In vitro stimulation of insulin release by SL 84.0418, a new alpha 2-adrenoceptor antagonist

Adrenoceptor Expression and Function in the Endocrine Pancreas

The sympathetic nervous system plays an important role in the regulation of endocrine pancreatic function, most importantly insulin release. Among the nine adrenoceptor (AR) subtypes, the α2A-AR appears to be the subtype most abundantly expressed in the human pancreas. While α2- and β-AR have opposing effects, the net response to sympathetic stimulation is inhibition of insulin secretion mediated by α2-AR located in the plasma membrane of pancreatic β cells. This inhibition may be present physiologically as evidenced by increased insulin secretion in healthy and diabetic humans and animals in response to α2-AR antagonists, a finding that was confirmed in all studies. Based on such data and on an association of an α2A-AR polymorphism, that increases receptor expression levels, with an elevated risk for diabetes, increased α2A-AR signaling in the pancreatic β cells has been proposed as a risk factor for the development of type 2 diabetes. Thus, the α2A-AR was proposed as a drug target for the treatment of some forms of type 2 diabetes. Drug research and development programs leveraging this mechanism have reached the clinical stage, but none have resulted in an approved medicine due to a limited efficacy. While β-AR agonists can increase circulating insulin levels in vivo, it remains controversial whether this includes a direct effect on β cells or occurs secondary to general metabolic effects. Therefore, the regulation of endocrine pancreatic function is physiologically interesting but may be of limited therapeutic relevance.

Methylamine Activates Glucose Uptake in Human Adipocytes Without Overpassing Action of Insulin or Stimulating its Secretion in Pancreatic Islets

Background: Methylamine, a natural soluble amine present in foods, is known to be a substrate of primary amine oxidase (PrAO) widely expressed in animal tissues. Methylamine has been reported to activate glucose transport in fat cells and to facilitate glucose disposal in rabbits but the interests and limits of such insulin-mimicking actions have not been further explored. This work aimed to perform a preclinical study of the inter-individual variations of these biological properties to study the putative link between PrAO activity and insulin resistance. Methods: Methylamine was tested on human adipocyte preparations and in rabbit pancreatic islets to determine its influence on glucose uptake and insulin release, respectively. PrAO activity and related responses were determined in adipose tissues obtained from two cohorts of non-obese and obese women. Results: Adipose tissue PrAO activity was negatively correlated with insulin resistance in high-risk obese women. PrAO-dependent activation of glucose uptake was negatively correlated with body mass index and reflected the decrease of insulin responsiveness of human fat cells with increasing obesity. Methylamine exhibited antilipolytic properties in adipocytes but was unable to directly activate insulin secretion in isolated pancreatic islets. Conclusions: PrAO activation by its substrates, e.g., methylamine, increases glucose utilization in human adipocytes in a manner that is linked to insulin responsiveness. Methylamine/PrAO interaction can therefore contribute to adipose tissue enlargement but should be considered as potentially useful for diabetes prevention since it could limit lipotoxicity and facilitate glucose handling, at the expense of favoring healthy fat accumulation.

Alpha2-adrenoceptor antagonist SL 84.0418 reduces the expression of neocortical spike-and-wave spindling episodes in DBA/2J mice

The present work was undertaken to study the effects of alpha(2)-adrenoceptor antagonist 2-(4,5-dihydro-1H-imidazol-2-yl)-1,2,4,5-tetrahydro-2-propyl-pyrrolo[3,2,1-hi]-indole hydrochloride (SL 84.0418) on the neocortical spike-and-wave spindling episodes (S and W) in the cortical electroencephalogram (ECoG) of DBA/2J mice. Our data indicate that SL 84.0418 (0.1-1.0-10 mg/kg ip) dose-dependently reduces the S and W of DBA/2J mice. This effect appears 30 min after drug administration and lasts for the duration of the recording period (240 min). The reducing effect of the SL 84.0418 on the S and W of mice was comparable to that induced by tolazoline (10-20-40 mg/kg ip), a well-known alpha(2)-adrenoceptor antagonist, whereas the alpha(2)-adrenoceptor agonist clonidine (0.02-0.1-0.5 mg/kg ip) significantly increased it. The present results indicate that SL 84.0418 administration induces significant reduction on S and W of DBA/2J mice suggesting a possible involvement of noradrenergic system in the development of S and W of DBA/2J mice.

