Ammonia Movement in the Small Intestine: Preferential Transport by the Ileum *

Abstract

Summary. Isolated, surviving sacs of everted small intestine were used to characterize ammonia transport in the golden hamster. Jejunal and ileal sacs incubated aerobically in ammonia-free test solution liberated the same quantity of ammonia as did sacs that were filled and immediately emptied of their contents, indicating no significant evolution of metabolic ammonia.Under aerobic conditions, ileal sacs transferred a solution of high ammonia content from the mucosal surface to the serosal surface against a concentration gradient. This transport was not glucose dependent and exhibited first-order Michaelis-Menten kinetics. Inhibition of absorption occurred with anaerobiosis, 2,4-dinitrophenol, and sodium cyanide. In jejunal segments ammonia was not transported against an adverse chemical gradient. Ileal ammonia absorption was accompanied by bicarbonate secretion and acidification of the serosal solution. Both bicarbonate movement and pH gradients were abolished by inhibitors of ammonia transport. In the jejunum, the absence of ammonia movement occurred in association with minimal bicarbonate secretion and no appreciable change in serosal pH.Despite the creation of hydrogen ion gradients tending to augment or to retard ammonia absorption by nonionic diffusion, ammonia movement was unaffected, i.e., relative acidification of serosal contents did not augment ammonia absorption, and relative alkalinization of serosal fluid caused no inhibition of ammonia transport. In the absence of bicarbonate ion, ammonia transport did not occur. The significance of these findings is discussed with consideration of both ionic and nonionic mechanisms of ammonia movement.

Expression of the ammonia transporter proteins Rh B glycoprotein and Rh C glycoprotein in the intestinal tract

“…Small intestinal ammonia transport may be mediated, at least in part, by the NH 4 ϩ /H ϩ exchange activity present in small intestinal brush-border vesicles that has a K m for ammonia of ϳ1 mM (32). This apical NH 4 ϩ /H ϩ exchange activity is consistent with the observation that increasing luminal H ϩ concentration increases small intestinal ammonia secretion (34,35,45,46). RhCG is a candidate protein to mediate this apical NH 4 ϩ /H ϩ exchange activity.…”

“…Specifically, small intestinal ammonia transport is energy-dependent, saturable, and exhibits Michaelis-Menton constant kinetics (34,35), and recalculation of published data suggest that the K m for luminal ammonia absorption is ϳ7 mM (35). Small intestinal ammonia transport may be mediated, at least in part, by the NH 4 ϩ /H ϩ exchange activity present in small intestinal brush-border vesicles that has a K m for ammonia of ϳ1 mM (32).…”

Ammonia inhibits cAMP-regulated intestinal Cl- transport. Asymmetric effects of apical and basolateral exposure and implications for epithelial barrier function.

“…The mechanism of ammonia absorption by the colon is not fully understood. Distal ileum is thought to possess transport pathways for the active absorption of NH' (35). Cohen et al, however, observed that in rat distal colon, NH3 is 400 times more permeant than NH', and suggested that passive absorption of NH3 may be quantitatively more important than transport of NH' at physiological fecal pH (33).…”

Physicochemical Fundamentals and Thermodynamics of the Membrane Transport of Drugs

“…III.B.). Moreover, some further criteria of active transport [126,268,[299][300][301][302]304,331] are concerned with its dependence on the intensity of cell metabolism [29,112,121,125,144,228,238,259,284], inhibition of it by a fall in temperature [261], and metabolic poisons [29,209,238], by O 2 , and substrate deficiency. The accumulation of a permeant in the cell interior, as proposed by LING [187], TROSHIN [295], HARRIS and PRANKERD [106], SHAW et aI.…”