Essential role of the imidazoline moiety in the insulinotropic effect but not the KATP channel-blocking effect of imidazolines; a comparison of the effects of efaroxan and its imidazole analogue, KU14R

Investigation of morin-induced insulin secretion in cultured pancreatic cells

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

Identification of morin as an agonist of imidazoline I-3 receptor for insulin secretion in diabetic rats

Morin is a flavonoid contained in guava that is known to reduce hyperglycemia in diabetics. Morin has been demonstrated to increase plasma insulin. However, the mechanism(s) remains unknown. The present study is designed to investigate the effect of morin on the imidazoline receptor (I-R) that regulates insulin secretion. We used Chinese hamster ovary (CHO) cells transfected with an I-R expression construct (NISCH-CHO-K1 cells) to identify the direct effect of morin on the I-R. Moreover, the imidazoline I₃ receptor (I-3R) is known to be present in pancreatic β cells and involved in insulin secretion. Therefore, we applied a specific antagonist (KU14R) to block I-3R in diabetic rats. Additionally, the effect of morin on insulin secretion was characterized in isolated pancreatic islets. Morin decreased blood glucose levels by increasing plasma insulin levels in diabetic rats. In CHO cells expressing an I-R, morin increased calcium influx in a dose-dependent manner. Additionally, KU14R dose-dependently inhibited the morin-induced effects, including hypoglycemia and the increase in insulin secretion and plasma C-peptide levels, in diabetic rats. Furthermore, morin enhanced insulin secretion from isolated pancreatic islets, and this effect was also dose-dependently inhibited by KU14R. Phospholipase C (PLC) is known to couple with the I-R, and a PLC inhibitor dose-dependently attenuated the insulin secretion induced by morin in isolated pancreatic islets. Taken together, these data suggest that morin can activate I-3R to enhance insulin secretion. Therefore, it would be useful to develop morin into a treatment for diabetic disorders.

Canavanine induces insulin release via activation of imidazoline I3 receptors

The aim of the present study was to identify the effect of canavanine on the imidazoline receptor because canavanine is a guanidinium derivative that has a similar structure to imidazoline receptor ligands. Transfected Chinese hamster ovary-K1 cells expressing imidazoline receptors (nischarin (NISCH)-CHO-K1 cells) were used to elucidate the direct effects of canavanine on imidazoline receptors. In addition, the imidazoline I3 receptor has been implicated in stimulation of insulin secretion from pancreatic β-cells. Wistar rats were used to investigate the effects of canavanine (0.1, 1 and 2.5 mg/kg, i.v.) on insulin secretion. In addition the a specific I3 receptor antagonist KU14R (4 or 8 mg/kg, i.v.) was used to block I3 receptors. Canavanine decreased blood glucose by increasing plasma insulin in rats. In addition, canavanine increased calcium influx into NISCH-CHO-K1 cells in a manner similar to agmatine, the endogenous ligand of imidazoline receptors. Moreover, KU12R dose-dependently attenuated canavanine-induced insulin secretion in HIT-T15 pancreatic β-cells and in the plasma of rats. The data suggest that canavanine is an agonist of I3 receptors both in vivo and in vitro. Thus, canavanine would be a useful tool in imidazoline receptor research.

Mechanisms of antihyperglycaemic action of efaroxan in mice: time for reappraisal of α2A-adrenergic antagonism in the treatment of type 2 diabetes?

AIMS/HYPOTHESIS

Inspired by recent speculation about the potential utility of α(2A)-antagonism in the treatment of type 2 diabetes, the study examined the contribution of α(2)-antagonism vs other mechanisms to the antihyperglycaemic activity of the imidazoline (±)-efaroxan.

METHODS

Effects of the racemate and its pure enantiomers on isolated pancreatic islets and beta cells in vitro, as well as on hyperglycaemia in vivo, were investigated in a comparative manner in mice.

