Ionic currents in mammalian fast skeletal muscle.

Abstract

The double sucrose-gap technique was applied to rat skeletal muscle fibers to study the ionic currents under voltage-clamp conditions. The iliacus muscle apparently of fast type according to characteristics of the twitch generated by an action potential. Micro-electrode measurements have shown that the intracellular potential is under good control even when an inward current develops. Components of an equivalent circuit with 2 time constants were estimated from records of the capacitive current. In rat muscle, between 15-21.degree. C, inward and outward currents were similar to Na and K currents found in frog muscle at lower temperature (1-3.degree. C). The inward current which depends on [Na]o and was abolished by tetrodotoxin was carried by Na+. Related to the mean value for the holding potential (-90.5 mV) this current reaches its maximum amplitude at +40 and +50 mV and reverses between +130 and +150 mV; its half inactivation occurs between +14 and +22 mV. The effect of low doses of tetrodotoxin suggests that 2 components participate in the Na current. The delayed outward current which shows inactivation was divided in 2 components: the fast had a linear instantaneous current-voltage relation and differed from the fast component of frog muscle in that its equilibrium potential was more negative than the resting potential; the slow had a linear instantaneous current-voltage relation, and the mean value for its equilibrium potential is 26 mV less negative than the resting potential. Inward-going rectification was present in rat muscle.