Temperature-Dependence of the Age-Related Difference in the Inotropic Effect of Norepinephrine in the Rat Myocardium

The influence of temperature on the myocardial inotropic response to norepinephrine was studied in rats of different ages. Right papillary muscles isolated from 3-, 12-, and 24-month-old rats were superfused at two temperatures (29 degrees C and 37 degrees C) with progressively larger doses of norepinephrine. At 29 degrees C, the contraction duration was longer and the inotropic effect of norepinephrine was significantly smaller in 24-month muscles than in 3- and 12-month preparations. In muscles superfused at 37 degrees C, these age-related differences in the contraction duration and in the response to norepinephrine disappeared. Analysis of variance and covariance showed that temperature, but not age, is responsible for the differences in the inotropic response to norepinephrine observed. These results show that experimental conditions, among which temperature may play a major role, deeply affect the responsiveness of cardiac preparations to beta-adrenergic agonists.