Abstract
The inhibitory effect of somatostatin (SST) on insulin secretion in vivo is attributed to a direct effect on pancreatic beta cells, but this is inconsistent with some in vitro results in which exogenous SST is ineffective in inhibiting secretion from isolated islets. We therefore investigated whether insulin secretion from the pancreatic islets may partly be regulated by an indirect effect of SST mediated via the CNS. Islet hormone secretion was assessed in vitro by perifusion and static incubations of isolated islets and in vivo by i.v. or i.c.v. administration of the SST analogue BIM23014C with an i.v. glucose challenge to conscious, chronically catheterised rats. Hormone content of samples was assessed by ELISA or RIA and blood glucose levels using a glucose meter. Exogenous SST14/SST28 or BIM23014C did not inhibit the release of insulin from isolated rodent islets in vitro, whereas peripheral i.v. administration of BIM23014C (7.5 μg) with glucose (1 g/kg) led to decreased plasma insulin content (2.3±0.5 ng insulin/ml versus 4.5±0.5 ng/ml at t=5 min, P<0.001) and elevated blood glucose levels compared with those of the controls (29.19±1.3 mmol/l versus 23.5±1.7 mmol/l, P<0.05). In contrast, central i.c.v. injection of BIM23014C (0.75 μg) had no significant effect on either plasma insulin (3.3±0.4 ng/ml, P>0.05) or blood glucose levels (23.5±1.7 mmol/l, P>0.05) although i.v. administration of this dose increased blood glucose concentrations (32.3±0.7 mmol/l, P<0.01). BIM23014C did not measurably alter plasma glucagon, SST, GLP1 or catecholamine levels whether injected i.v. or i.c.v. These results indicate that SST does not suppress insulin secretion by a centrally mediated effect but acts peripherally on islet cells.
Introduction
Somatostatin (SST) is synthesised and secreted by neuroendocrine cells in the CNS and in the gastrointestinal and immune systems (Reichlin 1983a,b). In the pancreatic islet, the peptide is produced by δ-cells, which comprise approximately 5% of the islet endocrine cells. SST exists as two biologically active forms, SST14 and SST28, which are derived by post-translational cleavage of a single prohormone. SST14 is expressed in neurones and most other SST-producing peripheral tissues, including δ-cells, while SST28 is primarily found at high levels in the gastrointestinal tract (Low 2004), and is the major form of circulating SST (Polonsky et al. 1983). The peptide often acts as an inhibitory regulator of endocrine systems, for example, as a hypothalamic factor to suppress growth hormone secretion, or as a local inhibitor of the release of gastrointestinal peptide hormones. The effect of SST is mediated through binding to SST receptors of which five different isoforms have been identified (reviewed by Moller et al. (2003)). In pancreatic islets, receptors have been identified on both α- and β-cells (Strowski et al. 2000, Cejvan et al. 2003) and exogenously administered SST inhibits both insulin and glucagon secretion (Schuit et al. 1989, Strowski et al. 2000, Cejvan et al. 2003), findings consistent with a negative regulatory role for SST in islet secretory function. In rodents, this effect is primarily mediated via receptors 5 and 2, respectively, although all five receptors are expressed in the islets (Ludvigsen et al. 2004). Results from studies using SSTR-deficient mice (SSTR1, SSTR2 or SSTR5) revealed changes in both basal and stimulated insulin secretion (Strowski et al. 2000, Wang et al. 2005, 2006), which is consistent with a role for SST in regulating hormone release. We have recently used islets from SST-deficient mice to demonstrate that SST released from the δ-cell exerts a tonic inhibitory input on hormone secretion from both α- and β-cells (Hauge-Evans et al. 2009) and it has further been shown that the release of islet SST, like insulin, is stimulated by glucose (Vieira et al. 2007, Hauge-Evans et al. 2012).
