Vascular smooth muscle cells (SMC) respond by relaxation to nitric oxide (NO) released from the endothelium which expresses a constitutive, Ca(2+)-dependent NO synthase (cNOS). SMC can, however, produce NO themselves upon stimulation by proinflammatory cytokines which induce expression of an inducible, Ca(2+)-independent NO synthase (iNOS). Protein kinase C represents another important second messenger system involved in the regulation of SMC contraction. We have investigated iNOS expression and NO synthesis in rat vascular SMC treated with the cytokines, IFN gamma and TNF alpha, in the presence or absence of the activator of protein kinase C, beta-phorbol-12-myristate 13-acetate (PMA). Treatment with PMA did not induce any significant accumulation of nitrite, a major stable metabolite of NO, in SMC. When added simultaneously with the cytokines, PMA significantly reduced nitrite accumulation induced by cytokine stimulation in a dose-dependent fashion. This inhibitory effect was mediated by activation of PKC since calphostin C, a specific PKC inhibitor, abolished the PMA effect. Further analysis of iNOS mRNA with a rat iNOS cDNA probe demonstrated that addition of PMA reduced expression of SMC iNOS mRNA, indicating that the antagonism in induction of NO synthesis between PMA and the proinflammatory cytokines acts on the transcriptional level. The inhibitory effect of PMA may be mediated via induction of a suppressor of iNOS expression, since pretreatment with PMA reduced NO production after subsequent treatment with cytokines. These observations suggest that activation of the PKC pathway is involved in a negative regulation of iNOS gene expression and this is compatible with the observation that vascular SMC contraction can be induced by PKC activation.