Cholinergic modification of glucose-induced biphasic insulin release in vitro

Adaptive short-term associative conditioning in the pancreatic β-cell

This study associates cholinergic stimulation of the pancreatic β-cell electrical activity with a short-term memory phenomenon. Glucose pulses applied to a basal glucose concentration induce depolarizing waves which are used to estimate the evolution of the β-cell glucose sensitivity. Exposure to carbamoylcholine (carbachol) increases the size of the glucose-induced depolarizing waves. This change appears after carbachol withdrawal and implies a temporal potentiation of sensitivity (TPS) lasting up to one hour. TPS induction requires the simultaneous action of carbachol and glucose. The substitution of glucose with the secretagogues glyceraldehyde or 2-ketoisocaproate mimics glucose-induced TPS, while palmitate does not. TPS is not produced if the membrane is kept hyperpolarized by diazoxide. Glucose can be replaced by tolbutamide, suggesting a role of depolarization and a subsequent increase in intracellular calcium concentration. A role for kinases is suggested because staurosporine prevents TPS induction. Cycloheximide does not impair TPS induction, indicating that de novo protein synthesis is not required. The fact that the two inputs acting simultaneously produce an effect that lasts up to one hour without requiring de novo protein synthesis suggests that TPS constitutes a case of short-term associative conditioning in non-neural tissue. The convergence of basal glucose levels and muscarinic activation happens physiologically during the cephalic phase of digestion, in order to later absorb incoming fuels. Our data reveals that the role of the cephalic phase may be extended, increasing nutrient sensitivity during meals while remaining low between them.

Negative Affect-Related Autonomic Arousal Mediates the Association between Baroreflex Dysfunction and Insulin Resistance in Non-Diabetic Young Adults

Autonomic dysfunction, in particular under-regulation of heart rate (HR) by the baroreflex, is implicated in development of insulin resistance (IR). According to reactivity hypothesis, sympathetic response to stressors may be more sensitive at predicting IR than baroreceptor sensitivity (BRS), a baseline measure of baroreflex functioning. Using ecological momentary assessment (EMA) of negative affect coupled with minute-to-minute HR and heart-rate variability (HRV) monitoring, we examined whether negative affect (NA)-related autonomic arousal mediates the association of BRS with IR. At baseline, BRS was measured, and fasting serum glucose and insulin levels were collected from 178 young adults (18-39 years old), from which homeostasis model assessment of IR (HOMA-IR) and beta-cell functioning (HOMA %B) were derived. Participants subsequently underwent one day of Holter HR and HRV monitoring while reporting negative affect levels via EMA. Multilevel modeling was used to assess the associations of momentary negative affect with HR and low- (LF) and high-frequency (HF) HRV during the 5-minute intervals following each EMA reading. Structural equation modeling was then used to determine whether individual differences in these associations mediated the association of BRS with IR, measured by HOMA-IR, HOMA %B, and insulin levels. As predicted, BRS was negatively associated with the IR (β = -.17, p = .024). However, NA-related autonomic arousal mediated their association, accounting for 56% of the covariance between BRS and IR. Not only do these results provide support for reactivity hypothesis, they reveal a potential point of intervention in the treatment of affective dysregulation.

Central Mechanisms of Glucose Sensing and Counterregulation in Defense of Hypoglycemia

Glucose homeostasis requires an organism to rapidly respond to changes in plasma glucose concentrations. Iatrogenic hypoglycemia as a result of treatment with insulin or sulfonylureas is the most common cause of hypoglycemia in humans and is generally only seen in patients with diabetes who take these medications. The first response to a fall in glucose is the detection of impending hypoglycemia by hypoglycemia-detecting sensors, including glucose-sensing neurons in the hypothalamus and other regions. This detection is then linked to a series of neural and hormonal responses that serve to prevent the fall in blood glucose and restore euglycemia. In this review, we discuss the current state of knowledge about central glucose sensing and how detection of a fall in glucose leads to the stimulation of counterregulatory hormone and behavior responses. We also review how diabetes and recurrent hypoglycemia impact glucose sensing and counterregulation, leading to development of impaired awareness of hypoglycemia in diabetes.

Muscarinic Agonist Ameliorates Insulin Secretion in Wfs1-Deficient Mice

OBJECTIVES

Similar to patients with Wolfram syndrome and to heterozygous Wolframin1 (Wfs1) mutation carriers, Wfs1-deficient mice exhibit impaired glucose tolerance and lower plasma insulin levels. Muscarinic receptor 3 agonists have previously been shown to potentiate glucose-stimulated insulin secretion. Therefore, the aim of this study was to investigate insulin-secretion dynamics in Wfs1-deficient mice and evaluate carbachol, muscarinic agonist and the ability to ameliorate the insulin secretion deficits caused by the Wfs1 mutation.

METHODS

Wild-type Wfs1 heterozygous and Wfs1 mutant mice were used. Blood glucose was measured after glucose and carbachol administration. Insulin secretion was measured from serum using ELISA.

RESULTS

Glucose administration causes hyperglycemia in Wfs1-deficient mice due to decreased insulin secretion. This deficit is abolished by administration of the muscarinic agonist carbachol.

CONCLUSIONS

Activation of the muscarinic pathway to potentiate insulin secretion may present a target to manage diabetes resulting from Wfs1 deficiency.

