The effects of prostaglandins on secretion of glucagon and insulin by the perfused rat pancreas

Rat prostaglandin EP3 receptor is highly promiscuous and is the sole prostanoid receptor family member that regulates INS-1 (832/3) cell glucose-stimulated insulin secretion

Chronic elevations in fatty acid metabolites termed prostaglandins can be found in circulation and in pancreatic islets from mice or humans with diabetes and have been suggested as contributing to the β‐cell dysfunction of the disease. Two‐series prostaglandins bind to a family of G‐protein‐coupled receptors, each with different biochemical and pharmacological properties. Prostaglandin E receptor (EP) subfamily agonists and antagonists have been shown to influence β‐cell insulin secretion, replication, and/or survival. Here, we define EP3 as the sole prostanoid receptor family member expressed in a rat β‐cell‐derived line that regulates glucose‐stimulated insulin secretion. Several other agonists classically understood as selective for other prostanoid receptor family members also reduce glucose‐stimulated insulin secretion, but these effects are only observed at relatively high concentrations, and, using a well‐characterized EP3‐specific antagonist, are mediated solely by cross‐reactivity with rat EP3. Our findings confirm the critical role of EP3 in regulating β‐cell function, but are also of general interest, as many agonists supposedly selective for other prostanoid receptor family members are also full and efficacious agonists of EP3. Therefore, care must be taken when interpreting experimental results from cells or cell lines that also express EP3.

Nutrient overload, lipid peroxidation and pancreatic beta cell function

Since the landmark discovery of α,β-unsaturated 4-hydroxyalkenals by Esterbauer and colleagues most studies have addressed the consequences of the tendency of these lipid peroxidation products to form covalent adducts with macromolecules and modify cellular functions. Many studies describe detrimental and cytotoxic effects of 4-hydroxy-2E-nonenal (4-HNE) in myriad tissues and organs and many pathologies. Other studies similarly assigned unfavorable effects to 4-hydroxy-2E-hexenal (4-HHE) and 4-hydroxy-2E,6Z-dodecadienal (4-HDDE). Nutrient overload (e.g., hyperglycemia, hyperlipidemia) modifies lipid metabolism in cells and promotes lipid peroxidation and the generation of α,β-unsaturated 4-hydroxyalkenals. Advances glycation- and lipoxidation end products (AGEs and ALEs) have been associated with the development of insulin resistance and pancreatic beta cell dysfunction and the etiology of type 2 diabetes and its peripheral complications. Less acknowledged are genuine signaling properties of 4-hydroxyalkenals in hormetic processes that provide defense against the consequences of nutrient overload. This review addresses recent findings on such lipohormetic mechanisms that are associated with lipid peroxidation in pancreatic beta cells. This article is part of a Special Issue entitled SI: LIPID OXIDATION PRODUCTS, edited by Giuseppe Poli.

Regulation of pancreatic β-cell function and mass dynamics by prostaglandin signaling

Prostaglandins (PGs) are signaling lipids derived from arachidonic acid (AA), which is metabolized by cyclooxygenase (COX)-1 or 2 and class-specific synthases to generate PGD₂, PGE₂, PGF_2α, PGI₂ (prostacyclin), and thromboxane A₂. PGs signal through G-protein coupled receptors (GPCRs) and are important modulators of an array of physiological functions, including systemic inflammation and insulin secretion from pancreatic islets. The role of PGs in β-cell function has been an active area of interest, beginning in the 1970s. Early studies demonstrated that PGE₂ inhibits glucose-stimulated insulin secretion (GSIS), although more recent studies have questioned this inhibitory action of PGE₂. The PGE₂ receptor EP3 and one of the G-proteins that couples to EP3, Gα_Z, have been identified as negative regulators of β-cell proliferation and survival. Conversely, PGI₂ and its receptor, IP, play a positive role in the β-cell by enhancing GSIS and preserving β-cell mass in response to the β-cell toxin streptozotocin (STZ). In comparison to PGE₂ and PGI₂, little is known about the function of the remaining PGs within islets. In this review, we discuss the roles of PGs, particularly PGE₂ and PGI₂, PG receptors, and downstream signaling events that alter β-cell function and regulation of β-cell mass.

