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1. The ionic currents in slow fibres 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 fibres isolated from the iliacus muscle. 2. The mean value of the resting potential in slow fibres is -70 mV. a value 8 mV more positive that the mean resting potential of fast fibres (-78 mV). 3. In slow muscle, a fast inward current which is blocked by tetrodotoxin and which depends on external sodium concentration is presumed to be carried by sodium ions. The characteristics of this current, which are time- and voltage-dependent, are similar to those of the iliacus fibres. 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 +/- S.E. of mean), reverses at +127.3 mV +/- 2.2 (mean +/- S.E. of mean), and its half inactivation occurs at +23.2 mV +/- 0.8 (mean +/- S.E. of mean). 4. The delayed outward current in slow fibres is unchanged by exposure to chloride free solution and has a time course very different from that found in fast fibres. This current reaches an initial peak in 5-10 msec and a second peak or steady level after 40-150 msec. The decay of the outward current is also very different, being ten times slower than that in fast fibres (1500-3000 msec). 5. Analysis of the tail currents reveals the existence of two components of delayed current in slow fibres. The faster component reverses at a potential of 11.3 mV +/- 0.9 (mean +/- S.E. of mean) positive to the holding potential (equivalent to a membrane potential of about -75 mV), in contrast to a reversal potential of 35.4 mV +/- 2.5 (mean +/- S.E. of mean) positive to the holding potential for the slower component (equivalent to a membrane potential of about -51 mV. 6. In L-glutamate solution the characteristics of the inward-going rectification are the same in the two types of muscle.
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