Ammonium transport in the colonic crypt cell line, T84: role for Rhesus glycoproteins and NKCC1.
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.
Worrell
Roger T
RT
Department of Surgery, University of Cincinnati, Cincinnati, OH 45219-0581, USA. Roger.Worrell@uc.edu
Merk
Lisa
L
Matthews
Jeffrey B
JB
D000251:Adenosine Triphosphatases / Q000378:metabolism; D001693:Biological Transport, Active / Q000502:physiology; D001798:Blood Proteins / Q000502:physiology*; D002460:Cell Line; D003106:Colon / Q000166:cytology* / Q000378:metabolism*; D000242:Cyclic AMP / Q000494:pharmacology; D018076:DNA, Complementary / Q000235:genetics; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008562:Membrane Glycoproteins / Q000502:physiology*; D008744:Methylamines / Q000378:metabolism; D010957:Plasmids / Q000235:genetics; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012414:Rubidium Radioisotopes; D028021:Sodium-Potassium-Chloride Symporters / Q000502:physiology*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D064506:Solute Carrier Family 12, Member 2
D001798:Blood Proteins; D018076:DNA, Complementary; D008562:Membrane Glycoproteins; D008744:Methylamines; D000644:Quaternary Ammonium Compounds; C498523:RHAG protein, human; D012414:Rubidium Radioisotopes; C578877:SLC12A2 protein, human; D028021:Sodium-Potassium-Chloride Symporters; D064506:Solute Carrier Family 12, Member 2; C027451:methylamine; D000242:Cyclic AMP; D000251:Adenosine Triphosphatases; D000254:Sodium-Potassium-Exchanging ATPase