| Abstract
| - Objective: The aim was to evaluate age related changes in myocardial contractility and α1 adrenergic responsiveness in myocardium from normal and cardiomyopathic hamsters. Methods: 50 male cardiomyopathic hamsters (CHF 147) and 45 golden Syrian hamsters were killed at 30, 50, 100, 200, or 300 days of age. Left ventricular papillary muscles were isolated and mounted in an isolated bath and isometric, isotonic, and unloaded contractions were recorded during a phenylephrine dose-response curve. The effects of increasing stimulation rate from 6 to 45·min−1 and of increasing temperature from 29°C to 35°C before and after the addition of phenylephrine were verified in 100 day old hamsters. Results: At 30 days of age, tension generation and shortening indices were similar in both strains. Thereafter, contractility increased in normal hamsters, to reach a peak at 50 d which was maintained until 300 d, when a slight decrease occurred. In cardiomyopathic hamsters, contractility increased more slowly, such that by 50 d tension was lower than in normals, at 15.8(SEM 1.9) v 26.5(2.2) mN·min−2 (p<0.01). This decrease persisted until 200 d, when it normalised and then decreased again by 300 d. The response of normal and cardiomyopathic muscles to phenylephrine was similar at 30 days of age. Thereafter, the response of cardiomyopathic muscles decreased more than in normals. This decrease was accompanied by a rightward shift in the dose-response curve. Increasing stimulation rate had no effect under basal conditions; however, once phenylephrine was added it caused tension to increase in cardiomyopathic muscles instead of decreasing, thereby rendering the relative effects of phenylephrine at 45 stimuli·min−1 similar in both groups. Conclusions: Force generating and shortening indices in normal and cardiomyopathic hamster myocardium are similarly immature at 30 days of age. Normal myocardium reaches its full potential by 50 d, but cardiomyopathic myocardium does not reach it until 200 d, and it decreases thereafter. A decrease in a, adrenergic responsiveness in cardiomyopathic myocardium at lower stimulation rates can be reversed by increasing the stimulation rate. Cardiovascular Research 1993;27:968-973
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