The role of cyclic AMP in insulin release

Dopamine-mediated autocrine inhibition of insulin secretion

The aim of the present research was to explore the mechanisms underlying the role of dopamine in the regulation of insulin secretion in beta cells. The effect of dopamine on insulin secretion was investigated on INS 832/13 cell line upon glucose and other secretagogues stimulation. Results show that dopamine significantly inhibits insulin secretion stimulated by both glucose and other secretagogues, while it has no effect on the basal secretion. This effect requires the presence of dopamine during incubation with the various secretagogues. Both electron microscopy and immunohistochemistry indicate that in beta cells the D2 dopamine receptor is localized within the insulin granules. Blocking dopamine entry into the insulin granules by inhibiting the VMAT2 transporter with tetrabenazine causes a significant increase in ROS production. Our results confirm that dopamine plays an important role in the regulation of insulin secretion by pancreatic beta cells through a regulated and precise compartmentalization mechanisms.

Rapid Quantification of First and Second Phase Insulin Secretion Dynamics using an In vitro Platform for Improving Insulin Therapy

High-throughput quantification of the first- and second-phase insulin secretion dynamics is intractable with current methods. The fact that independent secretion phases play distinct roles in metabolism necessitates partitioning them separately and performing high-throughput compound screening to target them individually. We developed an insulin-nanoluc luciferase reporter system to dissect the molecular and cellular pathways involved in the separate phases of insulin secretion. We validated this method through genetic studies, including knockdown and overexpression, as well as small-molecule screening and their effects on insulin secretion. Furthermore, we demonstrated that the results of this method are well correlated with those of single-vesicle exocytosis experiments conducted on live cells, providing a quantitative reference for the approach. Thus, we have developed a robust methodology for screening small molecules and cellular pathways that target specific phases of insulin secretion, resulting in a better understanding of insulin secretion, which in turn will result in a more effective insulin therapy through the stimulation of endogenous glucose-stimulated insulin secretion.

Cyclic AMP-Dependent Regulation of Kv7 Voltage-Gated Potassium Channels

Voltage-gated Kv7 potassium channels, encoded by KCNQ genes, have major physiological impacts cardiac myocytes, neurons, epithelial cells, and smooth muscle cells. Cyclic adenosine monophosphate (cAMP), a well-known intracellular secondary messenger, can activate numerous downstream effector proteins, generating downstream signaling pathways that regulate many functions in cells. A role for cAMP in ion channel regulation has been established, and recent findings show that cAMP signaling plays a role in Kv7 channel regulation. Although cAMP signaling is recognized to regulate Kv7 channels, the precise molecular mechanism behind the cAMP-dependent regulation of Kv7 channels is complex. This review will summarize recent research findings that support the mechanisms of cAMP-dependent regulation of Kv7 channels.

Allyl isothiocyanate increases carbohydrate oxidation through enhancing insulin secretion by TRPV1

The transient receptor potential (TRP) V1 is a cation channel belonging to the TRP channel family and it has been reported to be involved in energy metabolism, especially glucose metabolism. While, we have previously shown that intragastric administration of allyl isothiocyanate (AITC) enhanced glucose metabolism via TRPV1, the underlying mechanism has not been elucidated. In this study, we examined the relationship between insulin secretion and the increase in carbohydrate oxidation due to AITC. Intragastric administration of AITC elevated blood insulin levels in mice and AITC directly enhanced insulin secretion from isolated islets. These observations were not reproduced in TRPV1 knockout mice. Furthermore, AITC did not increase carbohydrate oxidation in streptozotocin-treated mice. These results suggest that intragastric administration of AITC could induce insulin secretion from islets via TRPV1 and that enhancement of insulin secretion was related to the increased carbohydrate oxidation due to AITC.

β-Cell glutamate signaling: Its role in incretin-induced insulin secretion

Insulin secretion from the pancreatic β-cell (referred to as β-cell hereafter) plays a central role in glucose homeostasis. Impaired insulin secretion is a major factor contributing to the development of diabetes and, therefore, is an important target for treatment of the disease. Cyclic adenosine monophosphate is a key second messenger in β-cells that amplifies insulin secretion. Incretins released by the gut potentiate insulin secretion through cyclic adenosine monophosphate signaling in β-cells, which is the basis for the incretin-based diabetes therapies now being used worldwide. Despite its importance, the interaction between glucose metabolism and incretin/cyclic adenosine monophosphate signaling in β-cells has long been unknown. A recent study showed that cytosolic glutamate produced by glucose metabolism in β-cells is a key signal in incretin-induced insulin secretion. Here we review the physiological and pathophysiological roles of β-cell glutamate signaling in incretin-induced insulin secretion.

β-Cell-specific protein kinase A activation enhances the efficiency of glucose control by increasing acute-phase insulin secretion

Acute insulin secretion determines the efficiency of glucose clearance. Moreover, impaired acute insulin release is characteristic of reduced glucose control in the prediabetic state. Incretin hormones, which increase β-cell cAMP, restore acute-phase insulin secretion and improve glucose control. To determine the physiological role of the cAMP-dependent protein kinase (PKA), a mouse model was developed to increase PKA activity specifically in the pancreatic β-cells. In response to sustained hyperglycemia, PKA activity potentiated both acute and sustained insulin release. In contrast, a glucose bolus enhanced acute-phase insulin secretion alone. Acute-phase insulin secretion was increased 3.5-fold, reducing circulating glucose to 58% of levels in controls. Exendin-4 increased acute-phase insulin release to a similar degree as PKA activation. However, incretins did not augment the effects of PKA on acute-phase insulin secretion, consistent with incretins acting primarily via PKA to potentiate acute-phase insulin secretion. Intracellular calcium signaling was unaffected by PKA activation, suggesting that the effects of PKA on acute-phase insulin secretion are mediated by the phosphorylation of proteins involved in β-cell exocytosis. Thus, β-cell PKA activity transduces the cAMP signal to dramatically increase acute-phase insulin secretion, thereby enhancing the efficiency of insulin to control circulating glucose.

