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1. In voltage clamped myelinated nerve fibres, the K+ conductance has been calculated from current recordings obtained in low and high K+ media, taking into account the changes in EK resulting from accumulation of depletion of K+ ions near to nodal membrane. 2. At the end of a depolarization, the instantaneous K+ current reverses at a potential (instantaneous reversal potential) differing from the Nernst potential calculated using the external and internal bulk concentrations (theoretical Nernst potential). During a depolarization, EK, as estimated from the instantaneous reversal potential, changes continuously. This change depends on the size, the duration and the direction of the time dependent K+ current. The variation of EK is attributed to continuous changes in K+ concentration near the membrane during voltage pulses which turn on the K+ conductance. 3. The chord conductance [GK = IK/(E-EK), as calculated using the instantaneous reversal potential values for EK, has been analysed as a function of time and membrane potential. As previously reported it increases with the initial K+ concentration in the external medium. 4. The time course of the K+ current depends on both the kinetics of the conductance increase and the rate of change in the driving force for K. The kinetics of the conductance increase can satisfactorily be described by a single exponential function following a delay after the onset of the depolarizing voltage clamp pulse. 5. This delay increases when the holding potential is made more negative. It decreases with membrane depolarization and it is independent of the external K+ concentration. At a given membrane potential, the turning on of the K+ conductance is found to be faster at high than at low external K+ concentrations. 6. At repolarization the turning off of the conductance cannot be described by a single exponential function. It is faster at low than at high external K+ concentrations. 7. The results suggest that the change in K+ conductance proceeds in a multi-step transition or (and) that the K+ conductance is determined by several types of K+ channels.

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