Changes in membrane potential and potassium and sodium activities during postnatal development of mouse skeletal muscle.

Dual-channel potassium-selective and single-channel sodium-selective microelectrodes were used to investigate the cause of changes in resting membrane potential of muscle fibers of the mouse during early development. The resting membrane of extensor digitorum longus fibers hyperpolarized during the postnatal period from -41.8 mV at 4 days of age to -76.4 mV at 27 days. During this period intracellular potassium activity increased by 42.1% from 82.5 mM at 8 days to 117.2 mM at 29 days. Intracellular sodium activity was high at 8 days, 23.7 mM, but decreased rapidly to adult values by 27 days when it was 9.98 mM, a 57.9% reduction in sodium activity. The time course of the change in resting membrane potential was different from that of the potassium equilibrium potential calculated from the data. If only potassium and sodium ions were to make significant contributions to the potential, then it was calculated that the permeability ratio PNa:PK would have to change from a value of 0.0659 at 8 days to 0.0227 at 27 days. The results indicated that other factors might be involved in generating the membrane potential inasmuch as, although both intracellular potassium and sodium activities did not change significantly after 27 to 30 days, the membrane potential had not attained adult values at that time. The possibility that increases in muscle activity during the postnatal period might initiate the changes in membrane polarization and intracellular ion activities is discussed together with possible complications in interpretation due to great variations in fiber diameters.