ADENYLYL-CYCLASE ISOFORMS AND VASOPRESSIN ENHANCEMENT OF AGONIST-STIMULATED CAMP IN VASCULAR SMOOTH-MUSCLE CELLS

The influence of arginine vasopressin (AVP) on agonist-stimulated aden osine 3',5'-cyclic monophosphate (cAMP) accumulation was investigated in vascular smooth muscle cells (VSMC) cultured from rat thoracic aort a. Incubation of VSMC with AVP for 60 s produced a 2- to 2.5-fold enha ncement of isoproterenol-induced cAMP formation. AVP also increased cA MP stimulation by the prostaglandin I-2 analogue iloprost. The effect of AVP to enhance agonist-stimulated cAMP formation was completely inh ibited in cells pretreated with a selective antagonist of V-1 vasopres sin receptors but was not affected by blockade of V-2 receptors. Inhib ition of protein kinase C activation failed to alter the action of AVP to potentiate cAMP stimulation, but treatment of cells with calmoduli n antagonists significantly attenuated this effect of the peptide. Mor eover, depletion of Ca2+ stores with thapsigargin decreased AVP enhanc ement of isoproterenol-stimulated cAMP by >70%. The action of AVP to i ncrease cAMP stimulation was also demonstrated in freshly isolated str ips of rat aorta where treatment with peptide produced a twofold incre ase in isoproterenol-stimulated cAMP formation. RNA blot analysis indi cated expression in VSMC of mRNA encoding type III adenylyl cyclase, a Ca2+-calmodulin-sensitive isoform of the effector. Furthermore, when detergent-solubilized membrane extract was subjected to calmodulin aff inity chromatography, a peak of adenylyl cyclase activity was identifi ed which had affinity for calmodulin matrix in the presence of Ca2+. T he results indicate that AVP activates V-1 receptors in VSMC to enhanc e agonist-stimulated cAMP formation by a Ca2+-calmodulin-dependent mec hanism and suggest that type III adenylyl cyclase may provide a focal point in the VSMC for cross talk between constrictor and dilator pathw ays.