Origin and Possible Significance of Alanine Production by Skeletal Muscle

Abstract

These experiments were undertaken to determine the source of alanine released by skeletal muscle and to clarify the possible relationships between this process and the degradation of branched chain amino acids, the release of glutamine, and protein turnover in this tissue. During incubation in vitro, the rat diaphragm underwent net protein breakdown and released amino acids into the medium at a linear rate. The diaphragm released larger amounts of alanine and glutamine and lower amounts of leucine, isoleucine, and valine than would be expected from the average composition of muscle protein. Addition of the branched chain amino acids increased the production of alanine, glutamate, lysine, but not glutamine by the diaphragm in a concentration-dependent manner. At the same time, the branched chain amino acids inhibited net protein breakdown; therefore the increased amounts of alanine, glutamate, and lysine must have resulted from de novo synthesis or reduced catabolism of these amino acids. All the other amino acids together failed to increase the production of alanine or glutamate. Alanine production in muscle appears related to the rapid oxidation of the branched chain amino acids. In diaphragms from fasted rats, both processes occurred at increased rates. Amino groups released on oxidation of branched chain amino acids could account for all nitrogen recovered in alanine. Alanine production by diaphragms from fasted rats increased upon addition of glucose and even further when insulin was present. The muscle incorporated 14C from [U-14C]glucose into alanine, and addition of branched chain amino acids increased the production of [14C]alanine from [14C]glucose. Therefore, alanine production does not reflect protein degradation in muscle but instead appears to be synthesized de novo primarily from exogenous glucose and from amino groups released by catabolism of branched chain amino acids. These findings suggest the existence of a branched chain amino acid-alanine cycle in the organism. During fasting, enhanced oxidation of branched chain amino acids by muscle would provide energy for muscle, while the concomitant synthesis of alanine from glucose would serve to shuttle ammonia and gluconeogenic precursors back to the liver.