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1 Epithelial Pathobiology Group, Department of Surgery, University Cincinnati, Cincinnati, OH, USA; Department of Molecular and Cellular Physiology, University Cincinnati, Cincinnati, OH, USA
2 Epithelial Pathobiology Group, Department of Surgery, University Cincinnati, Cincinnati, OH, USA
* To whom correspondence should be addressed. E-mail: Roger.Worrell{at}uc.edu.
A significant amount of 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 trans-epithelial 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. 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+]0/[NH4+]o ratio at the basolateral membrane.
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