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1. Experiments on sheep Purkinje fibres were designed to determine whether the current mechanisms responsible for delayed rectification at the pace-maker (negative to -50 mV) and plateau (positive to -50 mV) ranges of potential are kinetically separable and independent.

2. Hyperpolarizations from the plateau range were shown to produce decay of a single component of outward current within the plateau range, but two components were evident when the hyperpolarizations entered the pace-maker range.

3. The time courses of recovery of the two components were too similar at -25 mV to allow temporal resolution at this potential. Clear temporal resolution was, however, possible at potentials between -55 and -95 mV. An indirect method of resolving the two components at -25 mV was used.

4. The kinetic properties of the two components correspond to those previously described for the pace-maker potassium current, i_K2, and the outward plateau current, i_x1 (Noble & Tsien, 1968, 1969a).

5. The instantaneous (fully activated) current—voltage relation for i_K2 was reconstructed from the analysed current records. It was found that this relation shows a negative slope conductance at all potentials positive to -75 mV and that the current tends towards zero at zero membrane potential.

6. The results are compared with those predicted by two reaction models of the i_K2 and i_x1 mechanisms. It is concluded that i_K2 and i_x1 are kinetically separable but that it is not possible with present techniques to decide whether they are controlled by the same or completely independent membrane structures. It is also shown that the instantaneous current—voltage relation calculated for i_K2 does not depend on whether the controlling mechanisms are assumed to be independent or linked.

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