Indirect activation by internal calcium of chloride channels in endothelial cells

The action of internal Ca2+ on Cl- channels in endothelial cells was studied by the whole-cell clamp technique in cultured human aortic endothelial cells. Intracellular Ca2+ application by break-in of a Ca(2+)-containing pipette solution produced an outward-rectifying Cl- current after a few minutes delay. The amplitude of the Cl- current increased with the increase in the internal Ca2+ concentration, producing a maximal Cl- conductance as large as 2 nS/pF at pCa 5. The increase of the Ca(2+)-induced Cl- conductance was also dependent on the internal ATP concentration. At pCa 5, the Cl conductance per cell was 61 nS at 5 mM ATP and 24 nS at 1 mM. The calmodulin antagonists trifluoperazine and W-7 blocked the Cl- channel reversibly. The results suggest that Ca2+ activates the Cl- channels indirectly via a calmodulin-mediated pathway, and that binding of ATP to the channel is a prerequisite for activation.