Inhibition of adenosine metabolism increases myocardial interstitial adenosine concentrations and coronary flow

We employed an isolated guinea-pig heart model perfused at constant pressure (70 cmH₂O) to test the hypothesis that inhibition of adenosine metabolism increases interstitial adenosine concentrations (as measured with epicardial discs) and coronary flow. Iodotubercidin (ITU, 1 μm) and EHNA (erythro-9-[2-hydroxy-3-nonyl] adenine, 5 μm) were used to inhibit adenosine kinase and deaminase, respectively during control conditions and during metabolic stimulation with 1 μm isoproterenol. The adenosine receptor blocker 8-phenyltheophylline (8-PT) was used during control conditions to assess whether the response seen was adenosine specific. ITU plus EHNA decreased heart rate (202 ± 10 to 136 ± 11 beats/min) and increased coronary flow (8.2 ± 0.3 to 12.4 ± 0.9 ml/min/g) without a change in MVO₂, developed pressure or dP dt. ITU plus EHNA increased adenosine concentrations in epicardial fluid (0.24 ± 0.07 μm to 1.02 ± 0.09 μm) and venous effluent (40 ± 3 nm to 262 ± 32 nm) during control conditions, and adenosine release increased from 389 ± 96 pmols/min/g to 3480 ± 365 pmols/min/g. 8-PT infusion reversed the effects on heart rate and coronary flow and resulted in a persistent elevation of epicardial fluid adenosine concentrations. During metabolic stimulation with 1 μm isoproterenol, ITU plus EHNA significantly limited the increase in heart rate and ventricular developed pressure and dP dt while coronary flow increased to a significantly greater extent. Myocardial oxygen consumption was similar during metabolic stimulation between the two groups (vehicle vs. ITU plus EHNA). Epicardial fluid adenosine concentration in the vehicle-treated group increased from 0.17 ± 0.3 μm to 0.34 ± 0.02 μm at 15 min of isoproterenol stimulation whereas it increased from 1.10 ± 0.02 μm to 2.90 ± 0.46 μm in the ITU plus EHNA-treated group. Inhibition of adenosine metabolism during metabolic stimulation significantly increased venous adenosine concentrations and adenosine release and reduced inosine and hypoxanthine release proportionately. The release of adenosine + inosine + hypoxanthine was unchanged. Inhibition of adenosine metabolism provides evidence supporting the hypothesis that adenosine plays a role in regulating coronary vascular resistance as well as influencing heart rate and ventricular inotropy.