Stevia Nonsweetener Fraction Displays an Insulinotropic Effect Involving Neurotransmission in Pancreatic Islets

Stevia rebaudiana (Bert.) Bertoni besides being a source of noncaloric sweeteners is also an important source of bioactive molecules. Many plant extracts, mostly obtained with ethyl acetate solvent, are rich in polyphenol compounds that present insulinotropic effects. To investigate whether the nonsweetener fraction, which is rich in phenolic compounds isolated from Stevia rebaudiana with the solvent ethyl acetate (EAF), has an insulinotropic effect, including interference at the terminals of the autonomic nervous system of the pancreatic islets of rats. Pancreatic islets were isolated from Wistar rats and incubated with EAF and inhibitory or stimulatory substances of insulin secretion, including cholinergic and adrenergic agonists and antagonists. EAF potentiates glucose-stimulated insulin secretion (GSIS) only in the presence of high glucose and calcium-dependent concentrations. EAF increased muscarinic insulinotropic effects in pancreatic islets, interfering with the muscarinic receptor subfamily M3. Adrenergic inhibitory effects on GSIS were attenuated in the presence of EAF, which interfered with the adrenergic α2 receptor. Results suggest that EAF isolated from stevia leaves is a potential therapy for treating type 2 diabetes mellitus by stimulating insulin secretion only in high glucose concentrations, enhancing parasympathetic signal transduction and inhibiting sympathetic signal transduction in beta cells.

Food deprivation and nicotine correct akinesia and freezing in Na(+) -leak current channel (NALCN)-deficient strains of Caenorhabditis elegans

Mutations in various genes adversely affect locomotion in model organisms, and thus provide valuable clues about the complex processes that control movement. In Caenorhabditis elegans, loss-of-function mutations in the Na(+) leak current channel (NALCN) and associated proteins (UNC-79 and UNC-80) cause akinesia and fainting (abrupt freezing of movement during escape from touch). It is not known how defects in the NALCN induce these phenotypes or if they are chronic and irreversible. Here, we report that akinesia and freezing are state-dependent and reversible in NALCN-deficient mutants (nca-1;nca-2, unc-79 and unc-80) when additional cation channels substitute for this protein. Two main measures of locomotion were evaluated: spontaneous movement (traversal of >2 head lengths during a 5 second observation period) and the touch-freeze response (movement greater than three body bends in response to tail touch). Food deprivation for as little as 3 min stimulated spontaneous movement and corrected the touch-freeze response. Conversely, food-deprived animals that moved normally in the absence of bacteria rapidly reverted to uncoordinated movement when re-exposed to food. The effects of food deprivation were mimicked by nicotine, which suggested that acetylcholine mediated the response. Nicotine appeared to act on interneurons or motor neurons rather than directly at the neuromuscular junction because levamisole, which stimulates muscle contraction, did not correct movement. Neural circuits have been proposed to account for the effects of food deprivation and nicotine on spontaneous movement and freezing. The NALCN may play an unrecognized role in human movement disorders characterized by akinesia and freezing gait.

Decreased muscarinic M1 receptor gene expression in the cerebral cortex of streptozotocin-induced diabetic rats and Aegle marmelose leaf extract's therapeutic function

AIM

In the present study we have investigated the changes in the total muscarinic and muscarinic M1 receptor ([(3)H]QNB) binding and gene expression in the cerebral cortex of streptozotocin (STZ) induced diabetic, insulin and aqueous extract of Aegle marmelose leaf treated diabetic rats.

MATERIALS AND METHODS

Diabetes was induced in rats by intrafemoral injection of streptozotocin. Aegle marmelose leaves was given orally to one group of rats at a dosage of 1g/kg body weight per day for fourteen days. Blood glucose and plasma insulin level were measured. Muscarinic and Muscarinic M1 receptor binding studies were done in the cerebral cortex of experimental rats. Muscarinic M1 receptor gene expression was studied using real-time PCR.

RESULTS

Scatchard analysis for total muscarinic receptors in cerebral cortex showed that the B(max) was decreased significantly (p<0.001) in diabetic rats with a significant decrease (p<0.01) in the K(d) when compared to control group. Binding analysis of Muscarinic M1 receptors showed that B(max) was decreased significantly (p<0.001) in diabetic group when compared to control group. The K(d) also decreased significantly (p<0.01) when compared to control group. The binding parameters were reversed to near control by the treatment of diabetic rats with Aegle marmelose. Real-Time PCR analysis also showed a similar change in the mRNA levels of muscarinic M1 receptors.

CONCLUSION

The results showed that there is decrease in total muscarinic and muscarinic M1 receptors during diabetes which is up regulated by insulin and Aegle marmelose leaf extract treatment. This has clinical significance in therapeutic management of diabetes.

The dual effect of isoproterenol on insulin release is suppressed in pancreatic islets from hypothalamic obese rats

Hyperinsulinemia in obesity has been attributed to insulin oversecretion by pancreatic beta-cells. Beta-cells are equipped with cholinergic and adrenergic receptors; whereas overall acetylcholine action is to potentiate, catecholamines' effect is to inhibit glucose-induced insulin release (GIIR) via alpha2-adrenoceptor. However, it has been shown that beta-adrenergic agonists potentiate glucose response. GIIR was studied in pancreatic islets from hyperinsulinemic adult obese rats, obtained by L-glutamate monosodium (MSG) neonatal treatment. Islets from MSG-rats were more glucose responsive than control ones. Isoproterenol, a beta-adrenergic agonist, inhibited the GIIR in islets from MSG-obese rats. Results indicate that MSG treatment causes alteration on function of beta-cell adrenoceptors.

