Anabolic and catabolic mediators of intestinal protein turnover: a new experimental approach

Isotopic and modeling investigation of long-term protein turnover in rat tissues

Fractional synthesis rates (FSR) of tissue proteins (P) are usually measured using labeled amino acid (AA) tracer methods over short periods of time under acute, particular conditions. By combining the long-term and non-steady-state (15)N labeling of AA and P tissue fractions with compartmental modeling, we have developed a new isotopic approach to investigate the degree of compartmentation of P turnover in tissues and to estimate long-term FSR values under sustained and averaged nutritional and physiological conditions. We measured the rise-to-plateau kinetics of nitrogen isotopic enrichments (δ(15)N) in the AA and P fractions of various tissues in rats for 2 mo following a slight increase in diet δ(15)N. Using these δ(15)N kinetics and a numerical method based on a two-compartment model, we determined reliable FSR estimates for tissues in which P turnover is adequately represented by such a simple precursor-product model. This was the case for kidney, liver, plasma, and muscle, where FSR estimates were 103, 101, 58, and 11%/day, respectively. Conversely, we identified tissues, namely, skin and small intestine, where P turnover proved to be too complex to be represented by a simple two-compartment model, evidencing the higher level of subcompartmentation of the P and/or AA metabolism in these tissues. The present results support the value of this new approach in gaining cognitive and practical insights into tissue P turnover and propose new and integrated FSR values over all individual precursor AA and all diurnal variations in P kinetics.

Effect of energy substrates on protein degradation in isolated small intestinal enterocytes from rats

BACKGROUND

Nutrients affect small intestinal protein mass and metabolism, but studies on the effect of nutrients on small intestinal protein degradation are very limited due to a lack of a proper method. The objectives of this study were to establish a method to directly estimate protein degradation in isolated enterocytes from rats and to test the effect of energy substrates on protein degradation.

METHODS

Male Sprague-Dawley rats (150-200 g, n>or=8 per treatment) were used. Cell viability, tyrosine release as an indicator of protein degradation, and the effect of osmolarity, 50 mmol/L glucose, 20 mmol/L beta-hydroxybutyrate, 4.7 mmol/L butyrate, and 30 mmol/L glutamine on protein degradation were measured.

RESULTS

The average viability of enterocytes at time 30 minutes was 85.8% (range, 81%-94%). Tyrosine release was linear over the course of experiments, indicating constant protein degradation (R2=0.9943; p<.05). Osmolarity, glucose, and glutamine had no effect on protein degradation in isolated enterocytes. Beta-hydroxybutyrate significantly decreased it (-16%; p<.05), whereas butyrate slightly increased it (+5%; p<.05).

CONCLUSIONS

A high viability and constant protein degradation indicate a successful establishment of a method to estimate protein degradation in isolated small intestinal enterocytes from rats. The large effect of beta-hydroxybutyrate suggests a potential positive role for ketone bodies to limit the loss of small intestinal protein mass by decreasing protein degradation.

Atrofia mucosa/translocação bacteriana na sepse experimental em ratos Wistar

OBJETIVO: Avalizar a relação entre lesão mucosa e translocação bacteriana. MÉTODO: Utilizou-se 50 ratos distribuídos em 5 grupos: 1.Controle: injeção de inóculo padrão de Pseudomonas aeruginosa, 2.Dreno: injeção do inóculo padrão e drenagem da cavidade abdominal, após 6 horas, 3.Lavado: injeção do inóculo padrão e lavagem da cavidade abdominal, após 6 horas, 4.Lavado + dreno: injeção do inóculo padrão e após 6 horas, drenagem e lavagem da cavidade, 5.Normal: avaliação histológica da parede intestinal normal. Após o óbito, realizou-se hemocultura e cultura peritoneal. Realizou-se medida da espessura total da parede do jejuno e da camada mucosa em vilosidades seccionadas de forma longitudinal. RESULTADOS: Na hemocultura, houve crescimento de Pseudomonas aeruginosa e Escherichia coli em 90% e 52,5% dos animais. Na cultura peritoneal, houve crescimento de P. aeruginosa, E. coli e Klebsiella sp em 87,5%, 85% e 5% dos animais. Quanto a altura da camada mucosa e da parede intestinal, não houve alteração estatisticamente significativa entre os 5 grupos. CONCLUSÃO: A sepse aguda não causou alteração na camada mucosa do intestino delgado e a translocação ocorrida não pode ser considerada como decorrente de uma lesão da mucosa intestinal.

Key nutrients and growth factors for the neonatal gastrointestinal tract

The nutritional support of gastrointestinal growth and function is an important consideration in the clinical care of neonatal infants. In most health infants, the provision of either breast milk or formula seems to support normal intestinal mucosal growth, but the most significant advantages of breast milk may be for host defense or gut barrier-related functions that are involved in reducing infection. The specific effects of various milk-borne growth factors on key mucosal immune and barrier functions are likely to provide valuable new clues to the advantages of human milk. A substantial number of preterm, low-birth weight babies or those suffering from compromised intestinal function, however, often cannot tolerate oral feedings and instead receive TPN. The consequences of TPN on gastrointestinal function and how this contributes to morbidity of these infants warrants further study, with respect to both clinical and basic research questions. Although enteral nutrition seems to be a critical stimulus for intestinal function, the minimal amounts and composition of nutrients necessary to maintain specific intestinal functions remain to be established. The experimental tools exist to start defining the specific nutrient requirements for the infant gut and some of these nutrients are known (e.g., glutamate, glutamine, and threonine). Peptide growth factors and gut hormones clearly play a role in gut growth and in several ways mediate the trophic actions of enteral nutrition. Although a number of these growth factors are good candidates for therapeutic use, their clinical application in the management of gastrointestinal insufficiency and disease has been slow. The emergence of GLP-2 as a trophic peptide that seems to target the gut is a promising candidate on the horizon.