Somatostatin activates two types of inwardly rectifying K+ channels in MIN-6 cells

Somatostatin: Linking Cognition and Alzheimer Disease to Therapeutic Targeting

Over 4 decades of research support the link between Alzheimer disease (AD) and somatostatin [somatotropin-releasing inhibitory factor (SRIF)]. SRIF and SRIF-expressing neurons play an essential role in brain function, modulating hippocampal activity and memory formation. Loss of SRIF and SRIF-expressing neurons in the brain rests at the center of a series of interdependent pathological events driven by amyloid-β peptide (Aβ), culminating in cognitive decline and dementia. The connection between the SRIF and AD further extends to the neuropsychiatric symptoms, seizure activity, and inflammation, whereas preclinical AD investigations show SRIF or SRIF receptor agonist administration capable of enhancing cognition. SRIF receptor subtype-4 activation in particular presents unique attributes, with the potential to mitigate learning and memory decline, reduce comorbid symptoms, and enhance enzymatic degradation of Aβ in the brain. Here, we review the links between SRIF and AD along with the therapeutic implications. SIGNIFICANCE STATEMENT: Somatostatin and somatostatin-expressing neurons in the brain are extensively involved in cognition. Loss of somatostatin and somatostatin-expressing neurons in Alzheimer disease rests at the center of a series of interdependent pathological events contributing to cognitive decline and dementia. Targeting somatostatin-mediated processes has significant therapeutic potential for the treatment of Alzheimer disease.

Direct modulation of G protein-gated inwardly rectifying potassium (GIRK) channels

Ion channels play a pivotal role in regulating cellular excitability and signal transduction processes. Among the various ion channels, G-protein-coupled inwardly rectifying potassium (GIRK) channels serve as key mediators of neurotransmission and cellular responses to extracellular signals. GIRK channels are members of the larger family of inwardly-rectifying potassium (Kir) channels. Typically, GIRK channels are activated via the direct binding of G-protein βγ subunits upon the activation of G-protein-coupled receptors (GPCRs). GIRK channel activation requires the presence of the lipid signaling molecule, phosphatidylinositol 4,5-bisphosphate (PIP2). GIRK channels are also modulated by endogenous proteins and other molecules, including RGS proteins, cholesterol, and SNX27 as well as exogenous compounds, such as alcohol. In the last decade or so, several groups have developed novel drugs and small molecules, such as ML297, GAT1508 and GiGA1, that activate GIRK channels in a G-protein independent manner. Here, we aim to provide a comprehensive overview focusing on the direct modulation of GIRK channels by G-proteins, PIP2, cholesterol, and novel modulatory compounds. These studies offer valuable insights into the underlying molecular mechanisms of channel function, and have potential implications for both basic research and therapeutic development.

Somatostatin and the pathophysiology of Alzheimer's disease

Among the central features of Alzheimer's disease (AD) progression are altered levels of the neuropeptide somatostatin (SST), and the colocalisation of SST-positive interneurons (SST-INs) with amyloid-β plaques, leading to cell death. In this theoretical review, I propose a molecular model for the pathogenesis of AD based on SST-IN hypofunction and hyperactivity. Namely, hypofunctional and hyperactive SST-INs struggle to control hyperactivity in medial regions in early stages, leading to axonal Aβ production through excessive presynaptic GABAB inhibition, GABAB1a/APP complex downregulation and internalisation. Concomitantly, excessive SST-14 release accumulates near SST-INs in the form of amyloids, which bind to Aβ to form toxic mixed oligomers. This leads to differential SST-IN death through excitotoxicity, further disinhibition, SST deficits, and increased Aβ release, fibrillation and plaque formation. Aβ plaques, hyperactive networks and SST-IN distributions thereby tightly overlap in the brain. Conversely, chronic stimulation of postsynaptic SST2/4 on gulutamatergic neurons by hyperactive SST-INs promotes intense Mitogen-Activated Protein Kinase (MAPK) p38 activity, leading to somatodendritic p-tau staining and apoptosis/neurodegeneration - in agreement with a near complete overlap between p38 and neurofibrillary tangles. This model is suitable to explain some of the principal risk factors and markers of AD progression, including mitochondrial dysfunction, APOE4 genotype, sex-dependent vulnerability, overactive glial cells, dystrophic neurites, synaptic/spine losses, inter alia. Finally, the model can also shed light on qualitative aspects of AD neuropsychology, especially within the domains of spatial and declarative (episodic, semantic) memory, under an overlying pattern of contextual indiscrimination, ensemble instability, interference and generalisation.

Somatostatin peptide signaling dampens cortical circuits and promotes exploratory behavior

We sought to characterize the unique role of somatostatin (SST) in the prelimbic (PL) cortex in mice. We performed slice electrophysiology in pyramidal and GABAergic neurons to characterize the pharmacological mechanism of SST signaling and fiber photometry of GCaMP6f fluorescent calcium signals from SST neurons to characterize the activity profile of SST neurons during exploration of an elevated plus maze (EPM) and open field test (OFT). We used local delivery of a broad SST receptor (SSTR) agonist and antagonist to test causal effects of SST signaling. SSTR activation hyperpolarizes layer 2/3 pyramidal neurons, an effect that is recapitulated with optogenetic stimulation of SST neurons. SST neurons in PL are activated during EPM and OFT exploration, and SSTR agonist administration directly into the PL enhances open arm exploration in the EPM. This work describes a broad ability for SST peptide signaling to modulate microcircuits within the prefrontal cortex and related exploratory behaviors.

Consequences of somatic mutations of GIRK1 detected in primary malign tumors on expression and function of G-protein activated, inwardly rectifying, K+ channels

A search in the GDC Data Portal revealed 304 documented somatic mutations of the KCNJ3 gene in primary tumors (out of 10.202 cases). Most affected tumor types were carcinomas from uterus, skin and lung, while breast cancer exerted the lowest number of somatic mutations. We focused our research on 15 missense mutations within the region between TM1 and TM2, comprising the pore helix and ion selectivity signature. Expression was measured by confocal laser scan microscopy of eGFP tagged GIRK1 subunits, expressed with and without GIRK4 in oocytes of Xenopus laevis. GIRK ion currents were activated via coexpressed m2Rs and measured by the Two Electrode Voltage Clamp technique. Magnitude of the total GIRK current, as well as the fraction of current inducible by the agonist, were measured. Ion selectivity was gauged by assessment of the PNa+/PK+ ratio, calculated by the GIRK current reversal potential in extracellular media at different Na+ and K+ concentrations. None of the tested mutations was able to form functional GIRK1 homooligomeric ion channels. One of the mutations, G145A, which locates directly to the ion selectivity signature, exerted an increased PNa+/PK+ ratio. Generally, the missense mutations studied can be categorized into three groups: (i) normal/reduced expression accompanied by reduced/absent function (S132Y, F136L, E139K, G145A, R149Q, R149P, G178D, S185Y, Q186R), (ii) normal/increased expression as well as increased function (E140M, A142T, M184I) and (iii) miniscule expression but increased function relative to expression levels (I151N, G158S). We conclude, that gain of function mutations, identical or similar to categories (ii) and (iii), may potentially be involved in genesis and progression of malignancies in tissues that exert a high rate of occurrence of somatic mutations of KCNJ3.

