Effect of enkephalin and naloxone on gastric acid and serum gastrin and pancreatic polypeptide concentrations in humans

Research progress of opioid growth factor in immune-related diseases and cancer diseases

Methionine enkephalin (MENK) has an important role in both neuroendocrine and immune systems. MENK was known as an opioid growth factor (OGF) for its growth regulatory characteristics. OGF interacts with the OGF receptor (OGFr) to inhibit DNA synthesis by upregulating p16 and/or p21, which delays the cell cycle transition from G0/G1 to S phase, and inhibits cell proliferation. In addition, OGF combines with OGFr in immune cells to exert its immunomodulatory activity and regulate immune function. OGF has been studied as an immunomodulator in a variety of autoimmune diseases, including multiple sclerosis, inflammatory bowel disease, diabetes and viral infections, and has been proven to relieve symptoms of certain diseases in animal and in vitro experiments. Also, OGF and OGFr have various anti-tumor molecular mechanisms. OGF can be used as the primary therapy alone or combined with other drugs to treat tumors. This article summarizes the research progress of OGF in immune-related diseases and cancer diseases.

Effects of intra-gastric beta-casomorphin-7 on somatostatin and gastrin gene expression in rat gastric mucosa

AIM: To investigate the in vivo effect of beta-casomorphin-7 on the regulation of gastric somatostatin and gastrin messenger RNA in rat gastric mucosa.

METHODS: Somatostatin and gastrin mRNA were quantified by RT-PCR and in situ hybridization (ISH) in 24 rats. The rats were divided into three treatment groups: basal diet + physiological saline (n = 8), basal diet + beta-casomorphin-7 (7.5 × 10^-7 mol) (n = 8), and basal diet + poly-Gly-7 (containing equal mol of N with 7.5 × 10^-7 mol beta-casomorphin-7) (n = 8). After oral administration for 30 days, rats were killed by exsanguinations.

RESULTS: After intra-gastric administration of beta-casomorphin-7 for 30 d, gastrin mRNA increased by 52.8% (P < 0.05, n = 8), and somatostatin mRNA levels decreased by 30.7% compared with the controls (P < 0.01, n = 8). No significant differences in the expression of the two genes were observed in the poly-Gly-treated group, although gastrin mRNA expression was elevated by 35.6% as against the control group (P = 0.15, n = 8). The long-term oral administration of a casomorphin solution significantly decreased the even gray of D-cells, but did not lower the number of D-cells both in the antrum and fundus. Interestingly, the number of G-cells increased in the antrum and fundus, but its average density was augmented only in the antrum.

CONCLUSION: Beta-casomorphin-7 is capable of modulating gene expression of the regulatory peptides from G and D cells. Data from in situ hybridization studies indicate that beta-casomorphin-7 affects gastrin gene expression indirectly by means of the paracrine action of somatostatin, and depends on its intrinsic molecular function.

Expression Profiles Provide Insights into Early Malignant Potential and Skeletal Abnormalities in Multiple Endocrine Neoplasia Type 2B Syndrome Tumors

Identifying the molecular basis for genotype-phenotype correlations in human diseases has direct implications for understanding the disease process and hence for the identification of potential therapeutic targets. To this end, we performed microarray expression analysis on benign (pheochromocytomas) and malignant (medullary thyroid carcinomas, MTCs) tumors from patients with multiple endocrine neoplasia (MEN) type 2A or 2B, related syndromes that result from distinctive mutations in the RET receptor tyrosine kinase. Comparisons of MEN 2B and MEN 2A MTCs revealed that genes involved in the process of epithelial to mesenchymal transition, many associated with the tumor growth factor beta pathway, were up-regulated in MEN 2B MTCs. This MEN 2B MTC profile may explain the early onset of malignancy in MEN 2B compared with MEN 2A patients. Furthermore, chondromodulin-1, a known regulator of cartilage and bone growth, was expressed at high levels specifically in MEN 2B MTCs. Chondromodulin-1 mRNA and protein expression was localized to the malignant C cells, and its high expression was directly associated with the presence of skeletal abnormalities in MEN 2B patients. These findings provide molecular evidence that associate the previously unexplained skeletal abnormalities and early malignancy in MEN 2B compared with MEN 2A syndrome.

