Dynamic regulation of [Ca2+]iby plasma membrane Ca2+-ATPase and Na+/Ca2+exchange during capacitative Ca2+entry in bovine vascular endothelial cells

The dynamic regulation of Ca2+extrusion by the plasma membrane Ca2+-ATPase (PMCA) and Na+/Ca2+exchange (NCX) was investigated in single cultured calf pulmonary artery endothelial (CPAE) cells using indo-1 microfluorimetry to measure cytoplasmic Ca2+concentration ([Ca2+]i). The quantitative analysis of the recovery from an increase of [Ca2+]ielicited by activation of capacitative Ca2+entry (CCE) served to characterize kinetic parameters of these Ca2+extrusion systems in the intact cell. In CPAE cells the PMCA is activated in a Ca2+- and time-dependent manner. Full activation of the pump occurs only after [Ca2+]ihas been elevated for at least 1 min which results in an increase of the affinity of the pump for Ca2+and an increase in the apparent maximal extrusion rate (Vmax). Application of calmodulin antagonists W-7 and calmidazolium chloride (compound R 24571) revealed that calmodulin is a major regulator of PMCA activity in vivo. Sequential and simultaneous inhibition of PMCA and NCX suggested that both contribute to Ca2+extrusion in a non-additive fashion. The activity of one system is dynamically adjusted to compensate for changes in the extrusion rate by the alternative transporter. It was concluded that in vascular endothelial cells, the PMCA functions as a calmodulin-regulated, high-affinity Ca2+removal system. The contribution by the low-affinity NCX to Ca2+clearance became apparent at [Ca2+]i>~150 nM under conditions of submaximal activation of the PMCA.