Imidazolines stimulate release of insulin from RIN-5AH cells independently from imidazoline I1 and I2 receptors

Imidazoline Receptor System: The Past, the Present, and the Future

Imidazoline receptors historically referred to a family of nonadrenergic binding sites that recognize compounds with an imidazoline moiety, although this has proven to be an oversimplification. For example, none of the proposed endogenous ligands for imidazoline receptors contain an imidazoline moiety but they are diverse in their chemical structure. Three receptor subtypes (I₁, I₂, and I₃) have been proposed and the understanding of each has seen differing progress over the decades. I₁ receptors partially mediate the central hypotensive effects of clonidine-like drugs. Moxonidine and rilmenidine have better therapeutic profiles (fewer side effects) than clonidine as antihypertensive drugs, thought to be due to their higher I₁/α ₂-adrenoceptor selectivity. Newer I₁ receptor agonists such as LNP599 [3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride] have little to no activity on α ₂-adrenoceptors and demonstrate promising therapeutic potential for hypertension and metabolic syndrome. I₂ receptors associate with several distinct proteins, but the identities of these proteins remain elusive. I₂ receptor agonists have demonstrated various centrally mediated effects including antinociception and neuroprotection. A new I₂ receptor agonist, CR4056 [2-phenyl-6-(1H-imidazol-1yl) quinazoline], demonstrated clear analgesic activity in a recently completed phase II clinical trial and holds great promise as a novel I₂ receptor-based first-in-class nonopioid analgesic. The understanding of I₃ receptors is relatively limited. Existing data suggest that I₃ receptors may represent a binding site at the Kir6.2-subtype ATP-sensitive potassium channels in pancreatic β-cells and may be involved in insulin secretion. Despite the elusive nature of their molecular identities, recent progress on drug discovery targeting imidazoline receptors (I₁ and I₂) demonstrates the exciting potential of these compounds to elicit neuroprotection and to treat various disorders such as hypertension, metabolic syndrome, and chronic pain.

Imidazoline receptors agonists: possible mechanisms of endothelioprotection

Imidazoline receptor agonists are one of the groups of contemporary antihypertensive drugs with the pleiotropic cardiovascular effects. In this review, the historical, physiological, pathophysiological aspects concerning imidazoline receptor agonists and possible mechanisms for their participation in endothelioprotection were considered. Illuminated the molecular biology of each subtype of imidazoline receptors and their significance in the pharmacological correction of cardiovascular disease.

IR type 1 are localized in the brain nucleus, carrying out the descending tonic control of sympathetic activation, as well as in the endothelial cells of the vessels and kidneys. Their activation leads to a decrease in blood pressure, slowing the remodeling of the vascular wall and increasing sodium nares. IR type 2 is expressed predominantly in the adrenal gland, fat and muscle tissues. The physiological effects of their stimulation are associated with an increase in glucose utilization by peripheral tissues. IR type 3 are mainly present in pancreatic cells and are associated with the regulation of insulin secretion. Their stimulation leads to an increase in insulin liberation. Thus, IR agonists are able to improve endothelial function through various mechanisms, including blood pressure reduction, improvement in metabolic profile, and direct positive effects on the vascular wall.

Current information on the pharmacological effects of this group compounds allows us to conclude that they are a promising group for correcting endothelial dysfunction and complications associated with it.

Novel I1-Imidazoline Agonist S43126 Augment Insulin Secretion in Min6 Cells

The I₁-imidazoline receptor is a novel drug target for hypertension and insulin resistance which are major disorders associated with Type II diabetes. In the present study, we examined the effects of a novel imidazoline agonist S43126 on calcium fluxes and insulin secretion from Min6 β-cells. We also examined the effects of S43126 on the induction of IRAS, and phosphorylation of components in the I₁-imidazoline signaling pathways, namely ERK and PKB. Min6 β-cells were treated with varying doses of S43126 [10⁻⁸M to 10⁻⁵M] for various time (5–60mins). S43126 at higher dose [10⁻⁵M] stimulated insulin secretion under elevated glucose concentration compared to basal. In addition, insulin secretion and Ca²⁺ influx mediated by S43126 [10⁻⁵M] were decreased following co-treatment with efaroxan (I₁-antagonist) and nifedipine (L-type voltage-gated Ca²⁺-channel blocker) at various times (5–60mins). Furthermore, S43126 at [10⁻⁵M] increased Ca²⁺ oscillation, [Ca²⁺] and ⁴⁵Ca²⁺ uptake in a time and dose-dependent manner. Moreover, Western blot analysis of treated samples showed that S43126 caused an increased protein expression of IRAS as well as phosphorylation of both ERK1/2 and PKB in a concentration-dependent manner. We conclude that S43126 exerts its insulinotropic effect in a glucose dependent manner by a mechanism involving L-type calcium channels and imidazoline I₁-receptors.

Presynaptic I1-imidazoline receptors reduce GABAergic synaptic transmission in striatal medium spiny neurons

Imidazoline receptors are expressed widely in the CNS. In the present study, whole-cell patch-clamp recordings were made from medium spiny neurons in dorsal striatum slices from the rat brain, and the roles of I1-imidazoline receptors in the modulation of synaptic transmission were studied. Moxonidine, an I1-imidazoline receptor agonist, decreased the GABAA receptor-mediated IPSCs in a concentration-dependent manner. However, glutamate-mediated EPSCs were hardly affected. The depression of IPSCs by moxonidine was antagonized by either idazoxan or efaroxan, which are both imidazoline receptor antagonists containing an imidazoline moiety. In contrast, yohimbine and SKF86466 (6-chloro-2,3,4,5-tetrahydro-3-methyl-1H-3-benzazepine), which are alpha2-adrenergic receptor antagonists with no affinity for imidazoline receptors, did not affect the moxonidine-induced inhibition of IPSCs. Moxonidine increased the paired-pulse ratio and reduced the frequency of miniature IPSCs without affecting their amplitude, indicating that this agent inhibits IPSCs via presynaptic mechanisms. Moreover, the sulfhydryl alkylating agent N-ethylmaleimide (NEM) significantly reduced the moxonidine-induced inhibition of IPSCs. Thus, the activation of presynaptic I1-imidazoline receptors decreases GABA-mediated inhibition of medium spiny neurons in the striatum, in which NEM-sensitive proteins such as G(i/o)-type G-proteins play an essential role. The adenylate cyclase activator forskolin partly opposed IPSC inhibition elicited by subsequently applied moxonidine. Furthermore, the protein kinase C (PKC) activator phorbol 12,13-dibutyrate attenuated and the PKC inhibitor chelerythrine potentiated the moxonidine-induced inhibition of IPSCs. These results suggest that IPSC inhibition via presynaptic I1-imidazoline receptors involves intracellular adenylate cyclase activity and is influenced by static PKC activity in the striatum.