The effects of somatostatin analogues on glycaemia in the treatment of neuroendocrine tumours

Long-acting somatostatin analogues (SSAs) are the most commonly used drugs in the management of neuroendocrine tumours (NETs) because of their ability to control symptoms and prolong survival. SSA use is associated with changes in glucose metabolism. However, the impacts on glycaemic control and body mass index (BMI) caused by SSAs in NETs are largely unknown. In the present study, we evaluated the effects of SSA treatment on BMI and glycated haemoglobin (HbA1c) in our cohort of patients with NETs. We also assessed changes in glycaemic control and BMI before and after SSA treatment. In addition, we assessed the incidence of new diabetes or whether there was worsening of glycaemic control for patients with pre-existing diabetes. The study comprised a retrospective study of 279 patients with NETs who were treated with SSAs between January 2014 and January 2019. Glycaemic control was assessed by measuring changes in Hba1c. A number needed to harm analysis was used to look at new cases of diabetes within the study population. Treatment with SSAs was associated with a mean increase in HbA1c of 3.35 ± 6.30 mmol mol^-1 despite a mean decrease in BMI of -1.04 ± 2.79 kg m^-2 . There were 19 new cases of type 2 diabetes mellitus (T2DM) in the population of 209 with a number needed to harm of 12.5. Of the 34 patients with pre-existing T2DM, five had worsening of their mean HbA1c. Treatment with SSAs for NETs is associated with an increase in HbA1c, despite a reduction in BMI and, importantly, a risk of developing T2DM with a number needed to harm of 12.5. This project was registered with the National Health Service Clinical Audit and Registries. It has a CARMS number - 17666.

Analysis of Half a Billion Datapoints Across Ten Machine-Learning Algorithms Identifies Key Elements Associated With Insulin Transcription in Human Pancreatic Islet Cells

Machine learning (ML)-workflows enable unprejudiced/robust evaluation of complex datasets. Here, we analyzed over 490,000,000 data points to compare 10 different ML-workflows in a large (N=11,652) training dataset of human pancreatic single-cell (sc-)transcriptomes to identify genes associated with the presence or absence of insulin transcript(s). Prediction accuracy/sensitivity of each ML-workflow was tested in a separate validation dataset (N=2,913). Ensemble ML-workflows, in particular Random Forest ML-algorithm delivered high predictive power (AUC=0.83) and sensitivity (0.98), compared to other algorithms. The transcripts identified through these analyses also demonstrated significant correlation with insulin in bulk RNA-seq data from human islets. The top-10 features, (including IAPP, ADCYAP1, LDHA and SST) common to the three Ensemble ML-workflows were significantly dysregulated in scRNA-seq datasets from Ire-1αβ-/- mice that demonstrate dedifferentiation of pancreatic β-cells in a model of type 1 diabetes (T1D) and in pancreatic single cells from individuals with type 2 Diabetes (T2D). Our findings provide direct comparison of ML-workflows in big data analyses, identify key elements associated with insulin transcription and provide workflows for future analyses.

Somatostatin analogues for the treatment of hyperinsulinaemic hypoglycaemia

Hyperinsulinaemic hypoglycaemia (HH) is a biochemical finding of low blood glucose levels due to the dysregulation of insulin secretion from pancreatic β-cells. Under normal physiological conditions, glucose metabolism is coupled to β-cell insulin secretion so that blood glucose levels are maintained within the physiological range of 3.5-5.5 mmol/L. However, in HH this coupling of glucose metabolism to insulin secretion is perturbed so that insulin secretion becomes unregulated. HH typically occurs in the neonatal, infancy and childhood periods and can be due to many different causes. Adults can also present with HH but the causes in adults tend to be different. Somatostatin (SST) is a peptide hormone that is released by the delta cells (δ-cells) in the pancreas. It binds to G protein-coupled SST receptors to regulate a variety of location-specific and selective functions such as hormone inhibition, neurotransmission and cell proliferation. SST plays a potent role in the regulation of both insulin and glucagon secretion in response to changes in glucose levels by negative feedback mechanism. The half-life of SST is only 1-3 min due to quick degradation by peptidases in plasma and tissues. Thus, a direct continuous intravenous or subcutaneous infusion is required to achieve the therapeutic effect. These limitations prompted the discovery of SST analogues such as octreotide and lanreotide, which have longer half-lives and therefore can be administered as injections. SST analogues are used to treat different forms of HH in children and adults and therapeutic effect is achieved by suppressing insulin secretion from pancreatic β-cells by complex mechanisms. These treatments are associated with several side effects, especially in the newborn period, with necrotizing enterocolitis being the most serious side effect and hence SS analogues should be used with extreme caution in this age group.

GIRK1 triggers multiple cancer-related pathways in the benign mammary epithelial cell line MCF10A

Excessive expression of subunit 1 of GIRK1 in ER+ breast tumors is associated with reduced survival times and increased lymph node metastasis in patients. To investigate possible tumor-initiating properties, benign MCF10A and malign MCF7 mammary epithelial cells were engineered to overexpress GIRK1 neoplasia associated vital parameters and resting potentials were measured and compared to controls. The presence of GIRK1 resulted in resting potentials negative to the controls. Upon GIRK1 overexpression, several cellular pathways were regulated towards pro-tumorigenic action as revealed by comparison of transcriptomes of MCF10AGIRK1 with the control (MCF10AeGFP). According to transcriptome analysis, cellular migration was promoted while wound healing and extracellular matrix interactions were impaired. Vital parameters in MCF7 cells were affected akin the benign MCF10A lines, but to a lesser extent. Thus, GIRK1 regulated cellular pathways in mammary epithelial cells are likely to contribute to the development and progression of breast cancer.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST₂ and SST₅ receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST₂ is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

Cortistatin regulates glucose-induced electrical activity and insulin secretion in mouse pancreatic beta-cells

Although there is growing evidence that cortistatin regulates several functions in different tissues, its role in the endocrine pancreas is not totally known. Here, we aim to study the effect of cortistatin on pancreatic beta-cells and glucose-stimulated insulin secretion (GSIS). Exposure of isolated mouse islets to cortistatin inhibited GSIS. This effect was prevented using a somatostatin receptor antagonist. Additionally, cortistatin hyperpolarized the membrane potential and reduced glucose-induced action potentials in isolated pancreatic beta-cells. Cortistatin did not modify ATP-dependent K⁺ (K_ATP) channel activity. In contrast, cortistatin increased the activity of a small conductance channel with characteristics of G protein-coupled inwardly rectifying K⁺ (GIRK) channels. The cortistatin effects on membrane potential and GSIS were largely reduced in the presence of a GIRK channel antagonist and by down-regulation of GIRK2 with small interfering RNA. Thus, cortistatin acts as an inhibitory signal for glucose-induced electrical activity and insulin secretion in the mouse pancreatic beta-cell.

