Stimulation Of Somatostatin Release Research Articles

Characterization of the Glutamate Receptors Mediating Release of Somatostatin from Cultured Hippocampal Neurons

L-Glutamate, NMDA, DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), and kainate (KA) increased the release of somatostatin-like immunoreactivity (SRIF-LI) from primary cultures of rat hippocampal neurons. In Mg(2+)-containing medium, the maximal effects (reached at approximately 100 microM) amounted to 737% (KA), 722% (glutamate), 488% (NMDA), and 374% (AMPA); the apparent affinities were 22 microM (AMPA), 39 microM (glutamate), 41 microM (KA), and 70 microM (NMDA). The metabotropic receptor agonist trans-1-aminocyclopentane-1,3-dicarboxylate did not affect SRIF-LI release. The release evoked by glutamate (100 microM) was abolished by 10 microM dizocilpine (MK-801) plus 30 microM 1-aminophenyl-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466). Moreover, the maximal effect of glutamate was mimicked by a mixture of NMDA+AMPA. The release elicited by NMDA was sensitive to MK-801 but insensitive to GYKI 52466. The AMPA- and KA-evoked releases were blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQX) or by GYKI 52466 but were insensitive to MK-801. The release of SRIF-LI elicited by all four agonists was Ca(2+) dependent, whereas only the NMDA-evoked release was prevented by tetrodotoxin. Removal of Mg2+ caused increase of basal SRIF-LI release, an effect abolished by MK-801. Thus, glutamate can stimulate somatostatin release through ionotropic NMDA and AMPA/KA receptors. Receptors of the KA type (AMPA insensitive) or metabotropic receptors appear not to be involved.

Pancreatic somatostatin-14 and somatostatin-25 release in rainbow trout is stimulated by glucose and arginine

Previous work suggested that teleost fish possess two somatostatin (SS) genes. Regulation of differential secretion of SS gene I (SS-25) and SS gene II products (SS-14) by nutrients was studied using rainbow trout. Hemi-islets were cultured in medium containing different concentrations of glucose and arginine. SS-14 and insulin were determined by radioimmunoassay; SS-25 was determined by a novel enzyme-linked immunosorbent assay. Glucose stimulated SS-25 release in a concentration-related manner, with 10 mM being the most effective concentration; at higher (25 mM) glucose, release decreased. Insulin levels in the medium also were found to be more elevated from islets incubated in 10 mM glucose than those incubated in 25 mM glucose. Maximum SS-14 release was observed at lower (1-5 mM) glucose concentrations. Arginine stimulated SS release so that maximum release of both SS forms occurred at the lowest glucose concentrations (1 mM). A concentration-related effect of arginine was observed on SS-25 release. The results of this study indicate that glucose and arginine regulate secretion of both pancreatic SS gene I and gene II products from rainbow trout islets, suggesting that two different pancreatic SS interact in nutrient homeostasis in this species.

Calcitonin gene-related peptide modulates acid-mediated regulation of somatostatin and gastrin release from rat antrum

Background & Aims : Acid has been shown to stimulate calcitonin gene-related peptide (CGRP) release from peripheral sensory afferent nerve endings in the stomach. The aim of this study was to determine whether endogenous CGRP was involved, by a neurocrine mechanism, in acid-mediated stimulation of somatostatin and inhibition of gastrin release. Methods : A two-compartment sleeve of antral mucosal/submucosal tissue was perfused to determine sensory nerve and endocrine cell responses to luminal acid. CGRP receptor antagonist, CGRP 8–37, was used to inhibit the actions of endogenously released CGRP. Results : Perfusion of the antral sleeve lumen with media of increasing hydrogen ion concentration caused pH-dependent increases in CGRP and somatostatin release and decrease in gastrin release. CGRP 8–37 inhibited significantly basal somatostatin (−36%) and stimulated basal gastrin (+65%) release ( P < 0.02). Furthermore, CGRP 8–37 administration prevented luminal acid-mediated inhibition of gastrin release and stimulation of somatostatin release. These results indicate that CGRP 8–37 prevented acid-mediated feedback inhibition of gastrin release and acid-induced feedforward somatostatin release. Conclusions : These results suggest that CGRP plays an important role in the response of antral D and G cells to luminal acid and that local effector action of endogenous CGRP participates in regulation of antral regulatory peptide secretion.

