ADAPTATION IN MYOSIN EXPRESSION OF AVIAN SKELETAL-MUSCLE AFTER WEIGHTING AND UNWEIGHTING

Stretch-induced hypertrophy of the quail anterior latissimus dorsi is associated with decreased slow myosin 1 and increased slow myosin 2 ex pression and a small increase in expression of fast myosins. Because r educed neural activity has also been shown to accelerate expression fr om slow myosin 1 to slow myosin 2, we tested whether the increased exp ression of slow myosin 2 would be maintained when stretch was removed during a time when muscle activity would not be expected to increase. Quail anterior latissimus dorsi muscles were examined after 0-30 days of stretch overload and after 30 or 60 days of unweighting following 3 0 days of stretch. As expected, slow myosin 2 expression increased and slow myosin 1 expression decreased after 14-30 days of stretch. Novel findings were that slow myosin 1 and slow myosin 2 returned to contro l levels after unweighting. Furthermore, the expression of development al and fast myosin heavy chains were evident by day 7, and maintained throughout wing unweighting. These data are consistent with the hypoth esis that alterations in fast and developmental myosin expression resu lt from formation and subsequent maintenance of new fibres during hype rtrophy and regression. The relative amount and expression of myosin a ppears dependent upon mechanical stretch in the anterior latissimus do rsi muscle.