Intestinal ammonium transport by ammonium and hydrogen exchange.

BACKGROUND: Ionized ammonia (NH3) transport in the intestine has not been previously established as a mechanism of acidosis in urinary intestinal diversion or hepatic failure.

STUDY DESIGN: The purpose of this study was to establish that ionized transport of ammonium (NH4) occurs in the intestine and to characterize the mechanism of its transport using the methodology of brush border membrane vesicles and acridine orange fluorescence.

RESULTS: An NH4/H exchange was demonstrated and found to be the dominant mechanism causing a pH change when NH4 is transported across the lumenal membrane. Ionized NH4 transport was demonstrated to occur against an NH3 concentration gradient. The Km was 1.02 mmol and the Vmax was 247 U/sec. The Hill coefficient was 0.97, indicating a single port. Ammonium hydrogen exchange could be inhibited by amiloride but not by bumetanide. Sodium, potassium or chloride, or both, did not effect the NH4/H exchanger.

CONCLUSIONS: This study establishes that ionized NH4 transport occurs across the small intestine brush border in exchange for a hydrogen ion.