The effects of glucagon-like peptide-I (GLP-I) on hormone secretion from isolated human pancreatic islets.

Glucagon-like peptide-I (GLP-I) is a potent incretin hormone that is now considered as a new therapeutic tool in the treatment of diabetes mellitus. In this study we characterized the effects of GLP-I on peptide hormone release from isolated human pancreatic islets. GLP-I stimulated insulin release in the presence of 10 mM glucose (2.8 mM glucose, 100%; 10 mM glucose, 166%; 10 mM glucose + 10 nM GLP-I, 222%) but had only a weak insulinotropic effect (128%) at 2.8 mM glucose. Glucagon release was inhibited by 10 mM glucose (2.8 mM glucose, 100%; 10 mM glucose, 72%) and by 10 nM GLP-I at 2.8 mM glucose (67%). Somatostatin secretion was increased by 10 mM glucose (2.8 mM glucose, 100%; 10 mM glucose, 166%). GLP-I stimulated somatostatin release in the presence of 2.8 mM glucose (172%). Pancreatic polypeptide (PP) secretion was enhanced by 10 mM glucose (2.8 mM glucose, 100%; 10 mM glucose, 236%). GLP-I induced PP release only in the presence of 2.8 mM glucose (184%).

Role of nitric oxide in control of growth hormone release in the rat.

Previous experiments in this and other laboratories have revealed that nitric oxids (NO) plays a role in controlling the release of corticotropin-releasing hormone (CRH) and luteinizing-hormone-releasing hormone (LHRH). Therefore, we have investigated its role in control of growth hormone (GH) release in conscious rats by microinjecting NG-monomethyl-L-arginine (NMMA), an inhibitor of NO synthase (NOS), into the third ventricle (3V) of conscious, freely moving castrate male rats. An initial blood sample (0.3 ml) was drawn from an indwelling intra-atrial catheter just prior to injection of NMMA [1 mg in 5 microliters of 0.9% NaCl (saline)] into the 3V. To maintain the inhibitory action on NOS, a second injection of NMMA was administered into the 3V 60 min after the first. Additional blood samples (0.3 ml) were removed at 10 min intervals for 120 min. Other animals received injections of the diluent at the same times and volumes as NMMA. Interleukin (IL)-1 alpha (0.06 pmol in 2 microliters saline) was injected into the 3V immediately after the first injection of NMMA, whereas other animals received the NMMA diluent followed by IL-1 alpha. The effects of IL-1 alpha were almost identical to those of NMMA in that there was a dramatic lowering of plasma GH achieved primarily by a reduction in height of the GH pulses without a significant reduction in their number.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat gastric somatostatin and gastrin release: Interactions of exendin-4 and truncated glucagon-like peptide-1 (GLP-1) amide

The effect of exendin-4, a peptide of the secretin-glucagon family with high homology of amino acid sequence with glucagon-like peptide-1 (GLP-1), on gastric hormone release was investigated in the isolated perfused rat stomach. Exendin-4 dose dependently stimulated somatostatin release up to 9-fold at a concentration of 10 −7 M whereas gastrin release was inversely inhibited by up to 63%. These effects could partially be reduced by concomitant perfusion of truncated exendin-4, exendin(9–39)amide. Similarly, stimulation of somatostatin secretion and inhibition of gastrin release induced by GLP-1(7–36)amide was partially reversed by exendin-4 (9–39)amide. These data are consistent with the assumption that exendin-4 and truncated GLP-1amide exert their effects on gastric D and G cell by interaction with the same receptor.

