Radioimmunoassayable plasma motilin in man. Secretagogues, insulin-induced suppression, renal removal, and plasma components

In normal humans, significant motilin increases were found after meal ingestion, intraduodenal infusion of fat, and intraduodenal infusion of physiological HCl doses. Only a non-significant plasma motilin increase was found in response to intraduodenal infusion of cattle bile. Plasma motilin decreased significantly after an intravenous insulin injection. During routine cardiac catheterization in a group of 10 patients plasma motilin was significantly lower in the renal vein than in the femoral vein, femoral artery, right atrium, and hepatic vein, suggesting that the kidneys participate in the removal of motilin from the circulation. Fasting and oral fat-stimulated plasma motilin immunoreactivity eluted in two peaks on a Sephadex G-50 Fine column. The two peaks behaved identically with porcine motilin in dilution series.     

Effect of vagal blockade on food- and hormone-stimulated release of pancreatic polypeptide and motilin in dogs

Vagal control of food- and hormone-stimulated release of pancreatic polypeptide (PP) and motilin was investigated in four conscious dogs by examining the effect of cryogenic vagosympathetic blockade. The postprandial PP response of 189±7 pM was totally, although reversibly, inhibited to 58±11 pM with the vagi blocked. Similarly, bombesin-, CCK-OP-, or neurotensin-stimulated PP release was abolished. Although the PP response to intraduodenal perfusion of an elemental diet was also reduced by blockade, the 52±15% inhibition was less than observed with the meal. In contrast to PP, plasma motilin fell after the meal from a fasting level of 128±16 pM to a nadir of 52±7 pM. Vagal blockade reversed this decline as plasma motilin rose to a peak of 121±18 pM with a pattern resembling the motilin response in the interdigestive state. This motilin increment during blockade was inhibited by atropine and by infusion of porcine PP. Plasma motilin also fell with the elemental diet, but this response was not affected by blockade. During infusion of bombesin, plasma motilin rose by 60±9 pM; vagal blockade augmented this increment twofold. Thus, the PP response to a meal and to hormonal stimulation is controlled by a vagal cholinergic excitatory pathway. However, intestinal release of PP is mediated in part by the vagus and in part by a vagally independent mechanism which may be neural or hormonal. Alternatively, vagal noncholinergic inhibition is a major mechanism modulating the motilin response after oral food but motilin release exclusively from intestinal nutriments is mediated by nonvagal, noncholinergic mechanisms. That vagal blockade enhances bombesin-stimulated motilin release further supports control by a vagal inhibitory pathway. Since atropine inhibited the blockade-mediated rise of motilin after a meal, nonvagal cholinergic pathways also play a role in the regulation of motilin but may predominate only in the interdigestive state.Supported by a grant from the Medical Research Council of Canada (MA 6763). Dr. Greenberg is a recipient of a Medical Research Council of Canada Scholarship.This work was presented at the Annual Meeting of the American Gastroenterological Association in New Orleans, May 21–23, 1984, and a preliminary report was published in abstract form (Gastroenterology 86:1098, 1984).     

Evidence of somatostatin as a humoral modulator of motilin release in man. A study of plasma motilin and somatostatin during intravenous infusion of somatostatin, secretin, cholecystokinin, and gastric inhibitory polypeptide

In six healthy persons receiving graded intravenous infusions of synthetic somatostatin the plasma motilin concentrations decreased significantly (p less than 0.05) already with doses giving physiological plasma somatostatin levels, and a rebound of plasma motilin was observed after cessation of infusion of pharmacological somatostatin doses. After an intravenous secretin infusion (280 pmol/kg-h) producing pharmacological plasma secretin concentrations, a comparable plasma somatostatin increase was observed together with a substantial decrease in plasma motilin (p less than 0.05). Infusion of cholecystokinin in a pharmacological dose and of gastric inhibitory polypeptide (GIP) in doses giving plasma GIP levels in the physiological range had no effect on plasma somatostatin or motilin. Circulating plasma somatostatin may be a physiological modulator of the motilin release, and the plasma motilin fall seen during infusion of pharmacological doses of secretin may possibly be explained by the secretin-induced somatostatin release occurring simultaneously.     

