| Abstract: | The interaction between epinephrine and insulin in modulating in vivo glucose metabolism within individual tissues of the body has not previously been examined. This was investigated using the euglycemic hyperinsulinemic (120 milliunits/liter) clamp combined with administration of 3H]2-deoxyglucose and D-U-14C]glucose. Epinephrine produced whole body insulin resistance due to increased hepatic glucose output and reduced peripheral glucose disposal. Despite elevated insulin levels liver glycogen content was reduced by 50% during epinephrine infusion (5 nM). However, this effect was transient, occurring predominantly during the initial 60 min of study. These effects were prevented during beta-adrenergic blockade with propranolol and potentiated during alpha 1-adrenergic blockade with prazosin. The most significant effect of epinephrine in peripheral tissues was increased glycogenolysis in both oxidative and glycolytic skeletal muscle. A significant reduction in insulin-mediated 3H]2-deoxyglucose uptake (30%) was evident in 5 of 9 muscles tested during epinephrine infusion. This effect was most pronounced in the more insulin-sensitive oxidative muscles. The latter effect was probably indirectly mediated via increased glycogenolysis--increased accumulation of metabolites--inhibition of hexokinase. In addition, it is evident that insulin-mediated glycogen synthesis occurred during epinephrine infusion. All effects of epinephrine on muscle glucose metabolism were prevented by propranolol but not prazosin. Similar effects to that observed in muscle were not evident in adipose tissue. It is concluded that epinephrine may override many of the actions of insulin in vivo, and most of these effects are mediated via the beta-adrenergic receptor. In the intact rat there may be a complex interaction between alpha- and beta-adrenergic effects in regulating hepatic glucose output. |
