Inhibition Of Somatostatin Release Research Articles

196-OR: The Long-Chain Fatty-Acid Receptor FFA4 Stimulates Insulin Secretion via Inhibition of Somatostatin Release from the Delta Cell

The long-chain fatty-acid receptor FFA4 exerts beneficial effects on glucose homeostasis and insulin secretion and is considered a potential therapeutic target for type 2 diabetes. FFA4 is expressed in islets, but its precise mechanism of action remains unknown. Previous studies from our group suggest that FFA4 is expressed in delta cells and regulates somatostatin secretion. The objective of this study was to test the hypothesis that FFA4 agonists indirectly stimulate insulin secretion via inhibition of somatostatin release. In 1-h static incubations of isolated, wild-type mouse islets, the FFA4 agonist CpdA dose-dependently potentiated glucose-induced insulin secretion from 2.2±0.2 to 3.4±0.2 % of insulin content at the concentration of 50 μM (n=8; p<0.001), and simultaneously decreased somatostatin secretion from 28.4±2.1 to 10.5±0.9 pM (n=8; p<0.0001). No effect of CpdA on insulin or somatostatin secretion was observed in islets from mice that do not express somatostatin (insulin: 3.2±0.1 vs 2.9±0.4, n=6, NS; somatostatin: 11.4±1.2 vs 11.9±1.2, n=6, NS). No sex differences were observed. Likewise, in delta-cell deficient islets isolated from diphtheria toxin-treated male mice expressing the diphtheria toxin receptor under the somatostatin promoter, no effect of CpdA was observed on insulin or somatostatin release (insulin: 4.9±0.9 vs 4.1±0.8, n=3, NS; somatostatin: 10.1±4.9 vs 9.2±5.3, n=6, NS). We conclude that FFA4 stimulation of insulin secretion is exclusively mediated by inhibition of somatostatin secretion from delta cells. Disclosure L.Reininger: None. M.Ethier: None. C.Tremblay: None. J.Ghislain: None. M.Huising: Consultant; AstraZeneca, Research Support; Crinetics Pharmaceuticals, Inc. V.Poitout: Consultant; AstraZeneca. M.Flisher: None. Funding National Institutes of Health (R01DK132597)

FFA4 Regulates Insulin Secretion Via Inhibition of Somatostatin Secretion From Delta Cells

The long-chain fatty acid receptor FFA4 exerts beneficial effects on glucose homeostasis and insulin secretion and is considered a potential target for type 2 diabetes therapy. FFA4 is expressed in islets, but its precise mechanism of action remains unknown. Previous studies from our group suggest that FFA4 regulates somatostatin secretion by delta cells. The objective of this study was to test the hypothesis that FFA4 agonists indirectly stimulate insulin secretion via inhibition of somatostatin release.

Free fatty acid receptor 4 inhibitory signaling in delta cells regulates islet hormone secretion in mice

