Characterisation of somatostatin release from the pancreas: the role of calcium and acetylcholine

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

AIMS/HYPOTHESIS

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS/INTERPRETATIONS

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.

Oscillatory Ca2+ signaling in somatostatin-producing cells from the human pancreas

Oscillatory Ca2+ signaling was studied in human somatostatin-releasing pancreatic delta cells identified by immunostaining. A ratiometric fura-2 technique was used for measuring cytoplasmic concentrations of Ca2+ and Sr2+ in delta cells exposed to the respective cation. Rhythmic activity in terms of slow (frequency, 0.1 to 0.4 per minute) oscillations from close to the basal level was seen in the presence of 3 to 20 mmol/L glucose during superfusion with medium containing 2.6 to 5 mmol/L Ca2+ or 5 mmol/L Sr2. These oscillations could be transformed into a sustained increase by decreasing extracellular Ca2+ or adding 1 mmol/L tolbutamide or 20 nmol/L glucagon. Addition of glucagon to a medium containing 20 mmol/L glucose resulted in the generation of short (< 30 seconds) transients, which disappeared upon exposure to 100 nmol/L of the intracellular Ca(2+)-adenosine triphosphatase (ATPase) inhibitor thapsigargin. When analyzing small aggregates of islet cells, it became evident that oscillatory activity in delta cells can be synchronous with that in adjacent non-delta cells. It is concluded that secretion of pancreatic somatostatin in man involves Ca2+ signaling similar to that regulating the pulsatile release of insulin.

Presynaptic modulation by VIP, secretin and isoproterenol of somatostatin release from enriched enteric synaptosomes: role of cAMP

The release of somatostatin-like immunoreactivity was studied in isolated synaptosomes. A significant release of somatostatin-like immunoreactivity was observed in the presence of vasoactive intestinal polypeptide (VIP) (10(-6) M: 53.0 +/- 12.4 pg/mg, basal: 14.3 +/- 1.7 pg/mg, n = 5, P < 0.05), secretin (10(-6) M: 56.1 +/- 3.8 pg/mg, basal: 25.8 +/- 1.6 pg/mg, n = 6, P < 0.01) and isoproterenol (10(-5) M: 54.0 +/- 13.4 pg/mg, basal: 20.0 +/- 3.4 pg/mg, n = 8, P < 0.05). Forskolin, an unspecified activator of the adenylate cyclase, caused a significant release of somatostatin-like immunoreactivity (10(-6) M: 57.3 +/- 13.2 pg/mg, basal: 30.0 +/- 5.8 pg/mg, n = 13, P < 0.01) which was further augmented in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX 10(-4) M) (77.0 +/- 17.8 pg/mg, n = 13, P < 0.01). 3-Isobutyl-l-methylxanthine and N6, 2'-O-dibutyryladenosine-3',5'-cyclic monophosphate mimicked at effect of forskolin and VIP. The release of somatostatin was paralleled by an increase of cAMP immunoreactivity in the presence of VIP (10(-6) M: 37.1 +/- 9.4 pmol/mg, basal: 19.8 +/- 4.2 pmol/mg, n = 10, P < 0.05), isoproterenol (10(-5) M: 42.4 +/- 9.8 pmol/mg basal: 16.7 +/- 2.4 pmol/mg, n = 12, P < 0.01) and forskolin (10(-6) M: 47.1 +/- 12.4 pmol/mg, basal: 19.8 +/- 4.2 pmol/mg, n = 10, P < 0.01). The effect of nitric oxide (NO) which acts as an inhibitory neurotransmitter in the enteric nervous system was studied. NO is known to activate guanylate cyclase to induce transmitter release. The NO-generating compound sodium nitroprusside and bromoguanosine-3',5'-cyclic monophosphate (8-Br-cGMP) had no effect on the release of somatostatin-like immunoreactivity. These data demonstrate the stimulatory effect of VIP, secretin and isoproterenol on release of somatostatin-like immunoreactivity from enteric synaptosomes, which is presumably mediated by cAMP-dependent mechanisms. cGMP-dependent mechanisms seem to be of minor relevance.

Stimulation of somatostatin secretion by 3-O-methylglucose in the perfused dog pancreas

CONCLUSION

3-O-methylglucose stimulates somatostatin secretion from the dog pancreas by a glucose-dependent and glucose-like effect. Therefore, it is possible that 3-O-methylglucose-stimulated somatostatin secretion is dependent on glucose metabolism.

BACKGROUND

Somatostatin secretion from the endocrine pancreas is stimulated by glucose, glyceraldehyde, and dihydroxyacetone but not affected by fructose, galactose, or ribose. Whether the nonmetabolizable glucose analog, 3-O-methylglucose affects somatostatin secretion is, however, not known.

METHOD

We therefore, examined whether the glucose analog affects somatostatin secretion in the perfused dog pancreas.

