Effect of substance P on somatostatin release from the isolated perfused rat stomach

Neurotransmitters regulating acid secretion in the proventriculus of the Houbara bustard (Chlamydotis undulata): a morphological viewpoint

Endocrine cells containing somatostatin (Som), gastrin-releasing peptide (GRP), and neuronal nitric oxide synthase (nNOS) and nerve fibers containing choline acetyl transferase (ChAT), tyrosine hydroxylase (TH), galanin (Gal), substance P (SP), and vasoactive intestinal polypeptide (VIP) were immunolocalized in the proventriculus of the Houbara bustard, Chlamydotis undulata. While GRP-immunoreactive (GRP-IR) cells occur in the inner zone, somatostatin (Som-IR) and polyclonal nNOS (nNOS-IR) immunoreactive cells were localized mainly in the peripheral zone of submucosal glands. GRP-IR, Som-IR, and nNOS-IR cells were occasionally observed in the walls of the gastric glands. Endocrine cells are of the closed variety and usually possess apical processes extending along the basal surfaces of adjacent nonreactive cells. Ultrastructural features of these cells are typical. ChAT, Gal, SP, VIP, and TH were immunolocalized in nerve fibers and terminals in the walls of arterioles and capillaries at the periphery of submucosal glands. Immunoreactivity to monoclonal nNOS occurred mainly in neuronal cell bodies in ganglia located around the submucosal glands. ChAT and TH immunoreactive cell bodies were also occasionally seen around the submucosal glands in the peripheral region. Immunoreactivity to Gal, SP, and VIP, but not ChAT or TH, was discernible around the walls of gastric glands. It was concluded that the distribution of neurotransmitters in neuronal structures is similar, but that of the endocrine cells varies from that of some avian species. The roles of these neurotransmitters in the regulation of acid secretion are discussed.

Innervation of the gastric mucosa

A plethora of neuronal messengers ("classical" transmitters, gaseous messengers, amino acid transmitters, and neuropeptides) are capable of mediating or modulating gastric functions. Accordingly, the stomach is richly innervated. Gastric nerves are either intrinsic to the gastric wall, i.e., they have their cell bodies in the intramural ganglia and thus belong to the enteric nervous system, or they reach the stomach from outside, originating in the brainstem, in sympathetic ganglia, or in sensory ganglia. Topographically, the nerve fibers in the stomach reach all layers from the most superficial portions of the gastric glands to the outer smooth muscle layer. This wide distribution implies that virtually all different cell types may be reached by neuronal messengers. Within the gastric mucosa endocrine and paracrine cells (e.g., gastrin cells, ECL cells, somatostatin cells), exocrine cells (parietal cells, chief cells, mucous cells), smooth muscle cells, and stromal cells are regulated by neuronal messengers. The sensory innervation, responding to capsaicin, plays an important role in mucosal protection, and in ulcer healing. Presumably also other nerves are involved and a plasticity in the neuropeptide expression has been demonstrated at the margin of gastric ulcers. Taken together, available data indicate a complex interplay between hormones, paracrine messengers and neuronal messengers, growth factors and cytokines in the regulation of gastric mucosal activities such as secretion, local blood flow, growth, and restitution after damage.

Tachykinins in the gut. Part II. Roles in neural excitation, secretion and inflammation

The preprotachykinin-A gene-derived peptides substance (substance P; SP) and neurokinin (NK) A are expressed in intrinsic enteric neurons, which supply all layers of the gut, and extrinsic primary afferent nerve fibers, which innervate primarily the arterial vascular system. The actions of tachykinins on the digestive effector systems are mediated by three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Within the enteric nervous system, SP and NKA are likely to mediate, or comediate, slow synaptic transmission and to modulate neuronal excitability via stimulation of NK3 and NK1 receptors. In the intestinal mucosa, tachykinins cause net secretion of fluid and electrolytes, and it appears as if SP and NKA play a messenger role in intramural secretory reflex pathways. Secretory processes in the salivary glands and pancreas are likewise influenced by tachykinins. The gastrointestinal arterial system may be dilated or constricted by tachykinins, whereas constriction and an increase in the vascular permeability are the only effects seen in the venous system. Various gastrointestinal disorders are associated with distinct changes in the tachykinin system, and there is increasing evidence that tachykinins participate in the hypersecretory, vascular and immunological disturbances associated with infection and inflammatory bowel disease. In a therapeutic perspective, it would seem conceivable that tachykinin antagonists could be exploited as antidiarrheal, antiinflammatory and antinociceptive drugs.