ObjectiveMaintenance of glucose homeostasis requires the precise regulation of hormone secretion from the endocrine pancreas. Free fatty acid receptor 4 (FFAR4/GPR120) is a G protein-coupled receptor whose activation in islets of Langerhans promotes insulin and glucagon secretion and inhibits somatostatin secretion. However, the contribution of individual islet cell types (α, β, and δ cells) to the insulinotropic and glucagonotropic effects of GPR120 remains unclear. As gpr120 mRNA is enriched in somatostatin-secreting δ cells, we hypothesized that GPR120 activation stimulates insulin and glucagon secretion via inhibition of somatostatin release. MethodsGlucose tolerance tests were performed in mice after administration of selective GPR120 agonist Compound A. Insulin, glucagon, and somatostatin secretion were measured in static incubations of isolated mouse islets in response to endogenous (ω-3 polyunsaturated fatty acids) and/or pharmacological (Compound A and AZ-13581837) GPR120 agonists. The effect of Compound A on hormone secretion was tested further in islets isolated from mice with global or somatostatin cell-specific knock-out of gpr120. Gpr120 expression was assessed in pancreatic sections by RNA in situ hybridization. Cyclic AMP (cAMP) and calcium dynamics in response to pharmacological GPR120 agonists were measured specifically in α, β, and δ cells in intact islets using cAMPER and GCaMP6 reporter mice, respectively. ResultsAcute exposure to Compound A increased glucose tolerance, circulating insulin, and glucagon levels in vivo. Endogenous and/or pharmacological GPR120 agonists reduced somatostatin secretion in isolated islets and concomitantly demonstrated dose-dependent potentiation of glucose-stimulated insulin secretion and arginine-stimulated glucagon secretion. Gpr120 was enriched in δ cells. Pharmacological GPR120 agonists reduced cAMP and calcium levels in δ cells but increased these signals in α and β cells. Compound A-mediated inhibition of somatostatin secretion was insensitive to pertussis toxin. The effect of Compound A on hormone secretion was completely absent in islets from mice with either global or somatostatin cell-specific deletion of gpr120 and partially reduced upon blockade of somatostatin receptor signaling by cyclosomatostatin. ConclusionsInhibitory GPR120 signaling in δ cells contributes to both insulin and glucagon secretion in part by mitigating somatostatin release.

In Vitro Optogenetic Characterization of Neuropeptide Release from Prefrontal Cortical Somatostatin Neurons

Somatostatin is a neuropeptide thought to play a role in a variety of neuropsychiatric disorders, and is important for healthy aging and behavioral resiliency. Physiological conditions underlying somatostatin peptidergic release are not well-defined. Using a combination of optogenetic and biochemical approaches in transgenic mice, we demonstrate an assay for the induction and inhibition of somatostatin release in mouse acute brain slices.

GPR40 protein levels are crucial to the regulation of stimulated hormone secretion in pancreatic islets. Lessons from spontaneous obesity-prone and non-obese type 2 diabetes in rats

The role of islet GPR40 protein in the pathogenesis of diabetes is unclear. We explored the influence of GPR40 protein levels on hormone secretion in islets from two rat models of spontaneous type 2 diabetes displaying either hyperlipidaemia or hyperglycaemia.GPR40 expression was analysed by confocal microscopy, Western blot and qPCR in islets from preobese Zucker (fa/fa) rats, diabetic Goto-Kakizaki (GK) rats, and controls.Confocal microscopy of control islets showed expression of GPR40 protein in insulin, glucagon and somatostatin cells. GPR40 expression was strongly increased in islets of hyperlipidaemic fa/fa rats and coincided with a concentration-related increase in palmitate-induced release of insulin and glucagon and its inhibition of somatostatin release. Conversely, hyperglycaemic GK islets displayed an extremely faint expression of GPR40 as did high-glucose-cultured control islets. This was reflected in abolished palmitate-induced hormone response in GK islets and high-glucose-cultured control islets. The palmitate antagonist rosiglitazone promoted reappearance of GPR40 in high-glucose-cultured islets and served as partial agonist in glucose-stimulated insulin release.GPR40 protein is abundantly expressed in pancreatic islets and modulates stimulated hormone secretion. Mild hyperlipidaemia in obesity-prone diabetes creates increased GPR40 expression and increased risk for an exaggerated palmitate-induced insulin response and lipotoxicity, a metabolic situation suitable for GPR40 antagonist treatment. Chronic hyperglycaemia creates abrogated GPR40 expression and downregulated insulin release, a metabolic situation suitable for GPR40 agonist treatment to avoid glucotoxicity. GPR40 protein is interactively modulated by both free fatty acids and glucose and is a promising target for pharmacotherapy in different variants of type 2 diabetes.