Time-correlation between membrane depolarization and intracellular calcium in insulin secreting BRIN-BD11 cells: studies using FLIPR

Cytoplasmic Ca(2+) ([Ca(2+)](i)) and membrane potential changes were measured in clonal pancreatic beta cells using a fluorimetric imaging plate reader (FLIPR). KCl (30 mM) produced a fast membrane depolarization immediately followed by increase of [Ca(2+)](i) in BRIN-BD11 cells. l-Alanine (10 mM) but not l-arginine (10 mM) mimicked the KCl profile and also produced a fast membrane depolarization and elevation of [Ca(2+)](i). Conversely, a rise in glucose from 5.6 mM to 11.1 or 16.7 mM induced rapid membrane depolarization, followed by a slower and delayed increase of [Ca(2+)](i). GLP-1 (20 nM) did not affect membrane potential or [Ca(2+)](i). In contrast, acetylcholine (ACh, 100 microM) induced fast membrane depolarization immediately followed by a modest [Ca(2+)](i) increase. When extracellular Ca(2+) was buffered with EGTA, ACh mobilized intracellular calcium stores and the [Ca(2+)](i) increase was reduced by 2-aminoethoxydiphenyl borate but not by dantrolene, indicating the involvement of inositol triphosphate receptors (InsP(3)R). It is concluded that membrane depolarization of beta cells by glucose stimulation is not immediately followed by elevation of [Ca(2+)](i) and other metabolic events are involved in glucose induced stimulus-secretion coupling. It is also suggested that ACh mobilizes intracellular Ca(2+) through store operated InsP(3)R.

Cooperative enhancement of insulinotropic action of GLP-1 by acetylcholine uncovers paradoxical inhibitory effect of beta cell muscarinic receptor activation on adenylate cyclase activity

The cooperative effect of glucagon-like peptide 1 (GLP-1) and acetylcholine (ACh) was evaluated in a beta cell line model (BRIN BD11). GLP-1 (20 nM) and ACh (100 microM) increased insulin secretion by 24-47%, whereas in combination there was a further 89% enhancement of insulin release. Overnight culture with 100 ng/mL pertussis toxin (PTX) or 10nM PMA significantly reduced the combined insulinotropic action (P<0.05 and P<0.001, respectively) and the sole stimulatory effects of GLP-1 (PTX treatment; P<0.01) or ACh (PMA treatment; P<0.05). Under control conditions, ACh (50nM-1mM) concentration-dependently inhibited by up to 40% (P<0.001) the 10-fold (P<0.001) elevation of cyclic 3',5'-adenosine monophosphate (cAMP) induced by 20 nM GLP-1. The paradoxical inhibitory action of ACh was abolished by PTX pre-treatment, suggesting involvement of G(i) and/or G(o) G protein alpha subunit. Effects of selective muscarinic receptor antagonists on the concentration-dependent insulinotropic actions of ACh (50 nM-1 mM) on 20 nM GLP-1 induced insulin secretion revealed inhibition by rho-FHHSiD (M3 antagonist, P<0.05), stimulation with pirenzepine (M1 antagonist, P<0.001) and no significant effects of either methoctramine (M2 antagonist) or MT-3 (M4 antagonist). Antagonism of M2, M3 and M4 muscarinic receptor effects with methoctramine (3-100 nM), rho-FHHSiD (3-30 nM) or MT-3 (10-300 nM) did not significantly affect the inhibitory action of ACh on GLP-1 stimulated cAMP production. In contrast, M1 receptor antagonism with pirenzepine (3-30 0nM) resulted in a concentration-dependent decrease in the inhibitory action of ACh on GLP-1 stimulated cAMP production (P<0.001). These data indicate an important functional cooperation between the cholinergic neurotransmitter ACh and the incretin hormone GLP-1 on insulin secretion mediated through the M3 muscarinic receptor subtype. However, the insulinotropic action of ACh was associated with a paradoxical inhibitory effect on GLP-1 stimulated cAMP production, achieved through a novel PTX- and pirenzepine-sensitive M1 muscarinic receptor activated pathway. An imbalance between these pathways may contribute to dysfunctional insulin secretion.

Muscarinic receptor subtypes mediate stimulatory and paradoxical inhibitory effects on an insulin-secreting beta cell line

Acetylcholine (ACh), a major neurotransmitter from the autonomic nervous system, regulates the cholinergic stimulation of insulin secretion, through interactions with muscarinic receptors. The present study has characterised the individual involvement of muscarinic receptor subtypes in ACh-induced insulin secretion, using clonal beta cells and selective muscarinic receptor antagonists. BRIN BD11 cells clearly expressed mRNA encoding m1--m4 whereas m5 was not detected by RT-PCR. Insulin release was measured from BRIN BD11 cells treated with ACh in the presence of muscarinic receptor antagonists at concentrations ranging from 3 nM to 1 microM. 300 nM of muscarinic toxin-3 (M4 antagonist) and 1 microM of methoctramine (M2 antagonist) increased ACh (100 microM) stimulated insulin secretion by 168% and 50% respectively (ANOVA, P<0.05). The antagonists alone had no effect on insulin secretion. In contrast, 300 nM of pirenzepine (M1 antagonist) and 30 nM of hexahydro-sila-difenidol p-fluorohydrochloride (M3 antagonist) inhibited ACh stimulation by 91% and 84% respectively (ANOVA, P<0.01). It is concluded that ACh acts on different receptor subtypes producing both a stimulatory and an inhibitory action on insulin release.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Cytosolic Ca2+ gradients in pancreatic islet-cells stimulated by glucose and carbachol

Digital image analysis was employed to resolve the spatial differences in distribution of cytosolic free Ca2+ concentrations ([Ca2+]i) in mouse pancreatic islet-cells stimulated with glucose and carbachol. Using Indo-1 loaded mouse islet-cells, we have demonstrated that glucose induces steep spatial gradients of [Ca2+]i in isolated mouse islet-cells. Furthermore, the largest [Ca2+]i increase was always spatially restricted to a region just beneath the plasma membrane. Low concentrations of carbachol (0.6 microM) induced steep spatial gradients of [Ca2+]i which originated from the center of the cells. However, 10 microM carbachol increased [Ca2+]i to high levels collapsing the [Ca2+]i gradients in the center of the cells. Different patterns of [Ca2+]i oscillations were observed between dissociated pancreatic islet-cells and mouse pancreatic islets when challenged with 11 mM glucose. Under these conditions we could identify cells within the islet which oscillate with the same pattern as the whole islet. We postulate that "initiators" of insulin release, as glucose, induce greater [Ca2+]i increases at exocytotic sites than those induced by "potentiators", as carbachol.

