A procedure for measuring the contributions of intracellular and extracellular tyrosine pools to the rate of myocardial protein synthesis

Abstract

To determine the relative contribution of intracellular and extracellular amino acid sources as precursors for protein synthesis, rat left atria were incubated in l-[ 3H]tyrosine at medium tyrosine concentrations ranging from 0.05 to 2.5 m m. Under conditions where protein synthesis was shown to be constant, tyrosine incorporation rates calculated from either the intracellular or extracellular tyrosine concentration. This behavior is consistent with a model which postulates that amino acids inside and outside the cell simultaneously supply a common precursor pool. A method is presented for determining the contributions of the intracellular and extracellular pools to the total protein synthesis rate. Analysis of tyrosine incorporation data according to this procedure yields a total incorporation rate of 1.14 ± 0.18 ( s.e.m.) nmol tyrosine/mg protein/h, of which 61 ± 15% is attributed to a mixed intracellular source and 39 ± 7% to amino acids taken directly from the extracellular medium. During a 3-h incubation of the isolated atria protein synthesis and degradation rates were equal, and the oxygen consumption and ATP and creatine phosphate levels were similar to those of the beating, perfused heart.