SST and SST analogues inhibit insulin secretion when administered in vivo (Lins et al. 1980) and this response to exogenous SST is generally attributed to a direct inhibitory effect on the islet β-cells. There are, however, some inconsistencies in the literature regarding the effectiveness of SST in vitro. Thus, whereas the results of some studies indicate that SST inhibits stimulus-dependent hormone release from isolated rodent and human islets as well as the perfused pancreas (e.g. Casad & Adelman 1992, Zambre et al. 1999, Strowski et al. 2000), others have found that exogenous SST was ineffective in suppressing insulin or glucagon secretion from isolated mouse islets (Cejvan et al. 2003, Hauge-Evans et al. 2009). An analogous system has been reported in the exocrine pancreas, where exogenous SST did not inhibit enzyme secretion from isolated, arterially perfused pancreas, nor from acinar cells in vitro, but suppressed pancreatic enzyme release indirectly via binding to SSTR2 in the dorsal vagal complex (Liao et al. 2007).
These observations imply the existence of an additional mechanism through which exogenous SST can inhibit insulin secretion in vivo, and as all five SST receptors are widely expressed throughout the brain (Moller et al. 2003), we proposed an indirect, central effect of SST, potentially mediated by the sympathetic nervous system. We have therefore assessed the effect on insulin and glucagon secretion of i.v. or i.c.v. administration of BIM23014C, a broad-range SSTR agonist also known as Lanreotide, in parallel with an i.v. glucose challenge in rats.
Methods
Islet isolation
Islets were isolated from male Wistar rats or ICR mice (Harlan, Bicester, UK) by collagenase digestion (1 mg/ml, type XI, Sigma) and separated from exocrine pancreatic tissue on a histopaque gradient (Sigma), as described previously (Hauge-Evans et al. 2009). The islets were incubated overnight at 37 °C (5% CO2) in RPMI 1640 (10% FBS, 2 mmol/l glutamine, 100 units/ml penicillin/0.1 mg/ml streptomycin and 11 mmol/l glucose) before experiments were performed.
In vitro hormone secretion studies
For measuring the dynamics of insulin release, islets were transferred into chambers of a temperature-controlled multi-channel perifusion system as described previously (Jones et al. 1989) and perifused for 70 min at 37 °C with a low-glucose, bicarbonate-buffered physiological salt solution (Gey & Gey 1936). The tissue was subsequently perifused (0.5 ml/min) with salt solution containing agents of interest and fractions were collected at 2 min intervals. For static secretion experiments, islets were pre-incubated for 60 min in a buffer containing 1 mmol/l glucose after which batches of eight islets were incubated for 60 min in 0.4 ml salt solution containing agents of interest.
Animals and surgical procedures for in vivo studies
Adult male Wistar rats (approximately 300 g, Harlan) were housed under controlled conditions and provided with food and water and allowed to eat and drink ad libitum. All animal procedures were undertaken in accordance with the UK Home Office Regulations. All surgical procedures were carried out under anaesthesia induced with ketamine (100 mg/kg i.p.; Pharmacia and Upjohn Ltd, Crawley, UK) and Rompun (10 mg/kg i.p.; Bayer). Rats were fitted with a guide and a dummy cannula directed towards the left lateral cerebral ventricle, as described previously (Bowe et al. 2008) and, following a 10-day recovery period, the animals were fitted with two indwelling cardiac catheters via the external jugular veins. The catheters were exteriorised at the back of the head and secured to a cranial attachment and the animals were fitted with a 30-cm long metal spring tether, which was attached to a fluid swivel, allowing free movement of the rats in their enclosure. Experiments were commenced 3 days later.
