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

Cooperation between cAMP signalling and sulfonylurea in insulin secretion

Although glucose is physiologically the most important regulator of insulin secretion, glucose-induced insulin secretion is modulated by hormonal and neural inputs to pancreatic β-cells. Most of the hormones and neurotransmitters evoke intracellular signals such as cAMP, Ca²⁺ , and phospholipid-derived molecules by activating G protein-coupled receptors (GPCRs). In particular, cAMP is a key second messenger that amplifies insulin secretion in a glucose concentration-dependent manner. The action of cAMP on insulin secretion is mediated by both protein kinase A (PKA)-dependent and Epac2A-dependent mechanisms. Many of the proteins expressed in β-cells are phosphorylated by PKA in vitro, but only a few proteins in which PKA phosphorylation directly affects insulin secretion have been identified. On the other hand, Epac2A activates the Ras-like small G protein Rap in a cAMP-dependent manner. Epac2A is also directly activated by various sulfonylureas, except for gliclazide. 8-pCPT-2'-O-Me-cAMP, an Epac-selective cAMP analogue, and glibenclamide, a sulfonylurea, synergistically activate Epac2A and Rap1, whereas adrenaline, which suppresses cAMP production in pancreatic β-cells, blocks activation of Epac2A and Rap1 by glibenclamide. Thus, cAMP signalling and sulfonylurea cooperatively activate Epac2A and Rap1. This interaction could account, at least in part, for the synergistic effects of incretin-related drugs and sulfonylureas in insulin secretion. Accordingly, clarification of the mechanism of Epac2A activation may provide therapeutic strategies to improve insulin secretion in diabetes.

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

AIMS/HYPOTHESIS

Somatostatin from islet delta cells inhibits insulin and glucagon secretion, but knowledge of the regulation of pancreatic somatostatin release is limited. Some insulin secretagogues stimulate somatostatin secretion, and here we investigated whether delta cell secretory responses are indirectly regulated in a paracrine manner by insulin released from beta cells.

METHODS

Hormone release from static incubations of primary mouse islets or somatostatin-secreting TGP52 cells was measured by RIA. mRNA expression was assessed by RT-PCR.

RESULTS

Glucose and a range of other physiological and pharmacological agents stimulated insulin and somatostatin release, and insulin receptor mRNA was expressed in islets, MIN6 beta cells and TGP52 cells. However, exogenous insulin did not modulate basal or glucose-induced somatostatin secretion from islets, nor did pre-incubation with an antibody against the insulin receptor or with the insulin receptor tyrosine kinase inhibitor, HNMPA(AM)(3). Glucose and tolbutamide stimulated somatostatin release from TGP52 cells, whereas a range of receptor-operating agents had no effect, the latter being consistent with a lack of corresponding receptor mRNA expression in these cells. Parasympathetic activation stimulated insulin, but inhibited somatostatin release from mouse islets in accordance with differences in muscarinic receptor mRNA expression in islets, MIN6 and TGP52 cells. The inhibitory effect on somatostatin secretion was reversed by pertussis toxin or the muscarinic receptor 2 antagonist, methoctramine.

CONCLUSIONS/INTERPRETATIONS

A number of insulin secretagogues have analogous effects on insulin and somatostatin release, but this similarity of response is not mediated by an indirect, paracrine action of insulin on delta cells.

The dopamine receptor D2 agonist bromocriptine inhibits glucose-stimulated insulin secretion by direct activation of the alpha2-adrenergic receptors in beta cells