RESULTS

We found that when added to a medium containing 2.7 mM or 5.5 mM D-glucose, 3-O-methylglucose (10 mM) stimulated somatostatin secretion to the same extent as did an equivalent dose of D-glucose. The same stimulation was observed also with arginine at 2.5 mM in the perfusion medium. In contrast, 3-O-methylglucose did not stimulate somatostatin secretion in the absence of glucose in the perfusion medium. Mannoheptulose (5 mM), which inhibits glucose metabolism, completely blocked the secretion to both hexoses.

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

OBJECTIVE

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.

SUMMARY BACKGROUND DATA

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Gastrin releasing peptide stimulates the secretion of insulin, but not that of glucagon or somatostatin, from the isolated perfused dog pancreas

Gastrin releasing peptide (GRP) is an intrapancreatic peptide, but its physiological function is unknown. Previously, the peptide has been shown to increase plasma levels of insulin and glucagon in vivo in dogs, but no studies on the possible direct actions on islet hormone secretion from the dog pancreas have been undertaken. Therefore, we examined the effects of a 10-min perfusion of synthetic porcine GRP at four different dose rates over a wide range (0.1-50 nmol l-1) on the islet hormone release from the isolated dog pancreas (n = 5-6 in each group) at 5.5 mM glucose. We found that, at all four concentrations tested, GRP rapidly and markedly stimulated insulin secretion. The stimulation was, however, transient: the increased insulin secretion returned to basal levels within 7-8 min despite the ongoing GRP perfusion for 10 min. In contrast, GRP did not affect the pancreatic secretion of glucagon or somatostatin. We conclude that GRP stimulates insulin secretion by a direct pancreatic action without affecting the secretion of glucagon or somatostatin.

Dual effects of calcitonin gene-related peptide on insulin secretion in the perfused dog pancreas

Calcitonin gene-related peptide (CGRP) is an intrapancreatic neuropeptide with potential effects on islet hormone secretion. To investigate its pancreatic actions, we examined the effects of a 10 min perfusion of synthetic human CGRP on islet hormone release from the isolated dog pancreas (n = 6) at 5.5 mM glucose. At 0.1 nM, CGRP inhibited insulin secretion (P less than 0.01), which was already observed at 2 min after its introduction. After CGRP perfusion was stopped, a stimulatory off-response occurred. In contrast, at higher dose levels, CGRP stimulated insulin secretion. At 1.0 nM, the stimulation was weak and transient (P less than 0.01), occurring only during the first 3 min of CGRP perfusion. At 10 nM, the stimulation continued for 6 min (P less than 0.05), and at 50 nM, the stimulation was marked and sustained throughout the 10 min perfusion period (P less than 0.01). After the CGRP perfusion at 1.0 and 10 nM, but not at 50 nM, a marked stimulatory off-response in insulin secretion was seen. Glucagon and somatostatin secretion were not significantly affected by CGRP at any of the examined concentrations. We conclude that CGRP exerts dual effects on insulin secretion from the perfused dog pancreas: inhibition at low concentrations and stimulation at high concentrations. This pattern of effect might represent a new regulatory concept for neural influences on islet function: the qualitative response being determined by the amount of neurotransmitter released.

Stimulative effects of TMB-8 and trifluoperazine on pancreatic hormone release

The effects of 8-N-N-diethylamino octyl 3,4,5-trimethoxybenzoate (TMB-8) and trifluoperazine (TFP) on the early phase (10 min) of the release of pancreatic hormones from isolated rat islets were investigated. TMB-8 and TFP stimulated insulin, glucagon, and somatostatin release in a dose-dependent manner at a low glucose concentration (2.5 mM). The levels of glucagon and somatostatin release were also stimulated by these two agents at a high glucose concentration (10 mM). Their effects were independent of external calcium ion level. These two agents did not modify insulin release at the high glucose concentration. The stimulative effects of the two agents on the release of these hormones were partially suppressed when the islets were pretreated with 6-hydroxydopamine (6-OHDA), a chemical adrenergic denervator that acts at nerve endings. In this situation, the norepinephrine (NE) released from pancreatic islets decreased to 44% of that of non-treated islets (P less than 0.01). The addition of NE (10(-9) M) to the incubation medium increased insulin, glucagon, and somatostatin secretion by 20-30% over control levels (P less than 0.05). In conclusion, the early phase of pancreatic hormone release was stimulated by TMB-8 and TFP. Our results strongly suggest that these two drugs could be mediated by the NE released from nerve endings in the islets.