Glucagon-like peptide-1-(7-36) amide, oxyntomodulin, and glucagon interact with a common receptor in a somatostatin-secreting cell line

Glucagon-like peptide-1(7-36)amide (tGLP-1), oxyntomodulin (OXM), and glucagon are posttranslational end products of the glucagon gene expressed in intestinal L-cells. In vivo, these peptides are potent inhibitors of gastric acid secretion via several pathways, including stimulation of somatostatin release. We have examined the receptors through which these peptides stimulate somatostatin secretion using the somatostatin-secreting cell line RIN T3. tGLP-1, OXM, and glucagon stimulated somatostatin release and cAMP accumulation in RIN T3 cells to similar maximum levels, with ED50 values close to 0.2, 2, and 50 nM and 0.02, 0.3, and 8 nM, respectively. Binding of [125I]tGLP-1, [125I]OXM, and [125I]glucagon to RIN T3 plasma membranes was inhibited by the three peptides, with relative potencies as follows: tGLP-1 > OXM > glucagon. Whatever the tracer used, the IC50 for tGLP-1 was close to 0.15 nM and was shifted rightward for OXM and glucagon by about 1 and 2-3 orders of magnitude, respectively. Scatchard analyses for the three peptides were compatible with a single class of receptor sites displaying a similar maximal binding close to 2 pmol/mg protein. In the hamster lung fibroblast cell line CCL39 transfected with the receptor for tGLP-1, binding of [125I]tGLP-1 was inhibited by tGLP-1, OXM, and glucagon, with relative potencies close to those obtained with RIN T3 membranes. Chemical cross-linking of [125I]tGLP-1, [125I]OXM, and [125I]glucagon revealed a single band at 63,000 mol wt, the intensity of which was dose-dependently reduced by all three peptides. These data suggest that in the somatostatin-secreting cell line RIN T3, OXM and glucagon stimulate somatostatin release through a tGLP-1-preferring receptor. This suggests that some biological effects, previously described for these peptides, might be due to their interaction with this receptor.

Glucagon-like peptide-1-(7-36) amide, oxyntomodulin, and glucagon interact with a common receptor in a somatostatin-secreting cell line.

Glucagon-like peptide-1(7-36)amide (tGLP-1), oxyntomodulin (OXM), and glucagon are posttranslational end products of the glucagon gene expressed in intestinal L-cells. In vivo, these peptides are potent inhibitors of gastric acid secretion via several pathways, including stimulation of somatostatin release. We have examined the receptors through which these peptides stimulate somatostatin secretion using the somatostatin-secreting cell line RIN T3. tGLP-1, OXM, and glucagon stimulated somatostatin release and cAMP accumulation in RIN T3 cells to similar maximum levels, with ED50 values close to 0.2, 2, and 50 nM and 0.02, 0.3, and 8 nM, respectively. Binding of [125I]tGLP-1, [125I]OXM, and [125I]glucagon to RIN T3 plasma membranes was inhibited by the three peptides, with relative potencies as follows: tGLP-1 > OXM > glucagon. Whatever the tracer used, the IC50 for tGLP-1 was close to 0.15 nM and was shifted rightward for OXM and glucagon by about 1 and 2-3 orders of magnitude, respectively. Scatchard analyses for the three peptides were compatible with a single class of receptor sites displaying a similar maximal binding close to 2 pmol/mg protein. In the hamster lung fibroblast cell line CCL39 transfected with the receptor for tGLP-1, binding of [125I]tGLP-1 was inhibited by tGLP-1, OXM, and glucagon, with relative potencies close to those obtained with RIN T3 membranes. Chemical cross-linking of [125I]tGLP-1, [125I]OXM, and [125I]glucagon revealed a single band at 63,000 mol wt, the intensity of which was dose-dependently reduced by all three peptides. These data suggest that in the somatostatin-secreting cell line RIN T3, OXM and glucagon stimulate somatostatin release through a tGLP-1-preferring receptor. This suggests that some biological effects, previously described for these peptides, might be due to their interaction with this receptor.

