Effects of the beta-carbolines, harmane and pinoline, on insulin secretion from isolated human islets of Langerhans

The Other Side of the Perfect Cup: Coffee-Derived Non-Polyphenols and Their Roles in Mitigating Factors Affecting the Pathogenesis of Type 2 Diabetes

The global prevalence of type 2 diabetes (T2D) is 10.5% among adults in the age range of 20-79 years. The primary marker of T2D is persistent fasting hyperglycemia, resulting from insulin resistance and β-cell dysfunction. Multiple factors can promote the development of T2D, including obesity, inflammation, and oxidative stress. In contrast, dietary choices have been shown to prevent the onset of T2D. Oatmeal, lean proteins, fruits, and non-starchy vegetables have all been reported to decrease the likelihood of T2D onset. One of the most widely consumed beverages in the world, coffee, has also demonstrated an impressive ability to reduce T2D risk. Coffee contains a diverse array of bioactive molecules. The antidiabetic effects of coffee-derived polyphenols have been thoroughly described and recently reviewed; however, several non-polyphenolic molecules are less prominent but still elicit potent physiological actions. This review summarizes the effects of select coffee-derived non-polyphenols on various aspects of T2D pathogenesis.

Anti-obesity and anti-diabetic bioactive peptides: A comprehensive review of their sources, properties, and techno-functional challenges

The scourge of obesity arising from obesogens and poor dieting still ravages our planet as half of the global population may be overweight and obese by 2035. This metabolic disorder is intertwined with type 2 diabetes (T2D), both of which warrant alternative therapeutic options other than clinically approved drugs like orlistat with their tendency of abuse and side effects. In this review, we comprehensively describe the global obesity problem and its connection to T2D. Obesity, overconsumption of fats, the mechanism of fat digestion, obesogenic gut microbiota, inhibition of fat digestion, and natural anti-obesity compounds are discussed. Similar discussions are made for diabetes with regard to glucose regulation, the diabetic gut microbiota, and insulinotropic compounds. The sources and production of anti-obesity bioactive peptides (AOBPs) and anti-diabetic bioactive peptides (ADBPs) are also described while explaining their structure-function relationships, gastrointestinal behaviors, and action mechanisms. Finally, the techno-functional applications of AOBPs and ADBPs are highlighted.

Can Polyherbal Medicine be used for the Treatment of Diabetes? - A Review of Historical Classics, Research Evidence and Current Prevention Programs

Diabetes mellitus (DM), a chronic medical condition, has attained a global pandemic status over the last few decades affecting millions of people. Despite a variety of synthetic drugs available in the market, the use of herbal medicines for managing diabetes is gaining importance because of being comparatively safer. This article reviews the result of a substantial literature search on polyherbal formulations (PHFs) developed and evaluated with potential for DM. The accumulated data in the literature allowed us to enlist 76PHFs consisting of different parts of 147 plant species belonging to 58 botanical families. The documented plant species are laden with bioactive components with anti-diabetic properties and thus draw attention. The most favoured ingredient for PHFs was leaves of Gymnema sylvestre and seeds of Trigonella foenum-graecum used in 27 and 22 formulations, respectively. Apart from herbs, shilajit (exudates from high mountain rocks) formed an important component of 9 PHFs, whereas calcined Mytilus margaritiferus and goat pancreas were used in Dolabi, the most commonly used tablet form of PHF in Indian markets. The healing properties of PHFs against diabetes have been examined in both pre-clinical studies and clinical trials. However, the mechanism(s) of action of PHFs are still unclear and considered the pitfalls inherent in understanding the benefits of PHFs. From the information available based on experimental systems, it could be concluded that plant-derived medicines will have a considerable role to play in the control of diabetes provided the challenges related to their bioavailability, bioefficacy, optimal dose, lack of characterization, ambiguous mechanism of action, and clinical efficiency are addressed.

