Studies with isolated surviving rat hearts. Interdependence of free amino acids and citric-acid-cycle intermediates.

Abstract

The effects of various substrates on the concentrations of free amino acids, citrate cycle intermediates, acyl‐carnitines and on ammonia production by glucose‐perfused rat hearts were studied in order to assess the mode of regulation of the level of citrate cycle intermediates in this tissue.1. Pyruvate caused a ten‐fold increase in the level of citrate cycle intermediates which was accompanied by an increase in alanine and a decrease in glutamate.2. Acetate or acetoacetate effected a three‐fold increase in cycle intermediates and accumulation of acetyl‐carnitine. Accompanying these changes were decreases in the levels of alanine, aspartate and pyruvate, and a marked increase in the lactate/pyruvate ratio. The concentrations of other free amino acids and ammonia production were not significantly affected.3. l‐Adrenalin caused an elevation of cycle intermediates and a decrease in glutamate and more than doubled the rate of ammonia production.4. Perfusion with 2‐oxo‐3‐methylvalerate, 2‐oxoisovalerate or 2‐oxoisocaproate promoted increases in the level of citrate cycle intermediates, a marked drop in the concentration of glutamate, accumulation of branched‐chain acyl‐carnitines and the appearance of isoleucine, valine and leucine, respectively.5. Long‐term perfusion without substrate led to release of essential amino acids and appearance of propionyl and branched‐chain acyl‐carnitines.6. It is concluded that citrate cycle intermediates are produced by rat heart in at least three manners: (a) conditions which perturb the concentration of pyruvate cause shifts in the levels of alanine and cycle intermediates in the same direction via transamination reaction; (b) in addition to (a), acetate and acetoacetate are capable of eliciting an increase in cycle intermediates without increasing alanine or the rate of ammonia production and (c) l‐adrenalin promotes increases both in the level cycle intermediates and in ammonia release. An overall pathway for the production of citrate cycle intermediates and pyruvate through the degradation of protein amino acids is proposed.