Tolbutamide stimulates exocytosis of glucagon by inhibition of a mitochondrial-like ATP-sensitive K+ (KATP) conductance in rat pancreatic A-cells

Cellular and systemic mechanisms for glucose sensing and homeostasis

Glucose is a major source of energy in animals. Maintaining blood glucose levels within a physiological range is important for facilitating glucose uptake by cells, as required for optimal functioning. Glucose homeostasis relies on multiple glucose-sensing cells in the body that constantly monitor blood glucose levels and respond accordingly to adjust its glycemia. These include not only pancreatic β-cells and α-cells that secrete insulin and glucagon, but also central and peripheral neurons regulating pancreatic endocrine function. Different types of cells respond distinctively to changes in blood glucose levels, and the mechanisms involved in glucose sensing are diverse. Notably, recent studies have challenged the currently held views regarding glucose-sensing mechanisms. Furthermore, peripheral and central glucose-sensing cells appear to work in concert to control blood glucose level and maintain glucose and energy homeostasis in organisms. In this review, we summarize the established concepts and recent advances in the understanding of cellular and systemic mechanisms that regulate glucose sensing and its homeostasis.

K(ATP) channels and islet hormone secretion: new insights and controversies

ATP-sensitive potassium channels (K(ATP) channels) link cell metabolism to electrical activity by controlling the cell membrane potential. They participate in many physiological processes but have a particularly important role in systemic glucose homeostasis by regulating hormone secretion from pancreatic islet cells. Glucose-induced closure of K(ATP) channels is crucial for insulin secretion. Emerging data suggest that K(ATP) channels also play a key part in glucagon secretion, although precisely how they do so remains controversial. This Review highlights the role of K(ATP) channels in insulin and glucagon secretion. We discuss how K(ATP) channels might contribute not only to the initiation of insulin release but also to the graded stimulation of insulin secretion that occurs with increasing glucose concentrations. The various hypotheses concerning the role of K(ATP) channels in glucagon release are also reviewed. Furthermore, we illustrate how mutations in K(ATP) channel genes can cause hyposecretion or hypersecretion of insulin, as in neonatal diabetes mellitus and congenital hyperinsulinism, and how defective metabolic regulation of the channel may underlie the hypoinsulinaemia and the hyperglucagonaemia that characterize type 2 diabetes mellitus. Finally, we outline how sulphonylureas, which inhibit K(ATP) channels, stimulate insulin secretion in patients with neonatal diabetes mellitus or type 2 diabetes mellitus, and suggest their potential use to target the glucagon secretory defects found in diabetes mellitus.

Tolbutamide controls glucagon release from mouse islets differently than glucose: involvement of K(ATP) channels from both α-cells and δ-cells

We evaluated the role of ATP-sensitive K⁺ (K(ATP)) channels, somatostatin, and Zn²⁺ in the control of glucagon secretion from mouse islets. Switching from 1 to 7 mmol/L glucose inhibited glucagon release. Diazoxide did not reverse the glucagonostatic effect of glucose. Tolbutamide decreased glucagon secretion at 1 mmol/L glucose (G1) but stimulated it at 7 mmol/L glucose (G7). The reduced glucagon secretion produced by high concentrations of tolbutamide or diazoxide, or disruption of K(ATP) channels (Sur1(-/-) mice) at G1 could be inhibited further by G7. Removal of the somatostatin paracrine influence (Sst(-/-) mice or pretreatement with pertussis toxin) strongly increased glucagon release, did not prevent the glucagonostatic effect of G7, and unmasked a marked glucagonotropic effect of tolbutamide. Glucose inhibited glucagon release in the absence of functional K(ATP) channels and somatostatin signaling. Knockout of the Zn²⁺ transporter ZnT8 (ZnT8(-/-) mice) did not prevent the glucagonostatic effect of glucose. In conclusion, glucose can inhibit glucagon release independently of Zn²⁺, K(ATP) channels, and somatostatin. Closure of K(ATP) channels controls glucagon secretion by two mechanisms, a direct stimulation of α-cells and an indirect inhibition via somatostatin released from δ-cells. The net effect on glucagon release results from a balance between both effects.

Glucose decouples intracellular Ca2+ activity from glucagon secretion in mouse pancreatic islet alpha-cells

The mechanisms of glucagon secretion and its suppression by glucose are presently unknown. This study investigates the relationship between intracellular calcium levels ([Ca²⁺]_i) and hormone secretion under low and high glucose conditions. We examined the effects of modulating ion channel activities on [Ca²⁺]_i and hormone secretion from ex vivo mouse pancreatic islets. Glucagon-secreting α-cells were unambiguously identified by cell specific expression of fluorescent proteins. We found that activation of L-type voltage-gated calcium channels is critical for α-cell calcium oscillations and glucagon secretion at low glucose levels. Calcium channel activation depends on K_ATP channel activity but not on tetrodotoxin-sensitive Na⁺ channels. The use of glucagon secretagogues reveals a positive correlation between α-cell [Ca²⁺]_i and secretion at low glucose levels. Glucose elevation suppresses glucagon secretion even after treatment with secretagogues. Importantly, this inhibition is not mediated by K_ATP channel activity or reduction in α-cell [Ca²⁺]_i. Our results demonstrate that glucose uncouples the positive relationship between [Ca²⁺]_i and secretory activity. We conclude that glucose suppression of glucagon secretion is not mediated by inactivation of calcium channels, but instead, it requires a calcium-independent inhibitory pathway.

