Decrease in rat cardiac beta 1- and beta 2-adrenoceptors by training and endurance exercise

The cardiac beta-adrenoceptor adaptation to physical activity was investigated in rats which were subjected to a six-week endurance swimming training (ET; n = 7) and a training of high intensity (MT; n = 7). In addition, the effect of a single bout of endurance exercise without preceding training (EE; n = 7) was evaluated. These groups were compared with a sedentary control group (C; n = 9). Beta-adrenergic receptors in rat myocardial membranes were labelled using the high affinity antagonist radioligand (-)125iodocyanopindolol (ICYP). Computer modelling techniques provided estimates of the maximal binding capacity (Bmax) and the dissociation constants (KD). Tissue was constantly kept at temperatures of less than or equal to 4 degrees C and incubated at 4 degrees C for 18 h in buffer containing 100 microM GTP so as to prevent masking of beta-adrenoceptors by endogenous norepinephrine. In comparison with the C group (Bmax = 43.2 +/- 1.6 fmol/mg protein, KD = 11.7 +/- 1.5 pM) computerized coanalyses of saturation binding data of ET, MT, and EE revealed a 13.0%, 25.5%, and 16.6% decrease in Bmax (P less than 0.01), respectively, without significantly differing KD values (10.6 pM, 9.0 pM, 10.5 pM, respectively). We provide the first evidence that acute exercise lowers the sarcolemmal beta-adrenoceptor number in the rat heart. In the competition radioligand binding, CGP20712A and ICI118.551 were employed as subtype-selective antagonists of beta 1- and beta 2-adrenoceptors, respectively, to determine the relative proportions of the receptor subtypes. The ratio of beta 1-/beta 2-adrenoceptors in C was 67.5:32.5 and no statistically significant variation occurred in animals subjected to physical activity. On the basis of published data we assume that acute exercise induces a sequestration of beta-adrenoceptors from the cell surface to some intracellular compartment, whereas the molecular basis of the chronic beta-adrenoceptor down-regulation may involve a training-induced reduction in receptor synthesis. Our findings on cardiac beta-adrenoceptor adaptation to physical activity may represent one of the mechanisms underlying the relative bradycardia in trained subjects.