INSULIN ACUTELY INHIBITS CULTURED VASCULAR SMOOTH-MUSCLE CELL CONTRACTION BY A NITRIC-OXIDE SYNTHASE-DEPENDENT PATHWAY

Insulin acutely decreases contractile agonist-induced Ca2+ influx and contraction in endothelium-free cultured vascular smooth muscle (VSM) cells, but the mechanism is not known. Since it has been reported that insulin-induced vasodilation in humans is linked to nitric oxide synt hase activity, we wished to determine whether insulin inhibits Ca2+ in flux and contraction of cultured vascular smooth muscle cells by a nit ric oxide synthase-dependent pathway. Primary cultures of endothelial cell-free VSM cells from canine femoral artery were preincubated with and without 1 nmol/L insulin for 30 minutes, and the 5-minute producti on of cGMP was measured. Insulin alone did not affect cGMP production, but in the presence of 10(-5) mol/L serotonin insulin stimulated cGMP production by 60%. N-G-monomethyl-L-arginine (0.1 mmol/L), an inhibit or of nitric oxide synthase, inhibited the conversion of arginine to c itrulline by these cells, blocked insulin-stimulated cGMP production, and blocked the inhibition by insulin of 5-hydroxytryptamine (5-HT)-st imulated Mn+2 (a Ca2+ surrogate) influx and contraction. Insulin did n ot affect contraction of VSM cells grown under conditions designed to deplete the cells of tetrahydrobiopterin, an essential cofactor of nit ric oxide synthase. These studies demonstrate that insulin acutely inh ibits 5-HT-stimulated Ca2+ influx and contraction of endothelium-free cultured VSM cells by a nitric oxide synthase-dependent mechanism.