Glucagon stimulates in vitro liver phenylalanine (Phe) degradation, thus inducing net protein catabolism. Whether these effects occur also in vivo in humans is not known. Therefore, we studied the effects of physiological hyperglucagonemia on Phe rate of appearance (Rα), hydroxylation, and oxidation in seven normal volunteers during infusions of somatostatin with replacement doses of insulin and growth hormone. Steady-state Phe kinetics were evaluated using the l-[1-14C]Phe tracer both at the end of a 3-h basal glucagon replacement period (glucagon concentration: 212 ± 115 ng/l) and after a 3-h hormone infusion at the rate of ∼ 3 ng · kg−1 · min−1 (→654 ± 280 ng/l). Hyperglucagonemia did not change plasma Phe concentration and Ra but increased Phe oxidation by ∼ 30% (P < 0.01). Oxidation was also increased by ∼ 24% (P < 0.01) using plasma [14C]tyrosine (Tyr) specific activity as a precursor pool. Phe hydroxylation to Tyr estimated by assuming a fixed ratio of Tyr to Phe Rα (0.73) did not change. Nonhydroxylated Phe disposal decreased by ∼ 6% (P = 0.08). These data show that in humans in the postabsorptive state, hyperglucagonemia, with near maintenance of basal insulin and growth hormone concentrations, stimulates Phe oxidation but not Phe hydroxylation, suggesting a different regulation of these two Phe catabolic steps. Glucagon may also reduce Phe availability for protein synthesis.

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