Alkaloidal Phytoconstituents for Diabetes Management: Exploring the Unrevealed Potential

The main characteristic feature of diabetes mellitus is the disturbance of carbohydrate, lipid, and protein metabolism, which results in insulin insufficiency and can also lead to insulin resistance. Both the acute and chronic diabetic cases are increasing at an exponential rate, which is also flagged by the World Health Organization (WHO) and the International Diabetes Federation (IDF). Treatment of diabetes mellitus with synthetic drugs often fails to provide desired results and limits its use to symptomatic treatment only. This has resulted in the exploration of alternative medicine, of which herbal treatment is gaining popularity these days. Owing to their safety benefits, treatment compliance, and ability to exhibit effects without disturbing internal homeostasis, research in the field of herbal and ayurvedic treatments has gained importance. Medicinal phytoconstituents include micronutrients, amino acids, proteins, mucilage, critical oils, triterpenoids, saponins, carotenoids, alkaloids, flavonoids, phenolic acids, tannins, and coumarins, which play a dynamic function in the prevention and treatment of diabetes mellitus. Alkaloids found in medicinal plants represent an intriguing potential for the inception of novel approaches to diabetes mellitus therapies. Thus, this review article highlights detailed information on alkaloidal phytoconstituents, which includes sources and structures of alkaloids along with the associated mechanism involved in the management of diabetes mellitus. From the available literature and data presented, it can be concluded that these compounds hold tremendous potential for use as monotherapies or in combination with current treatments, which can result in the development of better efficacy and safety profiles.

Potential Role of Heterocyclic Aromatic Amines in Neurodegeneration

Heterocyclic aromatic amines (HAAs) are mainly formed in the pyrolysis process during high-temperature cooking of meat. Meat consumption is very typical of the western diet, and the amount of meat consumption in the eastern countries is growing rapidly; HAAs represents widespread exposure. HAAs are classified as possible human carcinogens; numerous epidemiological studies have demonstrated regular consumption of meat with HAAs as risk factor for cancers. Specific HAAs have received major attention. For example, 2-amino-1-methyl-6-phenylimidazo[4,5-b] pyridine has been extensively studied as a genotoxicant and mutagen, with emergent literature on neurotoxicity. Harmane has been extensively studied for a role in essential tremors and potentially Parkinson's disease (PD). Harmane levels have been demonstrated to be elevated in blood and brain in essential tremor patients. Meat consumption has been implicated in the etiology of neurodegenerative diseases; however, the role of toxicants formed during meat preparation has not been studied. Epidemiological studies are currently examining the association between HAAs and risk of neurodegenerative diseases such as essential tremors and PD. Studies from our laboratory and others have provided strong evidence that HAA exposure produces PD and Alzheimer's disease-relevant neurotoxicity in cellular and animal models. In this review, we summarize and critically evaluate previous studies on HAA-induced neurotoxicity and the molecular basis of potential neurotoxic effects of HAAs. The available studies provide strong support for the premise that HAAs may impact neurological function and that addressing gaps in understanding of adverse neurological outcomes is critical to determine whether these compounds are modifiable risk factors.

β-Carboline-based molecular hybrids as anticancer agents: a brief sketch

Cancer is a huge burden on the healthcare system and is foremost cause of mortality across the globe. Among various therapeutic strategies, chemotherapy plays an enormous role in overcoming the challenges of treating cancer, especially in late stage detection. However, limitations such as extreme side/adverse effects and drug resistance associated with available drugs have impelled the development of novel chemotherapeutic agents. In this regard, we have reviewed the development of β-carboline-based chemotherapeutic agents reported in last five years. The review mainly emphasizes on the molecular hybrids of β-carbolines with various pharmacophores, their synthetic strategies, and in vitro anticancer evaluation. In addition, the mechanisms of action, in silico studies, structural influence on the potency and selectivity among diverse cancer cell lines have been critically presented. The review updates readers on the diverse molecular hybrids prepared and the governing structural features of high potential molecules that can help in the future development of novel cytotoxic agents.