Novel insight into glucagon receptor action: lessons from knockout and transgenic mouse models

Using knockout and transgenic technology, genetically modified animal models allowed us to understand the role of glucagon signalling in metabolism. Mice with a global deletion of the glucagon receptor gene (Gcgr) were designed using gene targeting. The phenotype of Gcgr(-/-) mouse provided important clues about the role of Gcgr in foetal growth, pancreatic development and glucose and lipid homeostasis. The lack of Gcgr activation was associated with: (i) hypoglycaemic pregnancies, poor foetal growth and increased foetal-neonatal demise; (ii) altered cytoarchitecture of pancreatic islets; (iii) altered glucose, lipid and hormonal milieu; (iv) reduced gastric emptying; (v) altered body composition and protection from diet-induced obesity; (vi) altered energy state; (vii) impaired hepatocyte survival; (viii) altered metabolic response to prolonged fasting and exercise and (ix) prevented development of diabetes in insulin-deficient mice. In contrast, mice overexpressing the Gcgr on pancreatic β-cells displayed an increase insulin secretion, pancreatic insulin content and β-cell mass, and partially protected against hyperglycaemia and impaired glucose tolerance when fed a high-fat diet. These findings suggest that glucagon signalling plays a significant role in the regulation of glucose and lipid homeostasis. Treatment options designed to block Gcgr activation may have negative implications in the treatment of diabetes.

The granular chloride channel ClC-3 is permissive for insulin secretion

Insulin secretion from pancreatic beta cells is dependent on maturation and acidification of the secretory granule, processes necessary for prohormone convertase cleavage of proinsulin. Previous studies in isolated beta cells revealed that acidification may be dependent on the granule membrane chloride channel ClC-3, in a step permissive for a regulated secretory response. In this study, immuno-EM of beta cells revealed colocalization of ClC-3 and insulin on secretory granules. Clcn3(-/-) mice as well as isolated islets demonstrate impaired insulin secretion; Clcn3(-/-) beta cells are defective in regulated insulin exocytosis and granular acidification. Increased amounts of proinsulin were found in the majority of secretory granules in the Clcn3(-/-) mice, while in Clcn3(+/+) cells, proinsulin was confined to the immature secretory granules. These results demonstrate that in pancreatic beta cells, chloride channels, specifically ClC-3, are localized on insulin granules and play a role in insulin processing as well as insulin secretion through regulation of granular acidification.

Review. SUR1: a unique ATP-binding cassette protein that functions as an ion channel regulator

SUR1 is an ATP-binding cassette (ABC) transporter with a novel function. In contrast to other ABC proteins, it serves as the regulatory subunit of an ion channel. The ATP-sensitive (KATP) channel is an octameric complex of four pore-forming Kir6.2 subunits and four regulatory SUR1 subunits, and it links cell metabolism to electrical activity in many cell types. ATPase activity at the nucleotide-binding domains of SUR results in an increase in KATP channel open probability. Conversely, ATP binding to Kir6.2 closes the channel. Metabolic regulation is achieved by the balance between these two opposing effects. Precisely how SUR1 talks to Kir6.2 remains unclear, but recent studies have identified some residues and domains that are involved in both physical and functional interactions between the two proteins. The importance of these interactions is exemplified by the fact that impaired regulation of Kir6.2 by SUR1 results in human disease, with loss-of-function SUR1 mutations causing congenital hyperinsulinism and gain-of-function SUR1 mutations leading to neonatal diabetes. This paper reviews recent data on the regulation of Kir6.2 by SUR1 and considers the molecular mechanisms by which SUR1 mutations produce disease.

Evidence for KCNQ1 K+ channel expression in rat zymogen granule membranes and involvement in cholecystokinin-induced pancreatic acinar secretion

Secretion of enzymes and fluid induced by Ca(2+) in pancreatic acini is not completely understood and may involve activation of ion conductive pathways in zymogen granule (ZG) membranes. We hypothesized that a chromanol 293B-sensitive K(+) conductance carried by a KCNQ1 protein is expressed in ZG membranes (ZGM). In suspensions of rat pancreatic ZG, ion flux was determined by ionophore-induced osmotic lysis of ZG suspended in isotonic salts. The KCNQ1 blocker 293B selectively blocked K(+) permeability (IC(50) of approximately 10 microM). After incorporation of ZGM into planar bilayer membranes, cation channels were detected in 645/150 mM potassium gluconate cis/trans solutions. Channels had linear current-voltage relationships, a reversal potential (E(rev)) of -20.9 +/- 0.9 mV, and a single-channel K(+) conductance (g(K)) of 265.8 +/- 44.0 pS (n = 39). Replacement of cis 500 mM K(+) by 500 mM Na(+) shifted E(rev) to -2.4 +/- 3.6 mV (n = 3), indicating K(+) selectivity. Single-channel analysis identified several K(+) channel groups with distinct channel behaviors. K(+) channels with a g(K) of 651.8 +/- 88.0 pS, E(rev) of -22.9 +/- 2.2 mV, and open probability (P(open)) of 0.43 +/- 0.06 at 0 mV (n = 6) and channels with a g(K) of 155.0 +/- 11.4 pS, E(rev) of -18.3 +/- 1.8 mV, and P(open) of 0.80 +/- 0.03 at 0 mV (n = 3) were inhibited by 100 microM 293B or by the more selective inhibitor HMR-1556 but not by the maxi-Ca(2+)-activated K(+) channel (BK channel) inhibitor charybdotoxin (5 nM). KCNQ1 protein was demonstrated by immunoperoxidase labeling of pancreatic tissue, immunogold labeling of ZG, and immunoblotting of ZGM. 293B also inhibited cholecystokinin-induced amylase secretion of permeabilized acini (IC(50) of approximately 10 microM). Thus KCNQ1 may account for ZG K(+) conductance and contribute to pancreatic hormone-stimulated enzyme and fluid secretion.