Effect of cholecystokinin and secretin on somatostatin release from cultured antral cells

Background: Both secretin and Cholecystokinin (CCK) inhibit gastric acid secretion. However, their mode of action has yet to be determined. A newly developed primary culture of human antral epithelial cells has been used to examine the effect of secretin and Cholecystokinin on somatostatin release. Methods: Normal human antral epithelial cell cultures enriched for D cells were maintained in culture for 2 days before release studies. Results: Native human secretin at 10−8 mol/L stimulated somatostatin release threefold. Porcine secretin and the secretin analogs, Tyr10 human secretin, Tyr13 porcine secretin, and Tyr10,13 porcine secretin were equipotent to native human secretin. CCK stimulated somatostatin release with the greatest response (eight times basal) at 10−7 mol/L. The response to CCK was inhibited in a competitive manner by the addition of the benzodiazepine analog, MK-329. Addition of secretin in the presence of 10−8 mol/L CCK resulted in a potentiation of somatostatin release, with the greatest response at 106 mol/L secretin, resulting in a 12-fold increase above basal. Conclusions: The stimulation observed after the addition of CCK was the result of activation of the CCK-A receptor subtype. The secretin receptor resembles that of the pancreatic D cells and acts through increasing intracellular cyclic adenosine monophosphate levels. Finally, these data indicate that the inhibitory action of CCK and secretin on gastric acid secretion may result from a synergistic action on antral D cells to release somatostatin, which in turn decreases antral gastrin release.

Partial hepatectomy affects pancreatic size and function in rats.

The effect of partial hepatectomy (62 +/- 2% of liver mass) or sham laparotomy on the pancreas was studied in rats. Pancreatic contents of DNA, protein, and digestive enzymes were measured 14 days postoperatively. Pancreatic acini were prepared to study exocrine pancreatic function after hormonal stimulation. Islet hormone release was investigated in the isolated perfused pancreas. Liver regeneration reached 93 +/- 1% within 14 days. Water contents in liver and pancreas remained unaltered. Simultaneously, the pancreatic weight increased significantly. Pancreatic enzymes showed a parallel elevation, whereas DNA remained unaffected. Amylase secretion from pancreatic acini was unaltered. Stimulated insulin and somatostatin release from the perfused pancreas were both increased. Plasma cholecystokinin levels were elevated, whereas neurotensin was decreased. Insulin and gastrin remained unchanged. In conclusion, after partial hepatectomy, enhanced cholecystokinin and decreased neurotensin blood levels are suggested to contribute to a hypertrophic effect on the exocrine pancreas and an adaptive regulation of the endocrine gland.

Inhibitory actions of a somatostatin analog SMS 201-995 on endocrine secretion from the isolated perfused stomach and pancreas of the rat.

An analog of somatostatin, SMS 201-995 (SMS; Sandostatin), has been shown to have increased potency in the inhibition of hormone secretion in vivo. In this study, the effect of SMS on the secretion of immunoreactive insulin, gastrin and gastric somatostatin was examined in the rat using in situ vascularly perfused organ preparations. The analog was found to inhibit both basal somatostatin and gastrin secretion from the perfused stomach in a concentration-dependent manner. SMS (9.4 x 10(-8) mol/l) also inhibited somatostatin release stimulated by gastric inhibitory polypeptide (GIP), but did not suppress gastrin release under the same conditions; an opposite effect was obtained when l-isoproterenol was used to stimulate somatostatin release. In the perfused pancreas both SS-14 (6.1 x 10(-9) mol/l) and SMS (9.4 x 10(-9) mol/l) inhibited the GIP (2 x 10(-9) mol/l) or acetylcholine (1 x 10(-6) mol/l), but not 17.8 x 10(-9) mol/l glucose-stimulated insulin secretion. Insulin secretion stimulated by 17.8 x 10(-3) mol/l glucose was unaffected by either peptide at this concentration. These results show that SMS is a potent inhibitor of gastric and pancreatic endocrine secretion. However, the inhibitory effect of SMS on gastric endocrine secretion was uncoupled in the presence of different secretagogues, suggesting that the action of SMS was dependent on the activation of different mechanisms or pathways in the stomach. In the pancreas, SMS appears to be acting on the beta-cell via a similar mechanism as somatostatin.