Delta cell secretory responses to insulin secretagogues are not mediated indirectly by insulin

Somatostatin from islet delta cells inhibits insulin and glucagon secretion, but knowledge of the regulation of pancreatic somatostatin release is limited. Some insulin secretagogues stimulate somatostatin secretion, and here we investigated whether delta cell secretory responses are indirectly regulated in a paracrine manner by insulin released from beta cells. Hormone release from static incubations of primary mouse islets or somatostatin-secreting TGP52 cells was measured by RIA. mRNA expression was assessed by RT-PCR. Glucose and a range of other physiological and pharmacological agents stimulated insulin and somatostatin release, and insulin receptor mRNA was expressed in islets, MIN6 beta cells and TGP52 cells. However, exogenous insulin did not modulate basal or glucose-induced somatostatin secretion from islets, nor did pre-incubation with an antibody against the insulin receptor or with the insulin receptor tyrosine kinase inhibitor, HNMPA(AM)(3). Glucose and tolbutamide stimulated somatostatin release from TGP52 cells, whereas a range of receptor-operating agents had no effect, the latter being consistent with a lack of corresponding receptor mRNA expression in these cells. Parasympathetic activation stimulated insulin, but inhibited somatostatin release from mouse islets in accordance with differences in muscarinic receptor mRNA expression in islets, MIN6 and TGP52 cells. The inhibitory effect on somatostatin secretion was reversed by pertussis toxin or the muscarinic receptor 2 antagonist, methoctramine. A number of insulin secretagogues have analogous effects on insulin and somatostatin release, but this similarity of response is not mediated by an indirect, paracrine action of insulin on delta cells.

Palmitate Stimulation of Glucagon Secretion in Mouse Pancreatic α-Cells Results From Activation of l-Type Calcium Channels and Elevation of Cytoplasmic Calcium

We have investigated the short-term effects of the saturated free fatty acid (FFA) palmitate on pancreatic alpha-cells. Palmitate (0.5 or 1 mmol/l bound to fatty acid-free albumin) stimulated glucagon secretion from intact mouse islets 1.5- to 2-fold when added in the presence of 1-15 mmol/l glucose. Palmitate remained stimulatory in islets depolarized with 30 mmol/l extracellular K(+) or exposed to forskolin, but it did not remain stimulatory after treatment with isradipine or triacsin C. The stimulatory action of palmitate on secretion correlated with a 3.5-fold elevation of intracellular free Ca(2+) when applied in the presence of 15 mmol/l glucose, a 40% stimulation of exocytosis (measured as increases in cell capacitance), and a 25% increase in whole-cell Ca(2+) current. The latter effect was abolished by isradipine, suggesting that palmitate selectively modulates l-type Ca(2+) channels. The effect of palmitate on exocytosis was not mediated by palmitoyl-CoA, and intracellular application of this FFA metabolite decreased rather than enhanced Ca(2+)-induced exocytosis. The stimulatory effects of palmitate on glucagon secretion were paralleled by a approximately 50% inhibition of somatostatin release. We conclude that palmitate increases alpha-cell exocytosis principally by enhanced Ca(2+) entry via l-type Ca(2+) channels and, possibly, relief from paracrine inhibition by somatostatin released by neighboring delta-cells.

δ1-OPIOID receptor-mediated controlofacetylcholine (ACh) release in human neocortex slices

In slices of human neocortex, prelabelled with [3H]-choline, the release of [3H]-acetylcholine reflects the evoked release of endogenous acetylcholine whichwaselicited by the same electrical stimulation paradigm. [3H]-Acetylcholine releasewasdepressed by the δ-opioid receptor agonist d-Pen2-d-Pen5-enkephalin. When the nerve endings were depolarized byelevatingextracellular potassium the evoked [3H]-acetylcholine release was similarlydepressed by d-Pen2-d-Pen5-enkephalin in theabsence, but notin the presence, of tetrodotoxin which blocks action potential propagation.Therefore, the δ-opioid receptor inhibiting [3H]-acetylcholine release should notbe located tocholinergic nerve terminals, but rather to interneurons. The somatostatin2receptorpartial agonist octreotide per se did not influence action potential-evoked [3H]-acetylcholine release, but prevented the inhibition of release of [3H]-acetylcholine byd-Pen2-d-Pen5-enkephalin.Similarly, the δ1-opioid receptor antagonist 7-benzylidenenaltrexon perse did notinfluence [3H]-acetylcholine release, but prevented of the inhibition ofrelease by d-Pen2-d-Pen5-enkephalin.From the presentfindings we conclude : (1) The evoked release of [3H]-acetylcholine from humanneocortex slices reflects the release of endogenous acetylcholine.(2) It is inhibited in an indirectmanner by opioid receptors of the δ1-subtype,which (3) are not localized oncholinergic axon terminals but on soma and dendrites ofsomatostatin-containing interneurons,where they inhibit somatostatin release. (4) Theseinterneurons innervate cholinergic nerveendings in the human neocortex and appear to facilitateacetylcholine release via somatostatin2 receptors.

