INSULIN STIMULATES ENDOTHELIN BINDING AND ACTION ON CULTURED VASCULARSMOOTH-MUSCLE CELLS

Hyperinsulinemia has been implicated as a separate risk factor for the development of accelerated cardiovascular disease, but the mechanism is unknown. Recently, we and several other groups have shown that insu lin stimulates the production and secretion of the vasoconstrictor pep tide endothelin-1 (ET-1) from vascular endothelial cells, and hyperins ulinemia results in increased plasma ET levels in vivo. However, the i nteractive effects of diabetes, insulin, and glucose on ET target tiss ues, like those on vascular smooth muscle cells (VSMC), are not well d efined. In these studies, we examined the effects of the diabetic fact ors on ET receptors and [H-3]thymidine incorporation into cultured cel ls prepared from control, streptozocin-diabetic, insulin-treated diabe tic, and hyperinsulinemic rats. Scatchard analysis of saturation bindi ng studies revealed a 2-fold increase in ET receptor number in normal VSMC treated in vitro with insulin, whereas glucose had no significant effect. Neither treatment affected receptor affinity. Similarly, aort ic smooth muscle cells, brain capillary pericytes, and kidney afferent arteriolar smooth muscle cells from rats made hyperinsulinemic in viv o each showed approximately a 2-fold increase in receptor number. This increase in receptor density probably resulted from the stimulation o f receptor protein production, because insulin caused a maximal 2.3 +/ - 0.3 (+/-SEM) fold increase in the ET(A) receptor mRNA expressed in c ultured VSMC by 4 h. Both insulin and ET significantly increased thymi dine incorporation in aortic VSMC, but ET-1 was much more potent in th is regard. However, the combined effects of insulin plus ET-1 resulted in a 10-fold increase in this index of cell proliferation, significan tly different from the effects of either peptide alone. We postulate t hat hyperinsulinemia in vivo may potentiate ET release and receptor-me diated action, thereby contributing to vascular disease in the setting of diabetes.