Actions of alpha-adrenoceptor blocking agents on two types of intracellular calcium stores mobilized by noradrenaline in rat aorta.

In isolated rat aortic strips noradrenaline induces a biphasic contractile response in Ca-free medium, associated with two different intracellular calcium pools, one of which is common to caffeine. We analyzed the mechanisms involved in the depletion and repletion of both intracellular Ca pools sensitive to noradrenaline in different experimental procedures in presence of prazosin, phentolamine and yohimbine. At 37 degrees C the alpha-adrenergic blocking agents inhibited contractile responses to noradrenaline in Ca-free medium, with prazosin being highly selective. alpha 2-adrenoceptors probably do not participate in the release of Ca from internal stores, as no contractile response was observed after addition of clonidine in Ca-free medium. This indicates that noradrenaline-induced Ca-release from internal stores is mainly due to activation of alpha 1-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 sensitive to noradrenaline. This effect is attributable to a blockade of alpha 1-adrenoceptors and/or inhibition of receptor-mediated signal transduction.