypothesis for a revised asymmetric Starling principle in which the Starling force balance is applied locally across the endothelial glycocalyx layer rather than between lumen and tissue. These experiments were interpreted by a three-dimensional (3-D) mathematical model (Hu et al.; Microvasc Res 58: 281-304, 1999) to describe the coupled water and albumin fluxes in the glycocalyx layer, the cleft with its tight junction strand, and the surrounding tissue. This numerical 3-D model converges if the tissue is at uniform concentration or has significant tissue gradients due to tissue loading. However, for most physiological conditions, tissue gradients are two to three orders of magnitude smaller than the albumin gradients in the cleft, and the numerical model does not converge. A simpler multilayer one-dimensional (1-D) analytical model has been developed to describe these conditions. This model is used to extend Michel and Phillips's original 1-D analysis of the matrix layer (J Physiol 388: 421-435, 1987) to include a cleft with a tight junction strand, to explain the observation of Levick (Exp Physiol 76: 825-857, 1991) that most tissues have an equilibrium tissue concentration that is close to 0.4 lumen concentration, and to explore the role of vesicular transport in achieving this equilibrium. The model predicts the surprising finding that one can have steady-state reabsorption at low pressures, in contrast to the experiments in Michel and Phillips, if a backward-standing gradient is established in the cleft that prevents the concentration from rising behind the glycocalyx. endothelial glycocalyx; tight junction; capillary permeability; vesicular transport

Interstitium contains a matrix of fibrous molecules that creates considerable resistance to water and solutes in series with the microvessel wall. On the basis of our preliminary studies (Adamson et al., 1994, Microcirculation 1(4), pp. 251-265; Fu et al., 1995 Am. J. Physiol. 269(38), pp. H212 4-H2140), by using laser-scanning confocal microscopy and a theoretical model for interstitial transport, we determined both microvessel solute permeability (P) and solute tissue diffusion coefficient (Dt) of ce-lactalbumin (Stokes radius 2.01 nm) from the rate of tissue solute accumulation and the radial concentration gradient around individually perfused microvessel in frog mesentery, [P.sup.[alpha]-lactalbumin] is 1.7 [+ or -] 0.7(SD) x [10.sup.6] cm/s (n=6). [D.sub.t]/[D.sub.free] for [alpha]-lactalbumin is 27% [+ or -] 5% (SD) (n = 6). This value of [D.sub.t]/[D.sub.free] is comparable to that for small solute sodium fluorescein (Stokes radius 0.45 nm), while [P.sup.[alpha]-lactalbumin] is only 3.4% of [P.sup.sodium flourescein]. Our results suggest that frog mesenteric tissue is much less selective to solutes than the microvessel wall. [DOI: 10.I 115/1.1865186]

The Na-K-Cl cotransport of trabecular meshwork (TM) cells is necessary to regulate intracellular volume and TM permeability. The fault in Na-K-Cl cotransport may support the pathophysiology of glaucoma. Bovine and human TM cells show strong Na-K-Cl cotransport which is checked by bumetanide and ethacrynic acid. Various hormones and neurotransmitters modulate TM cell Na-K-Cl cotransport. This cotransport activity is stimulated by the shrinkage of the cells by hypertonic media and it starts a subsequent regulatory volume increase.