Glucose inhibits glucagon secretion by a direct effect on mouse pancreatic alpha cells

AIMS/HYPOTHESIS

The mechanisms by which glucose regulates glucagon release are poorly understood. The present study aimed to clarify the direct effects of glucose on the glucagon-releasing alpha cells and those effects mediated by paracrine islet factors.

MATERIALS AND METHODS

Glucagon, insulin and somatostatin release were measured from incubated mouse pancreatic islets and the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) recorded in isolated mouse alpha cells.

RESULTS

Glucose inhibited glucagon release with maximal effect at 7 mmol/l. Since this concentration corresponded to threshold stimulation of insulin secretion, it is unlikely that inhibition of glucagon secretion is mediated by beta cell factors. Although somatostatin secretion data seemed consistent with a role of this hormone in glucose-inhibited glucagon release, a somatostatin receptor type 2 antagonist stimulated glucagon release without diminishing the inhibitory effect of glucose. In islets exposed to tolbutamide plus 8 mmol/l K(+), glucose inhibited glucagon secretion without stimulating the release of insulin and somatostatin, indicating a direct inhibitory effect on the alpha cells that was independent of ATP-sensitive K(+) channels. Glucose lowered [Ca(2+)](i) of individual alpha cells independently of somatostatin and beta cell factors (insulin, Zn(2+) and gamma-aminobutyric acid). Glucose suppression of glucagon release was prevented by inhibitors of the sarco(endo)plasmic reticulum Ca(2+)-ATPase, which abolished the [Ca(2+)](i)-lowering effect of glucose on isolated alpha cells.

CONCLUSIONS/INTERPRETATION

Beta cell factors or somatostatin do not seem to mediate glucose inhibition of glucagon secretion. We instead propose that glucose has a direct inhibitory effect on mouse alpha cells by suppressing a depolarising Ca(2+) store-operated current.

Alpha-cells of the endocrine pancreas: 35 years of research but the enigma remains

Glucagon, a hormone secreted from the alpha-cells of the endocrine pancreas, is critical for blood glucose homeostasis. It is the major counterpart to insulin and is released during hypoglycemia to induce hepatic glucose output. The control of glucagon secretion is multifactorial and involves direct effects of nutrients on alpha-cell stimulus-secretion coupling as well as paracrine regulation by insulin and zinc and other factors secreted from neighboring beta- and delta-cells within the islet of Langerhans. Glucagon secretion is also regulated by circulating hormones and the autonomic nervous system. In this review, we describe the components of the alpha-cell stimulus secretion coupling and how nutrient metabolism in the alpha-cell leads to changes in glucagon secretion. The islet cell composition and organization are described in different species and serve as a basis for understanding how the numerous paracrine, hormonal, and nervous signals fine-tune glucagon secretion under different physiological conditions. We also highlight the pathophysiology of the alpha-cell and how hyperglucagonemia represents an important component of the metabolic abnormalities associated with diabetes mellitus. Therapeutic inhibition of glucagon action in patients with type 2 diabetes remains an exciting prospect.

Sulfonylureas correct trafficking defects of disease-causing ATP-sensitive potassium channels by binding to the channel complex

ATP-sensitive potassium (K(ATP)) channels mediate glucose-induced insulin secretion by coupling metabolic signals to beta-cell membrane potential and the secretory machinery. Reduced K(ATP) channel expression caused by mutations in the channel proteins: sulfonylurea receptor 1 (SUR1) and Kir6.2, results in loss of channel function as seen in congenital hyperinsulinism. Previously, we reported that sulfonylureas, oral hypoglycemic drugs widely used to treat type II diabetes, correct the endoplasmic reticulum to the plasma membrane trafficking defect caused by two SUR1 mutations, A116P and V187D. In this study, we investigated the mechanism by which sulfonylureas rescue these mutants. We found that glinides, another class of SUR-binding hypoglycemic drugs, also markedly increased surface expression of the trafficking mutants. Attenuating or abolishing the ability of mutant SUR1 to bind sulfonylureas or glinides by the following mutations: Y230A, S1238Y, or both, accordingly diminished the rescuing effects of the drugs. Interestingly, rescue of the trafficking defects requires mutant SUR1 to be co-expressed with Kir6.2, suggesting that the channel complex, rather than SUR1 alone, is the drug target. Observations that sulfonylureas also reverse trafficking defects caused by neonatal diabetes-associated Kir6.2 mutations in a way that is dependent on intact sulfonylurea binding sites in SUR1 further support this notion. Our results provide insight into the mechanistic and structural basis on which sulfonylureas rescue K(ATP) channel surface expression defects caused by channel mutations.

