Rat cardiac myocyte adenosine transport and metabolism

Based on the importance of myocardial adenosine and adenine nucleotide metabolism, the adenosine salvage pathway in ventricular myocytes was studied. Accurate estimates of transport rates, separate from metabolic flux, were determined. Adenosine influx was constant between 3 and 60 s. Adenosine metabolism maintained intracellular adenosine concentrations less than 10% of the extracellular adenosine concentrations and thus unidirectional influx could be measured. Myocytes transported adenosine via saturable [Michaelis constant = 6.2 +/- 2.1 microM and maximal velocity (Vmax) = 9.58 +/- 0.98 X 10(-1) pmol X mg protein-1 X s-1] and nonsaturable (rate constant = 1.8 X 10(-3)/s) processes. A minimum estimate of the Vmax of myocytic adenosine kinase (2 pmol X mg protein-1 X s-1) indicated the saturable component of adenosine influx was independent of adenosine kinase activity. Saturable transport was inhibited by nitrobenzylthioinosine and verapamil (inhibitor constant = 17 +/- 5 microM). Extracellular adenosine taken up by myocytes was rapidly phosphorylated to adenine nucleotides. Not all extracellular adenosine, though, was phosphorylated on entering myocytes, since free, as opposed to protein-bound, intracellular adenosine was detected after digitonin extraction of cells in the presence of 1 mM ethylene-diaminetetraacetic acid.