Regulation of insulin secretion by phospholipase C

1996 ◽  
Vol 271 (3) ◽  
pp. E409-E416 ◽  
Author(s):  
W. S. Zawalich ◽  
K. C. Zawalich
Keyword(s):  
Insulin Secretion ◽  
Phospholipase C ◽  
High Glucose ◽  
Human Islets ◽  
Secretory Response ◽  
Second Phase ◽  
Release Rates ◽  
Mouse Islets ◽  
Insulin Secretory Response ◽  
Hydrolysis Of

Biphasic insulin secretion in response to a sustained glucose stimulus occurs when rat or human islets are exposed to high levels of the hexose. A transient burst of hormone secretion is followed by a rising and sustained secretory response that, in the perfused rat pancreas, is 25- to 75-fold greater than prestimulatory insulin release rates. This insulin secretory response is paralleled by a significant five- to sixfold increase in the phospholipase C (PLC)-mediated hydrolysis of islet phosphoinositide (PI) pools by high glucose. In contrast, mouse islets, when stimulated under comparable conditions with high glucose, display a second-phase response that is flat and only slightly (two- to threefold) greater than prestimulatory release rates. The minimal second-phase insulin secretory response to high glucose is accompanied by the minimal activation of PLC in mouse islets as well. However, stimulation of mouse islets with the protein kinase C (PKC) activator tetradecanoyl phorbol acetate (TPA) or the muscarinic agonist carbachol, which significantly activates an isozyme of PLC distinct from that activated by high glucose, induces a rising and sustained second-phase insulin secretory response. When previously exposed to high glucose, both rat and human islets respond to subsequent restimulation with an amplified insulin secretory response. They display priming, sensitization, or time-dependent potentiation. In contrast, mouse islets primed under similar conditions with high glucose fail to display this amplified insulin secretory response on restimulation. Mouse islets can, however, be primed by brief exposure to either TPA or carbachol. Finally, whereas rat islets are desensitized by chronic exposure to high glucose, mouse islet insulin secretory responses are relatively immune to this adverse effect of the hexose. These and other findings are discussed in relationship to the role being played by agonist-induced increases in the PLC-mediated hydrolysis of islet phosphoinositide pools and the activation of PKC in these species-specific insulin secretory response patterns.

Differential effects of microtubule-altering agents on beta-cells during development

1983 ◽  
Vol 245 (4) ◽  
pp. E391-E400
Author(s):  
R. S. Hill ◽  
W. B. Rhoten
Keyword(s):  
Insulin Secretion ◽  
Beta Cells ◽  
Insulin Release ◽  
High Glucose ◽  
Deuterium Oxide ◽  
Inhibitory Effect ◽  
Secretory Response ◽  
Second Phase ◽  
Insulin Secretory Response

The effect of microtubule-altering agents on the insulin secretory response to glucose during the perinatal period was investigated with an in vitro perifusion system. Rat pancreatic mince from day 17 of gestation (D17G) to day 6 postnatally (D6PN) were perifused for 60 min in basal glucose followed by 45 min with high glucose (3.5 mg/ml) or with high glucose plus 10 mM arginine (D17G). The two phases of insulin secretion in response to high glucose developed in an age-dependent and asynchronous manner. The first phase matured between D17G and D18G, and maturation of the second phase occurred subsequently. Vinblastine (VB) (20 or 100 microM) had a differential effect on the insulin secretory response. VB did not inhibit stimulated insulin release at D17G. This absence of an inhibitory effect of VB at D17G could not be explained by the absence of polymerized tubulin because microtubules were present in the control beta-cells and, in addition, VB treatment resulted in the formation of paracrystalline deposits. Subsequently in development, and with isolated islets of the adult, VB inhibited stimulated insulin release. Heavy water (deuterium oxide, D2O) inhibited stimulated insulin secretion at D17G but blocked completely insulin release from the near-term beta-cell. The inhibition of insulin secretion by D2O was rapidly reversed when water replaced D2O in the perifusion media. The results indicate that the maturation of the second phase of insulin secretion coincides with the ability of the microtubule-altering agents to modify the insulin secretory response. One possible explanation for these findings is that at D17G the microtubules are not coupled physicochemically to other molecules or structures necessary for their role in insulin secretion to be expressed fully.