Motilin release in the pig

Motilin is found in the upper intestine of pig and man and in man is released by acid. A similar release by acid has now been found in the pig and is associated with markedly reduced immunostainable motilin in the upper small intestine. Clamping the arteries and veins to the stimulated segment immediately reversed the rise of plasma motilin, indicating the motilin release to be an entirely local phenomenon. The apparent half life of endogenous motilin was 3.9 minutes. No release of motilin was seen after a meal and the possible physiological role of motilin thus remains speculative.     

Changes in plasma motilin concentration and gastrointestinal contractile activity in conscious dogs

Simultaneous measurement of plasma motilin concentration and gastrointestinal contractile activity was made in 12 healthy dogs. Plasma motilin concentration was measured by radioimmunoassay and gastrointestinal contractile activity was monitored by means of chronically implanted force transducers. During the interdigestive state, it was found that the plasma motilin concentration increased in complete accordance with the cyclical interdigestive contractions of the stomach. Furthermore, plasma motilin concentration was lowered by the ingestion of food, and it remained low as long as the gastric motor activity was in the digestive pattern. Since, as reported previously, the interdigestive contractions can be induced by the exogenous administration of motilin, we concluded that (1) motilin is released at constant intervals during the interdigestive state, and this release is suppressed by the ingestion of food; (2) motilin induces the interdigestive contractions of the stomach and duodenum; and (3) motilin is an interdigestive hormone and is the only known polypeptide hormone of the gut whose release is not induced by a meal.Parts of this work were presented at the International Symposium on GI Hormones and Pathology of the Digestive Systems in June 18–21, 1977, in Rome and at Round Table of the XXVII International Congress of Physiological Sciences in July 18–23, 1977, in Paris.     

Motilin in plasma and tumor tissues from patients with the carcinoid syndrome. Possible involvement in the increased frequency of bowel movements

Motilin, normally present in a specific cell type in the upper small intestine, is believed to have a physiologic role in initiating the interdigestive migrating motor complex. Motilin may play a pathophysiologic role in the diarrhea in the irritable bowel syndrome, the dumping syndrome, chronic liver disease, and chronic renal failure. Furthermore, increased frequency of bowel movements is an important symptom in patients with the carcinoid syndrome. We have studied 73 patients with metastatic carcinoid tumors with regard to stool frequency and plasma concentration of motilin and neuropeptide K (NPK) and diurnal urinary excretion of 5-hydroxyindoleacetic acid (5-HIAA). Thirty-eight (52%) of the 73 patients had elevated (greater than 126 pmol/l) plasma concentrations of motilin, whereas 59 (81%) of the patients had diarrhea. The increased frequency of bowel motions correlated significantly (p less than 0.01) with the plasma concentrations of motilin, whereas no significant correlation with 5-HIAA and NPK was found. High-performance liquid chromatography of plasma extracts showed a single component eluting in the position of synthetic porcine motilin. However, extracts from five carcinoid tumors did not contain any significant levels of motilin. Carcinoid tumors are known to contain and secrete several biologically active substances such as serotonin, histamine, prostaglandins, and tachykinins, which are likely to cause disturbances of intestinal secretion and motility, which in turn might release motilin from the motilin-containing cells of the small intestine. The increased motilin levels might then participate in a vicious diarrhea circle together with the other agents.     

