Production of alanine and glutamine by atrial muscle from fed and fasted rats

During incubation in vitro, rat atrial and rabbit papillary muscles synthesized and released large amounts of alanine and even higher amounts of glutamine. The amino groups used in the synthesis of alanine and glutamine appeared to be derived from the catabolism of other amino acids, specifically, leucine, isoleucine, valine, aspartate, glutamate, asparagine, arginine, and proline. The total amount of glutamine and alanine synthesized increased by 45% in atrial muscle from fasted rats, in which the oxidation of branched-chain amino acids is enhanced. In atria from fed rats, addition of leucine stimulated by about 20% the production of both alanine and glutamine, probably by providing additional amino groups for transamination with α-ketoglutarate. However, in cardiac muscle from fasted rats and rabbits, leucine increased the production of glutamine, but not of alanine. Isoleucine and valine, by contrast, enhanced production of both alanine and glutamine in these tissues. In the presence of leucine, low levels of pyruvate seem to limit the production of alanine, apparently because leucine, unlike isoleucine or valine, promotes the conversion of pyruvate to lactate.