Cell signalling in insulin secretion: the molecular targets of ATP, cAMP and sulfonylurea

Clarification of the molecular mechanisms of insulin secretion is crucial for understanding the pathogenesis and pathophysiology of diabetes and for development of novel therapeutic strategies for the disease. Insulin secretion is regulated by various intracellular signals generated by nutrients and hormonal and neural inputs. In addition, a variety of glucose-lowering drugs including sulfonylureas, glinide-derivatives, and incretin-related drugs such as dipeptidyl peptidase IV (DPP-4) inhibitors and glucagon-like peptide 1 (GLP-1) receptor agonists are used for glycaemic control by targeting beta cell signalling for improved insulin secretion. There has been a remarkable increase in our understanding of the basis of beta cell signalling over the past two decades following the application of molecular biology, gene technology, electrophysiology and bioimaging to beta cell research. This review discusses cell signalling in insulin secretion, focusing on the molecular targets of ATP, cAMP and sulfonylurea, an essential metabolic signal in glucose-induced insulin secretion (GIIS), a critical signal in the potentiation of GIIS, and the commonly used glucose-lowering drug, respectively.

The anti-ulcer agent, irsogladine, increases insulin secretion by MIN6 cells

Insulin secretion by pancreatic islets is a multicellular process. In addition to other essential systems, gap junctions are an important component of cell-to-cell communication in pancreatic islets. It is well known that dysfunction of gap junctions causes inappropriate insulin secretion. The anti-ulcer agent, irsogladine, increases gap junctions in some cell types. To examine the effect of irsogladine on insulin secretion, we investigated insulin secretion by MIN6 cells treated with or without irsogladine. The expression of connexin 36 proteins and intracellular cAMP levels were also determined using immunoblotting and ELISA assays, respectively. Irsogladine had no effect on insulin secretion under 5.6mM glucose conditions. However, under 16.7 mM glucose conditions, irsogladine (1.0 × 10(-5)M) induced a 1.7 ± 0.20 fold increase in insulin secretion compared to the control (P<0.05). This effect of irsogladine on insulin secretion was inhibited by the addition of the gap junction inhibitor 18-beta-glycyrrhetinic acid. Irsogladine treatment increased the protein level of connexin 36 in the plasma membrane fraction. The intracellular cAMP level in MIN6 cells was significantly, but mildly, increased by irsogladine treatment. Furthermore, Rp-cAMP and H89 inhibited the effects of irsogladine on insulin secretion under high glucose conditions. Irsogladine increases insulin secretion under high glucose conditions. The up-regulation of gap junction channels and the increased level of intracellular cAMP induced by irsogladine treatment suggest that these phenomena are involved in irsogladine-induced increased insulin secretion.

Glucose increases extracellular [Ca2+] in rat insulinoma (INS-1E) pseudoislets as measured with Ca2+-sensitive microelectrodes

Secretory granules of pancreatic β-cells contain high concentrations of Ca2+ ions that are co-released with insulin in the extracellular milieu upon activation of exocytosis. As a consequence, an increase in the extracellular Ca2+ concentration ([Ca2+]ext) in the microenvironment immediately surrounding β-cells should be expected following the exocytotic event. Using Ca2+-selective microelectrodes we show here that both high glucose and non-nutrient insulinotropic agents elicit a reversible increase of [Ca2+]ext within rat insulinoma (INS-1E) β-cells pseudoislets. The glucose-induced increases in [Ca2+]ext are blocked by pretreatment with different Ca2+ channel blockers. Physiological agonists acting as positive or negative modulators of the insulin secretion and drugs known to intersect the secretory machinery at different levels also induce [Ca2+]ext changes as predicted on the basis of their described action on insulin secretion. Finally, the glucose-induced [Ca2+]ext increase is strongly inhibited after disruption of the actin web, indicating that the dynamic [Ca2+]ext changes recorded in INS-1E pseudoislets by Ca2+-selective microelectrodes occur mainly as a consequence of exocytosis of Ca2+-rich granules. In conclusion, our data directly demonstrate that the extracellular spaces surrounding β-cells constitute a restricted domain where Ca2+ is co-released during insulin exocytosis, creating the basis for an autocrine/paracrine cell-to-cell communication system via extracellular Ca2+ sensors.

cAMP-secretion coupling is impaired in diabetic GK/Par rat β-cells: a defect counteracted by GLP-1

cAMP-raising agents with glucagon-like peptide-1 (GLP-1) as the first in class, exhibit multiple actions that are beneficial for the treatment of type 2 diabetic (T2D) patients, including improvement of glucose-induced insulin secretion (GIIS). To gain additional insight into the role of cAMP in the disturbed stimulus-secretion coupling within the diabetic β-cell, we examined more thoroughly the relationship between changes in islet cAMP concentration and insulin release in the GK/Par rat model of T2D. Basal cAMP content in GK/Par islets was significantly higher, whereas their basal insulin release was not significantly different from that of Wistar (W) islets. Even in the presence of IBMX or GLP-1, their insulin release did not significantly change despite further enhanced cAMP accumulation in both cases. The high basal cAMP level most likely reflects an increased cAMP generation in GK/Par compared with W islets since 1) forskolin dose-dependently induced an exaggerated cAMP accumulation; 2) adenylyl cyclase (AC)2, AC3, and G(s)α proteins were overexpressed; 3) IBMX-activated cAMP accumulation was less efficient and PDE-3B and PDE-1C mRNA were decreased. Moreover, the GK/Par insulin release apparatus appears less sensitive to cAMP, since GK/Par islets released less insulin at submaximal cAMP levels and required five times more cAMP to reach a maximal secretion rate no longer different from W. GLP-1 was able to reactivate GK/Par insulin secretion so that GIIS became indistinguishable from that of W. The exaggerated cAMP production is instrumental, since GLP-1-induced GIIS reactivation was lost in the presence the AC blocker 2',5'-dideoxyadenosine. This GLP-1 effect takes place in the absence of any improvement of the [Ca(2+)](i) response and correlates with activation of the cAMP-dependent PKA-dependent pathway.

Dynamics of insulin secretion and the clinical implications for obesity and diabetes

Insulin secretion is a highly dynamic process regulated by various factors including nutrients, hormones, and neuronal inputs. The dynamics of insulin secretion can be studied at different levels: the single β cell, pancreatic islet, whole pancreas, and the intact organism. Studies have begun to analyze cellular and molecular mechanisms underlying dynamics of insulin secretion. This review focuses on our current understanding of the dynamics of insulin secretion in vitro and in vivo and discusses their clinical relevance.

Glycated albumin suppresses glucose-induced insulin secretion by impairing glucose metabolism in rat pancreatic β-cells

Background

Glycated albumin (GA) is an Amadori product used as a marker of hyperglycemia. In this study, we investigated the effect of GA on insulin secretion from pancreatic β cells.

