Summary
Previous work showed that amiloride partially inhibits the net gain of Na in cold-stored red cells of guinea pig and that the proportion of unidirectional Na influx sensitive to amiloride increases dramatically with cooling. This study shows that at 37°C amiloride-sensitive (AS) Na influx in guinea pig red blood cells is activated by cytoplasmic H+, hypertonic incubation, phorbol ester in the presence of extracellular Cat2+ and is correlated with cation-dependent H+ loss from acidified cells. Cytoplasmic acidification increases AS Na efflux into Na-free medium. These properties are consistent with the presence of a Na-H exchanger with a H+ regulatory site. Elevation of cytoplasmic free Mg2− above 3 mm greatly increases AS Na influx: this correlates with a Na-dependent loss of Mg2−, indicating the presence of a Na-Mg exchanger.
At 20°C activators of Na-H exchange have little or no further stimulatory effect on the already elevated AS Na influx. AS Na influx is much larger than either Na-dependent H+ loss or AS Na efflux at 20°C. The affinity of the AS Na influx for cytoplasmic H+ is greater at 20°C than at 37°C. Depletion of cytoplasmic Mg2+ does not abolish the high AS Na influx at 20°C.
Thus, elevation of AS Na influx with cooling appears to be due to increased activity of a Na-H exchanger (operating in a “slippage” mode) caused by greater sensitivity to H+ at a regulatory site.
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Zhao, Z., Willis, J.S. Cold activation of Na influx through the Na-H exchange pathway in guinea pig red cells. J. Membrain Biol. 131, 43–53 (1993). https://doi.org/10.1007/BF02258533
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DOI: https://doi.org/10.1007/BF02258533