TIME-DEPENDENT MODULATION OF CAPACITATIVE CA2-MEMBRANE CA2+ PUMP IN ENDOTHELIUM( ENTRY SIGNALS BY PLASMA)

In vascular endothelial cells, depletion of intracellular Ca2+ stores elicited capacitative Ca2+ entry (CCE) that resulted in biphasic chang es of intracellular Ca2+ concentration ([Ca2+](i)) with a rapid initia l peak of [Ca2+](i) followed by a gradual decrease to a sustained plat eau level. We investigated the rates of Ca2+ entry, removal, and seque stration during activation of CCE and their respective contributions t o the biphasic changes of [Ca2+](i). Ca2+ buffering by mitochondria, r emoval by Na+/Ca2+ exchange, and a fixed electrical driving force for Ca2+ (voltage-clamp experiments) had little effect on the CCE signal. The rates of entry of Mn2+ and Ba2+, used as unidirectional substitute s for Ca2+ entry through the CCE pathway, were constant and did not fo llow the concomitant changes of [Ca2+](i). Pharmacological inhibition of the plasma membrane Ca2+ pump, however, abolished the secondary dec ay phase of the CCE transient. The disparity between the biphasic chan ges of [Ca2+](i) and the constant rate of Ca2+ entry during CCE was th e result of a delayed, Ca2+-dependent activation of the pump. These re sults suggest an important modulatory role of the plasma membrane Ca2 pump in the net cellular gain of Ca2+ during CCE.