Cyclic AMP-elevating agents induce an inducible type of nitric oxide synthase in cultured vascular smooth muscle cells. Synergism with the induction elicited by inflammatory cytokines.

In cultured vascular smooth muscle cells, interferon gamma (IFN-gamma) induced the accumulation of nitrite, a stable metabolite of nitric oxide, in a dose- and time-dependent manner. In parallel with this reaction, this cytokine increased the mRNA and protein levels of an inducible macrophage-type of nitric oxide synthase (iNOS). Forskolin, a direct activator of adenylate cyclase, or dibutyryl cAMP alone caused small increases in nitrite accumulation and iNOS mRNA and protein levels and synergistically enhanced the IFN-gamma-stimulated reactions. 8-Bromo-cGMP neither increased by itself nor synergized with IFN-gamma to increase the same reactions. Prostaglandin E1 and beraprost, a stable analogue of prostaglandin I2, which by themselves showed only marginal effects on these reactions, also synergized with IFN-gamma to stimulate the reactions. Interleukin 1 beta or tumor necrosis factor alpha stimulated the same reactions which were similarly enhanced by forskolin. These results indicate that an elevation of intracellular cAMP, particularly in combination with inflammatory cytokines, positively regulates nitric oxide production at the level of iNOS mRNA expression in vascular smooth muscle cells.