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

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 Ca 2+ ‐free medium or in the presence of Ni 2+ , NaF failed to produce hyperpolarization. NaF‐induced hyperpolarization was substantially unaffected by deferoxamine, an Al 3+ 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. The selective inhibitors of the Ca 2+ ‐pump ATPase of endoplasmic reticulum, thapsigargin and cyclopiazonic acid, elicited hyperpolarization, whose properties were very similar to those of NaF. When intracellular Ca 2+ stores had been depleted with these inhibitors, NaF no longer generated hyperpolarization. In Ca 2+ ‐free medium, NaF (or thapsigargin) caused a transient increase in the cytosolic Ca 2+ concentration ([Ca 2+ ] i ) in cultured porcine aortic endothelial cells, and subsequent application of thapsigargin (or NaF) failed to increase [Ca 2+ ] i . 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. 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 Ca 2+ influx into endothelial cells that can be triggered by the emptying of intracellular Ca 2+ stores, as proposed for those of thapsigargin and cyclopiazonic acid.British Journal of Pharmacology (1999) 126 , 1567–1574; doi: 10.1038/sj.bjp.0702466