Endogenous adenosine improves work rate to oxygen consumption ratio in catecholamine stimulated isovolumic rat heart

This study examines the possibility that endogenous adenosine modulates efficiency in isovolumic perfused rat hearts stimulated with isoproterenol or norepinephrine. Efficiency in these hearts is calculated as the rate of pressure work divided by the myocardial oxygen consumption. Within 2 min of infusion of isoproterenol (50 nM), heart rate increased by 35%, the rate pressure product by 290%, oxygen consumption by 142%, and efficiency by 67%. Infusion of adenosine deaminase (2-4 IU/ml), or 8-phenyltheophylline (5 microM), into stimulated hearts augmented the increase in heart rate by 40-45%, rate-pressure product by 18-20%, and oxygen consumption by 50-55%. Efficiency was reduced by 30-35%. Adenosine release into the coronary venous effluent increased from 195 +/- 20 pmol/min/g to 2400 +/- 180 pmol/min/g after 5 min. A similar pattern of results was observed when norepinephrine (0.1 mM) was used. The results indicate that extracellular adenosine, released by catecholamine treatment, inhibits the effects of the catecholamines on rate and contractility. Consequently, adenosine reduces cardiac work (rate-pressure product), but in so doing, improves efficiency.