THAPSIGARGIN AND CYCLOPIAZONIC ACID-INDUCED ENDOTHELIUM-DEPENDENT HYPERPOLARIZATION IN RAT MESENTERIC-ARTERY

1 The present study was designed to determine whether putative, select ive inhibitors of the Ca2+-pump ATPase of endoplasmic reticulum, thaps igargin (TSG) and cyclopiazonic acid (CPA), induce endothelium-depende nt hyperpolarization in the rat isolated mesenteric artery. The membra ne potentials of smooth muscle cells of main superior mesenteric arter ies were measured by the microelectrode technique. 2 In tissues with e ndothelium, TSG (10(-8)-10(-5) M) caused sustained hyperpolarization i n a concentration-dependent manner. In tissues without endothelium, TS G did not cause any change in membrane potential. CPA (10(-5) M) also hyperpolarized the smooth muscle membrane, an effect that was endothel ium-dependent and long-lasting. 3 The hyperpolarizing responses to the se agents were not affected by indomethacin or N-G-nitro-L-arginine (L -NOARG). 4 In Ca2+-free medium, neither TSG nor CPA elicited hyperpola rization, in contrast to acetylcholine which generated a transient hyp erpolarizing response. 5 In rings of mesenteric artery precontracted w ith phenylephrine, TSG and CPA produced endothelium-dependent relaxati ons. 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 depolarizat ion. 6 These results indicate that TSG and CPA. can cause endothelium- dependent hyperpolarization in rat mesenteric artery possibly by relea sing endothelium-derived hyperpolarizing factor and that membrane hype rpolarization 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.