Block and activation of the pace-maker channel in calf Purkinje fibres: effects of potassium, caesium and rubidium

Abstract

1. The effects of low concentrations of Cs⁺ (0·01-3mM) on the fully activated I-V relation ī_f(E) for the pace-maker current in calf Purkinje fibres have been investigated. The action of Cs⁺ is two-fold: in the negative region of the I-V curve Cs⁺ induces a channel blockade; on the other hand, at more positive potentials Cs⁺ can produce the opposite effect, i.e. a current increase.

2. Cs⁺-induced blockade is concentration- and voltage-dependent, as observed on other cation channels. Data in the far negative voltage range (about - 150 to - 50 mV) can be fitted by a simple block model (Woodhull, 1973), which gives a mean value of 0·71 for the fraction of membrane thickness (δ) crossed by Cs⁺ ions before reaching the blocking site. The value of δ does not appear to be affected by either external Na or external K concentrations. Values for the dissociation constant of the blocking reaction at E = 0 mV (k₀) are found in the range 0·5-3·7 mM. In the positive region of the ī_f(E) relation the current depression caused by channel blockade vanishes. Unexpectedly, in this range the current can be observed to increase with Cs⁺, and ī_f(E) curves in different Cs⁺ concentrations show cross-over.

3. Changing external K⁺ also produces similar cross-over phenomena. Investigation of this effect reveals that the increase in slope of the I-V curve on raising the external K⁺ concentration follows Michaelis—Menten kinetics, and can be interpteted in terms of K⁺-induced channel activation. It is found that 44±6 mM-K⁺ half-saturates the channel activating reaction.

4. The Cs⁺-induced current increase is large in low-K⁺ solutions and vanishes in high-K⁺ solutions, suggesting a competition between Cs⁺ and K⁺ ions in their activating action. Increasing Na⁺ also limits the Cs⁺-induced current increase.

5. Rb⁺ also blocks the i_f channel, though less efficiently than Cs⁺. The block caused by Rb⁺ is, unlike that of Cs⁺, nearly voltage-independent, and is explained by assuming that the blocking reaction occurs near the external mouth of the channel (mean value of δ is 0·05). The zero-voltage dissociation constant (k₀) of the Rb⁺-blocking reaction ranges between 1·4 and 5·4 mM, and is lower in low-Na⁺, high-K⁺ solutions.

6. A possible characterization of the i_f channel which explains these results includes an inner `blocking' site, to which external Cs<sup>+</sup> ions bind, blocking the channel, and a more external `activatory' site, to which K⁺, Cs⁺, Rb⁺ and possibly Na⁺ ions bind. Binding of K⁺ to this site induces a current increase either by modulating the channel, or actually by opening the channel itself. A similar mechanism can apply to Cs⁺ and to Rb⁺ binding.