Role of mu-opioid receptors in insulin release in the presence of inhibitory and excitatory secretagogues

In mouse pancreatic islets incubated under static conditions, the inhibitory effects on glucose-evoked insulin release induced by adrenaline (1 microM), clonidine (2 microM) and UK 14,304 (brimonidine, 0.001-1 microM) were abolished by naloxone (30 nM). Only CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH(2), 0.1 microM), a very selective mu-opioid receptor antagonist, blocked the response to UK 14,304. Glucose-induced insulin secretion was attenuated by both beta-endorphin (0.01 microM) and endomorphin-1 (0.1 microM). Naloxone and CTOP prevented these inhibitory responses. The stimulatory effect of glibenclamide (1 microM) was also reduced by endomorphin-1. However, when islets were incubated in the presence of K(+) (30 mM), carbachol (100 microM) or forskolin (0.1 microM), neither the inhibitory effect induced by UK 14,304 was reversed by naloxone, nor endomorphin-1 altered the responses promoted by the excitatory agents. Thus, alpha(2)-adrenoceptor stimulation might inhibit glucose-induced insulin secretion by releasing endogenous opioids. Mu-Opioid receptor activation and opening of K(ATP) channels could be involved in the response.

Mechanisms of the hypoglycemic effects of the alpha2-adrenoceptor antagonists SL84.0418 and deriglidole

The effects of the alpha2-adrenoceptor antagonist SL84.0418 and its two enantiomers, (+) deriglidole and (-)SL86.0714 on glucose and insulin levels were examined in mice and in neonatal streptozotocin-induced diabetic rats. It was recently demonstrated in mouse pancreatic beta-cells that both deriglidole and SL86.0714 inhibit ATP-sensitive K+ channel with similar potency whereas alpha2-adrenoceptors are blocked only by deriglidole. In the present study, we showed, in vivo in mice, that SL84.0418 and deriglidole potently reduced glycemia and antagonized diazoxide-induced hyperglycemia, whereas SL86.0714 and tolbutamide were markedly less potent. In diabetic rats, SL84.0418 and deriglidole (10 mg/kg i.p.) fully normalized glucose tolerance whereas SL86.0714 and tolbutamide only slightly improved it. Five min after deriglidole administration in mice a marked and short lasting rise in insulin levels was observed, followed by a progressive reduction of glycemia. In diabetic rats, insulin and norepinephrine levels rose 15 min after deriglidole administration. Sympathetic outflow blockade by chlorisondamine, beta-adrenoceptor blockade by propranolol or their combination markedly reduced deriglidole-induced rise in insulin levels in a similar manner. Furthermore, in chlorisondamine-treated animals norepinephrine levels were strongly lowered and not modified by deriglidole and propranolol administration. However, in spite of sympathetic outflow and beta-adrenoceptor blockade, a moderate rise in insulinemia was still observed after deriglidole administration. Taken together these data demonstrate that deriglidole is the enantiomer that mediates the antihyperglycemic and insulin secretory effects of SL84.0418. Our study suggests that the major part of deriglidole effects is the consequence of the blockade of prejunctional alpha2-adrenoceptors that have reinforced the release of catecholamines in adrenergic nerve endings and indirectly activated postjunctional beta-adrenoceptors to further potentiate insulin secretion. However, it is also suggested that another undefined mechanism is involved in deriglidole potentiation of insulin secretion.