Involvement of Supraspinal Imidazoline Receptors and Descending Monoaminergic Pathways in Tizanidine-Induced Inhibition of Rat Spinal Reflexes

The neuronal pathways involved in the muscle relaxant effect of tizanidine were examined by measurement of spinal reflexes in rats. Tizanidine (i.v. and intra-4th ventricular injection) decreased the mono- and disynaptic (the fastest polysynaptic) reflexes (MSR and DSR, respectively) in non-spinalized rats. Depletion of central noradrenaline by 6-hydroxydopamine abolished the depressant effect of tizanidine on the MSR almost completely and attenuated the effect on the DSR. Co-depletion of serotonin by 5,6-dihydroxytryptamine and noradrenaline resulted in more prominent attenuation of tizanidine-induced inhibition of the DSR. Supraspinal receptors were then studied using yohimbine- and some imidazoline-receptor ligands containing an imidazoline moiety. Idazoxan (I1, I2, I3, and alpha2), efaroxan (I1, I3, and alpha2), and RX821002 (I3 and alpha2), but not yohimbine, an alpha2-adrenergic receptor antagonist with no affinity for I receptors, antagonized the inhibitory effects of tizanidine. Thus, supraspinal I receptors (most likely I3) and descending monoaminergic influences are necessary for tizanidine-induced inhibition of spinal segmental reflexes.

Attenuation of acute and chronic effects of morphine by the imidazoline receptor ligand 2-(2-benzofuranyl)-2-imidazoline in rat locus coeruleus neurons

1 The aim of this study was to determine if 2-(2-benzofuranyl)-2-imidazoline (2-BFI) interacts with the opioid system in the rat locus coeruleus, using single-unit extracellular recordings. 2 In morphine-dependent rats, acute administration of the selective imidazoline receptor ligands 2-BFI (10 and 40 mg kg(-1), i.p. and 100 micro g, i.c.v.) or valldemossine (10 mg kg(-1), i.p.) did not modify the naloxone-induced hyperactivity of locus coeruleus neurons compared with that observed in the morphine-dependent control group. 3 After chronic administration of 2-BFI (10 mg kg(-1), i.p., three times daily, for 5 days) and morphine, naloxone-induced hyperactivity and tolerance to morphine were attenuated. This effect was not observed when a lower dose of 2-BFI (1 mg kg(-1), i.p.) or valldemossine (10 mg kg(-1), i.p.) were used. 4 Acute administration of 2-BFI (10 and 40 mg kg(-1), i.p. and 100 micro g, i.c.v.) but not valldemossine (40 mg kg(-1), i.p.) diminished the potency of morphine to inhibit locus coeruleus neuron activity in vivo (ED(50) values increased by 2.3, 2.9; and 3.1 fold respectively). Similarly, the potency of Met(5)-enkephalin to inhibit locus coeruleus neurons was decreased when 2-BFI (100 micro M) was applied to rat brain slices (EC(50) increased by 5.6; P<0.05). 5 The present data demonstrate that there is an interaction between 2-BFI and the opioid system in the locus coeruleus. This interaction leads to an attenuation of both the hyperactivity of locus coeruleus neurons during opiate withdrawal and the development of tolerance to morphine when 2-BFI is chronically administered. These results suggest that imidazoline drugs may prove to be useful agents for the management of opioid dependence and tolerance.

Evidence that the novel imidazoline compound FT005 is an alpha(2)-adrenoceptor agonist

1: The aim of this study was to determine whether the hyperglycaemic action of the novel imidazoline compound FT005 could be mediated by activation of alpha(2)-adrenoceptors, using a variety of in vivo and in vitro methods including radioligand binding. 2: FT005 produced a dose-dependent increase in blood glucose levels of CBA/Ca mice (0.125-25 mg kg(-1), i.p.). The time course of this hyperglycaemic effect matched that of adrenaline (1 mg kg(-1)) more closely than glucagon (1 mg kg(-1)) or the K(ATP) channel opener diazoxide (25 mg kg(-1)). The hyperglycaemic effect of FT005 (1 mg kg(-1)) was significantly reduced by the alpha(2)-adrenoceptor antagonist rauwolscine (0.5 mg kg(-1)). 3: FT005 produced a significant reduction in plasma insulin levels of mice 30 min after administration. The hyperglycaemic effect of FT005 (25 mg kg(-1)), although still present, was significantly less in fasted mice in which insulin levels are lower, suggesting that a reduction of insulin secretion contributes to the action of FT005. 4: When studied in the mouse isolated vas deferens preparation, FT005 produced a complete inhibition of neurogenic contractions, which was blocked by rauwolscine. This is consistent with activation of pre-synaptic alpha(2)-adrenoceptors. 5: In radioligand binding studies FT005 completely displaced the alpha(2)-adrenoceptor antagonist [(3)H]-RX821002 from mouse whole brain homogenates. The displacement was best described by a two-site model of interaction comprising high and low affinity components. 6: The results indicate that FT005 is an agonist at alpha(2)-adrenoceptors. A reduction in insulin secretion contributes to the hyperglycaemic action of FT005, although an additional mechanism can not be excluded.

Desensitization of insulin secretory response to imidazolines, tolbutamide, and quinine. I. Secretory and morphological studies

The desensitization of pancreatic B-cells against stimulation by insulin secretagogues that inhibit ATP-dependent K(+) channels (K(ATP) channels) was investigated by measuring insulin secretion of perifused pancreatic islets. Additionally, the islet insulin content and the number of secretory granules per B-cell were determined. Prior to the measurement of secretion, islets were cultured for 18 h in the presence or absence of the test agents in a cell-culture medium containing 5 mM glucose. The effects of three imidazolines, phentolamine, alinidine, and idazoxan (100 microM each) were compared with those of the well-characterized sulfonylurea, tolbutamide (500 microM), and those of the ion channel-blocking alkaloid, quinine (100 microM). Insulin secretion was strongly reduced upon re-exposure to phentolamine, alinidine, tolbutamide, and quinine, whereas idazoxan, which stimulated secretion only weakly, had no significant effect. The imidazoline secretagogues phentolamine and alinidine induced a cross-desensitization against the stimulatory effect of tolbutamide and quinine. A long-term depolarization with 40 mM KCl was also able to induce a significant reduction of the secretory response to all of the above secretagogues. The insulin content of cultured islets was moderately, but significantly reduced by alinidine, whereas the reduction by phentolamine, tolbutamide, and quinine was not significant. In contrast to these observations, the ultrastructural examination revealed that tolbutamide-treated B-cells had a high degree of degranulation, whereas the other test agents and 40 mM KCl produced only a partial degranulation, except for phentolamine, which produced no significant degranulation at all. These results suggest that the desensitization of insulin secretion is a common property of all agents that stimulate insulin secretion by depolarisation of the plasma membrane. Depending on the specific secretagogue, additional mechanisms, proximal and distal to Ca(2+) influx, appear to contribute to the desensitization (see Rustenbeck et al., pages 1695-1703, this issue).

