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1. The single sucrose voltage clamp technique was used to control the membrane potential of strips of frog ventricular muscle and to measure the membrane current. The extracellular K accumulation was estimated from the after-potential observed after the release of the voltage clamp. 2. Comparing the time course of the membrane current to the time course of the development of the after-potential at different membrane potentials, it was found that all slow current changes are related to changes in the K current across the membrane. 3. Based on measurements of membrane current and the after-potential, the total membrane current was separated into two fractions: (a) the K current which gives rise to K accumulation and (b) the residual membrane current which is unrelated to K accumulation. The current-voltage relation for the residual membrane current is linear or slightly inwardly-rectifying. Residual current is zero at the resting potential and increases to about 1 microamperemeter/cm2 at -20 mV. 4. The measured membrane currents and after-potentials indicate qualitative differences between the K currents which dominate below and above -20 mV. More negative to -20 mV the after-potential develops rapidly while at potentials positive to -20 mV the after-potential develops with some delay. 5. The current dominating below -20 mV is inwardly-rectifying. The current-voltage relation has a maximum (about 2 microamperemeter/cm2) and a region with marked negative slope conductance. The outward current in the region of negative slope conductance is increased with increasing [K]o. 6. A model for the inwardly rectifying K current is described. The model accurately reproduces the shape of the measured current-voltage relations and their modification by alterations in the extracellular K concentration. The model is also compatible with the observation that all slow current changes below -20 mV are directly related to K accumulation. 7. The K current which dominates at potentials positive to -20 mV is activated by a potential and time dependent process which is unrelated to extracellular K accumulation. 8. Q10 for the magnitude of the inwardly rectifying K current is about 1.35 while the Q10 for the rate of increase of the time dependent K current is about 3--4. 9. Cs blocks the inwardly recitfying K current but has little effect on the time dependent K current. 10. The changes in the action potential duration caused by increasing the extracellular K concentration or addition of Cs to the perfusate can be explained by the effect of K and Cs on the inwardly rectifying K current.

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