Treatment with the dopamine receptor D2 (DRD2) agonist bromocriptine improves metabolic features in obese patients with type 2 diabetes by a still unknown mechanism. In the present study, we investigated the acute effect of bromocriptine and its underlying mechanism(s) on insulin secretion both in vivo and in vitro. For this purpose, C57Bl6/J mice were subjected to an intraperitoneal glucose tolerance test (ipGTT) and a hyperglycemic (HG) clamp 60min after a single injection of bromocriptine or placebo. The effects of bromocriptine on glucose-stimulated insulin secretion (GSIS), cell membrane potential and intracellular cAMP levels were also determined in INS-1E beta cells. We report here that bromocriptine increased glucose levels during ipGTT in vivo, an effect associated with a dose-dependent decrease in GSIS. During the HG clamp, bromocriptine reduced both first-phase and second-phase insulin response. This inhibitory effect was also observed in INS-1E beta cells, in which therapeutic concentrations of bromocriptine (0.5-50nM) decreased GSIS. Mechanistically, neither cellular energy state nor cell membrane depolarization was affected by bromocriptine whereas intracellular cAMP levels were significantly reduced, suggesting involvement of G-protein-coupled receptors. Surprisingly, the DRD2 antagonist domperidone did not counteract the effect of bromocriptine on GSIS, whereas yohimbine, an antagonist of the alpha2-adrenergic receptors, completely abolished bromocriptine-induced inhibition of GSIS. In conclusion, acute administration of bromocriptine inhibits GSIS by a DRD2-independent mechanism involving direct activation of the pancreatic alpha2-adrenergic receptors. We suggest that treatment with bromocriptine promotes beta cells rest, thereby preventing long-lasting hypersecretion of insulin and subsequent beta cell failure.

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.

The circulating hormonal milieu of the endocrine pancreas in healthy individuals, organ donors, and the isolated perfused human pancreas

Although basal circulating levels of individual islet cell hormones have been measured, few studies compared the molar ratios of the major hormones secreted by the endocrine pancreas. This study examined the basal levels of four major islet hormones: insulin, C-peptide (C-P), glucagon (G), and pancreatic polypeptide (PP) in normal subjects, in organ donors with brain death, and in the isolated perfused human pancreas. Basal blood samples were taken from normal, fasted control subjects (NCs). Pancreata were obtained from 17 organ donors (ODs) with donor portal vein (DPV) and radial arterial (DRA) blood samples taken before organ procurement. Single-pass perfusion was performed on the procured pancreata, and after rewarming and equilibration, basal samples were collected from the splenic vein (SV) for 30 min. Radioimmunoassays of insulin, C-P, G, and PP were performed on all samples, and basal levels of all hormones were expressed as a common unit, femtomoles per milliliter. The data suggest that in the basal state, these four major islet hormones circulate in a relatively constant molar ratio. The ratio of the hormones is altered in brain death and with in vitro perfusion of the pancreas. The isolated perfused human pancreas secretes a relatively constant molar ratio of these hormones; however, this ratio is markedly different from the circulating ratio seen in either the NC group or the OD group. We conclude that a relatively constant hormonal milieu is secreted from the normal endocrine pancreas, and this hormonal milieu is altered after brain death and with isolation and perfusion of the human pancreas.

Sympathetic inhibition, leptin, and uncoupling protein subtype expression in normal fasting rats

To further investigate neural effects on leptin and uncoupling proteins (UCPs), we studied in vivo perturbations intended to block adrenergic input to peripheral tissues. We examined plasma leptin, leptin mRNA, and adipose and muscle UCP subtype mRNA in rats treated with alpha-methyl-p-tyrosine methyl ester (AMPT-ME), which inhibits catecholamine synthesis and 6-hydroxydopamine (6HDA), which is toxic to catecholinergic nerve terminals but, unlike AMPT-ME, does not enter the central nervous system. Intraperitoneal AMPT-ME, 250 mg/kg, was administered at 1800 and 0700 the following day, and rats were killed at 1200-1400. All rats were fasted with free access to water during this time. Intraperitoneal AMPT-ME increased plasma leptin by 15-fold, increased interscapular brown adipose tissue (IBAT) and epididymal fat leptin mRNA by 2- to 2.5-fold, and also increased plasma insulin and glucose concentrations. Intraperitoneal AMPT-ME decreased IBAT UCP-3 mRNA to 40% of control, while it increased epididymal adipose UCP-3 mRNA approximately twofold. Intravenous AMPT-ME, 250 mg/kg, administered to conscious rats for 5 h decreased lumbar sympathetic nerve activity, increased plasma leptin (5.89 +/- 1.43 compared with 2.75 +/- 0.31 ng/ml in vehicle-treated rats, n = 7, P < 0.05), and decreased cardiac rate with no sustained change in blood pressure. Intraperitoneal 6HDA, 100 mg/kg, as a single dose at 1800, increased plasma leptin approximately twofold after 18-20 h, increased IBAT (but not epididymal fat) leptin mRNA by two- to threefold, and decreased IBAT UCP-3 mRNA to 30-40% of control. Neither AMPT-ME nor 6HDA significantly altered mRNA encoding gastrocnemius muscle UCP-3, IBAT UCP-1, or IBAT and epididymal UCP-2. In summary, AMPT-ME and 6HDA increased plasma leptin and upregulated leptin mRNA expression. AMPT-ME also resulted in complex tissue and subtype-specific modulation of adipose UCP mRNA. These data are consistent with interaction between leptin and sympathetic nerve activity (SNA) in regulation of fat cell energy utilization. However, the in vivo modulation of leptin and UCPs appears complex and, beyond a causal effect of SNA per se, may depend on concurrent changes in plasma insulin, glucose, and circulatory hemodynamics.

