Use of tributyltin to probe contribution of Cl(-)-HCO3- exchange to regulation of steady-state pHi in human neutrophils.

Organotin derivatives represent a class of artificial ionophores that mediate Cl(-)-OH- exchange and thereby facilitate the chemical equilibrium distribution of Cl- and H+ across biological membranes. Imposing different pH and Cl- gradients by varying extracellular pH (pHo) and extracellular [Cl-] in the presence of 1 microM tributyltin validated the above assumptions in human neutrophils. Under relatively alkaline conditions [intracellular pH (pHi) greater than or equal to 7.10 and pHo greater than or equal to 7.40], the cell's natural Cl(-)HCO3- exchanger mimicked the actions of the tributyltin compound and was the principal factor controlling steady-state pHi. However, with increasing extracellular acidification, there was a progressive deviation from the predicted equilibrium distribution in the case of the normal Cl(-)-HCO3- transport system, whereas tributyltin-treated cells followed theoretical expectations. Exposure of neutrophils to a number of inhibitors of Cl(-)-HCO3- exchange led to a fall in pHi, apparently confirming the impression that a net HCO3- influx through Cl(-)-HCO3- countertransport was chiefly responsible for maintaining steady-state pHi. However, this intracellular acidification could be satisfactorily ascribed to proton movements through a parallel pathway, namely nonionic diffusion of the free acid form of the drugs. These results imply that Cl(-)-HCO3- exchange is the dominant pH regulatory device only under relatively alkaline conditions and that other mechanisms in addition to Na(+)-H+ exchange are likely to play an important role in recovery from acidification and in maintaining steady-state pHi. The possibility that the lactate carrier may function in this capacity is discussed.