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 (GC1), whereas potassium conductance (GK) increased only slightly, at 16 and 29 months. Due to the lowered GC1, 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 GC1, although changes of this passive conductance alone cannot entirely account for the changes in the excitability characteristics recorded.