Inhibition by Ba of background current and of its modifications by carbachol in frog atrium.

In previous work on frog atrium [11], we have shown that the carbachol-induced K current (iCCh) and the inward rectifying K channel (iK1) are both inhibited by cesium ions. A more detailed study [1, 2] indicates that the dose-response and the voltage dependence of the inhibition by Cs of iK1 and iCCh are similar. These results are in favor of a modulation by acetylcholine of the iK1 channels. Such a conclusion has been put forward by [3] and by [9]. However, Cs ions can also block the pacemaker currents iK2 [8] and if [5]. An 'if like' component seems to be present in frog atrium (Bonvallet and Ojeda, unpublished results). For these reasons, we have studied the action of barium which can be considered as a more specific inhibitor of the inward rectifying K channels in skeletal muscle [13], [14] and in heart muscle [4]. We report here results obtained on two types of preparations: one in which carbachol increases the background conductance (typical effect) and another one (four fibres from the same frog heart) where carbachol decreases the background conductance (atypical effect) as observed by Carmeliet and Ramon (1980) in sheep Purkinje fibres. In both cases, the results show that, in presence of 5 mM barium, carbachol has no effect on the background current.