Phytotoxic Activity of the Natural Compound Norharmane on Crops, Weeds and Model Plants

Norharmane is a secondary metabolite that appears in different species of land plants. In this paper, we investigated for the first time the specificity of norharmane through germination and growth tests on some crops as Zea mays L. (maize), Triticum aestivum L. (wheat), Oryza sativa L. (rice) and Lactuca sativa L. (lettuce) and weeds as Amaranthus retroflexus L. (amaranth), Echinochloa crus-galli L. (barnyard grass), Plantago lanceolata L. (ribwort), Portulaca oleracea L. (common purslane) and Avena fatua L. (wild oat), and its phytotoxic capacity on the metabolism of adult Arabidopsis thaliana L. (thale cress) by measuring chlorophyll a fluorescence, pigment content, total proteins, osmotic potential and morphological analysis. Norharmane had an inhibitory effect on the germination of A. fatua and P. lanceolata, and the growth of P. oleracea, E. crus-galli and A. retroflexus. On adult A. thaliana plants, the compound was more effective to watering, leading to water stress that compromised the growth of the plants and ultimately affected the photosynthetic apparatus. Therefore, this research shows that norharmane not only affects seedlings’ metabolism, but also damages the metabolism of adult plants and can be a potential model for a future bioherbicide given its specificity.

A natural indole alkaloid, norharmane, affects PIN expression patterns and compromises root growth in Arabidopsis thaliana

Norharmane is an indole alkaloid that can be found in several terrestrial plants, as well as in some dinoflagellates and cyanobacteria. The aim of this study was to focus on the way this metabolite impacts the plant metabolism of the model species Arabidopsis thaliana. This metabolite caused increase of secondary and adventitious roots, as well as torsion, toxic effects, and a decrease in root length. Moreover, norharmane altered the cellular arrangement, resulting in unfinished cell walls, decreased auxin content and inhibited PIN proteins activity. All the alterations suggest that norharmane alters polar auxin transport by inhibiting PIN2, PIN3 and PIN7 transport proteins, thus causing a significant inhibitory effect on the growth of A. thaliana seedlings.

Harmane ameliorates obesity though inhibiting lipid accumulation and inducing adipocyte browning

Harmane, a compound derived from Peganum harmala L., was discovered as a lipid accumulation inhibitor with our triglyceride (TG) assay. To investigate the molecular mechanism underlying the role of harmane in the prevention of lipid accumulation, a number of biological experiments have been designed to determine if harmane reduces lipid accumulation by suppressing adipogenesis, lipogenesis, and regulating a specific signal transduction pathway. Our experimental data show that harmane inhibits TG accumulation though down-regulating the expression of adipogenic and lipogenic factors, up-regulating adipocyte browning markers and activating the SIRT1-LKB1-AMPK pathway. Therefore, harmane ameliorates obesity though inhibiting lipid accumulation and inducing adipocyte browning.

Harmane ameliorates obesity though inhibiting lipid accumulation and inducing adipocyte browning.

Antidiabetic phytoconstituents and their mode of action on metabolic pathways

Diabetes Mellitus, characterized by persistent hyperglycaemia, is a heterogeneous group of disorders of multiple aetiologies. It affects the human body at multiple organ levels thus making it difficult to follow a particular line of the treatment protocol and requires a multimodal approach. The increasing medical burden on patients with diabetes-related complications results in an enormous economic burden, which could severely impair global economic growth in the near future. This shows that today's healthcare system has conventionally been poorly equipped towards confronting the mounting impact of diabetes on a global scale and demands an urgent need for newer and better options. The overall challenge of this field of diabetes treatment is to identify the individualized factors that can lead to improved glycaemic control. Plants are traditionally used worldwide as remedies for diabetes healing. They synthesize a diverse array of biologically active compounds having antidiabetic properties. This review is an endeavour to document the present armamentarium of antidiabetic herbal drug discovery and developments, highlighting mechanism-based antidiabetic properties of over 300 different phytoconstituents of various chemical categories from about 100 different plants modulating different metabolic pathways such as glycolysis, Krebs cycle, gluconeogenesis, glycogen synthesis and degradation, cholesterol synthesis, carbohydrate metabolism as well as peroxisome proliferator activated receptor activation, dipeptidyl peptidase inhibition and free radical scavenging action. The aim is to provide a rich reservoir of pharmacologically established antidiabetic phytoconstituents with specific references to the novel, cost-effective interventions, which might be of relevance to other low-income and middle-income countries of the world.

