Adaptations of the β-adrenoceptor-adenylyl cyclase system in rat skeletal muscle to endurance physical training

 β-Adrenergic mechanisms may be important in the adaptation of skeletal muscle to endurance training. β-Adrenergic signal transduction was examined in the gastrocnemius muscle of rats submitted to a progressive, 12-week treadmill running program and compared with sedentary controls. β-Adrenoceptor density was significantly lower in exercised rats than in controls. The affinity constant for [¹²⁵I]-(-) iodocyanopindolol binding was not different among the various groups. Adenosine cyclic monophosphate formation was significantly decreased in trained animals when isoproterenol plus guanosine triphosphate or forskolin plus Mn²⁺ were used to stimulate adenylyl cyclase. Immunoblot analyses revealed that the amount of the α-subunit of stimulatory guanine nucleotide-binding protein (G_s,α), both the small and the large isoforms, also decreased with physical exercise. Thus, the present report shows that endurance training results in alterations in β-adrenergic receptor density, adenylyl cyclase activity and G_s protein level in rat gastrocnemius muscle. Received: 18 October 1996 / Received after revision: 11 March 1997 / Accepted: 20 June 1997