Imidazoline Binding Sites Research Articles

Relationship between imidazoline/guanidinium receptive sites and monoamine oxidase A and B

Imidazoline binding sites or imidazoline/guanidinium receptive sites (IGRS) recognize bioactive endogenous substances and a variety of pharmacologically active compounds containing imidazoline or guanidinium moieties. The family of imidazoline binding proteins consists of multiple membrane-associated proteins that differ in their tissue/subcellular localization, M r and ligand recognition properties. Two of the imidazoline binding proteins are identical to the mitochondrial enzyme monoamine oxidase (MAO) A and B isoforms, which contain imidazoline binding domains distinct from the enzyme active site. The relationship between the imidazoline binding proteins and monoamine oxidases was further characterized in the present report using a covalent probe (2-[3-azido-4[ 125I]iodophenoxy]methyl imidazoline, [ 125]-AZIPI) to label the imidazoline binding proteins in different species and following transient expression of MAO-A and -B in COS 7 cells. Species homologues of MAO-A and -B in rat and human differ in their apparent molecular weight by ~2000 Da. In rat and human liver [ 125I]-AZIPI identified peptides with apparent molecular weights similar to those of the species homologues of MAO. Peptides of M r ≈63,000 (MAO-A) and ≈59,000 (MAO-B) were also photolabeled in membranes prepared from COS-7 cells transfected with human cDNA clones encoding MAO-A or -B. Additional experiments indicate that the imidazoline binding domains on MAO-A and -B exhibit different ligand recognition properties. The covalent labeling of human liver MAO-B was more sensitive than that of placenta MAO-A to inhibition by the imidazoline 2-(4,5-dihydroimidaz-2-yl)-quinoline (BU224). These data indicate that the A and B isoforms of MAO possess imidazoline binding domains that differ in their ligand recognition properties. Allosteric regulation of the activity of MAO via the imidazoline binding domains may be of significance in various disease states associated with elevated enzyme expression or in which the enzyme is a therapeutic target.

Relationship between α2-Adrenergic Receptors and Imidazoline/Guanidinium Receptive Sites

Publisher Summary Compounds such as the α 1 -adrenergic receptor (AR) agonist/ α 2 -AR antagonist cirazoline, the α 2 -AR antagonist idazoxan, the α 2 -AR agonist guanabenz, the ion transport inhibitor amiloride, and other structurally related ligands that have an imidazoline or guanidinium moiety elicit a number of pharmacological effects. Although such ligands interact with known receptor systems, some of their functional effects are pharmacologically illdefined, such as their centrally mediated effects on blood pressure and the ability of certain imidazoline derivatives to augment glucose-induced insulin secretion from pancreatic β cells. Indeed several studies indicate that these molecules interact with distinct membrane-bound proteins variously termed imidazoline/ guanidinium receptive sites (IGRS), imidazoline binding proteins, I 1 and I 2 receptors, imidazoline receptors, nonadrenergic imidazoline binding sites, or imidazole receptors. These sites share the common property of not recognizing endogenous agonists for known monoamine receptors, but subgroups of these binding sites differ in their ligand recognition properties in various tissues. The synthetic ligands recognized by imidazoline binding proteins elicit several cell responses; however, the signaling pathways involved and the relative importance of these proteins in these events are unclear.

Identification and characterization of non-adrenergic binding sites in insulin-secreting cells with the imidazoline RX821002.

Recent studies have shown that certain compounds which possess an imidazoline moiety within their structure, are able to enhance the rate of insulin secretion, both in vivo and in vitro 1. 86Rb+ efflux experiments and electrophysiological studies2,3 have provided evidence that this response is due to the reduction in potassium flux via ATP-regulated K+ channels across the islet B-cell plasma membrane, resulting in membrane depolarization. The nature of the binding site mediating the effects of imidazolines in islets is unclear but it is significant that a class of binding sites known as “imidazoline-preferring” receptors has recently been described in a wide variety of tissues4. However, the pharmacology of the defined I1- and I2-imidazoline sites does not correlate with the observed responses to imidazolines in islets, although I2-receptors (also known as non-adrenoceptor idazoxan binding sites (NAIBS)) have been described in pancreatic islets5 and RINm5F insulinoma cells6. Moreover, we have shown that the secretory response induced by the imidazoline efaroxan shows stereoselectivity and is subject to down-regulation in the presence of agonist7. Thus, the possibilty remains the islet imidazoline receptor may represent a novel, third type of imidazoline binding site.

The I1-imidazoline receptor: from binding site to therapeutic target in cardiovascular disease.

