Effect of ingested carbohydrate, fat, and protein on the release of somatostatin-28 in humans

Gap junction coupling and islet delta-cell function in health and disease

The pancreatic islets contain beta-cells and alpha-cells, which are responsible for secreting two principal gluco-regulatory hormones; insulin and glucagon, respectively. However, they also contain delta-cells, a relatively sparse cell type that secretes somatostatin (SST). These cells have a complex morphology allowing them to establish an extensive communication network throughout the islet, despite their scarcity. Delta-cells are electrically excitable cells, and SST secretion is released in a glucose- and K_ATP-dependent manner. SST hyperpolarises the alpha-cell membrane and suppresses exocytosis. In this way, islet SST potently inhibits glucagon release. Recent studies investigating the activity of delta-cells have revealed they are electrically coupled to beta-cells via gap junctions, suggesting the delta-cell is more than just a paracrine inhibitor. In this Review, we summarize delta-cell morphology, function, and the role of SST signalling for regulating islet hormonal output. A distinguishing feature of this Review is that we attempt to use the discovery of this gap junction pathway, together with what is already known about delta-cells, to reframe the role of these cells in both health and disease. In particular, we argue that the discovery of gap junction communication between delta-cells and beta-cells provides new insights into the contribution of delta-cells to the islet hormonal defects observed in both type 1 and type 2 diabetes. This reappraisal of the delta-cell is important as it may offer novel insights into how the physiology of this cell can be utilised to restore islet function in diabetes.

Why fat is so preferable: from oral fat detection to inducing reward in the brain

Potential mechanisms underlying the high palatability of fat can be assessed by reviewing animal studies on fat detection and brain patterns during reward behavior. Fatty acids are likely recognized by receptors on taste buds, with the signals transmitted to the brain through taste nerves. Ingested oil is broken down and absorbed in the gastrointestinal tract, which also sends signals to the brain through unknown mechanisms. Information from both sensory receptors and peripheral tissue is integrated by the brain, resulting in a strong appetite for fatty foods via a reward system. Understanding mechanisms of fat recognition will prove valuable in the development of strategies to manage the high palatability of foods.

Postprandial inhibition of gastric ghrelin secretion by long-chain fatty acid through GPR120 in isolated gastric ghrelin cells and mice

Ghrelin is a gastric peptide hormone that controls appetite and energy homeostasis. Plasma ghrelin levels rise before a meal and fall quickly thereafter. Elucidation of the regulation of ghrelin secretion has been hampered by the difficulty of directly interrogating ghrelin cells diffusely scattered within the complex gastric mucosa. Therefore, we generated transgenic mice with ghrelin cell expression of green fluorescent protein (GFP) to enable characterization of ghrelin secretion in a pure population of isolated gastric ghrelin-expressing GFP (Ghr-GFP) cells. Using quantitative RT-PCR and immunofluorescence staining, we detected a high level of expression of the long-chain fatty acid (LCFA) receptor GPR120, while the other LCFA receptor, GPR40, was undetectable. In short-term-cultured pure Ghr-GFP cells, the LCFAs docosadienoic acid, linolenic acid, and palmitoleic acid significantly suppressed ghrelin secretion. The physiological mechanism of LCFA inhibition on ghrelin secretion was studied in mice. Serum ghrelin levels were transiently suppressed after gastric gavage of LCFA-rich lipid in mice with pylorus ligation, indicating that the ghrelin cell may directly sense increased gastric LCFA derived from ingested intraluminal lipids. Meal-induced increase in gastric mucosal LCFA was assessed by measuring the transcripts of markers for tissue uptake of LCFA, lipoprotein lipase (LPL), fatty acid translocase (CD36), glycosylphosphatidylinositol-anchored HDL-binding protein 1, and nuclear fatty acid receptor peroxisome proliferator-activated receptor-γ. Quantitative RT-PCR studies indicate significantly increased mRNA levels of lipoprotein lipase, glycosylphosphatidylinositol-anchored HDL-binding protein 1, and peroxisome proliferator-activated receptor-γ in postprandial gastric mucosa. These results suggest that meal-related increases in gastric mucosal LCFA interact with GPR120 on ghrelin cells to inhibit ghrelin secretion.