HIGH-INTENSITY EXERCISE ENHANCES GROWTH HORMONE (GH) SECRETION BY A GHRH-DEPENDENT MECHANISM

276 Aim of this study was to gain insight into the mechanism of exercise induced GH release with the help of pharmacological stimulation. In eight healthy, non smoking male subjects (age: 22.1±2.2 yr; BMI: 22.2±2.5 kg/m2; ˙VO2max: 52.2±1.5 ml/min/kg; mean±SD) maximal responses of serum GH were analysed to exercise (EX), to administration of GH-Releasing Hormone (GHRH, 100 mcg iv), to administration of Pyridostigmine (PD, 120 mg orally), which inhibits somatostatin release, as well as to coadministration of GHRH plus EX, and PD plus EX. The combination (PDplusEX) could be administered in only 5 out of 8 subjects. EX was performed on a bicycle ergometer with a stepwisely increased workload protocol with a duration of at least 25 min up to exhaustion. Venous blood for GH analysis (IRMA) was obtained by serial sampling for a time period of 120 min and the area under the response curve (AUC) was calculated. All tests were performed in random order after an overnight fast and at least 3 days apart. Peak GH values after administration of GHRH, PD, and EX alone were 42±9 mcg/1 (mean±SEM), 20±2 and 64±8, and the AUC-data were 3242±839 mcg/1.min, 1721±358 and 2472±408, respectively. A potentiating effect on the GH response was found when coadministering GHRH plus EX. The AUC(GHRHplusEX) of 7807±1221 mcg/1.min significantly exceeded the AUC obtained by arithmetic addition of AUC(GHRH) and AUC(EX): 5714±863, (p<0.05, Wilcoxon signed-ranks test). In contrast, the AUC(PDplusEX) of 3526±752 mcg/1.min was not significantly different from the AUC obtained by arithmetic addition of AUC(PD) and AUC(EX): 3747±706. Because maximally stimulating doses for GH responses are given, these results indicate that high-intensity EX acts via a GHRH-dependent mechanism, and not by a reduction in somatostatinergic tone. Based on the concept of a stratified signaling system for GH secretion, we hypothesize that during high-intensity EX GHRH operates as a primary factor for GH release, with the help of a secondary still unknown releasing factor as co-agonist.

Reduction of the somatotrope responsiveness to GHRH and Hexarelin but not to arginine plus GHRH in hyperprolactinemic patients.

