Lack of relationship between muscle sympathetic nerve activity and skeletal muscle vasodilation in response to insulin infusion.

Increases in plasma insulin concentration result in vasodilation in skeletal muscle but also in an increase in muscle sympathetic nerve activity (MSNA) which is thought to cause vasoconstriction. The increase in MSNA could therefore be a response to vasodilation (baroreflex), or MSNA could cause vasodilation via putative sympathetic vasodilatory fibres. To examine the relationship between vasodilation, MSNA and insulin action we studied nine non-diabetic Pima Indian men (age 29 +/- 7 years, weight 91 +/- 19 kg, 29 +/- 6% body fat, mean +/- SD) during sequential euglycaemic clamps at low and high insulin doses (80 and 600 mU.m-2.min-1). Leg blood flow was measured by thermodilution, leg glucose uptake by the balance technique, arterial pressure by invasive monitoring and MSNA by microneurography of the peroneal nerve. Whole body glucose uptake (M) ranged from 6.7 to 48.3 during low dose and from 9.4 to 67.7 mumol kg fat free mass-1.min-1 during high dose insulin infusion. At both insulin doses, incremental leg blood flow correlated with M (r = 0.63 and 0.71, respectively). No correlation was found between incremental MSNA and leg blood flow, M or leg glucose uptake. Blood pressure was unchanged throughout the study. MSNA increased after 15-40 min of insulin infusion in all the subjects, whereas leg blood flow started to increase only after 45 min in the most insulin sensitive but not in the most insulin resistant subjects. Thus, insulin stimulates MSNA more rapidly than vasodilation. In conclusion, insulin-mediated MSNA: 1) is neither a response to nor a cause of the vasodilation observed in insulin sensitive men, 2) has no net pressor effect even in the most insulin resistant men in whom insulin-mediated vasodilation was impaired. We conclude that the effect of insulin to stimulate MSNA is dissociated from its acute haemodynamic action.