title: The effects of external cations and ouabain on the intracellular sodium activity of sheep heart Purkinje fibres.
abstract: 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.
mesh_terms: D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000494:pharmacology; D006329:Heart Conduction System / Q000378:metabolism*; D066298:In Vitro Techniques; D008345:Manganese / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D010042:Ouabain / Q000494:pharmacology*; D011188:Potassium / Q000494:pharmacology; D011690:Purkinje Fibers / Q000378:metabolism* / Q000502:physiology; D012756:Sheep; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology / Q000502:physiology
chemical_list: D008345:Manganese; D010042:Ouabain; D012964:Sodium; D011188:Potassium; D002118:Calcium