HISTOLOGY AND COMPOSITION OF MUSCLES FROM NORMAL AND CALLIPYGE LAMBS

The histology and composition of muscles from normal (n = 10) and call ipyge (n = 11) wether lambs was compared. Normal Rambouillet ewes were mated with callipyge Dorset rams, and their progeny were visually cla ssified as callipyge or normal based on muscle definition in the loin and hind quarters. The muscles examined included three muscles that hy pertrophy in callipyge lambs (semitendinosus, longissimus, and gluteus medius) and one muscle believed not to hypertrophy (supraspinatus). T he hypertrophy-responsive muscles from callipyge lambs had a higher (P < .001) percentage of fast-twitch glycolytic (FG) fibers and lower (P < .001; P < .02 for SO in gluteus medius) percentages of slow-twitch oxidative (SO) and fast-twitch oxidative glycolytic (FOG) fibers. The diameters of the FG and FOG fibers were larger (P < .005 and P < .04, respectively) in hypertrophy-responsive muscles from callipyge lambs, but the SO fiber diameter was smaller (P < .05). Also, the protein:DNA ratio, an indicator of cell size, was greater (semitendinosus, P < .0 5; longissimus, P < .002; gluteus medius, P < .008) in the hypertrophy -responsive muscles from callipyge lambs. Thus, hypertrophy in callipy ge lambs was, at least in part, due to fiber type changes and muscle c ell enlargement. Hypertrophy was strongly associated with changes in t he FG fibers, the only fiber type that increased in both proportion an d average diameter in callipyge muscles. The protein:RNA ratio and RNA :DNA ratio, which are indicators of translational and transcriptional activity in the muscle cells, were not different between callipyge and normal muscles. This indicated that the accumulation of protein neces sary for myofiber enlargement occurred without differences in the tran slational or transcriptional activity of callipyge muscle.