Radioimmunoassay of motilin

A radioimmunoassay method of motilin was developed in our laboratory and was validated in dogs with a platinum monopolar electrode in the duodenum. We confirmed that a bolus infusion of 0.3 M tris-buffer solution or 0.1 N HCl solution in the duodenum produces a significant rise in plasma immunoreactive motilin (IRM) concentrations. This coincided with a marked increase in the percentage of spike potentials on slow waves of the duodenum, similar to phase III of interdigestive myoelectric-activity (MA). A possible relationship between plasma IRM and interdigestive MA of canine duodenum was studied. It was found that cyclic changes occurred in the fasting plasma IRM concentrations in dogs. While the peak motilin concentration was always observed in phase III, the lowest concentration of motilin was found in phase I of interdigestive MA in the duodenum. In dogs with the electrodes in the duodenum and jejunum, the peak IRM concentration did not correlate with phase III of interdigestive MA in the jejunum. A dose of synthetic porcine motilin, 0.06 g/kg/hr, which produced the plasma IRM concentration comparable to the peak fasting motilin concentration, could induce an identical phase III in the duodenum. These observations indicate that there is a relationship between cyclic changes in plasma IRM concentrations and interdigestive MA of the duodenum. It is suggested further that motilin is a hormone which may play an important role in inducing phase III of interdigestive MA in the duodenum.This work was supported by the Gastrointestinal Research Fund at The Genesee Hospital.     

Effect of bombesin on glucose-induced insulin release in humans

The effect of bombesin on basal and glucose-stimulated insulin release was studied in male healthy volunteers. Glucose was administered by oral, intravenous or intraduodenal route during saline or bombesin infusion (5 ng/kg/min for 60 min). The peptide had no significant effect on basal levels of glucose and insulin. However, during its administration, the insulin response and the expected rise in blood glucose after oral glucose load (50 g) were strongly inhibited, and the gastric emptying of liquids was significantly delayed. On the contrary, the insulin response to intravenous glucose (20 g) was significantly increased by bombesin without changes in plasma glucose levels. Finally, when glucose was infused into the duodenum, thus bypassing the stomach, the insulin response was significantly increased by the peptide. In this case, too, plasma glucose levels after glucose load were virtually identical during either bombesin or saline infusion. These data clearly demonstrate that the direct effect of bombesin on insulin release is stimulatory and suggest that the inhibitory effect observed after oral glucose is connected with the action of the peptide on gastric emptying, the delay of which slows the entry of glucose into the small bowel.     

Disturbed Motilin and Cholecystokinin Release in the Irritable Bowel Syndrome

Background: The irritable bowel syndrome (IBS) is associated with motor abnormalities in the small intestine and colon. Neuropeptides may have an important role in initiating and regulating the intestinal motility. Motilin has been proposed to initiate the peristaltic reflex in the small intestine and cholecystokinin the gastrocolic reflex. Methods: In 18 patients with IBS and 11 healthy control subjects plasma motilin and cholecystokinin (CCK) concentrations were measured after intraluminal stimulation of water and a fat-rich meal. Results: The IBS patients had reduced motilin secretion after both water intake and the fat meal. In contrast, the fat meal elicited an exaggerated and prolonged CCK release in the IBS patients. Conclusions: Disturbed motilin and CCK release may partly be responsible for the intestinal dysmotility in the IBS patients.     

The localization of the site(s) of neurotensin release in man and dog

The major source of neurotensin in the gut is the ‘N’ cell and this is found in the highest density in the ileum. The ingestion of food, particularly fat, causes the biphasic release of neurotensin-like immunoreactivity (NTLI) into the circulation. The early peak occurs sooner than expected if it is due to the presence of chyme in the ileum, suggesting that the early release of neurotensin is due to a more proximal concentration of N cells or that neurotensin is released from the ileum by a more proximal stimulus. This paper investigates the site(s) of neurotensin release by studying: three groups of patients with various resections of the small intestine and the fashioning of duodenostomies, jejunostomies and ileostomies; and the dog with chronic gastric, duodenal and ileal fistulae, and following ileal resection. The results indicate that the jejunum and ileum are critical in the release of neurotensin following fat stimulation and that the stomach and duodenum have no direct role. The stimulation of the jejunum by fat causes the early rise of plasma NTLI by releasing neurotensin from the ileum. If chyme is prevented from passing to the distal jejunum the magnitude of the early peak is diminished and the second, later, peak is abolished. The second peak of plasma NTLI is due to the direct luminal stimulation of the distal jejunum and ileum by the products of fat digestion. Ileal resection completely abolishes any release of plasma NTLI in response to fat. It was concluded that the source of neurotensin released by the ingestion of food is the ileum. The release of neurotensin from the distal gut is dependent upon a signal from the proximal to the distal gut. The identity of the signal is unknown, but it is either neural or humoral and previous studies suggest a cholinergic-dependent reflex.     

