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

✍ Scribed by Basilio A. Kotsias; Roque A. Venosa

• Publisher: John Wiley and Sons
• Year: 2001
• Tongue: English
• Weight: 211 KB
• Volume: 24
• Category: Article
• ISSN: 0148-639X
• DOI: 10.1002/mus.1106

✦ Synopsis

Abstract

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 μmol/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 ∼2.5 times greater than that of innervated muscles. This, most likely, generated the 30% increase in internal [Na^+^] concentration ([Na^+^]~I~) observed at this time. Tetrodotoxin reduced P~Na~ of both innervated and denervated muscles by about 25%. In 6‐day denervated muscles, virtually all 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 J~i~^Na^ 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 Na^+^, decreases the inward Na^+^ current and the extra J~i~^Na^ per AP. © 2001 John Wiley & Sons, Inc. Muscle Nerve 24: 1026–1033, 2001