Effect and mechanism of vagal nerve stimulation on somatostatin secretion in dogs

To investigate the effect of vagal nerve stimulation on the release of pancreatic somatostatin, we electrically stimulated (10 Hz, 5 ms, 13.5 mA, and 10 min) the thoracic vagi just below the heart in halothane anesthetized dogs (n = 15). The stimulation increased the pancreatic output of somatostatinlike immunoreactivity (SLI) (delta = +248 +/- 81 fmol/min, P less than 0.005; base-line levels = 455 +/- 150 fmol/min). min). Arterial plasma SLI levels increased as well (delta = +16 +/- 3 fmol/ml, P less than 0.001; base-line levels = 65 +/- 3 fmol/ml), reflecting stimulation of extrapancreatic SLI secretion. Significant vagal activation was verified by a fivefold increase of pancreatic output of pancreatic polypeptide (PP) (delta = +31.4 +/- 5.9 ng/min, P less than 0.001; base-line levels = 7.8 +/- 0.9 ng/min). Atropine pretreatment (n = 6) inhibited partially both the PP response (delta = +7.9 +/- 3.8 ng/min after atropine) and the pancreatic SLI response (delta = +92 +/- 29 fmol/min) to vagal nerve stimulation. However, atropine pretreatment did not modify the arterial SLI response (delta = +20 +/- 7 fmol/ml). Hexamethonium pretreatment (n = 9) completely abolished all three responses. We conclude that 1) electrical stimulation of the vagus stimulates pancreatic SLI, extrapancreatic SLI, and PP release in vivo in the dog; 2) both muscarinic and nonmuscarinic mechanisms mediate the PP and pancreatic SLI responses; 3) a nonmuscarinic mechanism mediates the extrapancreatic SLI response; and 4) all three responses are mediated via ganglionic nicotinic receptors.

Physiological and pathophysiological aspects of somatostatin

Somatostatin is found in the D-cells of organs that are exclusively responsible for the digestion, absorption, and metabolism of ingested nutrients. D-cells apparently release their secretory products both into the interstitial space (paracrine action) and into the circulation (endocrine action). Ingestion of all three basic nutrients--fat, carbohydrate, and particularly protein--elicits a significant increase in peripheral vein plasma somatostatin levels in dogs and humans. Acidification of a meal stimulates somatostatin release in dogs. Vagal, cholinergic, and adrenergic mechanisms exert a species-dependent effect on somatostatin release. Gut hormones also participate in the regulation of postprandial somatostatin release, and endogenous opioids have an effect that depends on the composition of the meal. Stimulation of postprandial somatostatin release by H2-receptor agonists and prostaglandins has been reported. Insulin inhibits and glucagon stimulates somatostatin release. Elevated levels of circulating glucose reduce the somatostatin response, an effect that cannot be entirely explained by the parallel augmentation of insulin secretion. Circulating nutrients also modify the effect of gut hormones on D-cell function. The physiological action of somatostatin is an inhibitory effect on virtually all gastrointestinal and pancreatic exocrine and endocrine functions. Secretory and/or motor activities are attenuated, thereby preventing an exaggerated and overshooting response. Alterations of tissue somatostatin content and plasma somatostatin levels have been observed in obesity and suggest that somatostatin deficiency may be a pathogenic factor. The observed changes of somatostatin may be secondary to alterations of other functions; nevertheless, hyposomatostatinaemia might facilitate nutrient assimilation.

Effects of a thiazide diuretic (hydroflumethiazide) and a loop diuretic (bumetanide) on the endocrine pancreas: studies in vitro

Treatment with thiazide diuretics causes an impairment of the glucose metabolism. To study whether this is due to a direct effect on the endocrine pancreas, the effects of the thiazide hydroflumethiazide on the release of glucagon, insulin, and somatostatin from the isolated perfused pancreas of normal and alloxan diabetic dogs were examined. Hydroflumethiazide at concentrations ranging from 1 to 50 micrograms/mL stimulated the normal secretion of glucagon (P less than 0.001), insulin (P less than 0.001), and somatostatin (P less than 0.001) in a dose-dependent manner. The normal hormone responses evoked by 50 micrograms/mL of the thiazide were, however, modified by the prevailing glucose level: higher insulin (P less than 0.05) and somatostatin (P less than 0.05) and lower glucagon (P less than 0.05) were obtained at the high glucose concentration of 11 mmol/L rather than at the low glucose concentration of 1.3 mmol/L. In alloxan diabetes, insulin secretion was almost extinct and did not respond to hydroflumethiazide, whereas glucagon was dose-dependently stimulated (P less than 0.001). In addition, we looked at the effect of the loop diuretic, bumetanide. The infusion of bumetanide at doses ranging from 0.5 to 3 micrograms/mL did not alter the release of glucagon, insulin, and somatostatin in the presence of 5.5 mmol/L glucose. The results suggest that hydroflumethiazide possesses the ability to directly stimulate A cell secretion in the normal and alloxan diabetic pancreas. Whether this effect is of clinical importance for the diminution in glucose tolerance observed during thiazide therapy remains, however, uncertain.

