Arteriovenous differences for amino acids across control and acid-secreting rat stomach in vivo

Branched-chain amino acids

The branched-chain amino acids (BCAA), isoleucine, leucine and valine, are unique in that they are principally metabolized extrahepatically in the skeletal muscle. This observation led to the investigation of these nutrients in a number of clinical scenarios. By far the most intensively studied applications for BCAA have been in patients with liver failure and/or patients in catabolic disease states. However, the resulting studies have not demonstrated a clear clinical benefit for BCAA nutritional supplements. In patients with liver failure, the BCAA did improve nitrogen retention and protein synthesis, but their effect on patient outcome was less clear. Similarly, in critically ill septic patients, BCAA did not improve either survival or morbidity. The BCAA are important nutrients, and it seems that any specific benefits associated with their use will be based upon a greater understanding of the underlying cellular biology. Potential areas of further research may include the combination of BCAA supplements with other anabolic factors (e.g. growth hormone) in managing patients with catabolic disease states.

Glutamine metabolism in the gastrointestinal tract of the rat assess by the relative activities of glutaminase (EC 3.5.1.2) and glutamine synthetase (EC 6.3.1.2)

The activities of the two key enzyme involved in glutamine metabolism, glutaminase (EC 3.5.1.2) and glutamine synthetase (EC 6.3.1.2), have been measured in the various tissues of the gastrointestinal (GI) tract of the rat, from the mouth to the rectum. Glutaminase activity was particularly high in the mucosa of the small intestine, where its activity accounted for more than 80% of the total activity of the GI tract. In contrast, the mouth and oesophagus had very low activities, accounting for less than 2% of the total. Glutamine synthetase was mainly confined to the lower part of the stomach, which accounted for almost 90% of the total activity of the GI tract. Activity in the small intestine was very low, accounting for less than 2% of the total, and similarly low levels were found in the mouth and oesophagus. The data provide the most complete information on the distribution of these enzymes in the GI tract of the rat and suggest: (a) that the mucosa of the small intestine has the highest capacity for glutamine breakdown but the lowest capacity for its synthesis, and so requires an external source of this amino acid; (b) that there is little potential for glutamine synthesis or breakdown in the mouth and oesophagus: and (c) that the lower stomach has a substantial capacity to synthesize glutamine, in contrast to the rest of the GI tract. The results of the investigation are relevant to sites of glutamine metabolism in therapeutic studies involving glutamine administration discussed with reference to reports of the effects of glutamine administration on GI tract injury.

Uptake of L-leucine and L-phenylalanine across the basolateral cell surface in isolated oxyntic glands

The time course, kinetic, specificity and sodium-dependence of L-leucine and L-phenylalanine uptake by rabbit isolated oxyntic glands were studied in order to identify the systems involved in the transport of branched-chain and aromatic neutral amino acids through the basolateral cell membrane. The uptake was measured directly in the disrupted cells after incubation of the glands with the 3H-labelled amino acid both in a sodium-containing and a sodium-free medium. The uptake of L-leucine was largely carrier-mediated whilst L-phenylalanine was taken up by either carrier-mediated and nonsaturable processes. Both amino acids were taken up by a Na(+)-independent process. The kinetic parameters of L-leucine and L-phenylalanine carrier-mediated influx were, respectively: Kt = 2.71 mM and Jmax = 1390 nmol mg-1 s-1, Kt = 1.03 mM and Jmax = 176 nmol mg-1 s-1. From cross-inhibition studies it can be inferred that L-leucine is primarily transported by a Na(+)-independent system which shows specificity for bulky side chains dipolar amino acids. The system displays similar affinities for L-phenylalanine (Ki = 2.81 mM) and L-isoleucine (Ki = 2.62 mM). Similar results were obtained from self-inhibition experiments: the Ki of the carrier-mediated uptake of L-leucine and L-phenylalanine were 2.12 and 2.40 mM (from a Hanes plot) or 3.2 and 0.8 mM (from a Dixon plot), respectively. It is concluded that a sodium-independent transport system, like Christensen's 'L' type, is shared by branched-chain and aromatic dipolar amino acids, which only shows slight differences in their affinities for the carrier.

Metabolism and gastric acid secretion. Substrate-dependency of aminopyrine accumulation in isolated rat parietal cells

The substrate-dependency of gastric acid secretion was investigated in isolated rat parietal cells by using the accumulation of the weak base aminopyrine as an index of acid secretion. Exogenous substrates enhanced accumulation of aminopyrine in rat parietal cells stimulated by secretagogues, and this effect was probably directly related to the provision of energy for acid secretion. At physiological concentrations, certain of the substrates (glucose, oleate, lactate, D-3-hydroxybutyrate, L-isoleucine, L-valine and acetoacetate) could support acid secretion, with glucose being the most effective. L-Leucine and acetate were only effective stimulators of parietal-cell aminopyrine accumulation at high concentrations (5mM). L-Glutamine was unable to stimulate aminopyrine accumulation even at high concentrations, and glutaminase activity in parietal cells was estimated to be low by comparison with small-intestinal epithelial cells. Variation in the concentrations of D-3-hydroxybutyrate and L-isoleucine, but not of glucose, within the physiological range affected their ability to support aminopyrine accumulation. The presence of 5 mM-L-isoleucine, 5 mM-lactate and combinations of certain substrates at physiological concentrations produced aminopyrine accumulation in stimulated parietal cells that was greater than that obtained in cells incubated with 5 mM-glucose alone. In conclusion, fulfillment of the metabolic requirements of the acid-secreting parietal cell under physiological circumstances requires a combination of substrates, and integration of the results with previous data [Anderson & Hanson (1983) Biochem. J. 210, 451-455; 212, 875-879] suggests that after overnight starvation in vivo metabolism of glucose, D-3-hydroxybutyrate and L-isoleucine may be of particular importance.

Substrate utilization by rat stomach in vivo. Arteriovenous differences for glucose, lactate, ketone bodies, fatty acids and glycerol under control and acid-secreting conditions

Arteriovenous differences for several potential metabolic substrates were measured across the fundic wall of the stomach of rats that had been starved overnight. There was an uptake of glucose and D-3-hydroxybutyrate, but no significant arteriovenous differences for acetoacetate, pyruvate, non-esterified fatty acids and glycerol were apparent. Lactate output represented a substantial fraction of glucose uptake when the arterial lactate concentration was within the resting physiological range, but when the arterial lactate concentration was above 1.3 mM, lactate was taken up by the stomach. Stimulation of acid secretion by pentagastrin did not affect the value of arteriovenous differences. Thus blood flow to the fundic mucosa and substrate metabolism may be similarly enhanced by pentagastrin. It is concluded that metabolism of glucose and D-3-hydroxybutyrate, and to a lesser extent of glutamine and branched-chain amino acids [Anderson & Hanson (1983) Biochem. J. 210, 451-455], could supply energy to power acid secretion.