Temporal and intra-horse consistency of circulating myostatin concentrations in Thoroughbred racehorses.

In Thoroughbred horses, a highly prevalent short interspersed nuclear element (SINE) mutation in the myostatin gene (MSTN) promoter influences circulating myostatin concentration and is associated with muscle morphology, fracture risk and optimal race distance. Our prior data reveal that within horses there is substantial variation in serum myostatin concentration, particularly in MSTN heterozygotes and wild type horses but it was unclear whether such variation relates to within-horse differences or to environmental or temporal effects. Here we report the intra-horse consistency of circulating myostatin, and investigated the hypothesis that it is affected by season. 49 racing Thoroughbreds under identical management were genotyped for the MSTN promoter SINE insertion and myostatin concentrations were measured in plasma with a validated enzyme-linked immunosorbent assay, from blood samples collected repeatedly over a year. As expected, genotype was significantly associated with circulating myostatin, with homozygotes having the lowest concentration, followed by heterozygous and then wild-type horses. Intra-horse plasma myostatin had low to moderate variability throughout the year, within genotyped groups (coefficient of variation: 17 to 23%): aside from a direct mutation effect, horses tended to be low, moderate, or higher myostatin expressers. Neither season, sex, nor age significantly influenced plasma myostatin concentration. Further work is necessary to determine whether an individual's plasma myostatin concentration is associated with any performance advantages, muscle characteristics or fracture risk within SINE genotyped groups.