Cytosolic Ca²⁺ and Phosphoinositide Hydrolysis Linked to Constitutively Active α_1d-Adrenoceptors in Vascular Smooth Muscle

Abstract

In the present study, we analyzed changes in intracellular Ca²⁺ levels and inositol phosphate accumulation related to a population of α_1d-adrenoceptors in rat aorta resembling constitutively active receptors. Following intracellular Ca²⁺ store depletion by noradrenaline in Ca²⁺-free medium and removal of the agonist, restoration of extracellular Ca²⁺ induced four signals: a biphasic (transient and sustained) increase in [Ca²⁺]_i, inositol phosphate accumulation, and a contractile response in the aorta. The transient increase in Ca²⁺, the inositol phosphate accumulation, and the contractile response were not observed in aortae incubated with prazosin or BMY 7378 [8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione] (a selective α_1d-adrenoceptor ligand), relating the three signals to α_1d-adrenoceptor activity. In the presence of nimodipine, only the sustained increase in Ca²⁺ and the inositol phosphate accumulation were observed, relating both signals to calcium entry through L-channels. The four signals were abolished by Ni²⁺. In the rat tail artery, where α_1d-adrenoceptors are not functionally active, restoration of extracellular Ca²⁺ after store depletion induced only a sustained increase in [Ca²⁺]_i without inositol phosphate accumulation nor contractile response. Taken together these results suggest that in the aorta, Ca²⁺ entry is required for the recovery of cytosolic calcium levels and the display of the membrane signals related to the constitutive activity of α_1d-adrenoceptors, i.e., inositol phosphate formation and Ca²⁺ entry through L-type channels, which maintains a contractile response once the agonist has been removed.