Regulation of Leucine and a-Ketoisocaproic Acid Metabolism in Skeletal Muscle

Effects of 3 days of starvation on catabolism of leucine and a-ketoisocaproic acid in skeletal muscle have been examined using a perfused hindquarter preparation. The perfusate contained 0.2 l l l ~ [l-"C]or @JClleucine, all other amino acids at approximately reported rat plasma concentrations, and 5.5 mM glucose. During 90 min of recirculation, there was net release into the perfusate of total leucine carbon ([Leultotal = [Leu] + [a-ketoisocaproic acid]) by hindquarters from starved rats, and no change with fed controls. After 30 min, insulin addition resulted in net uptake of leucine from and reduced accumulation of a-ketoisocaproic acid in the perfusate. In all perfusions, by 60 min, perfusate a-ketoisocaproic acid accounted for 21 to 25% of total circulating leucine carbon. When compared to fed controls, blood branchedchain amino acid concentrations were little affected by starvation; however, concentrations in muscle were elevated 1.9to 2.8-fold ( p < 0.002). Muscle and blood branched-chain a-keto acid concentrations were increased significantly by starvation. There was no intracellular accumulation of branched-chain a-keto acids which were 525 p ~ . Because of the larger muscle leucine pool in hindquarters from starved rats, equilibration of label between perfusate leucine and a-ketoisocaproic acid was slower. At all sampling times, specific radioactivity of perfusate [l-14C]a-ketoisocaproic acid equalled that of intracellular [l-14C]leucine. Without insulin, rates of oxidative decarboxylation of [l-'4C]a-ketoisocaproic acid increased continuously. By 90 min, apparent rates were 2.09 and 4.12 nmol of I4CO2 releasedg-' of muscle-min" in fed and starved rat hindquarters, respectively. With fed rat hindquarters, insulin addition resulted in a slower constant rate of oxidative decarboxylation, whereas in starved rat hindquarters it stimulated oxidation immediately. When insulin was present, decarboxylation rates were 4to &fold greater in hindquarters from starved versus fed rats. Leucine oxidation by fed rat hindquarters appeared, on the basis of comparisons of I4CO2 release from [1-14C]and [U-'4C]leucine, to be largely a-decarboxylation. There was more oxidation of the leucine molecule by starved rat hindquarters. Leucine s probably not a significant metabolic fuel for resting sketetal muscle. 14