Paradoxical stimulation of glucagon secretion by high glucose concentrations

Hypersecretion of glucagon contributes to the dysregulation of glucose homeostasis in diabetes. To clarify the underlying mechanism, glucose-regulated glucagon secretion was studied in mouse pancreatic islets and clonal hamster In-R1-G9 glucagon-releasing cells. Apart from the well-known inhibition of secretion with maximal effect around 7 mmol/l glucose, we discovered that mouse islets showed paradoxical stimulation of glucagon release at 25-30 mmol/l and In-R1-G9 cells at 12-20 mmol/l sugar. Whereas glucagon secretion in the absence of glucose was inhibited by hyperpolarization with diazoxide, this agent tended to further enhance secretion stimulated by high concentrations of the sugar. Because U-shaped dose-response relationships for glucose-regulated glucagon secretion were observed in normal islets and in clonal glucagon-releasing cells, both the inhibitory and stimulatory components probably reflect direct effects on the alpha-cells. Studies of isolated mouse alpha-cells indicated that glucose inhibited glucagon secretion by lowering the cytoplasmic Ca(2+) concentration. However, stimulation of glucagon release by high glucose concentrations did not require elevation of Ca(2+), indicating involvement of novel mechanisms in glucose regulation of glucagon secretion. A U-shaped dose-response relationship for glucose-regulated glucagon secretion may explain why diabetic patients with pronounced hyperglycemia display paradoxical hyperglucagonemia.

Effects of glimepiride and glyburide on glucose counterregulation and recovery from hypoglycemia

Severe hypoglycemia, the most serious side effect of sulfonylurea therapy, has been reported to occur more frequently with glyburide than glimepiride. The present studies were undertaken to test the hypothesis that a differential effect on glucagon secretion may be involved. We performed hyperinsulinemic hypoglycemic (approximately 2.5 mmol/L) clamps in 16 healthy volunteers who received in randomized order placebo, glyburide (10 mg), and glimepiride (4 mg) just before beginning the insulin infusion and measured plasma glucagon, insulin, C-peptide, glucagon, epinephrine, cortisol, and growth hormone levels during the clamp and during a 3-hour recovery period after discontinuation of the insulin infusion. Neither sulfonylurea altered glucagon responses or those of other counterregulatory hormones (except cortisol) during the clamp. However, glyburide delayed plasma glucose recovery from hypoglycemia (plasma glucose at end of recovery period: control, 4.9 +/- 0.2 mmol/L; glyburide, 3.7 +/- 0.2 mmol/L; P = .0001; glimepiride, 4.5 +/- 0.2 mmol/L; P = .08). Despite lower plasma glucose levels, glyburide stimulated insulin secretion during this period (0.89 +/- 0.13 vs 1.47 +/- 0.15 pmol x kg(-1) x min(-1), control vs glyburide; P = .001), whereas glimepiride did not (P = .08). Short-term administration of glyburide or glimepiride did not alter glucagon responses during hypoglycemia. In contrast, during recovery from hypoglycemia, glyburide but not glimepiride inappropriately stimulates insulin secretion at low plasma glucose levels. This differential effect on insulin secretion may be an important factor in explaining why glyburide causes severe hypoglycemia more frequently than glimepiride.

ATP-sensitive potassium channel (KATP channel) expression in the normal canine pancreas and in canine insulinomas

BACKGROUND

Pancreatic beta cells express ATP-sensitive potassium (KATP) channels that are needed for normal insulin secretion and are targets for drugs that modulate insulin secretion. The KATP channel is composed of two subunits: a sulfonylurea receptor (SUR 1) and an inward rectifying potassium channel (Kir6.2). KATP channel activity is influenced by the metabolic state of the cell and initiates the ionic events that precede insulin exocytosis. Although drugs that target the KATP channel have the expected effects on insulin secretion in dogs, little is known about molecular aspects of this potassium channel. To learn more about canine beta cell KATP channels, we studied KATP channel expression by the normal canine pancreas and by insulin-secreting tumors of dogs.

RESULTS

Pancreatic tissue from normal dogs and tumor tissue from three dogs with histologically-confirmed insulinomas was examined for expression of KATP channel subunits (SUR1 and Kir6.2) using RT-PCR. Normal canine pancreas expressed SUR1 and Kir6.2 subunits of the KATP channel. The partial nucleotide sequences for SUR1 and Kir6.2 obtained from the normal pancreas showed a high degree of homology to published sequences for other mammalian species. SUR1 and Kir6.2 expression was observed in each of the three canine insulinomas examined. Comparison of short sequences from insulinomas with those obtained from normal pancreas did not reveal any mutations in either SUR1 or Kir6.2 in any of the insulinomas.

