Ammonium effects on colonic Cl- secretion: anomalous mole fraction behavior.
A significant amount of ammonium (NH4+) is absorbed by the colon. The nature of NH4+ effects on transport and NH4+ transport itself in colonic epithelium is poorly understood. The goal of this study was to elucidate the effects of NH4+ on cAMP-stimulated Cl- secretion in the colonic cell line T84. In HEPES-buffered solutions, application of basolateral NH4+ resulted in a reduced level of Cl- secretory current. The effect of NH4+ appears to occur by at least three mechanisms: 1) basolateral membrane depolarization, 2) a competitive effect with K+, and 3) a long-term (>20 min) increase in transepithelial resistance (TER). The competitive effect with K+ exhibits anomalous mole fraction behavior. Transepithelial current relative to that in 10 mM basolateral K+ was inhibited 15% by 10 mM NH4+ alone and by 30% with a mixture of 2 mM K+ and 8 mM NH4+. A mole fraction mix of 2 mM K+:8 mM NH4+ produced a greater inhibition of basolateral membrane K+ current than pure K+ or NH4+ alone. Similar anomalous behavior was also observed for inhibition of bumetanide-sensitive 36Cl- uptake, e.g., Na+-K+-2Cl- -cotransporter (NKCC-1). No anomalous effect was observed on Na+-K+-ATPase current. Both NKCC-1 and Na+-K+-ATPase activity were elevated in 10 mM NH4+ with respect to 10 mM K+. The effect on TER did not exhibit anomalous mole fraction behavior. The overall effect of basolateral NH4+ on cAMP-stimulated transport is dependent on the [K+]o /[NH4+]o ratio at the basolateral membrane, where o is outside of the cell.
Worrell
Roger T
RT
Epithelial Pathobiology Group, Department of Surgery, University of Cincinnati, Cincinnati, OH 45219, USA. Roger.Worrell@uc.edu
Oghene
Jennifer
J
Matthews
Jeffrey B
JB
D002460:Cell Line; D002712:Chlorides / Q000378:metabolism*; D002713:Chlorine / Q000378:metabolism; D005576:Colforsin / Q000494:pharmacology; D003106:Colon / Q000187:drug effects / Q000378:metabolism*; D015232:Dinoprostone / Q000494:pharmacology; D004848:Epithelium / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology*; D011868:Radioisotopes; D028021:Sodium-Potassium-Chloride Symporters / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013268:Stimulation, Chemical
D002712:Chlorides; D015221:Potassium Channels; D000644:Quaternary Ammonium Compounds; D011868:Radioisotopes; D028021:Sodium-Potassium-Chloride Symporters; D005576:Colforsin; D002713:Chlorine; D000254:Sodium-Potassium-Exchanging ATPase; D015232:Dinoprostone; D011188:Potassium