Urea cycling and ammonia absorption in vivo in the digestive tract of the rat

Abstract

The transfer of blood urea into the digestive tract and ammonia absorption at that site have been studied in vivo in anesthetized rats. The vein-artery differences in urea and ammonia absorption were greater in the caecum than in the ileum. The urea load indicated that blood urea was an important factor in ammonia production. Carbohydrate substrates in the caecum (cell-wall carbohydrates or lactose) increased caecal size and volatile fatty acid production. In these conditions, blood urea uptake increased; moreover, acidification of the caecal content did not decrease ammonia absorption, despite a drop in the NH3 form, which is considered the most diffusible. Urea transfer and ammonia absorption were highest with high-protein diets when accompanied by considerable caecal fermentation. As compared to the predominant role of blood urea level and of caecal development (blood flow, surface of exchange, ureolytic activity), caecal pH seemed to be a less important factor. This observation could correspond to urea hydrolysis near the caecal wall in different conditions of pH and substrate than those found throughout the caecum.