Effects of lanthanum, calcium and barium on the resting membrane resistance of guinea-pig papillary muscles.

1. The influence of some cations upon the effective resting membranes resistance of guinea-pig isolated papillary muscles was investigated by means of the double sucrose-gap method. 2. Elevated Ca Concentrations and, to a higher degree La, increased the membrane resistance. The current-voltage relationship, however, revealed a marked difference. The action of Ca was restricted to cathodal pulses, whereas La decreased the permeability of both the hyper-and depolarized membrane. 3. With respect to the resting membrane properties La mimicked the action of Ba and of altered K concentrations, both of which are known to exert their effects mainly by changing the K permeability. 4. Taking into account the different actions of La and Ca on the current-voltage relationship and on the transmembrane potentials, it is suggested that in the resting state La, in contrast to Ca, mainly influences the K permeability. This difference should be considered when La is used as an agent for studying the cellular Ca exchange in heart muscle.