Effects of epinephrine infusion on leucine and alanine kinetics in humans

Infusion of epinephrine in humans increases glucose production and decreases plasma concentrations of some essential amino acids such as leucine, while not affecting the plasma concentration of the potential gluconeogenic amino acid alanine. To determine whether epinephrine alters alanine and leucine metabolism, rates of appearance [R(a)] and disappearance [R(d)] of glucose, alanine, and leucine were determined in postabsorptive volunteers using [³H]glucose, [²H₃]alanine, [¹⁵H]leucine, and [²H₃]leucine during a 180-min infusion of epinephrine (50 ng · kg^-1 · min^-1). Plasma glucose (90 ± 1 to 142 ± 5 mg/dl) and insulin (10 ± 1 to 16 ± 2 μg/ml) increased (P < 0.05), whereas plasma alanine concentrations did not change and plasma leucine concentrations increased (127 ± 5 to 72 ± 3 μM). Glucose R(a) increased transiently and returned to basal values by 120 min. In contrast, alanine R(a) and R(d) increased identically and progressively from 5.7 ± 0.5 to 14.5 ± 1.9 μmol · kg^-1 · min^-1 by 180 min. Although leucine nitrogen R(a) increased transiently and returned to basal values, leucine carbon R(a) and R(d) decreased (P < 0.05) during the infusion of epinephrine. The calculated rate and percent of leucine nitrogen going to alanine increased, whereas the percent of alanine nitrogen derived from leucine remained constant. The increase in alanine R(a) was entirely attributable to increased de novo synthesis because proteolysis, as estimated by leucine carbon flux, decreased. Because the only source of leucine in the postabsorptive state is presumed to be protein, these data demonstrate that physiological hyperrepinephrinemia in normal humans stimulates the production of potential glyconeogenic substrates and decreases endogenous proteolysis.