Methods

Islets were collected from male Wistar rats by collagenase digestion. Insulin secretion in the presence of non-glycated human albumin (HA) and GA was measured under three different glucose concentrations, 3 mM (G3), 7 mM (G7), and 15 mM (G15), with various stimulators. Insulin secretion was measured with antagonists of inducible nitric oxide synthetase (iNOS), and the expression of iNOS-mRNA was investigated by real-time PCR.

Results

Insulin secretion in the presence of HA and GA was 20.9 ± 3.9 and 21.6 ± 5.5 μU/3 islets/h for G3 (P = 0.920), and 154 ± 9.3 and 126.1 ± 7.3 μU/3 islets/h (P = 0.046), for G15, respectively. High extracellular potassium and 10 mM tolbutamide abrogated the inhibition of insulin secretion by GA. Glyceraldehyde, dihydroxyacetone, methylpyruvate, GLP-1, and forskolin, an activator of adenylate cyclase, did not abrogate the inhibition. Real-time PCR showed that GA did not induce iNOS-mRNA expression. Furthermore, an inhibitor of nitric oxide synthetase, aminoguanidine, and NG-nitro-L-arginine methyl ester did not abrogate the inhibition of insulin secretion.

Conclusion

GA suppresses glucose-induced insulin secretion from rat pancreatic β-cells through impairment of intracellular glucose metabolism.

Pancreatic beta-cell signaling: toward better understanding of diabetes and its treatment

Pancreatic beta-cells play a central role in the maintenance glucose homeostasis by secreting insulin, a key hormone that regulates blood glucose levels. Dysfunction of the beta-cells and/or a decrease in the beta-cell mass are associated closely with the pathogenesis and pathophysiology of diabetes mellitus, a major metabolic disease that is rapidly increasing worldwide. Clarification of the mechanisms of insulin secretion and beta-cell fate provides a basis for the understanding of diabetes and its better treatment. In this review, we discuss cell signaling critical for the insulin secretory function based on our recent studies.

Roles of cAMP signalling in insulin granule exocytosis

Insulin secretion is regulated by a series of complex events generated by various intracellular signals including Ca(2+), ATP, cAMP and phospholipid-derived signals. Glucose-stimulated insulin secretion is the principal mode of insulin secretion, and the mechanism potentiating the secretion is critical for physiological responses. Among the various intracellular signals involved, cAMP is particularly important for amplifying insulin secretion. Recently, glucagon-like peptide-1 (GLP-1) analogues and dipeptidyl peptidase-IV (DPP-IV) inhibitors have been developed as new antidiabetic drugs. These drugs all act through cAMP signalling in pancreatic beta-cells. Until recently, cAMP was generally thought to potentiate insulin secretion through protein kinase A (PKA) phosphorylation of proteins associated with the secretory process. However, it is now known that in addition to PKA, cAMP has other targets such as Epac (also referred to as cAMP-GEF). The variety of the effects mediated by cAMP signalling may be linked to cAMP compartmentation in the pancreatic beta-cells.

Critical role of the N-terminal cyclic AMP-binding domain of Epac2 in its subcellular localization and function

cAMP is a well-known regulator of exocytosis, and cAMP-GEFII (Epac2) is involved in the potentiation of cAMP-dependent, PKA-independent regulated exocytosis in secretory cells. However, the mechanisms of its action are not fully understood. In the course of our study of Epac2 knockout mice, we identified a novel splicing variant of Epac2, which we designate Epac2B, while renaming the previously identified Epac2 Epac2A. Epac2B, which lacks the first cAMP-binding domain A in the N-terminus but has the second cAMP-binding domain B of Epac2A, possesses GEF activity towards Rap1, as was found for Epac2A. Immunocytochemical analysis revealed that exogenously introduced Epac2A into insulin-secreting MIN6 cells was localized near the plasma membrane, while Epac2B was found primarily in the cytoplasm. Interestingly, cAMP-binding domain A alone introduced into MIN6 cells was also localized near the plasma membrane. In MIN6 cells, Epac2A was involved in triggering hormone secretion by stimulation with 5.6 mM glucose plus 1 mM 8-Bromo-cAMP, but Epac2B was not. The addition of a membrane-targeting signal to the N-terminus of Epac2B was able to mimic the effect of Epac2A on hormone secretion. Thus, the present study indicates that the N-terminal cAMP-binding domain A of Epac2A plays a critical role in determining its subcellular localization and potentiating insulin secretion by cAMP. J. Cell. Physiol. 219: 652-658, 2009. (c) 2009 Wiley-Liss, Inc.

Galphaz negatively regulates insulin secretion and glucose clearance

Relatively little is known about the in vivo functions of the alpha subunit of the heterotrimeric G protein Gz (Galphaz). Clues to one potential function recently emerged with the finding that activation of Galphaz inhibits glucose-stimulated insulin secretion in an insulinoma cell line (Kimple, M. E., Nixon, A. B., Kelly, P., Bailey, C. L., Young, K. H., Fields, T. A., and Casey, P. J. (2005) J. Biol. Chem. 280, 31708-31713). To extend this study in vivo, a Galphaz knock-out mouse model was utilized to determine whether Galphaz function plays a role in the inhibition of insulin secretion. No differences were discovered in the gross morphology of the pancreatic islets or in the islet DNA, protein, or insulin content between Galphaz-null and wild-type mice. There was also no difference between the insulin sensitivity of Galphaz-null mice and wild-type controls, as measured by insulin tolerance tests. Galphaz-null mice did, however, display increased plasma insulin concentrations and a corresponding increase in glucose clearance following intraperitoneal and oral glucose challenge as compared with wild-type controls. The increased plasma insulin observed in Galphaz-null mice is most likely a direct result of enhanced insulin secretion, since pancreatic islets isolated from Galphaz-null mice exhibited significantly higher glucose-stimulated insulin secretion than those of wild-type mice. Finally, the increased insulin secretion observed in Galphaz-null islets appears to be due to the relief of a tonic inhibition of adenylyl cyclase, as cAMP production was significantly increased in Galphaz-null islets in the absence of exogenous stimulation. These findings indicate that Galphaz may be a potential new target for therapeutics aimed at ameliorating beta-cell dysfunction in Type 2 diabetes.

PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis

Stimulus-secretion coupling is an essential process in secretory cells in which regulated exocytosis occurs, including neuronal, neuroendocrine, endocrine, and exocrine cells. While an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) is the principal signal, other intracellular signals also are important in regulated exocytosis. In particular, the cAMP signaling system is well known to regulate and modulate exocytosis in a variety of secretory cells. Until recently, it was generally thought that the effects of cAMP in regulated exocytosis are mediated by activation of cAMP-dependent protein kinase (PKA), a major cAMP target, followed by phosphorylation of the relevant proteins. Although the involvement of PKA-independent mechanisms has been suggested in cAMP-regulated exocytosis by pharmacological approaches, the molecular mechanisms are unknown. Newly discovered cAMP-GEF/Epac, which belongs to the cAMP-binding protein family, exhibits guanine nucleotide exchange factor activities and exerts diverse effects on cellular functions including hormone/transmitter secretion, cell adhesion, and intracellular Ca(2+) mobilization. cAMP-GEF/Epac mediates the PKA-independent effects on cAMP-regulated exocytosis. Thus cAMP regulates and modulates exocytosis by coordinating both PKA-dependent and PKA-independent mechanisms. Localization of cAMP within intracellular compartments (cAMP compartmentation or compartmentalization) may be a key mechanism underlying the distinct effects of cAMP in different domains of the cell.

PACAP and PACAP receptors in insulin producing tissues: localization and effects

We have studied the localization, receptor occupancy and potency of the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) in insulin-producing tissues. Immunocytochemistry showed that PACAP-like immunoreactivity (PACAP-IR) was localized to pancreatic nerves with accumulation in intrapancreatic ganglia in both mouse and rat. In contrast, PACAP-IR could not be demonstrated in endocrine cells. Furthermore, in situ hybridization, using oligodeoxyribonucleotide probes recognizing mRNA for PACAP receptors, demonstrated that mouse and rat pancreas, and the insulinoma cell lines HIT-T15 and RINm5F, expressed both the PACAP type 1 and the VIP2/PACAP receptors. Moreover, both PACAP27 and PACAP38 dose-dependently (0.1 nM to 100 nM) and equipotently stimulated insulin secretion in isolated mouse and rat islets and in HIT-T15 and RINm5F cells. Furthermore, in mouse islets, vasoactive intestinal polypeptide (VIP) was of equal potency as PACAP at stimulating insulin secretion. In mouse, PACAP also stimulated insulin secretion in a subfraction of the isolated islets also at the low dose of 1 fM. Thus, (1) PACAP is exclusively a neuropeptide in the pancreas, (2) insulin-producing cells express PACAP type 1 and VIP2/PACAP receptors and (3) the two forms of PACAP equipotently stimulate insulin secretion. Based on these results, we suggest that PACAP is involved in the neural regulation of insulin secretion.

Pancreatic beta cells synthesize and secrete nerve growth factor

Differentiation and function of pancreatic beta cells are regulated by a variety of hormones and growth factors, including nerve growth factor (NGF). Whether this is an endocrine or autocrine/paracrine role for NGF is not known. We demonstrate that NGF is produced and secreted by adult rat pancreatic beta cells. NGF secretion is increased in response to elevated glucose or potassium, but decreased in response to dibutyryl cAMP. Moreover, steady-state levels of NGF mRNA are down-regulated by dibutyryl cAMP, which is opposite to the effect of cAMP on insulin release. NGF-stimulated changes in morphology and function are mediated by high-affinity Trk A receptors in other mammalian cells. Trk A receptors are present in beta cells and steady-state levels of Trk A mRNA are modulated by NGF and dibutyryl cAMP. Taken together, these findings suggest endocrine and autocrine roles for pancreatic beta-cell NGF, which, in turn, could be related to the pathogenesis of diabetes mellitus where serum NGF levels are diminished.

Human islets of Langerhans express multiple isoforms of calcium/calmodulin-dependent protein kinase II

Previous studies have provided evidence for the presence of calcium/calmodulin-dependent protein kinase II (CaM kinase II) in rodent islets of Langerhans, and beta-cell CaM kinase II activity has been correlated with insulin secretion. In this study we provide the first conclusive evidence for the expression of CaM kinase II in human islets of Langerhans and show that multiple isoforms are expressed. Screening of a human islet cDNA library resulted in the isolation of a 999bp partial cDNA clone encoding CaM kinase II. The nucleotide sequence of the islet clone showed a high degree of homology (94.8%) to the two gamma isoforms of CaM kinase II previously isolated from human T lymphocytes (gammaB and gammaC). In order to obtain full length sequence for the islet clone, rapid amplification of cDNA ends (RACE) was used to amplify the 3' end of the islet clone from human islet poly A+ RNA. Two distinct gamma isoforms of CaM kinase II were amplified from the islet RNA. They were identified as gammaB and gammaE; the latter is distinguished from gammaB by a 114bp insertion within the association domain of the cDNA. Using reverse transcriptase polymerase chain reaction (RT-PCR) we also detected in human islets of Langerhans the novel beta3 isoform of CaM kinase II previously reported to be expressed in neonatal rat islets.

Impaired cytosolic Ca2+ response to glucose and gastric inhibitory polypeptide in pancreatic beta-cells from triphenyltin-induced diabetic hamster

Oral administration of a single dose of triphenyltin compounds induces diabetes with decreased insulin secretion in rabbits and hamsters after 2-3 days without any morphological changes in pancreatic islets. In the present study, to test the possibility that the impaired insulin secretion induced by triphenyltin compounds could result from an impaired Ca2+ response in pancreatic beta-cells, we investigated the effect of triphenyltin-chloride (TPTCl) administration on the changes in the cytoplasmic Ca2+ concentration ([Ca2+]i) induced by secretagogues, such as glucose, high K+, gastric inhibitory polypeptide (GIP), and acetylcholine (ACh) in hamster pancreatic beta-cells. TPTCl administration caused partial suppression in 10 mM K+-induced rise in [Ca2+]i without suppressing the increase in [Ca2+]i evoked by 20-50 mM K+. Administration of TPTCl strongly inhibited the rises in [Ca2+]i induced by 27.8 mM glucose, 100 microM ACh in the presence of 5.5 mM glucose, and by 100 nM GIP in the presence of 5.5 mM glucose. In the ACh-induced response, TPTCl administration strongly suppressed the late sustained phase, while weakly suppressing the initial rise in [Ca2+]i. TPTCl administration significantly suppressed the rise of cAMP content in islet cells induced by 100 nM GIP with 1 mM 3-isobutyl-1-methylxanthine in the presence of 5.5 mM glucose (P < 0.01, N = 5-11). TPTCl administration also impaired the insulin secretion in islet cells induced by 27.8 mM glucose, 100 nM GIP in the presence of 5.5 mM glucose, and 100 microM ACh in the presence of 5.5 mM glucose (P < 0.05, N = 9-16). We conclude that the pathology of triphenyltin-induced diabetes in hamsters involves a defect in cellular Ca2+ response due to a reduced Ca2+-influx through voltage-gated Ca2+ channels.

