Oxidation of Branched Chain Amino Acids by Isolated Hearts and Diaphragms of the Rat

Abstract Amino acid oxidation was studied in perfused hearts and incubated hemidiaphragms of rats fed ad libitum. Hearts were perfused with Krebs-Henseleit bicarbonate buffer containing glucose and 0.1 mm amino acids. The addition of 1 mm octanoate to the medium markedly stimulated 14CO2 production from 1-14Cleucine, isoleucine, and valine; inhibited 14CO2 production from 1-14Calanine, pyruvate, and α-ketoglutarate, and did not affect 14CO2 production from 14Chistidine, threonine, glutamate, ornithine, and phenylalanine. Hemidiaphragms incubated with 1 mm hexanoate or 0.1 to 1.0 mm octanoate increased the oxidation of the branched chain amino acids by 80 to 200% and decreased 14CO2 production from and pyruvate. Octanoate stimulated the oxidation of isoleucine by hemidiaphragms at isoleucine concentrations between 0.01 to 1.0 mm; when isoleucine exceeded 2 mm, octanoate became inhibitory. Octanoate stimulated the oxidation of the branched chain amino acids in the presence or absence of glucose and in the presence or absence of insulin from the medium, in diaphragms and hearts. Bovine serum albumin did not affect branched chain amino acid oxidation in the absence of added fatty acids; it inhibited the stimulatory effect of the latter. The inhibition was proportional to the albumin concentration. Palmitate and oleate (1 mm) added to albumin (15 mg per ml) stimulated branched chain amino acid oxidation by hemidiaphragms; the effect was smaller than that observed with equimolar octanoate and albumin. In hearts, no effect of 1 mm palmitate was observed. Butyrate (2 mm) did not affect branched chain amino acid oxidation by hemidiaphragms; 4 mm dl-β-OH butyrate and 4 mm acetate were inhibitory. In hearts and diaphragms, the omission of glucose from the incubation medium stimulated the oxidation of the branched chain amino acids. Pyruvate added to media containing glucose caused further inhibition of branched chain amino acid oxidation. Insulin stimulated 14CO2 production from 14Cleucine in hearts obtained from rats fasted for 48 hours and perfused without glucose. In hearts perfused with 14Cleucine, the addition of octanoate to the perfusate did not affect the tissue concentration of free leucine nor the labeling of the leucine pool. Octanoate stimulated 14CO2 production from leucine by hemidiaphragms which were preloaded with 14Cleucine, indicating stimulation of leucine oxidation beyond the transport of leucine into muscle cells. The apparent inverse relationship between the oxidation of pyruvate and that of the branched chain amino acids suggests that regulation of the latter may occur at the oxidative decarboxylation of the branched chain α-keto acids in muscles. It is suggested that regulation of branched chain amino acid catabolism in muscles may compliment the alanine cycle, and may play a role in homeostasis under conditions of limited availability of glucose (e.g. fasting) or increased utilization (e.g. exercise).