Chloride-dependence of the actions of phlorizin on frog skeletal muscle membrane.

ABSTRACT

The effects of phlorizin on the membrane potential changes induced by cevadine were compared in the presence and absence of external chloride anions in frog skeletal muscle. The action of the drug on 24Na-efflux was also studied in chloride-free medium. In accordance with previous results, it was found that phlorizin reduced the frequency of the membrane potential oscillation (1 mmol/l) or fully inhibited the rhythmic activity (2 mmol/l) in the presence of chloride anions. Replacing the total chloride content of bathing fluid with non-penetrating anions (glutamate, isethionate or sulphate) the inhibitory action of phlorizin on the membrane potential oscillation failed to appear while it reappeared rapidly if the chloride ions were partially restored in the incubating medium. The membrane potential changes evoked by changing the chloride concentration of Ringer solution at constant [K]0 were more expressed in the presence of phlorizin. The action of phlorizin on 24Na-transport proved to be a chloride-independent phenomenon. This finding indicates that the inhibitory effect of phlorizin on Na-transport processes may not be the reason of its blocking action on membrane potential oscillation. Furthermore, it suggests that failure of the drug to inhibit the membrane potential oscillation in the absence of chloride anions may not be accounted for the lack of phlorizin-binding under those circumstances. It is therefore assumed that the increase in chloride conductance may play a causal role in the inhibitory effect of phlorizin on membrane potential oscillation.