An analysis of the delayed outward current in single ventricular cells of the guinea-pig

Abstract

Properties of the delayed outward current (I _K) in ventricular myocytes of the guinea-pig were studied using the whole cell clamp method. The experiments were performed under conditions in whichI _K was enhanced by application of isoproterenol while the Ca²⁺ current was eliminated by Ca²⁺-removal and by the addition of Cd²⁺. The reversal potential (E _rev) ofI _K, determined from the current tails, was about 10 mV less negative than the K⁺ equilibrium potential. This was estimated by examining the reversal potential of the inward rectifier K⁺ current in Ba²⁺-containing solution, or from the Nernst equation. TheE _rev-log[K⁺]₀ relationship had a slope of 49 mV per tenfold change in [K⁺]₀. In Na⁺-free solution,E _rev became more negative. Thus, although the major charge carriers inI _K are K⁺ ions, Na⁺ ions may also contribute in part to this current. TheP _Na/P _K ratio inI _K, calculated by applying a Goldman-Hodgkin-Katz relation to the reversal potential, was 0.016. The activation ofI _K during depolarization showed a sigmoidal time course at the onset, while the time course of the current tails was monoexponential at voltages more negative than −50 mV, but biexponential at more positive voltages. These observations can be explained by the conductance equation of the Hodgkin-Huxley type in which the kinetic variable is raised to the second power. These and other features ofI _K observed in the ventricular cells are discussed in comparison to the properties of similar current systems reported in other cardiac preparations.