Seasonal variations in the content of catecholamines in carp heart (Cyprinus carpio)

1. Seasonal changes in epinephrine and norepinephrine contents were demonstrated in ventricular muscle tissues of carp heart. 2. Patterns of changes in epinephrine and norepinephrine contents were similar; high during late spring-to-summer and low in winter; however, the fluctuation in epinephrine content was greater than that of norepinephrine. 3. Tyramine caused a positive inotropic effect in isolated electrically-stimulated ventricular muscles of carp heart. The inotropic effect was totally blocked by reserpine, reduced by propranolol, but not altered by atenolol. 4. These results suggest that the catecholamines released from sympathetic nerve terminals may contribute to the regulation of the seasonal change in the function of cardiac muscles in the carp.