Attenuated in vitro coronary arteriolar vasorelaxation to insulin-like growth factor I in experimental hypercholesterolemia

Insulin and insulin-like growth factor (IGF) I affect coronary vasoactivity. Experimental hypercholesterolemia is associated with coronary atherogenesis and altered vasomotor regulation. Because the IGF axis is altered during atherogenesis, we postulated that experimental hypercholesterolemia is associated with an altered coronary vasoactive response to IGF- I in vitro. Coronary arteries and arterioles from pigs fed either a normal or high-cholesterol diet for 10 weeks were contracted with endothelin-1 and relaxed with cumulative concentrations of insulin or IGF-I (10^-12 to 10^-7 mol/L). Control arterioles were also incubated with the nitric oxide synthase inhibitor 10^-4 mol/L N(G)-monomethyl-L-arginine (L- NMMA) or the potassium channel blocker 10^-2 mol/L tetraethylammonium (TEA), contracted with endothelin-1, and relaxed with insulin or IGF-I. Experimental hypercholesterolemia (1) increased serum cholesterol (9.5± 1.0 versus 1.9±0.08 mmol/L; P<0.0001), (2) caused coronary arterial and arteriolar endothelial dysfunction in vitro (attenuated vasorelaxation to bradykinin), (3) did not alter the epicardial response to either insulin (P=0.80) or IGF- I (P=0.12), and (4) significantly attenuated the arteriolar response to IGF- I (maximal relaxation of 79±6% versus 42±8%; P=0.01) but not insulin (43±6% versus 53±7%; P=0.99). Control arteriolar vasorelaxation to IGF-I was attenuated by both L-NMMA (P<0.001) and TEA (P=0.01), whereas only L- NMMA attenuated insulin (P<0.001). Staining for IGF-I and IGF binding protein 2 was increased (P<0.05) in arterioles of cholesterol-fed pigs. IGF-I and insulin are therefore coronary arteriolar vasorelaxants through different mechanisms. Experimental hypercholesterolemia is associated with resistance to the coronary arteriolar vasorelaxing effects of IGF- I but not insulin, in conjunction with increased ligand and binding-protein expression. The IGF axis may contribute to the altered coronary vasoactivity in hypercholesterolemia.