Phorbol esters and arginine vasopressin in vascular smooth muscle cell activation

Vascular smooth muscle cell (VSMC) contraction is the result of several interacting mechanisms. In this study such interactions in rat aortic VSMC in culture were examined with a focus on the role of protein kinase C-mediated mechanisms. The change in shape of VSMC was used as a functional parameter representative of contraction. The protein kinase C agonist, phorbol myristate acetate (PMA), and arginine vasopressin (AVP) induced a dose-dependent, progressive change in VSMC shape. The effects of PMA differed from AVP in the delay time necessary to reach the plateau of the response and in the absence with PMA of a transient rise in cytosolic free Ca2+ [( Ca2+]i). The effect of PMA was potentiated by the Ca2+ ionophore, ionomycin, and involved a verapamil-inhibitable transmembrane Ca2+ transport system. Protein kinase C inhibition by either isoquinolin-sulfonyl-O-2-methylpiperazine or protein kinase C desensitization significantly reduced the cell shape change induced by either PMA or AVP. In the case of AVP, this inhibitory effect occurred without affecting the [Ca2+]i transient. Therefore, the [Ca2+]i transient appears to be independent of acute protein kinase C regulation, since it is apparently not affected by the absence of protein kinase C activity. Protein kinase C activation by PMA produced intracellular alkalinization that is blocked by the sodium transport antagonist, amiloride. Amiloride also blocked the cell shape change induced by PMA or AVP. The intracellular alkalinization, however, was not necessary for the cell shape change to occur. Specifically, with the use of VSMC preincubated with fetal calf serum, PMA did not induce cellular pH changes but still produced cell shape changes.