Node of Ranvier
Springer Online Journal Archives 1860-2000
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.
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