Somatostatin and the cerebral cortex

A unique subset of interneurons which are rich in immunoreactive somatostatin (IRS) exists in the cerebral cortex. The regulation of IRS secretion by these cells is reviewed. Acetylcholine, glutamic acid and several neuropeptides including VIP, CCK, and metenkephalin have been identified as IRS secretagogues. The types of molecules which stimulate IRS release, the electrophysiologic effects of somatostatin, and the recognition of abnormal IRS levels in human CNS diseases were all used to formulate a working model of the role of the somatostatinergic cell in ongoing cerebral cortical function.

Stimulation by calcium and barium of somatostatin release. Evidence for lower sensitivity of D- vis-à-vis B- and A-cells

To address the question whether a "second messenger" function of calcium differs between D-cells and other cells of the endocrine pancreas, we compared effects of calcium and barium (a calcium substitute) on somatostatin secretion to effects on insulin and glucagon secretion from the perfused pancreas of the rat. 6.5 mmol/l of calcium, when administered early during perfusion, failed to stimulate somatostatin release. 0.05 mmol/l of barium, when added to calcium-deprived media failed to affect somatostatin secretion while 0.5 induced a slight and 2.0 mmol/l a marked and sustained response. Barium-induced insulin release was left-shifted in relation to the somatostatin response, since 0.05 mmol/l of barium stimulated and 0.5 mmol/l evoked a near-maximal insulin response. All concentrations of barium evoked diphasic glucagon responses, i.e. a small (1 min) stimulation followed by sustained inhibition. Addition of 0.5 mmol/l of EGTA to calcium-deprived media abolished D- as well as B- and A-cell secretion. Reintroduction of 0.5-6.5 mmol/l of calcium stimulated somatostatin release; the secretory response was proportionate to the calcium concentration. In contrast, addition of calcium stimulated insulin and glucagon secretion maximally already at 0.5 mmol/l of calcium. We conclude that the D-cell is less sensitive than B- and A-cells to a regulatory effect on secretion exerted by extracellular calcium or barium.

A dual role of calcium in release of somatostatin from canine gastric antral mucosa

We have examined the factors regulating the mucosal release of somatostatin-like immuno-reactivity (SRIF-LI) from 1-cm2 sheets of isolated canine gastric antral mucosa mounted in a Ussing chamber. SRIF-LI was released predominantly into the luminal perfusate, was maximal at pH 2.5, 1,987 +/- 319 pg X ml-1 X h-1, and reached a nadir at pH 6.0 of 89 +/- 24 pg X ml-1 X h-1. Increasing extracellular Ca2+ to 10 mM stimulated the release of SRIF-LI at both high and low pH. The Ca2+ ionophore A23187 had no apparent effect at either pH 2.5 or 7.0. LaCl3 stimulated the release of SRIF-LI at pH 7.0 but not at pH 2.5. Ouabain and TMB-8 had no significant effect on the release of SRIF-LI. At pH 7.0, trifluoperazine (TFP) stimulated release of SRIF-LI (80 +/- 10 pg X ml-1 X h-1). EGTA stimulated release of SRIF-LI at pH 2.5 (1,134 +/- 137 pg X ml-1 X h-1) and at pH 7.0 (300 +/- 57 pg X ml-1 X h-1), which was reversed by replacement of Ca2+ (22 +/- 6 pg X ml-1 X h-1). Thus Ca2+ appears to exert a dual effect on the release of SRIF-LI: both an increase and depletion of extracellular Ca2+ release SRIF-LI.

Is phospholipase A2 a "glucose sensor" responsible for the phasic pattern of insulin release?

The mechanisms involved in the characteristic, normal biphasic pattern of glucose-induced insulin release (which is grossly altered in type II diabetics) have not been definitely elucidated. However, the temporal pattern of arachidonic acid release induced by cellular phospholipases precisely mimics that of first-phase insulin release, both being characterized by a burst of release peaking near 2-3 minutes and followed by a "trough" or apparent refractory period most apparent at 5-10 minutes. The latter appears temporally related not only to decreased arachidonate release but also to stimulation of its re-esterification. Pancreatic islets contain a glucose-sensitive phospholipase A2, and glucose has been shown to increase the accumulation of islet lipoxygenase-derived products which appear to be "third messengers" mediating insulin release. Blockade either of islet phospholipase(s) or of islet lipoxygenase totally abrogates glucose-induced insulin release. The hypothesis is therefore proposed that phospholipase A2 could be one beta cell "glucose sensor", and that the released arachidonate is coupled to an islet lipoxygenase. Labile oxygenated metabolites (lipid peroxides and epoxides) transduce the glucose signal into insulin release. The available data (albeit incomplete) are compatible with the formulation that the biphasic pattern of glucose-induced insulin release could be explained by dynamic changes in the availability of arachidonic acid and its consequent oxygenation.