To review previous work and present additional evidence characterizing the I1-imidazoline receptor and its role in cellular signaling, central cardiovascular control, and the treatment of metabolic syndromes. Second-generation centrally-acting antihypertensives inhibit sympathetic activity mainly via imidazoline receptors, whereas first-generation agents act via alpha2-adrenergic receptors. The I1 subtype of imidazoline receptor resides in the plasma membrane and binds central antihypertensives with high affinity. Radioligand binding assays have characterized I1-imidazoline sites in the brainstem site of action for these agents in the rostral ventrolateral medulla. Binding affinity at I1-imidazoline sites, but not at other classes of imidazoline binding sites, correlates closely with the potency of central antihypertensive agents in animals and in human clinical trials. The antihypertensive action of systemic moxonidine is eliminated by the I1/alpha2-antagonist efaroxan, but not by selective blockade of alpha2-adrenergic receptors. Until now, the cell signaling pathway coupled to I1-imidazoline receptors was unknown. Using a model system lacking alpha2-adrenergic receptors (PC12 pheochromocytoma cells) we have found that moxonidine acts as an agonist at the cell level and I1-imidazoline receptor activation leads to the production of the second messenger diacylglycerol, most likely through direct activation of phosphatidylcholine-selective phospholipase C. The obese spontaneously hypertensive rat (SHR; SHROB strain) shows many of the abnormalities that cluster in human syndrome X, including elevations in blood pressure, serum lipids and insulin. SHROB and their lean SHR littermates were treated with moxonidine at 8 mg/kg per day. SHROB and SHR treated with moxonidine showed not only lowered blood pressure but also improved glucose tolerance and facilitated insulin secretion in response to a glucose load. Because alpha2-adrenergic agonists impair glucose tolerance, I1-imidazoline receptors may contribute to the multiple beneficial effects of moxonidine treatment. The I1-imidazoline receptor is a specific high-affinity binding site corresponding to a functional cell-surface receptor mediating the antihypertensive actions of moxonidine and other second-generation centrally-acting agents, and may play a role in countering insulin resistance in an animal model of metabolic syndrome X.

3H]RX821002 (2-methoxyidazoxan) binds to α2-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 μM). Competition assays using (−)-adrenaline and the subtype-selective drugs ARC 239 (2-[2-[4-( o-methoxyphenyl)-piperazin-l-yl]-ethyl]4,4-dimethyl-1,3(2mH,4H-isoquinolindione), BRL 44408 (2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazole), oxymetazoline or prazosin for [ 3H]RX821002 binding sites revealed the presence of α 2B-adrenoceptors (33–51%), α 2D-adrenoceptors (15–28%) and an adrenaline-insensitive population (34–40%), sensitive to imidazolines. After the addition of (−)-adrenaline (3 μM) to mask α 2-adrenoceptors, [ 3H]RX821002 specifically identified a saturable binding site with high affinity ( K d = 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 > (−)-noradrenaline > cimetidine) is different to that of imidazoline I 1 or imidazoline I 2 binding sites. Alternative incubation in the presence of ARC 239 (50 nM) to mask α 2B-adrenoceptors or BRL 44408 (100 nm) to mask α 2D-adrenoceptors confirmed the existence of both α 2-adrenoceptor subtypes and a non-adrenoceptor imidazoline binding site.

Agmatine does not have activity at α2-adrenoceptors which modulate the firing rate of locus coeruleus neurones: an electrophysiological study in rat

Agmatine (decarboxylated arginine) has been proposed as an endogenous ligand for non-adrenoceptor, imidazoline binding sites, but also binds to α 2-adrenoceptors. The interaction of agmatine with α 2-adrenoceptors was evaluated by studying the effect of agmatine on the firing rate of locus coeruleus (LC) neurones using extracellular recordings in anesthetized rats and rat brain slices. In vivo, local application of agmatine into the LC caused a slight and short-lasting increase in cell firing rate ( P < 0.005). In vitro, agmatine failed to change the firing rate of LC neurones nor did it antagonize the inhibitory effect of noradrenaline on these cells. Since α 2-adrenoceptors are known to inhibit the firing of LC cells, we conclude that agmatine does not have agonist or antagonist properties at α 2-adrenoceptors of these neurones.

Imidazoline binding sites and signal transduction pathways.

