Effects of physical training on beta-adrenergic receptors in rat myocardial tissue

A diminished sympathetic activity has been related to training bradycardia seen at rest and during exercise. In order to evaluate if changes in heart adrenergic receptors can be one of the mechanisms by which the sympathetic responsiveness could be decreased by physical training, the number and affinity of beta-adrenergic receptors were determined in heart ventricular tissue of rats submitted to a 10-week running programme. Binding studies were done at different concentrations of (-)[3H] dihydroalprenolol (DHA) (0.5 to 14.4 nmol X litre-1) with ventricular membrane preparations from control and trained rats. Direct linear plot analysis revealed that physical training reduced the total number (1933 +/- 192 vs 2922 +/- 211 fmol X ventricles-1; P less than 0.01) density of beta-adrenergic receptors expressed either as fmol X mg-1 of membrane protein (34 +/- 3 vs 43 +/- 3; P less than 0.05) or as fmol X g-1 ventricle (1740 +/- 170 vs 2308 +/- 155; P less than 0.05). There was no significant change in the dissociation constant (3.11 +/- 0.14 vs 4.08 +/- 0.51 nmol X litre-1; P greater than 0.05). Basal plasma noradrenaline levels were not affected by training (116 +/- 18 vs 101 +/- 14 pg X cm-3; P greater than 0.10); however the adrenaline values were significantly higher in trained rats (91 +/- 16 vs 47 +/- 7 pg X cm-3; P less than 0.05). These data indicate that physical training induces changes at the level of beta-adrenergic receptors and this may partly explain the bradycardia seen in trained subjects and animals.