Ritodrine- and terbutaline-induced hypokalemia in preterm labor: mechanisms and consequences

The effects of ritodrine and terbutaline on potassium homeostasis, renal function, and cardiac rhythm were assessed in women treated with these drugs for preterm labor. Timed blood and urine samples were obtained for two hours before and during six hours of intravenous ritodrine (N = 5) and terbutaline (N = 5) administered in pharmacologically equivalent doses. No differences were found in any parameters affecting potassium homeostasis or renal function between these drugs. A decrease in mean plasma potassium of 0.9 mEq/liter occurred after 30 minutes of drug infusion (4.2 +/- 0.1 to 3.3 +/- 0.1 mEq/liter, P < 0.005) before any significant changes in plasma glucose (75.0 +/- 4.7 to 93.7 +/- 6.1 mg/dl, P = NS) or plasma insulin (12.4 +/- 6.0 to 28.4 +/- 5.1 mU/ml, P = NS). The mean plasma potassium after four hours of drug infusion was 2.5 +/- 0.1 mEq/liter. Plasma insulin rose to a level known to induce cellular potassium uptake (39.2 +/- 7.7 mU/ml) after 60 minutes of drug therapy and remained at this level for four hours. Hyperlactatemia occurred at four hours (4.7 +/- 0.8 mmol/liter) and the plasma lactate/pyruvate ratio increased in a 10:1 ratio. Both drugs significantly reduced glomerular filtration rate, sodium, potassium, and chloride excretion and urinary flow rate. Changes in acid-base homeostasis, plasma aldosterone, or renal potassium excretion did not contribute to ritodrine-or terbutaline-induced hypokalemia. In 83 women with preterm labor randomly assigned to ritodrine (N = 42) or terbutaline (N = 41), the maximum decrease in plasma potassium occurred after six hours of drug infusion. During Holter monitoring, 3 of 14 women treated with ritodrine or terbutaline developed symptomatic cardiac arrhythmias at the lowest plasma potassium while no women treated with saline and morphine (N = 12) developed cardiac arrhythmias (P = 0.14). We conclude that ritodrine and terbutaline induce profound hypokalemia by stimulating cellular potassium uptake and both drugs cause significant renal sodium and fluid retention and cardiac arrhythmias. Careful monitoring of electrolytes, fluid balance, and cardiac rhythm should occur during tocolytic therapy with ritodrine or terbutaline.

Differential expression of alpha 2-adrenoceptor subtypes in purified rat pancreatic islet A- and B-cells

In the endocrine pancreas, alpha 2-adrenoceptor stimulation reduces glucose-induced insulin secretion from islet B-cells. There is, however, controversy with regard to the effects of alpha 2-agonists on islet A-cell function. In this paper, we have used RNA samples prepared from whole rat islets and from FACS-purified rat A- and B-cells to study alpha 2-adrenoceptor subtype expression. RNase protection assays detected transcripts encoding alpha 2a and alpha 2b subtypes in the RNA pool of rat islets. Reverse transcription-PCR revealed that transcripts for all three alpha 2-adrenoceptor subtypes are present in rat islet cells in purified A-cell RNA. In contrast, RT-PCR of islet B-cell RNA yielded products corresponding to alpha 2a and alpha 2c, with no evidence for the presence of alpha 2b. Thus, the results reveal that both islet cell types express more than one receptor subtype and suggest that the distribution of the subtypes may differ between rat islet A- and B-cells.

