Abstract
What is the central question of this study?Does a common genetic variant in the beta(2)-adrenergic receptor (beta(2)-AR) have effects on skeletal muscle function in young, healthy men? What is the main finding and its importance?This study provides preliminary evidence that beta(2)-AR Arg16Gly genotype has a significant effect on fat-free mass, muscle strength and motor unit behaviour in recreationally trained men. These data might have important clinical and exercise-related implications. For example, beta(2)-AR (rs1042713) genotype might influence the responsiveness of skeletal muscle to clinical or exercise-based interventions or beta-AR agonist treatment. This study explored whether the beta(2)-adrenergic receptor (beta(2)-AR) single nucleotide polymorphism at amino acid 16 (Arg16Gly) has functional effects on skeletal muscle mass, torque production and motor unit behaviour in young, healthy men. Twenty-eight recreationally active men (mean +/- SD 23.1 +/- 1.3 years of age) were genotyped for Arg16Gly polymorphisms of beta(2)-AR as arginine homozygous (ArgArg; n = 5), glycine homozygous (GlyGly; n = 11) or arginine-glycine heterozygous (ArgGly; n = 12). The participants then completed body composition testing, assessments of leg extensor size and echo intensity, and evoked and voluntary isometric leg-extension muscle actions. During the evoked muscle actions, peak twitch torque, peak rate of torque development and peak relaxation rate were assessed. During the voluntary muscle actions, maximal voluntary isometric (MVIC) strength was assessed, and surface EMG signals were obtained during submaximal isometric muscle actions and later decomposed to examine motor unit firing behaviour. Fat-free mass and MVIC strength were greater (P = 0.004, d = 1.74 and P = 0.026, d = 1.10, respectively) in those expressing the GlyGly versus ArgArg allele. The slope of the mean firing rate versus recruitment threshold relationship was more negative in the GlyGly than the ArgArg allele carriers (P = 0.012, d = 1.68) at 50% MVIC, but was less negative in GlyGly and ArgGly versus ArgArg allele carriers (P = 0.013 and 0.016, respectively; d = 1.34 and 1.20, respectively) at 70% MVIC. These data provide preliminary evidence that beta(2)-AR Arg16Gly genotype has a significant effect on fat-free mass, muscle strength and motor unit behaviour in humans.