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MUCOSAL BIOLOGY

Ammonium transport in the colonic crypt cell line, T84: role for Rhesus glycoproteins and NKCC1

Departments of ¹Surgery and ²Molecular and Cellular Physiology, University of Cincinnati, Cincinnati, Ohio; ³Department of Surgery, University of Chicago, Chicago, Illinois

Submitted 8 June 2006 ; accepted in final form 15 November 2007

Although colonic lumen NH₄⁺ levels are high, 15–44 mM normal range in humans, relatively few studies have addressed the transport mechanisms for NH₄⁺. More extensive studies have elucidated the transport of NH₄⁺ in the kidney collecting duct, which involves a number of transporter processes also present in the distal colon. Similar to NH₄⁺ secretion in the renal collecting duct, we show that the distal colon secretory model, T84 cell line, has the capacity to secrete NH₄⁺ and maintain an apical-to-basolateral NH₄⁺ gradient. NH₄⁺ transport in the secretory direction was supported by basolateral NH₄⁺ loading on NKCC1, Na⁺-K⁺-ATPase, and the NH₄⁺ transporter, RhBG. NH₄⁺ was transported on NKCC1 in T84 cells nearly as well as K⁺ as determined by bumetanide-sensitive ⁸⁶Rb-uptake. ⁸⁶Rb-uptake and ouabain-sensitive current measurement indicated that NH₄⁺ is transported by Na⁺-K⁺-ATPase in these cells to an equal extent as K⁺. T84 cells expressed mRNA for the basolateral NH₄⁺ transporter RhBG and the apical NH₄⁺ transporter RhCG. Net NH₄⁺ transport in the secretory direction determined by ¹⁴C-methylammonium (MA) uptake and flux occurred in T84 cells suggesting functional RhG protein activity. The occurrence of NH₄⁺ transport in the secretory direction within a colonic crypt cell model likely serves to minimize net absorption of NH₄⁺ because of surface cell NH₄⁺ absorption. These findings suggest that we rethink the present limited understanding of NH₄⁺ handling by the distal colon as being due solely to passive absorption.

RhBG; RhCG; colon; hyperammonemia