Roles of GTP and phospholipase C in the potentiation of Ca2+-induced insulin secretion by glucose in rat pancreatic islets

1997 ◽  
Vol 153 (1) ◽  
pp. 61-71 ◽  
Author(s):  
J Vadakekalam ◽  
M E Rabaglia ◽  
S A Metz
Keyword(s):  
Insulin Secretion ◽  
Pancreatic Islets ◽  
Phospholipase C ◽  
Mycophenolic Acid ◽  
High Glucose ◽  
Secretory Response ◽  
Rat Pancreatic Islets ◽  
Isolated Rat Islets ◽  
Effect Of Glucose ◽  
Isolated Rat

Abstract Glucose can augment insulin secretion independently of K+ channel closure, provided cytoplasmic free Ca2+ concentration is elevated. A role for phospholipase C (PLC) in this phenomenon has been both claimed and refuted. Recently, we have shown a role for GTP in the secretory effect of glucose as well as in glucose-induced PLC activation, using islets pre-treated with GTP synthesis inhibitors such as mycophenolic acid (MPA). Therefore, in the current studies, we examined first, whether glucose augments Ca2+-induced PLC activation and second, whether GTP is required for this effect, when K+(ATP) channels are kept open using diazoxide. Isolated rat islets pre-labeled with [3H]myo-inositol were studied with or without first priming with glucose. There was a 98% greater augmentation of insulin secretion by 16·7 mm glucose (in the presence of diazoxide and 40 mm K+) in primed islets; however, the ability of high glucose to augment PLC activity bore no relationship to the secretory response. MPA markedly inhibited PLC in both conditions; however, insulin secretion was only inhibited (by 46%) in primed islets. None of these differences were attributable to alterations in labeling of phosphoinositides or levels of GTP or ATP. These data indicate that an adequate level of GTP is critical for glucose's potentiation of Ca2+-induced insulin secretion in primed islets but that PLC activation can clearly be dissociated from insulin secretion and therefore cannot be the major cause of glucose's augmentation of Ca2+-induced insulin secretion. Journal of Endocrinology (1997) 153, 61–71

scholarly journals Heterogeneous expression of CFTR in insulin-secreting β-cells of the normal human islet

PLoS ONE ◽  
2020 ◽  
Vol 15 (12) ◽  
pp. e0242749
Author(s):  
Mauricio Di Fulvio ◽  
Marika Bogdani ◽  
Myrian Velasco ◽  
Timothy S. McMillen ◽  
Cecilia Ridaura ◽  
...  
Keyword(s):  
Insulin Secretion ◽  
Human Islets ◽  
Secretory Response ◽  
Β Cells ◽  
Β Cell ◽  
Co Morbidity ◽  
Normal Human ◽  
Mouse Islets ◽  
Underlying Mechanisms ◽  
Cf Transmembrane Conductance Regulator

Cystic fibrosis (CF) is due to mutations in the CF-transmembrane conductance regulator (CFTR) and CF-related diabetes (CFRD) is its most common co-morbidity, affecting ~50% of all CF patients, significantly influencing pulmonary function and longevity. Yet, the complex pathogenesis of CFRD remains unclear. Two non-mutually exclusive underlying mechanisms have been proposed in CFRD: i) damage of the endocrine cells secondary to the severe exocrine pancreatic pathology and ii) intrinsic β-cell impairment of the secretory response in combination with other factors. The later has proven difficult to determine due to low expression of CFTR in β-cells, which results in the general perception that this Cl−channel does not participate in the modulation of insulin secretion or the development of CFRD. The objective of the present work is to demonstrate CFTR expression at the molecular and functional levels in insulin-secreting β-cells in normal human islets, where it seems to play a role. Towards this end, we have used immunofluorescence confocal and immunofluorescence microscopy, immunohistochemistry, RT-qPCR, Western blotting, pharmacology, electrophysiology and insulin secretory studies in normal human, rat and mouse islets. Our results demonstrate heterogeneous CFTR expression in human, mouse and rat β-cells and provide evidence that pharmacological inhibition of CFTR influences basal and stimulated insulin secretion in normal mouse islets but not in islets lacking this channel, despite being detected by electrophysiological means in ~30% of β-cells. Therefore, our results demonstrate a potential role for CFTR in the pancreatic β-cell secretory response suggesting that intrinsic β-cell dysfunction may also participate in the pathogenesis of CFRD.

