Joan P. W. Webb

Functional differentiation of human jejunum and ileum: A comparison of the handling of glucose, peptides, and amino acids

The characteristics of glucose, glycine, L-alanine, and glycyl-L-alanine absorption from the jejunum and ileum have been compared in normal human subjects. A perfusion technique has been used, and correct positioning of the perfusion tube has been confirmed by measuring the differential jejunal and ileal handling of bicarbonate. Glucose and glycine were absorbed faster from the jejunum than from the ileum of all subjects studied, and L-alanine was absorbed faster from the jejunum than from the ileum in five out of six subjects studied. In contrast, the dipeptide glycyl-L-alanine was absorbed at comparable rates from the jejunum and ileum. Higher concentrations of free amino acids were detected in the luminal contents aspirated during the ileal dipeptide perfusions. These results emphasize the importance of oligopeptide transport in the absorption of protein digestion products, especially in the human ileum, and the practical implications of these findings are discussed.

RESPONSE OF CIRCULATING SOMATOSTATIN, INSULIN, GASTRIN AND GIP, TO INTRADUODENAL INFUSION OF NUTRIENTS IN NORMAL MAN

We have studied the effect of direct infusion of nutrients into the duodenum of normal subjects on circulating plasma somatostatin, insulin, gastrin and gastric inhibitory polypeptide (GIP) levels. Six normal subjects were given on four separate occasions 150 ml of isotonic solutions containing 100 calories of carbohydrate, protein, or fat, and a control solution of saline, by infusion into the second part of the duodenum. Plasma somatostatin rose slightly after carbohydrate, mean basal 30 ± 3 pg/ml, peak 46±16 pg/ml at 15 min; and more markedly after protein, peak 57 ± 9 pg/ml at 30 min. However, fat was the most potent intraduodenal stimulus to plasma somatostatin release into circulation, peak 101 ± 11 pg/ml at 30 min. The plasma insulin rise was greatest after carbohydrate, peak 68 ± 10 i.u., but there was a significant rise after protein also, peak 34 ±6 i.u. Plasma gastrin rose significantly after protein only, peak 70 ± 22 pg/ml. Plasma GIP rose markedly after carbohydrate, basal 506 ± 50 pg/ml, peak 1480 ± 120 pg/ml. Protein was also a potent stimulus of circulating plasma GIP release, peak 1200 ± 190 pg/ml, while fat was the least potent, peak 730±190 pg/ml. Thus, calorie for calorie, fat is the most potent intraduodenal nutrient stimulus of circulating somatostatin. We postulate therefore that somatostatin may be an enterogastrone–a circulating hormone released by intraduodenal fat which inhibits gastric acid secretion. Fat is the least potent intraduodenal nutrient stimulus of circulating GIP release. This is evidence against the hypothesis that circulating GIP acts as an enterogastrone.

Ileal function in patients with untreated adult coeliac disease.

A double-lumen perfusion technique has been used to investigate jejunal and ileal absorption of glucose, water, and electrolytes in a group of patients with untreated adult coeliac disease. Correct positioning of the tube was confirmed by measuring the differential jejunal and ileal handling of bicarbonate. Eight control subjects and eight patients with coeliac disease were perfused with an isotonic electrolyte solution containing 50 mM glucose and 25 mM bicarbonate. The group of coeliac patients had impaired jejunal absorption of glucose (P less than 0.001), water (P less than 0.01), sodium (P less than 0.02), and chloride (P greater than 0.05) compared with the control group. In contrast the group of coeliac patients had normal ileal glucose and water absorption and increased ileal sodium (P greater than 0.01) and chloride (P greater than 0.05) absorption compared with the controls. Evidence for ileal adaptation was found in three individual patients who had absorptive values outside 2SD of the normal mean. The results indicate that the distal small intestine in coeliac disease has the ability to adopt to the damage and loss of absorptive capacity in the proximal small intestine.

Autonomic regulation of postprandial plasma somatostatin, gastrin, and insulin.