Classification and functions of enteroendocrine cells of the lower gastrointestinal tract

With over thirty different hormones identified as being produced in the gastrointestinal (GI) tract, the gut has been described as 'the largest endocrine organ in the body' (Ann. Oncol., 12, 2003, S63). The classification of these hormones and the cells that produce them, the enteroendocrine cells (EECs), has provided the foundation for digestive physiology. Furthermore, alterations in the composition and function of EEC may influence digestive physiology and thereby associate with GI pathologies. Whilst there is a rapidly increasing body of data on the role and function of EEC in the upper GI tract, there is a less clear-cut understanding of the function of EEC in the lower GI. Nonetheless, their presence and diversity are indicative of a role. This review focuses on the EECs of the lower GI where new evidence also suggests a possible relationship with the development and progression of primary adenocarcinoma.

sst1 mRNA is the prominent somatostatin receptor mRNA in the rat gastrointestinal tract: reverse transcription polymerase chain reaction and in situ-hybridization study

The inhibitory peptide hormone somatostatin and its receptors (sst1-sst5) regulate many physiological functions in the gastrointestinal tract. In an attempt to correlate the various effects of somatostatin in gastrointestinal physiology to individual sst subtypes sst1-sst5, mRNAs have been localized by semiquantitative reverse transcription polymerase chain reaction amplification and in situ hybridization of sst1 and sst3 in the rat alimentary tract. sst1-sst4 mRNAs were found throughout the gastrointestinal tract, sst1 mRNA being more abundant than sst2 and much more abundant than sst3 and sst4 mRNAs. sst5 transcripts were at the detection threshold. sst1 and sst3 mRNAs are present in enterocytes and enteric neurons suggesting a role of these subtypes in the somatostatin-mediated inhibition of acetylcholine release from myenteric neurons and of secretomotor neuron activity in the submucous plexus. The presence of sst3 mRNA in smooth muscle cells points to an additional role of this receptor in regulating gut motility.

The influence of the migrating motor complex on the postprandial endocrine response

BACKGROUND

The phase of the migrating motor complex (MMC) in the proximal small intestine at meal intake modulates gastric emptying, which is accelerated after intake during phase II. In the present study the relationship between phase of the MMC at meal intake and the postprandial endocrine response was studied.

METHODS

Eight healthy subjects ingested a caloric liquid meal of 2020 kJ during phase I and late phase II of the intestinal MMC, respectively, in a randomized order. Blood samples were drawn at regular intervals after meal intake and analysed for insulin, gastrin, neurotensin, cholecystokinin, motilin, and somatostatin by radioimmunoassays.

RESULTS

The area under the curve (AUC) until 15 min for serum insulin (P<0.05) and plasma neurotensin (P<0.02) and AUC until 120 min for serum gastrin (P<0.05) were higher after intake during late phase II than after phase I. Plasma cholecystokinin increased earlier (P<0.05) after intake during late phase II than after phase I. Plasma motilin and somatostatin were not influenced by preceding phase of the MMC.

CONCLUSIONS

This study shows that the phase of intestinal MMC at meal intake modulates the postprandial endocrine response, which may be explained by the prior entry of nutrients to the small intestine after intake during phase II.

Source of dietary protein influences kinetics of plasma gut regulatory peptide concentration in response to feeding in preruminant calves

The kinetics of the peripheral plasma concentrations of eight gut regulatory peptides were examined in response to feeding in preruminant calves. Two experiments were carried out in animals fed milk substitutes either based on milk protein (control diet) or in which casein had been replaced by hydrolyzed fish (fish diet in experiment 1) or whey (whey diet in experiment 2) protein concentrate. In contrast to the control diet, the latter two did not coagulate within the abomasum. No variation was observed in plasma concentrations of gut regulatory peptides during 1-1.4 hr before the morning meal regardless of the nature of the dietary protein. With the control diet, the meal was followed by an increase in cholecystokinin, gastrin and gastric inhibitory polypeptide and a fall in secretin, vasoactive intestinal polypeptide and motilin, whereas no significant change was observed for somatostatin and pancreatic polypeptide. The replacement of casein by protein substitutes did not greatly modify the pattern of plasma responses to feeding, but the prefeeding and postfeeding levels were highly affected. We conclude that the most important characteristic influencing plasma gut peptide concentrations is the ability of dietary protein to clot in the abomasum, consequently determining the pattern of gastric emptying, and that variations appear depending on the origin of protein substitutes in relation to the duodenal content and mainly to the digesta pH.