Autonomic nervous control of pancreatic somatostatin secretion

We studied the autonomic nervous control of pancreatic somatostatin secretion using isolated perfused pig pancreases prepared with either intact vagal or splanchnic nerve supply. Electrical stimulation of the vagus nerves increased pancreatic protein output 59-fold, whereas somatostatin output decreased to 57% of prestimulatory secretion. Acetylcholine mimicked the somatostatin response to vagal stimulation, and atropine abolished the inhibition. Splanchnic nerve stimulation increased perfusion pressure up to threefold, whereas somatostatin output decreased to 68%. Phenoxybenzamine abolished the pressure response to splanchnic nerve stimulation and reversed the inhibition to a 20% increase in somatostatin output. Propranolol did not influence the inhibitory effect of splanchnic stimulation but abolished the increase seen after phenoxybenzamine. It is concluded that both divisions of the autonomic nerve supply to the pancreas are inhibitory to somatostatin secretion, but increased secretion may be brought about by a beta-adrenergic mechanism.

Somatostatin release from dispersed hypothalamic cells - effects of membrane depolarization, calcium and glucose deprivation

We have developed a new short term in vitro system to examine hypothalamic somatostatin (SRIF) release. Hypothalamic cells were obtained from normal rats after trypsin or collagenase aided dispersion and released immuno-reactive (IR) SRIF which eluted in 3 molecular weight (MW) forms on gel chromatography. The smallest MW form, which constituted the major peak, co-eluted with synthetic cyclic 1-14 SRIF on gel and reverse phase high pressure liquid chromatography (HPLC). After 24 h in culture in medium containing heat inactivated fetal calf serum, cell viability was demonstrated by two techniques, (1) vital staining with trypan blue, and (2) incorporation of 32Pi into phospholipids. SRIF release was also studied at this time which was optimal in terms of responsivity of the cells to depolarizing stimuli. SRIF release increased in a time dependent manner, over 3 h. Membrane depolarization, induced either by potassium chloride 56 mM or ouabain (the Na+, K+-ATPase inhibitor) 10(-6) M or greater, markedly stimulated SRIF release. Incubation at 4 degrees C, or in the presence of EDTA 0.05 M or verapamil, the calcium channel blocker, 50 microM abolished these stimulatory effects. Glucose deprivation was induced by the addition of 2-deoxy-D-glucose (2-DG) to the experimental medium. 2-DG, at concentrations of up to 200 mg%, had no significant effect on SRIF release during incubation periods of up to 1 h.

Isolated perfused Brockmann body as a model for studying pancreatic endocrine secretion

The release of insulin, glucagon, and somatostatin was investigated in a newly developed in vitro perfusion preparation of the splenic Brockmann body (principal pancreatic islet) of the channel catfish (Ictalurus punctatus). The preparation was viable for up to 7 h by functional and morphological criteria. Glucose evoked a biphasic release of insulin and somatostatin, but had no effect on the release of glucagon. Arginine at basal glucose concentration caused a marked release of insulin and glucagon, but only a minor release of somatostatin.

Adrenergic modulation of pancreatic somatostatin, insulin, and glucagon secretion: evidence for differential sensitivity of islet A, B, and D cells

To characterize the adrenergic effects of epinephrine on somatostatin, insulin and glucagon release and to assess the potential interactions of islet A, B, and D cell function, isolated rat islets were incubated in vitro with epinephrine (0.05-20 microM) in the presence and absence of the alpha adrenergic antagonist, phentolamine (2 or 4 microM), and/or the beta adrenergic antagonist, propranolol (2 microM). At concentrations of epinephrine at or less than 1 microM, somatostatin and insulin release were inhibited while glucagon release was unaffected. At greater epinephrine concentrations, somatostatin and glucagon release were increased while insulin release was further suppressed. The threshold as well as the half-maximal effect of epinephrine occurred at lower concentrations for somatostatin release than for insulin and glucagon release. The inhibitory effect of 0.5 microM epinephrine on somatostatin and insulin release was completely reversed by phentolamine and was unaffected by propranolol. The stimulatory effect of 2 and 20 microM epinephrine on somatostatin and glucagon release was not observed when propranolol was included in the incubation medium along with epinephrine. These results demonstrate that rat islet A, B, and D cells differ in their sensitivity to alpha and beta adrenergic effects. At low concentrations of epinephrine, alpha adrenergic effects on D cells predominate over beta adrenergic effects whereas at greater concentrations of epinephrine alpha and beta effects appear to be equal; alpha adrenergic effects of epinephrine predominate over those of beta on the B cell at least up to 20 microM epinephrine: exclusively beta adrenergic effects of epinephrine are observed on the A cell at least up to 20 microM epinephrine.

