Abstract

Objective: Insulin and contraction each increase muscle microvascular blood volume (MBV) and glucose uptake. Inhibiting nitric oxide (NO) synthase blocks insulin's but not contraction's effects. We examined whether contraction could augment the MBV increase seen with physiologic hyperinsulinemia, and whether free fatty acid (FFA)-induced insulin resistance, differentially affects contraction vs. insulin-mediated increases in MBV.

Research design and Methods: Rats were fasted overnight. Plasma FFAs were increased by Intralipid/heparin infusion (3h), insulin was increased with a euglycemic clamp (3mU/min/kg), and hindlimb muscle contraction was electrically stimulated. Muscle MBV was measured using contrast-enhanced ultrasound (CEU). Insulin transport into muscle was measured using ¹²⁵I-insulin. BQ-123 (0.4 mg/h) was used to block the endothelin-1 (ET-1) receptor A (ET_A).

Results: Superimposing contraction on physiologic hyperinsulinemia increased MBV within 10 min by 37% and 67% for 0.1 or 1 Hz, respectively (P<0.01). FFA-elevation alone did not affect MBV while 0.1 Hz stimulation doubled MBV (P<0.05) and increased muscle insulin uptake (P<0.05) despite high FFA. Physiologic hyperinsulinemia during FFA-elevation paradoxically decreased MBV (P<0.05). This MBV decrease was reversed by either 0.1 Hz contraction or ET_A antagonism and the combination raised MBV above basal.

Conclusions: Contraction recruits microvasculature beyond that seen with physiologic hyperinsulinemia by a distinct mechanism that is not blocked by FFA-induced vascular insulin resistance. The paradoxical MBV decline seen with insulin+FFA may result from differential inhibition of insulin-stimulated NO-dependent vasodilation relative to ET-1 vasoconstriction. Our results implicate ET-1 as a potential mediator of FFA-induced vascular insulin resistance.