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

Abstract

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.

Molecular physiology of the Rh ammonia transport proteins

“…Rhcg is expressed in testes and epididymis and Rhcg deletion decreases male fertility [33]. In the colon, Rhcg may contribute to portal ammonia transport [34]. …”

Rh glycoprotein expression is modulated in pufferfish (Takifugu rubripes) during high environmental ammonia exposure

“…NKCC1 is thought to be involved in the basolateral uptake of NH 4 + in the kidney (Weiner and Hamm, 2006) as well as in colonic crypt cells together with Rh proteins (Worrell et al, 2008), and has also been implicated in ammonia-induced astrocyte swelling in the brain (Jayakumar et al, 2008). In seawater fish gills, NKCC1 is involved in salt secretion and has been detected in the basolateral regions of the MRCs in a number of species (Hwang and Lee, 2007).…”

Ammonia excretion inCaenorhabditis elegans: mechanism and evidence of ammonia transport of the Rhesus protein CeRhr-1

“…For the basolateral localized Na + /K + -ATPase, it was shown in 1960 by Jens Skou that this enzyme does accept NH 4 + as a substrate by replacing K + ions (Skou, 1960) and thereby is capable of actively pumping NH 4 + from the body fluids into the respective ammonia-transporting epithelial cell. The direct participation of this pump in the ammonia transport mechanism has now been identified for numerous systems, including those in gills of crustaceans (Furriel et al, 2004;Masui et al, 2002;Weihrauch et al, 1998;Weihrauch et al, 1999) and fish (Mallery, 1983;Nawata et al, 2010a;Wood et al, 2013), frog skin (Cruz et al, 2013), mammalian kidney (Garvin et al, 1985;Wall and Koger, 1994) and intestine (Worrell et al, 2008). The second pump often, if not always, involved in the ammonia transport processes is the V-ATPase.…”