Modulatory role of 1,25 dihydroxyvitamin D3 on pancreatic islet insulin release via the cyclic AMP pathway in the rat

1. Previous studies have shown that vitamin D3 deficiency impairs the insulin response to glucose via an alteration of signal transduction pathways, such as Ca2+ handling and the phosphoinositide pathway. In the present study the adenylyl cyclase pathway was examined in islets from 3 independent groups: normal rats, 4 weeks-vitamin D3 deficient rats and one week-1,25 dihydroxyvitamin D3 (1,25(OH)2D3) treated rats. 2. We found that the very low rate of insulin release observed in vitamin D3 deficient rats could be restored in vitamin D3 deficient islets only with high concentrations of dioctanoyl-cyclic AMP (DO-cyclic AMP), whereas 1,25(OH)2D3 improved the sensitivity of the islets to this exogenous cyclic AMP analogue. 3. The beneficial effect of 1,25(OH)2D3 observed with or without DO-cyclic AMP was protein kinase A-dependent, since the addition of N-[2-(p-bromocinnamylamino) ethyl]-5-isoquinolinesulphonamide (H-89), a specific inhibitor of cyclic AMP-dependent protein kinases, decreased the insulin release of treated rats back to the level seen in vitamin D3 deficient islets. 4. The low rate of insulin release could not be consistently related to an alteration in cyclic AMP content of the islets. Indeed, low insulin response to a barium+theophylline stimulus observed in vitamin D3 deficient islets was paradoxically associated with a supranormal cyclic AMP content in the islets. 5. This paradoxical increase in cyclic AMP observed in these conditions could not be attributed to a lower total phosphodiesterase (PDE) activity, although the portion of Ca(2+)-calmodulin-independent PDE was predominant in islets from vitamin D3 deficient rats. 6. On the other hand, the higher cyclic AMP content of vitamin D3 deficient islets could be related to an increase in glucagon-induced cyclic AMP synthesis in relation to the hyperglucagonaemia previously observed in vitamin D3 deficient rats. Since higher concentrations of exogenous glucagon and higher endogenous cyclic AMP concentrations were required in vitro to restore insulin release to normal values, the cyclic AMP-dependent pathways that usually potentiate insulin secretion appeared to be less efficient in relation to an alteration in the post cyclic AMP effector system. 7. 1,25(OH)2D3 exerted a stimulating effect on insulin release via protein kinase A activation but reduced the supranormal cyclic AMP synthesis, thus exerting a differential modulatory influence on biochemical disturbances in islets induced by vitamin D3 deficiency.

Neurotransmitters partially restore glucose sensitivity of insulin and glucagon secretion from perfused streptozotocin-induced diabetic rat pancreas

To elucidate the mechanisms of insensitivity of hormone secretion to glucose in streptozotocin-induced diabetic rat islets, we investigated the effects of acetylcholine (ACh) and norepinephrine on insulin and glucagon secretion in response to changes in glucose concentration, using perfused pancreas preparations. Basal insulin secretion at a blood glucose level of 5.6 mmol/l was significantly higher and basal glucagon secretion significantly lower in streptozotocin-induced diabetic rats than in controls, and neither high (16.7 mmol/l) nor low (1.4 mmol/l) blood glucose concentrations influenced insulin or glucagon secretion. Addition of 10(-6) mol/l ACh to the perfusate increased glucose-stimulated insulin secretion. Also, 10(-6) mol/l ACh, 10(-7) mol/l norepinephrine, as well as a combination of both, induced marked glucagon secretion, this was suppressed by high blood glucose level. Although simultaneous addition of 10(-6) mol/l ACh and 10(-7) mol/l norepinephrine induced only a slight increase in glucagon secretion in response to glucopenia, there was a significant increase in glucagon secretion in conjunction with an ambient decrease in insulin. Histopathological examination revealed a marked decline in acetylcholinesterase and monoamine-oxidase activities in the islets of streptozotocin-induced diabetic rats. We speculate that reduction of the potentiating effects of ACh and norepinephrine lessens glucose sensitivity of islet beta and alpha cells in this rat model of diabetes.

Interplay of glucose-stimulated Ca2+ sequestration and acetylcholine-induced Ca2+ release at the endoplasmic reticulum in rat pancreatic beta-cells

It is known that the stimulation with high glucose initially decreases as well as subsequently increases the cytosolic free Ca2+ concentration ([Ca2+]i) in pancreatic beta-cells. In the present study, we aimed at exploring the ionic mechanism and physiological role of the glucose-induced decrease in [Ca2+]i by measuring [Ca2+]i in single pancreatic beta-cells from normal rats. The glucose-induced decrease in [Ca2+]i in beta-cells was completely inhibited by thapsigargin (Tg), a specific inhibitor of the endoplasmic reticulum (ER) Ca2+ pump (Ca(2+)-ATPase). On the other hand, neither a Ca(2+)-free nor a low-Na+ condition significantly altered the glucose-induced decrease in [Ca2+]i. At basal glucose concentrations (1-4.5 mM), an insulin secretagogue acetylcholine (ACh) evoked a rather transient increase in [Ca2+]i in the presence and absence of extracellular Ca2+. A rise in glucose concentration from 1 to 4.5 mM produced a sustained decrease in [Ca2+]i and concomitantly augmented the ACh-evoked increase in [Ca2+]i. The resting [Ca2+]i level determined by glucose was tightly and reciprocally correlated with the peak of the [Ca2+]i response to ACh. Successive ACh pulses elicited repeated [Ca2+]i responses, which were progressively inhibited by Tg, suggesting that Ca2+ released by ACh was taken up by the ER Ca2+ pump and thus cycled. The results demonstrate that glucose decreases [Ca2+]i in pancreatic beta-cells mainly by activating the Ca2+ pump in ER from which ACh mobilizes Ca2+. Furthermore, the glucose-stimulated sequestration of Ca2+ by ER results in an augmented [Ca2+]i response to ACh, providing a mechanistic basis for the glucose-dependent action of ACh to initiate insulin secretion.