Arachidonic acid metabolism in isolated pancreatic islets. VI. Carbohydrate insulin secretagogues must be metabolized to induce eicosanoid release

Pancreatic islets stimulated with D-glucose are known to liberate arachidonic acid from membrane phospholipids and release prostaglandin E2 (PGE2). A component of the eicosanoid release induced by D-glucose has been demonstrated to occur without calcium influx and must be triggered by other coupling mechanisms. In this study, we have attempted to identify mechanisms other than calcium influx which might couple D-glucose stimulation to hydrolysis of arachidonate from membrane phospholipids in islet cells. We have found that occupancy of the beta cell plasma membrane D-glucose transporter is insufficient and that D-glucose metabolism is required to induce islet PGE2 release because 3-O-methylglucose fails to induce and mannoheptulose prevents PGE2 release otherwise induced by 17 mM D-glucose. The carbohydrate insulin secretagogues mannose and D-glyceraldehyde have also been found to induce islet PGE2 release, but the non-secretagogue carbohydrates L-glucose and lactate do not. Carbohydrate secretagogues are known to be metabolized to yield ATP and induce depolarization of the beta cell plasma membrane. We have found that depolarization by 40 mM KCl induces PGE2 release only in the presence and not in the absence of extracellular calcium, but exogenous ATP induces islet PGE2 release with or without extracellular calcium. Carbachol is demonstrated here to interact synergistically with increasing concentrations of glucose to amplify PGE2 release and insulin secretion. Pertussis toxin treatment is shown here not to prevent PGE2 release induced by glucose or carbachol but to increase the basal rate of PGE2 release and the islet cyclic AMP content. Theophylline (10 mM) exerts similar effects. Eicosanoid release in pancreatic islets can thus be activated by multiple pathways including muscarinic receptor occupancy, calcium influx, increasing cAMP content, and a metabolic signal derived from nutrient secretagogues, such as ATP.

Prostaglandins and the release of insulin. A review and a proposal

The effect of prostaglandins or the inhibition of their synthesis on the release of insulin is controversial. Dispute exists because there are apparently disparate experimental results. When the following factors are considered, however, much of the disparity is eliminated: 1) the experimental setting--in vitro or in vivo; 2) the experimental model--animal or human; 3) the experimental additive--the type of nonsteroidal antiinflammatory drug or specific prostaglandin; 4) the relationship of insulin levels to insulin secretion, degradation, and the observed hypoglycemic response. On the basis of such considerations the following conclusions are advanced. 1) From animal studies in vitro it appears that prostaglandins can directly augment insulin release. 2) Results from animal experiments in vivo, however, suggest that systemic prostaglandin administration diminishes insulin release. 3) No human studies have been performed in vitro which examine the insulin secretory response of pancreatic tissue to prostaglandins. 4) Prostaglandins reduce stimulated insulin levels in normal human subjects and in those with diabetes mellitus. Whether insulin secretion is reduced, or clearance is increased, is unknown. 5) Finally, the critical experiment remains to be done, that is the simultaneous examination of insulin, C-peptide, and glucose kinetics during an infusion of a prostaglandin.

Opiate-prostaglandin interactions in the regulation of insulin secretion from rat islets of Langerhans in vitro

The inadequate insulin secretory response to glucose stimulation in non-insulin dependent diabetes has been attributed to many factors including high PGE2 levels blunting the secretory response, and to the existence of inhibitory opiate activity in vivo. The purpose of the present work was to see if there was a connection between these two independent theories. Radioimmunoassayable PGE2 in islets of Langerhans was found to be proportional to islet number and protein content and was typically 4 to 5pg/micrograms islet protein. Indomethacin (2.8 X 10(-5) M), sodium salicylate (1.25 X 10(-3) M) and chlorpropamide (7.2 X 10(-5) M) all lowered islet PGE2 levels and stimulated insulin release in vitro. Dynorphin (1-13), stimulated insulin release at a concentration of 6 X 10(-9) M, while lowering islet PGE2. Conversely, at a higher concentration, (6 X 10(-7) M), dynorphin had no stimulatory effect on insulin secretion and did not lower PGE2 levels in islets or in the incubation media. The stimulatory effects of dynorphin and sodium salicylate on insulin secretion were blocked by exogenous PGE2 (10(-5) M). PGE2 at a lower concentration (10(-9) M) did not exert any inhibitory effect on dynorphin- or sodium salicylate-induced insulin release. This concentration of exogenous PGE2 stimulated insulin release in the presence of 6mM glucose. Results from these experiments suggest that since an opioid peptide can lower endogenous PGE2 production in islets and since the stimulatory effects of the opioid peptide are reversed by exogenous PGE2 there may be interactions between these two modulators of insulin secretion.