RESULTS

In isolated perifused islets, the two enantiomers of efaroxan were equally potent in counteracting inhibition of insulin release by the ATP-dependent K(+) (K(ATP)) channel-opener diazoxide but (+)-efaroxan, the presumptive carrier of α(2)-antagonistic activity, was by far superior in counteracting inhibition of insulin release by the α(2)-agonist UK14,304. In vivo, (+)-efaroxan improved oral glucose tolerance at 100-fold lower doses than (-)-efaroxan and, in parallel with observations made in vitro, was more effective in counteracting UK14,304-induced than diazoxide-induced hyperglycaemia. The antihyperglycaemic activity of much higher doses of (-)-efaroxan was associated with an opposing pattern (i.e. with stronger counteraction of diazoxide-induced than UK14,304-induced hyperglycaemia), which implicates a different mechanism of action.

CONCLUSIONS/INTERPRETATION

The antihyperglycaemic potency of (±)-efaroxan in mice is almost entirely due to α(2)-antagonism, but high doses can also lower blood glucose via another mechanism. Our findings call for reappraisal of the possible clinical utility of α(2A)-antagonistic compounds in recently identified subpopulations of patients in which a congenitally higher level of α(2A)-adrenergic activation contributes to the development and pathophysiology of type 2 diabetes.

Structural nucleotide analogs are potent activators/inhibitors of pancreatic β cell KATP channels: an emerging mechanism supporting their use as antidiabetic drugs

The 2H-1,4-benzoxazine derivatives are novel drugs structurally similar to nucleotides; however, their actions on the pancreatic β cell ATP-sensitive K+ (KATP) channel and on glucose disposal are unknown. Therefore, the effects of the linear/branched alkyl substituents and the aliphatic/aromatic rings at position 2 of the 2H-1,4-benzoxazine nucleus on the activity of these molecules against the pancreatic β cell KATP channel and the Kir6.2ΔC36 subunit were investigated using a patch-clamp technique. The effects of these compounds on glucose disposal that followed glucose loading by intraperitoneal glucose tolerance test and on fasting glycemia were investigated in normal mice. The 2-n-hexyl analog blocked the KATP (IC₅₀ = 10.1 × 10⁻⁹ M) and Kir6.2ΔC36 (IC₅₀ = 9.6 × 10⁻⁹ M) channels, which induced depolarization. In contrast, the 2-phenyl analog was a potent opener (drug concentration needed to enhance the current by 50% = 0.04 × 10⁻⁹ M), which induced hyperpolarization. The ranked order of the potency/efficacy of the analog openers was 2-phenyl > 2-benzyl > 2-cyclohexylmethyl. The 2-phenylethyl and 2-isopropyl analogs were not effective as blockers/openers. The 2-n-hexyl (2-10 mg/kg) and 2-phenyl analogs (2-30 mg/kg) reduced and enhanced the glucose areas under the curves, respectively, after glucose loading in mice. These compounds did not affect the fasting glycemia as is observed with glibenclamide. The linear alkyl chain and the aromatic ring at position 2 of the 1,4-benzoxazine nucleus are the determinants, which confer the KATP channel blocking action with glucose-lowering effects and the opening action with increased glucose levels, respectively. The opening/blocking actions of these compounds mimic those that were observed with ATP and ADP. The results support the use of these compounds as novel antidiabetic drugs.

Plasma membrane depolarization as a determinant of the first phase of insulin secretion

