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

Abstract

The effects of low concentrations of Cs+ (0.01-3 mM) on the fully activated I-V relation .hivin.if(E) for the pacemaker current in calf Purkinje fibers were investigated. The action of Cs+ is 2-fold: in the negative region of the I-V curve Cs+ induces a channel blockade; at more positive potentials Cs+ can produce the opposite effect, i.e., a current increase. Cs+-induced blockade is concentration and voltage dependent, as observed on other cation channels. Data in the far negative voltage range (.apprx. -150 to -50 mV) can be fitted by a simple block model, which gives a mean value of 0.71 for the fraction of membrane thickness (.delta.) crossed by Cs+ ions before reaching the blocking site. The value of .delta. does not appear to be affected by either external Na or external K concentrations. Values for the Kd 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 .hivin.if(E) relation the current depression caused by channel blockade vanishes. In this range the current can be observed to increase with Cs+, and .hivin.if(E) curves in different Cs+ concentrations show crossover. Changing external K+ also produces similar crossover phenomenon. 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 interpreted in terms of K+-induced channel activation; 44 .+-. 6 mM K+ half-saturates the channel activating reaction. 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. Rb+ also blocks the if 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 .delta. is 0.05). The zero-voltage Kd (k0) of the Rb+-blocking reaction ranges between 1.4-5.4 mM and is lower in low-Na+, high-K+ solutions. A possible characterization of the if channel which explains these results includes an inner blocking site, to which external Cs+ 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.