E-C coupling and contractile characteristics of mechanically skinned single fibres from young rats during rapid growth and maturation.

The postnatal growth of rats involves a developmental phase (0 to approximately 3 weeks), a rapid growth phase ( approximately 3 to approximately 10 weeks), and a slower maturation phase ( approximately 10 weeks+). In this study, we investigated the age-related changes in excitation-contraction (E-C) coupling characteristics of mammalian skeletal muscle, during rapid growth (4-10 weeks) and maturation (10-21 weeks) phases, using single, mechanically skinned fibres from rat extensor digitorum longus (EDL) muscle. Fibres from rats aged 4 and 8 weeks produced lower maximum T-system depolarization-induced force responses and fewer T-system depolarization-induced force responses to 75% run-down than those produced by fibres from rats aged 10 weeks and older. The sensitivity of the contractile apparatus to Ca(2+) in fibres from 4-week rats was significantly higher than that in fibres from 10-week rats; however, the maximum Ca(2+)-activated force per skinned fibre cross-sectional area (specific force) developed by fibres from 4-week rats was on average approximately 44% lower than the values obtained for all the other age groups. In agreement with the age difference in specific force, the MHC content of EDL muscles from 4-week rats was approximately 29% lower than that of 10-week rats. Thus, mechanically skinned fibres from rats undergoing rapid growth are less responsive to T-system depolarization and maximal Ca(2+) activation than fibres from rats at the later stage of maturation or adult rats. These results suggest that during the rapid growth phase in rats, the structure and function of elements involved in E-C coupling in fast-twitch skeletal muscle continue to undergo significant changes.