The steady-state potassium conductance of the ranvier node at various external K-concentrations

Summary

• 1. Voltage clamp experiments were carried out on single myelinated fibres of the frog to analyse the changes of the potassium conductance (g _K ) resulting from variations of the external K-concentration ([K]₀). The use ofg _K is justified by the observation that instantaneous K-currents vary linearly withE despite asymmetries between [K]₀ and [K]_i.

• 2. At constant membrane potential and for inward going currents,g _K increases as a Michaëlian function of [K]₀. We propose, therefore, that external K-ions bind specific membrane sites controllingg _K . The apparent equilibrium constant (K _app ) of the binding reaction decreases with depolarization following a double exponential function. The rate constants of this function depend on the external Ca-concentration.

• 3. An empirical equation is derived which satisfactorily describes the variation ofg _K as a function ofE and [K]₀. The fact that either the increase in [K]₀ or the membrane depolarization can lead to the opening of the channels is regarded as evidence that the binding of K to specific sites (“K-receptors”) is a necessary step for the formation of conducting channels. The affinity of the receptor-sites for K-ions depends on the electrical field in the membrane.

• 4. K-free Ringer solutions markedly reduce the steady-state K-current and its rate of activation. These effects are not observed if the lack of potassium is compensated by addition of caesium. It is concluded that Cs can replace K at the sites controlling the formation of the channel.

• 5. High external concentrations of Cs-ions reduce the inward K-currents when [K]₀ is high and tend to increase outward K-currents, especially when [K]₀ is low. These results are explained assuming that Cs-ions enter the channel with K-ions moving in single file. They stop the in-moving file when reaching a selectivity structure deeply located in the channel.

• 6. The fraction of the membrane thickness (δ) at which Cs-ions occupy a blocking position in the K-channel, when K and Cs are present at high concentrations in the external medium, is estimated to be: δ=0.6. From this result, it is deduced that the voltagesensitive K-receptors are located in the external half of the membrane.