Aim of the present study was to verify the maximal secretory capacity of somatotrope cells in patients with pathological hyperprolactinemia (HPRL) comparing it with that in normal age-matched women (NW). To this goal in 12 HPRL normal weight patients (age 28.6 +/- 2.6 yr, BMI 23.1 +/- 1.1 kg/m2) and 8 NW (27.2 +/- 0.8 yr, 22.8 +/- 0.8 kg/m2) we studied the GH response to GHRH (1 microgram/kg i.v.), GHRH plus arginine (ARG, 0.5 g/kg i.v.), an amino acid probably acting at the hypothalamic level inhibiting somatostatin release, and Hexarelin (HEX, 2 micrograms/kg i.v.), a synthetic hexapeptide belonging to GHRP family, which acts concomitantly at the pituitary and the hypothalamic level. IGF-I levels in HPRL were similar to those in NW (179.2 +/- 16.5 micrograms/l and 218.5 +/- 30.8 micrograms/l). In NW the GH response to GHRH (AUC: 1299.5 +/- 186.9 micrograms 90 min/l) was lower (p < 0.02) than those to GHRH + ARG (5252.7 +/- 846.3 micrograms 90 min/l) and HEX 3216.6 +/- 462.3 micrograms 90 min/l) which, in turn, were similar. In HPRL the GH response to GHRH (894.7 +/- 242.4 micrograms 90 min/l) was lower (p < 0.03) than that to HEX (1586.5 +/- 251.3 micrograms 90 min/l) and both were lower (p < 0.03) than that to GHRH + ARG (4468.8 +/- 941.7 micrograms 90 min/l). In HPRL the GH responses to GHRH and HEX were lower than those that in NW (p < 0.03) while that to GHRH + ARG was similar in both groups. These results demonstrate that the somatotrope responsiveness to GHRH and HEX is clearly reduced in patients with pathological hyperprolactinemia. On the other hand, in this condition the GH response to GHRH + ARG is normal. As arginine likely acts via inhibition of hypothalamic somatostatin release, these findings show that the maximal secretory capacity of somatotrope cells in hyperprolactinemia is preserved and indicate that partial refractoriness of somatotrope cells to GHRH and HEX could be due to somatostatinergic hyperactivity.

Tumour Necrosis Factor Alpha (Tnfα) Inhibits Somatostatin Release from Cultured Canine D-Cells in Primary Culture

Tumour Necrosis Factor Alpha (Tnfα) Inhibits Somatostatin Release from Cultured Canine D-Cells in Primary Culture

Effect of exogenous growth hormone administration on endogenous growth hormone secretion induced by a met-enkephalin analog

Exogenous growth hormone (hGH) administration in humans attenuates the endogenous growth hormone (GH) response to some pharmacological stimuli; in particular, pretreatment with hGH completely blocks the serum GH response to growth hormone-releasing hormone. In order to evaluate the mechanism(s) whereby opiods induce GH secretion in man, we gave the following treatments to six healthy male volunteers: (a) IV saline; (b) a met-enkephalin analog G-DAMME 250 micrograms IV as a bolus at time 0'; (c) hGH 2 IU as an IV bolus at time -180'; (d) G-DAMME as above, preceded by hGH as above. In our study, G-DAMME stimulated GH secretion both basally (peak 17.9 +/- 6.0 ng/ml) and, to a lesser extent, after hGH pretreatment (6.0 +/- 2.7 ng/ml). Since in our study G-DAMME was able to partially overcome the inhibitory effect of hGH administration, it is suggested that opioids act through an inhibition of somatostatin release and not through a GHRH-dependent pathway. However, an additional direct effect of hGH on pituitary somatotrophes cannot be excluded.

Hypothalamic α2A-adrenoceptors stimulate growth hormone release in the rat

Oxymetazoline, the relatively selective α 2A-adrenoceptor agonist (with more than 60-fold selectivity over the α 2B-adrenoceptor subtype), was administered into the lateral ventricle (i.c.v.) of rats and plasma growth hormone (GH) levels were measured. Oxymetazoline was more potent to release GH after i.c.v. administration than was clonidine; 0.01 μg i.c.v. oxymetazoline already caused a significant release of GH, while at least 0.1 μg clonidine had to be administered to cause a similar response. The dose-response curve was of an inverted U shape since with 10 μg of oxymetazoline the plasma GH did not rise. When oxymetazoline was injected i.c.v. to rats with somatostatin fibres to the median eminence transected by an anterolateral cut in the hypothalamus there was a significant rise in plasma GH, suggesting that oxymetazoline stimulated GHRH rather than inhibited somatostatin release. Pretreatment with CH-38083 (7,8-(methylenedioxy)-14-α-hydroxy-alloberban HCl, selective for α 2-adrenoceptors but not differentiating between α 2A and α 2B subtypes), prevented the plasma GH rise normally elicited by 1 μg i.c.v. oxymetazoline. The α 2A- and α 1-selective adrenoceptor antagonist, WB-4101 (2-(2,6-dimethoxyphenoxyethyl)-aminomethyl-1,4-benzodioxane hydrochloride), prevented the GH rise normally induced by oxymetazoline while prazosin, the α 2B- and α 1-selective adrenoceptor antagonist, prolonged the elevation occurring in the control rats between 30 and 60 min after oxymetazoline injection. Since both prazosin and WB-4101 are α 1-adrenoceptor antagonists but differ in their action on α 2A and α 2B subtypes as well as in their action on oxymetazoline-induced GH secretion, the antagonist studies suggest that oxymetazoline stimulates GH release through activation of α 2A-adrenoceptors stimulatory to GHRH release, and not by an action through α 2B- or α 2C- or α 1-adrenoceptors. Since WB-4101 also antagonized clonidine action on GH release we also suggest that the major component may be the stimulation of the α 2A-adrenoceptors in the clonidine action on GH release.