Regulation of insulin secretion via ATP-sensitive K+ channel independent mechanisms: role of phospholipase C

1997 ◽  
Vol 272 (4) ◽  
pp. E671-E677 ◽  
Author(s):  
W. S. Zawalich ◽  
K. C. Zawalich
Keyword(s):  
Insulin Secretion ◽  
Phospholipase C ◽  
Inositol Phosphates ◽  
Inhibitory Effect ◽  
Secretory Response ◽  
K Channel ◽  
Rat Islets ◽  
Protein Kinase C Inhibitor ◽  
Mouse Islets

Groups of rat or mouse islets were isolated and perifused with 20 mM glucose plus 200 microM diazoxide. The further addition of 30 mM K+ resulted in a rapid and sustained biphasic insulin secretory response. The onset of secretion in response to the addition of K+ was comparable in both species, but the magnitude of the response was significantly greater from rat islets. After the labeling of islet phosphoinositide pools with 2-[3H]inositol, the accumulation of labeled inositol phosphates (IP) in response to 30 mM K+ addition in the simultaneous presence of 20 mM glucose plus diazoxide was assessed. The addition of 30 mM K+ significantly increased IP accumulation approximately 300% in rat islets, whereas only an insignificant 25-30% increase was observed in mouse islets. The protein kinase C inhibitor staurosporine (50 nM) dramatically reduced the sustained secretory response from rat islets in the presence of 30 mM K+, 20 mM glucose, and diazoxide. Its effect was minimal on mouse islets and a significant inhibitory effect on insulin secretion was observed only during the final 5 min of the perifusion. The further addition of carbachol, an agonist that activates an isozyme of phospholipase C distinct from that activated by glucose, together with K+, 20 mM glucose, plus diazoxide resulted in a sustained amplification of insulin secretion from mouse but not rat islets. K+ (30 mM)-induced insulin secretion in the presence of 3 mM glucose was similar from perifused rat or mouse islets, a finding that would seem to preclude the activation of voltage-regulated Ca2+ channels as the pertinent difference. These results confirm previous observations with these species and document another anomaly that exists between the responses of rat islets compared with mouse islets. The inability to activate a nutrient- and calcium-regulated phospholipase C isozyme in mouse islets to the same extent as in rat islets appears to account, at least in part, for these different insulin secretory responses under these unique conditions.

Magnesium requirement for somatostatin inhibition of insulin secretion

1984 ◽  
Vol 105 (1) ◽  
pp. 83-86 ◽  
Author(s):  
Donald L. Curry ◽  
Leslie L. Bennett
Keyword(s):  
Insulin Secretion ◽  
Calcium Concentration ◽  
Inhibitory Effect ◽  
Secretory Process ◽  
Magnesium Ion ◽  
Second Phase ◽  
Intracellular Membrane ◽  
Calcium Magnesium ◽  
First Phase Insulin Secretion ◽  
Insulin Secretory Response

Abstract. Rat pancreas perfusions were performed using a perfusate with a fixed calcium concentration of 5 mEq/l and magnesium varying from 0 to 0.6 mEq/dl. Insulin secretion was stimulated by a constant glucose infusion of 300 mg/dl. This glucose concentration produces the typical biphasic insulin secretory response. We observed that in the absence of magnesium, somatostatin concentrations of 0.5 and 2.0 ng/ml were without effect on first phase insulin secretion. However, these same somatostatin levels produced 50% or more inhibition of insulin secretion in the presence of magnesium at 0.3 or 0.6 mEq/l. Similarly, in the absence of magnesium, somatostatin at 50 ng/ml failed to inhibit second phase insulin secretion, whereas this same somatostatin level produced about 50% inhibition of insulin secretion in the presence of magnesium at 0.3 mEq/l. Thus, altering perfusate magnesium concentrations without changing calcium is an important determinant of the degree of inhibition of secretion produced by somatostatin. In particular, in the absence of magnesium ion, somatostatin concentrations which would 'normally' produce 50% inhibition of secretion (ID50) are without effect. Therefore, magnesium ion is necessary for the full inhibitory effect of somatostatin to occur. These results suggest that inhibitors, as well as potentiators, of the insulin secretory process may act by altering intracellular/membrane calcium-magnesium ratios, but in opposite directions.

