The reversal potential for the pace-maker K current, iK2, was measured in sheep cardiac Purkinje fibres at extracellular K concentrations, [K]O, between 2-7 and 8 mM. The reversal potentials were found to be significantly more negative than the values predicted using the Nernst equation for any reasonable value of intracellular K+ concentration or activity. 2. It is suggested that this discrepancy may be explained by postulating that the extracellular K+ concentration [K]e in the cleft spaces between cells is smaller than [K]o as a result of ion pumping and restricted diffusion from the bulk extracellular medium. 3. In conformity with this hypothesis, it was shown that the value of [K]e may be further reduced by hyperpolarizing pulses, presumably as a consequence of K+ depletion during the passage of inward current. 4. The influence of temperature on the kinetics of the gating mechanism, s, controlling iK2 was investigated. The Q10 for the time constant, pis, of current change following voltage clamp steps was found to be about 17. This corresponds to an apparent activation enthalpy of 50 kcal/mole. 5. The Q10 of the maximum amplitude iotaK2, was found to 1-3. 6. The activation curve, s infinity (Em), spread slightly to more negative potentials by cooling from 37 to 30 degrees C and the curve became less steep. 7. There is a large decrease in the inward background current on cooling, as estimated by measuring the net membrane current when iK is presumed to be zero, i.e. at the reversal potential for iK2.