Plasma somatostatin-like immunoreactivity and somatostatin-28 levels in obese men

Somatostatin (SS) impairs nutrient absorption. It has been suggested that hyposomatostatinaemia may be involved in the pathogenesis of obesity. However, data on postprandial SS-like immunoreactivity (SLI) levels in obese subjects are controversial and the levels of SS-28, the main molecular form of circulating SLI in healthy subjects, have not been determined. To characterise the fasting and postprandial plasma pattern of SLI and SS-28 in obese men, we studied eight obese men (age 24-32 yr, BMI 33-42 kg/m2), with normal glucose tolerance test and normal gastric emptying of solids, and eight healthy men (age 24-39 yr, BMI 21-24 kg/m2). Blood samples were taken at regular intervals in fasting conditions and for 2 h after a standard solid-liquid meal (2.3 MJ). Plasma SLI and SS-28 were measured by RIA. Our results showed that fasting and postprandial plasma SLI and SS-28 levels were not significantly different in healthy and obese subjects. In conclusion SS-28 is the predominant form of circulating SLI in obese subjects. The normal pattern of fasting and postprandial plasma SLI and SS-28 levels in such subjects suggests that somatostatin does not have a pathogenetic role in obesity.

Endogenous somatostatin-28 modulates postprandial insulin secretion. Immunoneutralization studies in baboons

Somatostatin-28 (S-28), secreted into the circulation from enterocytes after food, and S-14, released mainly from gastric and pancreatic D cells and enteric neurons, inhibit peripheral cellular functions. We hypothesized that S-28 is a humoral regulator of pancreatic B cell function during nutrient absorption. Consistent with this postulate, we observed in baboons a two to threefold increase in portal and peripheral levels of S-28 after meals, with minimal changes in S-14. We attempted to demonstrate a hormonal effect of these peptides by measuring their concentrations before and after infusing a somatostatin-specific monoclonal antibody (mAb) into baboons and comparing glucose, insulin, and glucagon-like peptide-1 levels before and for 4 h after intragastric nutrients during a control study and on 2 d after mAb administration (days 1 and 2). Basal growth hormone (GH) and glucagon levels and parameters of insulin and glucose kinetics were also measured. During immunoneutralization, we found that (a) postprandial insulin levels were elevated on days 1 and 2; (b) GH levels rose immediately and were sustained for 28 h, while glucagon fell; (c) basal insulin levels were unchanged on day 1 but were increased two to threefold on day 2, coincident with decreased insulin sensitivity; and (d) plasma glucose concentrations were similar to control values. We attribute the eventual rise in fasting levels of insulin to its enhanced secretion in compensation for the heightened insulin resistance from increased GH action. Based on the elevated postmeal insulin levels after mAb administration, we conclude that S-28 participates in the enteroinsular axis as a decretin to regulate postprandial insulin secretion.

The diet and gut microflora influence the distribution of enteroendocrine cells in the rat intestine

Several functions of the gut are locally influenced by peptides and biogenic amines released from enteroendocrine cells. The aim of the present study was to assess whether the luminal stimulus of diet or microbial flora or diet-microbial interactions have an influence on the distribution of enteroendocrine cells along the crypt-surface axes of the small and large intestine. The effects of diet and indigenous flora were investigated by comparing the numbers of argyrophil and serotonin immunoreactive cells in the jejunum and colon of germ free and conventional rats fed either a purified diet containing fine ingredients or a commercial diet containing crude fibre of cereal origin. The effect of human flora were analysed in germ-free rats inoculated with human faecal organisms. 1. Feeding the commercial diet reduced the number of argyrophil endocrine cells in the jejunum and serotonin immunoreactive cells in the colon of germ-free animals but increased the serotonin immunoreactive cells in the colon of conventional animals. 2. The rat flora increased the serotonin immunoreactive cells in the colon of animals fed a commercial diet and decreased in those fed a purified diet. 3. Inoculation of human flora increased the numbers of serotonin immunoreactive cells both in the jejunum and colon. The results provide evidence that the dietary changes and diet-microbial interactions can affect the regional number of enteroendocrine cells.

