Parallel exchange of Na+-H+ and Cl-(-)HCO3- is thought to be central to the translocation of electrolytes and water during cell volume regulation and in transepithelial transport. Coupling between these transporters is thought to be indirect, through changes in the concentration of HCO3-, which result from alterations in cytosolic pH (pHi). The possibility of a more direct, HCO3-(-)independent interaction between Cl-(-)HCO3- exchange and the Na+-H+ antiport was studied in rat thymic lymphocytes. Measurements of radioactive Cl- flux and of HCO3-(-)dependent pHi changes demonstrated the presence of a Na+-independent, 4,4'diisothiocyanostilbene-2,2'-disulfonic acid-sensitive Cl-(-)HCO3- exchanger. At constant external pH, the rate of Cl-(-)HCO3- exchange was markedly accelerated by increasing pHi between 7.0 and 7.4. This activation was not related to variations in the concentration of HCO3- and is likely caused by a direct effect of intracellular H+ (OH-) on the exchanger. Osmotic shrinking of the cells induced a cytoplasmic alkalinization, due to activation of the Na+-H+ antiport. Concomitantly, the rate of anion exchange also increased. The stimulation of Cl-(-)HCO3- exchange was eliminated when the alkalinization caused by Na+-H+ exchange was precluded. These observations suggest that the exquisite pHi sensitivity of the Cl-(-)HCO3- exchange system provides a mechanism whereby the rates of cation and anion transport are closely coupled.

The purpose of the present experiments was to establish the factors that determine the intracellular Cl- concentration ([Cl-]i) of lymphocytes. Coulometric and isotope equilibration determinations indicated that [Cl-]i was in the range of 70-85 mmol/l cells. Since the membrane potential (Em) of these cells approximates -55 mV, [Cl-]i is severalfold higher than the level expected at electrochemical equilibrium (approximately 16 mM). This suggests that conductive pathways contribute marginally to the distribution of Cl-. Accordingly, altering the force driving conductive Cl- fluxes by manipulating Em had little effect on [Cl-]i. The possible role of electroneutral cation-Cl- cotransport in the accumulation of intracellular Cl- was also assessed. 36Cl- uptake was largely unaffected by omission of extracellular Na+ and K+ or by addition of bumetanide, a potent cotransport inhibitor. Moreover, [Cl-]i remained unaltered for at least 1 h in cells incubated without Na+ or K+ or in the presence of loop diuretics. Thus it appears unlikely that Cl(-)-anion cotransport plays a major role in maintaining [Cl-]i. A vigorous stilbene disulfonate-sensitive anion exchanger was detected in thymocytes. This system constitutes a large fraction of the Cl- flux pathways and is possibly a major contributor to the establishment of [Cl-]i. Accordingly, modifying the force driving Cl- through the exchanger, by altering pH at constant PCO2, resulted in changes in cellular Cl- content and associated changes in cell volume. These effects were markedly reduced in the nominal absence of HCO3- or in the presence of disulfonic stilbene derivatives, suggesting that they are mediated by Cl(-)-HCO3- exchange.(ABSTRACT TRUNCATED AT 250 WORDS)

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