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. 1981 May;314:457–480. doi: 10.1113/jphysiol.1981.sp013719

The effects of ATP on the interactions between monovalent cations and the sodium pump in dialysed squid axons

Luis Beaugé 1,2,3, Reinaldo Di Polo 1,2,3
PMCID: PMC1249445  PMID: 6273535

Abstract

1. The efflux of Na in dialysed axons of the squid has been used to monitor the sidedness of the interactions of the Na pump with Na+ ions, K+ ions and ATP. The axons were under conditions such that most of the Na efflux went through the Na pump by means of a complete cycle of ATP hydrolysis.

2. With 310 mm-Ki+, 70 mm-Nai+ and 10 mm-K+ artificial sea water (ASW) more than 97% of the Na efflux was abolished by removal of ATP. The efflux of Na was stimulated by ATP with a K½ of about 200 μm. This is similar to the K½ of 150 μm found for the ATP dependence of a ouabain-sensitive Na,K-ATPase activity in membrane fragments isolated from squid optical nerves.

3. A 100-fold reduction in the ATP concentration (from 3-5 mm to 30-50 μm) increased the apparent affinity of the Na pump for Ko+ about 8-fold. In addition, the maximal rate of Ko+-stimulated Na efflux was reduced by a similar factor. Analogous results were seen in axons dialysed with 310 mm-Ki+ or without Ki+.

4. The relative effectiveness of external monovalent cations as activators of the Na efflux was a function of the ATP concentration inside the axon. With 3-5 mm-ATP the order of effectiveness was K+ > NH4+ > Rb+. With 30-50 μm-ATP the sequence was NH4+ » K+ » Rb+. These results were not affected by the removal of Ki+.

5. When the ATP concentration was 3 mm and the Nai+ concentration 70 mm, the removal of Ki+ produced a slight and reversible increase in the total efflux of Na (15%) and no change in the ATP-dependent Na efflux. When the ATP concentration was reduced to 30-50 μm, or the Nai+ concentration lowered to 5-10 mm, the removal of Ki+ reversibly increased the total and the ATP-dependent efflux of Na. The largest increase in Na efflux was seen when both ATP and Nai+ were simultaneously reduced. The ATP-dependent extra Na efflux resulting from the exclusion of Ki+ was abolished by 10-4 m-ouabain in the sea waters.

6. The increase in the ATP-dependent Na efflux observed in axons dialysed with 0 Ki+ + 10 mm-K+ ASW was not seen in axons perfused with 310 mm-Ki+ + 450 mm-K+ ASW. However, both experimental conditions gave rise to a similar (and small) ATP-independent and ouabain-insensitive efflux of Na. This indicates that the effects on the Na pump of removing Ki+ are not due to the simultaneous membrane depolarization. In addition, it suggests that Ki+ has an inhibitory effect on the Na pump, and that that effect is antagonized by Nai+ and ATP.

7. The present results are consistent with the idea that the same conformation of the Na pump (and Na,K-ATPase) can be reached by interaction with external K+ after phosphorylation and with internal K+ before rephosphorylation. This enzyme conformation produces an enzyme—K complex from which K+ ions are not easily released unless high concentrations of ATP are present. This also stresses a non-phosphorylating regulatory role of ATP.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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