Involvement of brain catecholamines and acetylcholine in growth hormone hypersecretory states. Pathophysiological, diagnostic and therapeutic implications.

Secretion of growth hormone (GH) is excessive in acromegaly, but also in a number of other pathological states such as anorexia nervosa, insulin-dependent diabetes mellitus (IDDM), liver cirrhosis, depression, renal failure and GH-insensitivity syndrome. Abnormalities in the neuroendocrine control of GH secretion and/or a state of insensitivity to GH contribute to hypersecretion of GH in these states, with the possible exception of acromegaly, which appears to be a primary pituitary disease. GH hypersecretion may also occur in neonates or adolescents with tall stature, thus reflecting particular physiological or paraphysiological conditions. In the cohort of brain neurotransmitters, catecholamines and acetylcholine reportedly play a major role in the control of neurosecretory GH-releasing hormone (GHRH) and somatostatin (SS)-producing neurons, and hence GH secretion. Activation of alpha 2-adrenoceptors or of muscarinic cholinergic receptors in the hypothalamus stimulates GH release, probably through stimulation of GHRH and inhibition of SS release, respectively. Activation of dopamine receptors likewise stimulates GH release, while activation of beta-receptors inhibits GH release through stimulation of hypothalamic SS function. This review discusses the involvement of brain catecholamines and acetylcholine in GH hypersecretory states, including anorexia nervosa, acromegaly, IDDM, liver cirrhosis, depression, renal failure and GH insensitivity syndrome, with a view to providing a fuller understanding of their pathophysiology and, whenever possible, diagnostic and therapeutic implications.

Neuroactive steroids modulate GABA inhibition of hypothalamic somatostatin release.

The reduced steroids 3 alpha-hydroxy-5 alpha-pregnan-20-one (allopregnanolone) and 3 alpha,21-di-hydroxy-5 alpha-pregnan-20-one (allotetrahydroDOC) are potent ligands of GABAA receptors. This study examined the possible modulatory effect of these metabolites on GABA inhibition of somatostatin release in cultured hypothalamic neurones. Allopregnanolone potentiates GABA inhibition, and reversed picrotoxin and bicuculline-induced augmentation of somatostatin release in a dose-dependent manner. AllotetrahydroDOC also inhibits the stimulated release induced by the antagonists, but did not modify that induced by depolarizing concentrations of K+. Pregnenolone sulphate had no effect on picrotoxin-induced somatostatin release. These findings clearly establish that 3 alpha-hydroxysteroids modulate GABA inhibition of hypothalamic somatostatin release.