Protein kinase A-dependent and -independent stimulation of exocytosis by cAMP in mouse pancreatic B-cells

1. The mechanisms by which cAMP stimulates Ca(2+)-dependent insulin secretion were investigated by combining measurements of whole-cell Ca2+ currents, the cytoplasmic free Ca2+ concentration ([Ca2+]i) and membrane capacitance in single mouse B-cells maintained in tissue culture. 2. Cyclic AMP stimulated exocytosis > 4-fold in whole-cell experiments in which secretion was evoked by intracellular dialysis with a Ca(2+)-EGTA buffer with a [Ca2+]i of 1.5 microM. This effect was antagonized by inhibitors of protein kinase A (PKA). 3. Photorelease of cAMP from a caged precursor potentiated exocytosis at Ca2+ concentrations which were themselves stimulatory (> or = 60 nM), but was without effect in the complete absence of Ca2+. 4. Elevation of intracellular cAMP (by exposure to forskolin) evoked a 6-fold PKA-dependent enhancement of the maximal exocytotic response (determined as the maximum increase in cell capacitance that could be elicited by a train of depolarizations) in perforated-patch whole-cell recordings. 5. Exocytosis triggered by single depolarizations in standard whole-cell recordings was strongly potentiated by cAMP, but in this case the effect was unaffected by PKA inhibition. 6. When exocytosis was triggered by Ca2+ released from Ca(2+)-NP-EGTA ('caged Ca2+'), cAMP exerted a dual stimulatory effect on secretion: a rapid (initiated within 80 ms) PKA-independent phase and a late PKA-dependent component. 7. We conclude that cAMP stimulates insulin secretion both by increasing the release probability of secretory granules already in the readily releasable pool and by accelerating the refilling of this pool.

Low extracellular calcium enhances beta cell sensitivity to the stimulatory influence of 1,25-dihydroxyvitamin D3 on insulin release by islets from vitamin D3-deficient rats

The beneficial effect of 1,25-dihydroxyvitamin D3 [1,25 (OH)2 D3] on insulin secretion from beta cells in hypocalcemic vitamin D3-deficient rats is now well established. Moreover, few data concerning the mechanism of 1,25 (OH) 2D3 efficiency as a function of the severity of hypocalcemia. In the present experiment, we submitted islets from vitamin D3-deficient rats to in vitro exposure to a range of decreasing extracellular Ca2+ concentrations ([Ca2+]ex), from 0.5 mM to 0.6 mM, during a 6-h 10-8 M 1,25 (OH) 2D3 induction. Thereafter, we compared the effect of this pretreatment on the islets' insulin response to a given stimulus. Various stimuli were used, and we measured in parallel the variations of 86Rb+ and 45Ca2+ efflux and insulin release into the perifusion medium. In the presence of 1,25 (OH) 2D3, we observed an inverse correlation between the [Ca2+]ex pre-exposure and the amplitude of the insulin response to certain stimuli studied, suggesting that beta cells that were pre-exposed to low [Ca2+]ex became more sensitive to the beneficial effect of 1,25 (OH) 2D3 on insulin release. This effect was observed when beta cells were activated by acetylcholine but only during its second phase of stimulation, and more particularly with the barium plus theophylline stimulus. In contrast, insulin release was not affected by [Ca2+]ex pre-exposure during 1,25 (OH) 2D3 induction in response to acetylcholine during its first phase of stimulation, thus excluding any mechanism mediated via nutrient pathways, membrane depolarization, or inositol triphosphate (IP3)-dependent events. Moreover, the islets that were pre-exposed to a 10-fold [Ca2+]ex exhibited only a 50% lower 45Ca2+ content after 45Ca2+ loading, suggesting a different or relatively more efficient storage capacity in the presence of low extracellular calcium. Studies of 45Ca2+ efflux showed that the mobilization of Ca2+ stores induced by a barium plus theophylline stimulus, in the absence of calcium in the perifusion medium, was more efficient in islets pre-exposed to low [Ca2+]ex, whereas the acetylcholine-IP 3-induced mobilization of Ca2+ from reticular stores was not affected. These results generated the hypothesis that 1,25 (OH)2D3 may prepare the beta cells during their pre-exposure to low [Ca2+]ex to become more efficient as concerns insulin release via a more efficient mobilization of 45Ca2+ stores (mitochondrial?) and by an activation of release potentiating systems via protein kinase C protein kinase A pathways.

A truncated isoform of Ca2+/calmodulin-dependent protein kinase II expressed in human islets of Langerhans may result from trans-splicing

Calcium/calmodulin-dependent protein kinase II (CaM kinase II) has been proposed to play a key role in glucose stimulated insulin secretion. Using the rapid amplification of cDNA ends technique we amplified the 3' end of the CaM kinase II gamma gene from human islet RNA. A novel cDNA was detected composed of 5' sequence from the human CaM kinase II gamma gene joined to the 3' end of the human signal recognition particle 72 (SRP72) gene. We predict that this mRNA species will code for a truncated form of CaM kinase II, designated gammaSRP, comprising the entire catalytic and regulatory domains of the protein and with a predicted molecular weight of 37 kDa. We mapped the human SRP72 gene to chromosome 18 and, as the CaM kinase II gamma gene was previously mapped to human chromosome 10q22, we suggest this novel cDNA may have resulted from trans-splicing.

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.

Decreased glucose-induced cAMP and insulin release in islets of diabetic rats: reversal by IBMX, glucagon, GIP

The first aim of the study was to investigate the possibility that a defect on the islet adenosine 3',5'-cyclic monophosphate (cAMP) production could be involved in the failure of the glucose-induced insulin secretion in the neonatal streptozotocin diabetic rats. Exposure to glucose concentration that induced a rise of the cAMP content in the control islets did not elicit any significant increase in cAMP in diabetic islets. Forskolin, isobutyl methylxanthine (IBMX), glucagon, or pertussis toxin amplified the cAMP accumulation and the insulin release to the same extent in both types of islets. Somatostatin, prostaglandin E2, UK-14304, or galanin inhibited cAMP accumulation and insulin release to the same extent in both types of islets. Our second purpose was to investigate whether the use of activators of adenylate cyclase could restore the beta-cell competence to glucose in diabetic rats. The addition of IBMX, glucagon, or gastric inhibitory polypeptide (GIP) to perifused islets of diabetic rats amplified their insulin response to glucose, and a clear biphasic pattern of the release was regained. In conclusion, although there is no major alteration of the functionality of the adenylate cyclase in the beta-cells of the diabetic rats, we have identified a defective glucose-induced cAMP generation that could be explained by a block in the step(s) linking glucose metabolism and activation of adenylate cyclase.