Plasma motilin concentration and interdigestive migrating motor complex in diabetic gastroparesis: effect of metoclopramide

The objective of this study is to determine whether abnormal rhythm and amplitude of the oscillation in plasma motilin concentration are responsible for the abnormal motility observed in patients with diabetic gastroparesis. We also investigated the effect of metoclopramide on plasma motilin concentration and gastrointestinal motility in these patients. In healthy controls, basal plasma motilin concentration fluctuated in phase with the interdigestive migrating motor complex. All patients with diabetic gastroparesis did not have antral phase III activities and had significantly higher basal motilin concentrations, which maintained a normal oscillatory pattern. Administration of metoclopramide initiated antral phase III activities in healthy controls and in patients with diabetic gastroparesis. These were accompanied by a concurrent rise in plasma motilin concentration in healthy controls, contrasting with a fall in plasma motilin concentrations in patients with diabetic gastroparesis. We conclude that diabetic gastroparesis is associated with absence of antral interdigestive migrating motor complex and with elevated plasma motilin concentration with normal cyclic fluctuations. Our studies also show that metoclopramide initiates antral phase III activities in diabetic gastroparesis independent of plasma motilin concentration.     

Effects of corticotropin-releasing factor on plasma motilin and somatostatin levels and gastrointestinal motility in dogs

The effects of intracerebroventricular and intravenous administration of corticotropin-releasing factor (CRF) on gastrointestinal motility, motilin-induced gastric motor response, and plasma motilin and somatostatin levels were investigated in fasted dogs chronically prepared with strain gauge transducers on the antrum and proximal jejunum. Administered intracerebroventricularly at doses of 20 and 100 ng/kg in fasted dogs, CRF suppressed for 4-5 h the gastric cyclic migrating motor complex. A similar dose (100 ng/kg) administered intravenously was inactive. Corticotropin-releasing factor administration by the intravenous route at 100 ng/kg did not alter the cyclic plasma motilin and somatostatin variations associated with the cyclic gastric motor events. During the blockade of antral migrating motor complex induced by intracerebroventricular administration of CRF, cyclic peaks of plasma motilin were absent whereas those of somatostatin persisted. The gastrointestinal migrating motor complex induced by the intravenous administration of porcine motilin (0.25 microgram/kg) was abolished when motilin was injected 2 h after the intravenous administration of CRF (100 ng/kg), whereas a similar dose of CRF administered intracerebroventricularly did not abolish the antral and jejunal motor responses to motilin. It is concluded that in fasted dogs, CRF administered centrally affects the interdigestive gastric motility and the release of motilin but not that of somatostatin. These results also suggest that the intracerebroventricular CRF-induced blockade of motilin release is responsible for the inhibition of gastric migrating motor complex and circulating CRF is able to affect the gastric motor response to porcine motilin through a peripheral mechanism that does not involve somatostatin and motilin secretion.     