Stimulative effect of epinephrine on glucose production and utilization rates in sheep using a stable isotope

The rates of the production and utilization of blood glucose were measured during intravenous epinephrine infusion (2.0 nmol kg-1 min-1 for 1 hr) in sheep. An isotope dilution method with an infusion of [U-13C]glucose and nonsteady-state equations were used for the determination of blood glucose kinetics. Heart rate and concentrations of blood glucose, plasma free fatty acids, and lactate increased (P < 0.01), whereas plasma insulin concentrations tended to decrease (P < 0.06) during epinephrine infusion. The blood glucose turnover rate was 1.9 +/- 0.1 mg kg-1 min-1 before epinephrine infusion. The rate of blood glucose production increased (P < 0.01) to 6.4 +/- 0.5 mg kg-1 min-1 at 20 min after the initiation of epinephrine infusion. The blood glucose utilization rate increased (P < 0.05) gradually and reached 4.1 +/- 1.2 mg kg-1 min-1 at 60 min after the initiation of epinephrine infusion. These results suggest that in sheep, epinephrine stimulates the rates of both the production and the utilization of blood glucose and that hyperglycemia induced by epinephrine infusion is mainly due to a rapid enhancement in the rate of the production of blood glucose.

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

OBJECTIVE

The somatostatin-secreting delta cells in the islets of Langerhans appear to be regulated by neural mechanisms that have not been defined clearly. In this study, the celiac neural bundle of the human pancreas was electrically stimulated in the presence and absence of selective neural antagonists.

SUMMARY BACKGROUND DATA

The authors previously reported on studies of the splanchnic neural regulation of insulin, glucagon, and pancreatic polypeptide secretion. In these studies, alpha-adrenergic fibers appeared to have a predominant effect, strongly inhibiting the secretion of insulin, glucagon, and pancreatic polypeptide secretion. Cholinergic fibers appeared to stimulate strongly, although beta-adrenergic fibers weakly stimulated, the secretion of these hormones. Investigations of neural regulatory mechanisms governing human somatostatin release in vitro have not been previously reported.

METHODS

Pancreata were obtained from eight cadaveric organ donors. The isolated perfused human pancreas technique was used to assess the regulation of somatostatin secretion by the various neural fibers contained within the celiac plexus. The secretory response of somatostatin was examined in the presence of 16.7 mmol/L glucose, with and without neural stimulation, and specific neural antagonists.

RESULTS

The basal somatostatin secretion was 88 +/- 26 fmol/g/min and increased 131 +/- 23% (n = 8, p < 0.01) in response to 16.7 mmol/L glucose. The augmentation seen with glucose was inhibited 66 +/- 22% (n = 8, p < 0.05) during celiac neural bundle stimulation. Alpha-adrenergic blockade resulted in a 90 +/- 30% (n = 6, p < 0.01) augmentation of somatostatin release. Beta-adrenergic blockade caused a 13 +/- 2% (n = 6, p < 0.05) suppression of somatostatin release. Complete adrenergic blockade resulted in a 25 +/- 23% (n = 5, p = not significant) inhibition of somatostatin release. Cholinergic blockade resulted in a 40 +/- 10% (n = 6, p < 0.02) suppression of somatostatin release.

CONCLUSIONS

The predominant effect of celiac neural bundle stimulation was inhibition of somatostatin secretion through an alpha-adrenergic effect. Beta-adrenergic fibers stimulate somatostatin secretion; cholinergic fibers have a negligible effect on somatostatin secretion. These data suggest that the splanchnic innervation of the pancreas has a potent regulatory role in somatostatin release in this in vitro human model.

