Interactions of aromatic amino acids with gastric secretagogues in humans

Effects of parenteral nutrients on gastrointestinal motility and secretion

BACKGROUND

The stimulation of gastrointestinal motility and secretion during nutrient digestion is generally divided into a cephalic, gastric and intestinal phase. Little is known about the effects of macronutrients on gastrointestinal function during the postabsorptive or circulatory phase of digestion.

METHODS

Review of studies investigating the effects of circulating macro-nutrients such as fat, amino acids and glucose on gastrointestinal motility and secretion.

RESULTS

Intravenous infusion of fat emulsions delays gastric emptying and interrupts the interdigestive intestinal motor pattern. Intravenous amino acids, administered in high doses, stimulate gastric acid secretion, pancreatic secretion, gallbladder contraction, and intestinal motility. Patients receiving total parental nutrition (TPN) have inert gallbladders and are at risk of developing gallbladder sludge and stones. Administering a proportion of the daily amino acid requirement by rapid intravenous infusion may prove useful in the prevention of sludge and stone formation during TPN by promoting gallbladder contraction. Intravenous infusion of glucose, already at physiological postprandial plasma levels, inhibits gastrointestinal motility and secretion. The inhibitory effect of glucose is dose-dependent, that is, more pronounced at higher plasma glucose levels. Recent studies have indicated that in patients with diabetes mellitus alterations in gastrointestinal function are related to the degree of hyperglycaemia.

CONCLUSIONS

Nutrients during the circulatory phase of digestion influence gastrointestinal motility and secretion. Knowledge of these effects is relevant for conditions with increased plasma levels of macro-nutrients such as in patients with diabetes mellitus or during total parenteral nutrition.

Gastrointestinal motor and secretory responses to cholinergic stimulation in humans. Differential modulation by muscarinic and cholecystokinin receptor blockade

The present study investigated how a cholinergic agonist modifies interdigestive motility and secretion of the upper gastrointestinal tract and how muscarinic and cholecystokinin receptor blockade interfere with this direct cholinergic stimulation. In eight healthy volunteers, gastrointestinal motor and secretory responses to bethanechol (12.5, 25, and 50 micrograms kg-1 h-1) with and without a background of atropine (5 micrograms kg-1 h-1) or loxiglumide (10 mg kg-1 h-1) were studied. Stepdoses of bethanechol caused a parallel stimulation of antroduodenal motility and gastropancreatic secretion (P < 0.01) without inducing a fed pattern. However, duration of phase I was shortened (P < 0.05). Only high doses of bethanechol enhanced gastrin (P < 0.05), cholecystokinin (P < 0.05), and pancreatic polypeptide (P < 0.01) release. Atropine completely antagonized motor and secretory responses to cholinergic stimulation. Loxiglumide left cholinergically stimulated motility and pancreatic enzyme secretion unaltered. With co-infusion of bethanechol and loxiglumide, PP release dropped by 63% (P < 0.01); gastric acid output, gastrin and CCK release increased by 56%, 16%, and 25%, respectively (P < 0.05). We conclude that stimulation by a cholinergic agonist preserves the interdigestive pattern. Low dose muscarinic receptor blockade abolishes cholinergic stimulation over the full dose range. Inhibition of somatostatin release would explain stimulation of gastrin release and gastric acid secretion with co-infusion of bethanechol and loxiglumide. Endogenous CCK appears to interact with direct cholinergic stimulation at the pancreatic PP cell and the gastric D-cell but not at pancreatic acinar and antroduodenal smooth muscle cells.