Isoproterenol induces activity fronts in fed dogs through somatostatin release

We confirmed previous studies by others that intravenous somatostatin infusions (1.4 and 2.5 micrograms/kg X h) induced jejunal activity fronts in fed dogs. This finding, coupled with earlier demonstrations that somatostatin is released from the stomach and pancreas in vitro by isoproterenol, led us to investigate the possible role of somatostatin in mediating isoproterenol-induced activity fronts. Plasma somatostatin and motilin concentrations were determined after a meat-base meal, before and after isoproterenol infusions (0.5 micrograms/kg X min). Jejunal activity fronts were initiated in 15 of 17 experiments an average of 25 min after the start of the isoproterenol infusion. The concentration of motilin did not change, but somatostatin concentrations increased progressively beginning 15 min after starting the isoproterenol infusion. Thus, isoproterenol released somatostatin in vivo and this release was associated with jejunal activity fronts. The rise in plasma somatostatin concentration was similar to that obtained after somatostatin infusions, although in the latter experiments the activity fronts began soon after initiation of the somatostatin infusion. The results suggest that isoproterenol-induced activity fronts are mediated by somatostatin. We speculate that beta-adrenergic nerves may induce activity fronts through somatostatin release, possibly in situations such as extreme stress.     

Effect of motilin on the lower oesophageal sphincter.

The effect of motilin on lower oesophageal sphincter (LES) pressure has been studied in unanesthetised specially trained dogs using an infusion manometric technique. Motilin produced significant rises in resting pressure and contractions of the LES after doses ranging from 0-009 mug/kg to 0-05 mug/kg. Doses greater than 0-05 mug/kg resulted in repetitive high amplitude contractions. Atropine 30 mug/kg completely abolished the effect of the lower doses of motilin. Higher doses of motilin in atropinised dogs still caused a small rise in baseline pressure and contractile activity still appeared. Hexamethonium 2 mg/kg resulted in both a diminished rise in LES pressure and the disappearance of contractions after motilin. Hexamethonium and atropine together completely abolished the LES response to motilin. We conclude that motilin increases LES pressure by acting on preganglionic cholinergic neurones to release acetylcholine which excites other cholinergic neurones supplying the circular muscle of the LES.     

CIRCULATING SOMATOSTATIN AFTER FOOD AND GLUCOSE IN MAN

Using a recently validated radioimmunoassay, changes in circulating somatostatin have been measured in normal subjects after food (a standard breakfast), and oral and intravenous glucose. After the standard breakfast, a clear and sustained rise in plasma somatostatin was seen in all subjects from a mean value (+/-1 SE) of 28 +/- 7 pg/ml to a mean peak value, at 60 min of 57 +/- 11 pg/ml. When glucose was taken by mouth a significant but smaller rise was seen, but intravenous glucose caused no significant change in plasma somatostatin. A rise in circulating somatostatin after feeding has not previously been demonstrated in normal man and it is suggested that somatostatin may have an important endocrine role in the gut.     

Development of Neutrophilia by Serially Increasing Doses of Dexamethasone

Dexamethasone (4-8 mg/m² body surface area) was given orally or intravenously to six normal volunteers. The maximum neutrophil count occurred 4-6 h after oral or intravenous administration of dexamethasone and was due almost entirely to an increase in mature neutrophils; concomitantly there was a lymphocytopenia. A second rise in the neutrophil count occurred 24 h after oral ingestion of dexamethasone, coinciding with a lymphocytosis. Neutrophil alkaline phosphatase (NAP) activity during development fell as the neutrophil count rose. Other haematological values were unchanged except for small increments in erythrocyte sedimentation rate (ESR). Sodium concentration in serum and urine remained normal but urinary potassium excretion and urine volume increased after the intravenous dose. There was a direct relationship between plasma concentration of dexamethasone and the rise in neutrophil count following intravenous but not oral administration. The concentration of dexamethasone in plasma fell to half its peak value in 2-6 h. Dexamethasone-induced neutrophilia was similar to that induced by other corticosteroids. Dexamethasone in a dose of 6 mg/m² produced minimal discomfort while inducing an adequate neutrophilia in the volunteers.     

