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

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Abstract
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 β-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·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·ventricles−1; P−1 of membrane protein (34±3 vs 43±3; P−1 ventricle (1740±170 vs 2308±155; P0.05). Basal plasma noradrenaline levels were not affected by training (116±18 vs 101±14 pg·cm−3; P>0.10); however the adrenaline values were significantly higher in trained rats (91 ±16 vs 47±7 pg·cm−3; P<0.05). These data indicate that physical training induces changes at the level of β-adrenergic receptors and this may partly explain the bradycardia seen in trained subjects and animals.