Biochemical aspects in autism spectrum disorders: updating the opioid-excess theory and presenting new opportunities for biomedical intervention

Autism is a lifelong condition usually described as affecting social, cognitive and imaginative abilities. For many years, parents and some professionals have observed that in concordance with the behavioural and psychological symptoms of the condition, there are a number of physiological and biochemical correlates which may also be of relevance to the syndrome. One area of interest that encompasses many of these observations is the opioid-excess theory of autism. The main premise of this theory is that autism is the result of a metabolic disorder. Peptides with opioid activity derived from dietary sources, in particular foods that contain gluten and casein, pass through an abnormally permeable intestinal membrane and enter the central nervous system (CNS) to exert an effect on neurotransmission, as well as producing other physiologically-based symptoms. Numerous parents and professionals worldwide have found that removal of these exogenously derived compounds through exclusion diets can produce some amelioration in autistic and related behaviours. There is a surprisingly long history of research accompanying these ideas. The aim of this paper is to review the accompanying evidence in support of this theory and present new directions of intervention as a result of it.

Effects of intravenous famotidine on gastric acid secretion in patients undergoing cardiac surgery

OBJECTIVE

To determine the effects of cardiopulmonary bypass and famotidine on gastric acid secretion in adults undergoing cardiac surgery.

DESIGN

Prospective, randomized, double-blind, placebo-controlled study.

SETTING

University teaching hospital.

PARTICIPANTS

Eighteen patients undergoing elective cardiac surgery with cardiopulmonary bypass.

MAIN OUTCOME MEASURES

Famotidine 20 mg or NaCl 0.9% placebo was administered intravenously following induction of anesthesia and placement of a nasogastric pH probe. A second dose was given 12 hours after surgery in the intensive care unit. Gastric pH was measured continuously and gastric volume was measured every 4 hours for up to 24 hours after cardiopulmonary bypass.

RESULTS

Following famotidine administration, pH increased by 43% within 45 minutes and remained above 5.5 throughout the study period (p < 0.05 vs placebo and baseline). The gastric pH did not increase, but remained above 4.0 in most patients in the placebo group for up to 12 hours after cardiopulmonary bypass. Gastric volumes were on average 24% lower in the famotidine group (p > 0.05).

CONCLUSIONS

Gastric acid secretion is decreased during and for 12 hours after cardiopulmonary bypass. Perioperative administration of famotidine suppresses gastric secretion in cardiac surgery patients.

Effect of endogenous opioids on vagally induced release of gastrin, somatostatin and bombesin-like immunoreactivity from the perfused rat stomach

The aim of the present study was to evaluate the effect of the opiate receptor antagonist naloxone on vagally stimulated secretion of bombesin-like immunoreactivity (BLI), somatostatin and gastrin from the isolated rat stomach, which was perfused via the celiac artery with Krebs-Ringer buffer. Vagal stimulation was performed for 10 min with 1 ms, 10 V and 2, 5, 10 or 20 Hz, respectively. In control experiments BLI release increased significantly above basal secretion during a stimulation frequency of 10 Hz (1367 +/- 357 pg/10 min; P < 0.001) and 20 Hz (996 +/- 202 pg/10 min; P < 0.01), but not at 2 and 5 Hz. In comparison to the controls naloxone (10(-6) M) significantly increased BLI secretion at 5 Hz by 573 +/- 150 pg/10 min (P < 0.05), but attenuated the BLI response to higher stimulation frequencies of 10 and 20 Hz to 284 +/- 143 pg/10 min (P < 0.001) and 490 +/- 114 pg/10 min (P < 0.01), respectively. At 2 Hz naloxone had no effect on BLI release. As shown previously the cholinergic blocker atropine (10(-7) M) induced a significant BLI release during vagal stimulation at 2 Hz (680 +/- 233 pg/10 min; P < 0.01) and 5 Hz (935 +/- 324 pg/10 min; P < 0.05), but was without effect at 10 and 20 Hz compared to the controls. The effects of the combination of naloxone and atropine were similar to naloxone and atropine alone. Naloxone had no effect on vagal or GRP-induced regulation of gastrin and somatostatin release.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of acupuncture on gastric acid secretion in healthy male volunteers