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.

Genistein augments cyclic adenosine 3'5'-monophosphate(cAMP) accumulation and insulin release in MIN6 cells

Effects of genistein on insulin release were studied using MIN6 cells, a glucose-sensitive insulinoma cell line. At the non-stimulatory concentrations of glucose, genistein did not affect insulin release, however, at the stimulatory concentrations of glucose, genistein significantly increased insulin release in a dose-dependent manner up to 20 micrograms/ml. The content of cAMP in MIN6 cells was also elevated significantly by genistein and the dose-response relationship between the genistein and cAMP accumulation was consistent with the relationship between the genistein and insulin release. These effects were inhibited by calcium antagonists or by the omission of extracellular calcium. Isobutylmethylxanthine (IBMX;0.1mM) increased both cAMP accumulation and insulin release in MIN6 cells and there were no additive effects by the addition of genistein. The accumulation of cAMP might have, at least in part, resulted from phosphodiesterase inhibition by genistein. These results suggest that genistein augments glucose-induced insulin release by the contribution of cAMP accumulation and calcium modulation which depends on extracellular calcium.

Characterization of Ca2+/calmodulin-dependent protein kinase in rat pancreatic islets

We have attempted to identify islet Ca2+/calmodulin-dependent protein kinase (CaM kinase) by comparing its activity with purified brain CaM kinase II. Islet CaM kinase, in the presence of calmodulin and Ca2+, phosphorylated major endogenous substrates of 102, 57 and 53 kDa and also exogenous glycogen synthase; brain CaM kinase II phosphorylated glycogen synthase and peptides of 57 and 53 kDa. Alloxan (1 mM) inhibited the phosphorylation of glycogen synthase and the 102, 57 and 53 kDa islet peptides by islet CaM kinase; the phosphorylation of glycogen synthase and the 57 and 53 kDa substrates by brain CaM kinase II was also inhibited by alloxan. The Ca2+ and calmodulin-dependencies of phosphorylation of the endogenous islet substrates differed. In the presence of 400 nM calmodulin, half-maximal phosphorylation was attained at Ca2+ concentrations of 80 +/- 9, 401 +/- 61 and 459 +/- 59 nM for the 102, 57 and 53 kDa substrates respectively. In the presence of 10 microM Ca2+, half-maximal phosphorylation was attained at calmodulin concentrations of 9 +/- 2, 38 +/- 2.5 and 37 +/- 2 nM for the 102, 57 and 53 kDa substrates respectively. Differential centrifugation located the 102 kDa substrate in the post-100,000 g supernatant and the 57 and 53 kDa substrates in the particulate fraction. These data suggest that islet CaM kinase is similar to, if not identical with, brain CaM kinase II, but that phosphorylation of the endogenous 102 kDa substrate occurs by a distinct kinase which shows different sensitivities to Ca2+ and calmodulin. This kinase probably corresponds to CaM kinase III and the 102 kDa peptide to elongation factor 2 (EF-2), since the 102 kDa peptide was shown to undergo ADP-ribosylation in the presence of diphtheria toxin and NAD+.

Arachidonic acid-induced insulin secretion from rat islets of Langerhans is not mediated by protein phosphorylation

Arachidonic acid (AA) stimulated protein phosphorylation in electrically permeabilised islets, most notably of an islet protein of approximate molecular weight 18 kDa. This protein did not appear to be a substrate for cAMP-dependent protein kinase. The AA-induced protein phosphorylation was mediated by unmetabolised AA since the lipoxygenase inhibitor, nordihydroguaretic acid (NDGA), or the cyclooxygenase inhibitor, indomethacin, did not significantly reduce AA-induced phosphorylation. Although saturated fatty acids did not stimulate phosphorylation of islet proteins, a number of cis-unsaturated fatty acids, other than AA, induced 32P incorporation into an 18 kDa protein. However, some fatty acids which stimulated protein phosphorylation had no effect on insulin secretion in experiments where AA clearly stimulated insulin secretion. AA stimulated protein kinase C (PKC) activity extracted from islets but several fatty acids which induced protein phosphorylation had no significant effect on PKC activity in vitro. 50 nM staurosporine had no effect on AA-induced protein phosphorylation but this concentration of staurosporine markedly inhibited PKC activity. 200 nM staurosporine caused complete inhibition of the AA-induced phosphorylation without having any effect on AA-induced insulin secretion. These results suggest that AA and some other fatty acids can promote 32P incorporation into islet proteins, independently of PKC activation, and that AA-induced phosphorylation is not required for insulin secretory responses to AA.

Effects of okadaic acid on insulin secretion from rat islets of Langerhans

We have studied the effects of the phosphatase inhibitor, okadaic acid, on insulin secretion and protein phosphorylation in intact and electrically permeabilized pancreatic islets. Okadaic acid inhibited glucose-induced insulin secretion from intact islets, although this effect was probably non-specific since similar effects were obtained using 1-nor-okadaone, a virtually inactive analogue of okadaic acid. In permeabilized islets, okadaic acid enhanced basal and cyclic-AMP-induced insulin secretion and protein phosphorylation. These results indicate that protein phosphatases may play a role in the regulation of insulin release.

Effect of acute trauma on the levels of intracellular cAMP, cGMP and DNA: Studies on endocrinology and metabolism

In this experiment, we used morgrel dogs to study the effect of trauma on the neuro-endocrine and metabolic organs and the effects of hormones on metabolism by determining the levels of intracellular cAMP, cGMP and DNA. From the changes in intracellular nucleotides, we learned that many neuro-endocrine organs and the main metabolic organs are characteristic of predominance in alpha-adrenoreceptor after trauma, thereby leading to high blood sugar.

Does cyclic guanosine monophosphate mediate noradrenaline-induced inhibition of islet insulin secretion stimulated by glucose and palmitate?

