M. Mambrini

Enantiomers of clofibric acid analogs have opposite actions on rat skeletal muscle chloride channels

The S-(−) isomers of a series of clofibric acid analogs produced only a block of chloride conductance of rat skeletal muscle fibers with increasing concentrations until block was nearly complete. The R-(+) isomers, on the other hand, at low concentrations increased chloride conductance by as much as 9% to 39% and at higher concentrations decreased chloride conductance, but never by more than 27% of the control value. The actions of the enantiomeric pairs to either produce or inhibit myotonic excitability paralleled their ability to block or increase chloride conductance, respectively.

Membrane ionic conductances in normal and denervated skeletal muscle of the rat during development

The development of membrane ionic conductances of rat extensor digitorum longus (EDL) muscle fibers was studiedin vitro using intracellular recordings. At 7–8 days after birth, the potassium conductance (GK) dominated the total membrane conductance while the chloride conductance (GC1) was very low. A rapid increased of GC1 towards adult values was observed after few days (12–14 day old rats), whereas GK did not decrease up to day 23. Denervation at 7–8 days after birth suppressed the maturation of the electrical parameters measured, and 15 days after the nerve crush, GC1 was just detectable. These results suggest that the maturation of the electrical properties, and in particular that of the resting chloride conductance in mammalian striated muscle fibers, occurs during the first weeks of postnatal life and is dependent on innervation.

Changes in membrane ionic conductances and excitability characteristics of rat skeletal muscle during aging

Membrane electrical properties,component ionic conductances and excitability characteristics of extensor digitorum longus muscle from 3–4, 16 and 29 months old rats were measured “in vitro”. Fiber diameter, membrane resistance(Rm) and membrane capacitance, increased with aging,and the increase was significant at 29 months. The increase of Rm was mostly due to a decrease of chloride conductance(GCl),whereas potassium conductance(GK) increased only slightly, at 16 and 29 months. Due to the lowered GCl, the latency of action potential increased at both ages with a consequent prolongation of the duration of action potential. Nevertheless, a decrease in the firing capability was recorded in the aged fibers. Our results indicate,that during aging, the most affected parameter of skeletal muscle fibers is GCl, although changes of this passive conductance alone cannot entirely account for the changes in the excitability characteristics recorded.

Effect of Taurine on Chloride Conductance and Excitability of Rat Skeletal Muscle Fibers

Taurine is found in mammalian skeletal muscle fibers and all other excitable tissues (17), but its function is not well understood (16). Abnormal taurine contents whether pathologically or experimentally induced are often associated with changes of excitability in heart (9), nerve (8,21), and retina (4). The taurine content is increased in skeletal muscle from the dystrophic chicken (23).