Time-course of altered islet phospholipids and of calcium binding and ionophoretic properties of islet lipids following glucose stimulation

The time-course of alteration in islet cell phospholipid content following D-glucose exposure in islet cells and in islet cell membranes was related to the ability of lipids extracted from both cultured pancreatic islet cells and from plasma membranes isolated from the islet cells to translocate calcium in two model membrane systems. The first model system (bulk-phase system) detected lipid species with the ability to bind calcium, irrespective of their ability to enhance calcium transport across cell membranes. The second system (multilamellar membrane system) detected lipid species with the ability to both bind calcium and to enhance calcium transport across cell membranes (true ionophores). Pre-exposure to high D-glucose concentration led to a rapid (within 1 min) fall in membrane phosphoinositides. This was partially blocked by mannoheptulose. A concurrent fall in calcium binding activity of lipids from the plasma membrane was observed. In the whole islet cell fraction, D-glucose induced a marked increase in Ca2+ ionophoretic activity. Unlike the fall in membrane polyphosphoinositides and membrane Ca2+ binding activity, these changes were dependent on the presence of added extracellular calcium. L-Glucose was without effect on membrane phosphoinositide content. It is concluded that altered membrane and intracellular phospholipids may contribute to the increased availability of intracellular Ca2+ following D-glucose stimulation by virtue of their Ca2+ binding and ionophoretic properties.

Effects of inhibitors of eicosanoid synthesis on insulin release by neonatal pancreatic islets

In the pancreatic islet, eicosanoids may arise from both cyclooxygenase- and lipoxygenase-dependent metabolism of arachidonic acid. The inclusion of inhibitors of selective steps in these pathways indicated that in cultured neonatal rat islets, arachidonic acid may be metabolised through both pathways, concurrent with insulin release stimulated by D-glucose, D-glyceraldehyde and 2-ketoisocaproate. The effects of the inhibitors suggested that the products of the lipoxygenase pathway were necessary for the stimulatory effects of nutrients to be observed. In contrast to glucose, where insulin release was stimulated in the presence of inhibitors of cyclooxygenase, the stimulatory action of D-glyceraldehyde, 2-ketoisocaproate and melittin was only minimally affected by these inhibitors, although it was inhibited by lipoxygenase inhibition. These findings support a major stimulatory role for products of the lipoxygenase pathway of arachidonic acid metabolism in nutrient-induced secretion, and a negative or modulatory role of cyclooxygenase pathway products on glucose-stimulated insulin release in the neonatal islet.

Effects of prostacyclin and its stable analog, iloprost, upon insulin secretion in isolated pancreatic islets

We investigated the effects of prostacyclin in three concentrations of 2.7 nM, 53.8 nM and 267 nM on insulin release by isolated rat islets. The lowest concentration produced no significant effects. The middle concentration led to stimulation followed by inhibition of the reaction studied, while the highest concentration strongly depressed insulin release. These effects of prostacyclin appeared to be specific, because they were mimicked by its stable analog, iloprost, but not by its metabolite, 6-keto-PGF1 alpha. The results suggest that prostacyclin generated in islets might exert locally an influence upon insulin release.

Arachidonic acid metabolism in isolated pancreatic islets. I. Identification and quantitation of lipoxygenase and cyclooxygenase products

The metabolism of arachidonic acid by pancreatic islets has been studied with purified populations of large numbers of islets isolated from the rat. Sequential high-performance liquid chromatographic analyses of islet-derived metabolites of 3H-labeled arachidonate in both reversed and normal phases with 14C-labeled internal standards have demonstrated synthesis by the islets of the cyclooxygenase products prostaglandin E2, prostaglandin F2 alpha, thromboxane B2 and 12- hydroxyheptadecatrienoic acid as well as the lipoxygenase product 12-hydroxyeicosatetraenoic acid (12-HETE). Islet synthesis of these compounds was suppressed with appropriate inhibitors of arachidonate metabolism. Synthesis of the identified metabolites from endogenous arachidonate has also been quantitated with the use of deuterated internal standards, capillary column gas chromatographic analyses, and negative ion-chemical ionization mass spectrometric measurements. The relative abundances of metabolites derived from exogenous, radiolabeled arachidonate versus endogenous precursor differed considerably, and 12-HETE was by far the most abundant of these metabolites synthesized from endogenous arachidonate. Platelets contaminating the isolated islet preparations have been excluded as the source of the identified arachidonate metabolites. These studies establish that cells intrinsic to pancreatic islets synthesize a clearly characterized profile of arachidonate lipoxygenase and cyclooxygenase products. The sensitive and specific mass spectrometric methods for quantitation of these compounds permit detailed evaluation of their possible participation in insulin secretion from isolated islets.

