DISSOCIATION OF SYMPATHOEXCITATORY AND VASODILATOR ACTIONS OF MODESTLY ELEVATED PLASMA-INSULIN LEVELS

Objective: To determine sympathetic and vascular responses to modest i ncreases in plasma insulin level. Background: Most studies of sympathe tic and vascular actions of insulin have evaluated high plasma insulin levels (> 50 mu U/ml). Those levels increase sympathetic nerve activi ty but also cause vasodilation. Hypertension and obesity are associate d with only modestly elevated fasting insulin levels. Methods: We inve stigated the effects of a 90 min low-dose hyperinsulinemic euglycemic clamp on muscle sympathetic nerve activity (microneurography), forearm vascular resistance (plethysmography), heart rate, blood pressure and central venous pressure. Insulin and vehicle sessions were performed in 12 normal subjects. Results: Plasma insulin levels were elevated fr om values of 10 +/- 2 in the fasting state to 25 +/- 3 mu U/ml during insulin infusion. Insulin levels did not change during vehicle adminis tration. Muscle sympathetic nerve activity increased from 16 +/- 2 to 25 +/- 3 bursts/min during the insulin session and did not change duri ng vehicle administration. In contrast to muscle sympathetic nerve act ivity, forearm vascular resistance did not change during insulin admin istration (from 50 +/- 3 to 51 +/- 4 U). Forearm vascular resistance t ended to fall during vehicle administration (from 45 +/- 2 to 37 +/- 3 U). There were no changes in heart rate, blood pressure and central v enous pressure that could be attributed to insulin. Conclusion: Modest elevations of plasma insulin levels produce sympathetic activation si milar to that caused by high levels, but, in contrast to high levels, modest elevations in plasma insulin level do not decrease forearm vasc ular resistance. The present findings suggest a dissociation between s ympathoexcitatory and vascular actions of insulin at low plasma levels .