Thapsigargin‐ and cyclopiazonic acid‐induced endothelium‐dependent hyperpolarization in rat mesenteric artery

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Abstract
The present study was designed to determine whether putative, selective inhibitors of the Ca2+–pump ATPase of endoplasmic reticulum, thapsigargin (TSG) and cyclopiazonic acid (CPA), induce endothelium‐dependent hyperpolarization in the rat isolated mesenteric artery. The membrane potentials of smooth muscle cells of main superior mesenteric arteries were measured by the microelectrode technique. In tissues with endothelium, TSG (10−8–10−5m) caused sustained hyperpolarization in a concentration‐dependent manner. In tissues without endothelium, TSG did not cause any change in membrane potential. CPA (10−5 m) also hyperpolarized the smooth muscle membrane, an effect that was endothelium‐dependent and long‐lasting. The hyperpolarizing responses to these agents were not affected by indomethacin or NG–nitro‐L‐arginine (L‐NOARG). In Ca2+–free medium, neither TSG nor CPA elicited hyperpolarization, in contrast to acetylcholine which generated a transient hyperpolarizing response. In rings of mesenteric artery precontracted with phenylephrine, TSG and CPA produced endothelium‐dependent relaxations. L‐NOARG significantly inhibited the relaxations to these agents, but about 40–60% of the total relaxation was resistant to L‐NOARG. The L‐NOARG‐resistant relaxations were abolished by potassium depolarization. These results indicate that TSG and CPA can cause endothelium‐dependent hyperpolarization in rat mesenteric artery possibly by releasing endothelium‐derived hyperpolarizing factor and that membrane hyperpolarization can contribute to the endothelium‐dependent relaxations to these agents. The mechanism of hyperpolarization may be related to increased Ca2+ influx into endothelial cells triggered by depletion of intracellular Ca2+ stores due to inhibition of endoplasmic reticulum Ca2+–pump ATPase activity.