Difference in calcium dependency of insulin, glucagon and somatostatin secretion in response to glibenclamide in perfused rat pancreas

The extracellular calcium requirements for insulin, glucagon and somatostatin release induced by 1 microgram/ml of glibenclamide have been compared in the perfused, isolated rat pancreas. In the absence of glucose, the drug evoked insulin release equally well at physiological (2.6 mmol/l) and low (0.25 mmol/l) levels of total calcium. In contrast, glibenclamide evoked somatostatin release at 2.6 but not at 0.25 mmol/l of calcium. At 2.6 mmol/l of calcium, glibenclamide evoked bimodal effects (stimulation followed by inhibition) on glucagon secretion. At 0.25 mmol/l of calcium, basal secretory rates of glucagon were elevated and a small stimulatory effect of glibenclamide was seen. Addition of 0.5 mmol/l of EGTA to media with low calcium concentrations uniformly abolished the A, B and D cell secretory responses to glibenclamide. The possible modulation of calcium dependency by a non-stimulatory concentration of glucose was tested by its addition at 3.3 mmol/l to the perfusion media. Glucose enhanced glibenclamide-induced insulin secretion, both at 0.25 and 2.6 mmol/l of calcium. However, at 0.25 mmol/l of calcium, the enhancing effect of glucose was more pronounced than at 2.6 mmol/l. At 2.6 mmol/l of calcium, glucose diminished the somatostatin and abolished the glucagon response to glibenclamide. At 0.25 mmol/l of calcium, glucose did not influence somatostatin release while the presence of the sugar diminished basal and glibenclamide-induced glucagon secretion. The present data confirm the requirement of extracellular calcium for A, B and D cell secretion, demonstrating different calcium dependencies for the cell types and indicate that this dependency can, in part, be modulated by glucose.

Effects of neurotransmitters and cyclic AMP on somatostatin release from cultured cerebral cortical cells

The influence of cortical neurotransmitters and cyclic AMP on the release of immunoreactive somatostatin (IRS)from cultured cortical cells was examined. Cells were obtained by mechanoenzymatic dispersal of telencephalons of 17-day-old rat embryos and were maintained as monolayers in minimum essential medium with 10% heat-inactivated horse serum. After the cultures had stabilized morphologically and in cellular IRS content they were subjected to rapid sequential changes of a buffered salt solution with or without test substances added. The amount of somatostatin released was measured by a specific radioimmunoassay. Acetylcholine and the GABA antagonist, picrotoxin, both stimulated IRS release. The cholinergic stimulation was predominantly muscarinic. GABA and histamine, to a lesser extent, were inhibitory and norepinephrine and serotonin produced no net change in IRS release. Both cAMP and theophylline (DMX) stimulated IRS release. These results confirm the potential of intrinsic cortical somatostatinergic neurons to respond to endogenous neurotransmitters and further establishes somatostatin as a cortical neuromodulator.

Somatostatin, insulin, and glucagon secretion from isolated perfused pancreas of obese rats

A comparison of the somatostatin with the insulin and glucagon secretions in hypothalamic obesity and genetic obesity was made using the isolated perfused pancreas of rats. In our perfusion experiment, the somatostatin response to 19 mM arginine in the presence of 4.4 mM glucose was significantly greater in both ventromedial hypothalamus (VMH)-lesioned and Zucker fa/fa rats than in their controls, as was the perfusate insulin. The perfusate arginine-stimulated glucagon secretion appeared no different in obese and control rats. Because hyperinsulinemia in vivo and hyperresponses to arginine of perfusate insulin and somatostatin were observed in both VMH-lesioned and Zucker fa/fa rats, whereas the perfusate glucagon secretion in the presence of 4.4 mM glucose was unchanged by obesity, the secretory behavior of some pancreatic hormones seems similar in VMH-lesioned and Zucker fa/fa rats in certain conditions. These results suggest that some abnormalities of pancreatic hormone secretion may be caused by a mechanism common to obesity, whether caused experimentally or genetically.

Release of somatostatin-like immunoreactivity from the perfused canine thyroid. Selective stimulatory effect of calcium ions

