INSULIN DOES NOT REDUCE FOREARM ALPHA-VASOREACTIVITY IN OBESE HYPERTENSIVE OR LEAN NORMOTENSIVE MEN

Evidence supports the hypothesis that an impaired capacity of insulin to antagonize norepinephrine-induced vasoconstriction increases alpha-adrenergic tone in overweight young men with insulin resistance and mild hypertension. Therefore, the effects of regionally infused insulin at 100 muU/mL on forearm blood flow (milliliters per deciliter per minute) and responses to norepinephrine were measured in seven obese hypertensive and eight lean normotensive men younger than 45 years old. The obese hypertensive men were hyperinsulinemic and insulin resistant compared with the normotensive men, as evidenced by abnormal values for fasting insulin (15.5+/-1.6 versus 7.2+/-0.8 muU/mL, P<.001), the insulin area under the curve in response to a 2-hour oral glucose tolerance test (12.0+/-1.5 versus 6.7+/-1.1 mU x min/dL, P<.01), and the disappearance rate of glucose during a 15-minute insulin tolerance test (2.7+/-0.3 versus 4.1+/-0.3 mg%/min, P<.05). The logarithm of the norepinephrine EC50 was not significantly different in obese hypertensive men (mean, 95% confidence interval: -8.15, -8.42 to -7.87) versus lean normotensive men (-7.91, -8.23 to -7.59). The 2-hour regional insulin infusion at 100 muU/mL did not significantly alter the EC50 for norepinephrine in either group. Insulin at this concentration induced significant and similar increases of forearm blood How in the hypertensive and normotensive groups (1.7+/-0.4 versus 1.7+/-0.6 mL/100 mL per minute, P=NS). At approximately 100 muU/mL, insulin does not antagonize norepinephrine-induced vasoconstriction in the forearm circulation of either obese hypertensive or lean normotensive men. At this dose, the direct forearm vasodilator actions of insulin, in contrast to indexes of insulin-mediated glucose disposal, are comparatively well preserved in obese young men with mild hypertension. Thus, abnormalities of the regional vascular actions of insulin do not explain the previously reported elevation of forearm vascular alpha-adrenergic tone in obese, mildly hypertensive young men.