Effects of cholinergic agonists on diacylglycerol and intracellular calcium levels in pancreatic beta-cells

We have studied the effects of cholinergic agonists on the rates of insulin release and the concentrations of diacylglycerol (DAG) and intracellular free Ca2+ ([Ca2+]i) in the beta-cell line MIN6. Insulin secretion was stimulated by glucose, by glibenclamide and by bombesin. In the presence of glucose, both acetylcholine (ACh) and carbachol (CCh) produced a sustained increase in the rate of insulin release which was blocked by EGTA or verapamil. The DAG content of MIN6 beta-cells was not affected by glucose. Both CCh and ACh evoked an increase in DAG which was maximal after 5 min and returned to basal after 30 min; EGTA abolished the cholinergic-induced increase in DAG. ACh caused a transient rise in [Ca2+]i which was abolished by omission of Ca2+ or by addition of devapamil. Thus, cholinergic stimulation of beta-cell insulin release is associated with changes in both [Ca2+]i and DAG. The latter change persists longer than the former and activation of protein kinase C and sensitization of the secretory process to Ca2+ may underlie the prolonged effects of cholinergic agonists on insulin release. However, a secretory response to CCh was still evident after both [Ca2+]i and DAG had returned to control values suggesting that additional mechanisms may be involved.

Muscarinic modulation of insulin secretion by single pancreatic beta-cells

We have used a reverse hemolytic plaque assay and frequency distribution of immunoplaque areas to analyze the effect of carbachol (CCh, 100 nM), on insulin secretion by single pancreatic beta-cells. The CCh effect was strongly dependent on the extracellular glucose concentration. Compared with the respective controls in each condition, when glucose was omitted from the incubation medium, CCh induced a 85% increase in the insulin secretion index. In 5.6 mM glucose, CCh induced a 100% increase in the insulin secretion index and this effect was characterized by (1) amplification of the response to glucose, and (2) recruitment of previously silent cells to secretory activity. However, at high glucose concentrations (20.6 mM), the insulin secretion index decreased 49%. CCh effects were blocked by atropine (1 microM). CCh effects were not uniform among beta-cells. The functional subpopulation of beta-cells with the highest secretion rate was preferentially affected by the muscarinic agonist. The specific sodium channel blocker tetrodotoxin prevented CCh-stimulated insulin secretion in basal media, suggesting that voltage-dependent sodium channels are involved in CCh stimulation-secretion coupling in single beta-cells.

Glucose-, calcium- and concentration-dependence of acetylcholine stimulation of insulin release and ionic fluxes in mouse islets

Mouse islets were used to define the glucose-dependence and extracellular Ca2+ requirement of muscarinic stimulation of pancreatic beta-cells. In the presence of a stimulatory concentration of glucose (10 mM) and of Ca2+, acetylcholine (0.1-100 microM) accelerated 3H efflux from islets preloaded with myo-[3H]inositol. It also stimulated 45Ca2+ influx and efflux, 86Rb+ efflux and insulin release. In the absence of Ca2+, only 10-100 microM-acetylcholine mobilized enough intracellular Ca2+ to trigger an early but brief peak of insulin release. At a non-stimulatory concentration of glucose (3 mM), 1 microM- and 100 microM-acetylcholine increased 45Ca2+ and 86Rb+ efflux in the presence and absence of extracellular Ca2+. However, only 100 microM-acetylcholine marginally increased 45Ca2+ influx and caused a small, delayed, stimulation of insulin release, which was abolished by omission of Ca2+. At a maximally effective concentration of glucose (30 mM), 1 microM- and 100 microM-acetylcholine increased 45Ca2+ influx and efflux only slightly, but markedly amplified insulin release. Again, only 100 microM-acetylcholine mobilized enough Ca2+ to trigger a peak of insulin release in the absence of Ca2+. The results thus show that only high concentrations of acetylcholine (greater than or equal to 10 microM) can induce release at low glucose or in a Ca2+-free medium. beta-Cells exhibit their highest sensitivity to acetylcholine in the presence of Ca2+ and stimulatory glucose. Under these physiological conditions, the large amplification of insulin release appears to be the result of combined effects of the neurotransmitter on Ca2+ influx, on intracellular Ca2+ stores and on the efficiency with which Ca2+ activates the releasing machinery.