Hepatic regulation of pancreatic alpha-cell function

The direct feedback regulation between the endocrine gland and its target organ is an expected biological relationship. However, such a phenomenon is far from being well established in the case of the endocrine pancreas and its major target organ, the liver, especially since plasma glucose has been established as the prime regulator. In this perspective, I have reexamined the feedback regulation between plasma glucose and glucagon secretion by the pancreatic alpha cell. Surprisingly, available data in the literature appear to document a frequent breakdown of this well-established interdependence between plasma glucose and pancreatic alpha cells, as reflected by a sustained elevation of plasma glucagon levels in several physiologic and pathologic states with concurrent euglycemia or hyperglycemia. Moreover, normal or low glucagon concentrations in the presence of fasting hypoglycemia in patients with insulinoma or non-islet cell tumors secreting insulin-like peptides and in patients with hepatic glycogen storage disorders may enhance our hypothesis that plasma glucose level may not be the major regulator of glucagon secretion. Extensive data in the literature show that hyperglucagonemic states are characterized by a unique metabolic environment, namely hepatic glycogen depletion. Similarly, hepatic glycogen stores are abundant in the presence of normal or low glucagon concentrations. These findings imply a distinct relationship between hepatic glycogen content and plasma glucagon level.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 2-adrenergic modulation of pancreatic glucagon secretion in rats

The present study was designed to clarify the mechanism of adrenergic modulation of pancreatic glucagon secretion in rats under physiological conditions by 1) epinephrine infusion alone or together with adrenergic blockers and 2) administration of adrenergic agonists. Intravenous infusion of epinephrine alone (1 microgram/kg/min, equal to 0.7 nmol/kg/min) caused a significant increase in glucagon secretion. Phentolamine (an alpha blocker) or yohimbine (an alpha 2 blocker) administration completely inhibited the increase of glucagon secretion caused by epinephrine infusion, but neither the administration of bunazosin (an alpha 1 blocker) nor beta blockers inhibited it. Infusion of clonidine (an alpha 2 agonist) caused significant increase of glucagon secretion even at a low dose of 0.5 nmol/kg/min, although infusion of neither an alpha 1 nor a beta 2 agonist caused it even at the high dose of 40.0 nmol/kg/min. It is concluded that the alpha 2 receptor mechanism plays the most important role in the adrenergic modulation of glucagon secretion in rats under physiological conditions.

Beta-cell dysfunction in hyperglycaemic rat models: recovery of glucose-induced insulin secretion with lowering of the ambient glucose level

Glucose-induced insulin secretion is lost in the face of chronic hyperglycaemia. The same defect is present when normal rats are made hyperglycaemic by 48-h glucose infusions. Insulin secretory responses were mapped out during the post-infusion period in order to determine how long it takes for normal Beta-cell function to recover, and to identify factors which influence the rate of recovery. Male Sprague Dawley rats weighing 200-250 g were infused with 50% glucose or 77 mmol/l NaCl for 48 h. The glucose-infused rats were mildly hypoglycaemic for 14 h after the infusion ceased. Glucose-induced insulin secretion, absent at the end of the glucose infusion, was normal 6 h post-infusion. Such rapid recovery was not because of the short duration of hyperglycaemia; mild hypoglycaemia from a 5-h insulin infusion in 90% pancreatectomized rats resulted in a four-fold rise in glucose-induced insulin secretion. Under in vitro conditions, extreme glucose deprivation caused by perfusing the pancreas of glucose-infused rats with buffer devoid of glucose restored glucose-induced insulin secretion in just 37 min. Therefore, the suppression of glucose-induced insulin release by chronic hyperglycaemia is a dynamic situation that requires ongoing hyperglycaemia to prevent the reappearance of glucose responsiveness. This study shows recovery of glucose-induced insulin secretion after just 6 h of mild hypoglycaemia in vivo and even faster recovery with more severe glucose deprivation in vitro. Our results suggest that there is an inverse relationship between the rate of return of Beta-cell glucose responsiveness and the ambient glucose concentration.

Selective stimulation of glucagon secretion by beta 2-adrenoceptors in isolated islets of Langerhans of the rat

1. In rat isolated islets of Langerhans the selective beta 2-adrenoceptor agonist, clenbuterol (1 to 20 microM), significantly increased the level of adenosine 3':5'-cyclic monophosphate (cyclic AMP) within 2 min of incubation. 2. The cyclic AMP response to clenbuterol was inhibited in the presence of the selective beta 2 adrenoceptor antagonist, ICI 118551 (0.1 or 10 microM) but remained unchanged when the beta 1-antagonist, atenolol (0.1 microM) was administered. 3. Despite causing an elevation in cyclic AMP, clenbuterol (up to 20 microM) failed to influence insulin secretion at any glucose concentration tested, even in the presence of a phosphodiesterase inhibitor. 4. By contrast, clenbuterol elicited a dose-dependent rise in the rate of glucagon secretion; the maximal agonist-induced increase in secretion was two fold, a response equivalent to that observed with 20 mM L-arginine. 5. ICI 118551 significantly inhibited the rise in glucagon secretion induced by clenbuterol (up to 20 microM). 6. The results indicate that the rat islet A cell population is equipped with functional beta 2-adrenoceptors which influence glucagon secretion via the second messenger cyclic AMP, but that the B cells are deficient in functional beta-receptors.