The role of endogenous atrial natriuretic peptide in resting and stress-induced release of corticotropin, prolactin, growth hormone, and thyroid-stimulating hormone.

Our previous studies have shown that stimulation of the anteroventral third ventricle region increases atrial natriuretic peptide (ANP) release, whereas lesions of the anteroventral third ventricle or median eminence block the release of ANP from blood volume expansion, suggesting a critical central nervous system participation in this response. ANP is also produced within neurons that have cell bodies in the rostral hypothalamus and axons that extend to the median eminence and neural lobe. In addition to its natriuretic effect, the peptide can inhibit the release of corticotropin (ACTH) and prolactin, anterior pituitary hormones that are released during stress. To determine the physiologic significance of ANP in the control of basal and stress-induced release of anterior pituitary hormones, highly specific antiserum against the peptide (AB-ANP) was microinjected into the third cerebral ventricle of conscious freely moving male rats to immunoneutralize hypothalamic ANP. In the initial experiment, the antiserum or control normal rabbit serum (NRS) was injected into the third cerebral ventricle to determine the effect of the antiserum on basal release of pituitary hormones. The antiserum had no effect on the concentrations of plasma ACTH, prolactin, or thyroid-stimulating hormone for 3 hr after the injection; however, plasma growth hormone concentration, although unchanged for 2 hr, was markedly elevated at 3 hr. These results indicate that although ANP appears to have no effect on the basal release of the other hormones, it has a physiologically significant inhibitory effect on growth hormone release. The delay of the effect is probably related to the time required for the antiserum to diffuse to the site of action of the peptide, presumably at some distance from the ventricle. Since this effect was demonstrable only after 3 hr, in the stress experiment, the antiserum or NRS was microinjected into the third ventricle 3 hr prior to application of ether stress. The rapid elevation of plasma ACTH in NRS-injected rats was markedly augmented by AB-ANP. Ether also induced a rapid increase in plasma prolactin in the NRS-injected animals, as expected. Contrary to the ACTH response, the maximal increase in plasma prolactin after ether was attenuated in animals preinjected with AB-ANP. In the NRS-injected animals, there was a significant decline in plasma growth hormone after the application of ether that was significantly accentuated by AB-ANP, but this was probably the result of the higher initial levels of plasma growth hormone in the ANP-AB group followed by its disappearance with a half-time similar to that of the NRS-injected group. The decline in plasma thyroid-stimulating hormone after ether stress was unaltered in the animals injected with AB-ANP. The results of these immunoneutralization studies suggest that endogenous ANP does not play a role in thyroid-stimulating hormone release. On the other hand, the endogenous peptide appears to have a physiologically significant inhibitory role in suppressing ACTH release during stress, mediated at least partly by suppression of vasopressin release. Endogenous ANP has a pathophysiologic role in augmenting the prolactin release in stress either by inhibiting release of prolactin-inhibiting factors or, alternatively, by enhancing release of prolactin-releasing factors. Endogenous ANP appears to inhibit resting, without altering stress-induced inhibition of growth hormone release by stimulating somatostatin release and/or inhibiting growth hormone-releasing hormone release or by both actions.