Direct muscarinic cholinergic inhibition of hepatic glucose production in humans

To explore the potential role of the parasympathetic nervous system in human glucoregulatory physiology, responses to the muscarinic cholinergic agonist bethanechol (5.0 mg s.c.) and antagonist atropine (1.0 mg i.v.) were measured in normal humans. There were no changes in the plasma glucose concentration or rates of glucose production or utilization following atropine administration. After bethanechol administration there were no changes in the plasma glucose concentration or fluxes despite increments in plasma glucagon (75 +/- 7 to 103 +/- 10 pg/ml, P less than 0.02). There were no changes in insulin or C-peptide levels. To test the hypothesis that direct muscarinic inhibition of glucose production was offset by an indirect action of the agonist, specifically increased glucagon secretion with consequent stimulation of glucose production, bethanechol was administered while glucagon levels were held constant with the islet clamp technique (somatostatin infusion with insulin, glucagon and growth hormone replacement at fixed rates). Under that condition the muscarinic agonist induced a 25% decrement in the plasma glucose concentration (101 +/- 8 to 75 +/- 8 mg/dl, P less than 0.05). When compared with separate clamp control studies (with placebo rather than bethanechol injection) both the rate of glucose production and the glucose concentration were reduced (P less than 0.05) following bethanechol injection; the rate of glucose utilization was unaltered. Thus, we conclude: Withdrawal of parasympathetic tone does not appear to be an important glucoregulatory process in humans. Direct muscarinic cholinergic inhibition of hepatic glucose production occurs in humans but during generalized muscarinic activation this is offset by an indirect muscarinic action, increased glucagon secretion with consequent stimulation of glucose production. Thus, particularly if regional neuronal firing occurs, the parasympathetic nervous system may play an important role in human glucoregulatory physiology.

Acetylcholine stimulates glucose metabolism by pancreatic islets

Acetylcholine stimulates insulin secretion in the presence of physiological concentrations of glucose. Stimulation of insulin secretion by acetylcholine is accompanied by an increase in glucose usage by isolated rat islets. Acetylcholine increased glucose usage by 38%, 28%, and 12% at 3.5 mM, 5.5 mM, and 10 mM glucose, respectively, compared to glucose usage by isolated islets incubated with glucose alone. Data showing increased glucose usage in islets treated with acetylcholine converge with data from an earlier report (J. Biol. Chem. 254 3921-3929 [1979]) showing a crossover point for glycolytic metabolites at phosphofructokinase to indicate that activation of glycolysis by acetylcholine results from increased phosphofructokinase activity and coordinate activation of hexokinase in intact islets.

Potentiating synergism between adenosine diphosphate or triphosphate and acetylcholine on insulin secretion

The interaction between adenosine di- or triphosphate (ADP or ATP) and acetylcholine (ACh) was studied on insulin secretion. The experiments were performed on the isolated perfused rat pancreas, in the presence of a physiological nonstimulating glucose concentration (5.5 mM). ADP or ATP (1.65 microM) and ACh (0.05 microM) elicited a comparable peak of insulin secretion. When either ATP or ADP was simultaneously infused with ACh, insulin secretory response was significantly higher than the sum of the responses of each agonist applied separately. Similar effects were obtained with stable structural analogues of ATP and ADP (adenylimidodiphosphate, AMP-PNP, and alpha, beta-methyleneadenosine 5'-diphosphate, alpha, beta-methylene ADP) whether they acted alone or in combination with ACh. In contrast, adenosine was ineffective. Furthermore, the study of combined half doses of ATP (or ADP) and ACh allowed us to establish a potentiating synergism between both agonists. These data indicate a potentiating synergism on the beta-cell between ACh and ATP or ADP, the substances acting, respectively, via muscarinic cholinergic receptors and purinergic P2 receptors. So, ATP and ADP by activating P2 receptors could be involved in the parasympathetic control of insulin secretion.

Cholinergic stimulation of ion fluxes in pancreatic islets

Cholinergic agents are known to stimulate the hydrolysis of polyphosphoinositides in pancreatic islets. The effect of carbamylcholine upon ion fluxes in the islet cells was investigated. Carbamylcholine provoked a rapid but poorly sustained increase in 45Ca and 86Rb outflow from perifused islets. Such a cationic response was observed at different glucose concentrations (zero to 16.7 mM), at three concentrations of carbamylcholine (10 microM, 100 microM and 1.0 mM), and in the absence or presence of extracellular Ca2+. It coincided with a biphasic stimulation of insulin release, both the cationic and secretory responses being abolished in the presence of atropine (10 microM). At variance with nutrient secretagogues, carbamylcholine failed to affect the net production of cyclic AMP and caused a transient decrease in 32P outflow from islets prelabelled with [32P]phosphate. It is proposed that cholinergic agents mobilize Ca2+ from intracellular sites, possibly through generation of inositol, 1,4,5-triphosphate from phosphatidylinositol 4,5-bisphosphate. The intracellular redistribution of Ca2+ does not appear sufficient, however, to account fully for the secretory response, which may also involve activation of protein kinase C by diacylglycerol.

Adrenergic control of insulin release from isolated islet tissue in the rainbow trout, Salmo gairdneri R

Immunoreactive insulin levels (IRI) were measured by a homologous fish insulin radioimmunoassay. An in vitro pancreatic islet superfusion technique was employed to monitor the changes in IRI in the presence and absence of specific adrenergic agonists and antagonists. Exogenous adrenaline at low concentrations (10(-10) M) inhibited IRI release but evoked an IRI stimulation at high concentrations (10(-6) M). The stimulation of IRI by adrenaline is thought to involve beta-adrenoceptors located postsynaptically on the beta-cell membrane as the effect of adrenaline was mimicked by the beta-agonist, isoproterenol, and abolished by the beta-antagonist, propranolol. Phentolamine (an alpha-antagonist) potentiated the adrenergic stimulation of IRI, whereas yohimbine (an alpha 2-antagonist) was without effect. Phenylephrine (alpha 1-adrenoceptor agonist) inhibited IRI release suggesting the presence of alpha 1-inhibitory adrenoceptors which exert a modulatory influence on adrenaline-stimulated insulin release.