In vitro mechanism of action on insulin release of S-22068, a new putative antidiabetic compound

1. The MIN6 cell line derived from in vivo immortalized insulin-secreting pancreatic beta cells was used to study the insulin-releasing capacity and the cellular mode of action of S-22068, a newly synthesized imidazoline compound known for its antidiabetic effect in vivo. 2. S-22068, was able to release insulin from MIN6 cells in a dose-dependent manner with a half-maximal stimulation at 100 micronM. Its efficacy (8 fold over the basal value), which did not differ whatever the glucose concentration (stimulatory or not), was intermediate between that of sulphonylurea and that of efaroxan. 3. Similarly to sulphonylureas and classical imidazolines, S-22068 blocked K(ATP) channels and, in turn, opened nifedipine-sensitive voltage-dependent Ca2+ channels, triggering Ca2+ entry. 4. Similarly to other imidazolines, S-22068 induced a closure of cloned K(ATP) channels injected to Xenopus oocytes by interacting with the pore-forming Kir6.2 moiety. 5. S-22068 did not interact with the sulphonylurea binding site nor with the non-I1 and non-I2 imidazoline site evidenced in the beta cells that is recognized by the imidazoline compounds efaroxan, phentolamine and RX821002. 6. We conclude that S-22068 is a novel imidazoline compound which stimulates insulin release via interaction with an original site present on the Kir6.2 moiety of the beta cell K(ATP) channels.

Non-adrenergic binding of [3H]atipamezole in rat kidney--regional distribution and comparison to alpha2-adrenoceptors

1 Atipamezole (4-(2-ethyl-2,3-dihydro-1H-inden-2-yl)-1H-imidazole) was first introduced as a potent and specific alpha2-adrenoceptor antagonist, but in some tissues [3H]atipamezole identifies an additional population of binding sites, distinct from both classical alpha2-adrenoceptors and I1- and I2-imidazoline receptors identified with [3H]para-aminoclonidine or [3H]idazoxan. 2 In the present study we have characterized [3H]atipamezole binding sites in rat kidney by receptor autoradiography and membrane binding assays and determined whether they are pharmacologically identical with the previously described binding sites for [3H]para-aminoclonidine and [3H]idazoxan. [3H]RX821002 and [3H]rauwolscine were used to compare the regional distribution of alpha2-adrenoceptors to that of non-adrenergic binding sites of [3H]atipamezole. 3 Comparative autoradiographic experiments demonstrated the differential localisation of [3H]atipamezole, [3H]RX821002 and [3H]rauwolscine binding sites in rat kidney. The pattern of distribution of non-adrenergic [3H]atipamezole binding sites is clearly distinct from that of alpha2-adrenoceptors. 4 The non-adrenergic binding of [3H]atipamezole in rat kidney does not fall into any of the previously identified three classes of imidazoline receptors studied with [3H]para-aminoclonidine, [3H]idazoxan and [3H]RX821002. 5 Atipamezole had no inhibitory effect on MAO-A or MAO-B activity in renal membranes, which speaks against the involvement of MAOs in the observed radioligand binding.

Inhibition of Ca2+-activated K+ current by clotrimazole in rat anterior pituitary GH3 cells

The ionic mechanism of clotrimazole, an imidazole antimycotic P-450 inhibitor, was examined in rat anterior pituitary GH3 cells. In perforated-patch whole-cell recording experiments, clotrimazole reversibly caused an inhibition of the Ca2+-activated K+ current in a dose-dependent manner. The IC50 value of the clotrimazole-induced inhibition of I(K(Ca)) was 3 microM. In the outside-out configuration of single channel recording, application of clotrimazole (10 microM) into the bath medium did not change the single channel conductance of large conductance Ca2+-activated K+(BK(Ca)) channels, but it suppressed the channel activity significantly. The change in the kinetic behavior of BK(Ca) channels caused by clotrimazole in these cells is found to be due to a decrease in mean open time and an increase in mean closed time. Other structurally distinct P-450 inhibitors (e.g. ketoconazole or econazole) also effectively suppressed the amplitude of I(K(Ca)). Clotrimazole (10 microM) blocked both the inactivating and non-inactivating components of the voltage-dependent K+ outward current (I(K(V))), but it produced a slight reduction of L-type Ca2+ inward current (I(Ca,L)) without altering the current-voltage relationship of I(Ca,L). Clotrimazole (10 microM) also increased the firing rate of action potentials. These results provide direct evidence that clotrimazole is capable of suppressing the activity of BK(Ca) channel in GH3 cells. Because of the non-selective inhibitory effect of clotrimazole on I(K(Ca)) and I(K(V)), this inhibition is mainly, if not entirely, due to a direct channel blockade. Thus, the present study implies that the blockade of these ionic channels by clotrimazole would affect hormonal secretion and neuronal excitability.

Protection by imidazol(ine) drugs and agmatine of glutamate-induced neurotoxicity in cultured cerebellar granule cells through blockade of NMDA receptor

This study was designed to assess the potential neuroprotective effect of several imidazol(ine) drugs and agmatine on glutamate-induced necrosis and on apoptosis induced by low extracellular K+ in cultured cerebellar granule cells. Exposure (30 min) of energy deprived cells to L-glutamate (1-100 microM) caused a concentration-dependent neurotoxicity, as determined 24 h later by a decrease in the ability of the cells to metabolize 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT) into a reduced formazan product. L-glutamate-induced neurotoxicity (EC50=5 microM) was blocked by the specific NMDA receptor antagonist MK-801 (dizocilpine). Imidazol(ine) drugs and agmatine fully prevented neurotoxicity induced by 20 microM (EC100) L-glutamate with the rank order (EC50 in microM): antazoline (13)>cirazoline (44)>LSL 61122 [2-styryl-2-imidazoline] (54)>LSL 60101 [2-(2-benzofuranyl) imidazole] (75)>idazoxan (90)>LSL 60129 [2-(1,4-benzodioxan-6-yl)-4,5-dihydroimidazole](101)>RX82 1002 (2-methoxy idazoxan) (106)>agmatine (196). No neuroprotective effect of these drugs was observed in a model of apoptotic neuronal cell death (reduction of extracellular K+) which does not involve stimulation of NMDA receptors. Imidazol(ine) drugs and agmatine fully inhibited [3H]-(+)-MK-801 binding to the phencyclidine site of NMDA receptors in rat brain. The profile of drug potency protecting against L-glutamate neurotoxicity correlated well (r=0.90) with the potency of the same compounds competing against [3H]-(+)-MK-801 binding. In HEK-293 cells transfected to express the NR1-1a and NR2C subunits of the NMDA receptor, antazoline and agmatine produced a voltage- and concentration-dependent block of glutamate-induced currents. Analysis of the voltage dependence of the block was consistent with the presence of a binding site for antazoline located within the NMDA channel pore with an IC50 of 10-12 microM at 0 mV. It is concluded that imidazol(ine) drugs and agmatine are neuroprotective against glutamate-induced necrotic neuronal cell death in vitro and that this effect is mediated through NMDA receptor blockade by interacting with a site located within the NMDA channel pore.

