Acetylcholine‐evoked depolarization reduces EDHF in rat mesenteric arteries during hypertension

Abstract

After blockade of endothelium dependent hyperpolarizing factor (EDHF) with Ca2+ activated K+ channel inhibitors, acetylcholine (ACh) evokes a depolarization in the smooth muscle of mesenteric arteries from spontaneously hypertensive rats (SHR), but not from normotensive WKY rats. The aim of the present study was to use electrophysiology to identify the mechanism underlying this response and to determine whether it could affect EDHF-mediated hyperpolarization. Data showed that the depolarization was accompanied by contraction and was abolished by removal of the endothelium, atropine and U-73122 (phospholipase C inhibitor). Cyclopiazonic acid (Ca2+-ATPase inhibitor) or removal of extracellular Ca2+ also markedly diminished the response, while inhibitors of protein kinase C or nonselective cation channels (gadolinium, lanthanum and amiloride) were without effect. The depolarization was inhibited by niflumic and flufenamic acid (Ca2+ activated Cl− channel inhibitors) or by replacement of external Cl− with impermeant isethionate anion. Moreover, EDHF-mediated hyperpolarization was significantly augmented by these Cl− channel blockers. Results suggest that ACh-evoked depolarization arises from muscarinic receptor activation of phospholipase C and opening of Ca2+ activated Cl− channels following release of intracellular Ca2+. Since ACh-evoked depolarization acts to reduce EDHF-mediated hyperpolarization in SHR arteries, Ca2+ activated Cl− channels may provide a novel therapeutic target to improve endothelial function during hypertension. Supported by the NHMRC and Heart Foundation of Australia.