Phorbol ester inhibition of chicken intestinal brush-border sodium-proton exchange

Phorbol esters, specific activators of protein kinase C, inhibit amiloride-sensitive Na uptake from the mucosal medium in intact intestinal mucosa as well as in isolated chicken villus enterocytes. In isolated cells, maximal inhibition is observed at 60 s, and influx returns to control values within 15 min. This effect can be measured either as initial 22Na influx rates or by following changes in intracellular pH using the pH-sensitive fluorescent dye 5,6-carboxyfluorescein. The effects of amiloride and phorbol esters were not additive, suggesting inhibition of a common transport system, i.e., Na-H exchange. In brush-border membrane vesicles (BBMV) made from villus enterocytes, amiloride-sensitive Na-H exchange activity was significantly inhibited in phorbol ester-treated cells. The degree of inhibition of 22Na uptake by BBMV had the same time course and dose-effect relationship as phorbol ester-inhibited cellular Na uptake. Similarly, the time course of protein kinase C translocation from cytosol to particulate or brush-border membrane fractions correlated with Na uptake measurements made in whole cells and BBMV. These results suggest that protein kinase C activation in chicken villus enterocytes inhibits brush-border membrane Na-H exchange activity.

Effect of intravenous omeprazole on intragastric pH during intravenous infusion of amino acids

Intravenous amino acids stimulate gastric acid secretion by an unknown mechanism. In patients on parenteral nutrition, this amino acid-induced gastric acid secretion might contribute to the failure of H2-receptor antagonists to raise intragastric pH above 4.0, a level thought to be needed to prevent stress ulceration. Therefore we studied the effect of single and repeated doses of the H+/K(+)-ATPase blocker omeprazole on the intragastric pH during a 3-hr infusion of amino acids in 10 healthy volunteers; 5% glucose was used as a control infusion. Amino acids significantly decreased intragastric pH when compared to glucose infusion (P less than 0.05). After intravenous administration of 40 mg, 80 mg and 2 X 40 mg omeprazole, this amino acid-induced fall in pH was significantly inhibited (P less than 0.01). No advantage of the 80-mg dose over the 40-mg dose could be demonstrated. The repeated dose of 40 mg showed a tendency to higher pH values compared to the single-dose experiments, which reached significance in the amino acid experiments only (P less than 0.05). Neither during the infusion of amino acids nor the glucose infusion omeprazole was able to continuously raise intragastric pH above 4.0. In conclusion, this study shows that intravenous omeprazole prevents gastric acid stimulation by intravenous amino acids but fails to continuously raise intragastric pH above 4.

Cerebroventricular calcitonin gene-related peptide inhibits rat duodenal bicarbonate secretion by release of norepinephrine and vasopressin

Proximal duodenal bicarbonate secretion is an important factor in humans and animals protecting the mucosa against acid-peptic damage. This study examined the mechanisms responsible for the central nervous system regulation of duodenal bicarbonate secretion by calcitonin gene-related peptide (CGRP) in unrestrained rats. Cerebroventricular administration of rat CGRP significantly inhibited basal duodenal bicarbonate secretion as well as the stimulatory effects of vasoactive intestinal peptide, neurotensin, a luminal PGE1 analogue, misoprostol, and hydrochloric acid. The inhibitory effects of cerebroventricular CGRP were abolished by ganglionic blockade with chlorisondamine, significantly attenuated by noradrenergic blockade with bretylium, and enhanced by vagotomy. Inhibition of duodenal bicarbonate secretion induced by CGRP coincided with significant increases in plasma norepinephrine (NE) and vasopressin concentrations. The alpha adrenergic receptor antagonist, phentolamine, and the vasopressin V1 receptor antagonist, (1-deaminopenicillamine, 2-[O-methyl]Tyr, 8-Arg)-vasopressin, given intravenously reversed the central inhibitory effect of CGRP by approximately 50% each. Pretreatment of the animals with both phentolamine and the vasopressin antagonist completely abolished the central inhibitory effect of CGRP. Peripheral vasopressin and NE significantly decreased duodenal bicarbonate secretion, and their inhibitory effects were additive and prevented by phentolamine and the vasopressin antagonist, respectively. We conclude that cerebroventricular CGRP inhibits rat duodenal bicarbonate secretion by activation of sympathetic efferents and subsequent release of NE and vasopressin that act on alpha adrenergic and vasopressin receptors, respectively.