Dissociation of the effects of epinephrine and insulin on glucose and protein metabolism

The separate and combined effects of insulin and epinephrine on leucine metabolism were examined in healthy young volunteers. Subjects participated in four experimental protocols: 1) euglycemic insulin clamp (+80 μU/ml), 2) epinephrine infusion (50 ng · kg^-1 min^-1) plus somatostatin with basal replacement of insulin and glucagon, 3) combined epinephrine (50 ng · kg^-1 · min^-1) plus insulin (+80 μU/ml) infusion, and 4) epinephrine and somatostatin as in study 2 plus basal amino acid replacement. Studies were performed with a prime-continuous infusion of [1-¹⁴C]leucine and indirect calorimetry. Our results indicate that 1) hyperinsulinemia causes a generalized decrease in plasma amino acid concentrations, including leucine; 2) the reduction in plasma leucine concentration is primarily due to an inhibition of endogenous leucine flux; nonoxidative leucine disposal decreases after insulin infusion; 3) epinephrine, without change in plasma insulin concentration, reduces plasma amino acid levels; 4) combined epinephrine-insulin infusion causes a greater decrease in plasma amino levels than observed with either hormone alone; this is because of a greater inhibition of endogenous leucine flux; and 5) when basal amino acid concentrations are maintained constant with a balanced amino acid infusion, epinephrine inhibits the endogenous leucine flux. In conclusion, the present results do not provide support for the concept that epinephrine is a catabolic hormone with respect to amino acid-protein metabolism. In contrast, epinephrine markedly inhibits insulin-mediated glucose metabolism.