The role of plasma membrane depolarization as a determinant of the initial phase of insulin secretion was investigated. NMRI mouse islets and beta-cells were used to measure the kinetics of insulin secretion, ATP and ADP content, membrane potential, and cytosolic free Ca(2+) concentration ([Ca(2+)](i)). The depolarization of metabolically intact beta-cells by KCl corresponded closely to the theoretical values. In contrast to physiological (glucose) or pharmacological (tolbutamide) ATP-sensitive K(+) (K(ATP)) channel block, KCl depolarization did not induce action potential spiking. The depolarization by 15 mM K(+) (21 mV) corresponded to the plateau depolarization by 50 or 500 microM tolbutamide; that by 40 mM K(+) (41 mV) corresponded to the action potential peaks. Nifedipine and diazoxide abolished action potentials but not KCl depolarization, suggesting that the depolarizing strength of 15, but not 40 mM K(+) corresponds to that of K(ATP) channel closure. K(+) (40 mM) induced a massive secretory response in the presence of 5 mM glucose, whereas 15 mM K(+), like 50 microM tolbutamide, was only slightly effective, even though a marked increase in [Ca(2+)](i) was produced. Raising glucose from 5 to 10 mM in the continued presence of 15 mM K(+) resulted in a strongly enhanced biphasic response. The depolarization pattern of this combination could be mimicked by combining basal glucose with 15 mM K(+) and 50 microM tolbutamide; however, the secretory response to these nonnutrients was much weaker. In conclusion, the initial secretory response to nutrient secretagogues is largely influenced by signaling mechanisms that do not involve depolarization.

Triggering and amplification of insulin secretion by dimethyl alpha-ketoglutarate, a membrane permeable alpha-ketoglutarate analogue

Cytosolic alpha-ketoglutarate is a potential signalling compound at late steps of stimulus-secretion-coupling in the course of insulin secretion induced by glucose and other fuels. This hypothesis is mainly based on the insulin-releasing effect of the membrane permeable ester dimethyl alpha-ketoglutarate which enters the beta-cell and is cleaved to produce cytosolic monomethyl alpha-ketoglutarate and eventually alpha-ketoglutarate. The present study tested this hypothesis. Insulin release, K(ATP) channel currents, membrane potential, ATP/ADP ratio and fluorescence of NAD(P)H (reduced pyridine nucleotides) were measured in mouse pancreatic islets and beta-cells. At a substimulatory glucose concentration (5 mM), dimethyl alpha-ketoglutarate (15 mM) produced a sustained insulin release, but no change of the islet ATP/ADP ratio and NAD(P)H fluorescence. In the absence of glucose, however, dimethyl alpha-ketoglutarate (15 mM) did not stimulate insulin release although it increased the ATP/ADP ratio and NAD(P)H fluorescence. Insulin secretion induced by a maximally effective concentration of the K(ATP) channel-blocking sulfonylurea glipizide was strongly amplified by dimethyl alpha-ketoglutarate in the presence of 5 mM glucose, but only moderately in the absence of glucose. Dimethyl alpha-ketoglutarate directly inhibited K(ATP) channels in inside-out membrane patches, depolarized the plasma membrane of intact beta-cells and generated action potentials. In conclusion, the stimulation of insulin secretion by extracellularly applied dimethyl alpha-ketoglutarate depends on inhibition of beta-cell K(ATP) channels by direct action of dimethyl alpha-ketoglutarate. The metabolism of alpha-ketoglutarate generated intracellularly by ester cleavage contributes to stimulation of insulin secretion both by indirect K(ATP) channel inhibition (via activation of ATP production) and by an amplifying effect.

The insulinotropic effect of fluoroquinolones

Antimicrobial fluoroquinolones induce, with strongly varying frequency, life-threatening hypoglycemias, which is explained by their ability to block K(ATP) channels in pancreatic B-cells and thus to initiate insulin secretion. In apparent contradiction to this, we observed that none of the fluoroquinolones in this study (gatifloxacin, moxifloxacin, ciprofloxacin, and a number of fluorophenyl-substituted compounds) initiated insulin secretion of perifused mouse islets when the glucose concentration was basal (5mM). Only when the glucose concentration was stimulatory by itself (10mM), the fluoroquinolones enhanced secretion. The fluoroquinolones were ineffective on SUR1 Ko islets, which do not have functional K(ATP) channels. All of these fluoroquinolones depolarized the membrane potential of mouse B-cells (patch-clamping in the whole-cell mode). Using metabolically intact B-cells (perforated-patch mode) however, 100microM of gatifloxacin, ciprofloxacin or moxifloxacin were unable to depolarize when the glucose concentration was 5mM, whereas other K(ATP) channel blockers (tolbutamide and efaroxan) remained effective. Only at a very high concentration (500microM) gatifloxacin and moxifloxacin, but not ciprofloxacin induced repetitive depolarizations which could be antagonized by diazoxide. In the presence of 10mM glucose all fluoroquinolones which enhanced secretion markedly elevated cytosolic calcium concentration ([Ca(2+)](i)). In the presence of 5mM glucose gatifloxacin and moxifloxacin at 500microM but not at 100microM elevated [Ca(2+)](i). It is concluded that fluoroquinolones in the clinically relevant concentration range are not initiators, but rather enhancers of glucose-induced insulin secretion. The block of K(ATP) channels appears necessary but not sufficient to explain the hypoglycemic effect of fluoroquinolones.