Noradrenaline inhibits in rat islets the stimulation of insulin secretion induced by glucose and its potentiation by palmitate, but the signalling system responsible remains unknown. We have tested the hypothesis that noradrenaline-induced inhibition is mediated by an elevation of cyclic GMP (cGMP) levels. The analogue 8-Br-cGMP decreases dose-dependently the potentiation by palmitate of glucose-induced insulin secretion, whereas it only slightly affects the proper effect of glucose. Similarly, it abolishes palmitate acceleration of glucose-induced 45Ca2+ uptake without modifying the sugar effect. Finally, 8-Br-cGMP completely inhibits the stimulation of the lipid synthesis de novo induced by palmitate, but not that caused by glucose alone. On the other hand, noradrenaline increases dose-dependently islet cGMP content, with alpha 2-adrenergic specificity. As noradrenaline-induced elevation of cGMP is sensitive to pertussis toxin, it probably results from alpha 2-adrenoceptor activation of islet guanylate cyclase through a guanine nucleotide regulatory protein. It is concluded that the elevated cGMP levels mediate noradrenaline inhibition of lipid synthesis de novo, and hence of acceleration by palmitate of 45Ca2+ uptake and insulin secretion in the presence of glucose.

Effects of glucose, insulin, and cAMP on transcription of the serine dehydratase gene in rat liver

Starvation and diabetes both caused a dramatic induction of hepatic L-serine dehydratase (SDH) (EC 4.2.1.13) in rats. Increases in the activity of the enzyme which had been demonstrated in several previous studies were found to be associated with increases in the amount of SDH protein and its mRNA in our studies reported herein. Nuclear run-on experiments with isolated liver nuclei demonstrated that the increases in SDH activity were mainly the result of increases in the rate of SDH gene transcription. Refeeding of glucose to starved rats or the administration of insulin to diabetic rats caused a marked reduction in the amount of SDH mRNA. The rates of transcription as measured in isolated nuclei were reduced to uninduced levels within 30 min of either treatment. Following the administration of Bt2-cAMP, the transcription rates of the SDH gene returned to the original induced rates within 40 min both in glucose-refed rats and in diabetic rats administered insulin. The results of these experiments indicate that the induction of SDH in rat liver in vivo is controlled predominantly at the level of gene transcription by the reciprocal action of cAMP and insulin.

Identification of the calmodulin-regulated protein phosphatase, calcineurin, in rat pancreatic islets

The aim of this work was the identification of the calmodulin-stimulated protein phosphatase, calcineurin, in rat pancreatic islets. For this purpose, a high-affinity calcineurin antibody and the Western blotting technique were used to detect the presence of calcineurin in freshly collagenase-isolated islets. The calcineurin content detected by this method was about 0.30 ng islet (approx. 0.07% of the total islet protein). The subunit composition and Mr of islet calcineurin were similar to those of bovine brain calcineurin. Incubation of nitrocellulose membranes of the Western blotting, containing the islet protein fractions, with 125I-labeled calmodulin and 45Ca2+ demonstrated that the A subunit bound calmodulin, while the B subunit bound Ca2+. The presence of calcineurin in the islets of Langerhans would suggest its possible participation, as a counterpart of the kinases effect, in the regulatory mechanism of insulin secretion.

On the mechanism of impaired insulin secretion in chronic renal failure

It has been suggested that a sustained rise in resting levels of cytosolic calcium [Ca2+]i of pancreatic islets is responsible for impaired insulin secretion in chronic renal failure (CRF). Evidence for such an event is lacking and the mechanisms through which it may affect insulin secretion are not known. Studies were conducted in normal, CRF, and normocalcemic, parathyroidectomized (PTX) CRF rats to answer these questions. Resting levels of [Ca2+]i of islets from CRF rats were higher (P less than 0.01) than in control of CRF-PTX rats. [3H]2-deoxyglucose uptake and cAMP production by islets were not different in the three groups. Insulin content of, and glucose-induced insulin secretion by islets from CRF rats was lower (P less than 0.01) than in control and CRF-PTX rats. In contrast, glyceraldehyde-induced insulin release by CRF islets was normal. Basal ATP content, both glucose-stimulated ATP content and ATP/ADP ratio, net lactic acid output, Vmax of phosphofructokinase-1, and Ca2+ ATPase of islets from CRF rats were lower (P less than 0.02-less than 0.01) than in normal or CRF-PTX animals. Data show that: (a) Glucose but not glyceraldehyde-induced insulin secretion is impaired in CRF; (b) the impairment in glucose-induced insulin release in CRF is due to a defect in the metabolism of glucose; (c) this latter defect is due to reduced ATP content induced partly by high [Ca2+]i of islets; and (d) the high [Ca2+]i in islets of CRF rats is due to augmented PTH-induced calcium entry into cells and decreased calcium extrusion from the islets secondary to reduced activity of the Ca2+ ATPase.

Glucose-stimulated insulin secretion is not dependent on activation of protein kinase A

The involvement of cyclic AMP-dependent protein kinase A (PKA) in the exocytotic release of insulin from rat pancreatic islets was investigated using the Rp isomer of adenosine 3',5'-cyclic phosphorothioate (Rp-cAMPS). Preincubation of electrically permeabilised islets with Rp-cAMPS (1 mM, 1 h, 4 degrees C) inhibited cAMP-induced phosphorylation of islet proteins of apparent molecular weights in the range 20-90 kDa, but did not affect basal (50 nM Ca2+) nor Ca2(+)-stimulated (10 microM) protein phosphorylation. Similarly, Rp-cAMPS (500 microM) inhibited both cAMP- (100 microM) and 8BrcAMP-induced (100 microM) insulin secretion from electrically permeabilised islets without affecting Ca2(+)-stimulated (10 microM) insulin release. In intact islets, Rp-cAMPS (500 microM) inhibited forskolin (1 microM, 10 microM) potentiation of insulin secretion, but did not significantly impair the insulin secretory response to a range of glucose concentrations (2-20 mM). These results suggest that cAMP-induced activation of PKA is not essential for either basal or glucose-stimulated insulin secretion from rat islets.

Time-course of Ca2+-induced insulin secretion from perifused, electrically permeabilised islets of Langerhans: effects of cAMP and a phorbol ester

The pattern of insulin secretion from electrically permeabilised islets was studied in a perifusion system. Increases in intracellular Ca2+ stimulated insulin secretion in a dose-related manner, but the secretory response to Ca2+ was only transient, with permeabilised islets becoming rapidly insensitive to a stimulatory concentration of Ca2+. Cyclic AMP and the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA), both stimulated Ca2+-induced insulin secretion from perifused permeabilised islets by increasing the maximum secretory response to Ca2+, and by maintaining elevated rates of secretion when the permeabilised islets had become insensitive to the stimulatory effects of Ca2+. These results suggest that cAMP and PMA may act partly by modifying the magnitude of the secretory response to Ca2+, and also by Ca2+-independent effects on the secretory mechanism.

