Time-dependent modulation of capacitative Ca2+ entry signals by plasma membrane Ca2+ pump in endothelium

In vascular endothelial cells, depletion of intracellular Ca2+ stores elicited capacitative Ca2+ entry (CCE) that resulted in biphasic changes of intracellular Ca2+ concentration ([Ca2+](i)) with a rapid initial peak of [Ca2+](i) followed by a gradual decrease to a sustained plateau level. We investigated the rates of Ca2+ entry, removal, and sequestration during activation of CCE and their respective contributions to the biphasic changes of [Ca2+](i). Ca2+ buffering by mitochondria, removal 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 substitutes for Ca2+ entry through the CCE pathway, were constant and did not follow the concomitant changes of [Ca2+](i). Pharmacological inhibition of the plasma membrane Ca2+ pump, however, abolished the secondary decay phase of the CCE transient. The disparity between the biphasic changes of [Ca2+](i) and the constant rate of Ca2+ entry during CCE was the result of a delayed, Ca2+-dependent activation of the pump. These results suggest an important modulatory role of the plasma membrane Ca2+ pump in the net cellular gain of Ca2+ during CCE.