Effect of carbachol on luminal release of somatostatin from isolated perfused rat duodenum

The dynamic release of somatostatin-like immunoreactivity (SLI) from duodenum into the lumen was studied in the isolated, vascularly perfused rat duodenum. The luminal release of duodenal SLI was stimulated by a cholinergic agonist, carbachol, and the carbachol-induced release of SLI was completely blocked by atropine, but not by hexamethonium. These data suggest that the luminal release of SLI from rat duodenum is under the control of a cholinergic muscarinic stimulation. The ratio of somatostatin-14 to somatostatin-28 in picograms was about 1 during basal release but increased to approximately 2 during carbachol stimulation.

Release of substance P-like immunoreactivity from the vascularly perfused rat stomach

The release of gastric substance P-like immunoreactivity (SP-LI) has been studied in the vascularly perfused rat stomach. In the presence of 20 microM bacitracin and captopril, basal release of SP-LI was sustained throughout the experiments. Gastric SP-LI release was stimulated in a concentration-dependent manner by increasing the concentration of KCl in the perfusion medium. This stimulated release was reduced by the omission of Ca2+, indicating that a Ca2(+)-dependent mechanism was involved. Naloxone did not alter basal SP-LI secretion. [Met5]Enkephalin also had no significant effect on K(+)-stimulated secretion suggesting that enkephalinergic mechanisms are not involved. Gastric SP-LI release was also increased by capsaicin perfusion but this was not sustained. In conclusion, the present results provide the first evidence for the release of SP-LI into the rat stomach vasculature.

Effect of galanin on endocrine secretions from the isolated perfused rat stomach and pancreas

Nerves containing galanin immunoreactivity have been shown to be present in the stomach and pancreas. The present experiments were designed to test the effect of galanin on the release of gastric somatostatin-like immunoreactivity (SLI) and immunoreactive gastrin (IR-G), pancreatic SLI and immunoreactive insulin (IR-I) from the isolated perfused rat stomach and pancreas respectively. Galanin (2 X 10(-10)-5 X 10(-8) M) inhibited gastric SLI and IR-G release dose dependently. At a concentration of 10(-8) M, galanin also suppressed IR-I release stimulated by gastric inhibitory polypeptide (2 X 10(-10) M) in the presence of 8.9 X 10(-3) M glucose. Pancreatic SLI release under this condition was not altered. Thus the present study suggests that galanin might be involved in the neural regulation of gastric and pancreatic endocrine secretions.

The effects of substance P, histamine and histamine antagonists on somatostatin and gastrin release from the isolated perfused rat stomach

Secretion of somatostatin-like immunoreactivity (SLI) from the isolated perfused rat stomach has been shown to be inhibited by substance P. The present study was initiated to examine the possibility that this action of substance P was mediated via release of histamine. Substance P (1 microM) reduced basal secretion of SLI in agreement with earlier studies. Neither pyrilamine nor cimetidine influenced this action. Basal immunoreactive gastrin (IRG) secretion was unaffected by substance P. Addition of pyrilamine during substance P perfusion increased IRG secretion whereas addition of cimetidine resulted in a delayed decrease on removal of both compounds. Histamine (1 and 10 microM) increased SLI secretion and reduced IRG secretion. Pyrilamine increased and cimetidine decreased IRG secretion but neither drug influenced SLI secretion. Pyrilamine had no effect on histamine-stimulated SLI secretion but inhibition of IRG secretion by histamine was converted to stimulation. Cimetidine potentiated histamine stimulation of SLI secretion and inhibition of IRG secretion.

IN CONCLUSION

(1) substance P inhibition of SLI secretion is unlikely to be mediated via release of histamine. (2) The gastrin cell appears to have both H1- and H2-receptors which mediate opposite actions but H1-receptor-mediated inhibition is predominant. (3) Histamine weakly stimulates SLI secretion but there may be both inhibitory and stimulatory pathways acting via H2- and H1-receptors, respectively.

