Patch-clamp study of developmental changes in voltage-dependent ion channels of mouse skeletal muscle fibers

Denervation-activated inward rectifier in frog slow skeletal muscle fibers

We tested whether the absence of an inward rectifier channel in slow skeletal muscle fibers of the frog is regulated by innervation. Normal and denervated slow fibers were identified according to their passive electrical properties. In current-clamp experiments, anomalous rectification was quantified as the ratio of effective resistances for hyperpolarizing and depolarizing pulses. In isotonic potassium solution, this ratio was 0.45 +/- 0.1 (n = 14) for twitch fibers, whereas slow fibers displayed linear behavior [ratio = 1.0 +/- 0.05 (n = 15)]. However, denervated slow fibers showed anomalous rectification (ratio, 0.48 +/- 0.07; n = 5). This finding was supported by voltage-clamp experiments in which denervated slow fibers displayed (1) an inward rectifier current during hyperpolarizing pulses, (2) an increase in this current when [K(+)](o) was increased, and (3) a current inhibition after application of Ba(2+). These results suggest that frog slow fibers, which normally do not possess inward rectifier channels, can express them after denervation.

Expression of L-type calcium channels associated with postnatal development of skeletal muscle function in mouse

Several factors have an influence on the improvement of muscle activity and motor co-ordination of mammals during post-natal development. One of them is voltage sensitive L-type calcium channel function. In striated muscles of adult mammals these channels are located in T-tubule membranes thus linking the on-coming action potential to the molecular process of muscle contraction. The postnatal development of L-type calcium channels is therefore critical not only for contraction but also for all subsequent motor learning. We used high affinity enantiomer of dihydropyridine labelled with a fluorophore in order to show the relative amount of L-type calcium channels by histofluorescence in tissue. We found by qualitative microscopical analysis that the amount of L-type calcium channels increased during the postnatal development in the mouse skeletal muscle (m. rectus femoris and m. gastrocnemius). We also noted variation between different fibre types in the increase of the amount of L-type calcium channels, as judged by the intensity of histofluorescence. We showed by histochemical staining and statistical analysis that the high density of L-type calcium channels in adult muscles is correlated with fast oxidative glycolytic fibre type of striated muscles rather than slow oxidative or fast glycolytic fibres. Based on this finding we propose that the development of L-type calcium channels can be considered as one of the factors determining the different physiological properties of fibre types.