Investigation of morin-induced insulin secretion in cultured pancreatic cells

Morin is a flavonoid contained in guava that is known to reduce hyperglycemia in diabetes. Insulin secretion has been demonstrated to increase following the administration of morin. The present study is designed to investigate the potential mechanism(s) of morin-induced insulin secretion in the MIN6 cell line. First, we identified that morin induced a dose-dependent increase in insulin secretion and intracellular calcium content in MIN6 cells. Morin potentiated glucose-stimulated insulin secretion (GSIS). Additionally, we used siRNA for the ablation of imidazoline receptor protein (NISCH) expression in MIN6 cells. Interestingly, the effects of increased insulin secretion by morin and canavanine were markedly reduced in Si-NISCH cells. Moreover, we used KU14R to block imidazoline I₃ receptor (I-3R) that is known to enhance insulin release from the pancreatic β-cells. Without influence on the basal insulin secretion, KU14R dose-dependently inhibited the increased insulin secretion induced by morin or efaroxan in MIN6 cells. Additionally, effects of increased insulin secretion by morin or efaroxan were reduced by diazoxide at the dose sufficient to open K_ATP channels and attenuated by nifedipine at the dose used to inhibit L-type calcium channels. Otherwise, phospholipase C (PLC) is introduced to couple with imidazoline receptor (I-R). The PLC inhibitor dose-dependently inhibited the effects of morin in MIN6 cells. Similar blockade was also observed in protein kinase C (PKC) inhibitor-treated cells. Taken together, we found that morin increases insulin secretion via the activation of I-R in pancreatic cells. Therefore, morin would be useful to develop in the research and treatment of diabetic disorders.

Mechanism of action of natural products used in the treatment of diabetes mellitus

Diabetes mellitus (DM) is a metabolic disorder caused by insufficient or inefficient insulin secretary response and it is characterized by increased blood glucose levels (hyperglycemia). DM is a heterogonous group of syndromes. Glucose is the main energy source for the body, and in the case of DM, management of glucose becomes irregular. There are three key defects in the onset of hyperglycemia in DM, namely increased hepatic glucose production, diminished insulin secretion, and impaired insulin action. Conventional drugs treat diabetes by improving insulin sensitivity, increasing insulin production and/or decreasing the amount of glucose in blood. This article provides a comprehensive review of the mode of action of most popular hypoglycemic herbs, such as ginseng, bitter melon, fenugreek, banaba, Gymnema sylvestre and Coptis chinensis. The herbs act by increasing insulin secretion, enhancing glucose uptake by adipose and skeletal muscle tissues, inhibiting intestinal glucose absorption and inhibiting hepatic glucose production. Although evidence from animals and humans consistently supports the therapeutic effect of these phytomedicines, multicenter large-scale clinical trials have not been conducted to evaluate the safety and efficacy of these herbal medicines and their interaction with conventional drugs when administered simultaneously.

Modulation of stress by imidazoline binding sites: implications for psychiatric disorders

In this review, we present evidence for the involvement of imidazoline binding sites (IBS) in modulating responses to stress, through central control of monoaminergic and hypothalamo-pituitary-adrenal (HPA) axis activity. Pharmacological and physiological evidence is presented for differential effects of different IBS subtypes on serotoninergic and catecholaminergic pathways involved in control of basal and stress-stimulated HPA axis activity. IBS ligands can modulate behavioural and neuroendocrine responses in animal models of stress, depression and anxiety, and a body of evidence exists for alterations in central IBS expression in psychiatric patients, which can be normalised partially or fully by treatment with antidepressants. Dysfunction in monoaminergic systems and the HPA axis under basal and stress-induced activation has been extensively reported in psychiatric illnesses. On the basis of the literature, we suggest a potential therapeutic role for selective IBS ligands in the treatment of depression and anxiety disorders.

