ACTIONS OF ALPHA-ADRENOCEPTOR BLOCKING-AGENTS ON 2 TYPES OF INTRACELLULAR CALCIUM STORES MOBILIZED BY NORADRENALINE IN RAT AORTA

In isolated rat aortic strips noradrenaline induces a biphasic contrac tile response in Ca-free medium, associated with two different intrace llular calcium pools, one of which is common to caffeine. We analyzed the mechanisms involved in the depletion and repletion of both intrace llular Ca pools sensitive to noradrenaline in different experimental p rocedures in presence of prazosin, phentolamine and yohimbine. At 37-d egrees-C the alpha-adrenergic blocking agents inhibited contractile re sponses to noradrenaline in Ca-free medium, with prazosin being highly selective. Alpha2-adrenoceptors probably do not participate in the re lease of Ca from internal stores, as no contractile response was obser ved after addition of clonidine in Ca-free medium. This indicates that noradrenaline-induced Ca-release from internal stores is mainly due t o activation of alpha1-adrenoceptors. At 25-degrees-C, these compounds failed to inhibit caffeine-induced contraction in Ca-free medium, but abolished the release of Ca from an intracellular store only sensitiv e to noradrenaline. This effect is attributale to a blockade of alpha1 -adrenoceptors and/or inhibition of receptor-mediated signal transduct ion.