L-leucine or its keto acid potentiate but do not initiate insulin release in chicken

In the isolated perfused chicken pancreas, 20 and 40 mM L-leucine or 10-40 mM alpha-ketoisocaproic acid (alpha-KIC) did not initiate insulin release. In the presence of 14 mM glucose (a noninsulinotropic concentration), 20 mM L-leucine and 10 mM alpha-KIC evoked a slight biphasic insulin release. The response to 20 mM L-leucine was further increased when 14 mM glucose was combined with 10 mM L-glutamine (10 mM glutamine alone did not induce insulin release and did not potentiate the response to 10 mM L-leucine). At 1 mM, 8-bromo-adenosine 3',5'-cyclic monophosphate (8-BrcAMP) alone caused a slight and progressive increase in insulin secretion but did not sensitize the pancreas to either 20 mM L-leucine or 10 mM alpha-KIC, whereas it facilitated a marked insulin release in response to 14 mM glucose. On the other hand, 10-40 mM K+ or 20 mM L-arginine induced a rapid monophasic insulin output. In conclusion, L-leucine or alpha-KIC, which do not initiate insulin release alone and are not potentiated by 8-BrcAMP, may not be regarded as primary insulinotropic agents in the chicken. This result, together with the previously documented resistance of the chicken pancreas to glucose alone or to D-glyceraldehyde, strongly suggests that the mechanisms initiating insulin secretion are different in chickens and mammals, whereas potentiating mechanisms (low glucose concentration, arginine, acetylcholine, and cAMP) and membrane depolarization events (K+ and arginine) are present in both chickens and mammals.

Protein phosphorylation in electrically permeabilized islets of Langerhans. Effects of Ca2+, cyclic AMP, a phorbol ester and noradrenaline

The incorporation of 32P from [gamma-32P]ATP into intracellular proteins was studied in electrically permeabilized rat islets of Langerhans. Ca2+ (10 microM), cyclic AMP (100 microM) and a protein kinase C-activating phorbol ester, phorbol 13-myristate 12-acetate (PMA; 100 nM) produced marked changes in the phosphorylation state of a number of proteins in permeabilized islets after incubation for 1 min at 37 degrees C. Ca2+ modified the effects of cyclic AMP and PMA on protein phosphorylation. Noradrenaline (10 microM) had no detectable effects on Ca2+-dependent protein phosphorylation, but significantly inhibited Ca2+-induced insulin secretion from electrically permeabilized islets. These results suggest that electrically permeabilized islets offer a useful model in which to study rapid events in protein phosphorylation as a mechanism of stimulus-secretion coupling. If the rapid Ca2+-induced effects on protein phosphorylation are involved in the control of insulin secretion, the results of this study also imply that part of the catecholamine inhibition of insulin secretion occurs at a stage in the secretory pathway beyond the activation of the regulated protein kinases.

Forskolin-induced block of delayed rectifying K+ channels in pancreatic beta-cells is not mediated by cAMP

K+ channels in the membrane of murine pancreatic beta-cells were studied using the patch-clamp technique. The delayed outward current was activated in whole-cell experiments by depolarizing voltage pulses to potentials between -30 mV and 0 mV. Forskolin blocked the current rapidly (less than 5 s) and reversibly with 50% inhibition at 13 microM. The inhibition did not depend on a stimulation of the adenylate cyclase since it occurred even in presence of 1 mM cAMP in the pipette solution which replaced the cytoplasm. Membrane permeant cAMP analogues and phosphodiesterase inhibitors did not influence the delayed outward current. In experiments on outside-out patches forskolin (100 microM) shortened the openings of a channel of about 10 pS conductance at 0 mV and a time course of activation and inactivation similar to the whole-cell current. Another smaller, slowly activating channel and the Ca2+- and ATP-dependent K+ channels were influenced only weakly or not at all. It is therefore concluded that the 10-pS channel generates most of the delayed outward K+ current in murine pancreatic beta-cells. The Ca2+-independent part of the delayed outward current in bovine adrenal chromaffin cells was also blocked by forskolin (100 microM).

Na channels and two types of Ca channels in rat pancreatic B cells identified with the reverse hemolytic plaque assay

The reverse hemolytic plaque assay (RHPA) was used to study the secretory properties of single rat pancreatic B cells, and to identify insulin-secreting cells for patch-clamp experiments. In secretion studies using the RHPA, we find that the percentage of secreting B cells and the amount of insulin secreted per B cell increase as the glucose concentration is raised from 0 to 20 mM. Using the whole-cell variation of the patch-clamp technique, we find that identified B cells have three types of channels capable of carrying inward current: (a) tetrodotoxin-sensitive, voltage-dependent Na channels, which are nearly completely inactivated at -40 mV, (b) fast deactivating (FD) Ca channels, and (c) slowly deactivating (SD) Ca channels. We have shown that Na channels are functionally significant to the B cell, because tetrodotoxin partially inhibits glucose-induced insulin secretion. The properties of FD and SD Ca channels differ in several respects. FD channels deactivate at -80 mV, with a time constant of 129 microseconds, they are half-maximally activated near +15 mV, they do not inactivate during 100 ms, they conduct Ba2+ better than Ca2+, and they are very sensitive to washout during intracellular dialysis. SD channels, on the other hand, deactivate with a time constant of 2.8 ms, they are half-maximally activated near -5 mV, they inactivate rapidly, they conduct Ba2+ and Ca2+ equally well, and they are insensitive to washout.

Morphological changes in pancreatic B-cells after intravenous administration of forskolin

The morphological changes in rat pancreatic islets were studied after intravenous injection of forskolin (1.5 mg/kg), an activator of adenylate cyclase, to male Wistar rats. A high amount of insulin containing granules was observed on PAP immunostained paraffin sections. Ultrastructural study of islet B-cells demonstrated enlarged perinuclear space, dilated cisternae of rough endoplasmic reticulum and activated Golgi complex with numerous vesicles. The results suggest that intravenous administration of forskolin produces a modest increase of B-cell synthetic activity in the pancreatic islets of rats. It is assumed that forskolin affects pancreatic B-cells via increased cAMP.