Transport of caesium in frog muscle.

1. The entry of caesium into sartorius muscle cells is strongly suppressed by the presence of 10(-5)M strophanthidin in Ringer solution.2. The amount by which caesium entry is reduced in the presence of strophanthidin is dependent on the intracellular sodium concentration and is greater the higher the intracellular sodium concentration.3. The magnitude of caesium influx in the absence of strophanthidin is highly dependent on the intracellular sodium concentration.4. Caesium uptake by muscles in which sodium has been largely replaced by lithium is reduced to very low values.5. Caesium can promote the extrusion of sodium from muscles with high intracellular sodium concentrations. The effects of 25 mM caesium and 5 mM potassium on sodium extrusion are roughly the same.6. External potassium inhibits the entry of caesium ions into muscle cells presumably by competing for transport sites.7. The drug strophanthidin has no effect on (134)Cs efflux provided that muscles have been loaded with tracer ions for a long period of time. Caesium efflux from the intracellular compartment appears to occur by a process not mediated by metabolism.8. The action of strophanthidin on (134)Cs efflux from muscles exposed to tracer for short times suggests that caesium ions are transported inwardly by an active process after first accumulating in a superficial reservoir.