Inhibitory Effects of Insulin on Cytosolic Ca²⁺Level and Contraction in the Rat Aorta

To determine the mechanism of the inhibitory effect of insulin on vascular tone, contraction was measured simultaneously with endothelial and smooth muscle cytosolic Ca2+ level ([Ca2+]i) in the isolated rat aorta. Insulin (200 mU/mL) increased endothelial [Ca2+]i and decreased resting muscle tone. The removal of endothelium abolished the effects of insulin. In the aorta precontracted with norepinephrine, insulin (3 to 120 mU/mL) induced concentration-dependent inhibition of contraction. The relaxant effect followed the increase in endothelial [Ca2+]i and decrease in smooth muscle [Ca2+]i. The relaxant effect was attenuated by removal of endothelium or by the addition of 10(-5) mol/L NG-monomethyl-L-arginine but not by 10(-5) mol/L indomethacin. In the absence of endothelium, the relaxant effect of insulin followed the decrease in smooth muscle [Ca2+]i. These results suggest that insulin inhibits vascular contraction by dual mechanisms in the isolated rat aorta: (1) Insulin acts on vascular endothelium by increasing endothelial [Ca2+]i and releasing NO, which decreases smooth muscle [Ca2+]i and the Ca2+ sensitivity of the contractile elements. (2) Insulin also directly acts on smooth muscle and decreases smooth muscle [Ca2+]i.