Skeletal muscle

Abstract

Consistent with LS individuals, GHR-/- mice have reduced muscle mass. The animal study on skeletal muscle greatly complements the human data, where studies are limited due to ethnic issues. For instance, GHR-/- mice had reduced muscle fiber size, but not the number of muscle fibers, resulting in reduced muscle mass. Contradictory data on fiber type proportion have been reported. Skeletal muscle IGF-1 mRNA was decreased in GHR-/- mice, but protein level remained the same compared to that in control mice, thus suggesting that autocrine actions of IGF-1 are not sufficient to rescue the sarcopenia phenotype of GHR-/- mice. GH/IGF-1 increased muscle mass independently of androgens and glucocorticords in WT mice. Muscle insulin sensitivity was increased in GHR-/- mice, but some studies found no change in key insulin signaling molecules or even delayed insulin response in GHR-/- muscle. Conflicting results may arise because of differences in age, gender, genetic background, muscle groups investigated, and technique differences. To address these issues and elucidate the gender- and age-specific effects of GH in skeletal muscle, it will be important to analyze animals of both genders across greater age spans. Also, it will be necessary to find out if GH exerts differential effects on different muscles. If so, then a representative skeletal muscle or group of muscles may be designated for use by all laboratories, so that legitimate comparisons may be made across studies. Ultimately, a clear mechanism of GH on muscle biology will be helpful in treating LS individuals who -suffer from weakened muscle as well as other human diseases involving skeletal muscle such as muscle wasting and muscle atrophy. © Springer-Verlag Berlin Heidelberg 2011.