Physiologic and histologic features of muscle development in the hamster
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This study determined to what extent the hind limb muscles of hamsters resemble those of other mammals in undergoing changes in physiologic, morphologic, and histochemical properties as a function of age. Maximal isometric twitch and tetanic responses were evoked in soleus and plantaris muscles of hamsters aged 13 days to 6 months; all experiments were conducted in vivo under sodium pentobarbital anesthesia. In keeping with findings in the cat and rat, both hamster muscles had relatively prolonged twitches in the youngest animals; the twitches became briefer during development, that of plantaris having a minimum mean contraction time of 15.4 +/- 2.4 ms at 20 days and that of soleus, 28.3 +/- 3.5 ms at 46 days. In both muscles there was a subsequent slight prolongation of the twitch. The two muscles had similar masses at 13 and 20 days; thereafter the plantaris became considerably larger and stronger than the soleus and developed more tetanic tension per unit cross-sectional area. In keeping with its briefer contraction, plantaris had a more rapid rate of rise of tetanic tension than soleus and was more susceptible to fatigue; whereas the soleus developed depression of the twitch after a tetanus, the plantaris exhibited potentiation. Histological and histochemical studies showed that the plantaris had significantly more muscle fibers than the soleus and a much greater proportion of type II fibers (91 and 39%, respectively, in 120- to 180-day-old animals). Whereas the type II fibers had similar cross-sectional areas in the two muscles, the type I fibers were significantly smaller in plantaris than in soleus.