Variation In Muscle Mass Research Articles

Frequent Sequence Variation in the Human Myostatin (GDF8) Gene as a Marker for Analysis of Muscle-Related Phenotypes

Myostatin is a recently identified member of the transforming growth factor-β family of regulatory factors, also known as growth and differentiation factor 8 (GDF8). The nucleotide sequence of human myostatin was determined in 40 individuals. The invariant promoter contains a consensus MyoD binding site, and the coding sequence contains five missense substitutions in conserved amino acid residues (A55T, K153R, E164K, P198A, and I225T). Two of these, A55T in exon 1 and K153R in exon 2, are polymorphic in the general population with significantly different allele frequencies in Caucasians and African Americans (P < 0.001). Neither of the common polymorphisms had a significant impact on muscle mass response to strength training in either Caucasians or African Americans, although skewed allele frequencies preclude detection of small effects. These allelic variants provide markers for examining association between the myostatin gene and interindividual variation in muscle mass and differences in loss of muscle mass with aging.

Influences of myogenin genotypes on birth weight, growth rate, carcass weight, backfat thickness, and lean weight of pigs.

Lean weight is related to muscle fiber number. Muscle fiber formation (myogenesis) occurs only during embryonic development when it is under the control of the MyoD gene family consisting of myogenin, MyoD1, myf-5, and myf-6. Myogenin has a central position within the MyoD gene family because myogenin expression abrogates myoblast proliferation potential and regulates the differentiation of single nucleated myoblasts into multinucleated myofibers. Thus, myogenin genotype could be related to variation in the number of muscle fibers formed, leading to variation in muscle mass and, thus, lean weight. A polymorphism at the porcine myogenin locus was associated with birth weight, growth rate, lean weight at 200 d, and backfat thickness. Yorkshire pigs from two commercial lines were genotyped, and crosses between heterozygous pigs and heterozygous and homozygous pigs were made. Resulting litters were genotyped, and phenotypic data were collected. Significant differences were found between the two homozygous myogenin genotypes for birth weight, growth rate, and lean weight, but not for backfat thickness. Variation at the myogenin locus explained 4% of the total phenotypic variation in birth weight, growth rate, and carcass weight, and 5.8% of the total variation in lean weight. We conclude that myogenin genotype influences porcine growth rate and muscle mass.

Regional variation in canine intestinal muscle mass and function

Our aim was to investigate the contribution of variations in intestinal muscle morphology or function to regional differences in motor properties in vivo. We quantitated intestinal muscle thickness and surface area along the canine gut and compared the in vitro contractile properties of the jejunum and ileum. The thickness and cross-sectional surface area of both circular and longitudinal muscle demonstrated a parabolic distribution along the intestine, with the greatest values occurring in the proximal and distal regions. The terminal ileum had the greatest circular (885 +/- 194 microns) and longitudinal muscle (367 +/- 135 microns) thickness. Circular muscle was 2.5-3 times thicker than longitudinal muscle at all points. Passive tension was similar in muscle strips from the mid-jejunum, mid-ileum, and terminal ileum (2.8 +/- 0.8, 2.5 +/- 0.4, and 2.3 +/- 0.8 vs 2.5 +/- 0.5, 1.9 +/- 0.5, and 2.8 +/- 1.0, longitudinal and circular, respectively). Active and total tension, however, were significantly greater in longitudinal than circular muscle in mid-jejunum (active; 8.5 +/- 1.4 vs 5.6 +/- 1.2, P < 0.05 and total 11.3 +/- 1.7 vs 8.1 +/- 1.2) and in mid-ileum (active 9.5 +/- 1.6 vs 5.8 +/- 1.2 and total 12.0 +/- 1.6 vs 7.7 +/- 1.2). Values for each layer were similar in both sites. In contrast, in the terminal ileum, longitudinal and circular muscle strips demonstrated similar active (10.1 +/- 1.7 vs 9.0 +/- 2.7, NS) and total tension (12.4 +/- 2.0 vs 11.9 +/- 3.4, NS). Dose-response curves to carbachol (10(-8)-10(-2) M) were similar in all these regions. We conclude (1) there are regional variations in muscle mass but contractile properties are similar in jejunum and ileum; and (2) the unique motor properties of the terminal ileum may be related more to differences in muscle morphology and neural input than intrinsic function.

Correlation of Flight-Muscle Size and Body Mass in Cooper’s Hawks: A Natural Analogue of Power Training

ABSTRACT Muscle hypertrophy in response to increased load (e.g. power training) is well documented in laboratory animals and in man (see, for example, Thorstensson, 1976; Gonyea, Ericson &amp; Bonde-Peterson, 1977), but this phenomenon has not been quantitatively linked to changes in the use of muscles by wild animals. In free-living animals, and sometimes even in those used in laboratory studies, predicting the stress placed on an individual muscle is difficult. Thus, evaluating a compensatory response in relation to a change in muscle use is also difficult. The flight muscles of birds, however, represent a system in which changes in muscle size can be quantitatively compared to the power output required for flight. We describe here a correlation of flight-muscle mass with body mass in Cooper’s hawks (Accipiter cooperii). We believe this correlation represents a natural analogue of power training. The variation in muscle mass between individuals can be directly related to differences in the power requirements for flight caused by differences in body mass.