Female Sprague-Dawley rats were subjected to endurance-training programs, and the effect of training on myocardial beta-adrenergic receptor number, receptor-binding characteristics, and adenylate cyclase (AC) activities associated with the receptor were examined. Training produced a 45% (P less than 0.01) increase in the succinate dehydrogenase activity of the plantaris muscle. Specific (-)-[3H]dihydroalprenolol (DHA)-binding data were subjected to Scatchard plot analysis to quantify beta-adrenergic receptor number and DHA-binding characteristics of myocardial membranes. The DHA concentrations at which 50% of the total binding sites were occupied were similar for membranes from sedentary (1.95 +/- 0.51) and trained (1.59 +/- 0.34 nM) groups. Total DHA-binding sites of membranes from control (91.6 +/- 13.3) and trained (83.1 +/- 7.6 fmol/mg) groups were also similar. Basal and maximally stimulated AC activities were also unchanged by endurance training. Fluoride-stimulated AC activities of crude homogenate and 10,000 g fractions decreased 47 and 49%, respectively, with training. No differences were observed in a 40,000 g fraction. The specific activities of a ouabain-sensitive Na+-K+-ATPase (a sarcolemmal membrane marker) of crude homogenate, 10,000 g, and 40,000 g membrane fractions were similar. These data indicate that training produces no detectable difference in the potential for adrenergic responses at the receptor level.
Thyroid hormone regulation of beta-adrenergic receptor number.