Ablation of somatostatin cells leads to impaired pancreatic islet function and neonatal death in rodents

The somatostatin (SST)-secreting cells were mainly distributed in the pancreatic islets, brain, stomach and intestine in mammals and have many physiological functions. In particular, the SST-secreting δ cell is the third most common cell type in the islets of Langerhans. Recent studies have suggested that dysregulation of paracrine interaction between the pancreatic δ cells and β cells results in impaired glucose homeostasis and contributes to diabetes development. However, direct evidence of the functional importance of SST cells in glucose homeostasis control is still lacking. In the present study, we specifically ablated SST-secreting cells by crossing Sst-cre transgenic mice with R26 ^DTA mice (Sst ^Cre R26 ^DTA ). The Sst ^Cre R26 ^DTA mice exhibited neonatal death. The life spans of these mice with severe hypoglycemia were extended by glucose supplementation. Moreover, we observed that SST cells deficiency led to increased insulin content and excessive insulin release, which might contribute to the observed hypoglycemia. Unexpectedly, although SST is critical for the regulation of insulin content, factors other than SST that are produced by pancreatic δ cells via their endogenous corticotropin-releasing hormone receptor 2 (CRHR2) activity play the main roles in maintaining normal insulin release, as well as neonatal glucose homeostasis in the resting state. Taken together, our results identified that the SST cells in neonatal mouse played critical role in control of insulin release and normal islet function. Moreover, we provided direct in vivo evidence of the functional importance of the SST cells, which are essential for neonatal survival and the maintenance of glucose homeostasis.

Editor's Highlight: Therapeutic Concentrations of Antidepressants Inhibit Pancreatic Beta-Cell Function via Mitochondrial Complex Inhibition

Diabetes mellitus risk is increased by prolonged usage of antidepressants (ADs). Although various mechanisms are suggested for their diabetogenic potential, whether a direct effect of ADs on pancreatic β-cells is involved is unclear. We examined this idea for 3 ADs: paroxetine, clomipramine and, with particular emphasis, fluoxetine, on insulin secretion, mitochondrial function, cellular bioenergetics, KATP channel activity, and caspase activity in murine and human cell-line models of pancreatic β-cells. Metabolic assays showed that these ADs decreased the redox, oxidative respiration, and energetic potential of β-cells in a time and concentration dependent manner, even at a concentration of 100 nM, well within the therapeutic window. These effects were related to inhibition of mitochondrial complex I and III. Consistent with impaired mitochondrial function, lactate output was increased and insulin secretion decreased. Neither fluoxetine, antimycin nor rotenone could reactivate KATP channel activity blocked by glucose unlike the mitochondrial uncoupler, FCCP. Chronic, but not acute, AD increased oxidative stress and activated caspases, 3, 8, and 9. A close agreement was found for the rates of oxidative respiration, lactate output and modulation of KATP channel activity in MIN6 cells with those of primary murine cells; data that supports MIN6 as a valid model to study beta-cell bioenergetics. To conclude, paroxetine, clomipramine and fluoxetine were all cytotoxic at therapeutic concentrations on pancreatic beta-cells; an action suggested to arise by inhibition of mitochondrial bioenergetics, oxidative stress and induction of apoptosis. These actions help explain the diabetogenic potential of these ADs in humans.

Lipophilicity predicts the ability of nonsulphonylurea drugs to block pancreatic beta-cell KATP channels and stimulate insulin secretion; statins as a test case

AIMS

KATP ion channels play a key role in glucose-stimulated insulin secretion. However, many drugs block KATP as "off targets" leading to hyperinsulinaemia and hypoglycaemia. As such drugs are often lipophilic, the aim was to examine the relationship between drug lipophilicity (P) and IC 50 for KATP block and explore if the IC 50's of statins could be predicted from their lipophilicity and whether this would allow one to forecast their acute action on insulin secretion.

MATERIALS AND METHODS

A meta-analysis of 26 lipophilic, nonsulphonylurea, blockers of KATP was performed. From this, the IC 50's for pravastatin and simvastatin were predicted and then tested experimentally by exploring their effects on KATP channel activity via patch-clamp measurement, calcium imaging and insulin secretion in murine beta cells and islets.

RESULTS

Nonsulphonylurea drugs inhibited KATP channels with a Log IC 50 linearly related to their logP. Simvastatin blocked KATP with an IC 50 of 25 nmol/L, a value independent of cytosolic factors, and within the range predicted by its lipophilicity (21-690 nmol/L). 10 μmol/L pravastatin, predicted IC 50 0.2-12 mmol/L, was without effect on the KATP channel. At 10-fold therapeutic levels, 100 nmol/L simvastatin depolarized the beta-cell membrane potential and stimulated Ca2+ influx but did not affect insulin secretion; the latter could be explained by serum binding.

CONCLUSIONS

The logP of a drug can aid prediction for its ability to block beta-cell KATP ion channels. However, although the IC 50 for the block of KATP by simvastatin was predicted, the difference between this and therapeutic levels, as well as serum sequestration, explains why hypoglycaemia is unlikely to be observed with acute use of this statin.

KCNJ3 is a new independent prognostic marker for estrogen receptor positive breast cancer patients

Numerous studies showed abnormal expression of ion channels in different cancer types. Amongst these, the potassium channel gene KCNJ3 (encoding for GIRK1 proteins) has been reported to be upregulated in tumors of patients with breast cancer and to correlate with positive lymph node status. We aimed to study KCNJ3 levels in different breast cancer subtypes using gene expression data from the TCGA, to validate our findings using RNA in situ hybridization in a validation cohort (GEO ID GSE17705), and to study the prognostic value of KCNJ3 using survival analysis. In a total of > 1000 breast cancer patients of two independent data sets we showed a) that KCNJ3 expression is upregulated in tumor tissue compared to corresponding normal tissue (p < 0.001), b) that KCNJ3 expression is associated with estrogen receptor (ER) positive tumors (p < 0.001), but that KCNJ3 expression is variable within this group, and c) that ER positive patients with high KCNJ3 levels have worse overall (p < 0.05) and disease free survival probabilities (p < 0.01), whereby KCNJ3 is an independent prognostic factor (p <0.05). In conclusion, our data suggest that patients with ER positive breast cancer might be stratified into high risk and low risk groups based on the KCNJ3 levels in the tumor.

Overexpression of KCNJ3 gene splice variants affects vital parameters of the malignant breast cancer cell line MCF-7 in an opposing manner

BACKGROUND

Overexpression the KCNJ3, a gene that encodes subunit 1 of G-protein activated inwardly rectifying K(+) channel (GIRK1) in the primary tumor has been found to be associated with reduced survival times and increased lymph node metastasis in breast cancer patients.

METHODS

In order to survey possible tumorigenic properties of GIRK1 overexpression, a range of malignant mammary epithelial cells, based on the MCF-7 cell line that permanently overexpress different splice variants of the KCNJ3 gene (GIRK1a, GIRK1c, GIRK1d and as a control, eYFP) were produced. Subsequently, selected cardinal neoplasia associated cellular parameters were assessed and compared.

RESULTS

Adhesion to fibronectin coated surface as well as cell proliferation remained unaffected. Other vital parameters intimately linked to malignancy, i.e. wound healing, chemoinvasion, cellular velocities / motilities and angiogenesis were massively affected by GIRK1 overexpression. Overexpression of different GIRK1 splice variants exerted differential actions. While GIRK1a and GIRK1c overexpression reinforced the affected parameters towards malignancy, overexpression of GIRK1d resulted in the opposite. Single channel recording using the patch clamp technique revealed functional GIRK channels in the plasma membrane of MCF-7 cells albeit at very low frequency.