Six randomized, placebo controlled studies were performed to investigate the effect of electroacupuncture on gastric acid output in 38 healthy males. Electroacupuncture decreased basal acid output when compared to placebo acupuncture [from 3.50 +/- 0.59 mmol/hr to 2.54 +/- 0.56 mmol/hr (P < 0.05)] as well as sham feeding-stimulated acid output [from 18.52 +/- 2.25 mmol/hr to 5.38 +/- 2.11 mmol/hr (P < 0.005)], but had no effect on the pentagastrin stimulated acid output. The inhibitory effect of acupuncture on sham feeding-stimulated acid output was not affected by local anesthesia of the acupoint, but was prevented by a prior intravenous naloxone injection. Acupuncture did not alter plasma gastrin levels (20.7 +/- 7.6 micrograms/liter, vs control 21.2 +/- 7.2 micrograms/liter) but naloxone increased it (26.1 +/- 14.5 micrograms/liter) (P < 0.05). We conclude that the antisecretory effects of electroacupuncture do not result from decreased gastrin release or decreased parietal cell sensitivity to gastrin, but are mediated through naloxone-sensitive opioid neural pathways and vagal efferent pathways.

Effect of beta-casomorphins on intestinal propulsion in the guinea-pig colon

beta-Casomorphins are a family of opioid peptides originally isolated from beta-casein. In view of a possible physiological significance of these milk-derived compounds, the effects of bovine beta-casomorphin-5 (beta-CM-5), beta-casomorphin-4 (beta-CM-4) and D-Ala2-beta-casomorphin-4-NH2 (D-Ala2-beta-CM-4-NH2) have been investigated on the peristaltic reflex in the guinea-pig isolated colon and compared with morphine. beta-CM-5 and D-Ala2-beta-CM-4-NH2 each dose-dependently inhibited the velocity of propulsion of an intraluminal bolus; beta-CM-4 was ineffective. IC50 values were 0.30, 5.21 and 0.29 microM for morphine, beta-CM-5 and D-Ala2-beta-CM-4-NH2, respectively. The potency ratios vs morphine were 0.06 and 0.96 for beta-CM-5 and D-Ala2-beta-CM-4-NH2, respectively. Blockade of the peristaltic reflex by beta-CM-5 or D-Ala2-beta-CM-4-NH2 was reversed by the opioid antagonist naloxone. D-Ala2-beta-CM-4-NH2 also dose-dependently inhibited resting acetylcholine output (IC50 = 5.69 microM; potency ratio vs morphine: 0.63). In conclusion, certain beta-casomorphins inhibit intestinal propulsion and cholinergic neurotransmission in the guinea-pig colon, probably by acting at opioid receptors.

Endogenous opiates in the mediation of early meal-induced jejunal absorption of water and electrolytes

The purpose of this study was to investigate the role of endogenous opiates in mediating meal-stimulated jejunal absorption. Jejunal Thiry-Vella loops, 25 cm long, were studied in awake conditioned dogs, using luminal perfusion with carbon-14 polyethylene glycol. Fluxes of water, sodium, and chloride were calculated every 15 minutes over a 1-hour basal period, followed by a 3-hour experimental period. The animals were divided into four groups: control, naloxone, meal, and meal plus naloxone. In the control and naloxone groups, the fluxes did not change over the 4-hour observation period. Meal alone immediately stimulated the absorption of water and electrolytes in the Thiry-Vella loop (p less than 0.05). The addition of naloxone infusion to the meal stimulus resulted in significantly reduced absorption during the first hour after the meal (p less than 0.05). We concluded that endogenous opiates play a role in meal-stimulated jejunal absorption.

Thiorphan and acetorphan inhibit gastric secretion by a central, non-opioid mechanism in the rat

Thiorphan and its prodrug, acetorphan, are two inhibitors of enkephalinase (EC 3.4.24.11), a membrane-bound peptidase which plays an important role in the metabolic degradation of enkephalins. Since exogenous opioids have been reported to both stimulate and inhibit gastric secretion, the effects of thiorphan and acetorphan were studied in conscious rats equipped with chronic gastric fistulas. While i.v. thiorphan had no effect, both i.c.v. thiorphan and i.v. acetorphan potently inhibited the basal gastric acid output and the acid output stimulated by pentagastrin. Conversely, neither drug affected the histamine- or methacholine-induced stimulation of acid secretion. Neither thiorphan or acetorphan had any effect on the acid secretion stimulated by a combination of pentagastrin and acetylcholine in vagotomized rats. The results strongly suggest that both drugs inhibit gastric secretion through an effect at the level of the central nervous system, which decreases the vagal drive to the stomach. However, the effects of thiorphan and acetorphan were not prevented by naloxone. This is at variance with most of the effects of these drugs reported to date, including the inhibition of gastric secretion in cats. Furthermore, these effects were observed at doses which could inhibit other enzymes apart from enkephalinase. This suggests that the antisecretory action in rats is related to the protection of some non-opioid peptide(s) from degradation. In conclusion, both peptidase inhibitors inhibit gastric secretion of the rat through a central mechanism involving unknown, non-opioid peptide(s).

