Abstract
Isometric twitch and tetanic contractile properties and velocity of unloaded shortening (V0) of whole avian posterior latissimus dorsi muscle (PLD) were examined between embryonic day 15 and the first 2 wk after hatching. The time to peak twitch force, time to half-relaxation of the twitch response, and time to half-peak tetanic force all change significantly during the final week in ovo but do not change during the first 2 wk ex ovo. Comparisons with previously published reports by others indicate that the twitch half-relaxation time at hatching is approximately the same as that of the adult PLD. The velocity of unloaded shortening increases 2.3-fold during the period studied. It has previously been shown by other that the velocity of shortening is well correlated with a muscle's myosin ATPase activity. Therefore, the observed changes in V0 suggest that the myosin ATPase activity of the avian PLD increases between embryonic day 15 and the first 2 wk posthatching, and this change could account, at least in part, for some of the changes in the isometric properties that were measured.
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