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

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 ( k 0 ) 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 ( k 0 ) 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 + 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.