Small- and intermediate-conductance calcium-activated K+ channels provide different facets of endothelium-dependent hyperpolarization in rat mesenteric artery

Abstract

Activation of both small-conductance (SKCa) and intermediate-conductance (IKCa) Ca2+-activated K+ channels in endothelial cells leads to vascular smooth muscle hyperpolarization and relaxation in rat mesenteric arteries. The contribution that each endothelial K+ channel type makes to the smooth muscle hyperpolarization is unknown. In the presence of a nitric oxide (NO) synthase inhibitor, ACh evoked endothelium and concentration-dependent smooth muscle hyperpolarization, increasing the resting potential (approx. –53 mV) by around 20 mV at 3 mM. Similar hyperpolarization was evoked with cyclopiazonic acid (10 mM, an inhibitor of sarcoplasmic endoplasmic reticulum calcium ATPase (SERCA)) while 1-EBIO (300 mM, an IKCa activator) only increased the potential by a few millivolts. Hyperpolarization in response to either ACh or CPA was abolished with apamin (50 nM, an SKCa blocker) but was unaltered by 1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole (1 mM TRAM-34, an IKCa blocker). During depolarization and contraction in response to phenylephrine (PE), ACh still increased the membrane potential to around –70 mV, but with apamin present the membrane potential only increased just beyond the original resting potential (circa –58 mV). TRAM-34 alone did not affect hyperpolarization to ACh but, in combination with apamin, ACh-evoked hyperpolarization was completely abolished. These data suggest that true endothelium-dependent hyperpolarization of smooth muscle cells in response to ACh is attributable to SKCa channels, whereas IKCa channels play an important role during the ACh-mediated repolarization phase only observed following depolarization.