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 (Gs,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 Gs protein level in rat gastrocnemius muscle.