Neural control of islet function by norepinephrine and sympathetic neuropeptides

It is clear that the sympathoadrenal system has a role in the regulation of endocrine pancreatic function and that the sympathetic nerves of the pancreas can change pancreatic hormone secretion to increase the availability of metabolic fuels. It seems likely that the classical sympathetic neurotransmitter, NE, acts in concert with peptide co-transmitters, such as galanin and NPY. Each is released during the stimulation of pancreatic sympathetic nerves and each is capable of influencing either islet function or pancreatic blood flow. There is considerable indirect evidence that the sympathetic innervation of the pancreas is activated during acute stress and influences the endocrine pancreas. However, proving such a physiologic role is difficult because of redundant mechanisms that influence the secretion of the metabolically-crucial hormones, insulin and glucagon. Such definitive proof therefore awaits the development of new techniques to dissect and dissociate these mechanisms.

Mechanism of sympathetic neural regulation of insulin, somatostatin, and glucagon secretion

The effects of electrical stimulation of the left splanchnic nerve on insulin, somatostatin, and glucagon secretion from the isolated perfused rat pancreas were investigated. Electrical splanchnic nerve stimulation (SNS), performed by square-wave impulses, produced a 25% decrease in effluent flow and a 10-fold increase in perfusate norepinephrine. Both insulin and somatostatin output in the presence of 16.7 mM glucose were inhibited during SNS by 85 and 56% of the basal value, respectively. Glucagon output in the presence of 5.5 mM glucose was stimulated 20-fold by SNS. Perfusion with 10(-6) M propranolol further decreased insulin and somatostatin output during SNS, when expressed as the total decrement beneath basal during stimulation. The glucagon response to SNS tended to be enhanced, although not significantly, by simultaneous infusion of 10(-6) M propranolol. However, 10(-6) M phentolamine (Phe) attenuated the SNS-induced inhibition of insulin and somatostatin output by 50 and 40%, respectively. However, insulin output remained decreased after SNS with Phe. The SNS-induced glucagon response was completely abolished by 10(-6) M Phe alone or by 10(-6) M Phe plus 10(-6) M propranolol. With 10(-6) M Phe plus 10(-6) M propranolol, insulin and somatostatin output remained decreased after SNS. These results suggest that insulin and somatostatin secretions induced by glucose are inhibited during SNS through the alpha-adrenergic mechanism and also that the beta-adrenergic mechanism exerts a stimulatory action. SNS-induced glucagon secretion occurs mainly through alpha-adrenergic activation.(ABSTRACT TRUNCATED AT 250 WORDS)

An insulin-releasing property of imidazoline derivatives is not limited to compounds that block alpha-adrenoceptors

As we have demonstrated previously phentolamine stimulates the release of additional insulin from isolated mouse islets and raises plasma insulin levels in the whole rat. This effect was independent of the well known property of phentolamine to block alpha-adrenoceptors. In experiments on isolated pancreatic islets from mice we now demonstrate that tolazoline and antazoline which are chemically closely related to phentolamine, share its ability to potentiate insulin release. The following results were taken as evidence that this effect does not result from an alpha-adrenoceptor blocking action of imidazoline compounds. More than 10 times higher concentrations of phentolamine were required to liberate additional insulin from isolated islets than were effective in counteracting the inhibitory effect of clonidine on insulin release. The newly introduced alpha 2-adrenoceptor antagonist BDF 8933, which is an imidazoline derivative, stimulates insulin release as well, while the irreversible alpha-adrenoceptor blocking agent benextramine of different structure failed to do so, even when being present in concentrations blocking the alpha 2-adrenoceptor-mediated effects of clonidine. Antazoline shared the ability of phentolamine to stimulate insulin release despite having no or only very little alpha-adrenoceptor blocking activity. When used under our conditions, it almost entirely failed to alleviate the inhibition of insulin release induced by clonidine. We conclude that the response of the islet cells to imidazoline derivatives is not limited to those capable of blocking alpha-adrenoceptors. On the other hand, alpha-adrenoceptor blocking agents of different chemical structure fail to induce the release of additional insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonadrenergic sympathetic neural influences on basal pancreatic hormone secretion