Vagal stimulation exaggerates the inhibitory ghrelin response to oral fat in humans

Ghrelin, the growth hormone secretagogue receptor ligand, is a key regulator of adiposity and food intake. However, the regulation of ghrelin in response to dietary fat intake remains largely unclear. Furthermore, cephalic elevation of ghrelin may influence fat absorption and postprandial lipaemia. Therefore, the aim of this study was to examine the effect of fat ingestion and vagal stimulation on the regulation of plasma ghrelin.Vagal stimulation was achieved by modified sham feeding (MSF). Eight healthy subjects (four male/four female) consumed a 50 g fat load on two separate occasions. On one occasion, the fat load was preceded by the MSF of a meal for 1 h. Blood, appetite and breath were analysed for 5 h postprandially.A 25% (S.E.M. 3.4) suppression in ghrelin concentration was observed after fat ingestion (P<0.001), without an increase in glucose or insulin. MSF in addition to oral fat enhanced ghrelin suppression further, as well as elevating plasma triacylglycerol (P<0.001) and reducing appetite (P<0.001). The fasting ghrelin concentration was inversely correlated with gastric half-emptying time (P=0.036).We conclude that ghrelin release may be influenced directly by both vagal stimulation and oral fat ingestion.     

Substrate,hormone, and temperature responses in males and females to a common breakfast

To evaluate the response to a mixed meal we studied oral temperature, metabolite, and hormonal responses to a common American breakfast containing 11 kcal/kg body weight (carbohydrate 43%, fat 42%, and protein 15%) in 12 normal volunteers (6 males and 6 females). There was a significant rise in oral temperature during the postcibal period. This change in oral temperature did not depend upon food consumption in males but was meal-dependent in females. Food ingestion caused increases in the peripheral circulating concentrations of glucose, lactate, pyruvate, and amino acids and reciprocal decreases in the concentrations of free fatty acids, glycerol, and urea nitrogen. Acetoacetate and beta-hydroxybutyrate decreased during the postcibal period but the changes were not statistically significant. Although peripheral venous serum insulin and plasma glucagon concentrations were indistinguishable between the sexes, males had higher concentrations of plasma triglycerides, plasma amino acids, and serum urea nitrogen. Peripheral venous plasma somatostatin and secretin remained unchanged, but pancreatic polypeptide hormone showed a large biphasic response to the meal. After breakfast the blood glucose concentration tended to be greater in males than in females and this difference was significant at 60 and 120 min postcibal. Furthermore, every female had a 120 min postcibal glucose concentration that was lower than her basal fasting glucose concentration. This suggests that postcibal glucose concentrations should be related to gender in making the diagnosis of carbohydrate intolerance or reactive hypoglycemia.     

Failure of oral water loading and intravenous hypotonic saline to suppress plasma prolactin in man.

To test the effect of acute changes in plasma osmolality on plasma prolactin concentrations, the hormone was measured before and during oral water loading, hypotonic saline infusion, and hypertonic saline infusion in normal subjects. In 10 normal men there was a small but statistically significant rise in mean prolactin, from 7.6 to 12.3 ng/ml, occurring within half an hour after the ingestion of a water load of 20 ml/kg. A small but statistically insignificant rise in mean plasma prolactin was observed in 11 normal women, although there was a significant negative correlation between plasma prolactin and plasma osmolality in these subjects. No effect of the intravenous infusion of either hypotonic saline (0.45%) or hypertonic saline (5%) on mean plasma prolactin was noted in 5 normal men. These studies do not confirm a previously reported suppression of prolactin concentrations by oral water loading of hypotonic saline infusions in normal subjects. While the data suggest a negative correlation between plasma osmolality and plasma prolactin, at least after water loading, they do not support a physiological role for prolactin in the short-term regulation of plasma osmolality in humans.     

LACK OF EFFECT OF ETHANOL ON PLASMA CORTISOL IN MAN

The effects of acute administration of ethanol were studied in a total of 18 normal male subjects. Sixteen men were given oral ethanol (2·5 ml/kg vodka or gin) and four received intravenous infusions of ethanol (1 ml/kg), at two times of day, 0900 h and 1800 h. Neither intravenous nor oral ethanol caused a specific rise in plasma cortisol, in contrast to previously reported work. There was no evidence for circadian variation in response. Since ethanol does not release plasma cortisol in normal individuals, the pathogenesis of ‘alcohol-induced pseudo-Cushing's syndrome’ should be re-considered.