DISCUSSION

We conclude that GIRK1d acts as a dominant negative constituent of functional GIRK complexes present in the plasma membrane of MCF-7 cells, while overexpression of GIRK1a and GIRK1c augmented their activity. The core component responsible for the cancerogenic action of GIRK1 is apparently presented by a segment comprising aminoacids 235-402, that is present exclusively in GIRK1a and GIRK1c, but not GIRK1d (positions according to GIRK1a primary structure).

CONCLUSIONS

The current study provides insight into the cellular and molecular consequences of KCNJ3 overexpression in breast cancer cells and the mechanism upon clinical outcome in patients suffering from breast cancer.

Somatostatin 4 receptor activation modulates G-protein coupled inward rectifying potassium channels and voltage stimulated calcium signals in dorsal root ganglion neurons

Somatostatin has a wide biological profile resulting from its actions on the five receptor subtypes (sst1-5). Recently somatostatin was shown to exert analgesic effects via activation of the sst4 receptor. Although the analgesia in pain models is established, the precise molecular mechanism has yet to be fully elucidated. This research aimed to identify possible anti-nociceptive mechanisms, showing functional links of the sst4 receptor to G-protein coupled inward rectifying potassium (GIRK) channels and reduction of voltage stimulated calcium influx within the pain processing pathway. Whole cell voltage clamp experiments and calcium imaging experiments were conducted on DRG neurons prepared from adult rats. Application of an sst4 receptor selective agonist, J-2156, on DRG neurons induced a GIRK modulated potassium current, and inhibited voltage sensitive calcium current. Both mechanisms are thought to contribute to the analgesic properties of sst4 receptor agonists.

The somatostatin receptor in human pancreatic β-cells

The peptide hormone somatostatin (SST) is produced in the brain, the gut, and in δ-cells in pancreatic islets of Langerhans. SST secretion from δ-cells is stimulated by glucose, amino acids, and glucagon-like peptide-1. Exogenous SST strongly inhibits the secretion of the blood glucose-regulating hormones insulin and glucagon from pancreatic β-cells and α-cells, respectively. Endogenous SST secreted from δ-cells is a paracrine regulator of insulin and glucagon secretion, although the exact physiological significance of this regulation is unclear. Secreted SST binds to specific receptors (SSTRs), which are coupled to Gi/o proteins. In both β- and α-cells, activation of SSTRs suppresses hormone secretion by reducing cAMP levels, inhibiting electrical activity, decreasing Ca²⁺ influx through voltage-gated Ca²⁺ channels and directly reducing exocytosis in a Ca²⁺ and cAMP-independent manner. In rodents, β-cells express predominantly SSTR5, whereas α-cells express SSTR2. In human islets, SSTR2 is the dominant receptor in both β- and α-cells, but other isoforms also contribute to the SST effects. Evidence from rodent models suggests that SST secretion from δ-cells is dysregulated in diabetes mellitus, which may contribute to the metabolic disturbances in this disease. SST analogues are currently used for the treatment of hyperinsulinism and other endocrine disorders, including acromegaly and Cushing's syndrome.

The cellular building blocks of breathing

Respiratory brainstem neurons fulfill critical roles in controlling breathing: they generate the activity patterns for breathing and contribute to various sensory responses including changes in O2 and CO2. These complex sensorimotor tasks depend on the dynamic interplay between numerous cellular building blocks that consist of voltage-, calcium-, and ATP-dependent ionic conductances, various ionotropic and metabotropic synaptic mechanisms, as well as neuromodulators acting on G-protein coupled receptors and second messenger systems. As described in this review, the sensorimotor responses of the respiratory network emerge through the state-dependent integration of all these building blocks. There is no known respiratory function that involves only a small number of intrinsic, synaptic, or modulatory properties. Because of the complex integration of numerous intrinsic, synaptic, and modulatory mechanisms, the respiratory network is capable of continuously adapting to changes in the external and internal environment, which makes breathing one of the most integrated behaviors. Not surprisingly, inspiration is critical not only in the control of ventilation, but also in the context of "inspiring behaviors" such as arousal of the mind and even creativity. Far-reaching implications apply also to the underlying network mechanisms, as lessons learned from the respiratory network apply to network functions in general.

The role of voltage-gated potassium channels Kv2.1 and Kv2.2 in the regulation of insulin and somatostatin release from pancreatic islets

The voltage-gated potassium channels Kv2.1 and Kv2.2 are highly expressed in pancreatic islets, yet their contribution to islet hormone secretion is not fully understood. Here we investigate the role of Kv2 channels in pancreatic islets using a combination of genetic and pharmacologic approaches. Pancreatic β-cells from Kv2.1(-/-) mice possess reduced Kv current and display greater glucose-stimulated insulin secretion (GSIS) relative to WT β-cells. Inhibition of Kv2.x channels with selective peptidyl [guangxitoxin-1E (GxTX-1E)] or small molecule (RY796) inhibitors enhances GSIS in isolated wild-type (WT) mouse and human islets, but not in islets from Kv2.1(-/-) mice. However, in WT mice neither inhibitor improved glucose tolerance in vivo. GxTX-1E and RY796 enhanced somatostatin release in isolated human and mouse islets and in situ perfused pancreata from WT and Kv2.1(-/-) mice. Kv2.2 silencing in mouse islets by adenovirus-small hairpin RNA (shRNA) specifically enhanced islet somatostatin, but not insulin, secretion. In mice lacking somatostatin receptor 5, GxTX-1E stimulated insulin secretion and improved glucose tolerance. Collectively, these data show that Kv2.1 regulates insulin secretion in β-cells and Kv2.2 modulates somatostatin release in δ-cells. Development of selective Kv2.1 inhibitors without cross inhibition of Kv2.2 may provide new avenues to promote GSIS for the treatment of type 2 diabetes.

SSTR2 is the functionally dominant somatostatin receptor in human pancreatic β- and α-cells

Somatostatin-14 (SST) inhibits insulin and glucagon secretion by activating G protein-coupled somatostatin receptors (SSTRs), of which five isoforms exist (SSTR1-5). In mice, the effects on pancreatic β-cells are mediated by SSTR5, whereas α-cells express SSTR2. In both cell types, SSTR activation results in membrane hyperpolarization and suppression of exocytosis. Here, we examined the mechanisms by which SST inhibits secretion from human β- and α-cells and the SSTR isoforms mediating these effects. Quantitative PCR revealed high expression of SSTR2, with lower levels of SSTR1, SSTR3, and SSTR5, in human islets. Immunohistochemistry showed expression of SSTR2 in both β- and α-cells. SST application hyperpolarized human β-cells and inhibited action potential firing. The membrane hyperpolarization was unaffected by tolbutamide but antagonized by tertiapin-Q, a blocker of G protein-gated inwardly rectifying K⁺ channels (GIRK). The effect of SST was mimicked by an SSTR2-selective agonist, whereas a SSTR5 agonist was marginally effective. SST strongly (>70%) reduced depolarization-evoked exocytosis in both β- and α-cells. A slightly weaker inhibition was observed in both cell types after SSTR2 activation. SSTR3- and SSTR1-selective agonists moderately reduced the exocytotic responses in β- and α-cells, respectively, whereas SSTR4- and SSTR5-specific agonists were ineffective. SST also reduced voltage-gated P/Q-type Ca²⁺ currents in β-cells, but normalization of Ca²⁺ influx to control levels by prolonged depolarizations only partially restored exocytosis. We conclude that SST inhibits secretion from both human β- and α-cells by activating GIRK and suppressing electrical activity, reducing P/Q-type Ca²⁺ currents, and directly inhibiting exocytosis. These effects are mediated predominantly by SSTR2 in both cell types.