Imidazolines and pancreatic hormone secretion

A range of imidazoline derivatives are known to be effective stimulators of insulin secretion, and this response correlates with closure of ATP-sensitive potassium channels in the pancreatic beta-cell. However, mounting evidence indicates that potassium channel blockade may form only part of the mechanism by which imidazolines exert their effects on insulin secretion. Additionally, it remains unclear whether members of this class of drugs can bind directly to potassium channel components and whether occupation of a single binding site accounts for their functional activity. This review considers recent developments in the field and highlights evidence that does not fit readily with the concept that a single mechanism of action is sufficient to mediate the effects of imidazolines on pancreatic hormone secretion.

Design and synthesis of imidazoline derivatives active on glucose homeostasis in a rat model of type II diabetes. 2. Syntheses and biological activities of 1,4-dialkyl-, 1,4-dibenzyl, and 1-benzyl-4-alkyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazines and isosteric analogues of imidazoline

Piperazine derivatives have been identified as new antidiabetic compounds. Structure-activity relationship studies in a series of 1-benzyl-4-alkyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazines resulted in the identification of 1-methyl-4-(2', 4'-dichlorobenzyl)-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazine, PMS 812 (S-21663), as a highly potent antidiabetic agent on a rat model of diabetes, mediated by an important increase of insulin secretion independently of alpha2 adrenoceptor blockage. These studies were extended to find additional compounds in these series with improved properties. In such a way, substitution of both piperazine N atoms was first optimized by using various alkyl, branched or not, and benzyl groups. Second, some modifications of the imidazoline ring and its replacement by isosteric heterocycles were carried out, proceeding from PMS 812, to evaluate their influence on the antidiabetic activity. The importance of the distance between the imidazoline ring and the piperazine skeleton was studied third. Finally, the influence of the N-benzyl moiety was also analyzed compared to a direct N-phenyl substitution. The pharmacological evaluation was performed in vivo using glucose tolerance tests on a rat model of type II diabetes. The most active compounds were 1,4-diisopropyl-2-(4', 5'-dihydro-1'H-imidazol-2'-yl)piperazine (41a), PMS 847 (S-22068), and 1,4-diisobutyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazine (41b), PMS 889 (S-22575), which strongly improved glucose tolerance without any side event or hypoglycemic effect. More particularly, PMS 847 proved to be as potent after po (100 micromol/kg) as after ip administration and appears as a good candidate for clinical investigations.

Stimulation of locus coeruleus neurons by non-I1/I2-type imidazoline receptors: an in vivo and in vitro electrophysiological study

1. Imidazoline binding sites have been reported to be present in the locus coeruleus (LC). To investigate the role of these sites in the control of LC neuron activity, we studied the effect of imidazolines using in vivo and in vitro single-unit extracellular recording techniques. 2. In anaesthetized rats, local (27 pmoles) and systemic (1 mg kg(-1), i.v.) administrations of 2-(2-benzofuranyl)-2-imidazoline (2-BFI), a selective I-imidazoline receptor ligand, increased the firing rate of LC cells (maximal increase: 22+/-5%, P<0.001 and 16+/-7%, P<0.001 respectively). Chronic pretreatment with the irreversible monoamine oxidase inhibitor clorgyline (3 mg kg(-1), i.p., every 12 h for 14 days) abolished this effect. 3. In rat midpontine brain slices containing the LC, bath application (1 mM) of the imidazolines 2-BFI, 2-(4,5-dihydroimidaz-2-yl)-quinoline (BU224), idazoxan, efaroxan, phentolamine and (2-2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline (RX821002) reversibly stimulated LC cells. The maximal effect was approximately 90% except for RX821002 and efaroxan which induced smaller maximal effects (approximately 58% and approximately 35% respectively). Simultaneous application of idazoxan and 2BFI did not lead to additive effects. 4. Bath application of the alpha2-adrenoceptor antagonists, yohimbine (1 - 10 microM) and N-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ) (10 microM), failed to modify LC activity. The irreversible blockade of alpha2-adrenoceptors with EEDQ (10 microM) did not alter the effect of idazoxan or that of efaroxan. Previous application of clorgyline (10 microM) did not modify the excitatory effect of 2-BFI or efaroxan. 5. Changes in the pH of the bathing solution (6.84-7.84) did not influence the effect caused by idazoxan. Bath application of 2-BFI (1 mM) reversed the inhibition induced by diazoxide (300 microM), an ATP-sensitive K+ channel opener, whereas application of glibenclamide (3 microM), an ATP-sensitive K+ channel blocker, partially blocked the effect of 2-BFI. 6. This study shows that imidazoline compounds stimulate the firing rate of LC neurons. This effect is not mediated by alpha2-adrenoceptors nor by I1 or I2-imidazoline receptors but involves a different subtype of imidazoline receptor. Our results indicate that this receptor is located extracellularly and modulates ATP-sensitive K+ channels.

Clonidine-displacing substance and its putative role in control of insulin secretion: a minireview

1. Imidazoline-binding sites, or I-sites, are a class of recently defined nonadrenoceptor recognition sites whose most potent ligands are imidazolines and related compounds. 2. The pancreatic islet beta-cell I-site, which mediates imidazoline-induced stimulation of insulin release, appears to be the first site to be pharmacologically defined with selective agonists and antagonists. 3. The natural ligand for imidazoline recognition sites is still unknown. The strongest candidate is clonidine-displacing substance (CDS), originally identified in extracts of rat and bovine brain. However, the bioactive molecule has not been identified definitively. Agmatine, a decarboxylated derivative of arginine, also binds to both I-sites and alpha2-adrenoceptors (Li et al., 1994), and is, by definition, a CDS molecule. 4. In the endocrine pancreas, agmatine is a weak insulin secretagogue, which induces a slowly developing secretory response. However, this profile does not correlate with interaction at the islet I-site, and thus agmatine is unlikely to be an endogenous secretagogue acting functionally at the islet I-site. 5. Crude preparations of CDS from rat brain can potentiate glucose-induced insulin release and reverse the effects of diazoxide in rat and human islets of Langerhans. These two effects are also subject to blockade by the imidazoline antagonists RX801080 and KU14R. Furthermore, islets that were desensitized to the effects of the imidazoline secretagogue efaroxan (after 18-hr culture with imidazoline) were refractory to the actions of CDS. 6. Overall, CDS displays many characteristics expected of an endogenous regulator of insulin secretion acting through the islet beta-cell imidazoline site. This evidence strengthens the hypothesis that the islet beta-cell imidazoline site mediating control of insulin release in the endocrine pancreas is a biologically relevant receptor. Furthermore, a physiological role of CDS in the endocrine pancreas cannot be excluded.