CONCLUSION

Canine pancreatic KATP channels have the same subunit composition as those found in the endocrine pancreases of humans, rats, and mice, suggesting that the canine channel is regulated in a similar fashion as in other species. SUR1 and Kir6.2 expression was found in the three insulinomas examined indicating that unregulated insulin secretion by these tumors does not result from failure to express one or both KATP channel subunits.

Increasing the decrement in insulin secretion improves glucagon responses to hypoglycemia in advanced type 2 diabetes

OBJECTIVE

In advanced beta-cell failure, counterregulatory glucagon responses may be impaired due to a reduced decrement in insulin secretion during the development of hypoglycemia. The present studies were therefore undertaken to test the hypothesis that these may be improved by increasing this decrement in insulin secretion.

RESEARCH DESIGN AND METHODS

Twelve subjects with type 2 diabetes who have been insulin requiring were studied as a model of advanced beta-cell failure. Glucagon responses were examined during a 90-min hypoglycemic clamp (approximately 2.8 mmol/l) on two separate occasions. On one occasion, tolbutamide was infused for 2 h before the clamp so that the decrement in insulin secretion during the induction of hypoglycemia would be increased. On the other occasion, normal saline was infused as a control.

RESULTS

Before the hypoglycemic clamp, infusion of tolbutamide increased insulin secretion approximately 1.9-fold (P < 0.001). However, during hypoglycemia, insulin secretion decreased to similar rates on both occasions (P = 0.31) so that its decrement was approximately twofold greater following the tolbutamide infusion (1.63 +/- 0.20 vs. 0.81 +/- 0.17 pmol x kg(-1) x min(-1), P < 0.001). This was associated with more than twofold-greater glucagon responses (42 +/- 11 vs. 19 +/- 8 ng/l, P < 0.002) during the hypoglycemic clamp but unaltered glucagon responses to intravenous arginine immediately thereafter (449 +/- 50 vs. 453 +/- 50 ng/l, P = 0.78).

CONCLUSIONS

Increasing the decrement in insulin secretion during the development of hypoglycemia improves counterregulatory glucagon responses in advanced beta-cell failure. These findings further support the concept that the impaired counterregulatory glucagon responses in advanced beta-cell failure may at least partially be due to a reduced decrement in insulin secretion.

Conserved mechanisms of glucose sensing and regulation by Drosophila corpora cardiaca cells

Antagonistic activities of glucagon and insulin control metabolism in mammals, and disruption of this balance underlies diabetes pathogenesis. Insulin-producing cells (IPCs) in the brain of insects such as Drosophila also regulate serum glucose, but it remains unclear whether insulin is the sole hormonal regulator of glucose homeostasis and whether mechanisms of glucose-sensing and response in IPCs resemble those in pancreatic islets. Here we show, by targeted cell ablation, that Drosophila corpora cardiaca (CC) cells of the ring gland are also essential for larval glucose homeostasis. Unlike IPCs, CC cells express Drosophila cognates of sulphonylurea receptor (Sur) and potassium channel (Ir), proteins that comprise ATP-sensitive potassium channels regulating hormone secretion by islets and other mammalian glucose-sensing cells. They also produce adipokinetic hormone, a polypeptide with glucagon-like functions. Glucose regulation by CC cells is impaired by exposure to sulphonylureas, drugs that target the Sur subunit. Furthermore, ubiquitous expression of an akh transgene reverses the effect of CC ablation on serum glucose. Thus, Drosophila CC cells are crucial regulators of glucose homeostasis and they use glucose-sensing and response mechanisms similar to islet cells.

Glucagon stimulates exocytosis in mouse and rat pancreatic alpha-cells by binding to glucagon receptors

Glucagon, secreted by the pancreatic alpha-cells, stimulates insulin secretion from neighboring beta-cells by cAMP- and protein kinase A (PKA)-dependent mechanisms, but it is not known whether glucagon also modulates its own secretion. We have addressed this issue by combining recordings of membrane capacitance (to monitor exocytosis) in individual alpha-cells with biochemical assays of glucagon secretion and cAMP content in intact pancreatic islets, as well as analyses of glucagon receptor expression in pure alpha-cell fractions by RT-PCR. Glucagon stimulated cAMP generation and exocytosis dose dependently with an EC50 of 1.6-1.7 nm. The stimulation of both parameters plateaued at concentrations beyond 10 nm of glucagon where a more than 3-fold enhancement was observed. The actions of glucagon were unaffected by the GLP-1 receptor antagonist exendin-(9-39) but abolished by des-His1-[Glu9]-glucagon-amide, a specific blocker of the glucagon receptor. The effects of glucagon on alpha-cell exocytosis were mimicked by forskolin and the stimulatory actions of glucagon and forskolin on exocytosis were both reproduced by intracellular application of 0.1 mm cAMP. cAMP-potentiated exocytosis involved both PKA-dependent and -independent (resistant to Rp-cAMPS, an Rp-isomer of cAMP) mechanisms. The presence of the cAMP-binding protein cAMP-guanidine nucleotide exchange factor II in alpha-cells was documented by a combination of immunocytochemistry and RT-PCR and 8-(4-chloro-phenylthio)-2'-O-methyl-cAMP, a cAMP-guanidine nucleotide exchange factor II-selective agonist, mimicked the effect of cAMP and augmented rapid exocytosis in a PKA-independent manner. We conclude that glucagon released from the alpha-cells, in addition to its well-documented systemic effects and paracrine actions within the islet, also represents an autocrine regulator of alpha-cell function.