Cloning and characterisation of GIRK1 variants resulting from alternative RNA editing of the KCNJ3 gene transcript in a human breast cancer cell line

The aim of this study was to investigate the impact of increased mRNA levels encoding GIRK1 in breast tumours on GIRK protein expression. mRNA levels encoding hGIRK1 and hGIRK4 in the MCF7, MCF10A and MDA-MB-453 breast cancer cell lines were assessed and the corresponding proteins detected using Western blots. cDNAs encoding for four hGIRK1 splice variants (hGIRK1a, 1c, 1d and 1e) were cloned from the MCF7 cell line. Subcellular localisation of fluorescence labelled hGIRK1a-e and hGIRK4 and of endogenous GIRK1 and GIRK4 subunits was monitored in the MCF7 cell line. All hGIRK1 splice variants and hGIRK4 were predominantly located within the endoplasmic reticulum. Heterologous expression in Xenopus laevis oocytes and two electrode voltage clamp experiments together with confocal microscopy were performed. Only the hGIRK1a subunit was able to form functional GIRK channels in connection with hGIRK4. The other splice variants are expressed, but exert a dominant negative effect on heterooligomeric channel function. Hence, alternative splicing of the KCNJ3 gene transcript in the MCF7 cell line leads to a family of mRNA's, encoding truncated versions of the hGIRK1 protein. The very high abundance of mRNA's encoding GIRK1 together with the presence of GIRK1 protein suggests a pathophysiological role in breast cancer.

Ion channels and the hallmarks of cancer

Plasma membrane (PM) ion channels contribute to virtually all basic cellular processes and are also involved in the malignant phenotype of cancer cells. Here, we review the role of ion channels in cancer in the context of their involvement in the defined hallmarks of cancer: 1) self-sufficiency in growth signals, 2) insensitivity to antigrowth signals, 3) evasion of programmed cell death (apoptosis), 4) limitless replicative potential, 5) sustained angiogenesis and 6) tissue invasion and metastasis. Recent studies have indicated that the contribution of specific ion channels to these hallmarks varies for different types of cancer. Therefore, to determine the importance of ion channels as targets for cancer diagnosis and treatment their expression, function and regulation must be assessed for each cancer.

N-type Ca(2+) -channels in murine pancreatic beta-cells are inhibited by an exclusive coupling with somatostatin receptor subtype 1

Somatostatin (SRIF) is a well-established inhibitor of insulin secretion, an effect in part mediated by a direct inhibition of voltage-operated Ca(2+)-channels. However, the identity of the somatostatin receptor subtypes (SSTRs) and voltage-operated Ca(2+)-channels involved in this process are unknown. Whole-cell perforated patch-clamp methods were applied to the murine pancreatic beta-cell line, MIN6, to explore the molecular pharmacology of this problem. SRIF-14 inhibited voltage-gated Ca(2+) currents (ICa(2+)) by 19 +/- 3% (n=24) with a pEC(50) = 9.05 (95% confidence limits 9-9.1). This action was mimicked solely by 100 nm CH-275, a selective agonist at the somatostatin type 1 receptor (SSTR1), but not by 100 nm BIM-23027, L-362855, or NNC-269100; agonists selective for the other four SSTRs known to exist in MIN6. The inhibition of ICa(2+) produced by SRIF and CH-275 was insensitive to pertussis toxin but was reversed by a prepulse to +100 mV. The inhibition of ICa(2+) by SRIF-14 was unaffected by 20 microm nifedipine, an inhibitor of L-type Ca(2+) channels. Application of the specific N-type Ca(2+) channel (Ca(v)2.2) inhibitor omega-conotoxin GV1A at 100 nm mimicked, and as a consequence abolished, the inhibitory effect of SRIF-14 on ICa(2+). SRIF selectively inhibits N-type Ca(2+)-channels in murine pancreatic beta-cells via exclusive coupling with SSTR1. These findings help explain how SSTR1 activation can inhibit insulin secretion in pancreatic beta-cells and suggest a possible new therapeutic lead for treatment of hyperinsulinemia.

Adrenaline-induced hyperpolarization of mouse pancreatic islet cells is mediated by G protein-gated inwardly rectifying potassium (GIRK) channels

Insulin secretion inhibitors (ISI) such as adrenaline and somatostatin act on the pancreatic beta-cell by a number of mechanisms, one of which is plasma membrane hyperpolarization. Despite the ample evidence for this effect, the principal underlying channels have not been identified thus far. The G protein-gated inwardly rectifying potassium (Kir3.x/GIRK) channels, which are responsible for hyperpolarization in other excitable tissues, are likely candidates. In this paper, we show that GIRK channels are expressed and functional in mouse pancreatic islet cells. Reverse transcription polymerase chain reaction analysis revealed all four GIRK gene products in islet tissue. Immunofluorescent labeling of pancreatic sections demonstrated exclusive islet localization of all GIRK subunits, in part within insulin-expressing cells. Using the whole-cell configuration of the patch clamp technique, we found that the application of tertiapin-Q, a selective inhibitor of the GIRK channels, abolishes adrenaline-mediated inward currents and strongly attenuates adrenaline-induced hyperpolarization in a reversible manner. These results imply that GIRK channels are responsible for a major part of the electrical response to adrenaline in islet cells and suggest a role for these channels in pancreatic physiology.

Adrenaline-induced hyperpolarization of mouse pancreatic islet cells is mediated by G protein-gated inwardly rectifying potassium (GIRK) channels

Insulin secretion inhibitors (ISI) such as adrenaline and somatostatin act on the pancreatic beta-cell by a number of mechanisms, one of which is plasma membrane hyperpolarization. Despite the ample evidence for this effect, the principal underlying channels have not been identified thus far. The G protein-gated inwardly rectifying potassium (Kir3.x/GIRK) channels, which are responsible for hyperpolarization in other excitable tissues, are likely candidates. In this paper, we show that GIRK channels are expressed and functional in mouse pancreatic islet cells. Reverse transcription polymerase chain reaction analysis revealed all four GIRK gene products in islet tissue. Immunofluorescent labeling of pancreatic sections demonstrated exclusive islet localization of all GIRK subunits, in part within insulin-expressing cells. Using the whole-cell configuration of the patch clamp technique, we found that the application of tertiapin-Q, a selective inhibitor of the GIRK channels, abolishes adrenaline-mediated inward currents and strongly attenuates adrenaline-induced hyperpolarization in a reversible manner. These results imply that GIRK channels are responsible for a major part of the electrical response to adrenaline in islet cells and suggest a role for these channels in pancreatic physiology.