'Seeing through a glass darkly': casting light on imidazoline 'I' sites

Although imidazoline sites have been the subject of research for several years, there is still controversy about their structure, diversity and physiology. The I1 site is thought to exist principally as a binding site and is widely purported to play a role in controlling systemic blood pressure, although this is still unclear. The majority of I2 sites are widely accepted as being allosteric sites on monoamine oxidase; however, even with selective ligands, their exact function remains to be determined. A putative I3 site modulates insulin secretion and could represent the first functional site to be pharmacologically defined with selective agonists and antagonists. The structure and relevance of the proposed endogenous ligand 'clonidine-displacing substance' remains elusive. A potential candidate for this substance is agmatine; however, although it is capable of displacing bound clonidine from imidazoline sites, it lacks the functionality ascribed to the clonidine-displacing substance. In this review, Richard M. Eglen and colleagues assess our knowledge of imidazoline sites in the light of recent data.

Characterisation of new efaroxan derivatives for use in purification of imidazoline-binding sites

The insulin secretagogue activity of certain imidazoline compounds is mediated by a binding site associated with ATP-sensitive K+ (K(ATP)) channels in the pancreatic beta-cell. We describe the effects of a series of structural modifications to efaroxan on its activity at this site. Substitution of amino-, nitro- or azide- groups onto the 5-position of the benzene ring of efaroxan did not significantly affect the functional interaction of the ligand with the islet imidazoline binding site. Modification of the imidazoline ring to an imidazole to generate 2-(2-ethyl-2,3-dihydrobenzo[b]furan-2-yl)-1H-imidazole (KU14R) resulted in loss of secretagogue activity. Indeed, this reagent appeared to act as an imidazoline antagonist since it blocked the secretory responses to imidazoline compounds and also inhibited the blockade of beta-cell K(ATP) channels by efaroxan in patch clamp experiments. Application of KU14R alone resulted in a modest reduction in K(ATP) channel opening, suggesting that it may display weak partial agonism, at least in patch-clamp experiments.

Characterisation and localisation of [3H]2-(2-benzofuranyl)-2-imidazoline binding in rat brain: a selective ligand for imidazoline I2 receptors

In rat whole brain homogenates, saturation binding analysis revealed that both [3H]2-BFI (2-(2-benzofuranyl)-2-imidazoline) and [3H]idazoxan (in the presence of 5 microM rauwolscine) bound with high affinity to an apparent single population of sites. However, the Kd for [3H]2-BFI (1.74+/-0.14 nM) was significantly (P < 0.01) less than that for [3H]idazoxan (10.4+/-2.68 nM). In competition studies idazoxan, 2-BFI, BU224 (2-(4,5-dihydroimidaz-2-yl)-quinoline), amiloride and guanabenz displayed high affinity (Ki values = 7.32, 1.71, 2.08, 21.80 and 14.90 nM, respectively) for 70-80% of sites, and low microM affinity for the remaining 20-30% of sites labelled by [3H]2-BFI. In contrast, several alpha2-adrenoceptor, imidazoline I1 receptor and histamine receptor ligands exhibited only micromolar affinity for the [3H]2-BFI labelled site. Quantitative receptor autoradiography revealed high binding by [3H]2-BFI to discrete brain nuclei, notably the area postrema, interpeduncular nucleus, arcuate nucleus, mammillary peduncle, ependyma and pineal gland. These data indicate that [3H]2-BFI recognises imidazoline I2 receptors in rat brain with higher affinity and selectivity than [3H]idazoxan and thus represents a superior radioligand to [3H]idazoxan for the study of imidazoline I2 receptors.

Sigma receptor ligands and imidazoline secretagogues mediate their insulin secretory effects by activating distinct receptor systems in isolated islets

The effects of two potent sigma receptor agonists (+)-3-PPP ((R)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine) and DTG (N,N'-di-(o-tolyl)guanidine) on the insulin secretory responses in rat islets of Langerhans were investigated. Both sigma receptor ligands were able to potentiate the insulin secretory response of islets incubated at 6 mM glucose, in a dose-dependent manner and were also able to reverse the effects of diazoxide on insulin release. When islets were treated with efaroxan, a well-characterised imidazoline insulin secretagogue, and either (+)-3-PPP or DTG together, there was an unexpected and profound absence of stimulation of insulin release as compared to when islets were incubated with each compound alone. Experiments performed with islets where there was desensitization of DTG/sigma receptor or efaroxan/imidazoline binding site mediated responses suggest that at least two distinct receptor systems appear to be involved. The complex interactions of these two classes of drug require further investigation.

Signaling and sites of interaction for RX-871024 and sulfonylurea in the stimulation of insulin release

The objective of this study was to compare effects of RX-871024, a compound with imidazoline structure, and the sulfonylurea glibenclamide, representatives of two groups of ATP-dependent potassium channel (KATP) blockers, on insulin secretion and cytoplasmic free calcium concentration ([Ca2+]i). Furthermore, we studied the interaction of the compounds on these two parameters. The experiments were performed in the perfused rat pancreas, isolated rat pancreatic islets, and dispersed beta-cells. At maximal effective concentrations of the compounds, RX-871024 had a more pronounced insulinotropic effect than glibenclamide, but the increase in [Ca2+]i was similar. Glibenclamide enhanced the insulinotropic effect of suboptimal concentrations of RX-871024 at 3.3 and 16.7 mM glucose. Notably, glibenclamide and RX-871024 also stimulated insulin secretion under Ca(2+)-clamped conditions, i.e., during plasma membrane depolarization with KCl and glucose or in permeabilized islets. The magnitudes of insulin stimulation under the latter types of conditions were similar for both compounds. It is concluded that RX-871024 and the sulfonylurea glibenclamide promote insulin secretion by two mechanisms, namely closure of KATP channels and a direct stimulation of exocytosis. At a similar increase in [Ca2+]i, the maximal stimulatory effect of RX-871024 on insulin secretion was stronger than that of glibenclamide, implying that RX-871024 also affects insulin secretion by a signal transduction pathway that is not activated by glibenclamide.

Design and synthesis of imidazoline derivatives active on glucose homeostasis in a rat model of type II diabetes. 1. Synthesis and biological activities of N-benzyl-N'-(arylalkyl)-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazines

The physiopathology of non-insulin-dependent diabetes mellitus is associated with a dysfunction in the regulation of insulin secretion. The alpha 2-adrenoceptors have been reported to be involved in this alteration, although alpha 2-antagonists containing an imidazoline ring may stimulate insulin secretion independently of alpha 2-adrenoceptor blockage. Recently, a new "imidazoline-binding site" involved in the control of K(+)-ATP channels in the B cell has been proposed. In the course of searching for new antidiabetic agents, 1-alkyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)-4-benzylpiperazines, 1-benzyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)-4-alkylpiperazines, and 1-benzyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)-4-benzylpiperazines have been designed and evaluated as potential adrenoceptor antagonists. Pharmacological evaluation was performed in vivo using glucose tolerance tests performed on a rat model of type II diabetes obtained by injection of a low dose (35 mg/kg) of streptozotocin (STZ). For some compounds, binding experiments were performed on alpha 2 adrenoceptors and I1 and I2 imidazoline-binding sites. The biological and physicochemical data have been combined with molecular modeling studies to establish structure-activity relationships. The most active compound was 1-(2',4'-dichlorobenzyl)-2-(4',5'-dihydro-1'H-imidazol-2'-yl)- 4-methylpiperazine (7f); intraperitoneal administration (100 mumol/kg) of 7f strongly improved glucose tolerance in STZ diabetic rats. This effect seemed at least partly mediated by a significant increase of insulin secretion. Other compounds of the same family (7b, 16f, 23b) have also shown potent activity. We found no correlation between in vivo antihyperglycemic properties and in vitro affinities for alpha 2-adrenoceptors or I1, and I2 binding sites. These compounds can be considered as antihyperglycemic agents potentially useful for treatment of type II diabetes and are currently under complementary investigation.