Evidence for peptidergic innervation of the islets of Langerhans is increasing, yet the role of neuropeptides in mediating neurally induced changes of islet function is not clear. To determine if nonadrenergic transmitters make an important contribution to sympathetic neural effects on basal pancreatic hormone secretion, we examined the effect of local sympathetic nerve stimulation (SNS) on the output of immunoreactive insulin (IRI), immunoreactive glucagon (IRG), and somatostatin (SLI) from the duodenal lobe of the pancreas in situ in halothane-anesthetized dogs, under conditions where the actions of the classical transmitter norepinephrine (NE) should be blocked by propranolol (PROP) and yohimbine (YO). In the absence of adrenergic antagonists, SNS rapidly reduced the output of IRI (delta = -1.34 +/- 0.91 mU/min) and SLI (delta = -600 +/- 350 fmol/min) and stimulated that of IRG (delta = +1.39 +/- 0.57 ng/min). In the presence of PROP and YO, SNS induced similar changes of hormone secretion: delta IRI, -1.30 +/- 0.53 mU/min; delta SLI, -480 +/- 180 fmol/min; delta IRG = +1.89 +/- 0.63 ng/min. Because PROP and YO abolished the pancreatic effects of high dose infusions of NE (1 microgram.kg-1.min-1 iv), we suggest that the antagonists produced sufficient, combined adrenergic blockade at the level of the islet, and we conclude that a nonadrenergic neurotransmitter or modulator plays a major role in mediating sympathetic neural effects on basal islet hormone secretion.

Splanchnic neural regulation of insulin and glucagon secretion in the isolated perfused human pancreas

The isolated perfused human pancreas was employed as a model in which electrical stimulation of the celiac mixed neural bundle was performed in the presence or absence of selective neural blockers. The insulin and glucagon responses to hyperglycemia alone or in the presence of splanchnic nerve stimulation were similar in magnitude to the results obtained in a preliminary report on isolated human pancreatic function and in studies using animal models. Stimulation of the celiac neural bundle in the presence of hyperglycemia resulted in an inhibition of insulin release and in an augmentation of glucagon release. alpha-adrenergic stimulation resulted in a strong suppression of insulin secretion and a mild suppression of glucagon secretion. beta-adrenergic fiber stimulation caused a mild augmentation of both insulin and glucagon release, whereas the cholinergic fibers strongly stimulated both alpha- and beta-cell secretion. The predominant effects of celiac neural bundle stimulation are insulin inhibition by was of an alpha-adrenergic effect and glucagon stimulation by way of a cholinergic effect. Thus, in this in vitro human model, our data confirm that the splanchnic innervation of the pancreas has a potent regulatory role on pancreatic hormone release in human subjects.

Effects of isoprenaline and glucagon on insulin secretion from pancreatic islets

The effects of isoprenaline and glucagon on insulin secretion from pancreatic islets were investigated. In the presence of high concentrations of isoprenaline (10-50 mumol/l), glucose-induced (20 mmol/l) insulin secretion from isolated perifused mouse islets was inhibited. This inhibition was apparently mediated by alpha 2-adrenoceptors, as it was antagonized by rauwolscine. At low concentrations isoprenaline (0.1 or 1 mumol/l) did not affect glucose-induced (2.5; 10 or 20 mmol/l) insulin secretion from perifused mouse or rat islets, even if alpha 2-adrenoceptors were blocked by rauwolscine. A stimulatory effect of isoprenaline on insulin secretion was also not observed in the perfused rat pancreas. However, when incubated mouse islets were exposed to glucose (10 mmol/l), insulin secretion was further enhanced by isoprenaline (0.5 mumol/l). To elucidate the underlying mechanism, the effects of glucagon on insulin secretion were investigated, because glucagon is released from the pancreatic A-cells during stimulation with isoprenaline and is accumulated in the islets and the surrounding medium during incubations of pancreatic islets. Indeed, glucagon stimulated insulin secretion from perifused mouse islets in the presence of high glucose (10 or 15 mmol/l) concentrations but not of low glucose (5 mmol/l) concentrations. Thus it is concluded that direct beta-adrenergic stimulation of pancreatic B-cells does not occur in mouse or rat pancreatic islets. Augmentation of glucose-induced insulin secretion by isoprenaline observed in incubation systems can be explained as a result of stimulation by glucagon, which is released from pancreatic A-cells by isoprenaline.