Both G i and G o heterotrimeric G proteins are required to exert the full effect of norepinephrine on the beta-cell K ATP channel

The effects of norepinephrine (NE), an inhibitor of insulin secretion, were examined on membrane potential and the ATP-sensitive K+ channel (K ATP) in INS 832/13 cells. Membrane potential was monitored under the whole cell current clamp mode. NE hyperpolarized the cell membrane, an effect that was abolished by tolbutamide. The effect of NE on K ATP channels was investigated in parallel using outside-out single channel recording. This revealed that NE enhanced the open activities of the K ATP channels approximately 2-fold without changing the single channel conductance, demonstrating that NE-induced hyperpolarization was mediated by activation of the K ATP channels. The NE effect was abolished in cells preincubated with pertussis toxin, indicating coupling to heterotrimeric G i/G o proteins. To identify the G proteins involved, antisera raised against alpha and beta subunits (anti-G alpha common, anti-G beta, anti-G alpha i1/2/3, and anti-G alpha o) were used. Anti-G alpha common totally blocked the effects of NE on membrane potential and K ATP channels. Individually, anti-G alpha i1/2/3 and anti-G alpha o only partially inhibited the action of NE on K ATP channels. However, the combination of both completely eliminated the action. Antibodies against G beta had no effects. To confirm these results and to further identify the G protein subunits involved, the blocking effects of peptides containing the sequence of 11 amino acids at the C termini of the alpha subunits were used. The data obtained were similar to those derived from the antibody work with the additional information that G alpha i3 and G alpha o1 were not involved. In conclusion, both G i and G o proteins are required for the full effect of norepinephrine to activate the K ATP channel.

An update on somatostatin receptor signaling in native systems and new insights on their pathophysiology

The peptide somatostatin (SRIF) has important physiological effects, mostly inhibitory, which have formed the basis for the clinical use of SRIF compounds. SRIF binding to its 5 guanine nucleotide-binding proteins-coupled receptors leads to the modulation of multiple transduction pathways. However, our current understanding of signaling exerted by receptors endogenously expressed in different cells/tissues reflects a rather complicated picture. On the other hand, the complexity of SRIF receptor signaling in pathologies, including pituitary and nervous system diseases, may be studied not only as alternative intervention points for the modulation of SRIF function but also to exploit new chemical space for drug-like molecules.

Ketogenic diet metabolites reduce firing in central neurons by opening K(ATP) channels

A low-carbohydrate ketogenic diet remains one of the most effective (but mysterious) treatments for severe pharmacoresistant epilepsy. We have tested for an acute effect of physiological ketone bodies on neuronal firing rates and excitability, to discover possible therapeutic mechanisms of the ketogenic diet. Physiological concentrations of ketone bodies (beta-hydroxybutyrate or acetoacetate) reduced the spontaneous firing rate of neurons in slices from rat or mouse substantia nigra pars reticulata. This region is thought to act as a "seizure gate," controlling seizure generalization. Consistent with an anticonvulsant role, the ketone body effect is larger for cells that fire more rapidly. The effect of ketone bodies was abolished by eliminating the metabolically sensitive K(ATP) channels pharmacologically or by gene knock-out. We propose that ketone bodies or glycolytic restriction treat epilepsy by augmenting a natural activity-limiting function served by K(ATP) channels in neurons.

Unregulated insulin secretion by pancreatic beta cells in hyperinsulinism/hyperammonemia syndrome: role of glutamate dehydrogenase, ATP-sensitive potassium channel, and nonselective cation channel

The hyperinsulinism/hyperammonemia (HI/HA) syndrome is caused by "gain of function" of glutamate dehydrogenase (GDH). Several missense mutations have been found; however, cell behaviors triggered by the excessive GDH activity have not been fully demonstrated. This study was aimed to clarify electrophysiological mechanisms underlying the dysregulated insulin secretion in pancreatic beta cells with GDH mutations. GDH kinetics and insulin secretion were measured in MIN6 cells overexpressing the G446D and L413V. Membrane potentials and channel activity were recorded under the perforated-patch configuration that preserved intracellular environments. In mutant MIN6 cells, sensitivity of GDH to guanosine triphosphate (GTP) was reduced and insulin secretion at low glucose concentrations was enhanced. The basal GDH activity was elevated in L413V bearing a mutation in the antenna-like structure. The L413V cells were depolarized without glucose, often accompanying by repetitive Ca2+ firings. The depolarization was maintained in the presence of adenosine triphosphate (ATP) and disappeared by depleting ATP, suggesting that the depolarization depended on intracellular ATP. In L413V cells, the ATP-sensitive potassium channel (K(ATP) channel) was suppressed and the nonselective cation channel (NSCC) was potentiated, while sensitivity of the channels to their specific blockers or agonists was not impaired. These data suggest that the L413V cells increase the intracellular ATP/adenosine diphosphate (ADP) ratio, which in turn causes sustained depolarization not only by closure of the K(ATP) channel, but also by opening of the NSCC. The resultant activation of the voltage-gated Ca2+ channel appears to induce hyperinsulinism. The present study provides evidence that multiple channels cooperate in unregulated insulin secretion in pancreatic beta cells of the HI/HA syndrome.

Somatostatin inhibits oxidative respiration in pancreatic beta-cells

Somatostatin potently inhibits insulin secretion from pancreatic beta-cells. It does so via activation of ATP-sensitive K+-channels (KATP) and G protein-regulated inwardly rectifying K+-channels, which act to decrease voltage-gated Ca2+-influx, a process central to exocytosis. Because KATP channels, and indeed insulin secretion, is controlled by glucose oxidation, we investigated whether somatostatin inhibits insulin secretion by direct effects on glucose metabolism. Oxidative metabolism in beta-cells was monitored by measuring changes in the O2 consumption (DeltaO2) of isolated mouse islets and MIN6 cells, a murine-derived beta-cell line. In both models, glucose-stimulated DeltaO2, an effect closely associated with inhibition of KATP channel activity and induction of electrical activity (r > 0.98). At 100 nm, somatostatin abolished glucose-stimulated DeltaO2 in mouse islets (n = 5, P < 0.05) and inhibited it by 80 +/- 28% (n = 17, P < 0.01) in MIN6 cells. Removal of extracellular Ca2+, 5 mm Co2+, or 20 microm nifedipine, conditions that inhibit voltage-gated Ca2+ influx, did not mimic but either blocked or reduced the effect of the peptide on DeltaO2. The nutrient secretagogues, methylpyruvate (10 mm) and alpha-ketoisocaproate (20 mm), also stimulated DeltaO2, but this was unaffected by somatostatin. Somatostatin also reversed glucose-induced hyperpolarization of the mitochondrial membrane potential monitored using rhodamine-123. Application of somatostatin receptor selective agonists demonstrated that the peptide worked through activation of the type 5 somatostatin receptor. In conclusion, somatostatin inhibits glucose metabolism in murine beta-cells by an unidentified Ca2+-dependent mechanism. This represents a new signaling pathway by which somatostatin can inhibit cellular functions regulated by glucose metabolism.