Purification and radioimmunoassays for superoxide dismutases in the mouse: tissue concentrations in different strains

Both murine cuprozinc and manganosuperoxide dismutases were purified to electrophoretic homogeneity from liver; the former (dimer) had a sp. act. of 2600 U/mg and a subunit mol. wt. of 17,000, the latter (tetramer) 2300 U/mg and mol. wt. 23,000. Heterologous radioimmunoassays had sensitivities of 3.1 ng/ml for the former enzyme and 2.5 ng/ml for the latter, and were appicable across murine genetic lines (Balb/c, ob/ob, db/db). Islets had the lowest concentrations of both enzymes among the tissues studied, and this could explain their vulnerability to free-radical damage.

Altered endocrine pancreatic function following vagotomy: possible behavioral and metabolic bases for assessing completeness of vagotomy

Total vagotomy induces multiple alterations in the function of the endocrine pancreas. The temporal evolution of selected behavioral and pancreatic alterations following subdiaphragmatic vagotomy was investigated. Sustained modifications were observed in the circadian rhythm of plasma insulin, beta-cell responses to glucose and acetylcholine, whole system response to oral, but not intravenous, glucose and meal frequency. Transient alterations occurred in plasma insulin concentrations, peak glucose response to oral glucose, body weight, food intake, dark-phase meal size and light/dark food intake ratio. Since a major methodological problem in studies of the effects of vagotomy remains the assessment of the completeness of nerve section, the efficiency of several validation tests based on pancreatic alterations were investigated. Of these possible tests, the marked changes in the time-course of blood glucose following oral glucose administration and the in vivo assessment of acetylcholine sensitivity appear to be the most promising. Among other tests investigated, the water intake/100 g body weight and the stomach weight/body weight ratio had high efficiencies. It is recommended that a combination of objective tests, appropriate to the specific study, with varying degrees of invasiveness be used to assess the completeness of vagotomy.

Organophosphate poisoning presenting as diabetic ketoacidosis

A 3-year-old boy was admitted to hospital following rapid-onset coma. Laboratory tests demonstrated hyperglycemia, glycosuria and keto-acidosis. Organophosphorus poisoning was the cause of the coma since he had been in contact with Parathion, serum cholinesterase activity was undetectable and his condition returned to normal under atropine therapy.

Modification of chemically induced diabetes in rats by vitamin E. Supplementation minimizes and depletion enhances development of diabetes

Administration of the antioxidant vitamin E to rats, prior to administration of either streptozotocin or alloxan, provided protection against the diabetogenic effect of both these agents. This was demonstrated by their response to a glucose load, their pancreatic insulin content and light microscopy findings. In addition, rats whose antioxidant state was depleted, by being maintained on a vitamin E and selenium-deficient diet, demonstrated increased diabetogenic susceptibility to normally nondiabetogenic doses of streptozotocin. These findings provide indirect support for the suggestion that the chemical agents streptozotocin and alloxan may exert their diabetogenic effect by acting as oxidants or free radical producers.

The modulatory effect of the hypothalamus on glucagon and insulin secretion in the rat

Rats were provided both with brain cannulas, to permit infusion of fluids in the ventromedial and lateral hypothalamic areas (resp. VMH and LH), and with two heart catheters. In this way infusions of fluids and withdrawal of blood could be done in unanesthetized free moving animals. Infusion of norepinephrine (NE) in the VMH elicited glucagon release during the whole period of NE infusion whereas insulin levels did not change. This glucagon release could not be suppressed by α- and β-receptor blockade but it was suppressed by hexamethonium, a blocking agent of signal transmission in peripheral ganglia of the autonomic nervous system. On the other hand, infusion of NE in the LH elicited insulin release during the whole period of NE infusion whereas glucagon levels remained unchanged. It is argued that [1] the sympathetic nervous system is involved in glucagon release, [2] no α- and β-receptor mechanisms are involved during this glucagon release, [3] the parasympathetic nervous system is involved in insulin release during noradrenergic LH stimulation.

Insulin secretory dynamics during development of rat pancreas

The dynamics of insulin secretion during development of the fetal rat pancreas were investigated. The time of onset of glucose-induced insulin secretion was of special interest. Pancreases from 15- to 22-day-old fetal rats were perifused in vitro with low (0.5 or 0.9 mg/ml) or high (5 mg/ml) concentrations of glucose in the presence or absence of arginine and leucine. Levels of insulin in the perifusate were determined by radioimmunoassay. At day 17, a significant increase in perfusate insulin level was observed in response to arginine and leucine (each at 5 mM), This response was independent of a high concentration of glucose. In addition, perifusate insulin levels were augmented when the concentration of amino acids were kept constant and the glucose concentration was changed from a high level to a low level. On day 20, a monophasic, rapid-onset short-duration rise in insulin release with a high glucose concentration was observed. This response was enhanced by acetylcholine (2.7 x 10(-9) M). At days 21 and 22, insulin levels rose rapidly in the presence of high glucose and remained elevated. The results show that there is considerable precision in the timing of the onset and maturation of the glucose-induced insulin secretory response prenatally and reaffirm that insulin secretion by the fetal beta-cell varies with the stimulus applied.

Influence of the vagus and splanchnic nerves on insulin secretion and glycemia

The role of the autonomic nervous system in control of the acute and the late-phase of insulin secretion was studied in mongrel dogs before and after bilateral severance of the vagus, splanchnic, and vagus plus splanchnic nerves. Glucose-stimulated acute insulin secretion increased after splanchnicotomy or transection of both autonomic nerves but remained unchanged after vagotomy alone. Late-phase insulin secretion was not altered by transection of any of the nerves alone or in combination; similar results were obtained as to fasting insulin levels. In contrast, fasting glycemia was increased after vagotomy and decreased after splanchnicotomy. These findings disclose tonic sympathetic and parasympathetic influences on blood glucose basal concentration and indicate an acute inhibitory role of the sympathetic system on insulin secretion.

