INCREASED MYOCARDIAL ADENOSINE PRODUCTION AND REDUCTION OF BETA-ADRENERGIC CONTRACTILE RESPONSE IN AGED HEARTS

The contractile response of the aged adult heart of β-adrenergic stimulation is known to be reduced compared with the young adult heart. Since endogenous adenosine exerts an antiadrenergic action in the heart, this study was undertaken to determine if the basal endogenous levels of myocardial adenosine increases with age and whether this increase mediates the reduced responsiveness of aged heart to β-adrenergic stimulation. Young (3-5 months) and aged (12-22 months) Sprague-Dawley adult rat hearts of CD and SD stock were perfused at constant pressure and paced at 270 contractiosn/min. The two age groups had a similar level of +dP/dt(max) (index of contractility) under control conditions. Adenosine release into the coronary effluent was 30 ± 3 nmol/min/g dry wt from young and 54 ± 9 nmol/min/g dry wt from aged hearts. Inosine release was also greater from aged hearts. Isoproterenol (10-8 M) stimulation increased contractile state by 113% in young hearts and only 69% in aged hearts. Isoproterenol further increased the adenosine and inosine release from both age groups. Theophylline (5 x 10-5 M), an adenosine antagonist, prevented the difference in the contractile response to isoproterenol stimulation between the young and aged hearts. Elevation of external calcium from 2 to 4 mM increased contractility equally in both age groups without influencing adenosine release. Myocardial oxygen consumption, coronary effluent PO2, oxygen supply-demand ratio, and lactate release were similar for both age groups, indicating that under the conditions studied the elevated release of adenosine by the aged hearts was not due to hypoxia. Aged (10-14 months) adult guinea pig hearts also displayed a reduced responsiveness to the isoproterenol stimulation and released more adenosine compared with young (3-4 months) adult guinea pig hearts. These findings suggest that enhanced adenosine levels that are present in the aged myocardium are responsible, in part, for the reduced contractile responsiveness of the older adult heart to β-adrenergic stimulation.