Serotonin inhibits nitric oxide synthesis in rat vascular smooth muscle cells stimulated with interleukin-1.

We investigated the effects of serotonin (5-hydroxytryptamine; 5-HT) on nitric oxide (NO) synthesis in vascular smooth muscle cells. We measured the production of nitrite, a stable metabolite of NO, and the expression of inducible NO synthase protein in cultured rat vascular smooth muscle cells. Incubation of the cultures with interleukin-1beta (10 ng/ml) caused a significant increase in nitrite production. 5-HT inhibited nitrite production by interleukin-1beta -stimulated vascular smooth muscle cells in a concentration-dependent manner (10(-8)-10(-5) M). 5-HT-induced inhibition of nitrite production was accompanied by decreased inducible NO synthase protein accumulation in vascular smooth muscle cells. Addition of the 5-HT2 receptor antagonist ketanserin, but not the 5-HT1A receptor antagonist spiroxatrine, inhibited the effect of 5-HT. On the other hand, the 5-HT2 receptor agonist alpha-methyl-5-HT, but not the 5-HT1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin, decreased interleukin-1beta-induced nitrite production by vascular smooth muscle cells. 5-HT significantly increased protein kinase C activity in vascular smooth muscle cells, and the protein kinase C inhibitor calphostin C dose-dependently abolished the effect of 5-HT on nitrite production. After protein kinase C activity was functionally depleted by treatment of cells with phorbol 12-myristate 13-acetate for 24 h, the effect of 5-HT was abolished. These results indicate that 5-HT acts on 5-HT2 receptors and inhibits NO synthesis in interleukin-1beta-stimulated vascular smooth muscle cells at least partially through a protein kinase C-dependent pathway.