Loxiglumide inhibits cholecystokinin stimulated somatostatin secretion and simultaneously enhances gastric acid secretion in humans

In vitro studies have demonstrated that cholecystokinin releases somatostatin from the gastric mucosa. To date, there is no information about the in vivo significance of this finding in man. Therefore, we have studied the effect of infusion of cholecystokinin resulting in plasma concentrations within the range found after meal-stimulation, on somatostatin release and on gastric acid secretion. In addition we have studied these functions during infusion of the type A cholecystokinin receptor antagonist loxiglumide. In eight healthy subjects, basal gastric acid secretion was distinctly stimulated by cholecystokinin. The effect of cholecystokinin on gastric acid secretion was markedly enhanced by loxiglumide. Cholecystokinin also significantly stimulated somatostatin output into the gastric lumen, but not into the systemic circulation. Somatostatin output into the gastric lumen during infusion of cholecystokinin was abolished by loxiglumide. The data indicate that on the one hand circulating cholecystokinin, like gastrin, stimulates gastric acid secretion probably by binding to less specific type B receptors on parietal cells that are not blocked by loxiglumide, but on the other hand that cholecystokinin, in contrast to gastrin, also inhibits gastric acid secretion probably by binding to specific type A receptors present on somatostatin producing D-cells in the gastric mucosa, that are blocked by loxiglumide.

Dietary fat and adipose tissue metabolism in ruminants, pigs, and rodents: a review

Effects of dietary fat on dairy cows are reviewed. Dietary fat did not affect gain in BW or body condition score after peak lactation but tended to increase BW loss during early lactation and body fat deposition in growing cattle. Dietary fat decreased de novo fatty acid synthesis in adipose tissue. Basal FFA release from adipose tissue in vitro and beta-adrenergic lipolytic responses were increased by protected polyunsaturated fatty acids. Dietary fat increased body fat in growing pigs and decreased BW loss in lactating sows. Dietary fat decreased de novo fatty acid synthesis and basal glycerol release in adipose tissue and tended to increase simultaneously beta-adrenergic lipolytic responses to increased membrane fluidity. Dietary fat increased body fat in rats. Polyunsaturated fatty acids were sometimes less efficient than saturated ones in increasing body fat. Lipoprotein lipase activity in adipose tissue generally decreased. Hepatic fatty acid synthesis was decreased sharply by polyunsaturated fatty acids, and adipose tissue response was less important. beta-Adrenergic-stimulated lipolysis decreased, and fatty acid esterification increased, particularly from saturated fatty acids. A trend toward insulin resistance, which was more marked with saturated fatty acids, occurred in adipose tissue.

Physiology and pathophysiology of gallbladder motility

Most significant gallbladder disease is associated with gallbladder stasis. Gallbladder motility is controlled by a complex interplay of hormonal and neural factors. Experimental and clinical studies have demonstrated impaired motility in gallstone disease, and experimental evidence indicates that motility disturbances precede gallstone formation. The ability to measure gallbladder motility clinically has also resulted in better diagnosis and treatment for patients with chronic acalculous cholecystitis.

Regulation of somatostatin-14 and -28 secretion by gastric acid in dogs: differential role of cholecystokinin

BACKGROUND

Prosomatostatin-derived peptides include two principle bioactive molecular forms, somatostatin 28 (S-28) and somatostatin 14 (S-14). This study examined whether there is a functional relationship between gastric acid secretion and the release of S-28 and S-14 into the circulation.

METHODS

In conscious dogs with gastric and duodenal cannulas, S-28 and S-14 responses, measured after extraction of acidified plasma and separation by gel chromatography, were evaluated by administration of nutrients and acid-inducing secretagogues without and with omeprazole.

RESULTS

Ingestion of a solid meal caused equivalent plasma elevations of S-28 and S-14, whereas infusions of histamine and gastrin selectively increased plasma S-14. Omeprazole decreased meal-stimulated S-28 (-67% +/- 8%; P < 0.01) and S-14 (-56 +/- 9%; P < 0.01) and abolished S-14 increases to histamine and gastrin. Intraduodenal perfusions of a liquid protein meal increased S-28 above S-14, comprising approximately 71% of total somatostatin-like immunoreactivity released, and omeprazole suppressed S-28 (-87% +/- 5%; P < 0.01) without influencing S-14. Similar responses occurred after exogenous cholecystokinin. Moreover, pretreatment of the intraduodenal protein meal with the cholecystokinin-A receptor antagonist MK-329 abolished increases of S-28 and S-14 and caused a further twofold increase of gastric acid (P < 0.025).