It is well accepted that the C cells of the thyroid contain somatostatin, but the role in local endocrine function has not yet been firmly established in this organ, and it has not been proved that thyroidal somatostatin is released into the circulation. We have measured the contents of somatostatin-like immunoreactivity in the effluent of canine thyroid glands perfused without recirculation with a synthetic buffer medium. During basal conditions a definite release was consistently found in the order of 10 pg/ml corresponding to 12 pg/min. The somatostatin-like immunoreactivity was studied in dilution experiments and by gel-filtration chromatography, and found to have properties identical to those of synthetic cyclic somatostatin, which was also recovered quantitatively when added to sampling tubes. Various compounds were infused in concentrations that are highly active in pancreas perfusion experiments. 14-min infusion of arginine, 5 and 11.5 mmol/liter; isoproterenol, 10 and 23.7 nmol/liter and 68.7 mumol/liter; acetylcholine, 5 mumol/liter, carbamylcholine, 10 and 100 mumol/liter; glucagon, 1 and 30 nmol/liter; and porcine calcitonin, 1 and 100 ng/ml did not affect the basal release of somatostatin-like immunoreactivity significantly. Neither did an increase from the control level of 4 mmol/liter glucose of 10 or 20 mmol/liter, nor an increase in the control level of 4.4 mmol/liter K+ to 7.5 or 14.4 mmol/liter. Each of these compounds were tested in three or four dogs. The effect of an increase in Ca++ from the control level of 1.5 mmol/liter to 2.25, 3.0, and 4.5 mmol/liter was tested in random order in five thyroid lobes. All three doses elicited an immediate increase in effluent somatostatin-like immunoreactivity. In most experiments the response was biphasic with an early spike, followed by a stable level that was maintained during prolonged Ca++ infusion. The secretory response was not diminished through a series of repeated short pulses of calcium infusion. The response to 3.0 mmol/liter Ca++ (control period 8.4 +/- 1.5, test period 337 +/- 110 pg/ml, mean +/- SE) and 4.5 mmol/liter Ca++ (control period 9.5 +/- 1.4, test period 386 +/- 125) were significantly higher than 2.25 mmol/liter Ca++ (control period 7.2 +/- 1.0 test period 140 +/- 39), while there was no significant difference between responses to the two high doses. Infusion of salmon calcitonin, 10 ng/ml and 1 microgram/ml; or porcine calcitonin, 1 microgram/ml during calcium stimulation (2.25 mmol/liter of Ca++) did not induce alterations in the release of somatostatin-like immunoreactivity. The results demonstrate that thyroidal somatostatin is mobilizable, and it appears to be selectively sensitive to calcium stimulation, indicating a possible role in calcitonin release control.

Somatostatin and diabetes

Somatostatin, a tetradecapeptide originally isolated from the hypothalamus on the basis of its ability to inhibit the secretion of growth hormone, is now known to be widely distributed in various endocrine and gastrointestinal tissues and to have diverse actions, including inhibition of insulin and glucagon secretion. The location of somatostatin in pancreatic islet D cells suggests that it may act as a local regulator of insulin and glucagon secretion. Changes in islet D-cell function in experimentally-induced and spontaneous diabetes in animals suggest that the peptide may be involved in the pathogenesis of diabetes. Clinical studies with the peptide have provided insight into the physiologic roles of glucagon and growth hormone, and have indicated a potential therapeutic use for somatostatin in diabetes in man.

Autonomic function and control of pancreatic somatostatin

In the canine pancreas alpha and beta adrenergic receptors exist on D cells with α stimulation inhibiting and β stimulation increasing somatostatin release. There are no dopaminergic receptors on D cells. Stimulation of muscarinic receptors causes mild inhibition of somatostatin secretion. Autonomic receptors on the D cell may be physiologically stimulated in vivo via local ganglionic and/or central autonomic drivers.

Increased somatostatin secretion from pancreatic islets of streptozotocin-diabetic rats in response to glucose

Glucose stimulates somatostatin release from perifused pancreatic islets of diabetic rats 42-47 days after the induction of diabetes, and 48 h after withdrawal of insulin replacement therapy. The glucose effect is augmented by theophylline or glucagon. Basal somatostatin release and glucose-induced secretion are significantly higher in diabetic islets than in controls. It is suggested that glucose promotes somatostatin release by directly interacting with islet D cells but not via indirect pathways. Glucose-induced stimulation appears to be modulated by a D-cell adenylate cyclase/phosphodiesterase system. Reasons responsible for increased somatostatin secretion by diabetic islets include reduction in B-cell mass, suggesting that B cells may normally suppress the secretory activity of D cells.

Effect of ventromedial hypothalamic lesions on the secretion of somatostatin, insulin, and glucagon by the perfused rat pancreas

Somatostatin, insulin, and glucagon secretion by the perfused pancreas were studied in adult female rats 10 days after ventromedial hypothalamic (VMH) lesions and in sham operated controls to assess the role of their hypothalamic control. Insulin secretion was significantly greater in VMH-lesioned rats both under basal conditions and after stimulation by theophylline and arginine plus theophylline. Basal glucagon secretion was greater in VMH-lesioned rats as was the glucagon response to theophylline alone and in combination with arginine. Basal somatostatin secretion with similar in VMH and control rats but somatostatin secretion induced by theophylline and by arginine plus theophylline was significantly increased in VMH-lesioned rats. Both the pancreatic content and concentration of somatostatin were increased in VMH-lesioned rats. These results indicate the presence of hyperresponsiveness of A, B, and D cells following VMH destruction and provide new evidence for a role of the hypothalamus in the regulation of pancreatic somatostatin secretion.