In vivo experimental procedure
Before experimental tests, the rats were fasted overnight. On the day of the experiment, conscious rats received injections either i.c.v. with physiological saline or with the SSTR agonist, BIM23014C (Lanreotide acetate, Sigma), dissolved in physiological saline (0.75 μg) or i.v. with BIM23014C (7.5 μg) in parallel with i.v. administration of glucose (1 g/kg) at time point 0. Blood samples were withdrawn with heparinised syringes via cardiac catheters at −10, −5, 0, 5, 10, 20, 30, 45 and 60 min following BIM23014C and glucose administration at a volume of 100 μl except at time points −5, 10 and 30 min, where 300 μl were drawn. Blood glucose concentration was measured using a glucose meter (Accuchek Aviva, Roche) after which the plasma was separated by centrifugation (2351 g, 5 min at 4 °C) and frozen for later assay of islet hormone content. Each treatment group contained two animals per experimental day and the experiment was repeated every 3–4 days with animals exposed to different treatments each time to give a total number of 4–11 per treatment group.
Hormone immunoassays
The islet hormone and catecholamine content of plasma samples were assessed by ELISA, using commercially available kits (Insulin: Millipore Ltd, Watford, UK; glucagon: Mercodia, Uppsala, Sweden; SST and GLP1: USCN Life Science, Inc., Wuhan, China; catecholamine: Bioassay Technology Laboratory, Shanghai, China). Islet hormone concentrations in the incubation medium from in vitro experiments were assessed by RIA using an in-house insulin assay as described previously (Hauge-Evans et al. 2009).
Statistical analysis
Data are expressed as means±s.e.m. and analysed using Student's t-test or one- or two-way ANOVA as appropriate and Bonferroni's multiple comparisons test. The choice of test is indicated in the figure legends. Differences between treatments were considered significant at P<0.05.
Results
We have previously shown that exogenous SST14 had little inhibitory effect on dynamic insulin secretion from isolated mouse islets in response to 20 mmol/l glucose (Hauge-Evans et al. 2009), and herein we confirm these findings under a range of conditions. These include intermediate and high concentrations of both glucose and SST14 in static and dynamic experimental settings for measurements of insulin release as well as with isolated islets from a different species, the rat (Figs 1 and 2). No effect on dynamic insulin release was observed, whether islets were exposed to SST14 after an initial stimulation by glucose (Fig. 1A), or whether they were co-perifused with glucose and SST14 simultaneously (Fig. 1B and C). In parallel experiments, SST28 was similarly ineffective in modulating dynamic glucose-induced insulin secretion from isolated rodent islets (Fig. 1D) as was the stable SST analogue, BIM23014C (Fig. 2A). In addition, BIM23014C did not inhibit arginine-induced glucagon secretion (Fig. 2B).
To assess whether SST exerts an effect on insulin release indirectly via a centrally mediated mechanism, the SST analogue, BIM23014C, was injected either peripherally (i.v.) or centrally (i.c.v.), the latter at a tenfold lower dose to compensate for the smaller volume of the ventricular system compared with the systemic circulation. Assuming approximate blood and cerebrospinal fluid (CSF) volumes of distribution of 20 ml and 300 μl, respectively, the predicted concentrations of BIM23014C in blood and CSF will be in the order of 300 nmol/l and 2 μmol/l respectively. This is well above that required to activate SST receptors, including those less preferentially targeted by the agonist (Meyerhof 1998) and similar to doses used in other studies (Rettig et al. 1989, Karasawa et al. 2014). As shown in Fig. 3A, i.v. administration of BIM23014C at t=0 together with glucose (1 g/kg) led to an immediate and significant decrease in plasma insulin levels in response to the glucose challenge compared with controls with saline and glucose only, and this decrease was paralleled with significantly increased blood glucose levels (Fig. 4A and B). The effect of i.v. BIM23014C on insulin release was observed primarily during the initial 10 min of the response and was reflected in a decreasing trend of AUC for 30 min compared with control as shown in Fig. 3B. Despite the stable nature of the compound (half-life: 90 min), there was no difference in plasma insulin levels compared with controls once the elevated blood glucose concentrations had been cleared from the circulation (P>0.2 for t=60 and 120 min). In contrast to peripheral administration of BIM23014C, i.c.v. injection of the compound at a high central concentration had no significant effect on the time course of insulin release or the overall AUC (Fig. 3A and B). Accordingly, no parallel changes in blood glucose concentration were observed when BIM was administered simultaneously with- (Fig. 4A and B) or 5 min before i.v. glucose injection (control AUC: 195.7±14.3 mmol/l×min; i.c.v. BIM23014C and i.v. glucose co-treatment: 186.9±24.1mmol/l×min; i.c.v. BIM23014C pre-treatment: 126.2±8.8, n=3–4, P>0.05). In contrast, i.v. administration of the same dose (0.75 μg) led to increased blood glucose levels compared with controls (AUC: 331.3±32.7 mmol/l×min versus 193.1±20.9 mmol/l×min, P<0.05). These results thus indicate that central activation of SSTRs by the SST analogue had no modulatory effect on insulin release from the pancreas and that the i.v. effect of the SST analogue was therefore unlikely to be mediated indirectly by a CNS-dependent mechanism.
