Insulin and insulin-like growth factor I (IGF-I) exhibit vasoactivity. To examine the role of the endothelium in mediating the vascular responses to insulin and IGF-I, we exposed both isolated intact rat mesenteric arteries and rat aortic rings to these growth factors in the presence and absence of endothelium. Perfusion of rat mesenteric arteries with insulin, IGF-I, or IGF-II resulted in the potentiation of arginine vasopressin (AVP)-induced vasoconstriction. Of these growth factors, IGF-I was the most potent, with a significant effect at 0.6 nM and maximal effects at 6.0 nM, followed by IGF-II and insulin. Endothelial denudation or addition of cycloheximide prevented the growth-factor effects. Tissue cGMP levels in the mesenteric artery were minimally affected by growth factors. Insulin and IGF-I vascular effects were not inhibited by BQ123, an endothelin (ET) antagonist that blocked ET-1 enhancement of AVP response. Perfusion of mesenteric arteries with IGF-I for 1 h did not alter vessel ET-1 or ET-1 mRNA contents. Addition of indomethacin markedly inhibited the IGF-I effect on AVP contraction. Thus, the mesenteric vascular effect of insulin and IGF-I is not associated with ET-1 release but appears to link to an increased release of an endothelial-derived contracting factor or the decreased production of an endothelial-derived relaxing factor from the cyclooxygenase pathway. In contrast to their action in the mesenteric artery, insulin (exceeding 100 nM) and IGF-I (1-30 nM) attenuated AVP- and norepinephrine-induced contraction in rat aortic rings. Endothelial-denudation abolished this effect.(ABSTRACT TRUNCATED AT 250 WORDS)