EVIDENCE FOR DEACTIVATION OF BOTH ECTOSOLIC AND CYTOSOLIC 5'-NUCLEOTIDASE BY ADENOSINE A(1), RECEPTOR ACTIVATION IN THE RAT CARDIOMYOCYTES

Adenosine, an important regulator of many cardiac functions, is produced by ectosolic and cytosolic 5'-nucleotidase. The activity of these enzymes is influenced by several ischemia-sensitive metabolic factors, e.g., ATP, ADP, H+, and inorganic phosphate. However, there is no clear evidence that adenosine itself affects 5'-nucleotidase activity. This study tested whether adenosine decreases the activity of ectosolic and cytosolic 5'-nucleotidase. Cardiomyocytes were isolated from adult male Wistar rats and suspended in the modified Hepes-Tyrode buffer solution. After stabilization, isolated cardiomyocytes were incubated with and without adenosine (10(-9)-10(-4) M). Ectosolic and cytosolic 5'-nucleotidase activity was decreased by exogenous adenosine (ectosolic 5'-nucleotidase activity, 20.6+/-2.3 vs. 8.6+/-1.6 mu mol/min per 10(6) cells [P < 0.05]; cytosolic 5'-nucleotidase activity, 2.47+/-0.58 vs. 1.61+/-0.54 mu mol/min per 10(6) cells [P < 0.05] at 10(-6) M adenosine) after 30 min. The decrease in ectosolic and cytosolic 5'-nucleotidase activity was inhibited by 8-phenyltheophylline and pertussis toxin, and was mimicked by N-6-cyclohexyladenosine, an adenosine A(1) receptor agonist. Neither CGS21680C, and A(2) receptor agonist, nor cycloheximide deactivated ectosolic and cytosolic 5'-nucleotidase. Thus, we conclude that activation of adenosine A(1) receptors is coupled to G(i) proteins and attenuates ectosolic and cytosolic 5'-nucleotidase activity in rat cardiomyocytes.