Pinoline may be used as a probe for CYP2D6 activity

Pinoline, 6-methoxy-1,2,3,4-tetrahydro-beta-carboline, is a serotonin analog that selectively inhibits the activity of monoamine oxidase-A and shows antidepressant activity. Our previous study using a panel of recombinant cytochrome P450 (P450) enzymes suggests that pinoline O-demethylation may be selectively catalyzed by polymorphic CYP2D6. The current study, therefore, aimed to delineate the impact of CYP2D6 status on pinoline metabolism. Enzyme kinetic studies using recombinant CYP2D6 allelic isozymes revealed that CYP2D6.2 exhibited 5-fold lower enzyme efficiency (V(max)/K(m)) toward pinoline compared with CYP2D6.1, and CYP2D6.10 did not show any catalytic activity. Inhibition study showed that quinidine (1 microM) completely blocked pinoline O-demethylase activity in human liver microsomes, whereas other P450 isoform-selective inhibitors had no or minimal effects. Pinoline O-demethylase activities in 10 human liver microsomes showed significantly strong correlation with bufuralol 1'-hydroxylase activities (R(2)=0.93; p<0.0001) and CYP2D6 contents (R(2)=0.82; p=0.005), whereas no appreciable correlations with enzymatic activities of other P450 enzymes were found. Furthermore, we compared pinoline urinary metabolic ratio (pinoline/6-hydroxy-1,2,3,4-tetrahydro-beta-carboline) between CYP2D6-humanized and wild-type control mice after intraperitoneal injection of pinoline (30 mg/kg). Results indicated that the two genotyped mice were clearly distinguished by pinoline metabolic ratio (mean +/- S.D.), which was much higher in wild-type mice (0.29+/-0.19, n=4) than in CYP2D6-humanized transgenic mice (0.0070+/-0.0048, n=4). Our findings suggest that pinoline O-demethylation is governed by CYP2D6 status, and pinoline, at a proper concentration or dose, may be a good probe to evaluate CYP2D6 activity.

A spectroscopic study of the interaction of the fluorescent beta-carboline-3-carboxylic acid N-methylamide with DNA constituents: nucleobases, nucleosides and nucleotides

Interaction between beta-carboline-3-carboxylic acid N-methylamide, betaCMAM, and nucleobases, nucleosides and nucleotides is studied in the ground state with UV-visible, (1)H NMR and (31)P NMR spectroscopies and in the first excited state, with steady-state and time-resolved fluorescence spectroscopy. Job plots show a predominant 1:1 interaction in both electronic states. Association constants are estimated from changes in the absorption spectra, and show that the strongest interaction is produced with the nucleosides: 2'-deoxyadenosine (dAdo) and thymidine (Thd), and with the mononucleotides: 2'-deoxycytidine 5'- monophosphate (5'-dCMP) and uridine 5'- monophosphate (5'-UMP). These results are corroborated by the upfield shifts of two (1)H NMR resonances of the betaCMAM indole group. The (31)P NMR resonance of nucleotides is shifted downfield, suggesting the presence of electrostatic or hydrogen bond interaction with betaCMAM. In the first electronic singlet excited state, static and dynamic quenching of betaCMAM emission is achieved upon addition of nucleobases, nucleosides and nucleotides. This has been analysed using Stern-Volmer kinetics.

Targeting fat to prevent diabetes

An emerging view is that obesity causes metabolic problems when adipose tissue fails to meet the increased demands for fat storage. A study in this issue of Cell Metabolism (Waki et al., 2007) has identified harmine as a proadipogenic small molecule that promotes energy expenditure in white adipose tissue and delays the onset of obesity-associated diabetes.