Oxyntomodulin and related peptides control somatostatin secretion in RIN T3 cells

We studied the effects of oxyntomodulin (OXM), of its C-terminal (19–37) fragment (OXM (19–37)) and of glucagon (GLU) on somatostatin release, cyclic AMP accumulation and inositol phosphate turnover in somatostatin-secreting RIN T3 cells in culture. Rapid changes in cellular free Ca 2+ were also measured using fura-2. Carbachol was used as a control test agent for the parameters involving the inositol phosphate/Ca 2+ cascade. OXM, GLU and OXM (19–37) were all able to stimulate somatostatin release with relative ED 50 of approx. 1, 22 and 45, respectively. OXM and GLU stimulated cyclic AMP levels with relative ED 50 of approx. 1 and 30, respectively, whereas OXM (19–37) was totally ineffective on this parameter. In contrast to cariachiol, none of the peptides significantly modified the inositol phosphate turnover or induced rapid changes in cellular free Ca 2+. We conclude that the RIN T3 cells contain a receptor-cyclic AMP system similar to that found in gastric mucosa and that this system is linked to somatostatin release. Another receptor-second messenger mechanism linked to somatostatin release is triggered by the (19–37) fragment. This mechanism is not the inositol phosphate/Ca 2+ cascade triggered in the same cells by cholinergic agents.

Glutamate Peripherally Administered Exerts Somatostatin-Releasing Action in the Conscious Rat

Abstract This study was designed to determine whether glutamate is able to stimulate somatostatin release from in vivo conscious animals when somatostatin release is monitored in unanaesthetized rats stereotaxically implanted with a push-pull cannula in the median eminence. One week after implantation, the median eminence was perfused with artificial cerebrospinal fluid alone or with the addition of either CGS 19755, an N-methyl-D-aspartate (NMDA) receptor antagonist (1(10(-6) M), or glutamate (10(-5) M). The latter (which is able to cross the brain-blood barrier at large doses) was also peripherally administered (1 g/kg ip). Median eminence perfusate samples were collected every 15 min and somatostatin was measured by a sensitive radioimmunoassay. In rats receiving ip glutamate injection, somatostatin release from the median eminence was significantly increased (73.3+/-10.4 versus 24.8 +/- 6.2; P < 0.01; n = 5) when compared to baseline levels measured in the same animals, but no effect was observed when local perfusion of the median eminence with glutamate (10(-4) to 10(-5) M; n = 6) was performed. Glutamate-induced somatostatin release was completely blunted (n = 5) by prior local administration of CGS 19755 (10(-5) M), a potent NMDA-type receptor antagonist able to cross the blood-brain barrier. In contrast, administration of glutamic acid diethylester, a competitive antagonist of non-NMDA receptors, at doses of 10(-4) M (n = 4), was not able to alter this response. Our results are the first in vivo evidence in favour of a neuroendocrine role for glutamate on somatostatin release whose site of action seems to exclude the median eminence.

“Gastrin” and “CCK” receptors on histamine-and somatostatin-containing cells from rabbit fundic mucosa—II: Characterization by means of selective antagonists (L-364, 718 and L-365, 260)

In the preceding paper, by means of selective agonists to gastrin (HG-17) and cholecystokinin (CCK-39), we evidenced the existence of “gastrin-type” receptors that could regulate histamine release and “CCK-type” receptors that could stimulate somatostatin release in isolated rabbit fundic nonparietal cells (F1 cells). Furthermore, these receptors could induce phosphoinositide breakdown. To confirm the involvement of these receptor types in these biological and biochemical processes, we used selective antagonists, L-364,718 (3-(benzoylamino)-benzodiazepine) specific to “CCK-A-type” receptor and L-365,260 (3-(acylamino)-benzodiazepine) specific to “gastrin/CCK-B-type” receptor. Neither L-364,718 nor L-365,260 alone caused any significant stimulation of [ 3H]inositol phosphate ([ 3H] InsP) production and release of histamine or somatostatin-like immunoreactivity (SLI). Each analogue inhibited in a dose-dependent manner [ 125I] HG-17 or [ 125I]CCK-39 binding to F1 cells, [ 3H]InsP accumulation and histamine and SLI release stimulated by HG-17 or CCK-39. L-365, 260 appeared to be 30–70 times more potent than L-364,718 in inhibiting [ 125I]HG-17 binding to F1 cells, as well as HG-17-induced [ 3H]InsP accumulation and HG-17- or CCK-39-enhanced histamine release ( ic 50 values: ≈5–20 nM for L-365,260 and ≈200–1500 nM for L-364,718 ). In contrast, L-364,718 was 200 to 400 times more potent than L-365,260 in inhibiting [ 125I]CCK-39 binding to F1 cells, CCK-39-induced [ 3H]-InsP accumulation and SLI release stimulated by CCK-39 or HG-17 ( ic 50 values: ≈0.3–1 nM for L-364, 718 and 100–200 nM for L-365,260 ). These results led to conclude: (i) the existence of a “gastrintype” receptor related to histamine release; (ii) the existence of a “CCK-A-type” receptor related to somatostatin release; (iii) the existence of “gastrin type” and “CCK-A-type” receptors linked to the phosphoinositide breakdown pathway.