Sustained oscillations of insulin, glucagon, and somatostatin from the isolated canine pancreas during exposure to a constant glucose concentration

Canine pancreata were perfused in vitro to examine whether hormone cycles could be demonstrated without hepatic or central nervous influence. Insulin, glucagon, and somatostatin demonstrated regular sustained cyclic secretion from the in vitro canine pancreas. Oscillations were noted for over 200 min during the infusion of a constant glucose concentration. Insulin demonstrated a 10-min period with a range of 8-12 min/cycle. Somatostatin had a 10-min period with a range of 8-11 min. Glucagon had a period of 8.6 min with range of 6-10 min. These periods do not allow glucagon to be consistently 90 degrees out of phase with insulin and somatostatin. When glucose was increased from 88 to 200 mg/dl, insulin cycles persisted but on an elevated base line, demonstrating that cycles react to glucose changes but are not dependent upon them. Cycles were disrupted by infusions of dopamine, apomorphine, epinephrine, and acetylcholine, but were reestablished. Autonomic blockade by both single and combined infusions of atropine (cholinergic), propranolol, and dibenzyline (adrenergic) had no effect on cycles. These results suggest that, in vitro, there is an intrinsic rhythm of hormone secretion by the pancreas despite a constant glucose level. The production of in vitro cycles requires the presence of a driving oscillator or pacemaker within the pancreas and the coordination of islets by pace-maker-islet communication, presumably by a non-adrenergic neural system. In vitro oscillations may Indicate that the pancreas is the driver or Zeitgeber of in vivo glucose-insulin cycles.

Low somatostatin content in cerebrospinal fluid in multiple sclerosis. An indicator of disease activity?

In 27 patients with multiple sclerosis (MS), and in 10 control subjects of comparable age, percent ideal body weight and sex ratio, the cerebrospinal fluid (CSF) content of somatostatin was measured by radioimmunoassay. The results showed that the group of patients in relapse (n = 16) had significantly lower somatostatin content in CSF (95 +/- 4.1 (SEM) pg/ml) than both the control group (142 +/- 8.4 pg/ml) and the group of MS patients (n = 11), who had been in a clinical stable phase for more than 6 months (131 +/- 3.2 pg/ml). Duration of the disease and degree of neurological impairment were apparently without relation to the reduction of somatostatin content in the CSF. There was no relationship between CSF content of somatostatin and the content of total protein or IgG, neither of which showed any relationship to the activity of the disease.

Differential sensitivity to somatostatin of pancreatic polypeptide, glucagon and insulin secretion from the isolated perfused canine pancreas

This dose-response study deals with the relative inhibitory effect of somatostatin on the acetylcholine-stimulated release of pancreatic polypeptide (PP), glucagon, and insulin from the isolated canine pancrease. Somatostatin in picomolar doses potently inhibited insulin and glucagon secretion, whereas PP secretion was relatively insensitive. Also, in the absence of acetylcholine, somatostatin exerted a preferential inhibition of the release insulin and glucagon compared with PP. These findings point to a physiologically important role of somatostatin for the secretion of insulin and glucagon, but probably not for PP.

Characterization of somatostatin release from the pancreas: the role of potassium

The effect of potassium on somatostatin secretion from the isolated perfused canine pancreas was studied. Potassium stimulated dose-dependent somatostatin release in a monophasic response pattern. The effect of potassium was abolished in the absence of calcium. Perfusion of 1 micronmol/l atropine and 1 micronmol/l propranolol was without effect on the potassium induced somatostatin release. The results suggest that the stimulatory effect of potassium on somatostatin release is secondary to increases in calcium influx into the D cell. The sympathetic and parasympathetic nerve endings in the pancreas are apparently not involved in the potassium mediated secretory processes.

Streptozotocin diabetes: a glucoreceptor dysfunction affecting D cells as well as B and A cells

Somatostatin release from the isolated pancreas of 3 normal and 6 streptozotocin diabetic dogs has been measured in response to various stimuli to determine whether abnormalities in somatostatin release are present in the diabetic pancreas. Simultaneous measurement of glucagon secretion was also made. In the pancreas from normal dogs increases in perfusate glucose from 25 to 200 mg/100 ml induced a 2--3 fold increase in somatostatin release and a two thirds decrease in glucagon secretion. In contrast, in the diabetic pancreas glucose caused no change in the secretion of the two hormones. In the diabetic pancreas addition of insulin to the perfusate (25,000 microU/ml) for periods from 10 to 75 minutes aimed at restoring normal extracellular insulin levels in the islets failed to restore either somatostatin or glucagon secretion to normal. In contradistinction to the lack of effect of glucose, the somatostatin and glucagon responses to arginine (5 mmol/l), isoproterenol (2 ng/ml) and calcium (5 mmol/l) were normal in the diabetic pancreas. The data suggests the presence of a selective glucoreceptor abnormality of the D as well as of B and A cells in the streptozotocin diabetic dog.