Effect of naloxone on vagally-induced gastric acid secretion in rats

Previous studies in man and dogs have demonstrated that endogenous opioids participate in the stimulation of cephalic phase gastric acid secretion during indirect activation of the vagus. Since the effect of naloxone during direct vagal activation is unknown gastric acid secretion was assessed during electrical stimulation of the distal cut ends of both vagal nerves. In overnight fasted anesthetized rats the distal ends of the bisectioned cervical vagi were stimulated with 10V, 5Hz, 5 msec for 15 min. Vagal stimulation elicited an increase of gastric acid secretion by 5.6 mumol/min. Naloxone (1 mumol/kg.h) augmented gastric acid secretion significantly. Since this effect of naloxone was in contrast to previous data possible mechanisms of action of naloxone were examined that might help to explain this apparently inhibitory action of endogenous opioids on vagally-induced gastric acid secretion. The additional infusion of atropine or hexamethonium abolished the stimulatory effect of naloxone on vagally-induced acid secretion completely indicating that the action of naloxone depends on cholinergic background activity. Combined blockade of alpha- and beta-adrenergic receptors with phetolamine and propanolol reduced vagally-induced acid secretion in controls and during naloxone to a similar degree. Both adrenergic blocking agents also reduced the residual acid secretion observed during atropine or atropine + naloxone infusion. Measurements of plasma gastrin levels suggested that the naloxone-induced changes of acid secretion were not due to alterations of gastrin secretion. In summary these data demonstrate that vagally-induced acid secretion in anesthetized rats is largely due to cholinergic mechanisms with a small but separate contribution of adrenergic mechanisms. Endogenous opioids are activated during peripheral vagal stimulation attenuating vagally-induced acid secretion by modulation of cholinergic but not adrenergic mechanisms.

Interaction between endogenous opioids, cholinergic and adrenergic mechanisms during vagally-induced gastrin release in rats

Endogenous opioids are present in neurons of the vagus and the intrinsic nervous system and they are colocalized with gastrin in antral G-cells. This raises the possibility that endogenous opioids modulate gastrin release. Stimulation of both cervical vagi (10V, 5Hz, 5ms) elicited an increase of arterial plasma gastrin levels at intragastric pH7 or pH2. The response at pH2 was 30% of that at luminal pH7. Atropine reduced vagally stimulated gastrin levels substantially. At luminal pH2 the small residual noncholinergic response was mediated neither by adrenergic mechanisms nor by endogenous opioids. At luminal pH 7 adrenergic blockade with phentolamine and propranolol reduced vagally stimulated gastrin by 60%. In the presence of atropine adrenergic blockade elicited only a small inhibitory effect suggesting that vagal activation of adrenergic mechanisms depends on atropine-sensitive cholinergic pathways. Blockade of opiate receptors by naloxone had no effect on vagal gastrin release, however, the noncholinergic gastrin response was reduced significantly by naloxone, suggesting that cholinergic mechanisms normally restrain activation of endogenous opioids during vagal stimulation. Naloxone had no effect on the noncholinergic, nonadrenergic stimulation of gastrin levels. These data suggest that endogenous opioids can contribute to vagal gastrin release provided the cholinergic restraint is blocked and adrenergic mechanisms stimulate endogenous opioids. In conclusion a major role of endogenous opioids in the regulation of vagal gastrin release can not be detected.

