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1. Voltage clamp experiments on sartorius muscle fibres at 3° C showed that the potassium current is divisible into three components, namely:

(a) Current in the delayed rectifier channel, which reached a maximum in about 0·1 sec at -30 mV, and declined with a time constant of about 4 msec when the fibre was repolarized to -100 mV; this component had an approximately linear instantaneous current—voltage relation and an equilibrium potential E₁ at 10-15 mV positive to the resting potential.

(b) A slow component which reached a maximum in about 3 sec at -30 mV, and declined with a time constant of about 0·5 sec when the fibre was repolarized to -100 mV; this component had an approximately linear instantaneous current—voltage relation and a mean equilibrium potential E₂ at -83 mV in fibres where E₁ averaged -75 mV.

(c) Current in the inward rectifier channel which decreased with a time constant of about 0·25 sec when the fibre was hyperpolarized to -150 mV. This component had an equilibrium potential close to the resting potential and an instantaneous current—voltage relation which was that of an inward rectifier.

2. The general characteristics of the late after-potential in muscles in hypertonic solutions at 3° C are consistent with those of the slow conductance change. The sign of the late after-potentials was reversed by depolarizing below -80 mV.

3. The decline of current during a maintained hyperpolarization cannot be attributed solely to a decrease in tubular potassium concentration, since there may be a large decrease in current without much alteration of equilibrium potential. The negative slope conductance often seen at -150 mV is also difficult to reconcile with the tubular depletion hypothesis.

4. Replacement of 10 mM-K by 10 mM-Rb abolished inward rectification but had less effect on the fast and slow components of the potassium conductance.

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