Calcium influx into endothelial cells and formation of endothelium-derived relaxing factor is controlled by the membrane potential

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Abstract
We studied the role of the membrane potential in the control of the intracellular free calcium concentration ([Ca2+]i) and release of the two autacoids endothelium-derived relaxing factor (EDRF = nitric oxide) and prostaglandin I2 in endothelial cells. ATP (3 μmol/l) and bradykinin (1 nmol/l) evoked rapid increases (sixfold) in [Ca2+]i in cultured endothelial cells. [Ca2+]i remained elevated over several minutes. When the cells were depolarized, either by K+ (70–90 mmol/l) or by preincubation with the blocker of K+ channels tetraethylammonium (3 mmol/l), the initial peak of [Ca2+]i remained unaffected but [Ca2+]i returned significantly faster to resting levels, indicating a reduction in Ca2+ influx. In native, freshly isolated endothelial cells, K+ abolished increases in [Ca2+]i induced by acetylcholine (3 μmol/l). Release of EDRF in response to bradykinin (cultured cells) and acetylcholine (native cells) was inhibited by K+ (by 70%), whereas release of prostaglandin I2 was not significantly reduced. Preincubation of cultured endothelial cells with the receptor-independent stimulus thimerosal (5 μmol/l, 40 min) evoked a long-lasting release of EDRF and small elevations of [Ca2+]i (twofold) after washout of the drug. Depolarization with K+ decreased thimerosal-induced EDRF release and [Ca2+]i in a reversible manner. In patch-clamped endothelial cells, bradykinin (1 nmol/l) induced transient hyperpolarizations that were significantly prolonged by BRL 34915 (1 μmol/l), an activator of K+ channels. BRL 34915 also elicited increases in [Ca2+]i, particularly in thimerosal-stimulated endothelial cells. These effects were abolished by K+. We conclude that the initial rise in [Ca2+]i in response to receptor-binding agonists, caused by mobilization of Ca2+ from intracellular stores, activates K+ channels, thereby inducing hyperpolarization. This hyperpolarization provides the driving force for transmembrane Ca2+ influx into endothelial cells and is thus an important signal for synthesis and release of EDRF.