The insulin secretagogues glibenclamide and repaglinide do not influence growth hormone secretion in humans but stimulate glucagon secretion during profound insulin deficiency

In vitro data have recently suggested that sulfonylureas (SUs) enhance GH secretion by modulating the effects of GHRH and somatostatin in pituitary cells. The present study was undertaken to explore in more detail a possible influence of a single dose of SU (glibenclamide) and a non-SU (repaglinide) insulin secretagogue on circulating GH dynamics. Ten C-peptide-negative type 1 diabetic individuals were examined on three occasions in random order. Either glibenclamide (10.5 mg), repaglinide (8 mg), or placebo was administered after overnight normalization of plasma glucose by iv insulin infusion. Subsequently, GH concentrations were measured regularly after stimulation with GHRH (bolus 0.1 micro g/kg) alone and during concomitant infusion with somatostatin (7 ng.kg(-1).min(-1)). Insulin was replaced at baseline levels (0.25 mU.kg(-1).min(-1)) and plasma glucose clamped at 5-6 mmol/liter. Overall, there were no significant statistical differences in GH responses determined as either GH peak concentrations, integrated levels of GH, or secretory burst mass of GH during the experimental protocol. In contrast, plasma glucagon concentrations were significantly increased during glibenclamide and repaglinide exposure. The present experimental design does not support the hypothesis that acute administration of pharmacological doses of the oral antihyperglycemic agents glibenclamide and repaglinide per se enhance GH release in humans. Additionally, this study shows that these potassium channel inhibitors seem to stimulate glucagon secretion in people who have severe intraislet insulin deficiency (e.g. type 1 diabetes). However, extrapolation of our findings to type 2 diabetic individuals should be done with some caution.

Relationship between a HCO3- -permeable conductance and a CLCA protein from rat pancreatic zymogen granules

Ca(2+)-induced enzyme secretion in the exocrine pancreas is not completely understood. We have proposed that Ca(2+)-induced enzyme secretion in the exocrine pancreas involves activation of ion conductances in the membrane of zymogen granules (ZG). Here we have identified a Ca(2+)-activated anion conductance in rat pancreatic ZG membranes (ZGM). Ca(2+) (2.5-50 microM) increased the conductance for I(-), NO(3)(-), Br(-), or HCO(3)(-), but not for Cl(-), as determined by the rate of valinomycin-induced osmotic lysis of ZG suspended in isotonic K(+)-salts. 4,4'-Diisothiocyanatodihydrostilbene-2,2'-disulfonate (100 microM) or 25 microM dithiothreitol strongly inhibited Ca(2+)-dependent lysis. The permeability sequence, Ca(2+) dependence, and inhibitor sensitivity of ZG anion conductance are reminiscent of a family of epithelial Ca(2+)-activated anion channels (CLCA). CLCA expression was confirmed by RT-PCR with rat pancreatic mRNA and mouse CLCA1 primers. A PCR product (580bp) exhibited 81%, 77%, and 57% amino acid similarity to the three mouse isoforms mCLCA-1, -2, and -3 (mgob-5), respectively. Antibodies against bovine tracheal CLCA1 showed CLCA expression in ZGM by immunoblotting, immunoperoxidase light microscopy, and immunogold labeling. These findings suggest that a CLCA-related protein could account for the Ca(2+)-activated HCO(3)(-) conductance of rat pancreatic ZGM and contribute to hormone-stimulated enzyme secretion.

Mechanisms of exocytosis in insulin-secreting B-cells and glucagon-secreting A-cells

In pancreatic B- and A-cells, metabolic stimuli regulate biochemical and electrical processes that culminate in Ca2+-influx and release of insulin or glucagon, respectively. Like in other (neuro)endocrine cells, Ca2+-influx triggers the rapid exocytosis of hormone-containing secretory granules. Only a small fraction of granules (<1% in insulin-secreting B-cells) can be released immediately, while the remainder requires translocation to the plasma membrane and further "priming" for release by several ATP- and Ca2+-dependent reactions. Such functional organization may account for systemic features such as the biphasic time course of glucose-stimulated insulin secretion. Since this release pattern is altered in type-2 diabetes mellitus, it is conceivable that disturbances in the exocytotic machinery underlie the disease. Here I will review recent data from our laboratory relevant for the understanding of these processes in insulin-secreting B-cells and glucagon-secreting A-cells and for the identification of novel targets for antidiabetic drug action. Two aspects are discussed in detail: 1) The importance of a tight interaction between L-type Ca2+-channels and the exocytotic machinery for efficient secretion; and 2) the role of intragranular acidification for the priming of secretory granules and its regulation by a granular 65-kDa sulfonylurea-binding protein.

