Down-regulation of adenylate cyclase coupled to adrenomedullin receptor in vascular smooth muscle cells.

Adrenomedullin activates receptor-mediated adenylate cyclase to cause vasorelaxation. To elucidate whether desensitization of adenylate cyclase coupled to vascular adrenomedullin receptors occurs, we studied the adenylate cyclase activity after treatment with rat adrenomedullin in cultured rat aortic vascular smooth muscle cells. Cyclic AMP (cAMP) generation induced by adrenomedullin was markedly decreased by pretreatment with adrenomedullin: a maximal reduction (approximately 80%) was induced after 2 h and persisted during 24 h. Desensitization was independent of protein kinase A, protein kinase C, protein tyrosine kinase or receptor sequestration, because pretreatment with either isoproterenol, forskolin, tetradecanoylphorbol acetate, cytochalasin D, or colchicine did not affect the adrenomedullin-stimulated cAMP response. Furthermore, preincubation with inhibitors for these protein kinases prior to pretreatment with adrenomedullin failed to affect the adrenomedullin-induced decrease in cAMP response following the second stimulation with adrenomedullin. The present results provide the evidence for the existence of desensitization of adenylate cyclase coupled to vascular adrenomedullin receptors.