Phentolamine block of KATP channels is mediated by Kir6.2

The ATP-sensitive K+-channel (KATP channel) plays a key role in insulin secretion from pancreatic beta cells. It is closed both by glucose metabolism and the sulfonylurea drugs that are used in the treatment of noninsulin-dependent diabetes mellitus, thereby initiating a membrane depolarization that activates voltage-dependent Ca2+ entry and insulin release. The beta cell KATP channel is a complex of two proteins: Kir6.2 and SUR1. The former is an ATP-sensitive K+-selective pore, whereas SUR1 is a channel regulator that endows Kir6.2 with sensitivity to sulfonylureas. A number of drugs containing an imidazoline moiety, such as phentolamine, also act as potent stimulators of insulin secretion, but their mechanism of action is unknown. We have used a truncated form of Kir6.2, which expresses independently of SUR1, to show that phentolamine does not inhibit KATP channels by interacting with SUR1. Instead, our results argue that phentolamine may interact directly with Kir6.2 to produce a voltage-independent reduction in channel activity. The single-channel conductance is unaffected. Although the ATP molecule also contains an imidazoline group, the site at which phentolamine blocks is not identical to the ATP-inhibitory site, because phentolamine block of an ATP-insensitive mutant (K185Q) is normal. KATP channels also are found in the heart where they are involved in the response to cardiac ischemia: they also are blocked by phentolamine. Our results suggest that this may be because Kir6.2, which is expressed in the heart, forms the pore of the cardiac KATP channel.

Stimulation of insulin release from the MIN6 cell line by a new imidazoline compound, S-21663: evidence for the existence of a novel imidazoline site in beta cells

1. The MIN6 cell line derived from in vivo immortalized insulin-secreting pancreatic beta cells was used to study the insulin-releasing capacity and the cellular mode of action of S-21663, a newly synthesized imadizoline compound known for its antidiabetic effect in vivo and its ability to release insulin from perfused pancreas. 2. S-21663, at concentrations ranging from 10(-5) M to 10(-3) M was able to release insulin from MIN6 cells; its activity peaked at 10(-4) M, a drop in the stimulant factor being noted between 10(-4) and 10(-3) M. Its efficacy, which did not differ whatever the glucose concentration (stimulant or not), was higher than that of the other secretagogues tested, glucose, sulphonylureas or the peptide tGLP-1. 3. In contrast to tGLP-1, S-21663 did not change the cyclic AMP content, whereas it increased Ca2+ influx via verapamil- and nifedipine-sensitive voltage-dependent calcium channels, the insulin release being a direct consequence of this Ca2+ entry. The S-21663-induced Ca2+ influx appears to be essentially the consequence of closure of K+ channels which differ from the ATP-dependent K+ (K-ATP) channels as determined by measurement of 86Rb efflux and use of a K-ATP channel opener. 4. Comparison of the effects of S-21663 to that of efaroxan, another imidazoline compound shown to act on insulin release in a glucose-dependent way via binding sites distinct from the imidazoline I1 and I2 sites, suggested that S-21663 acts through a novel site which displays a remarkably stable expression along the cell culture. 5. It is concluded that S-21663 is a very efficient, glucose-independent insulin secretagogue acting through a novel imidazoline site, linked to K+ channels, distinct from the I1, I2 and 'efaroxan' binding sites. In vitro and in vivo features of S-21663 indicate that this compound, or new drugs derived from it, might be the basis for a new pharmacological approach to the mangement of type II (non insulin-dependent) diabetes.

Inhibition of intestinal Cl- secretion by clotrimazole: direct effect on basolateral membrane K+ channels

We evaluated the effects of clotrimazole and clofibrate on Ca(2+)- and adenosine 3',5'-cyclic monophosphate (cAMP)-mediated Cl- secretion in the colonic cell line, T84. We used 1-ethyl-2-benzimidazolinone (1-EBIO) to activate the Ca(2+)-dependent K+ channel (KCa) in these cells to induce a sustained Cl- secretory current (Isc). Clotrimazole potently inhibited the KCa-dependent Isc, with an inhibition constant (Ki) of 0.27 +/- 0.02 microM. Clofibrate also inhibited the 1-EBIO-induced Isc albeit with lower affinity (Ki = 6.5 +/- 1.2 microM). Clotrimazole (10 microM) inhibited the Isc response to the Ca(2+)-mediated agonist, carbachol, by 82%. Similarly, both clotrimazole and clofibrate inhibited cAMP-mediated Cl- secretion, with Ki values of 5.2 +/- 1.0 and 6.7 +/- 1.1 microM, respectively. We used nystatin to permeabilize the apical or basolateral membrane to determine the effects of clotrimazole and clofibrate on the basolateral K+ (IK) and apical Cl- (ICl) currents following stimulation by either 1-EBIO or forskolin. Both clotrimazole and clofibrate inhibited the 1-EBIO- and forskolin-induced IK without affecting ICl. We determined the effects of clotrimazole and clofibrate on KCa using 86Rb+ uptake studies into membrane vesicles. Both clotrimazole and clofibrate inhibited the 1-EBIO-induced 86Rb+ uptake, with Ki values of 0.31 +/- 0.08 and 10.8 +/- 5.5 microM, respectively. Similarly, clotrimazole inhibited the Ca(2+)-induced 86Rb+ uptake with a Ki of 0.51 +/- 0.15 microM. Charybdotoxin inhibited both the 1-EBIO- and Ca(2+)-induced 86Rb+ uptakes with similar affinities (Ki values of 0.57 +/- 0.07 and 0.47 +/- 0.08 nM, respectively), suggesting 1-EBIO and Ca2+ activate the same channel (KCa) in this assay. In excised, single-channel recordings both clotrimazole and clofibrate inhibited KCa, demonstrating a direct inhibition of the channel by these compounds. We demonstrate that clotrimazole blocks the intestinal KCa, thereby inhibiting Cl- secretion. These results suggest that clotrimazole may be useful as an antidiarrheal.