Factors influencing the norharman and harman contents in espresso coffee

Espresso coffee (EC) brews were analyzed for beta-carboline [norharman (NH) and harman (H)] contents, by RP-HPLC with fluorescence detection. The influence of the coffee species (arabica or robusta), the roast degree, and the brew length was studied. The results show that the content of NH and H in EC is dependent primarily on the coffee species, followed by brew length. The roast degree has only a minor influence on the final content of NH and H in EC. When compared with other coffee brews, EC has an amount of these beta-carbolines (in micrograms per liter) similar to that of mocha coffee, both being more concentrated than filter and press-pot coffees. Therefore, the consumer's preferences will determine the amount of NH and H ingested daily. For the caffeinated 30 mL of EC, the arabica coffees contain about 4.08 microg of NH and 1.54 microg of H. Commercial blends (usually with a maximum of 30% robusta) range from the cited arabica values to 10.37 microg of NH and 4.35 microg of H.

Screening of microalgae for species excreting norharmane, a manifold biologically active indole alkaloid

A screening of microalgae strains is described, with the objective to discover more species besides the known cyanobacterium Nodularia harveyana which excrete the manifold biologically active and co-mutagenic indole alkaloid norharmane (9H-pyrido(3,4-b)indole) into their environment. Seven more cyanobacterial species, Anabaena cylindrica, Anabaena inaequalis, Anabaenopsis siamensis, Chroococcus minutus, Nostoc carneum, Nostoc commune and Phormidium foveolarum, were newly discovered. The norharmane concentrations detected for cyanobacterial culture media varied in a species-dependent manner from less than 1 up to 525 microg l(-1). The risk for humans and livestock, resulting from the natural appearance of norharmane as an extracellular metabolite of various cyanobacteria, is discussed.

Leads from Indian medicinal plants with hypoglycemic potentials

Diabetes mellitus is caused due to deficiency in production of insulin by the pancreas, or by the ineffectiveness of the insulin produced. It is a global problem and number of those affected is increasing day by day. The plants provide a potential source of hypoglycemic drugs because many plants and plant derived compounds have been used in the treatment of diabetes. Several medicinal plants have found potential use as hypoglycemic in the Indian system of medicines, including ayurveda. Many Indian plants have been investigated for their beneficial use in different types of diabetes and reports occur in numerous scientific journals. This article aims to provide a comprehensive review on various plant species from Indian biosphere and their constituents, which have been shown to display potent hypoglycemic activity. The use of herbs as hypoglycemic is a major avenue in Indian perspectives particularly for treating diabetes, which require to be explored more effectively as there are so many literatures available on these aspects. This paper describes the chemistry, activity and usage of the constituents isolated from these plants from India for the treatment of diabetes.

Antialgal, antibacterial and antifungal activity of two metabolites produced and excreted by cyanobacteria during growth

The antimicrobial activity of two cyanobacterial exometabolites, norharmane (9H-pyrido(3,4-b)indole) and 4,4'-dihydroxybiphenyl, was determined in suspension assays. Good anticyanobacterial activities (concentrations of 8-80 microg ml(-1)) and moderate antibacterial (16-160 microg ml(-1)) and antifungal (32-40 microg ml(-1)) activities were found for both compounds. The natural function as allelopathic chemicals and a possible applicability as antifouling agents or leads for the development of new antifouling chemicals are discussed.

In vitro and ex vivo distribution of [3H]harmane, an endogenous beta-carboline, in rat brain

The endogenous beta-carboline, harmane, has been shown to bind to monoamine oxidase A (MAO-A) and a separate, high affinity, non-MAO site. Research in our laboratory has shown that harmane is an active component of clonidine-displacing substance (CDS), the proposed endogenous ligand for imidazoline binding sites (IBS). In the present study we have investigated the distribution of [3H]harmane in rat brain, and related the binding profile to the distribution of the MAO-A selective ligand [3H]Ro41-1049 and the I2BS ligand [3H]2-BFI. The in vivo distribution of [3H]harmane following intravenous administration was also investigated. Receptor autoradiography revealed a highly significant correlation for the distribution of [3H]harmane and [3H]Ro41-1049, and a significant correlation for [3H]harmane and the I2BS ligand [3H]2-BFI. The in vivo distribution of [3H]harmane suggests that the ligand accumulates in the adrenal gland and throughout the brain with the primary route of excretion occurring via the duodenum. In conclusion, these studies have shown that [3H]harmane labels a population of binding sites that reflect the distribution of MAO-A. Further evidence for a non-MAO, IBS [3H]harmane population has not been shown but the high level of expression of the MAO-A site is likely to have masked the much smaller population of I2BS.