CONCLUSIONS

In the fed state, gastric acid causes direct release of S-14 from the stomach, but the acid-dependent component of S-28 secretion requires cholecystokinin as a cofactor. Negative feedback regulation between somatostatin and gastric acid secretory responses to nutrients may include S-28 modulated, in part, by cholecystokinin.

Diurnal profile of oxyntomodulin-like immunoreactivity in duodenal ulcer patients

Plasma concentrations of oxyntomodulin-like immunoreactivity, a group of intestinal peptides capable of mediating an enterogastrone signal, were measured during a 24-h period in 6 duodenal ulcer patients and compared with those of 16 age-matched controls. Each subject was submitted to 18 oxyntomodulin-like immunoreactivity determinations. Four standardized meals were given during the test. Furthermore, each patient was evaluated for peak acid output after pentagastrin stimulation. The values of the duodenal ulcer subjects were predominantly within normal acid secretion limits. Fasting levels, meal-induced variations, and nocturnal production of oxyntomodulin-like immunoreactivity were similar in the two groups. A negative correlation was observed between peak acid output and oxyntomodulin-like immunoreactivity evaluated either as nocturnal production or as maximum nyctohemeral concentration. We conclude that, taken as a whole, duodenal ulcer disease is not caused by a defect in oxyntomodulin-like immunoreactivity secretion. However, this study does not rule out the possibility of a selective deficiency of these peptides in some duodenal ulcer subgroups such as hypersecretory patients.

Plasma molecular forms of gastrin, neurotensin and somatostatin in pregnancy and gestational diabetes after an oral glucose load or a mixed meal

At present the physiological role of gastrin, neurotensin and somatostatin in pregnancy and gestational diabetes is scarcely known. We have measured their different molecular forms in plasma of six female controls, six normal pregnant (NP) women and six gestational diabetic (GD) women under basal conditions and 30 min after an oral glucose load (100 g) and a liquid mixed meal in order to study if their alteration could contribute to the impaired glucose tolerance in GD. Total basal concentrations of neurotensin and somatostatin were higher in GD than in controls and NP, and no change was found after the glucose load or mixed meal in GD. Neurotensin-1-13 was the main molecular form of all neurotensins at basal time in the three groups studied, being higher in GD in comparison with controls and NP. Somatostatin-1-14 was the predominant molecular form in controls and GD under basal conditions and did not show any change any change after stimuli. In NP, somatostatin-1-14 showed a significant increase following both kinds of stimuli. Total gastrin concentrations in NP and GD showed a significant increase after the glucose load, which was not observed in controls. Gastrin-17 was the main molecular form at basal time and 30 min post-stimuli in GD but not in NP and controls. We suggest that the basal elevation of neurotensin and somatostatin levels could contribute to the impaired glucose tolerance observed in gestational diabetes, as well as to the lack of post-stimuli responses for neurotensin and somatostatin in GD.(ABSTRACT TRUNCATED AT 250 WORDS)

Intestinal fat does not inhibit gastric function through a hormonal somatostatin mechanism in dogs

In awake dogs with chronic gastric, duodenal, and jejunal fistulas, F(ab)1 fragments of somatostatin monoclonal antibody (mAb S607) were administered intravenously (IV) to test the hypothesis that intraintestinal lipid inhibits peptone-stimulated gastric acid secretion and emptying by a hormonal somatostatin mechanism. Plasma somatostatin was increased significantly by duodenal and jejunal perfusion with 20% lipid. Somatostatin administered IV caused dose-dependent inhibition of meal-stimulated gastric acid secretion and gastric emptying similar to that seen after intestinal perfusion with lipid. Administration of mAb S607 F(ab)1 fragments significantly reversed somatostatin (400 pmol.kg-1.h-1, IV)-induced inhibition of peptone-stimulated acid output and gastric emptying. Acid output inhibited by intraduodenal lipid was reversed partially after F(ab)1 administration, but the inhibitory effect of intrajejunal lipid was not altered. Inhibition of acid secretion by IV somatostatin and by intraintestinal fat was not caused by a decrease in circulating gastrin concentrations. Gastric emptying delayed by intraintestinal lipid was unaffected by antibody administration. Somatostatin does not appear to be a major hormonal mediator of intestinal fat-induced inhibition of gastric acid secretion or delayed gastric emptying in dogs.

