Colon luminal content and epithelial cell morphology are markedly modified in rats fed with a high-protein diet.

Hyperproteic diets are used in human nutrition to obtain body weight reduction. Although increased protein ingestion results in an increased transfer of proteins from the small to the large intestine, there is little information on the consequences of the use of such diets on the composition of large intestine content and on epithelial cell morphology and metabolism. Rats were fed for 15 days with either a normoproteic (NP, 14% protein) or a hyperproteic isocaloric diet (HP, 53% protein), and absorptive colonocytes were observed by electron microscopy or isolated for enzyme activity studies. The colonic luminal content was recovered for biochemical analysis. Absorbing colonocytes were characterized by a 1.7-fold reduction in the height of the brush-border membranes (P = 0.0001) after HP diet consumption when compared with NP. This coincided in the whole colon content of HP animals with a 1.8-fold higher mass content (P = 0.0020), a 2.2-fold higher water content (P = 0.0240), a 5.2-fold higher protease activity (P = 0.0104), a 5.5-fold higher ammonia content (P = 0.0008), and a more than twofold higher propionate, valerate, isobutyrate, and isovalerate content (P < 0.05). The basal oxygen consumption of colonocytes was similar in the NP and HP groups, but ammonia was found to provoke a dose-dependent decrease of oxygen consumption in the isolated absorbing colonocytes. The activity of glutamine synthetase (which condenses ammonia and glutamate) was found to be much higher in colonocytes than in small intestine enterocytes and was 1.6-fold higher (P = 0.0304) in colonocytes isolated from HP animals than NP. Glutaminase activity remained unchanged. Thus hyperproteic diet ingestion causes marked changes both in the luminal environment of colonocytes and in the characteristics of these cells, demonstrating that hyperproteic diet interferes with colonocyte metabolism and morphology. Possible causal relationships between energy metabolism, reduced height of colonocyte brush-border membranes, and reduced water absorption are discussed.

Colonic luminal ammonia and portal blood L-glutamine and L-arginine concentrations: a possible link between colon mucosa and liver ureagenesis.

The highest ammonia concentration in the body is found in the colon lumen and although there is evidence that this metabolite can be absorbed through the colonic epithelium, there is little information on the capacity of the colonic mucosa to transfer and metabolize this compound. In the present study, we used a model of conscious pig with a canula implanted into the proximal colon to inject endoluminally increasing amounts of ammonium chloride and to measure during 5 h the kinetics of ammonia and amino acid concentration changes in the portal and arterial blood. By injecting as a single dose from 1 to 5 g ammonia into the colonic lumen, a dose-related increase in ammonia concentration in the portal blood was recorded. Ammonia concentration remained unchanged in the arterial blood except for the highest dose tested, i.e. 5 g which thus apparently exceeds the hepatic ureagenesis capacity. By calculating the apparent net ammonia absorption, it was determined that the pig colonic epithelium has the capacity to absorb 4 g ammonia. Ammonia absorption through the colonic epithelium was concomitant with increase of L-glutamine and L-arginine concentrations in the portal blood. This coincided with the expression of both glutamate dehydrogenase and glutamine synthetase in isolated colonic epithelial cells. Since L-glutamine and L-arginine are known to represent activators for liver ureagenesis, we propose that increased portal concentrations of these amino acids following increased ammonia colonic luminal concentration represent a metabolic link between colon mucosa and liver urea biosynthesis.