Dynamics of glucose-induced insulin secretion in normal human islets

2015 ◽  
Vol 309 (7) ◽  
pp. E640-E650 ◽  
Author(s):  
Jean-Claude Henquin ◽  
Denis Dufrane ◽  
Julie Kerr-Conte ◽  
Myriam Nenquin
Keyword(s):  
Insulin Secretion ◽  
Stimulation Index ◽  
Human Islets ◽  
Diabetic Patients ◽  
Second Phase ◽  
Biphasic Insulin ◽  
Biphasic Insulin Secretion ◽  
Normal Human ◽  
Second Phases

The biphasic pattern of glucose-induced insulin secretion is altered in type 2 diabetes. Impairment of the first phase is an early sign of β-cell dysfunction, but the underlying mechanisms are still unknown. Their identification through in vitro comparisons of islets from diabetic and control subjects requires characterization and quantification of the dynamics of insulin secretion by normal islets. When perifused normal human islets were stimulated with 15 mmol/l glucose (G15), the proinsulin/insulin ratio in secretory products rapidly and reversibly decreased (∼50%) and did not reaugment with time. Switching from prestimulatory G3 to G6–G30 induced biphasic insulin secretion with flat but sustained (2 h) second phases. Stimulation index reached 6.7- and 3.6-fold for the first and second phases induced by G10. Concentration dependency was similar for both phases, with half-maximal and maximal responses at G6.5 and G15, respectively. First-phase response to G15–G30 was diminished by short (30–60 min) prestimulation in G6 (vs. G3) and abolished by prestimulation in G8, whereas the second phase was unaffected. After 1–2 days of culture in G8 (instead of G5), islets were virtually unresponsive to G15. In both settings, a brief return to G3–G5 or transient omission of CaCl2 restored biphasic insulin secretion. Strikingly, tolbutamide and arginine evoked immediate insulin secretion in islets refractory to glucose. In conclusion, we quantitatively characterized the dynamics of glucose-induced insulin secretion in normal human islets and showed that slight elevation of prestimulatory glucose reversibly impairs the first phase, which supports the view that the similar impairment in type 2 diabetic patients might partially be a secondary phenomenon.

scholarly journals Functional cytochrome P450 1A enzymes are induced in mouse and human islets following pollutant exposure

Diabetologia ◽  
2019 ◽  
Vol 63 (1) ◽  
pp. 162-178 ◽  
Author(s):  
Muna Ibrahim ◽  
Erin M. MacFarlane ◽  
Geronimo Matteo ◽  
Myriam P. Hoyeck ◽  
Kayleigh R. C. Rick ◽  
...  
Keyword(s):  
Enzyme Activity ◽  
Insulin Secretion ◽  
Beta Cell ◽  
Human Islets ◽  
High Dose ◽  
Single High Dose ◽  
Direct Exposure ◽  
Mouse Islets

Abstract Aims/hypothesis Exposure to environmental pollution has been consistently linked to diabetes incidence in humans, but the potential causative mechanisms remain unclear. Given the critical role of regulated insulin secretion in maintaining glucose homeostasis, environmental chemicals that reach the endocrine pancreas and cause beta cell injury are of particular concern. We propose that cytochrome P450 (CYP) enzymes, which are involved in metabolising xenobiotics, could serve as a useful biomarker for direct exposure of islets to pollutants. Moreover, functional CYP enzymes in islets could also impact beta cell physiology. The aim of this study was to determine whether CYP1A enzymes are activated in islets following direct or systemic exposure to environmental pollutants. Methods Immortalised liver (HepG2) and rodent pancreatic endocrine cell lines (MIN6, βTC-6, INS1, α-TC1, α-TC3), as well as human islets, were treated in vitro with known CYP1A inducers 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 3-methylcholanthrene (3-MC). In addition, mice were injected with either a single high dose of TCDD or multiple low doses of TCDD in vivo, and islets were isolated 1, 7 or 14 days later. Results CYP1A enzymes were not activated in any of the immortalised beta or alpha cell lines tested. However, both 3-MC and TCDD potently induced CYP1A1 gene expression and modestly increased CYP1A1 enzyme activity in human islets after 48 h. The induction of CYP1A1 in human islets by TCDD was prevented by cotreatment with a cytokine mixture. After a systemic single high-dose TCDD injection, CYP1A1 enzyme activity was induced in mouse islets ~2-fold, ~40-fold and ~80-fold compared with controls after 1, 7 and 14 days, respectively, in vivo. Multiple low-dose TCDD exposure in vivo also caused significant upregulation of Cyp1a1 in mouse islets. Direct TCDD exposure to human and mouse islets in vitro resulted in suppressed glucose-induced insulin secretion. A single high-dose TCDD injection resulted in lower plasma insulin levels, as well as a pronounced increase in beta cell death. Conclusions/interpretation Transient exposure to TCDD results in long-term upregulation of CYP1A1 enzyme activity in islets. This provides evidence for direct exposure of islets to lipophilic pollutants in vivo and may have implications for islet physiology.

