Search

Although colonic lumen NH(4)(+) levels are high, 15-44 mM normal range in humans, relatively few studies have addressed the transport mechanisms for NH(4)(+). More extensive studies have elucidated the transport of NH(4)(+) in the kidney collecting duct, which involves a number of transporter processes also present in the distal colon. Similar to NH(4)(+) secretion in the renal collecting duct, we show that the distal colon secretory model, T84 cell line, has the capacity to secrete NH(4)(+) and maintain an apical-to-basolateral NH(4)(+) gradient. NH(4)(+) transport in the secretory direction was supported by basolateral NH(4)(+) loading on NKCC1, Na(+)-K(+)-ATPase, and the NH(4)(+) transporter, RhBG. NH(4)(+) was transported on NKCC1 in T84 cells nearly as well as K(+) as determined by bumetanide-sensitive (86)Rb-uptake. (86)Rb-uptake and ouabain-sensitive current measurement indicated that NH(4)(+) is transported by Na(+)-K(+)-ATPase in these cells to an equal extent as K(+). T84 cells expressed mRNA for the basolateral NH(4)(+) transporter RhBG and the apical NH(4)(+) transporter RhCG. Net NH(4)(+) transport in the secretory direction determined by (14)C-methylammonium (MA) uptake and flux occurred in T84 cells suggesting functional RhG protein activity. The occurrence of NH(4)(+) transport in the secretory direction within a colonic crypt cell model likely serves to minimize net absorption of NH(4)(+) because of surface cell NH(4)(+) absorption. These findings suggest that we rethink the present limited understanding of NH(4)(+) handling by the distal colon as being due solely to passive absorption.