Studies of caesium uptake by rat soleus and vastus lateralis musclesin vivo and of its efflux rate relative to potassiumin vitro

In rats given drinking water containing 40 mMCsCl for periods of up to 15 days, uptake of Cs+ into fibres was much more rapid in red soleus than in pale vastus lateralis muscles. On the 3rd day the concentration gradient between fibre water and plasma was 2.5 times greater for Cs+ than for K+ in soleus while in vastus the former had not yet reached equality with the latter. On the 5th day mean membrane potential measured by microelectrodein vivo was —78 mV in vastus and —79 mV in soleus. Rate coefficients for net efflux of Cs+ and K+ from isolated Cs-enriched muscles into anaerobic K-free Krebs containing ouabain were not significantly different from one another in several muscle types examined. When Cs-rich muscles were immersed for 30 min in plasma from the same animal containing ouabain and bubbled with nitrogen and membrane notentials were measured in addition to net efflux of Cs+ and K+, the calculated permeability coefficients for K+ efflux was about 4 times that of Cs+ in all muscles examined. The results suggest that the very striking accumulation of Cs+ in red muscles in contrast to pale ones was not due to slower efflux of Cs+ relative to K+ in the former muscles, but to a much faster influx rate for this cation.