CONTRASTING AUTONOMIC AND HEMODYNAMIC-EFFECTS OF INSULIN IN HEALTHY ELDERLY VERSUS YOUNG SUBJECTS

Acute increases in plasma insulin produce both sympathoexcitation and vasodilation in normal young adults. Aging is associated with insulin resistance and may alter the sympathetic or the vascular responses to insulin. Therefore, we assessed sympathetic and vascular responses to acute physiological increases in plasma insulin levers in IO healthy, normotensive elderly (65+/-2 years) and 12 normal young (27+/-1 years) subjects matched for body mass index (25+/-1 kg/m(2) in both groups). We measured muscle sympathetic nerve activity (microneurography), FBF (plethysmography), heart rate, and blood pressure and calculated fore arm vascular resistance and insulin sensitivity (M value) during a 90- minute hyperinsulinemic/euglycemic clamp. M values were 4.3+/-0.4 mg . kg(-1). min(-1) in the elderly and 8.4+/-1.4 mg . kg(-1). min(-1) in the young subjects (P<.05). Baseline muscle sympathetic nerve activity was higher in the elderly subjects (33+/-3 versus 15+/-2 bursts per m inute, P<.05); however, the absolute and percent increases in muscle s ympathetic nerve activity were smaller in the elderly than in the youn g subjects (+10+/-1 versus +15+/-1 bursts per minute, or +37+/-11% ver sus +110+/-16%, P<.05). Forearm vascular resistance decreased with ins ulin from 46+/-2 to 31+/-3 units in the young but increased with insul in in the elderly subjects from 37+/-3 to 47+/-7 units (P<.05). Heart rate increased in young but not in elderly subjects. Insulin did not c hange blood pressure in either group. Tn conclusion, as opposed to vas odilation in young adults, insulin caused vasoconstriction in healthy elderly individuals. The failure of the vasodilator action of insulin in the elderly may permit even modest insulin-induced sympathoexcitati on to elicit vasoconstriction. We speculate that the vasoconstrictor r esponse to insulin may further potentiate insulin resistance in the el derly.