The kinetics and rectifier properties of the slow potassium current in cardiac Purkinje fibres

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Abstract
The reversal potential of the slow outward current in Purkinje fibres varies with [K]o [external K concentration] in accordance with the expected K equilibrium potential. Virtually all of this current may be carried by K ions. The magnitude of the current is determined by 2 separable factors. The 1st factor is directly proportional to a variable obeying 1st-order voltage-dependent kinetics of the Hodgkin-Huxley type but with extremely long time constants. The time constants of this variable are extremely sensitive to temperature and the Q10 over the range 26-38[degree] C is 6. The 2nd factor shows inward-going rectification with a marked negative slope in the current-voltage relation beyond about 25 mV positive to the K equilibrium potential. The current-voltage relations measured at different values of [K]o cross each other on the outward current side of the equilibrium potential. The changes in slow K current during pace-maker activity were calculated. The mechanism of the pacemaker potential differs in several important respects from that described by Noble''s model. The negative slope in the current-voltage relation appears to be an important factor in generating the last phase of pace-maker depolarization. The role of the slow K current during the action potential and the consequences of the high temperature dependence of the kinetics are discussed.