Effects of acute alpha 2-blockade on insulin action and secretion in humans

We tested whether acute alpha 2-blockade affects insulin secretion, glucose and fat metabolism, thermogenesis, and hemodynamics in humans. During a 5-h epinephrine infusion (50 ng.min-1.kg-1) in five volunteers, deriglidole, a selective alpha 2-receptor inhibitor, led to a more sustained rise in plasma insulin and C-peptide levels (+59 +/- 14 vs. +28 +/- 6, and +273 +/- 18 vs. +53 +/- 14 pM, P < 0.01 vs. placebo) despite a smaller rise in plasma glucose (+0.90 +/- 0.4 vs. +1.5 +/- 0.3 mM, P < 0.01). Another 10 subjects were studied in the postabsorptive state and during a 4-h hyperglycemic (+4 mM) clamp, coupled with the ingestion of 75 g of glucose at 2 h. In the postabsorptive state, hepatic glucose production, resting energy expenditure, and plasma insulin, free fatty acid (FFA), and potassium concentrations were not affected by acute alpha 2-blockade. Hyperglycemia elicited a biphasic rise in plasma insulin (to a peak of 140 +/- 24 pM), C-peptide levels (1,520 +/- 344 pM), and insulin secretion (to 410 +/- 22 pmol/min); superimposed glucose ingestion elicited a further twofold rise in insulin and C-peptide levels, and insulin secretion. However, alpha 2-blockade failed to change these secretory responses. Fasting blood beta-hydroxybutyrate and glycerol and plasma FFA and potassium concentrations all declined with hyperglycemia; time course and extent of these changes were not affected by alpha 2-blockade. Resting energy expenditure (+25 vs. +16%, P < 0.01) and external cardiac work (+28% vs. +19%, P < 0.01) showed larger increments after alpha 2-blockade. We conclude that acute alpha 2-blockade in humans 1) prevents epinephrine-induced inhibition of insulin secretion, 2) does not potentiate basal or intravenous- or oral glucose-induced insulin release, 3) enhances thermogenesis, and 4) increases cardiac work.

BTS 67 582 stimulates insulin secretion from perifused rat pancreatic islets

The novel antidiabetic agent BTS 67 582 (1,1-dimethyl-2-[2-(4-morpholinophenyl)]guanidine monofumarate) demonstrated a concentration-dependent stimulation of insulin release in perifused rat pancreatic islets. EC50 values of 7.7 microM and 6.3 microM were obtained for BTS 67 582 in the presence of 8 mM glucose, after islets were pre-equilibrated with 4 and 8 mM glucose respectively. In contrast, there was little or no stimulation of insulin release at substimulatory (4 mM) or maximal stimulatory (15 mM) glucose concentrations. The plasma EC50 value for the glucose lowering effect of BTS 67 582 in fasted normal rats was 3.9 microM indicating a similar potency in vivo. In islets, BTS 67 582 completely antagonised (EC50 value of 13.2 microM) the actions of the selective ATP-dependent K+ channel opener diazoxide indicating K+ channel blocking activity. BTS 67 582 only weakly reversed the alpha2-adrenoceptor mediated inhibition of insulin release in islets (EC50 of 83 microM). BTS 67 582, like other imidazoline/guanidine insulin releasing agents, appears to promote insulin release via an effect on the islet ATP-dependent K+ channel which is not mediated by binding to the sulphonylurea receptor.

Involvement of capsaicin-sensitive nerves in the regulation of glucose tolerance in diabetic rats

The role of afferent sensory nerves in glucose tolerance and glucose stimulated insulin secretion was investigated in normal (N) or neonatal streptozotocin-induced diabetic (D) rats treated with capsaicin (NC and DC, respectively) during the neonatal period. In capsaicin treated animals, oral glucose tolerance was markedly improved both in NC and DC whereas insulin secretion was not affected. The effect of alpha 2-adrenergic blockade was investigated in D and DC rats. The alpha 2-adrenoceptor antagonist deriglidole (3 mg/kg i.p.) administered 30 min before glucose load slightly reduced glucose levels and markedly increased insulin levels both in D and DC rats. Thereafter, the high insulin levels were maintained or further increased following glucose administration. As a consequence, glucose tolerance was further improved in D and in DC, which already expressed an improved glucose tolerance after capsaicin administration. These results suggest that sensory nerves are implicated in the control of glucose tolerance in normal and neonatal streptozotocin-induced diabetic rats through a mechanism independent of insulin release. It is also suggested that afferent sensory nerves are not implicated in the adrenergic control of insulin secretion from pancreatic beta-cells.

