Alanine and glutamine synthesis and release from skeletal muscle. III. Dietary and hormonal regulation

Abstract

Alanine and glutamine formation and release were studied using the intact epitrochlaris preparation of rat skeletal muscle. Alanine release from skeletal muscle was increased by fasting (65%), cortisone (145%), thyroxine (200%), and diabetes (185%). Glutamine release was decreased by cortisone (37%) and diabetes (23%) but not significantly affected by fasting or thyroxine. Tissue levels of alanine were unchanged but tissue glutamine levels were markedly reduced (30 to 60%) in all treatment groups. Insulin added in vitro did not affect amino acid release even with preparations obtained from diabetic animals. Inhibition of glycolysis with 0.2 mM iodoacetate had no effect on the rate of alanine and glutamine formation in any treatment group. Pyruvate generation was increased by all treatments even in the presence of the inhibitor. Total skeletal muscle alanine, aspartate, and branched chain aminotransferase, glutamate dehydrogenase, and malic enzyme activities were not significantly altered in any treatment groups. The addition of 10 mM aspartate, cysteine, branched chain amino acids, and serine significantly increased alanine formation, whereas the maximal rate of glutamine formation in the presence of stimulating amino acids was reduced in each treatment groups--the most marked effects were noted with cortisone and diabetic preparations. Although accelerated muscle proteolysis is an important factor regulating alanine formation in skeletal muscle, the redirection of carbon flow from glutamine toward alanine formation observed in fasting, cortisone, thyroxine-treated, and diabetic rats, indicates that factors other than proteolysis also participate in the control of amino acid release from muscle.