Free fatty acid receptor 1 (FFA(1)R/GPR40) and its involvement in fatty-acid-stimulated insulin secretion

Free fatty acids (FFA) have generally been proposed to regulate pancreatic insulin release by an intracellular mechanism involving inhibition of CPT-1. The recently de-orphanized G-protein coupled receptor, FFA(1)R/GPR40, has been shown to be essential for fatty-acid-stimulated insulin release in MIN6 mouse insulinoma cells. The CPT-1 inhibitor, 2-bromo palmitate (2BrP), was investigated for its ability to interact with mouse FFA(1)R/GPR40. It was found to inhibit phosphatidyl inositol hydrolysis induced by linoleic acid (LA) (100 muM in all experiments) in HEK293 cells transfected with FFA(1)R/GPR40 and in the MIN6 subclone, MIN6c4. 2BrP also inhibited LA-stimulated insulin release from mouse pancreatic islets. Mouse islets were subjected to antisense intervention by treatment with a FFA(1)R/GPR40-specific morpholino oligonucleotide for 48 h. Antisense treatment of islets suppressed LA-stimulated insulin release by 50% and by almost 100% when islets were pretreated with LA for 30 min before applying the antisense. Antisense treatment had no effect on tolbutamide-stimulated insulin release. Confocal microscopy using an FFA(1)R/GPR40-specific antibody revealed receptor expression largely localized to the plasma membrane of insulin-producing cells. Pretreating the islets with LA for 30 min followed by antisense oligonucleotide treatment for 48 h reduced the FFA(1)R/GPR40 immunoreactivity to background levels. The results demonstrate that FFA(1)R/GPR40 is inhibited by the CPT-1 inhibitor, 2BrP, and confirm that FFA(1)R/GPR40 is indeed necessary, at least in part, for fatty-acid-stimulated insulin release.

The PDZ/coiled-coil domain containing protein PIST modulates insulin secretion in MIN6 insulinoma cells by interacting with somatostatin receptor subtype 5

The multi-domain protein PIST (protein interacting specifically with Tc10) interacts with the SSTR5 (somatostatin receptor 5) and is responsible for its intracellular localization. Here, we show that PIST is expressed in pancreatic beta-cells and interacts with SSTR5 in these cells. PIST expression in MIN6 insulinoma cells is reduced by somatostatin (SST). After stimulation with SST, SSTR5 undergoes internalization together with PIST. MIN6 cells over-expressing PIST display enhanced glucose-stimulated insulin secretion and a decreased sensitivity to SST-induced inhibition of insulin secretion. These data suggest that PIST plays an important role in insulin secretion by regulating SSTR5 availability at the plasma membrane.

Pertussis toxin-sensitive pathway inhibits glucose-stimulated Ca2+ signals of rat islet β-cells by affecting L-type Ca2+ channels and voltage-dependent K+ channels

A role of pertussis toxin (PTX)-sensitive pathway in regulation of glucose-stimulated Ca2+ signaling in rat islet beta-cells was investigated by using clonidine as a selective agonist to alpha2-adrenoceptors which link to the pathway. An elevation of extracellular glucose concentration from 5.5 to 22.2 mM (glucose stimulation) increased the levels of [Ca2+]i of beta-cells, and clonidine reversibly reduced the elevated levels of [Ca2+]i. This clonidine effect was antagonized by yohimbine, and abolished in beta-cells pre-treated with PTX. Clonidine showed little effect on membrane currents including those through ATP-sensitive K+ channels induced by voltage ramps from -90 to -50 mV. Clonidine showed little effect on the magnitude of whole-cell currents through L-type Ca2+ channels (ICa(L)), but increased the inactivation process of the currents. Clonidine increased the magnitude of the voltage-dependent K+ currents (IVK). These clonidine effects on ICa(L) and IVK were abolished in beta-cells treated with PTX or GDP-betaS. These results suggest that the PTX-sensitive pathway increases IVK activity and decreases ICa(L) activity of islet beta-cells, resulting in a decrease in the levels of [Ca2+]i elevated by depolarization-induced Ca2+ entry. This mechanism seems responsible at least in part for well-known inhibitory action of PTX-sensitive pathway on glucose-stimulated insulin secretion from islet beta-cells.

Epinephrine-induced hyperpolarization of islet cells without KATP channels

This study examines the effect of epinephrine, a known physiological inhibitor of insulin secretion, on the membrane potential of pancreatic islet cells from sulfonylurea receptor-1 (ABCC8)-null mice (Sur1KO), which lack functional ATP-sensitive K+ (KATP) channels. These channels have been argued to be activated by catecholamines, but epinephrine effectively inhibits insulin secretion in both Sur1KO and wild-type islets and in mice. Isolated Sur1KO beta-cells are depolarized in both low (2.8 mmol/l) and high (16.7 mmol/l) glucose and exhibit Ca(2+)-dependent action potentials. Epinephrine hyperpolarizes Sur1KO beta-cells, inhibiting their spontaneous action potentials. This effect, observed in standard whole cell patches, is abolished by pertussis toxin and blocked by BaCl2. The epinephrine effect is mimicked by clonidine, a selective alpha2-adrenoceptor agonist and inhibited by alpha-yohimbine, an alpha2-antagonist. A selection of K+ channel inhibitors, tetraethylammonium, apamin, dendrotoxin, iberiotoxin, E-4130, chromanol 293B, and tertiapin did not block the epinephrine-induced hyperpolarization. Analysis of whole cell currents revealed an inward conductance of 0.11 +/- 0.04 nS/pF (n = 7) and a TEA-sensitive outward conductance of 0.55 +/- 0.08 nS/pF (n = 7) at -60 and 0 mV, respectively. Guanosine 5'-O-(3-thiotriphosphate) (100 microM) in the patch pipette did not significantly alter these currents or activate novel inward-rectifying K+ currents. We conclude that epinephrine can hyperpolarize beta-cells in the absence of KATP channels via activation of low-conductance BaCl2-sensitive K+ channels that are regulated by pertussis toxin-sensitive G proteins.

Hyperinsulinism in Infancy: From Basic Science to Clinical Disease

Dunne, Mark J., Karen E. Cosgrove, Ruth M. Shepherd, Albert Aynsley-Green, and Keith J. Lindley. Hyperinsulinism in Infancy: From Basic Science to Clinical Disease. Physiol Rev 84: 239–275, 2004; 10.1152/physrev.00022.2003.—Ion channelopathies have now been described in many well-characterized cell types including neurons, myocytes, epithelial cells, and endocrine cells. However, in only a few cases has the relationship between altered ion channel function, cell biology, and clinical disease been defined. Hyperinsulinism in infancy (HI) is a rare, potentially lethal condition of the newborn and early childhood. The causes of HI are varied and numerous, but in almost all cases they share a common target protein, the ATP-sensitive K+channel. From gene defects in ion channel subunits to defects in β-cell metabolism and anaplerosis, this review describes the relationship between pathogenesis and clinical medicine. Until recently, HI was generally considered an orphan disease, but as parallel defects in ion channels, enzymes, and metabolic pathways also give rise to diabetes and impaired insulin release, the HI paradigm has wider implications for more common disorders of the endocrine pancreas and the molecular physiology of ion transport.

