Adenine nucleotide metabolism in the heart

Abstract

When hypertrophied hearts were subjected to ischemia, the rate and magnitude of adenine nucleotide degradation were enhanced, but nucleotides did not appear to escape from intact ischemic cells. 5' Nucleotidase, the enzyme that catalyzes the conversion of adenosine monophosphate (AMP) to adenosine, was associated with the sarcolemma, transverse tubules, and intercalated discs of the myocardial cells. Hence, adenosine is formed at the cell margins, and that fraction that escapes into the interstitial fluid can act locally to dilate the coronary resistance vessels. Most of the adenosine formed probably reenters the myocardial cells and is rephosphorylated to AMP. However, adenosine that crosses the vascular endothelium undergoes further degradation to inosine and hypoxanthine by enzymes located in the vascular endothelium. The rate of de novo synthesis of cardiac nucleotides is quite slow and is greatly accelerated by cardiac ischemia or hypoxia, but falls somewhat short of the calculated loss during control and ischemic or hypoxic conditions. In contrast to de novo synthesis, rephosphorylation of nucleosides (the salvage pathway) proceeds at a rapid rate. Studies on red cell ghosts revealed that: adenosine enters the cells by facilitated diffusion as well as by simple diffusion; at extracellular concentrations of adenosine up to 10 μM, practically all the adenosine is phosphorylated by adenosine kinase to nucleotides; inosine and hypoxanthine formation predominate at extracellular levels of adenosine above 10 μM and reach equal concentrations in cells and medium; and free adenosine is not present within the ghost cells until pharmacological levels (1 mM) of adenosine are reached in the medium. The reason for the preferential phosphorylation of adenosine at low extracellular concentrations is presumably a greater affinity of adenosine kinase than of adenosine deaminase for adenosine. Similar studies with dispersed embryonic chick heart cells indicated that adenosine was incorporated into myocardial cell nucleotides, but formation of inosine and hypoxanthine was much greater than in red cell ghosts. Free adenosine was not found within the embryonic heart cells even at 1 mM concentrations of adenosine. In the isolated perfused guinea pig heart uniformly labeled adenosine was found to be directly phosphorylated by adenosine kinase to AMP without prior degradation. In contrast, inosine was first degraded to hypoxanthine prior to its incorporation into nucleotides. These observations are in agreement with the concept that adenosine formed at the myocardial cell margins by the action of 5' nucleotidase is either released into the interstitial fluid where it serves as a metabolic vasodilator or it is taken up again by the myocardial cells to replenish the cellular stores of adenine nucleotides.