Relationship between NaF- and thapsigargin-induced endothelium-dependent hyperpolarization in rat mesenteric artery

1 In isolated rat mesenteric artery with endothelium, NaF caused slowly developing hyperpolarization. The hyperpolarizing effect was unchanged in the presence of N-G-nitro-L-arginine (L-NOARG) and indomethacin, but was markedly reduced by high K+. In Ca2+-free medium or in the presence of Ni2+, NaF failed to produce hyperpolarization. 2 NaF-induced hyperpolarization was substantially unaffected by deferoxamine, an Al3+ chelator, okadaic acid and calyculin A, phosphatase inhibitors, and preincubation with pertussis toxin, suggesting that neither the action of fluoroaluminates as a G protein activator nor inhibition of phosphatase activity contributes to the hyperpolarizing effect. 3 The selective inhibitors of the Ca2+-pump ATPase of endoplasmic reticulum, thapsigargin and cyclopiazonic acid, elicited hyperpolarization, whose properties were very similar to those of NaF. When intracellular Ca2+ stores had been depleted with these inhibitors, NaF no longer generated hyperpolarization. 4 In Ca2+-free medium, NaF (or thapsigargin) caused a transient increase in the cytosolic Ca2+ concentration ([Ca2+](i)) in cultured porcine aortic endothelial cells, and subsequent application of thapsigargin (or NaF) failed to increase [Ca2+](i). 5 In arterial rings precontracted with phenylephrine, NaF produced endothelium-dependent relaxation followed by sustained contraction even in the presence of L-NOARG and indomethacin. The relaxant response was abolished by high K+ or cyclopiazonic acid. 6 These results indicate that NaF causes endothelium-dependent hyperpolarization, thereby leading to smooth muscle relaxation of rat mesenteric artery. This action appears to be mediated by the promotion of Ca2+ influx into endothelial cells that can be triggered by the emptying of intracellular Ca2+ stores, as proposed for those of thapsigargin and cyclopiazonic acid.