Pharmacological agents that directly modulate insulin secretion

Blood glucose levels are sensed and controlled by the release of hormones from the islets of Langerhans in the pancreas. The beta-cell, the insulin-secreting cell in the islet, can detect subtle increases in circulating glucose levels and a cascade of molecular events spanning the initial depolarization of the beta-cell membrane culminates in exocytosis and optimal insulin secretion. Here we review these processes in the context of pharmacological agents that have been shown to directly interact with any stage of insulin secretion. Drugs that modulate insulin secretion do so by opening the K(ATP) channels, by interacting with cell-surface receptors, by altering second-messenger responses, by disrupting the beta-cell cytoskeletal framework, by influencing the molecular reactions at the stages of transcription and translation of insulin, and/or by perturbing exocytosis of the insulin secretory vesicles. Drugs acting primarily at the K(ATP) channels are the sulfonylureas, the benzoic acid derivatives, the imidazolines, and the quinolines, which are channel openers, and finally diazoxide, which closes these channels. Methylxanthines also work at the cell membrane level by antagonizing the purinergic receptors and thus increase insulin secretion. Other drugs have effects at multiple levels, such as the calcineurin inhibitors and somatostatin. Some drugs used extensively in research, e.g., colchicine, which is used to study vesicular transport, have no effect at the pharmacological doses used in clinical practice. We also briefly discuss those drugs that have been shown to disrupt beta-cell function in a clinical setting but for which there is scant information on their mechanism of action.

Growth hormone-releasing peptide-2 reduces inward rectifying K+ currents via a PKA-cAMP-mediated signalling pathway in ovine somatotropes

Inward-rectifying potassium (Kir) channels are essential for maintaining the resting membrane potential near the K(+) equilibrium and they are responsible for hyperpolarisation-induced K(+) influx. We characterised the Kir current in primary cultured ovine somatotropes and examined the effect of growth hormone-releasing peptide-2 (GHRP-2) on this current and its related intracellular signalling pathways. The Kir current was, in most cases, isolated using nystatin-perforated patch-clamp techniques. In bath solution containing 5 mM K(+), the Kir current was composed of both transient (fast activated) and delayed (slowly activated) components. An increase in the external K(+) concentration from 5 to 25 mM induced an augmentation of approximately 4-fold in the delayed part of the Kir current and both BaCl(2) and CsCl dose-dependently inhibited this current, confirming the presence of the Kir current in ovine somatotropes. Moreover, this specific effect of high K(+) on the Kir current was only observed in the cells that showed positive staining with anti-growth hormone (GH) antibodies, or in GC cells that belong to a rat somatotrope cell line. Application of GHRP-2 (100 nM) reversibly and significantly reduced the Kir current in bath solutions with 5 or 25 mM K(+) in ovine somatotropes. In addition, we found that the reduction in the Kir current mediated by GHRP-2 was totally abolished by the pretreatments with H89 (1 microM) or Rp-cAMP (100 microM) or by intracellular dialysis of a specific protein kinase A (PKA) inhibitory peptide PKI (10 microM). The specific PKC blocker chelerythrine (1 microM) or inhibitory peptide PKC(19-36) (10 microM) did not show any effects on the GHRP-2-induced decrease in the Kir current. These results suggest that the inhibition of Kir current through PKA-cAMP pathways may play an integral role in GHRP-2-induced depolarisation and GH release in ovine somatotropes.

Somatostatin inhibits thalamic network oscillations in vitro: actions on the GABAergic neurons of the reticular nucleus

We examined the effects of somatostatin (SST) on neurons in the thalamic reticular nucleus (RT) using whole-cell patch-clamp techniques applied to visualized neurons in rat thalamic slices. SST, acting via sst(5) receptors and pertussis toxin-sensitive G-proteins, activated an inwardly rectifying K(+) (GIRK) current in 20 of 28 recorded cells to increase input conductance 15 +/- 3% above control and inhibited N-type Ca(2+) currents in 17 of 24 neurons via voltage-dependent mechanisms. SST reversibly depressed evoked EPSCs (eEPSCs) to 37 +/- 8% of control without altering their kinetics. SST-mediated inhibition of eEPSCs showed short-term relief from block during 25 Hz stimulus trains. SST also reduced the frequency (33 +/- 8%) but not the amplitude of miniature EPSCs (mEPSCs). These data indicate that SST mediates presynaptic inhibition of glutamate release onto RT neurons. In current-clamp recordings, SST preferentially inhibited burst discharges mediated by near-threshold corticothalamic EPSPs and intracellularly applied depolarizing currents. SST had powerful effects on in vitro intrathalamic rhythms, which included a shortening of the duration and a reduction in spike count within each oscillatory event. Furthermore, there was a paradoxical increase in the synchrony of epileptiform oscillations, likely mediated by a suppression of the responses to weak synaptic inputs in RT. We conclude that SST suppresses discharges in RT neurons, via presynaptic inhibition of glutamate release and postsynaptic activation of GIRK channels, leading to the dampening of both spindle-like and epileptiform thalamic network oscillations. SST may act as an important endogenous regulator of physiological and pathological thalamocortical network activities.

Block of Ca(2+)-channels by alpha-endosulphine inhibits insulin release

1. alpha-Endosulphine, isolated as an endogenous equivalent for sulphonylureas, is a 121-amino acids protein of 19 kDa apparent molecular mass, member of a cyclic AMP-regulated phosphoprotein family. We have previously shown that alpha-endosulphine inhibits sulphonylurea binding and K(ATP) channel activity, thereby stimulating basal insulin secretion. 2. We now describe that in the perfused rat pancreas, no stimulation was detected and that alpha-endosulphine inhibited glucose stimulated insulin release. This inhibition was dose-dependent and affected both phases of insulin secretion. 3. This inhibitory effect of alpha-endosulphine also occurred on MIN6 beta-cells when insulin release was stimulated either by glucose, sulphonylureas or a high K(+) depolarization. Inhibition was concentration-dependent with a half-maximal inhibition at 0.5 microM and was mirrored by inhibition of calcium influx. 4. Electrophysiological experiments demonstrated, in comparison to the effects of the sulphonylurea tolbutamide, that these inhibitory effects were linked to a direct inhibition of L-type Ca(2+)-channels and were independent from a regulation of K(ATP) channels. 5. Although alpha-endosulphine is able to stimulate insulin release under specific conditions acting via modulation of K(ATP) channel activity, the present study suggests that, under physiological conditions, the peptide mainly acts to block voltage-gated Ca(2+)-channels. This block leads to the inhibition of calcium influx and triggers inhibition of insulin release. 6. We conclude that alpha-endosulphine is not exclusively an endogenous equivalent for sulphonylureas and not solely a K(ATP) channel regulator.

The biology and physiology of somatostatin receptors

The biology and therapeutic application of somatostatin and its receptors are reviewed. The focus is on recent literature and patents, especially with regard to the specific function of each somatostatin receptor subtype. Detailed mechanisms mediating the effects of somatostatin and its analogues remain to be elucidated. Nevertheless, progress is being made towards a clear picture of the cellular signalling and physiological changes regulated by somatostatin and its receptors.