Adenosine Uptake by Canine Heart

Abstract

The rate of myocardial adenosine uptake was studied in anesthetized openchest dogs to assess the importance of the process in the regulation of extracellular fluid concentrations of this coronary vasodilator. 8-¹⁴C-adenosine solutions were infused into the left coronary artery at rates that yielded concentrations in coronary artery plasma of 0.4-13.3 µM. The adenosine uptake rate was calculated as the product of the adenosine infusion rate and the extraction fraction of radioactivity assumed to be adenosine. Uptake appeared to follow Michaelis-Menten kinetics over the range of plasma adenosine concentrations tested, and the apparent values of K_m and V_max were 11.6 ± 1.4 (SE) µM and 4.9 ± 0.5 (SE) nmoles/g left ventricle min^-1, respectively. The K_m and the tissue activity of dog heart adenosine deaminase were 43 µM and 1.2 µmoles/g left ventricle min^-1, respectively, and those of adenosine kinase were 0.4 µM and 23 nmoles/g left ventricle min^-1, values sufficiently different from the parameters of uptake to suggest that the limiting step in uptake is permeation into the cell. Uptake was inhibited by 8-13 µM 6-(p-nitrobenzylthio) guanoside, an adenosine analogue that inhibits the facilitated diffusion of a variety of nucleosides in erythrocytes, and by dipyridamole, a compound that inhibits nucleoside uptake in erythrocytes, platelets, and tissue culture cells. Uptake was not inhibited by cardiotoxic doses of ouabain. These findings are evidence against uptake by simple diffusion but are consistent with uptake mediated by a carrier. The rate of cellular adenosine uptake from the extracellular space of the heart appears to be rapid enough to be important in the regulation of myocardial levels of this coronary vasodilator metabolite.