Influences of age on the positive inotropic effect mediated by alpha- and beta-adrenoceptors in rat ventricular strips.

The positive inotropic effect of phenylephrine (in the presence of atenolol) and isoprenaline was studied in isolated right ventricular strips obtained from Wistar rats aged 2 weeks, 3 months, and 26 months. The positive inotropic effects induced by phenylephrine and isoprenaline were higher in the 2-week-old than in the 3- or 26-month-old rats. The typical 'alpha-type' or 'beta-type' responses were induced by phenylephrine (plus atenolol) or isoprenaline, in all the age groups. The EC50 values for the alpha-mediated positive inotropic effect decreased with age, while the EC50 value for the positive inotropic effect of isoprenaline increased in the senescent heart. These results suggest an enhanced alpha-adrenergic sensitivity in the senescent heart.