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The kinetic properties of the if channel in shortened sheep Purkinje fibres were investigated using a two-micro-electrode voltage-clamp technique in the presence of Ba2+, Mn2+ and tetrodotoxin (TTX). The time course of the hyperpolarization-activated if currents (DiFrancesco, 1981 b) at potentials within the activation range was found to depend on whether the channel was switching 'on' or 'off'. At potentials positive to the half-activation point if decay was faster than if onset; at potentials negative to the half-activation point if onset was faster than if decay. The time courses of if onset and decay become similar only at potentials close to the centre of the activation range. If a single exponential was fitted to the time course of if switching, the time constant (tau y) was found to vary as a function of potential from approximately 50 msec to several seconds. The tau y - voltage relation is an extremely steep bell-shaped distribution. Reducing external Na+ (range 140-17.5 mM) did not alter the voltage dependence of the if time course. Increasing external K+ (range 5-60 mM) shifted the if time constants and activation curve by similar amounts in a depolarizing direction. The temperature dependence of if was investigated over the range 27.5-41 degrees C. Cooling reversibly slowed the time course of if activation with a Q10 of 3.13 (S.D. +/- 0.85, n = 62). A reversible reduction in the slope of the fully-activated current-voltage relation was observed on cooling, the Q10 being 1.35 (S.D. +/- 0.07), and was usually accompanied by a small depolarizing shift of the half-activation point and the reversal potential Ef. It is concluded that the if time course shows a marked potential dependence and does not obey Hodgkin-Huxley kinetics. Its temperature dependence resembles that of if in the sino-atrial node (DiFrancesco & Ojeda, 1980).

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