Functional coupling of Na(+)-K(+)-2Cl- cotransport and Ca(2+)-dependent K+ channels in vascular endothelial cells

To determine whether the activation of Na(+)-K(+)-2Cl- cotransport by Ca(2+)-mobilizing agonists is a direct effect of Ca2+ or is secondary to activation of Ca(2+)-dependent K+ channels [via cell shrinkage or decreased intracellular Cl- concentration ([Cl-]), we measured K+ fluxes in aortic endothelial cells in response to ATP and bradykinin. With either agonist there was an immediate bumetanide-insensitive efflux inhibitable by the K+ channel blockers tetrabutylammonium (TBA, 23 mM) and quinidine (1 mM), followed several minutes later by increased bumetanide-sensitive efflux or influx (Na(+)-K(+)-2Cl- cotransport). ATP induced a loss of cell K+ that was prevented by TBA and augmented by bumetanide. Both TBA and quinidine prevented the stimulation of cotransport by agonists but not by hypertonic shrinkage. Raising medium [K+] to prevent K+ loss also blocked activation of cotransport by agonists. The results indicate that the stimulation of Na(+)-K(+)-2Cl- cotransport by Ca2+ is not direct but instead is indirect via activation of Ca(2+)-dependent K+ channels and a resulting decrease in cell volume and intracellular [Cl-]. This suggests that at least one role of Na(+)-K(+)-2Cl- cotransport in endothelial cells is to maintain cell volume and intracellular [Cl-] during agonist stimulation.