Induction of c-fos protein by activation of vasopressin receptors in smooth muscle cells

Stimulation of vasopressin (V1) receptors of rat aortic smooth muscle cells (A-10, ATCC CRL 1476) results in the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) to inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) with the mobilization of intracellular calcium. When A-10 cells are exposed to arginine vasopressin (AVP), there is an increase in the level of c-fos oncoprotein. The extent of induction of c-fos oncoprotein depends on both the time of exposure of the cells to AVP, reaching a maximum at 60 min after which there is a slow decline, and the concentration of AVP used, with an approximate EC50 of 1 nM which corresponds well with the Kd of vasopressin binding to these receptors. This vasopressin-mediated increase in c-fos protein level is inhibited by a V1/V2 antagonist (SKF 101498) suggesting that this is a receptor-mediated event. In addition dDAVP, a V2 selective agonist, is much less effective than AVP in inducing c-fos protein suggesting that AVP mediates its effect via V1 receptors. Desensitization of vasopressin receptors by prolonged exposure to AVP resulted in no additional induction of c-fos protein level in response to second challenge of AVP. In addition to AVP, phorbol dibutyrate (PDBu), an activator of protein kinase C (PKC), also stimulates the accumulation of c-fos protein although to a lesser extent than AVP. The above data suggest that c-fos protein levels in smooth muscle cells are regulated by AVP and the hormonal effect may be mediated through PI turnover and DAG, IP3 and Ca2+ signals.