Gastrointestinal motor and secretory responses to cholinergic stimulation in humans. Differential modulation by muscarinic and cholecystokinin receptor blockade

The present study investigated how a cholinergic agonist modifies interdigestive motility and secretion of the upper gastrointestinal tract and how muscarinic and cholecystokinin receptor blockade interfere with this direct cholinergic stimulation. In eight healthy volunteers, gastrointestinal motor and secretory responses to bethanechol (12.5, 25, and 50 micrograms kg-1 h-1) with and without a background of atropine (5 micrograms kg-1 h-1) or loxiglumide (10 mg kg-1 h-1) were studied. Stepdoses of bethanechol caused a parallel stimulation of antroduodenal motility and gastropancreatic secretion (P < 0.01) without inducing a fed pattern. However, duration of phase I was shortened (P < 0.05). Only high doses of bethanechol enhanced gastrin (P < 0.05), cholecystokinin (P < 0.05), and pancreatic polypeptide (P < 0.01) release. Atropine completely antagonized motor and secretory responses to cholinergic stimulation. Loxiglumide left cholinergically stimulated motility and pancreatic enzyme secretion unaltered. With co-infusion of bethanechol and loxiglumide, PP release dropped by 63% (P < 0.01); gastric acid output, gastrin and CCK release increased by 56%, 16%, and 25%, respectively (P < 0.05). We conclude that stimulation by a cholinergic agonist preserves the interdigestive pattern. Low dose muscarinic receptor blockade abolishes cholinergic stimulation over the full dose range. Inhibition of somatostatin release would explain stimulation of gastrin release and gastric acid secretion with co-infusion of bethanechol and loxiglumide. Endogenous CCK appears to interact with direct cholinergic stimulation at the pancreatic PP cell and the gastric D-cell but not at pancreatic acinar and antroduodenal smooth muscle cells.

Blunted GH Response to Growth Hormone-Releasing Hormone (GHRH) Alone or Combined with Arginine in Non-Insulin-Dependent Diabetes Mellitus

An increased spontaneous and stimulated growth hormone (GH) secretion is well documented in insulin-dependent diabetes mellitus. On the contrary, in non-insulin-dependent diabetes mellitus (NIDDM) conflicting results arise from literature. In 14 patients with NIDDM, 7 normal weight (NWD) and 7 obese (OD), we investigated the somatotrope responsiveness to GHRH (1 microgram/kg) alone or combined with arginine (ARG, 0.5 g/kg), which is able to enhance the GH response to GHRH, probably by inhibiting somatostatin release from hypothalamus. Baseline IGF-I, IRI FFA and glucose levels were also determined. Twelve healthy normal subjects (NS) and 12 obese patients (OP) were evaluated as control groups. GH but not IGF-I levels were higher (p < 0.05) in NS than in OP (1.5 +/- 0.5 vs 0.5 +/- 0.2 microgram/l). Insulin levels were higher (p < 0.05) in OP than in NS, NWD and OD (18.7 +/- 1.8 vs 8.7 +/- 0.5, 6.4 +/- 1.9 and 11.8 +/- 1.2 microU/l). FFA were higher (p < 0.05) in NWD. OD and OP than in NS (0.69 +/- 0.04, 0.70 +/- 0.04 and 0.65 +/- 0.06 vs 0.39 +/- 0.03 mmol/l). Plasma glucose was higher (p < 0.05) in diabetic patients than in normal and obese subjects. GH responses to GHRH in NWD, OD and OP were similar (AUC: 221.6 +/- 33.3, 206.0 +/- 35.9 and 177.2 +/- 57.3 micrograms/l/min, respectively) and all lower (p < 0.05) than that in NS (776.7 +/- 206.5 micrograms/l/min). ARG determined a significant increase of GHRH-induced GH release in all groups (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Splanchnic neural regulation of somatostatin secretion in the isolated perfused human pancreas.