Regulation of somatostatin secretion in man: study of the role of free fatty acids and ketone bodies

We have investigated in normal subjects the possible role of plasma free fatty acids (FFA) and blood ketone bodies (KB) in the regulation of human somatostatin secretion. Heparin injected during the intravenous infusion of a fat emulsion raised FFA levels acutely from 0.4 +/- 0.1 to near 3 mmol/L. Plasma somatostatin-like immunoreactivity (SLI) rose from a mean (+/- SEM) basal value of 9.2 +/- 1.0 ng Eq S14/L to 20.0 +/- 6.0 ng Eq S14/L (P less than 0.05). Plasma immunoreactive insulin (IRI) level was unchanged and glucagon (IRG) concentration decreased from 156 +/- 20 to 107 +/- 2 ng/L (P less than 0.05). During this test, there was a rise not only in FFA but also in plasma triglycerides (TG) and in blood glycerol and KB levels. The infusion of a fat emulsion alone increased triglyceride and glycerol levels to a similar extent but induced also a mild rise of FFA (0.37 +/- 0.05 to 1.13 +/- 0.5 mmol/L, P less than 0.01), KB (78 +/- 12 to 360 +/- 45 mumol/L, P less than 0.01), and SLI (14.8 +/- 4.6 to 23.8 +/- 7.1 ng Eq S14/L, P less than 0.05). The induction by DL-Na-3-hydroxybutyrate infusion of a rise of KB was associated with a decrease of FFA (P less than 0.05) and SLI (P less than 0.05) without modification of IRI or IRG levels. Phentolamine infusion did not modify the SLI or glucagon response to acute elevations of FFA, whereas propranolol suppressed the increase of SLI without preventing the concomitant decrease of IRG.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic regulation of insulin secretion from the chicken pancreas in vitro

Adrenergic regulation of insulin secretion in the chicken was studied using a perifused pancreas fragment preparation. Beta-adrenergic stimulation by 50 microM isoproterenol potentiated theophylline-stimulated insulin secretion. Glucose at 19.5 mM did not stimulate insulin secretion, a finding consistent with previous reports of chicken pancreas sensitivity in vitro. Pretreatment with 50 microM isoproterenol did not alter this glucose insensitivity. Alpha-adrenergic stimulation by 50 microM epinephrine in the presence of beta blockade by sotalol or by 50 microM phenylephrine did not alter insulin secretion. Inhibition of insulin secretion by somatostatin could be demonstrated, however. Epinephrine, 50 and 0.164 microM, potentiated theophylline-stimulated insulin release and at 50 microM stimulated insulin secretion as an off-response even in the absence of theophylline. It is concluded that adrenergic regulation of insulin secretion in the chicken is primarily mediated through beta-adrenergic receptors, resulting in stimulation of insulin secretion.

Morphological and functional aspects of pancreatic islet innervation

Pancreatic islets are collections of 4 functionally-related endocrine cells distributed nonrandomly in the pancreas. Their major physiological actions center about the regulation of metabolic homeostasis. Experimental evidence shows that, in addition to circulating substates, the islets are controlled by outflow from the central nervous system communicated through autonomic nerves. Islet cells also interact with one another via hormonal messengers and, possibly, electrotonic impulses producing a complex--yet well-controlled--system for the integration of numerous types of signals. This paper is a brief review of some of the numerous interactions between the autonomic nervous system and the endocrine pancreas. Particular emphasis is placed on the role of recently discovered autonomic factors and newly recognized autonomic centers in the brain.