Evidence for a physiological role of NH4+ transport on the secretory Na(+)-K(+)-2Cl- cotransporter.

The secretory Na(+)-K(+)-2Cl- cotransporter in salivary acinar cells is responsible for driving the transepithelial Cl- fluxes that give rise to fluid secretion. We demonstrate that the application of the muscarinic agonist carbachol to rat parotid acini results in an intracellular acid load that can be blocked by bumetanide, a specific inhibitor of the cotransporter. One component of this bumetanide-sensitive acid load is ouabain-sensitive while a second is dependent on the presence of sub-millimolar concentrations of NH4+ in our media. Our data indicate that this latter effect arises from NH4+ entry on the cotransporter operating in a Na(+)-NH4(+)-2Cl- cotransport mode and that at physiological NH4+ levels in the rat (approximately 0.1 mM), 10-15% of the acinar Cl- entry occurs via this route. We suggest that Na(+)-NH4(+)-2Cl- cotransport may also play a significant physiological role in other cell types and that this mode of operation of the secretory Na(+)-K(+)-2Cl- cotransporter could account for the currently unexplained presence of this protein in a number of tissues.