INTRACELLULAR CALCIUM, CURRENTS, AND STIMULUS-RESPONSE COUPLING IN ENDOTHELIAL-CELLS

Vascular endothelium appears to be a unique organ. It not only responds to numerous hormonal and chemical signals but also senses changes in physical parameters such as shear stress, producing mediators that modulate the responses of numerous cells, including vascular smooth muscle, platelets, and leukocytes. In many cases, the initial response of endothelial cells to these diverse signals involves elevation of cytosolic Ca2+ and activation of Ca2+-dependent enzymes, including nitric oxide synthase and phospholipase A2. Both the release of Ca2+ from intracellular stores, most likely the endoplasmic reticulum, and the influx of Ca2+ from the extracellular space contribute to the [Ca2+]i increase. The most important trigger for Ca2+ release is inositol 1,4,5-trisphosphate, which is generated by the action of phospholipase C, a plasmalemmal enzyme activated in many cases by the receptor-G protein cascade. Ca2+ influx appears to be related to the activity of receptor-G protein-enzyme complex and to the degree of fullness of the endoplasmic reticulum but does not involve voltage-gated Ca2+ channels. The magnitude of the Ca2+ influx depends on the electrochemical gradient, which is modulated by the membrane potential, V(m). Under basal conditions, V(m) is dominated by a large inward rectifier K+ current. Some stimuli, e.g., acetylcholine, have been shown to hyperpolarize V(m), thus increasing the electrochemical gradient for Ca2+, which appears to be modulated by activation of Ca2+-dependent K+ and Cl- currents. However, the lack of potent and specific blockers for many of the described or postulated channels (e.g., nonselective cation channel, Ca2+-activated Cl- channel) makes an estimation of their effect on endothelial cell function rather difficult. Possible future directions of research and clinical implications are discussed.