Acetorphan, an enkephalinase inhibitor, decreases gastric secretion in cats

Acetorphan is an inhibitor of "enkephalinase" (EC 3.4.24.11) which has been shown to reduce in vivo and in vitro the degradation of enkephalins and other peptides. The effects of acetorphan on gastric secretion were studied in cats fitted with gastric fistulae and Heidenhain pouches. Acetorphan inhibited by 40-60% the acid secretion from the gastric fistulae after stimulation by submaximal doses of pentagastrin, histamine and 2 deoxy-D-glucose. These inhibitions were reduced or suppressed by naloxone. The meal-stimulated secretion from the fistulae was not changed after acetorphan. Acetorphan slightly and progressively reduced the pentagastrin-stimulated acid output from the Heidenhain pouches, and this effect was naloxone resistant. No change was found in the secretion from Heidenhain pouches under histamine stimulation, while meal-induced secretion of the pouches was increased by acetorphan, and this increase was not prevented by naloxone. Endogenous opioids probably exert an inhibitory regulatory control upon the gastric secretion of cats. In addition, non-opioid factors may be involved in the effect of acetorphan on meal-stimulated secretion.

Do enkephalins participate in vagal activation of gastric acid secretion in man?

The effects of the anticholinergic drug benzilonium bromide and the opiate receptor blocker naloxone, given alone or in combination, on the acid secretory response and on plasma gastrin releasing peptide (GRP) response to sham feeding was tested in eight duodenal ulcer (DU) patients. Naloxone alone had no effect on the acid secretion after sham feeding. Benzilonium reduced basal acid secretion and the acid response to sham feeding but did not abolish the response. The combination of benzilonium and naloxone was not more effective than benzilonium alone. Neither drug, nor the combination had any effect on plasma GRP following sham feeding. It is concluded that enkephalins are unlikely to participate in the acid response to sham feeding in patients with DU.

Opioids and opioid receptors in peripheral tissues

Opioid peptides belonging to the enkephalin, beta-endorphin or dynorphin family, acting on specific opiate receptors may be found in peripheral tissues. Enkephalins have a widespread peripheral distribution, while beta-endorphin and dynorphin may be found locally in the enteric nervous system. The peptides of the various families are formed from specific precursor molecules. Apart from the enteric nervous system, opioids are also found in the adrenal medulla as well as in several autonomic ganglia. There is some evidence of three different classes of opioid receptors in peripheral tissues, i.e. mu-, delta- and kappa-receptors. These receptors are not only found on enteric nervous and mucosa cells but also on various cells in the immune system where opioid peptides seem to have important actions and appear to link the neuroendocrine and immune systems to control immunological functions. The physiological as well as the pathophysiological role of opioid peptides in the periphery is gradually being elucidated and, based on such knowledge, new therapeutic implications in gastrointestinal or immune diseases may be developed.

Autoradiographic localization of mu- and delta-type opioid receptors in the gastrointestinal tract of the rat and guinea pig

The distribution of delta- and mu-type opioid binding sites in the gastrointestinal tract of the rat and guinea pig was studied by autoradiography after in vitro incubation of tissue slices with 3H-D-Ala2,D-Leu5-enkephalin, and 3H-naloxone or 3H-dihydromorphine to locate delta- and mu-type opioid receptors, respectively. In the gastric fundus, both mu- and delta-type binding sites were found to occur associated with the circular muscle, muscularis mucosae, and submucosal plexus, whereas in the corpus and antrum, binding was located primarily in the submucosal plexus, deep muscular plexus, and mucosa. Some mu-type opioid receptor sites were present in the myenteric plexus. A dense distribution of both mu- and delta-type binding sites was observed throughout the mucosa of the duodenum and ileum of the rat. In guinea pig ileal tissue, however, only mu-type binding could be demonstrated, occurring in the submucosal plexus and diffusely over the muscle layers. Endogenous opioid peptides, acting at these receptors sites, might be involved in the control of gastrointestinal motility, endocrine and exocrine secretions, as well as intestinal fluid and electrolyte transport.

Release of [Leu5]enkephalin immunoreactivity from the isolated perfused rat stomach

The release of [Leu5]enkephalin immunoreactivity ([Leu5]enk-IR) from the isolated perfused rat stomach was demonstrated under basal conditions in the presence of peptidase inhibitors (0.1 microM thiorphan, 1 microM captopril and 2 microM bestatin). Depolarization with 50 mM KCl resulted in a four-fold increase in both [Leu5]enk-IR and gastrin (IR-gastrin) levels. Administration of the nicotinic agonist, 1,1-dimethyl-4-phenyl-piperazinium (DMPP) (10 microM) stimulated the release of [Leu5]enk-IR in a calcium-dependent manner. The muscarinic acetylcholine receptor agonist methacholine (10 microM) had no effect on [Leu5]enk-IR release.

