Small- and intermediate-conductance Ca2+-activated K+ channels directly control agonist-evoked nitric oxide synthesis in human vascular endothelial cells

The contribution of small-conductance (SKCa) and intermediate-conductance Ca2+-activated K+ (IKCa) channels to the generation of nitric oxide (NO) by Ca2+-mobilizing stimuli was investigated in human umbilical vein endothelial cells (HUVECs) by combining single-cell microfluorimetry with perforated patch-clamp recordings to monitor agonist-evoked NO synthesis, cytosolic Ca2+ transients, and membrane hyperpolarization in real time. ATP or histamine evoked reproducible elevations in NO synthesis and cytosolic Ca2+, as judged by 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) and fluo-3 fluorescence, respectively, that were tightly associated with membrane hyperpolarizations. Whereas evoked NO synthesis was unaffected by either tetraethylammonium (10 mmol/l) or BaCl2 (50 mu mol/l) + ouabain (100 mu mol/l), depleting intracellular Ca2+ stores by thapsigargin or removing external Ca2+ inhibited NO production, as did exposure to high ( 80 mmol/l) external KCl. Importantly, apamin and charybdotoxin (ChTx)/triarylmethane (TRAM)-34, selective blockers SKCa and IKCa channels, respectively, abolished both stimulated NO synthesis and membrane hyperpolarization and decreased evoked Ca2+ transients. Apamin and TRAM-34 also inhibited an agonist-induced outwardly rectifying current characteristic of SKCa and IKCa channels. Under voltage-clamp control, we further observed that the magnitude of agonist-induced NO production varied directly with the degree of membrane hyperpolarization. Mechanistically, our data indicate that SKCa and IKCa channel-mediated hyperpolarization represents a critical early event in agonist-evoked NO production by regulating the influx of Ca2+ responsible for endothelial NO synthase activation. Moreover, it appears that the primary role of agonist-induced release of intracellular Ca2+ stores is to trigger the opening of both KCa channels along with Ca2+ entry channels at the plasma membrane. Finally, the observed inhibition of stimulated NO synthesis by apamin and ChTx/TRAM-34 demonstrates that SKCa and IKCa channels are essential for NO-mediated vasorelaxation.