Effects of lipoxygenase and cyclooxygenase inhibitors on glucose-stimulated insulin secretion from the isolated perfused rat pancreas

Some of the metabolites of arachidonic acid formed in the lipoxygenase and cyclooxygenase pathways stimulate insulin release. We studied the relative importance of each of these pathways in the modulation of glucose-induced insulin release by using inhibitors of arachidonate metabolism. Perfusion of the isolated rat pancreas with two chemically different inhibitors of cyclooxygenase, flurbiprofen and sodium salicylate, markedly inhibited prostaglandin E2 release, but had little effect on glucose-induced insulin release or on potentiation of insulin release caused by prior exposure to glucose. On the other hand, nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, not only inhibited both phases of glucose-induced insulin release but also abolished the potentiation effect. These effects of NDGA prevailed, when it was administered together with flurbiprofen, which caused profound inhibition of prostaglandin E2 release. We conclude that 1) lipoxygenase pathways play a dominant role in glucose-stimulated insulin release, and 2) endogenous lipoxygenase metabolites influence the potentiating effect of glucose on the release of insulin in response to a subsequent stimulation.

Leukotrienes stimulate insulin release from the rat pancreas

We and others had observed that drugs that inhibit the lipoxygenase enzymes inhibit insulin release. The aim of the study was to search for evidence that, among the products of lipoxygenase-activated metabolism of arachidonic acid, leukotrienes (LTs) promote insulin release. Synthetic LTB4, LTC4, LTD4, and LTE4 were administered over 1-min periods to the isolated, perfused rat pancreas in the presence of 5.6 mM glucose. Perfusion effluent levels of insulin and glucagon were measured by radioimmunoassay. LTB4 and LTC4 and, to a lesser extent, LTE4 and LTD4 stimulated insulin release in a dose-related manner, in the concentration range of 10(-11) to 10(-7) M. Only 10(-7) M LTC4 stimulated glucagon release. We conclude that (i) among the arachidonic acid metabolites, LTs are involved in the modulation of secretion of pancreatic islet hormones and (ii) LTs preferentially promote insulin release.

Modulation by prostaglandin D2 of glucagon and insulin secretion in the perfused rat pancreas

Effects of prostaglandin (PG) D2 on insulin and glucagon secretion from perfused rat pancreas were examined. In the presence of 2.8 mM glucose, only glucagon release was strongly stimulated by 14 microM of PGD2. When the glucose concentration was elevated to 11.2 mM, insulin release was accelerated by 14 microM of PGD2 but there was no effect upon glucagon release. Both glucagon and insulin releases induced by 19 mM arginine with PGD2 were not different from those without PGD2 in the presence of 2.8 mM glucose. But in the presence of 11.2 mM glucose, glucagon release induced by 19 mM arginine was augmented by 14 microM PGD2. Since the distribution of PGD2 has been reported to be in neuroendocrine organs, these results suggest that PGD2 is a possible candidate as a modulator in the neural control of endocrine pancreas.

Glucose dependent stimulation by prostaglandin D2 of glucagon and insulin in perfused rat pancreas

Effects of prostaglandin D2 on pancreatic islet function in perfused rat pancreas were examined in comparison with those of prostaglandin E2, which has hitherto been suggested to be a modifier of pancreatic hormone release. In the presence of 2.8 mM glucose, only glucagon release was strongly stimulated by 14 microM of prostaglandin D2, while release of both glucagon and insulin was augmented by 14 microM of prostaglandin E2. When the glucose concentration was elevated to 11.2 mM, insulin release was accelerated by 14 microM of prostaglandin D2 but there was no effect upon glucagon release. Again, release of both glucagon and insulin was augmented by 14 microM of prostaglandin E2 in the presence of 11.2 mM of glucose. The regulation of glucagon and insulin release through prostaglandin D2 is apparently adapted to glycemic changes, and may be a physiological modulator of pancreatic islet function.