Influence of opiates on alkali secretion by amphibian gastric and duodenal mucosa in vitro

Experiments were performed to study the effects of opiates on gastric and duodenal alkali secretion in amphibian mucosa in vitro. Alkali secretion by fundic mucosa of Rana temporaria, or antral mucosa of Rana catesbeiana, was unaffected by morphine, methionine-enkephalin and leucine-enkephalin, two enkephalin analogues, or the opiate antagonist naloxone. Acid secretion by fundic mucosa in vitro was not influenced by 10(-5) and 10(-4) M morphine, or by 10(-7) to 10(-5) M naloxone. However, duodenal alkali secretion in Rana catesbeiana was stimulated by opiates while the electrical potential difference was unaffected. Morphine stimulated secretion, maximally at 10(-5) M, by 33% over basal values, 30 min after exposure to the drug, whereas the maximal response to methionine-enkephalin occurred at 10(-6) M and was obvious within the first 15 min after administration. The effects of these opiates were prevented by pretreatment with naloxone or with the more specific opiate receptor antagonist ICI 154129. The response to morphine was inhibited when bicarbonate in the nutrient-side solution was replaced by the impermeant anion HEPES or by removal of chloride from the bathing media. Furosemide (10(-3) M) also inhibited the response of duodenal mucosa to morphine. The nerve-blocker tetrodotoxin (10(-7) M) prevented the morphine-induced response. These data suggest that opiates can stimulate duodenal alkali secretion, probably by activating an electrically neutral Cl-/HCO3- exchange. It seems likely that the effect of opiates is mediated by a neurologic intermediary, and the results suggest the possibility that duodenal alkali secretion may be under some neurologic control.

Central and peripheral involvement of mu receptors in gastric secretory effects of opioids in the dog

The effects of dermorphin and morphine on gastric acid secretion were studied in conscious dogs with both gastric fistulas (GF) and Heidenhain pouches (HP). Under basal conditions dermorphin and morphine, infused systemically at graded doses, produced a significant increase in acid secretion from both GF and HP. This increase was significantly inhibited by naloxone, naltrexone methylbromide and N-methyl-levallorphan methanesulphonate. Dermorphin did not modify the acid output stimulated by 2-deoxy-D-glucose from GF, while morphine significantly inhibited it; on the contrary acid secretion from HP was increased in this test by both dermorphin and morphine. Acid secretion from GF stimulated by pentagastrin was unaffected by morphine and significantly enhanced by dermorphin. Under these conditions a significant increase in acid secretion from HP was recorded with dermorphin and morphine. Naloxone and N-methyl-levallorphan methanesulphonate, given during pentagastrin-stimulated secretion, significantly inhibited acid output 'per se' from GF and HP and prevented the stimulatory effect of dermorphin and morphine. Bethanechol-induced secretion from GF and HP was significantly increased by both dermorphin and morphine. The present results demonstrate that opioids have simultaneous yet opposite effects on acid secretion in the dog and that mu receptors are involved in both the excitatory and inhibitory effects. Excitatory effects do not seem to be mediated via a vagal pathway (peripheral ?), in contrast to the inhibitory effects (central ?). The inhibitory effects of opiate antagonists on pentagastrin-stimulated secretion suggest a physiological role of peripheral opioid receptors in gastric acid secretion.

Effect of oral nalmefene, an opiate-receptor antagonist, on meal-stimulated gastric acid secretion and serum gastrin concentration in man

We evaluated whether nalmefene, an orally administered opiate-receptor antagonist, would inhibit gastric acid secretion in response to a meal in healthy humans. On separate days either 50 mg nalmefene or a placebo tablet was administered by mouth 90 min before a blenderized steak meal was infused into the stomach through a nasogastric tube. Compared to placebo, nalmefene inhibited meal-stimulated acid secretion in each of 6 subjects studied (P less than 0.05). During the second and third hours after the meal, nalmefene inhibited mean acid secretion by 16%. Nalmefene also resulted in significantly higher meal-stimulated serum gastrin concentrations than placebo (P less than 0.05) even though intragastric pH was kept constant at 5.0 in both experiments. These studies indicate that an orally administered opiate-receptor antagonist can inhibit gastric acid secretion in response to a meal in humans, yet increase meal-stimulated serum gastrin concentrations.