Glucagon is a major regulator of glucose homoeostasis and both glucose and SST inhibit glucagon secretion from α-cells within the islet. Results from the assessment of plasma glucagon levels following i.v. or i.c.v. administration of BIM23014C together with i.v. glucose indicated that the compound under either experimental setting did not further repress the already very low levels of plasma glucagon, as shown in Fig. 5A. These results were further confirmed in a second experimental cohort, where no significant difference was observed between either of the three treatment groups at time points 0, 5 and 30 min (n=6 animals per group, P>0.05). In addition, there were no significant changes in plasma catecholamine levels between treatment groups at these time points (Fig. 5B, P>0.05), indicating that the observed increases in blood glucose levels following i.v. BIM23014C administration were a consequence of changes in plasma insulin levels rather than a potential indirect effect of BIM23014C on glucagon release mediated by changes in levels of circulating catecholamines. Consistent with the route of administration, i.v. injection of glucose did not affect the plasma content of GLP1 or SST at the 10 or 30 min time points and i.v. or i.c.v. BIM23014C did not further alter the basal levels of these hormones (results not shown).
Discussion
The effect of SST on the secretion of peptides from the exocrine pancreas and on intestinal uptake of water and electrolytes is mediated indirectly via the CNS (Primi & Bueno 1987, Li & Owyang 1993, Liao et al. 2007) and other islet hormones have similarly been shown to convey their effects indirectly via the CNS in addition to their direct action on target tissues. For example, insulin acts indirectly via insulin receptors expressed on the ventromedial hypothalamus to modulate peripheral glucose homoeostasis in addition to its direct action on target tissues (Diggs-Andrews et al. 2010, Paranjape et al. 2010). We therefore proposed the hypothesis that the inhibitory effect of SST on insulin release may similarly be at least partly dependent on central activation of SSTRs by SST.
To test our theory, the SST analogue, BIM23014C, was administered either i.v. or i.c.v. The i.c.v. injection of compounds into the ventricular system by is a well-established method and was chosen because it allows for several brain regions to be affected simultaneously, rather than limiting the target area to one specific part of the brain. While i.v. injection of the compound gave the expected decrease in plasma insulin and a concomitant increase in blood glucose levels consistent with reduced insulin action on target tissues, no significant effects were observed following infusion via the i.c.v. route, despite the estimated local concentration being higher after i.c.v. than after i.v. injection due to the much smaller distribution volume of the CSF (Pheng et al. 2009). The use of a lower overall dose i.c.v. versus i.v. also minimised the likelihood of leakage of BIM23014C into the peripheral circulation by clearance from the CSF, which could mask a central action of the compound. The rationale behind this approach was supported by findings by Tannenbaum & Patel (1986), indicating that centrally administered SST affected not only GH secretion, but also increased basal blood glucose levels. However, this was attributed to measurable leakage of SST from the cerebral ventricles into the general circulation, which could be measured peripherally within minutes of i.c.v. administration of high peptide concentrations. Based on the dosage used in this study, this was unlikely to be the case in our experiments, and we saw no effect of i.c.v. BIM23014C on basal glucose levels, although i.v. administration of the same dose led to an increase in blood glucose concentrations following i.v. glucose administration when compared with controls.