The somatostatin-secreting delta cells in the islets of Langerhans appear to be regulated by neural mechanisms that have not been defined clearly. In this study, the celiac neural bundle of the human pancreas was electrically stimulated in the presence and absence of selective neural antagonists. The authors previously reported on studies of the splanchnic neural regulation of insulin, glucagon, and pancreatic polypeptide secretion. In these studies, alpha-adrenergic fibers appeared to have a predominant effect, strongly inhibiting the secretion of insulin, glucagon, and pancreatic polypeptide secretion. Cholinergic fibers appeared to stimulate strongly, although beta-adrenergic fibers weakly stimulated, the secretion of these hormones. Investigations of neural regulatory mechanisms governing human somatostatin release in vitro have not been previously reported. Pancreata were obtained from eight cadaveric organ donors. The isolated perfused human pancreas technique was used to assess the regulation of somatostatin secretion by the various neural fibers contained within the celiac plexus. The secretory response of somatostatin was examined in the presence of 16.7 mmol/L glucose, with and without neural stimulation, and specific neural antagonists. The basal somatostatin secretion was 88 +/- 26 fmol/g/min and increased 131 +/- 23% (n = 8, p < 0.01) in response to 16.7 mmol/L glucose. The augmentation seen with glucose was inhibited 66 +/- 22% (n = 8, p < 0.05) during celiac neural bundle stimulation. Alpha-adrenergic blockade resulted in a 90 +/- 30% (n = 6, p < 0.01) augmentation of somatostatin release. Beta-adrenergic blockade caused a 13 +/- 2% (n = 6, p < 0.05) suppression of somatostatin release. Complete adrenergic blockade resulted in a 25 +/- 23% (n = 5, p = not significant) inhibition of somatostatin release. Cholinergic blockade resulted in a 40 +/- 10% (n = 6, p < 0.02) suppression of somatostatin release. The predominant effect of celiac neural bundle stimulation was inhibition of somatostatin secretion through an alpha-adrenergic effect. Beta-adrenergic fibers stimulate somatostatin secretion; cholinergic fibers have a negligible effect on somatostatin secretion. These data suggest that the splanchnic innervation of the pancreas has a potent regulatory role in somatostatin release in this in vitro human model.

GABA inhibition of somatostatin gene expression in cultured hypothalamic neurones

GABAA receptors mediate the inhibition of somatostatin release in hypothalamic neurones. To study the possible effect of GABA on somatostatin biosynthesis, somatostatin and preprosomatostatin mRNA levels were evaluated after exposure of hypothalamic neurones to muscimol or bicuculline. Muscimol (50 microM) decreased preprosomatostatin mRNA levels, by 25% after 4 h and 30% after 24 h treatment. Bicuculline (50 microM and 100 microM) increased preprosomatostatin mRNA levels by 1.4 and 1.5 fold after 4 h and by 1.3 and 1.7 fold after 24 h treatment. Somatostatin content was not modified after muscimol or bicuculline exposure. Total DNA content, used to control cellular viability, was not modified under any experimental conditions. Our findings suggest that the GABAergic inhibition of mRNA levels could be a consequence of the GABA inhibition of somatostatin release, thus allowing limited changes in peptide steady-state levels.

Effects of aging on gastrin and somatostatin secretion from isolated perfused rat stomach.

We have examined the release of gastrin and somatostatin from the isolated perfused stomach of rats of three different age groups (4 months, 12 months, and 24 months old) in response to bombesin and carbachol. The basal release of gastrin was diminished in 24-month-old rats. Basal somatostatin release showed an age-related decrease. Bombesin (10(-10) and 10(-9) M) and carbachol (10(-8) and 10(-7) M) stimulated gastrin release in each age group. The integrated release of gastrin in response to bombesin (10(-10) and 10(-9) M) or carbachol (10(-8) M) did not differ among the three age groups, although integrated gastrin release in response to carbachol (10(-7) M) decreased in 24-month-old rats. Bombesin-stimulated release of somatostatin decreased in 12- and 24-month-old rats. Carbachol inhibited release of somatostatin in each age group. Compared with 4-month-old rats, the inhibition of somatostatin release by carbachol was less in 24-month-old rats at 10(-8) and 10(-7) M, and less in 12-month-old rats at 10(-7) M. The decreased basal gastrin secretion and well-preserved gastrin response were further confirmed in conscious aged rats tested by means of oral gavage with 10% peptone. Our findings indicate that gastrin response to the stimuli is well preserved with aging, whereas the response of somatostatin diminishes in an age-related manner. Aging has different effects on the release of gastrin and somatostatin from the rat stomach.