Mechanism of SNAP potentiating antiproliferative effect of calcitonin gene-related peptide in cultured vascular smooth muscle cells.

We previously showed that CGRP inhibits cell proliferation which correlates with an elevation of cAMP levels in rabbit aortic vascular smooth muscle cells (VSMCs). The present study determined the effects of S-nitroso-N-acetylpenicillamine (SNAP, a nitric oxide donor) on CGRP-induced antiproliferative effects and cellular mechanism in cultured rabbit aortic VSMCs. The cells (in fifth-sixth passage) were exposed to 2.5% fetal bovine serum for 24 h in the presence or absence of SNAP, hCGRP or both.(3)H-thymidine incorporation was used to measure DNA synthesis. The results showed that SNAP (60-100 microm) significantly inhibited the proliferation and elevated cGMP levels in cultured rabbit aortic VSMCs. In combination, however, SNAP (30 microm) potentiated hCGRP (10-100 n m)-induced antiproliferation. SNAP (30 microm) and hCGRP (10-100 n m) or forskolin (10 microm), an activator of adenylate cyclase, caused more than additive cAMP elevations, but not cGMP elevations, in these cells. Quazinone, an inhibitor of cGMP-inhibited-phosphodiesterase (cGI-PDE, PDE3), or SNAP plus quazinone caused a similar potentiation as SNAP of the hCGRP-induced elevations of cAMP levels. The data indicate that SNAP-induced potentiation of CGRP's effects likely involves inhibition of cGI-PDE, thus allowing enhanced accumulation of cAMP that mediates the antiproliferative effects of hCGRP in cultured rabbit aortic VSMCs.