Comparison of the effect of somatostatin and an analogue, SMS 201-995, on gastrin and insulin secretion from isolated perfused rat stomach and pancreas

The effects of a somatostatin analogue, SMS 201-995, and somatostatin-14 on gastrin and insulin release from the isolated perfused rat stomach and pancreas were studied. Equipotent effects were observed on the inhibition of basal gastrin release from the stomach. SMS 201-995 was approximately 14 times less effective than somatostatin-14 (molar basis) in the pancreas. At a high glucose concentration (17.6 mM), neither the analogue nor the peptide inhibited insulin release, but at a glucose concentration of 8.8 mM, there was significant inhibition. In the presence of 10 mM arginine plus glucose at a concentration of either 8.8 or 17.6 mM, insulin secretion was reduced by both SMS 201-995 and somatostatin-14 to levels obtained with glucose alone. However, when gastric inhibitory polypeptide (10 ng/ml) in the presence of 8.8 mM glucose was used to stimulate insulin release, somatostatin-14 completely inhibited the insulinotropic action of gastric inhibitory polypeptide while SMS 201-995 was without effect. Studies with this analogue suggest that arginine and gastric inhibitory polypeptide stimulate insulin release via different mechanisms.

Effect of pentagastrin on gastric acid secretion in the totally isolated vascularly perfused rat stomach stimulated with the phosphodiesterase inhibitor isobutyl methylxanthine

The development of a very sensitive stomach biomodel, using the totally isolated rat stomach, is reported. Pentagastrin-stimulated acid secretion in anesthetized and conscious rats was also studied, and the capacity to produce acid and the sensitivity towards pentagastrin in the different rat stomach models were compared. The secretory capacity and the sensitivity were evaluated by means of the maximal acid output (MAO) and threshold dose (TD), respectively. Conscious rats provided with gastric fistulas had a MAO of 48 +/- 6.3 mu eq/10 min and a TD of 6 micrograms/kg-h. In anesthetized rats with luminally perfused stomach MAO was reduced to 11.6 +/- 1.2 mu eq/10 min, whereas the sensitivity was increased, as indicated by a TD of 0.5 micrograms/kg-h. Totally isolated vascularly perfused rat stomachs without isobutyl methylxanthine (IMX) produced even less acid (MAO, 2.2 +/- 0.4 mu eq/10 min) without any change in demand for stimulant (TD, 0.25 micrograms/stomach-h). Addition of IMX to the vascular perfusate in the isolated stomach preparation increased the MAO to 8.0 +/- 1.0 mu eq/10 min and induced a remarkable reduction in demand for stimulant, with a TD obtained already at a pentagastrin dose of 0.008 micrograms/stomach-h, corresponding to a concentration of pentagastrin in the vascular perfusate medium of 84.5 pM. Accordingly, the totally isolated vascularly perfused rat stomach stimulated with IMX is the most sensitive bioassay for (penta)gastrin so far reported.

Gastrointestinal somatostatin: distribution, secretion and physiological significance

Somatostatin-like immunoreactivity (SLI) has been found throughout the gastrointestinal tract in all species examined. In the stomach it is mainly present in endocrine-type D-cells whereas in the intestine there is also an extensive distribution in enteric neurones. In all regions of the gastrointestinal tract multiple forms of somatostatin exist. A precursor (prosomatostatin) has been partially sequenced, three forms with 20 (SS-20), 25 (SS-25) and 28 (SS-28) amino acids completely sequenced, and somatostatin-14 (SS-14) demonstrated by radioimmunoassay. Both SS-14 and SS-28 exert a wide range of actions on the gastrointestinal tract and there is strong supportive evidence for a role in the regulation of gastric acid and gastrin secretion, gastrointestinal motility and intestinal transport. Both in vivo and in vitro studies on the secretion of gastric SLI into the vasculature have shown that nutrients initiate the process but that subsequent events are regulated by a complex interplay between hormonal and neuronal pathways. GIP is one of the most potent hormonal secretagogues. In the stomach, acetylcholine, opioid peptides and substance P are probably involved in parasympathetic inhibitory pathways and gastrin releasing peptide in stimulatory pathways. The sympathetic nerves are also stimulatory. Regulation of secretion of intestinal SLI has not been so extensively studied. Although SLI is also found in the gastrointestinal lumen the significance is unclear. Despite these advances the exact route of delivery of somatostatin to its target organs is uncertain and paracrine, endocrine and neural pathways may all be involved.