In vitro effect of delta 9-tetrahydrocannabinol to stimulate somatostatin release and block that of luteinizing hormone-releasing hormone by suppression of the release of prostaglandin E2.

Previous in vivo studies have shown that delta 9-tetrahydrocannabinol (THC), the principal active ingredient in marijuana, can suppress both luteinizing hormone (LH) and growth hormone (GH) secretion after its injection into the third ventricle of conscious male rats. The present studies were designed to determine the mechanism of these effects. Various doses of THC were incubated with either stalk median eminence fragments (MEs) or mediobasal hypothalamic (MBH) fragments in vitro. Although THC (10 nM) did not alter basal release of LH-releasing hormone (LHRH) from MEs in vitro, it completely blocked the stimulatory action of dopamine or norepinephrine on LHRH release. The effective doses to block LHRH release were associated with a blockade of synthesis and release of prostaglandin E2 (PGE2) from MBH in vitro. In contrast to the suppressive effect of THC on LHRH release, somatostatin release from MEs was enhanced in a dose-related manner with a minimal effective dose of 1 nM. Since PGE2 suppresses somatostatin release, this enhancement may also be related to the suppressive effect of THC on PGE2 synthesis and release. We speculate that these actions are mediated by the recently discovered THC receptors in the tissue. The results indicate that the suppressive effect of THC on LH release is mediated by a blockade of LHRH release, whereas the suppressive effect of the compound on growth hormone release is mediated, at least in part, by a stimulation of somatostatin release.

Role of gastrin in bombesin-stimulated somatostatin release

The intermediary pathways in the bombesin-induced somatostatin release were examined in isolated perfused rat stomach obtained from male rats that were fasted overnight. The stomachs were perfused by way of the celiac artery. On coinfusion of 1.0 μmol/L tetrodotoxin and 1 nmol/L bombesin, a significant depression in release of somatostatin was observed compared with that observed with bombesin alone. The 5-minute integrated somatostatin response after treatment with tetrodotoxin and bombesin was 173% ± 14% of basal, which was significantly lower than that observed with bombesin alone (394% ± 59% of basal, P < 0.05) but significantly higher than that observed with medium-199 alone (95% ± 7% of basal, P < 0.05); this indicated that ~70% of the bombesin-stimulated somatostatin release was indirectly mediated through neural pathways, while a significant (~30%) segment of it was mediated by nonneural mechanisms. To test if the 30% somatostatin release was secondary to gastrin release in response to bombesin, gastrin antiserum and bombesin (1 nmol/L) were coadministrated in the presence or absence of tetrodotoxin (1 μmol/L). Gastrin antiserum alone did not significantly affect basal release of somatostatin but caused a significant inhibition (~23%) of bombesin-provoked somatostatin release. Coadministration of gastrin antiserum and tetrodotoxin attenuated bombesin-stimulated somatostatin release. Gastrin (1 μmol/L) alone significantly stimulated somatostatin release (150% ± 10% of basal), which was completely attenuated in the presence of gastrin antiserum. Tetrodotoxin did not affect bombesin-elicited gastrin release, confirming that bombesin-stimulated gastrin release was directly mediated. To determine the nature of the neural pathways mediating the bombesin-induced somatostatin release, atropine (100 nmol/L) was used. Atropine inhibited bombesin-induced somatostatin release to the same extent as tetrodotoxin, indicating that cholinergic pathways mediated bombesin-induced somatostatin release. These results show that almost all the somatostatin response to bombesin is indirectly mediated, and is composed of a major neural (cholinergic) and a minor nonneural pathway. The nonneural mechanism appears to be contributed primarily by gastrin released in response to bombesin, which apparently has a short paracrine positive feedback effect on somatostatin release.