Insulin release, cGMP, cAMP, and membrane potential in acetylcholine-stimulated islets

Acetylcholine potentiated the glucose-induced insulin release from microdissected mouse islets of Langerhans but had no effect on basal insulin release. Significant potentiation was obtained with 0.1 micron acetylcholine in the presence of 10 micron eserine and with 1 micron or more acetylcholine in the absence of a choline esterase inhibitor. Carbamylcholine, too, potentiated insulin release. Potentiation was blocked by methylatropine, whereas methylatropine alone had no effect on insulin release. Acetylcholine or carbamylcholine (5-500 micron) had no obvious effect on cyclic GMP or cyclic AMP in the islets. In the presence of 11.1 mM D-glucose, the membrane potential of beta-cells oscillated slowly between a polarized silent state of -50 to -55 mV and a depolarized active state of -33 to -39 mV, at which a fast spike activity occurred. Acetylcholine made the potential stay at the plateau and induced a continuous spike activity pattern. Atropine inhibited the electrical effects of acetylcholine but not those of glucose alone. It is suggested that cholinergic potentiation of insulin release is mediated by changes of transmembrane ionic fluxes, probably without the intervention of cyclic GMP or cyclic AMP.

Insulin infusion into the portal and peripheral circulations of unanaesthetized dogs

Changes in glucose, phosphate, potassium, non-esterified fatty acids (NEFA) and insulin in peripheral venous blood were compared in five unrestrained fasted dogs during different rates of insulin infusion by the portal and peripheral circulation. The 'low' rate of insulin infusion was that required to produce a fall in plasma glucose of 0.56-0.83 mmol/l (0.024 u/kg/h female, 0.012 u/kg/h male). Two-fold and four-fold higher rates are referred to as 'medium' and 'high' rates, respectively. In all dogs, dose-related reductions in the glucose concentration of peripheral blood resulted from increasing the rate of insulin by either route. At 'low' and 'high' rates of infusion the net response was independent of the route of administration, but the 'medium' rate of insulin infusion led to a greater degree of hypoglycaemia when given peripherally than intraportally. As expected, insulin infused peripherally resulted in graded increases in peripheral insulin levels as the rate of infusion was increased from low to high while, paradoxically, intraportal administration at low and medium infusion rates resulted in a mean decrease of peripheral insulin levels from control, the low-dose producing the most consistent fall. The fact that hypoglycaemia accompanied this fall during portal infusion suggests that these low doses of insulin had a direct effect on plasma phosphate resulting from the change in route of administration of insulin (i.e. increase during portal and decrease during peripheral infusion) also suggests a direct hepatic effect of insulin given intraportally.

A quantitative study of the insulin release induced by vagal stimulation in anesthetized cats

Insulin was released by vagal stimulation in anesthetized and eviscerated cats. The plasma insulin concentration and blood flow in the portal vein were determined concomitantly and the insulin output was calculated. Stimulation of either the right or the left cervical vagus released the same amount of insulin, whereas bilateral stimulation released twice as much. Following a stimulation that depleted the "vagally-releasable pool", a recovery period of 15--20 min was needed before the same maximal output could be obtained again. With shorter interstimulatory periods the amounts of insulin released were reduced. When less than 2--3 00 impulses were applied during a stimulation period, the amount of insulin released per impulse was constant. Atropine (0.2--2 mg/kg) did not reduce the vagally-induced insulin release.

In vitro release of gastrin from isolated perfused antrum

Gastrin is released from isolated perfused rat antra in a regular fashion by a variety of stimuli, including acetylcholine, glycine, and calcium. Two peaks are seen with the physiologic stimulus, acetylcholine, suggesting perhaps two different antral gastrin populations.

Interactions of acetylcholine and epinephrine on the dynamics of insulin release in vitro

An in vitro system for perifusion of rat pancreatic islets has been utilized to define the effects of epinephrine on acetylcholine-induced insulin release over varying concentrations of the two agents. Perifusion of islets with epinephrine before challenge with acetycholine produced marked enhancement of both phases of cholinergically induced insulin release; enhancement of the first phase being more marked with increase in acetylcholine concentration and the converse being observed with the second phase. Perifusion of islets with epinephrine during stimulation with acetylcholine produced inhibition of insulin release, an effect dependent upon the concentration of epinephrine and of acetylcholine. There was an order of difference in the acetycholine concentration needed to overcome significant epinephrine-mediated inhibition of the first phase of insulin release (5 X 10(-4) mug/ml) and that needed to overcome inhibition of the second phase (5 X 10(-3) mug/ml). Comparison of the effects of various concentrations of epinephrine on glucose- and acetyl-choline-induced insulin release revealed that epinephrine was a less potent inhibitor of the first phase of acetylcholine-induced insulin release than of the first phase of glucose-induced insulin release. These data provide some insight into the potential interactions between cholinergic and adrenergic autonomic systems in modifying insulin release.

Portal and cubital serum insulin during oral, portal and cubital glucose tolerance tests

Oral, intracubital, and intraportal glucose tolerance tests have been performed on 7 non-obese non-diabetics, and glucose and insulin concentrations have been followed in the peripheral and portal blood. A significant rise in portal glucose and insulin was found 1/2-2 min after oral glucose intake. There was no lag between the rise in insulin and glucose concentrations. The portal glucose concentration after oral glucose intake was significantly higher than after cubital glucose infusion for 45 min, although the peripheral glucose concentrations were identical. In the cubital vein the insulin concentration after oral glucose intake was significantly higher than after i.v. glucose infusion, but in the portal blood there was no difference. After portal glucose infusion the cubital insulin concentration did not differ significantly from the concentration after i.v. glucose infusion. Thus, it seems unlikely that a high portal glucose concentration is responsible for the higher peripheral insulin concentration after oral glucose intake. A high portal glucose concentration does not seem to influence the hepatic uptake or release of glucose.