Pathways in beta-cell stimulus-secretion coupling as targets for therapeutic insulin secretagogues

Physiologically, insulin secretion is subject to a dual, hierarchal control by triggering and amplifying pathways. By closing ATP-sensitive K+ channels (KATP channels) in the plasma membrane, glucose and other metabolized nutrients depolarize beta-cells, stimulate Ca2+ influx, and increase the cytosolic concentration of free Ca2+ ([Ca2+]i), which constitutes the indispensable triggering signal to induce exocytosis of insulin granules. The increase in beta-cell metabolism also generates amplifying signals that augment the efficacy of Ca2+ on the exocytotic machinery. Stimulatory hormones and neurotransmitters modestly increase the triggering signal and strongly activate amplifying pathways biochemically distinct from that set into operation by nutrients. Many drugs can increase insulin secretion in vitro, but only few have a therapeutic potential. This review identifies six major pathways or sites of stimulus-secretion coupling that could be aimed by potential insulin-secreting drugs and describes several strategies to reach these targets. It also discusses whether these perspectives are realistic or theoretical only. These six possible beta-cell targets are 1) stimulation of metabolism, 2) increase of [Ca2+]i by closure of K+ ATP channels, 3) increase of [Ca2+]i by other means, 4) stimulation of amplifying pathways, 5) action on membrane receptors, and 6) action on nuclear receptors. The theoretical risk of inappropriate insulin secretion and, hence, of hypoglycemia linked to these different approaches is also envisaged.

The putative imidazoline receptor agonist, harmane, promotes intracellular calcium mobilisation in pancreatic β-cells

beta-Carbolines (including harmane and pinoline) stimulate insulin secretion by a mechanism that may involve interaction with imidazoline I(3)-receptors but which also appears to be mediated by actions that are additional to imidazoline receptor agonism. Using the MIN6 beta-cell line, we now show that both the imidazoline I(3)-receptor agonist, efaroxan, and the beta-carboline, harmane, directly elevate cytosolic Ca(2+) and increase insulin secretion but that these responses display different characteristics. In the case of efaroxan, the increase in cytosolic Ca(2+) was readily reversible, whereas, with harmane, the effect persisted beyond removal of the agonist and resulted in the development of a repetitive train of Ca(2+)-oscillations whose frequency, but not amplitude, was concentration-dependent. Initiation of the Ca(2+)-oscillations by harmane was independent of extracellular calcium but was sensitive to both dantrolene and high levels (20 mM) of caffeine, suggesting the involvement of ryanodine receptor-gated Ca(2+)-release. The expression of ryanodine receptor-1 and ryanodine receptor-2 mRNA in MIN6 cells was confirmed using reverse transcription-polymerase chain reaction (RT-PCR) and, since low concentrations of caffeine (1 mM) or thimerosal (10 microM) stimulated increases in [Ca(2+)](i), we conclude that ryanodine receptors are functional in these cells. Furthermore, the increase in insulin secretion induced by harmane was attenuated by dantrolene, consistent with the involvement of ryanodine receptors in mediating this response. By contrast, the smaller insulin secretory response to efaroxan was unaffected by dantrolene. Harmane-evoked changes in cytosolic Ca(2+) were maintained by nifedipine-sensitive Ca(2+)-influx, suggesting the involvement of L-type voltage-gated Ca(2+)-channels. Taken together, these data imply that harmane may interact with ryanodine receptors to generate sustained Ca(2+)-oscillations in pancreatic beta-cells and that this effect contributes to the insulin secretory response.