Results from our previous studies with SST KO mice indicated that SST primarily affects insulin release under stimulatory conditions (Hauge-Evans et al. 2009) and i.v. administration of glucose enabled us to study this with minimal changes to incretin hormone and endogenous SST levels from the gastrointestinal tract otherwise observed following oral or intraduodenal glucose administration. This is relevant as both affect overall plasma insulin levels. We did not observe any significant change in the incretin hormone GLP1 or plasma SST 10 min after i.v. glucose injection, consistent with the route of administration, and BIM23014C administration (i.v. or i.c.v.) had no significant effect on their basal levels, indicating that the influence of peripheral factors from the intestine was limited in the present experimental protocol.
Our results indicated that i.v. glucose administration led to immediate changes in both plasma glucose and insulin levels and we have previously reported that this takes place within 2–3 min of injection (Bowe et al. 2014), confirming an instantaneous response of pancreatic β-cells to hyperglycaemia, which in this study was promptly modulated by i.v. administration but not by i.c.v. administration of BIM (within 5 min). In contrast, Yavropoulou et al. (2014) reported that bolus i.c.v. infusion of SST14 with a simultaneous and continuous intraduodenal glucose infusion in dogs caused a delayed reduction of both plasma insulin and glucose levels 45–90 min after administration despite the very short plasma half-life of SST14 (2–3 min). It seems likely that the different experimental outcomes reflect differences in the experimental protocols, in particular with regard to the route of glucose administration, where a local incretin effect as well as a centrally mediated effect of SST on the intestine cannot be excluded. Under conditions where intestinal influences are minimised, as in our experimental model, we did not observe any significant changes in plasma insulin and blood glucose levels in response to a locally high i.c.v. BIM23014C concentration.
In summary, while measurement of glucose homoeostasis by i.v. administration cannot be used to rule out a subtle, long-term effect of i.c.v. SST under physiological circumstances, this study using conscious, unrestrained experimental animals could find no evidence for an important centrally mediated effect of SST in regulating glucose-induced insulin secretion in vivo, although we cannot exclude the possibility of the presence of other indirect, but peripheral pathways mediating the effect of SST on islet endocrine cells to suppress hormone secretion. Given the discrepancies within the literature regarding an inhibitory action of SST in vitro, it is also possible that the lack of effect of exogenous SST or SST analogues in suppressing glucose-induced insulin (and glucagon) release from isolated islets in vitro is a consequence of islet isolation and/or culture, perhaps through alterations in SST receptor expression or via changes in the tonic inhibitory input from islet δ-cells. Whatever the underlying cause(s), the very different effect of SST on insulin secretion between in vivo and in vitro experimental models is a potential confounding factor worth considering when designing and interpreting studies in this area.
Declaration of interest
The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.
Funding
This work was funded by Diabetes UK and ACHE is a Diabetes UK RD Lawrence Fellow (BDA: 10/0003980).
