DIRECT DEMONSTRATION OF INSULIN-LIKE GROWTH FACTOR-I-INDUCED NITRIC-OXIDE PRODUCTION BY ENDOTHELIAL-CELLS

Several lines of evidence indicate that insulin-like growth factor-I ( IGF-I) is a potent mediator of vasodilation. To elucidate the mechanis m and site of action of IGF-I, we performed continuous monitoring of n itric oxide (NO) release from endothelial cells using a highly-sensiti ve amperometric NO-sensor. Two types of cultured cells were used: huma n umbilical vein endothelial cells and immortalized rat renal interlob ar artery endothelial cells. In separate experiments, [Ca2+]i changes in response to IGF-I were measured spectrofluorometrically in fura-2-l oaded cells. Stimulation with IGF-I resulted in a rapid, dose-dependen t increase in [NO] as detected by the NO-probe positioned 1 mm above t he monolayers, followed by a sustained elevation lasting for at least five minutes. The effect of IGF-I was significantly suppressed by pret reatment with anti-IGF-I antibody, suggesting that it was specific for IGF-I. N(G)-nitro-L-arginine methyl ester, an inhibitor of NO synthes is, significantly blunted responses to IGF-I, but dexamethasone preinc ubation did not reduce the IGF-I-induced release of NO. These results indicate that the observed IGF-I-induced release of NO is a result of activation of the constitutive, rather than the inducible type of NO s ynthase in endothelial cells. Genistein, a tyrosine kinase inhibitor, resulted in a profound suppression of the IGF-I-induced release of NO. IGF-I did not affect [Ca2+]i in either type of cells. Therefore, IGF- I-induced NO production by both types of endothelial cells is mediated via a tyrosine kinase-dependent mechanism. IGF-I may be an important regulator of vascular tone in vivo through NO release from the vascula r endothelium, and may contribute to the regulation of renal blood flo w under physiological and pathophysiological conditions.