Possible evidence for endogenous production of a novel galanin-like peptide

Galanin mRNA and peptide are not detectable in normal islets. We studied the effect of galanin antagonists on insulin secretion in the rat beta cell line, RIN5AH, and in perifused rat islets. In RIN cell membranes galanin and its antagonists showed high affinity for 125I-galanin binding sites [Kd: (galanin) 0.03+/-0.01; Ki for galanin antagonists: (C7) 0.12+/- 0.02, (M35) 0.21+/-0.04, and (M40) 0.22+/-0.03 nM, mean+/- SEM, n = 4]. Galanin (1 microM) inhibited glucose-induced insulin release in islets (control 21.2+/-1.5 vs. galanin 4.5+/-0.2 fmol/islet per min, P < 0.001, n = 6) and RIN5AH cells (control 0.26+/-0.01 vs. galanin 0.15+/-0.02 pmol/10(6) cells per h, P < 0.001, n = 9). In RIN5AH cells, all antagonists blocked the inhibitory effects of galanin and stimulated insulin release in the absence of galanin. C7 and M40 (1 microM) alone significantly stimulated glucose-induced insulin secretion. Purified porcine galanin antibody (GAb) enhanced glucose-induced insulin release from islets (control 100+/- 16.3% vs. GAb 806.1+/-10.4%, P < 0.001, n = 6), and RIN5AH cells (control 100+/-9.6% vs. GAb 149+/-6.8%, P < 0. 01, n = 6). Western blotting of dexamethasone-treated islet extracts using GAb showed a specific band of similar molecular weight to porcine galanin not detected using a rat specific galanin antibody. One possible explanation for these results is the presence of an endogenous galanin-like peptide.

The effect of the putative endogenous imidazoline receptor ligand, clonidine-displacing substance, on insulin secretion from rat and human islets of Langerhans

1. The effects of a rat brain extract containing clonidine-displacing substance (CDS), a putative endogenous imidazoline receptor ligand, on insulin release from rat and human isolated islets of Langerhans were investigated. 2. CDS was able to potentiate the insulin secretory response of rat islets incubated at 6 mM glucose, in a dose-dependent manner. The magnitude of this effect was similar to that in response to the well-characterized imidazoline secretagogue, efaroxan. 3. CDS, like other imidazoline secretagogues, was also able to reverse the inhibitory action of diazoxide on glucose-induced insulin release, in both rat and human islets. 4. These effects of CDS on secretion were reversed by the imidazoline secretagogue antagonists, RX801080 and the newly defined KU14R, providing the first evidence that imidazoline-mediated actions of CDS can be blocked by specific imidazoline antagonists. 5. The effects of CDS on insulin secretion were unaffected when the method of preparation involved centri-filtration through a 3,000 Da cut-off membrane or when the extract was treated with protease. These results confirm that the active principle is of low molecular weight and is not a peptide. 6. Overall, the data suggest that CDS behaves as a potent endogenous insulin secretagogue acting at the islet imidazoline receptor.

Central imidazoline (I1) receptors as targets of centrally acting antihypertensives: moxonidine and rilmenidine

Clonidine, guanfacine, guanabenz and alpha-methyl-dioxyphenylalanine (DOPA), the prototypes of centrally acting antihypertensives, are assumed to induce peripheral sympathoinhibition and a reduction in blood pressure via the stimulation of alpha2-adrenoceptors in the brain stem. More recently, central imidazoline (I1)-receptors have been recognized to be another target of centrally acting antihypertensive drugs. Clonidine is considered to be a mixed agonist that stimulates both alpha2- and I1-receptors. Moxonidine and rilmenidine are considered to be moderately selective I1-receptor stimulants, although it still remains unknown whether these agents act directly on the receptor as genuine agonists. A survey is given on the location, characteristics and functional aspects of imidazoline I1-receptors as targets of centrally acting antihypertensives. Furthermore, the pharmacology and clinical potential of selective I1-receptor agonists such as moxonidine and rilmenidine are discussed. Although far from perfect, these compounds have shown that it may potentially be possible to develop agents with which the well-known side effects caused by alpha2-receptor agonists can be separated from the central antihypertensive mechanism.

[3H]RX821002 (2-methoxyidazoxan) binds to alpha 2-adrenoceptor subtypes and a non-adrenoceptor imidazoline binding site in rat kidney

The binding of [3H]RX821002 (2-methoxyidazoxan) was evaluated in rat kidney membranes. [3H]RX821002 (0.13-16 nM) recognized a single, saturable binding site with high affinity. Different binding site densities were calculated depending on non-specific binding as defined by (-)-adrenaline or RX821002 (10 microM). Competition assays using (-)-adrenaline and the subtype-selective drugs ARC 239 (2-[2-[4-(o-methoxyphenyl)-piperazin-1-yl]-ethyl]-4,4-dimethyl-1,3 (2H,4H)-isoquinolindione), BRL 44408 (2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidaz ole), oxymetazoline or prazosin for [3H]RX821002 binding sites revealed the presence of alpha 2B-adrenoceptors (33-51%), alpha 2D-adrenoceptors (15-28%) and an adrenaline-insensitive population (34-40%), sensitive to imidazolines. After the addition of (-)-adrenaline (3 microM) to mask alpha 2-adrenoceptors, [3H]RX821002 specifically identified a saturable binding site with high affinity (Kd = 4.9 +/- 1.5 nM). The pharmacological profile of this non-adrenoceptor, [3H]RX821002 binding site (potencies: efaroxan > clonidine > guanabenz > BRL 44408 > ARC 239 > BU 224 (2-(4,5-dihydroimidaz-2-yl)quinoline) > moxonidine > (-)-nor-adrenaline > cimetidine) is different to that of imidazoline I1 or imidazoline I2 binding sites. Alternative incubation in the presence of ARC 239 (50 nM) to mask alpha 2B-adrenoceptors or BRL 44408 (100 nM) to mask alpha 2D-adrenoceptors confirmed the existence of both alpha 2-adrenoceptor subtypes and a non-adrenoceptor imidazoline binding site.

Elevation of cytosolic calcium by imidazolines in mouse islets of Langerhans: implications for stimulus-response coupling of insulin release

1. Microfluorimetry techniques with fura-2 were used to characterize the effects of efaroxan (200 microM), phenotolamine (200-500 microM) and idazoxan (200-500 microM) on the intracellular free Ca2+ concentration ([Ca2+]i) in mouse isolated islets of Langerhans. 2. The imidazoline receptor agonists efaroxan and phentolamine consistently elevated cytosolic Ca2+ by mechanisms that were dependent upon Ca2+ influx across the plasma membrane; there was no rise in [Ca2+]i when Ca2+ was removed from outside of the islets and diazoxide (100-250 microM) attenuated the responses. 3. Modulation of cytosolic [Ca2+]i by efaroxan and phentolamine was augmented by glucose (5-10 mM) which both potentiated the magnitude of the response and reduced the onset time of imidazoline-induced rises in [Ca2+]i. 4. Efaroxan- and phentolamine-evoked increases in [Ca2+]i were unaffected by overnight pretreatment of islets with the imidazolines. Idazoxan failed to increase [Ca2+]i under any experimental condition tested. 5. The putative endogenous ligand of imidazoline receptors, agmatine (1 microM-1 mM), blocked KATP channels in isolated patches of beta-cell membrane, but effects upon [Ca2+]i could not be further investigated since agmatine disrupts fura-2 fluorescence. 6. In conclusion, the present study shows that imidazolines will evoke rises in [Ca2+]i in intact islets, and this provides an explanation to account for the previously described effects of imidazolines on KATP channels, the cell membrane potential and insulin secretion in pancreatic beta-cells.