alpha2A-adrenoceptor antagonism increases insulin secretion and synergistically augments the insulinotropic effect of glibenclamide in mice

BACKGROUND AND PURPOSE

The imidazoline-type alpha2-adrenoceptor antagonists (+/-)-efaroxan and phentolamine increase insulin secretion and reduce blood glucose levels. It is not known whether they act by antagonizing pancreatic beta-cell alpha2-adrenoceptors or by alpha2-adrenoceptor-independent mechanisms. Many imidazolines inhibit the pancreatic beta-cell KATP channel, which is the molecular target of sulphonylurea drugs used in the treatment of type II diabetes. To investigate the mechanisms of action of (+/-)-efaroxan and phentolamine, alpha2A-adrenoceptor knockout (alpha2A-KO) mice were used.

EXPERIMENTAL APPROACH

Effects of (+/-)-efaroxan, 5 mg kg(-1), and phentolamine, 1 mg kg(-1), on blood glucose and insulin levels were compared with those of the non-imidazoline alpha2-adrenoceptor antagonist [8aR,12aS,13aS]-5,8,8a,9,10,11,12,12a,13,13a-decahydro-3-methoxy-12-(ethylsulphonyl)-6H-isoquino[2,1-g][1,6]naphthyridine (RS79948-197), 1 mg kg(-1), and the sulphonylurea glibenclamide, in alpha2A-KO and control (wild type (WT)) mice.

KEY RESULTS

In fed WT mice, (+/-)-efaroxan, phentolamine and RS79948-197 reduced blood glucose and increased insulin levels. Fasting abolished these effects. In fed alpha2A-KO mice, (+/-)-efaroxan, phentolamine and RS79948-197 did not alter blood glucose or insulin levels, and in fasted alpha2A-KO mice, blood glucose levels were increased. Glibenclamide, at a dose only moderately efficacious in WT mice (5 mg kg(-1)), caused severe hyperinsulinaemia and hypoglycaemia in alpha2A-KO mice. This was mimicked in WT mice by co-administration of RS79948-197 with glibenclamide.

CONCLUSIONS AND IMPLICATIONS

These results suggest that (+/-)-efaroxan and phentolamine increase insulin secretion by inhibition of beta-cell alpha2A-adrenoceptors, and demonstrate a critical role for alpha2A-adrenoceptors in limiting sulphonylurea-induced hyperinsulinaemia and hypoglycaemia.

Fuel-induced amplification of insulin secretion in mouse pancreatic islets exposed to a high sulfonylurea concentration: role of the NADPH/NADP+ ratio

AIMS/HYPOTHESIS

The aim of this study was to examine whether the cytosolic NADPH/NADP+ ratio of beta cells serves as an amplifying signal in fuel-induced insulin secretion and whether such a function is mediated by cytosolic alpha-ketoglutarate.

METHODS

Pancreatic islets and islet cells were isolated from albino mice by collagenase digestion. Insulin secretion of incubated or perifused islets was measured by ELISA. The NADPH and NADP+ content of incubated islets was determined by enzymatic cycling. The cytosolic Ca2+ concentration ([Ca2+]c) in islets was measured by microfluorimetry and the activity of ATP-sensitive K+ channels in islet cells by patch-clamping.

