Regulation of leucine and alpha-ketoisocaproic acid metabolism in skeletal muscle. Effects of starvation and insulin.

Abstract

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 @J-Clleucine, 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 2 1 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 1.9-to 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 4-to &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 is probably not a significant metabolic fuel for resting sketetal muscle.14