References
Bowe JE, Li XF, Kinsey-Jones JS, Brain SD, Lightman SL & O'Byrne KT 2008 The role of corticotrophin-releasing hormone receptors in the calcitonin gene-related peptide-induced suppression of pulsatile luteinising hormone secretion in the female rat. Stress 11 312–319. (doi:10.1080/10253890701801448)
Bowe JE, Franklin ZJ, Hauge-Evans AC, King AJ, Persaud SJ & Jones PM 2014 Assessing glucose homeostasis in rodent models. Journal of Endocrinology 222 G13–G25. (doi:10.1530/JOE-14-0182)
Casad RC Jr & Adelman RC 1992 Aging enhances inhibitory action of somatostatin in rat pancreas. Endocrinology 130 2420–2422. (doi:10.1210/endo.130.4.1347743)
Cejvan K, Coy DH & Efendic S 2003 Intra-islet somatostatin regulates glucagon release via type 2 somatostatin receptors in rats. Diabetes 52 1176–1181. (doi:10.2337/diabetes.52.5.1176)
Diggs-Andrews KA, Zhang X, Song Z, Daphna-Iken D, Routh VH & Fisher SJ 2010 Brain insulin action regulates hypothalamic glucose sensing and the counterregulatory response to hypoglycemia. Diabetes 59 2271–2280. (doi:10.2337/db10-0401)
Gey GO & Gey MK 1936 Maintenance of human normal cells in continuous culture, preliminary report: cultivation of mesoblastic tumours and normal cells and notes on methods of cultivation. American Journal of Cancer 27 45–76. (doi:10.1158/ajc.1936.45)
Hauge-Evans AC, King AJ, Carmignac D, Richardson CC, Robinson IC, Low MJ, Christie MR, Persaud SJ & Jones PM 2009 Somatostatin secreted by islet δ-cells fulfills multiple roles as a paracrine regulator of islet function. Diabetes 58 403–411. (doi:10.2337/db08-0792)
Hauge-Evans AC, Anderson RL, Persaud SJ & Jones PM 2012 Delta cell secretory responses to insulin secretagogues are not mediated indirectly by insulin. Diabetologia 55 1995–2004. (doi:10.1007/s00125-012-2546-9)
Jones PM, Persaud SJ & Howell SL 1989 Time-course of Ca2+-induced insulin secretion from perifused, electrically permeabilised islets of Langerhans: effects of cAMP and a phorbol ester. Biochemical and Biophysical Research Communications 162 998–1003. (doi:10.1016/0006-291X(89)90772-9)
Karasawa H, Seeichi Y, Wang L, Stengel A, Rivier J & Tache Y 2014 Brain somatostatin receptor 2 mediates the dipsogenic effect of central somatostatin and cortistatin in rats: role in drinking behaviour. American Journal of Physiology. Regulatory, Integrative and Comparative Physiology 307 R793–R801. (doi:10.1152/ajpregu.00248.2014)
Li Y & Owyang C 1993 Somatostatin inhibits pancreatic enzyme secretion at a central vagal site. American Journal of Physiology 265 G251–G257.
Liao Z, Li ZS, Lu Y & Wang WZ 2007 Microinjection of exogenous somatostatin in the dorsal vagal complex inhibits pancreatic secretion via somatostatin receptor-2 in rats. American Journal of Physiology. Gastrointestinal and Liver Physiology 292 G746–G752. (doi:10.1152/ajpgi.00174.2006)
Lins PE, Efendic S, Meyers CA, Coy DH, Schally A & Luft R 1980 Selective effect of some somatostatin analogs on glucagon as opposed to insulin release in rats in vivo. Metabolism 29 728–731. (doi:10.1016/0026-0495(80)90194-8)
Low MJ 2004 Clinical endocrinology and metabolism. The somatostatin neuroendocrine system: physiology and clinical relevance in gastrointestinal and pancreatic disorders. Best Practice & Research. Clinical Endocrinology & Metabolism 18 607–622. (doi:10.1016/j.beem.2004.08.005)
Ludvigsen E, Olsson R, Stridsberg M, Janson ET & Sandler S 2004 Expression and distribution of somatostatin receptor subtypes in the pancreatic islets of mice and rats. Journal of Histochemistry and Cytochemistry 52 391–400. (doi:10.1177/002215540405200310)
