Objective: To determine sympathetic and vascular responses to modest increases in plasma insulin level.
Background: Most studies of sympathetic and vascular actions of insulin have evaluated high plasma insulin levels ( > 50 microU/ml). Those levels increase sympathetic nerve activity but also cause vasodilation. Hypertension and obesity are associated with only modestly elevated fasting insulin levels.
Methods: We investigated 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 from values of 10 +/- 2 in the fasting state to 25 +/- 3 microU/ml during insulin infusion. Insulin levels did not change during vehicle administration. Muscle sympathetic nerve activity increased from 16 +/- 2 to 25 +/- 3 burst/min during the insulin session and did not change during vehicle administration. In contrast to muscle sympathetic nerve activity, forearm vascular resistance did not change during insulin administration (from 50 +/- 3 to 51 +/- 4 U). Forearm vascular resistance tended to fall during vehicle administration (from 45 +/- 2 to 37 +/- 3 U). There were no changes in heart rate, blood pressure and central venous pressure that could be attributed to insulin.
Conclusions: Modest elevations of plasma insulin levels produce sympathetic activation similar to that caused by high levels, but, in contrast to high levels modest elevations in plasma insulin level do not decrease forearm vascular resistance. The present findings suggest a dissociation between sympathoexcitatory and vascular actions of insulin at low plasma levels.