The postnatal growth of rats involves a developmental
phase (0 to ∼3 weeks), a rapid growth phase (∼3 to
∼10 weeks), and a slower maturation phase (∼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 depolarizationinduced
force responses and fewer T-system depolarizationinduced
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 Ca2+ in fibres from
4-week rats was significantly higher than that in fibres from
10-week rats; however, the maximum Ca2+-activated force
per skinned fibre cross-sectional area (specific force)
developed by fibres from 4-week rats was on average
∼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 ∼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 Ca2+
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.