Sodium influx during action potential in innervated and denervated rat skeletal muscles

Resting Na+ influx (J(i)(Na)) was measured in innervated and denervated (1-6 days) rat extensor digitorum longus muscle in the absence and presence of 2 mu mol/L tetrodotoxin (TTX). The mean value of Na+ permeability (P-Na) in innervated muscles was 49.6 +/- 2.6 pm.s(-1). At the second day postdenervation, it decreased by about 45%, This was followed, between the second and fourth days, by a sharp rise, which by the sixth day reached a steady value similar to2.5 times greater than that of innervated muscles. This, most likely, generated the 30% increase in internal [Na+] concentration ([Na+](l)) observed at this time. Tetrodotoxin reduced P-Na of both innervated and denervated muscles by about 25%. In 6-day denervated muscles, virtually ail the TTX effect on P-Na represents the blockage of TTX-resistant Na' channels. Denervation produced a depolarization of about 20 mV by the sixth day. The extra JiNa per action potential (AP) decreased monotonically with time after denervation from 20.0 +/- 3.8 in innervated to 11.1 +/- 1.0 nmol.g(-1).AP(-1) in 6-day denervated muscles, The overshoot of the AP decreased from 15 +/- 1 in innervated to 7 +/- 1 mV in 6-day denervated muscles. Likewise, the maximum rate of rise (+dV/dt), an expression of the inward Na' current, fell from 305 +/- 14 in innervated to 188 +/- 18 V.s(-1) in 6-day denervated muscles, The estimated 6-day denervated/innervated ratio of peak Na+ conductance (g(Na)) was 0.67. The changes in AP parameters promoted by denervation were substantially reduced when both innervated and denervated fibers were hyperpolarized to -90 mV. These results suggest that the depolarization, mainly due to the increase in P-Na /P-K ratio, increases Na+ inactivation and consequently reduces peak g(Na) in spite of the absolute increment in resting TTX-sensitive P-Na. This, in addition to the moderate reduction in the inward driving force on Naf, decreases the inward Na' current and the extra JiNa per AP. (C) 2001 John Wiley & Sons, Inc.