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In newborn rat skeletal extensor digitorum longus (EDL) muscle, it has been found that an influx of calcium from the extracellular medium is necessary for contraction, in contrast to the situation observed in adult EDL muscle. The aim of the present study was to determine the influence of the extracellular calcium concentration ([Ca]o) upon the contractile responses elicited in developing as well as in regenerating (notexin-injected) soleus (SOL) muscle. A morphological study was performed to follow the steps of postnatal development and regeneration in SOL muscle. In nominally calcium-free solution, the amplitudes of the twitch and tetanic tensions were greatly reduced in 1-14-day-old developing SOL muscles, as well as in notexin-injected SOL muscles. With longer times after birth, twitch and tetanic tensions of SOL muscle were less affected by the absence of calcium. This contrasts with notexin-injected SOL muscle in which the amplitudes of the contractions remained strongly dependent on [Ca]o. The present finding suggests that some functional characteristics are different in regenerating muscle fibers and may be of interest in the evaluation of the contractile properties of muscles in which injections of genetically engineered or not autologous myoblasts or viral vector have been performed.

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