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

Abstract

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-sensitive86Rb-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 by14C-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.