To evaluate the neural regulation of postprandial somatostatin release we studied the effect of blockade of (a) alpha-adrenergic and beta-adrenergic and (b) cholinergic receptors on the plasma somatostatin, gastrin and insulin responses to a standard meal in two groups of five fasting healthy male volunteers. Thymoxamine (0.1 mg/kg iv over two minutes then 10 mg/hour for two hours) and propranolol (0.15 mg/kg iv over two minutes, then 0.75 mg/kg/hour for two hours) were started just before eating while atropine (0.04 mg/kg/im) was given at 15 minutes on completion of the meal. There was a prompt and sustained rise in plasma somatostatin after a control meal in all experiments. This rise was arrested by atropine but not altered by either thymoxamine or propranolol. The plasma gastrin response to a meal was moderately enhanced by thymoxamine and markedly enhanced by atropine. Postprandial insulin release was not affected by alpha- or beta-adrenergic blockade but was abolished by atropine. The effect of atropine on the postprandial plasma somatostatin rise might have been mediated through reduction in gastric acidity or delay in gastric emptying. Hence we gave five fasting male volunteers and intraduodenal infusion of fat emulsion (25 calories in 30 minutes) on two occasions both alone and after atropine. Plasma somatostatin rose during the fat infusion alone and this rise was abolished by atropine. These data suggest that (a) cholinergic but not adrenergic mechanisms are important modulators of plasma somatostatin release after orally ingested and intraduodenally infused nutrients (b) atropine abolishes plasma somatostatin release independently of its effects on gastric acidity and motility and (c) are consistent with the hypothesis that atropine potentiates postprandial gastrin release through reduction of somatostatin mediated inhibition.

Does gastric acid release plasma somatostatin in man

Food and insulin hypoglycaemia raise plasma concentrations of somatostatin. Both also stimulate gastric acid secretion but it is not clear whether gastric acid itself has any effect on somatostatin secretion. We, therefore, studied the effect on plasma concentrations of somatostatin of infusion of 0.1 N HC1 into the stomach and duodenum of healthy subjects. Plasma somatostatin did not rise with a small dose of HC1 given intragastrically (15 mmol) or intraduodenally (4 mmol). After an intraduodenal infusion of 60 mmol HC1 over 30 minutes, sufficient to reduce intraluminal pH to 2, plasma somatostatin rose moderately in five subjects from a mean value (+/- SEM) of 32 +/- 3 pg/ml to a peak at 10 minutes of 54 +/- 11 pg/ml. It is concluded that: (a) intragastric acid infusions do not release circulating somatostatin in man; and (b) that intraduodenal acidification albeit at grossly supraphysiological doses is a moderate stimulus of plasma somatostatin release. Therefore, gastric acid is unlikely to be a major factor mediating postprandial plasma somatostatin release in man.

The assay of dipeptides using fluorescamine and its application to determining dipeptidase activity.

Abstract The differing effect of pH on the reaction of fluorescamine with amino acids and peptides has been utilized to develop a method for the quantitative estimation of peptides in the presence of amino acids. This method has been applied to the measurement of dipeptidase activities in human jejunal aspirates.

Limitation of the use of inert gases in the measurement of small gut mucosal blood flow.

The role of blood flow in absorption from the small gut would be best studied by a method able to measure exclusively blood flow in the small gut mucosa because this might vary independently of total small gut blood flow. Such a method should be repeatable to give values in control and experimental periods, and one would hope that it would be adaptable for use in man. We had hoped to meet these criteria by measuring the rate of removal of 133Xenon from fluid in the lumen of the small gut. Kety (1951) has described the factors involved in the use of inert gases to measure tissue blood flow. In muscle (e.g., Lassen, H0edt-Rasmussen, Lindjberg, Pedersen, and Munck, 1963), myocardium (e.g., Ross, Ueda, Lichtlen, and Rees, 1964), and brain (e.g., Lassen and Klee, 1956) blood flow has been measured from the rate of disappearance of radioactive inert gases out of the tissue. The validity of these methods depends upon the assumption that diffusion of gas through the tissue is so fast that it does not delay its removal, which is therefore dependent only upon blood flow. In the gut, if the same assumption holds true, then when the gas is dissolved in fluid placed in the gut lumen, the rate of its removal will be proportional to blood flow in the mucosa, and will be unaffected by blood flow in any other part of the gut wall.

