Direct vasodilator effects of physiological hyperinsulin-aemia in human skeletal muscle

Systemic hyperinsulinaemia induces vasodilatation in human skeletal muscle. This effect is gradual in onset, and at low insulin levels not maximal until at least 3 h. To investigate whether the vasodilator response to insulin results from a direct vascular effect, we infused insulin directly into the cannulated brachial artery (perfused forearm technique) in a total of 30 experiments in 20 healthy, lean, normotensive volunteers. Local, intra-arterial, infusion of insulin (180 min, 0.3 mU dL-1 forearm volume min-1, n = 15, forearm venous insulin concentration approximately 540 pmol L-1) induced a gradual increase in forearm blood flow (FBF; venous occlusion plethysmography) from 1.86 +/- 0.17 to 3.64 +/- 0.64 mL dL-1 min-1 after 180 min (ANOVA P < 0.001). Percentage increases in FBF after 60, 120 and 180 min averaged 14.4 +/- 5.9, 59.4 +/- 25.5 and 124.6 +/- 51.2% respectively. Forearm glucose uptake increased from 0.24 +/- 0.05 to a maximum of 1.98 +/- 0.28 micromol dL-1 min (P < 0.001). Furthermore, insulin infusion increased forearm lactate release and potassium uptake. In 10 out of these 15 individuals, the forearm glucose uptake was further increased in a second, separate, repeat experiment with concomitant intra-arterial infusion of glucose 5% (0.2 mL dL-1 min-1), resulting in forearm venous glucose concentrations of approximately 15 mmol L-1. This combined infusion achieved a similar vasodilator response to the infusion of insulin alone. The individual vascular responses of the two paired experiments showed a strong correlation (r = 0.87, P < 0.01). In five subjects time and vehicle control experiments were performed, showing no changes in FBF or metabolism during the 180 min. We conclude that the slow vasodilator response to insulin (as observed during systemic infusion) can, at least partly, be explained by a direct vascular effect of insulin. Insulin-mediated skeletal muscle glucose uptake precedes this effect, but seems not to be an important determinant of the vasodilator response to insulin.