Gliclazide directly inhibits arginine-induced glucagon release

Arginine-stimulated insulin and somatostatin release is enhanced by the sulfonylurea gliclazide. In contrast, gliclazide inhibits the glucagon response. The aim of the present study was to investigate whether this inhibition of glucagon release was mediated by a direct suppressive effect of gliclazide or was secondary to the paracrine effect of released somatostatin. To eliminate the paracrine effects of somatostatin, we first perfused isolated rat pancreata with a medium supplemented with 23% of the standard calcium content. Second, we perifused isolated rat islets with a novel and highly specific antagonist of type 2 somatostatin receptor, DC-41-33 (2 micro mol/l), which fully antagonizes the suppressive somatostatin effect on rat A cells. Gliclazide (30 micro mol/l) inhibited glucagon release by 54% in the perfusion experiments, whereas the somatostatin response was nearly abolished. In islet perifusions with DC-41-33, arginine-induced glucagon release was inhibited by 66%. We therefore concluded that gliclazide inhibits glucagon release by a direct action on the pancreatic A cell.

Ion channels in secretory granules of the pancreas and their role in exocytosis and release of secretory proteins

Regulated secretion in exocrine and neuroendocrine cells occurs through exocytosis of secretory granules and the subsequent release of stored small molecules and proteins. The introduction of biophysical techniques with high temporal and spatial resolution, and the identification of Ca(2+)-dependent and -independent "docking" and "fusion" proteins, has greatly enhanced our understanding of exocytosis. The cloning of families of ion channel proteins, including intracellular ion channels, has also revived interest in the role of secretory granule ion channels in exocytotic secretion. Thus secretory granules of pancreatic acinar cell express a ClC-2 Cl(-) channel, a HCO-permeable member of the CLCA Ca(2+)-dependent anion channel family, and a KCNQ1 K(+) channel. Evidence suggests that these channels may facilitate the release of digestive enzymes and/or prevent exocytosed granules from collapsing during "kiss and run" recycling. In pancreatic beta-cells, a granular ClC-3 Cl(-) channel provides a shunt pathway for a vacuolar-type H(+)-ATPase. Acidification "primes" the granules for Ca(2+)-dependent exocytosis and release of insulin. In summary, secretory granules are equipped with specific sets of ion channels, which modulate regulated exocytosis and the release of macromolecules. These channels could represent excellent targets for therapeutic interventions to control exocytotic secretion in relevant diseases, such as pancreatitis, cystic fibrosis, or diabetes mellitus.

Differential interactions of nateglinide and repaglinide on the human beta-cell sulphonylurea receptor 1

Repaglinide and nateglinide represent a new class of insulin secretagogues, structurally unrelated to sulphonylureas, that were developed for the treatment of type 2 diabetes. The inhibitory effect of these drugs was investigated on recombinant wild-type and mutant Kir6.2/SUR1 channels expressed in HEK293 cells. Nateglinide and repaglinide dose-dependently inhibited whole-cell Kir6.2/SUR1 currents with half-maximal inhibitory concentration (IC(50)) values of 800 and 21 nmol/l, respectively. Mutation of serine 1237 in SUR1 to tyrosine (S1237Y) abolished tolbutamide and nateglinide block, suggesting that these drugs share a common point of interaction on the SUR1 subunit of the ATP-sensitive K(+) channel. In contrast, repaglinide inhibition was unaffected by the S1237Y mutation (IC(50) = 23 nmol/l). Radioligand binding studies revealed a single high-affinity binding site for [(3)H]repaglinide on membranes prepared from HEK293 cells expressing wild-type (equilibrium dissociation constant [K(D)] = 0.40 nmol/l) or mutant (K(D) = 0.31 nmol/l) Kir6.2/SUR1 channels. Nateglinide and tolbutamide displaced [(3)H]repaglinide binding to wild-type channels with IC(50) values of 0.7 and 26 micro mol/l, respectively, but produced <10% displacement of [(3)H]repaglinide bound to mutant channels. This is consistent with the idea that binding of nateglinide and tolbutamide, but not repaglinide, is abolished by the SUR1[S1237Y] mutation and that the binding site for repaglinide is not identical to that of nateglinde/tolbutamide. These results are discussed in terms of a conformational analysis of the drug molecules.

Nuclear KATP channels trigger nuclear Ca(2+) transients that modulate nuclear function

Glucose, the principal regulator of endocrine pancreas, has several effects on pancreatic beta cells, including the regulation of insulin release, cell proliferation, apoptosis, differentiation, and gene expression. Although the sequence of events linking glycemia with insulin release is well described, the mechanism whereby glucose regulates nuclear function is still largely unknown. Here, we have shown that an ATP-sensitive K(+) channel (K(ATP)) with similar properties to that found on the plasma membrane is also present on the nuclear envelope of pancreatic beta cells. In isolated nuclei, blockade of the K(ATP) channel with tolbutamide or diadenosine polyphosphates triggers nuclear Ca(2+) transients and induces phosphorylation of the transcription factor cAMP response element binding protein. In whole cells, fluorescence in situ hybridization revealed that these Ca(2+) signals may trigger c-myc expression. These results demonstrate a functional K(ATP) channel in nuclei linking glucose metabolism, nuclear Ca(2+) signals, and nuclear function.

