Role of endothelial Ca2+ stores in the regulation of hydraulic conductivity of Rana microvessels in vivo

Cytosolic Ca2+ concentration and rate of increase of the cytosolic Ca2+ concentration in the regulation of vascular permeability in Rana in vivo

Vascular permeability is assumed to be regulated by the cytosolic Ca(2+) concentration ([Ca(2+)](c)) of the endothelial cells. When permeability is increased, however, the maximum [Ca(2+)](c) appears to occur after the maximum permeability increase, suggesting that Ca(2+)-dependent mechanisms other than the absolute Ca(2+) concentration may regulate permeability. Here we investigate whether the rate of increase of the [Ca(2+)](c) (d[Ca(2+)](c)/dt) may more closely approximate the time course of the permeability increase. Hydraulic conductivity (L(p)) and endothelial [Ca(2+)](c) were measured in single perfused frog mesenteric microvessels in vivo. The relationships between the time courses of the increased L(p), [Ca(2+)](c) and d[Ca(2+)](c)/dt were examined. L(p) peaked significantly earlier than [Ca(2+)](c) in all drug treatments examined (Ca(2+) store release, store-mediated Ca(2+) influx, and store-independent Ca(2+) influx). When L(p) was increased in a store-dependent manner the time taken for L(p) to peak (3.6 +/- 0.9 min during store release, 1.2 +/- 0.3 min during store-mediated Ca(2+) influx) was significantly less than the time taken for [Ca(2+)](c) to peak (9.2 +/- 2.8 min during store release, 2.1 +/- 0.7 min during store-mediated influx), but very similar to that for the peak d[Ca(2+)](c)/dt to occur (4.3 +/- 2.0 min during store release, 1.1 +/- 0.5 min during Ca(2+) influx). Additionally, when the increase was independent of intracellular Ca(2+) stores, L(p) (0.38 +/- 0.03 min) and d[Ca(2+)](c)/dt (0.30 +/- 0.1 min) both peaked significantly before the [Ca(2+)](c) (1.05 +/- 0.31 min). These data suggest that the regulation of vascular permeability by endothelial cell Ca(2+) may be regulated by the rate of change of the [Ca(2+)](c) rather than the global [Ca(2+)].

The role of endothelial cell Ca2+store release in the regulation of microvascular permeabilityin vivo

Microvascular permeability is regulated by changes in intracellular calcium concentration. The mechanism by which this increase in calcium determines permeability under normal conditions and during stimulation with agonists remains to be elucidated. In order to determine whether calcium release from intracellular stores could contribute towards the regulation of vascular permeability, hydraulic conductivity (Lp) was measured in frog mesenteric microvessels during stimulation of the endothelial cells of these vessels with agonists that release calcium from the intracellular stores. ATP (which acts through activation of inositol 1,4,5-trisphosphate (IP3) receptors) increased Lp in the absence of calcium influx across the plasma membrane 2.3 +/- 0.3 fold (mean +/- s.e.m., P < 0.01, n = 8), which was less than the increase in the presence of calcium influx (3.1 +/- 1.1 fold). Caffeine (which acts through activation of ryanodine receptors) also increased Lp in the absence of calcium influx across the plasma membrane 3.8 +/- 1.0 fold (P < 0.01, n = 9), but by at least as much as it does in the presence of calcium influx (2.8 +/- 0.5 fold). It is surprising that there was a strong positive correlation between the size of the response during store release and the baseline permeability (r = 0.91 for ATP, r = 0.75 for caffeine). This suggests that the filling state of the stores may regulate the baseline permeability of the microvessels.

Hyperosmolarity enhances the lung capillary barrier

Although capillary barrier deterioration underlies major inflammatory lung pathology, barrier-enhancing strategies are not available. To consider hyperosmolar therapy as a possible strategy, we gave 15-minute infusions of hyperosmolar sucrose in lung venular capillaries imaged in real time. Surprisingly, this treatment enhanced the capillary barrier, as indicated by quantification of the capillary hydraulic conductivity. The barrier enhancement was sufficient to block the injurious effects of thrombin, TNF-alpha, and H2O2 in single capillaries, and of intratracheal acid instillation in the whole lung. Capillary immunofluorescence indicated that the hyperosmolar infusion markedly augmented actin filament formation and E-cadherin expression at the endothelial cell periphery. The actin-depolymerizing agent latrunculin B abrogated the hyperosmolar barrier enhancement as well as the actin filament formation, suggesting a role for actin in the barrier response. Furthermore, hyperosmolar infusion blocked TNF-alpha-induced P-selectin expression in an actin-dependent manner. Our results provide the first evidence to our knowledge that in lung capillaries, hyperosmolarity remodels the endothelial barrier and the actin cytoskeleton to enhance barrier properties and block proinflammatory secretory processes. Hyperosmolar therapy may be beneficial in lung inflammatory disease.