Potassium currents in frog ventricular muscle: evidence from voltage clamp currents and extracellular K accumulation.

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Abstract
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.