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

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