A physicochemical study of fat absorption in rats. Limitation of methods in vitro.

Abstract 1. 1. The hypothesis that the first stage of normal fatty acid absorption represents a partition between a luminal bile salt mixed micellar phase and mucosal membrane lipid phase has been investigated. 2. 2. Everted segments of rat jejunum incubated with [14C]oleic acid in 10 mM sodium taurocholate accumulated a greater amount of 14C-labelled free fatty acid when incubated at pH 5.8 then at pH 7.3 but this was unrelated to incorporation into jejunal [14C]triglyceride. 3. 3. Uptake of [14C]oleic acid into mucosal free fatty acid from 15 mM taurocholate (20 μmoles/ml) in vitro was less than from 6 mM taurocholate, but the triglyceride accumulation was greater. 4. 4. pH had no effect on the uptake in vitro of the non-ionized oleyl alcohol from taurocholate mixed micellar solution but with increasing taurocholate concentration there was decreased uptake of [14C] oleyl alcohol. 5. 5. Increasing concentrations of [14C]oleic acid in bile salt mixed micelles caused an increase both of 14C-labelled free fatty acid and [14C]triglyceride in vitro. 6. 6. Absorption of [14C]oleic acid from perfused or closed loops of jejunum in vivo was increased with increased oleic acid concentration but was not influenced by pH or bile salt concentration. 7. 7. These findings suggest that although uptake of free fatty acid by the mucosal cell in vitro may be predicted on the basis of a partition between micelle and lipid membrane via a molecular phase, this is not an important mechanism during normal fatty acid absorption.

The absorption of tristearin and stearic acid and tripalmitin and palmitic acid: Studies on the rate-limiting steps in rats

Abstract 1. 1. In free feeding experiments in rats the absorption of labelled tristearin was poor. It remained constant over a wide dose range, but fell with a high dose. The absorption in bile fistula rats was diminished. 2. 2. The addition of triolein increased the absorption of labelled tristearin in control rats but not in bile fistula rats, suggesting that poor solubility in bile salt solution is an important step hindering the absorption of tristearin and that triolein stimulates absorption by increasing this solubility. 3. 3. Jejunal segments incubated in a micellar solution of equal concentration of labelled stearic acid and oleic acid in 10 mM sodium taurocholate took up stearic acid at a slightly faster rate than oleic in a ratio of 0.84, but incorporation into triglyceride was greater for oleic acid in the ratio of 1.3. 4. 4. When a micellar solution of equal concentration of labelled oleic and stearic acids were infused into the duodenum the content of oleic acid in lymph triglyceride was greater than that of stearic acid in a ratio of 1.35. 5. 5. When equal weights of labelled oleic and stearic acids were fed, mixed with bran, the content of oleic acid in lymph triglyceride was much greater than that for stearic acid, in a ratio of 8.9 in the first hour, falling to 4.5 by the fourth hour. These ratios are much greater than those during lymphatic absorption from equimolar micellar solution. 6. 6. We conclude that micellar solubilisation is a major rate-limiting step in the absorption of tristearin and stearic acid, that mucosal cell uptake is not rate-limiting and that esterification is only a minor rate-determining step. 7. 7. The absorption of palmitic acid by control rats was slightly increased by a large dose of triolein added to the feed, but that of tripelmitin was unaltered. Reasons for the difference in behaviour between tripalmitin and tristearin are discussed.

Oleic Acid Absorption from Micellar Solutions and Emulsions in the Rat

Summary Closed loops of rat jejunum in vivo have been used to study the absorption of oleic acid from nonmicellar and micellar taurocholate solutions and compared with that from micellar solutions of a nonionic detergent Pluronic Acid F 68. (a) Absorption from 15 mM taurocholate was greater than from 1 mM taurocholate over a range of oleic acid concentrations. (b). Absorption of oleic acid was proportional to its total and not its micellar concentration. (c) Pluronic acid micelles were as efficient as taurocholate micelles in promoting absorption from a 10 mM oleic acid solution but had no advantage over a nonmicellar solution at low oleic acid concentration 0.1 mM.