Endothelial cell hyperpolarization increases [Ca²⁺](i) and venular microvessel permeability

We tested the hypothesis that Ca²⁺ influx into endothelial cells forming the walls of intact venular microvessels was increased when the cell membranes were hyperpolarized. Cytoplasmic free Ca²⁺ concentration ([Ca²⁺](i)) was measured after the endothelial cells forming the microvessel wall were loaded with fura 2, endothelial cell membrane potential was measured with the membrane potential dye bis-oxonol, and hydraulic conductivity (L(p)) of the vessels was measured by the modified Landis technique to follow changes in microvessel permeability. When microvessels were exposed to low-K⁺ (0.1 mM) Ringer solution, the membrane of the endothelial cells was hyperpolarized ~27 mV and [Ca²⁺](i) increased from 47 nM to a peak value of 151 ± 28 nM. Under the same experimental conditions, L(p) increased to a peak 6.3 times control. In the presence of ionomycin (5 μM), the initial peak [Ca²⁺](i) measured with low-K⁺ Ringer solution was 347 ± 58 nM compared with 252 ± 58 nM with ionophore and normal Ringer solution. The corresponding initial increases in L(p) were 28 times control and 10 times control, respectively. The results conform to the hypothesis that vasoactive substances that hyperpolarize the endothelial cell membrane may initiate and/or potentiate the inflammatory response in venular microvessels.