The effects of external cations and ouabain on the intracellular sodium activity of sheep heart Purkinje fibres.

1. The intracellular Na activity of sheep heart Purkinje fibres has been measured using recessed-tip Na(+)-sensitive glass micro-electrodes.2. The internal Na activity was 7.2 +/- 2.0 mM (mean +/- S.D., n = 32) at the normal external Na concentration, [Na](o), in these experiments of 140 mM (equivalent to an external Na activity of 105 mM). The equilibrium potential for Na across the fibre membrane was therefore approximately + 70 mV.3. When the [K](o) was altered the internal Na activity changed, reaching a new level within about 20 min. Increasing the [K](o) from 4 to 25 mM decreased the internal Na by approximately 30%, while decreasing the [K](o) from 4 to 1 mM increased internal Na by 20%.4. The removal of external K produced an easily reversible increase in the internal Na with an initial rate equivalent to a concentration change of 0.24 +/- 0.07 m-mole/min (mean +/- S.D., n = 8).5. Ouabain produced increases in the internal Na activity that were only very slowly reversible. The threshold concentration for producing an increase was approximately 10(-7)M.6. When [Na](o) was reduced the internal Na activity fell rapidly with a single exponential time course (time constant 3.3 +/- 0.8 min, mean +/- S.D., n = 16) to a new, relatively stable level. The recovery of internal Na on return to the normal [Na](o) did not have a simple time course. It was normally complete within 10-30 min.7. The relationship of the stabilized level of the internal Na activity to the [Na](o) was approximately linear over the range 140-14 mM-[Na](o). When [Na](o) was reduced from 140 to 14 mM the internal Na activity fell by 72 +/- 5% (mean +/- S.D., n = 21).8. When the [Na](o) was reduced, the decrease in the internal Na activity was partially inhibited by Mn or by removal external Ca.9. When the [Ca](o) was altered over the range 0.2-16 mM the internal Na activity was reduced by approximately 50% for a tenfold increase in the [Ca](o).10. The relationship between internal Na and contractility is discussed.