Sulphonylureas do not increase insulin secretion by a mechanism other than a rise in cytoplasmic Ca2+ in pancreatic B-cells

The following sequence of events is thought to underlie the stimulation of insulin release by hypoglycaemic sulphonylureas. Interaction of the drugs with a high-affinity binding site (sulphonylurea receptor) in the B-cell membrane leads to closure of ATP-sensitive K+ channels, depolarization, opening of voltage-dependent Ca2+ channels, Ca2+ influx and rise in cytoplasmic [Ca2+]i. Recent experiments using permeabilized islet cells or measuring changes in B-cell membrane capacitance have suggested that sulphonylureas can increase insulin release by a mechanism independent of a change in [Ca2+]i. This provocative hypothesis was tested here with intact mouse islets. When B-cells were strongly depolarized by 60 mM K+, [Ca2+]i was increased and insulin secretion stimulated. Under these conditions, tolbutamide did not further increase [Ca2+]i or insulin release, whether it was applied before or after high K+, and whether the concentration of glucose was 3 or 15 mM. This contrasts with the ability of forskolin and phorbol 12-myristate 13-acetate (PMA) to increase release in the presence of high K+. Tolbutamide also failed to increase insulin release from islets depolarized with barium (substituted for extracellular Ca2+) or with arginine in the presence of high glucose. Glibenclamide and its non-sulphonylurea moiety meglitinide were also without effect on insulin release from already depolarized B-cells. In the absence of extracellular Ca2+, acetylcholine induced monophasic peaks of [Ca2+]i and insulin secretion which were both unaffected by tolbutamide. Insulin release from permeabilized islet cells was stimulated by raising free Ca2+ (between 0.1 and 23 microM). This effect was not affected by tolbutamide and inconsistently increased by glibenclamide. In conclusion, the present study does not support the proposal that hypoglycaemic sulphonylureas can increase insulin release even when they do not also raise [Ca2+]i in B-cells.

The imidazoline SL 84.0418 shows stereoselectivity in blocking alpha 2-adrenoceptors but not ATP-sensitive K+ channels in pancreatic B-cells

The novel alpha 2-adrenoceptor antagonist SL 84.0418 (2-(4,5-dihydro-1H-imidazol-2-yl)-1,2,4,5-tetrahydro-2-propyl-pyrrolo[3, 2,1- hi]-indole hydrochloride) is a racemic mixture of a (-) enantiomer (SL 86.0714) and a (+) enantiomer (SL 86.0715 or deriglidole). It was recently reported to inhibit alpha 2-adrenoceptors and ATP-sensitive K+ channels in mouse pancreatic B-cells, and to increase insulin release. We have now studied the stereospecificity of these responses with isolated mouse islets. Both enantiomers were equipotent in potentiating insulin release induced by 15 mM glucose alone. SL 86.0714 and deriglidole were also equally effective in inhibiting 86Rb efflux from islets perifused with a low-glucose medium, and in reversing the inhibition of glucose-induced insulin release caused by the opening of ATP-sensitive K+ channels with diazoxide. In contrast, deriglidole was approximately 100-fold more potent than SL 86.0714 in reversing the inhibition of insulin release caused by the activation of alpha 2-adrenoceptors with clonidine. The effects of SL 84.0418 are thus stereoselective on alpha 2-adrenoceptors, but not on ATP-sensitive K+ channels of pancreatic B-cells.