Intraislet hyperinsulinemia prevents the glucagon response to hypoglycemia despite an intact autonomic response

Because absence of the glucagon response to falling plasma glucose concentrations plays a key role in the pathogenesis of iatrogenic hypoglycemia in patients with insulin-deficient diabetes and the mechanism of this defect is unknown, and given evidence in experimental animals that a decrease in intraislet insulin is a signal to increased glucagon secretion, we examined the role of endogenous insulin in the physiological glucagon response to hypoglycemia. We tested the hypothesis that intraislet hyperinsulinemia prevents the glucagon response to hypoglycemia despite an intact autonomic-adrenomedullary, sympathetic neural, and parasympathetic neural-response and a low alpha-cell glucose concentration. Twelve healthy young adults were studied on three separate occasions. Insulin was infused in hourly steps in relatively low doses (1.5, 3.0, 4.5, and 6.0 pmol.kg(-1).min(-1)) from 60 through 300 min on all three occasions. Plasma glucose levels were clamped at euglycemia ( approximately 5.0 mmol/l, approximately 90 mg/dl) on one occasion and at hourly steps of approximately 4.7, 4.2, 3.6, and 3.0 mmol/l ( approximately 85, 75, 65, and 55 mg/dl) from 60 through 300 min on the other two occasions. On one of the latter occasions, the beta-cell secretagogue tolbutamide was infused in a dose of 1.0 g/h from 60 through 300 min. Hypoglycemia with tolbutamide infusion, compared with similar hypoglycemia alone, was associated with higher (P < 0.0001) C-peptide levels (final values of 1.0 +/- 0.2 vs. 0.1 +/- 0.0 nmol/l), higher (P < 0.0001) rates of insulin secretion (final values of 198 +/- 60 vs. 15 +/- 4 pmol/min), and higher (P < 0.0001) insulin levels (final values of 325 +/- 30 vs. 245 +/- 20 pmol/l) as expected. The glucagon response to hypoglycemia was prevented during tolbutamide infusion (P < 0.0001). Glucagon levels were 17 +/- 1 pmol/l at baseline on both occasions, 14 +/- 1 vs. 15 +/- 1 pmol/l, respectively, during the initial hyperinsulinemic euglycemia, and 15 +/- 1 vs. 22 +/- 2 pmol/l, respectively, during hypoglycemia with and without tolbutamide infusion. Autonomic-adrenomedullary (plasma epinephrine), sympathetic neural (plasma norepinephrine), and parasympathetic neural (plasma pancreatic polypeptide)-responses to hypoglycemia were not reduced during tolbutamide infusion. We conclude that intraislet hyperinsulinemia prevents the glucagon response to hypoglycemia despite an intact autonomic response and a low alpha-cell glucose concentration.

Somatostatin inhibits exocytosis in rat pancreatic alpha-cells by G(i2)-dependent activation of calcineurin and depriming of secretory granules

1. Measurements of cell capacitance were used to investigate the molecular mechanisms by which somatostatin inhibits Ca(2+)-induced exocytosis in single rat glucagon-secreting pancreatic alpha-cells. 2. Somatostatin decreased the exocytotic responses elicited by voltage-clamp depolarisations by 80 % in the presence of cyclic AMP-elevating agents such as isoprenaline and forskolin. Inhibition was time dependent and half-maximal within 22 s. 3. The inhibitory action of somatostatin was concentration dependent with an IC(50) of 68 nM and prevented by pretreatment of the cells with pertussis toxin. The latter effect was mimicked by intracellular dialysis with specific antibodies to G(i1/2) and by antisense oligonucleotides against G proteins of the subtype G(i2). 4. Somatostatin lacked inhibitory action when applied in the absence of forskolin or in the presence of the L-type Ca(2+) channel blocker nifedipine. The size of the omega-conotoxin-sensitive and forskolin-independent component of exocytosis was limited to 60 fF. By contrast, somatostatin abolished L-type Ca(2+) channel-dependent exocytosis in alpha-cells exposed to forskolin. The magnitude of the latter pool amounted to 230 fF. 5. The inhibitory effect of somatostatin on exocytosis was mediated by activation of the serine/threonine protein phosphatase calcineurin and was prevented by pretreatment with cyclosporin A and deltamethrin or intracellularly applied calcineurin autoinhibitory peptide. Experiments using the stable ATP analogue AMP-PCP indicate that somatostatin acts by depriming of granules. 6. We propose that somatostatin receptors associate with L-type Ca(2+) channels and couple to G(i2) proteins leading to a localised activation of calcineurin and depriming of secretory granules situated close to the L-type Ca(2+) channels.

Priming of insulin granules for exocytosis by granular Cl− uptake and acidification

ATP-dependent priming of the secretory granules precedes Ca2+-regulated neuroendocrine secretion, but the exact nature of this reaction is not fully established in all secretory cell types. We have further investigated this reaction in the insulin-secreting pancreatic B-cell and demonstrate that granular acidification driven by a V-type H+-ATPase in the granular membrane is a decisive step in priming. This requires simultaneous Cl− uptake through granular ClC-3 Cl− channels. Accordingly, granule acidification and priming are inhibited by agents that prevent transgranular Cl− fluxes, such as 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS) and an antibody against the ClC-3 channels, but accelerated by increases in the intracellular ATP:ADP ratio or addition of hypoglycemic sulfonylureas. We suggest that this might represent an important mechanism for metabolic regulation of Ca2+-dependent exocytosis that is also likely to be operational in other secretory cell types.