Effect of antiepileptic drugs on somatostatin release in vitro

In the present study we investigated the effect of antiepileptic drugs on high potassium (50 mM) stimulated somatostatin release in rat cortical slices in a superfusion system. The somatostatin-like immunoreactivity (SLI) in superfusate was determined by radioimmunoassay. The antiepileptic drugs studied, vigabatrin, valproate, carbamaxepine, phenobarbital, primidone, clonazepam and phenytoin were tested at a concentration range of 1–1000 μM). Of the drugs used vigabatrin had the most significant inhibitory effect on SLI release (IC 50 = 240 μM). Vigabatrin also caused a concomitant, dose-dependent increase in superfusate gamma-amino butyric acid (GABA) level. A 30% decrease in the release of SLI followed incubation with valproate and carbamazepine, but only at high drug concentrations (1000 μM). Phenobarbital, primidone, clonazepam and phenytoin did not affect SLI release. Addition of GABA to superfusate caused a dose-dependent decrease in the amount of SLI release (IC 50 = 56 μM). In conclusion, at low concentrations the antiepileptic drugs had only minor effects on SLI release. At higher concentrations, however, vigabatrin and valproate decreased the release of SLI, which may relate to their ability to elevate tissue levels of GABA.

Gastric somatostatin: A paracrine regulator of acid secretion

The presence of acid in the lumen of the gastric fundus induces release of somatostatin close to the parietal cells; this acts to attenuate acid secretion in response to secretagogues, such as histamine and gastrin. The release of somatostatin within the stomach is further regulated by the activity of cholinergic neurons that inhibit somatostatin release and thus augment acid secretion (disinhibition), and noncholinergic (bombesin) neurons that stimulate somatostatin release and thus attenuate acid secretion. The influence of these neurons and the participation of somatostatin as a paracrine regulator of acid secretion has been probed and validated by the use of selective antagonists (atropine and a bombesin antagonist), somatostatin antiserum and pertussis toxin. Similar mechanisms exist in the distal antral segment of the stomach for the paracrine regulation of gastrin release by somatostatin.

Sex difference in growth hormone feedback in the rat

Growth hormone inhibits its own secretion in animals and man but the mechanism for this inhibition is unclear: both stimulation of somatostatin release and inhibition of GH-releasing factor (GRF) release have been implicated. We have now studied the GRF responsiveness of conscious male and female rats under conditions of GH feedback induced by constant infusion of exogenous human GH (hGH). Intravenous infusions of hGH (60 micrograms/h) were maintained for 3 to 6 h whilst serial injections of GRF(1-29)NH2 (0.2-1 microgram) were given at 45-min intervals. The GH responses were studied by assaying blood samples withdrawn at frequent intervals using an automatic blood sampling system. We have confirmed that male and female rats differ in their ability to respond to a series of GRF injections; female rats produced consistent GH responses for up to 13 consecutive GRF injections, whereas male rats showed a 3-hourly pattern of intermittent responsiveness. In female rats, multiple injections of GRF continued to elicit uniform GH responses during hGH infusions, whereas hGH infusions in male rats disturbed their intermittent pattern of responsiveness to GRF, and their regular 3-hourly cycle of refractoriness was prolonged. We suggest that this sex difference in GH feedback may be due to GH altering the pattern of endogenous somatostatin release differentially in male and female rats. Such a mechanism of GH autofeedback could be involved in the physiological control of the sexually differentiated pattern of GH secretion in the rat.