Cyclic AMP phosphodiesterase activity in the hearts of trained rats

Running exercise trained rats at either 60 or 76% of their [Formula: see text] caused myocardial cyclic AMP phosphodiesterase (PDE) activity to be increased above control levels for at least 24 h following work. Neither training nor the exercise had any effect on the total concentration of calmodulin in heart tissues. The affinity of PDE for cyclic AMP was not changed by the exercise or training. The chelating agent, EGTA, had the same influence on PDE activity regardless of whether it was present in assays of control or exercised heart extract. Km and EGTA results suggest that calcium-bound calmodulin does not account for the higher PDE activity in the hearts of exercised rats. Supernatants from hearts homogenized in the presence of charcoal, to remove nucleotides from the extract, did not eliminate the exercise-associated increase in PDE activity. These results suggest that the elevated activity was not caused by an in vitro nucleotide activation. Preincubation of the enzyme from exercised and control rat hearts with snake venom activated PDE when assays were performed with the low concentration of cyclic AMP (1 μM). Moreover, the activity reached in the extract of exercisers (23.3 pmol∙100 μL−1∙min−1) was significantly greater than the activity found in control hearts (17.59 pmol∙100 μL−1∙min−1). Exercise increases PDE activity in the myocardium of trained rats. The results presented suggest that the increased PDE activity resulting from exercise is not dependent upon exercise intensity when the work is in excess of 60% of [Formula: see text]. In addition, the data obtained using indirect probes suggest that the increased enzyme activity was not caused by metabolites, endogenous nucleotides, or calmodulin changes in the hearts of exercised animals.