Effects of the adrenergic nervous system on training-induced cardiac enlargement, and on the intrinsic rate and phenylephrine sensitivity of isolated rat atria.

Atria isolated from rats after 6 to 7 weeks running training or swim training in water at 30 degrees C beat at a slower rate than did the atria from sedentary control animals. If the rats received noradrenaline injections with each running session their atrial rate was even lower, while propranolol injections did not lead to lower intrinsic atrial rate. Repeated noradrenaline or propranolol injections alone or swim training in warm water (38 degrees C) did not result in altered atrial rate. A plot of the intrinsic atrial rate against heart weight yielded a significant (r = -0.803) negative relationship (y = 311 - 0.91 x). Swim training in cold water, or chronic noradrenaline injections led to a lessened sensitivity of isolated atria to phenylephrine while chronic beta blockade tended to increase the sensitivity. The results suggest that an intense, although transient, cardiac stimulation, typical of the combined action of running training and noradrenaline injections, results in a lower intrinsic atrial rate and greater cardiac enlargement, while a more sustained type of cardiac stimulation, as produced typically by repeated injections of noradrenaline, leads to less sensitivity to phenylephrine.