Effects of ammonia on acid-base transport by the B-type intercalated cell.

Ammonia, in addition to its role as a constituent of urinary net acid excretion, stimulates cortical collecting duct (CCD) net bicarbonate reabsorption. The current study sought to begin determining the cellular transport processes through which ammonia regulates bicarbonate reabsorption by testing whether ammonia stimulates B-type intercalated cell bicarbonate secretion, bicarbonate reabsorption, or both. The effects of ammonia on single CCD intercalated cells was studied by use of measurements of intracellular pH taken from in vitro microperfused CCD segments after luminal loading of the pH-sensitive fluorescent dye BCECF. These results showed, first, that ammonia inhibited B-cell unidirectional bicarbonate secretion and that this occurred despite no effect of ammonia on apical Cl(-)/HCO(3)(-) exchange activity. Second, ammonia increased the contribution of a SCH28080-sensitive apical H(+)-K(+)-ATPase to basal intracellular pH regulation and it stimulated basolateral Cl(-)/HCO(3)(-) exchange activity. Thus, ammonia activated both apical proton secretion and basolateral base exit, consistent with stimulation of unidirectional bicarbonate reabsorption. It was concluded that ammonia regulates CCD net bicarbonate reabsorption, at least in part, through the coordinated regulation of the separate processes of B-cell bicarbonate reabsorption and bicarbonate secretion. These effects do not reflect a general activation of ion transport but, instead, reflect coordinated and specific regulation of ion transport.