Somatostatin 28 and coupling of human interdigestive intestinal motility and pancreatic secretion

To determine the effects of small increases in somatostatin 28 plasma concentrations on human interdigestive gastrointestinal motility and pancreatic secretion, six fasting volunteers were intubated with gastroduodenal multilumen tubes and motility and pancreatic enzyme secretion were measured. Subjects received intravenous NaCl and somatostatin 28 at 11 and 44 pmol.kg-1.h-1 for 120 minutes or at least one interdigestive cycle. The two doses increased plasma somatostatin 28 levels within the physiological or into the supraphysiological range, respectively. Somatostatin 28 at 11 and 44 pmol.kg-1.h-1 decreased the length of the interdigestive motility cycle by 50% and 67% compared with controls, respectively (both P less than 0.002). Propagation velocity of the migrating motor complex (P less than 0.01) and plasma motilin were decreased (P less than 0.01). The smaller and larger dose decreased pancreatic enzyme outputs by 50% and 65%, respectively (P less than 0.005), but with the smaller dose, phase III-associated enzyme outputs were greater than phase I outputs. These findings suggest that small changes in somatostatin 28 plasma concentrations modulate human interdigestive motility and pancreatic enzyme output while coupling of motor and secretory events is preserved.

Evidence for hormonal inhibition of exocrine pancreatic function by somatostatin 28 in humans

Somatostatin 28 (S-28), originating in gastrointestinal cells, is secreted into the circulation and increases in humans after ingestion of a mixed meal. To evaluate the possibility that the increased levels of S-28 post cibum might modulate the release of enzymes and bicarbonate from the exocrine pancreas, S-28 was infused intravenously into healthy volunteers to levels seen after food intake. During S-28 infusion, the output of lipase, trypsin, amylase, and bicarbonate stimulated by either exogenous cholecystokinin octapeptide or endogenous signals from intraduodenal administration of tryptophan or a mixture of amino acids was significantly reduced. It is concluded that S-28 released from the gut during food intake modulates pancreatic exocrine function in humans.

Effects of oleic acid and endogenous bile on duodenal secretion of somatostatin in man

We studied the effects of intraduodenal oleic acid on the release of somatostatin to plasma and the correlation between endogenous bile output and plasma somatostatin. In five normal persons infusion of 0, 5, 10, 20, and 40 mM oleic acid dose-dependently increased the levels of somatostatin during as well as after gallbladder emptying. The difference between somatostatin concentration during and after gallbladder emptying was not significant. The amylase secretion also was significantly correlated to the dose of fat, whereas the output of bile salts was the same for all fat doses used. Our observations indicate that intraduodenal oleic acid--and not bile salts--releases somatostatin from the gut.

Neural and hormonal mechanisms mediate the enterogastric reflex: a study in intestinal transplants in rats

To determine the relative contributions of neural reflexes and intestinal hormones to the inhibition of gastric acid secretion by intestinal fat, rats with an extrinsically denervated, transplanted segment of jejunum and those with an innervated segment of jejunum were studied. Postoperatively, meal-stimulated gastric acid secretion was measured by extragastric titration. When secretion reached a plateau, graded doses of oleic acid or saline were instilled into the jejunal segments. In both groups, acid secretion was inhibited by intrajejunal fat but not saline. At doses of 0.4 and 0.08 mmol oleic acid, there was a 25% and 17% greater maximal inhibition of plateau acid response in the innervated rats than in the transplanted rats, presumably because of the neural contribution. To examine the hormonal mediators, the effects of a somatostatin monoclonal antibody and a cholecystokinin A receptor antagonist (L-364,718) on fat-induced inhibition of gastric acid secretion were studied in transplanted rats. Treatment of the patients with transplants with a somatostatin monoclonal antibody (2.18 mg IV) or L-364,718 (1 mg/kg IV) reduced the fat-induced inhibition of acid secretion by 95% and 28%, respectively. In conclusion, both neural and hormonal mechanisms mediate fat-induced inhibition of gastric acid secretion, with the hormonal mechanism predominating. Somatostatin, and to a lesser extent cholecystokinin, contribute to the hormonal mechanism.