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

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 Rb-86-uptake. Rb-86-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 C-14-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.