RESULTS

Both 30 mmol/l glucose and 10 mmol/l alpha-ketoisocaproate stimulated insulin secretion and elevated the NADPH/NADP+ ratio of islets preincubated in the absence of fuel. The increase in the NADPH/NADP+ ratio was abolished in the presence of 2.7 micromol/l glipizide (closing all ATP-sensitive K+ channels). However, alpha-ketoisocaproate, but not glucose, still stimulated insulin secretion. That glipizide did not inhibit alpha-ketoisocaproate-induced insulin secretion was not the result of elevated [Ca2+]c, as glucose caused a more marked [Ca2+]c increase. Insulin release triggered by glipizide alone was moderately amplified by dimethyl alpha-ketoglutarate (which is cleaved to produce cytosolic alpha-ketoglutarate), but there was no indication of a signal function of cytosolic alpha-ketoglutarate.

CONCLUSIONS/INTERPRETATION

The results strongly suggest that the NADPH/NADP+ ratio in the beta cell cytosol does not serve as an amplifying signal in fuel-induced insulin release. The study supports the view that amplification results from the intramitochondrial production of citrate by citrate synthase and from the associated export of citrate into the cytosol.

Antagonism of the insulinotropic action of first generation imidazolines by openers of KATP channels

The antagonism between K(ATP) channel-blocking insulinotropic imidazolines - phentolamine, alinidine, idazoxan and efaroxan - and K(ATP) channel openers, diazoxide and nucleoside diphosphates, was studied in mouse pancreatic islets and B-cells. In inside-out patches from B-cells, 500muM MgGDP abolished the inhibitory effect of the imidazolines. 300muM diazoxide further increased channel activity. The depolarizing effect of all imidazolines (100muM) on the B-cell membrane potential was practically completely antagonized by 300muM diazoxide. In contrast, diazoxide was unable to decrease the cytosolic Ca(2+) concentration ([Ca(2+)](i)) which was elevated by phentolamine, whereas the [Ca(2+)](i) increases induced by the other imidazolines were promptly antagonized. The effects on [Ca(2+)](i) were reflected by the secretory activity in that the stimulatory effects of alinidine, idazoxan and efaroxan, but not that of phentolamine were antagonized by diazoxide. Metabolic inhibition of intact B-cells by 250muM NaCN, most likely by a decrease of the ATP/ADP ratio, significantly diminished the K(ATP) channel-blocking effect of a low concentration of alinidine (10muM), whereas efaroxan proved to be susceptible even at a highly effective concentration (100muM). This may explain the oscillatory pattern of the [Ca(2+)](i) increase typically produced by efaroxan in pancreatic B-cells. In conclusion, the inhibitory effect of imidazolines on K(ATP) channels, which is exerted at the pore-forming subunit, Kir6.2, is susceptible to the action of endogenous and exogenous K(ATP) channel openers acting at the regulatory subunit SUR, which confers tissue specificity. With intact cells this antagonism can be obscured, possibly by intracellular accumulation of some imidazolines.

Rhes expression in pancreatic beta-cells is regulated by efaroxan in a calcium-dependent process

The monomeric G-protein Rhes has been described to be present in pancreatic beta-cells, and a putative role in the control of insulin release has been proposed. Here, we show that treatment of beta-cells with the imidazoline insulin secretagogue efaroxan resulted in a concentration- and time-dependent increase in the expression of Rhes, which peaked after 4h of efaroxan exposure; thereafter, Rhes mRNA levels decreased. Marked stereoselectivity was displayed, with (-)-efaroxan (the selectively insulinotropic enantiomer) being much more effective than (+)-efaroxan at raising Rhes transcript levels. The mechanism by which Rhes gene expression is activated in beta-cells appears to require the influx of extracellular calcium and de novo protein synthesis, and is not directly associated with the release of insulin. The present results confirm our earlier proposal that Rhes is an imidazoline-regulated transcript in pancreatic beta-cells. Studies to understand the role of Rhes as a regulator of beta-cell function are, thus, warranted.