Ionic currents in slow twitch skeletal muscle in the rat.

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Abstract
The ionic currents in slow fibers isolated from rat soleus muscle have been studied under voltage-clamp conditions with a double sucrose-gap method and the results are compared to those obtained from fast fibers isolated from the iliacus muscle. The mean value of the resting potential in slow fibers is -70 mV, a value 8 mV more positive that the mean resting potential of fast fibers (-78 mV). In slow muscle, a fast inward current which is blocked by tetrodotoxin and which depends on external Na concentration is presumably carried by Na+. The characteristics of this current, which are time- and voltage-dependent, are similar to those of the iliacus fibers. From a holding potential at -86 mV, this inward current is maximal (2.6 mA/cm2 .+-. 0.3) at +49.1 mV .+-. 1.5 (mean .+-. standard error of mean [SEM]), reverses at +127.3 mV .+-. 2.2 (mean .+-. SEM), and its half inactivation occurs at +23.2 mV .+-. 0.8 (mean .+-. SEM). The delayed outward current in slow fibers is unchanged by exposure to chloride free solution and has a time course very different from that found in fast fibers. This current reaches an initial peak in 5-10 ms and a second peak or steady level after 40-150 ms. The decay of the outward current is also very different, being 10 times slower than that in fast fibers (1500-3000 ms). Analysis of the tail currents reveals the existence of 2 components of delayed current in slow fibers. The faster component reverses at a potential of 11.3 mV .+-. 0.9 (mean .+-. SEM) positive to the holding potential (equivalent to a membrane potential of .apprx. -75 mV), in contrast to a reversal potential of 35.4 mV .+-. 2.5 (mean .+-. SEM) positive to the holding potential for the slower component (equivalent to a membrane potential of .apprx. -51 mV. In L-glutamate solution the characteristics of the inward-going rectification are the same in the 2 types of muscle.