Basal release of nitric oxide and its interaction with endothelin-1 on single vessel hydraulic permeability.

BACKGROUND: Both endothelin-1 (ET-1) and nitric oxide (NO) are released by the endothelium and are implicated in modulating the permeability of the endothelial barrier. The present study was designed to examine the interaction between ET-1 and NO and its influence on microvascular permeability as well as the role of NO in maintaining microvascular permeability. To isolate the direct effect of ET-1 and NO, experiments were conducted under conditions where hydraulic and oncotic pressures were controlled. METHODS: Postcapillary venules in the rat mesentery were perfused in situ and paired measurements of hydraulic permeability (Lp) obtained using the modified Landis micro-occlusion method. The effect of basal endogenous NO was tested by measuring the effects of perfusion with the NO synthase inhibitor Nw-nitro-L-arginine-methyl-ester (L-NAME) (100 micromol/L) on Lp (n = 6). In addition, Lp measured after a 15-minute perfusion with L-NAME (100 micromol/L) was compared with measures of Lp obtained after perfusion with a combined mixture of L-NAME (100 micromol/L) and ET-1 (80 pmol/L) (n = 6). RESULTS: Units for Lp are mean +/- SE x 10(-8) cm x sec(-1) x cm H2O(-1). Under basal conditions, in the absence of exogenous ET-1, NO synthase inhibition led to a significant increase in Lp from 5.7 +/- 0.5 to 9.8 +/- 1.4 (p = 0.02). Compared with L-NAME alone, ET-1 + L-NAME significantly decreased Lp from 10.3 +/- 0.8 to 5.7 +/- 0.6 (p = 0.006). CONCLUSION: Constitutive release of NO from the microvascular endothelium plays a role in maintaining a basal level of microvascular permeability. Decreases in microvascular permeability seen with the administration of ET-1 are not mediated via the release of NO. These findings suggest important roles for ET-1 and NO in maintaining and modulating microvascular permeability.

Basal release of endothelin-1 and the influence of the ETB receptor on single vessel hydraulic permeability.

BACKGROUND: Endothelin-1 (ET-1) has a direct permeability decreasing effect on the microvasculature. The present study was designed to test the hypothesis that this effect is mediated via the endothelin B (ETB) receptor located on the microvascular endothelium and to determine whether basal microvascular permeability is dependent on constitutive release of ET-1. To isolate the direct effect of ET-1, experiments were conducted under conditions in which hydraulic and oncotic pressures were controlled. METHODS: Postcapillary venules in the rat mesentery were perfused in situ, and paired measurements of hydraulic permeability (Lp) were obtained using the modified Landis micro-occlusion method. Lp measured after a 15-minute perfusion with the ETB receptor blocker BQ-788 (1 micromol/L) was compared with measures of Lp obtained after perfusion with a combined mixture of BQ-788 and ET-1 (80 pmol/L) (n = 6). In addition, the effect of basal endogenous ET-1 was tested by measuring the effects of BQ-788 perfusion on Lp (n = 6). RESULTS: Units for Lp are mean +/- SE x 10(-8) cm x s(-1) cm H2O(-1). ETB receptor blockade prevented any decrease in Lp induced by ET-1 (BQ-788 alone = 7.9 +/- 0.7; BQ-788 + ET-1 = 8.2 +/- 0.8;p = 0.5). Under basal conditions and in the absence of exogenous ET-1, ETB receptor blockade led to a significant increase in Lp from 6.8 +/- 0.9 to 9.7 +/- 1.2 (p = 0.001). CONCLUSION: Decreases in microvascular permeability in single postcapillary venules after the administration of ET-1 are mediated via the ETB receptor. Constitutive release of ET-1 from the microvascular endothelium also plays a role in maintaining basal levels of permeability. These findings suggest important roles for ET-1 in maintaining and modulating microvascular permeability.

Direct actions of endothelin-1 on single vessel hydraulic permeability.

BACKGROUND: There is evidence that endothelin-1 (ET-1) increases extravasation of fluid and protein into vascular beds. The present study was designed to determine the direct effects of ET-1 on hydraulic permeability (Lp) when microvascular hydraulic and oncotic pressures are controlled. METHODS: Postcapillary venules in the rat mesentery were perfused in situ and paired measurements of Lp obtained by using the modified Landis micro-occlusion method. Lp measured after a 15-minute perfusion with Ringer's albumin solution (control) was compared with Lp after a subsequent 15-minute perfusion with one of three treatments: control (n = 4), 8 pM ET-1 (n = 6), or 80 pM ET-1 (n = 6). RESULTS: Baseline L for all vessels averaged (+/- SE) 8.1 +/-0.8 x 10(-8) cm x sec(-10 x cm H2O(-1) and was not significantly different between groups. Perfusion with either control or 8 pM ET-1 did not significantly change the Lp of any of the vessels. Significant decreases in Lp of 40 to 60% were observed in venules perfused with 80 pM ET-1. The average Lp in this group was 9.9 +/- 1.4 during baseline and decreased to 5.0 +/- 0.7 during ET-1 perfusion (p = 0.003). Washout of 80 pM ET-1 for periods of up to 15 minutes did not return Lp to baseline values. CONCLUSION: Low-dose ET-1 did not directly increase Lp in postcapillary venules. ET-1 at 80 pM, however, significantly decreased Lp. These data implicate factors other than a direct permeability-increasing effect in ET-1. At higher concentrations, ET-1 may have a protective effect on endothelial barrier function.

Methods for studying microvascular barrier function in ischemia-reperfusion injury.

Ischemia-reperfusion (I/R) injury is accompanied by functional disturbances in the microcirculation that often translate into significant morbidity and mortality. We present an overview of methods to evaluate microvascular barrier properties and how these methods can be applied to the study of I/R injury. Included is a summary of techniques for measuring 1) fluid and macromolecule flux from the vascular compartment into the interstitial space of tissues and 2) the permeability parameters of microvascular wall. Several experimental models will be surveyed including whole animal, isolated organ, individually perfused microvessels and cultured endothelial cells using examples from our own studies and work of other investigators relating to I/R-induced endothelial injury. The strengths and weaknesses of these different approaches are discussed.