Involvement of capsaicin-sensitive nerves in regulation of insulin secretion and glucose tolerance in conscious mice

1994 ◽  
Vol 267 (4) ◽  
pp. R1071-R1077 ◽  
Author(s):  
S. Karlsson ◽  
A. J. Scheurink ◽  
A. B. Steffens ◽  
B. Ahren
Keyword(s):  
Insulin Secretion ◽  
Glucose Tolerance ◽  
Insulin Response ◽  
Secretory Response ◽  
Sensory Nerves ◽  
Plasma Norepinephrine ◽  
Plasma Catecholamine ◽  
Intravenous Glucose ◽  
The Impact ◽  
Insulin Secretory Response

The impact of sensory nerves in glucose-stimulated insulin secretion and glucose tolerance was investigated in conscious mice treated neonatally with either capsaicin (Cap) or vehicle (Veh). At 10-12 wk after Cap, both the early (1 min) insulin secretory response to intravenous glucose (2.8 mmol/kg) (by 67%) and glucose elimination were potentiated (P < 0.05). In contrast, basal insulin, glucagon, and glucose were not affected by Cap. Plasma norepinephrine and epinephrine levels did not differ between Cap- and Veh-treated animals, whereas the increase in plasma insulin levels normally induced by alpha-adrenoceptor blockade by phentolamine was absent after Cap treatment. In isolated islets, the insulin secretory response to glucose (20 mmol/l), carbachol (0.1 mmol/l), or phentolamine (0.5 mmol/l) was not affected after Cap. It is concluded that sensory denervation by Cap results in increased glucose tolerance, which is in part because of a potentiated early insulin response to glucose. This potentiation does not seem secondary to altered plasma catecholamine levels or to altered islet secretory capacity. The results suggest rather that Cap-sensitive nerves, by a local effector function and/or as the afferent loop of a neural reflex, exert inhibitory influences on insulin secretion.

REGULATION OF INSULIN SECRETION IN THE OVINE FOETUS IN UTERO. EFFECTS OF SODIUM VALERATE ON SECRETION AND OF ADRENOCORTICOTROPHIN-INDUCED PREMATURE PARTURITION ON THE INSULIN SECRETORY RESPONSE TO GLUCOSE

1973 ◽  
Vol 56 (1) ◽  
pp. 13-25 ◽  
Author(s):  
J. M. BASSETT ◽  
G. D. THORBURN ◽  
DIANNE H. NICOL
Keyword(s):  
Insulin Secretion ◽  
Plasma Insulin ◽  
Short Chain Fatty Acid ◽  
Plasma Concentrations ◽  
Insulin Level ◽  
In Utero ◽  
Secretory Response ◽  
Lower Plasma ◽  
Insulin Secretory Response ◽  
Corticosteroid Secretion

SUMMARY Intravenous infusions of glucose into lambs in utero (130–150 days) and after birth, confirmed the marked post-natal increase in the magnitude of the response of plasma insulin to glucose. These studies also suggest that insulin secretion in foetal lambs is stimulated by glucose at lower plasma concentrations than in lambs after birth. The short-chain fatty acid, valeric acid, given as the sodium salt, caused a very rapid increase in the plasma insulin level of foetal lambs, when given either by intravenous injection or infusion. When birth was induced after only 135 days of gestation by i.v. infusion of a synthetic adrenocorticotrophin preparation (Synacthen) into foetal lambs there was also a prematurely induced maturation of the insulin secretory response to glucose. In these prematurely born lambs the insulin secretory response to i.v. glucose infusion was similar to that of normal lambs after birth and differed greatly from that of normal foetuses of similar age. The results indicate that maturation of the insulin secretory mechanism in the lamb is associated with parturition and suggest that these changes may be consequences of the increasing corticosteroid secretion in the foetus during the last few days of gestation.

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