Dissociation of vascular resistance with endocrine pancreas secretion: the effects of epoxymethano analogs of PGH2

The epoxymethano analogs of PGH2 caused rapid and persistent increase in perfusion pressure in isolated rat pancreata without significant effect on glucagon and insulin secretory responses to PGH2 and PGE2. The changes in perfusion pressure are interpreted as alterations in vascular resistance since the flow rate was kept constant at 2.5 mL per min. PGH2 alone caused significant elevation in pressure. However, PGH2 administration superimposed upon an infused epoxymethano analog of PGH2, decreased perfusion pressure significantly, whereas PGH2 induced hormone release was not decreased. The analogs neither stimulated nor inhibited the endocrine pancreas secretion. These studies provide evidence for complete dissociation of vascular constriction from pancreatic hormone release and further suggest that the effects of PGH2 on islet hormone secretion may result from the conversion of PGH2 to other prostanoids.

Prostaglandin synthesis and metabolism in isolated pancreatic islets of the rat

Isolated pancreatic islets of Langerhans of the rat which were sonicated and incubated with radiolabeled arachidonic acid for 1 hr synthesized several species of prostaglandins (PGs). Both thin-layer and high-performance liquid (HPLC) chromatographic techniques demonstrated the synthesis by islet sonicates of PGF2 alpha and PGE2 equivalents, in addition to the 15-keto-13, 14-dihydro metabolites of these primary PGs. In addition, HPLC allowed the identification of 6-keto-PGF1 alpha (the metabolite of prostacyclin) as a major PG synthesized from arachidonate by this tissue. Islet vascular elements, as well as endocrine cells, may contribute to the synthesis of the latter compound. Lesser amounts of arachidonate were incorporated into PG-like compounds eluting as thromboxane. The synthesis of PGs was sensitive to the protein concentration of islet sonicate, and a five-fold dilution of protein resulted in a comparable reduction in arachidonate incorporation into PGs. Labeled arachidonate was also incorporated into compounds which elute as hydroxy or hydroperoxyeicosatetraenoic acids on HPLC. Thus, isolated pancreatic islets synthesize a variety of PGs which may have a physiological role in hormone secretion from this endocrine organ.

Insulin and glucagon secretion in essential fatty acid deficient rats

Insulin and glucagon secretion in response to common secretagogues were ascertained in the perfused pancreas isolated from essential fatty acid deficient rats. The pattern of insulin secretory response to glucose (16.7 mmol/L) by isolated rat pancreas perfused for 30 min was biphasic in EFA-deficient and control rat pancreas. The amplitude of glucose-stimulated acute secretion (phase I) was significantly greater (p less than 0.01) in magnitude and amplitude in EFA-deficient rats than in the control rats. There was no significant difference in the second phase of glucose-stimulated insulin secretion in the two groups. Glucagon secretion in EFA-deficient and control rats was inhibited by glucose (16.7 mmol/l). Glucagon secretion induced by L-arginine (10 mmol/l) was not significantly different in EFA-deficient and in control rat pancreata (p greater than 0.05). However, arginine (10 mmol/l)-stimulated insulin release was significantly higher in EFA-deficient than in control rats. Growth hormone (100 mumol/l)-induced glucagon and insulin secretion was variable in the two groups but significantly higher than basal secretion. The level of L-leucine (10 mmol/l)-stimulated glucagon and insulin secretion in EFA-deficient rats was minimal but significant. Our results show that isolated pancreata of rats devoid of precursors for endogenous prostaglandin synthesis secreted insulin and glucagon in response to common secretagogues. On the basis of our data, it is concluded that endogenous prostaglandins are probably not obligatory for normal secretory functions of islets of Langerhans.

Prostacyclin does not affect insulin secretion in humans

The effects of Prostacyclin (PGI2) infusion on insulin secretion and glucose tolerance were investigated in 7 healthy subjects. PGI2 infusion caused no statistically significant changes of either glucose or insulin concentration, over the range 2.5-20 ng/Kg/min. A constant PGI2 infusion (10 ng/Kg/min) did not inhibit acute insulin responses to a glucose (20 g i.v.) pulse (response before PGI2 = 612 +/- 307%; during PGI2 = 515 +/- 468%, mean +/- SD, mean change 3-5 min insulin, % basal; P=NS). Glucose disappearance rates were similar after the first and second glucose pulse. Thus, in contrast to PGE2, PGI2 does not affect insulin secretion nor glucose disposal at doses producing platelet and vascular changes. It is hypothesized that an altered PGI2/PGE2 balance in diabetes may represent a link between vascular, platelet and metabolic changes.