CALCIUM-ENTRY ACTIVATED BY STORE DEPLETION IN HUMAN UMBILICAL VEIN ENDOTHELIAL-CELLS

We have used the patch clamp technique combined with simultaneous measurement of intracellular Ca2+ to record ionic currents activated by depletion of intracellular Ca2+-stores in endothelial cells from human umbilical veins. Two protocols were used to release Ca2+ from intracellular stores, i.e. loading of the cells via the patch pipette with Ins(1,4,5)P-3, and extracellular application of thapsigargin. Ins(1,4,5)P-3 (10 mu M) evoked a transient increase in [Ca2+](i) in cells exposed to Ca2+-free extracellular solutions. A subsequent reapplication of extracellular Ca2+ induced an elevation of [Ca2+](i). These changes in [Ca2+](i) were very reproducible. The concomitant membrane currents were neither correlated in time nor in size with the changes in [Ca2+](i). Similar changes in [Ca2+](i) and membrane currents were observed if the Ca2+-stores were depleted with thapsigargin. Activation of these currents was prevented and holding currents at -40 mV were small if store depletion was induced in the presence of 50 mu M NPPB. This identifies the large currents, which are activated as a consequence of store-depletion, as mechanically activated Cl- currents, which have been described previously [1,2]. Loading the cells with Ins(1,4,5)P-3 together with 10 mM BAPTA induced only a very short lasting Ca2+ transient, which was not accompanied by activation of a detectable current, even in a 10 mM Ca2+-containing extracellular solution. Also thapsigargin does not activate any membrane current if the pipette solution contains 10 mM BAPTA (ruptured patches). The contribution of Ca2+-influx to the membrane current during reapplication of 10 mM extracellular calcium to thapsigargin-pretreated cells was estimated from the first time derivative of the corresponding Ca2+ transients at different holding potentials. These current values showed strong inward rectification, with a maximal amplitude of 1.0 +/- 0.3 pA at -80 mV (n = 8: membrane capacitance 59 +/- 9 pF). We conclude that store depletion in endothelial cells may activate an extremely small Ca2+ current, that is consistent with CRAC currents described in mast cells [3]. However, a non-electrogenic component and/or Ca2+-entry through a non-selective pathway cannot be ruled out.