Meyerhof W 1998 The elucidation of somatostatin receptor functions: a current view. Reviews of Physiology, Biochemistry and Pharmacology 133 55–108. (doi:10.1007/Bfb0000613)
Moller LN, Stidsen CE, Hartmann B & Holst JJ 2003 Somatostatin receptors. Biochimica et Biophysica Acta 1616 1–84. (doi:10.1016/S0005-2736(03)00235-9)
Paranjape SA, Chan O, Zhu W, Horblitt AM, McNay EC, Cresswell JA, Bogan JS, McCrimmon RJ & Sherwin RS 2010 Influence of insulin in the ventromedial hypothalamus on pancreatic glucagon secretion in vivo. Diabetes 59 1521–1527. (doi:10.2337/db10-0014)
Pheng V, Uenoyama Y, Homma T, Inamoto Y, Takase K, Yoshizawa-Kumagaye K, Isaka S, Watanabe TX, Ohkura S & Tomikawa J et al. 2009 Potencies of centrally- or peripherally-injected full-length kisspeptin or its C-terminal decapeptide on LH release in intact male rats. Journal of Reproduction and Development 55 378–382. (doi:10.1262/jrd.20240)
Polonsky KS, Shoelson SE & Docherty HM 1983 Plasma somatostatin 28 increases in response to feeding in man. Journal of Clinical Investigation 71 1514–1518. (doi:10.1172/JCI110907)
Primi MP & Bueno L 1987 Influence of centrally administered somatostatin and two related peptides on intestinal absorption of water and electrolytes in conscious dogs. Peptides 8 619–623. (doi:10.1016/0196-9781(87)90035-0)
Reichlin S 1983a Somatostatin. New England Journal of Medicine 309 1495–1501. (doi:10.1056/NEJM198312153092406)
Reichlin S 1983b Somatostatin (second of two parts). New England Journal of Medicine 309 1556–1563. (doi:10.1056/NEJM198312223092506)
Rettig R, Geist R, Sauer U, Rohmeiss P & Unger T 1989 Central effects of somatostatin: pressor response AVP release, and sympathoinhibition. American Journal of Physiology. Regulatory, Integrative and Comparative Physiology 257 R588–R594.
Schuit FC, Derde MP & Pipeleers DG 1989 Sensitivity of rat pancreatic A and B cells to somatostatin. Diabetologia 32 207–212. (doi:10.1007/BF00265096)
Strowski MZ, Parmar RM, Blake AD & Schaeffer JM 2000 Somatostatin inhibits insulin and glucagon secretion via two receptors subtypes: an in vitro study of pancreatic islets from somatostatin receptor 2 knockout mice. Endocrinology 141 111–117. (doi:10.1210/endo.141.1.7263)
Tannenbaum GS & Patel YC 1986 On the fate of centrally administered somatostatin in the rat: massive hypersomatostatinemia resulting from leakage into the peripheral circulation has effects on growth hormone secretion and glucoregulation. Endocrinology 118 2137–2143. (doi:10.1210/endo-118-5-2137)
Vieira E, Salehi A & Gylfe E 2007 Glucose inhibits glucagon secretion by a direct effect on mouse pancreatic alpha cells. Diabetologia 50 370–379. (doi:10.1007/s00125-006-0511-1)
Wang XP, Norman M, Yang J, Liu SH, Magnusson J, DeMayo FJ & Brunicardi FC 2005 The effect of global SSTR5 gene ablation on the endocrine pancreas and glucose regulation in aging mice. Journal of Surgical Research 129 64–72. (doi:10.1016/j.jss.2005.05.024)
Wang XP, Norman M, Yang J, Magnusson J, Kreienkamp HJ, Richter D, DeMayo FJ & Brunicardi FC 2006 Alterations in glucose homeostasis in SSTR1 gene-ablated mice. Molecular and Cellular Endocrinology 247 82–90. (doi:10.1016/j.mce.2005.11.002)
Yavropoulou MP, Kotsa K, Pikilidou M, Keisisoglou I & Yovos JG 2014 Effects of intracerebroventricular infusion of somatostatin-14 on peripheral glucoregulation in dogs. Journal of Neuroendocrinology 26 11–17. (doi:10.1111/jne.12124)
Zambre Y, Ling Z, Chen MC, Hou X, Woon CW, Culler M, Taylor JE, Coy DH, Van Schravendijk C & Schuit F et al. 1999 Inhibition of human pancreatic islet insulin release by receptor-selective somatostatin analogs directed to somatostatin receptor subtype 5. Biochemical Pharmacology 57 1164. (doi:10.1016/S0006-2952(99)00010-6)