Imidazoli(di)ne compounds interact with the phencyclidine site of NMDA receptors in the rat brain

The effects of several imidazoli(di)ne compounds on the binding of the non-competitive NMDA receptor antagonist [3H](+)-MK-801 (dizocilpine) to rat brain membranes were studied. These compounds fully inhibit radioligand binding with potencies in the micromolar range. The obtained profile of drug affinity correlated well with the potency of the same compounds promoting insulin release by blocking ATP-sensitive K+ channels in the rat insulinoma cell line RIN-5AH. It is suggested that imidazoli(di)ne compounds interact with cation channels sharing a common phencyclidine binding site (e.g. NMDA receptors, K+ channels and nicotinic acetylcholine receptors) and that this could be the basis of some biological effects of imidazoli(di)nes.

Pharmacological modulation of immunoreactive imidazoline receptor proteins in rat brain: relationship with non-adrenoceptor [3H]-idazoxan binding sites

1. The densities of various imidazoline receptor proteins (with apparent molecular masses of approximately 29/30-45- and 66-kDa) were quantitated by immunoblotting in the rat cerebral cortex after various drug treatments. The modulation of these imidazoline receptor proteins was then compared with the changes in the density of non-adrenoceptor [3H]-idazoxan binding sites (I2-sites) induced by the same drug treatments. 2. Chronic treatment (7 days) with the I2-selective imidazol(in)e drugs idazoxan (10 mg kg-1), cirazoline (1 mg kg-1) and LSL 60101 (10 mg kg-1) differentially increased the immunoreactivity of imidazoline receptor proteins. The levels of the 29/30-kDa protein were increased by idazoxan and LSL 60101 (23%), the levels of the 45-kDa protein only by cirazoline (44%) and those of the 66-kDa protein only by idazoxan (50%). These drug treatments also increased the density of I2-sites (32-42%). 3. Chronic treatment (7 days) with efaroxan (10 mg kg-1), RX821002 (10 mg kg-1) and yohimbine (10 mg kg-1), which possess very low affinity for I2-imidazoline receptors, did not alter either the immunoreactivity of imidazoline receptor proteins or the density of I2-sites. 4. Chronic treatment (7 days) with the monoamine oxidase (MAO) inhibitors clorgyline (10 mg kg-1) and phenelzine (10 mg kg-1) decreased the immunoreactivity of the 29/30-kDa (17-24%), 45-kDa (19%) and 66-kDa (23-31%) imidazoline receptor proteins. The alkylating agent N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline (1.6 mg kg-1, 6 h) also decreased the levels of the three imidazoline receptor proteins (20-47%). These drug treatments consistently decreased the density of I2-sites (31-57%). 5. Significant correlations were found when the mean percentage changes in immunoreactivity of imidazoline receptor proteins were related to the mean percentage changes in the density of I2-sites after the various drug treatments (r = 0.92 for the 29/30-kDa protein, r = 0.69 for the 45-kDa protein and r = 0.75 for the 66-kDa protein). 6. In the rat cerebral cortex the I2-imidazoline receptor labelled by [3H]-idazoxan is heterogeneous in nature and the related imidazoline receptor proteins (29/30-, 45- and 66-kDa) detected by immunoblotting contribute differentially to the modulation of I2-sites after drug treatment.

Clotrimazole and efaroxan inhibit red cell Gardos channel independently of imidazoline I1 and I2 binding sites

In the present report, we investigated the potential involvement of imidazoline I1 and I2 binding sites in the inhibition of the Ca(2+)-activated K+ channel (Gardos channel) by clotrimazole in human red cells. Ca(2+)-activated 86Rb influx was inhibited by clotrimazole and efaroxan but not by the imidazoline binding site ligands clonidine, moxonidine, cirazoline and idazoxan (100 microM). Binding studies with [3H]idazoxan and [3H]p-aminoclonidine did not reveal the expression of I1 and I2 binding sites in erythrocytes. These data indicate that the effects of clotrimazole and efaroxan on the erythrocyte Ca(2+)-activated K+ channel may be mediated by a 'non-I1/non-I2' binding site.

The imidazoline I1 receptor agonist, moxonidine, inhibits insulin secretion from isolated rat islets of Langerhans

In order to study the pharmacology of the putative imidazoline receptor involved in stimulation of insulin secretion, the potent and selective imidazoline I1 receptor agonist, moxonidine, was employed. Surprisingly, this agent caused a rapid and complete inhibition of glucose-induced insulin secretion in isolated rat islets of Langerhans. This response was reversible upon removal of the compound but was only partially attenuated under conditions of complete alpha 2 blockade, suggesting that it did not derive entirely from the weak alpha 2-adrenoceptor agonist activity of moxonidine. Furthermore, the response could not be attributed to activation of imidazoline I1 receptors since it was not reproduced by a second potent imidazoline I1 receptor agonist, cimetidine, and could not be alleviated by the imidazoline I1 receptor antagonist efaroxan. The results confirm that the imidazoline receptor involved in control of insulin secretion differs from the I1 subclass and suggest that moxonidine inhibits insulin secretion by a mechanism unrelated to imidazoline I1 receptor agonism.

Molecular characterization and isolation of a 45-kilodalton imidazoline receptor protein from the rat brain

Imidazoli(di)nes bind to molecular entities different from alpha 2-adrenoceptors: the so-called imidazoline receptors (IRs). Two main types of IRs have been described, the clonidine- and the idazoxan-preferring types, as well as other IRs whose pharmacological properties do not fit either type, but little is known about the molecular features of these receptors. In this study, IR proteins have been solubilized from the rat brain, using the zwitterionic detergent CHAPS, and analyzed by pharmacological and immunological means two of the four peak discriminated by gel filtration chromatography using [3H]idazoxan binding and a specific antibody. The IR eluted in the first peak accounted for 80% of the specific binding of [3H]idazoxan to solubilized brain membranes, and its pharmacological features corresponded to the non-adrenoceptor component of [3H]idazoxan binding in rat brain native membranes. The elution volume of this peak corresponded to a 130-140-kDa protein, but immunoblot analysis with a specific anti-IR antiserum showed the presence of a approximate 45-kDa IR protein, suggesting that this receptor is either an oligomeric protein complex or that it is associated with other proteins. This result was in agreement with the isolation and immunodetection of a 45-kDa peptide by affinity chromatography, which supported the relationship between this protein and a rat brain imidazoline binding site. The second peak, accounting for 15% of the specific binding of [3H]idazoxan to solubilized membranes, had a Mr of approximately 65-70,000, as determined by gel filtration chromatography and immunoblotting.(ABSTRACT TRUNCATED AT 250 WORDS)