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

Activation of both small-conductance (SK Ca ) and intermediate-conductance (IK Ca ) Ca 2+ -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 μ m . Similar hyperpolarization was evoked with cyclopiazonic acid (10 μ m , an inhibitor of sarcoplasmic endoplasmic reticulum calcium ATPase (SERCA)) while 1-EBIO (300 μ m , an IK Ca activator) only increased the potential by a few millivolts. Hyperpolarization in response to either ACh or CPA was abolished with apamin (50 n m , an SK Ca blocker) but was unaltered by 1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole (1 μ m TRAM-34, an IK Ca 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 SK Ca channels, whereas IK Ca channels play an important role during the ACh-mediated repolarization phase only observed following depolarization.