INHIBITION OF INSULIN-MEDIATED GLUCOSE-UPTAKE IN RAT HINDLIMB BY AN ALPHA-ADRENERGIC VASCULAR EFFECT

The vasoconstrictor norepinephrine, at high doses, inhibits oxygen uptake (VO2) in the perfused hindlimb, possibly by opening vascular shunts and reducing nutrient access. Thus, in the present study, the effect of norepinephrine on insulin-mediated glucose uptake (IMGU) was assessed. Rat hindlimbs were perfused at constant flow with medium containing 8.3 mM glucose and a tracer amount of 2-deoxy-D-[1-H-3]glucose (2-DG) with and without 15 nM insulin, 10 mu M norepinephrine (NE), and combinations of the adrenergic blockers propranolol (Prop) and prazosin. Perfusions were also conducted at a lower dose of 1 mu M NE where VO2 is stimulated. NE (10 mu M) inhibited IMGU > 80%, and this inhibition, when measured by 2-DG uptake, was most pronounced in muscles rich in white fibers. The inhibitory effect of NE on IMGU comprised a beta-adrenergic component also partly evident at lower concentrations of NE (i.e., 1 mu M) and an alpha-adrenergic component only evident at 10 mu M NE. In contrast to the results for the hindlimb, 10 mu M NE plus Prop (alpha-adrenergic combination) had no significant effect on insulin-mediated 2-DG uptake by isolated incubated soleus or extensor digitorum longus muscles. It is concluded that NE, at doses likely to occur at sympathetic vasoconstrictor synapses in muscle, impairs IMGU by a Vascular effect to cause shunting and reduce access.