Insulin influences the nitric oxide cyclic nucleotide pathway in cultured human smooth muscle cells from corpus cavernosum by rapidly activating a constitutive nitric oxide synthase

Abstract

Aims: We have evaluated, in cultured human cavernosal smooth muscle cells, the expression and activity of calcium-dependent constitutive nitric oxide synthase (cNOS) and the ability of insulin to induce nitric oxide (NO) production and to increase intracellular cyclic nucleotides guanosine 3′,5′-cyclic monophosphate (cGMP) and adenosine 3′,5′-cyclic monophosphate (cAMP). Methods: cNOS mRNA was detected by RT-PCR amplification, cNOS protein by immunofluoresence, cNOS activity as L-[3H]-citrulline production from L-[3H]-arginine and cyclic nucleotides by radioimmunoassay. Results: cNOS mRNA and cNOS protein were found in cultured cells; cNOS activity was increased by 5-min exposure to 1 μmol/l calcium ionophore ionomycin (from 0.1094±0.0229 to 0.2685±0.0560 pmol/min per mg cell protein, P = 0.011) and to 2 nmol/l insulin (from 0.1214±0.0149 to 0.2045±0.0290 pmol/min per mg cell protein, P = 0.041). Insulin increased both cGMP and cAMP in a dose- and time-dependent manner (i.e. with 2 nmol/l insulin, cGMP rose from 2.71±0.10 to 6.80±0.40 pmol/106 cells at 30 min, P = 0.0001; cAMP from 1.26±0.06 to 3.02±0.30 pmol/106 cells at 60 min, P = 0.0001). NOS inhibitor NG-monomethyl-L-arginine and phosphatidylinositol 3-kinase (PI 3-kinase) inhibitors wortmannin and LY 294002 blunted these effects of insulin. The action of insulin on cyclic nucleotides persisted in the presence of phosphodiesterase inhibition, guanylate cyclase activation by NO donors and adenylate cyclase activation by Iloprost or forskolin. Conclusion: Human cavernosal smooth muscle cells, by expressing cNOS activity, are a source of NO and not only its target; in these cells, insulin rapidly activates cNOS through a PI 3-kinase pathway, with a consequent increase of both cyclic nucleotides, thus directly influencing the mechanisms involved in penile vascular tone and interplaying with classical haemodynamic mediators.