1. Discrete, non-adrenergic binding sites for imidazolines have been characterized in the brain and periphery. The I1 clonidine-preferring site is mainly distributed in the brain and brain stem, while the I2 idazoxan-preferring site is more widely distributed. 2. The I1 site appears to be associated with modulation of blood pressure. Imidazolines act within the rostral ventrolateral medulla to produce hypotension. The underlying signal transduction mechanism is poorly understood. 3. The imidazolines clonidine and cirazoline inhibited nicotine-stimulated calcium entry into rat phaeochromocytoma (PC-12) cells by a non-adrenergic mechanism. This effect was not attributable to the stimulation of protein kinases. 4. Similarly, clonidine and cirazoline inhibited nicotine-stimulated inward currents into PC-12 cells. This inhibitory action was not altered by inhibitors of signal transducing G-proteins. 5. Clonidine and cirazoline displaced the ion channel ligand [3H]-phencyclidine from nicotinic acetylcholine receptors, suggesting that these drugs act by direct blockade of the intrinsic ion channel of the nicotinic acetylcholine receptor. 6. This ion channel-blocking activity represents a novel action of these imidazolines and may underlie some of the proposed physiological actions of I1 sites.

Identification and characterization of imidazoline-binding sites from calf striatum

‘Non-adrenoceptor’-binding sites for [ 3H]clonidine (I 1-sites) and [ 3H]idazoxan (I 2-sites) are identified in calf striatum membranes. The pharmacological profile of both subtypes was investigated by competition binding with the imidazolines idazoxan, cirazoline, Bu 224 (2-(4,5-dihydroimidaz-2-yl)-quinoline) and Bu 239 (2-(4,5-dihydroimidaz-2-yl)-quinoxaline); the guanidino derivatives clonidine, moxonidine, guanabenz, amiloride and agmatine; the oxazoline rilmenidine and the imidazole histamine. The competition experiments indicate that both populations of imidazoline-binding sites in calf striatum consist of a high- (H) and a low-affinity (L) compartment. The monoamine oxidase (MAO) inhibitors pargyline (non-selective) and deprenyl (MAO-B-selective) have micromolar affinity for the I 1-sites and much lower affinity for the I 2-sites. The venom of the marine snail Conus geographus is the most potent of the 13 tested Conus venom preparations. None of the tested venoms is able to discriminate between both sites.

S-26-5 - Psychopharmacology of pregnant and postpartum women

Imidazoline compounds are known to interact with α2-adrenoceptors as well as with specific non-adrenergic binding sites. Such binding sites are present in the brain and in peripheral tissues. Hypotensive effects of imidazolines were shown to be related to specific interaction with imidazoline binding sites within the brainstem. Heterogeneity of these sites based on differences in selectivities was reported. In order to facilitate the characterization of human brain imidazoline receptors, we synthesized new ligands by substitutions on the cirazoline phenyl ring. Affinities of these cirazoline derivatives were determined in two imidazoline binding site models, namely the human brain and the rabbit kidney. Interaction of these compounds with imidazoline binding sites from the human brain appeared more sensitive to structural variations of the imidazoline than those with rabbit kidney sites. Moreover, no correlation was found between affinities for imidazoline binding sites and those for α2-adrenoceptors of the rat brain. Arylazide derivative of 2-(5-amino-2-methyl-phenoxymethyl)-imidazoline exhibited a higher affinity for human brain imidazoline binding sites than for human brain α2-adrenoceptors. Photoincorporation of this azido-compound in human brain imidazoline binding sites was achieved and blockade of [3H]idazoxan imidazoline specific binding observed. These new tools may allow fine characterization of the different subtypes of imidazoline binding proteins.

Polyclonal anti-idiotypic antibodies to idazoxan and their interaction with human brain imidazoline binding sites

Polyclonal antibodies were raised in rabbits against purified polyclonal anti-idazoxan antibodies. The anti-idiotypic antibodies thus obtained, proved able to inhibit [ 3H]idazoxan specific binding to anti-idazoxan antibodies. Applied to human nucleus reticularis lateralis membrane preparations, these antibodies (20 μg) inhibited about 50 and 70% of the imidazoline specific binding of [ 3H]idazoxan and [ 3H]clonidine, respectively. Furthermore, they specifically immunoprecipitated 50% of [ 3H]idazoxan binding activity of imidazoline binding sites solubilized from the same tissue. [ 3H]Rauwolscine binding to α 2-adrenoceptors in rat cortex was not significantly affected by these antibodies. The antibodies labeled a 43 kDa protein in Western blots of partially purified imidazoline binding sites from human brain. In conclusion, these anti-idiotypic antibodies recognize imidazoline binding sites from human brain and allow the detection of a 43 kDa binding protein associated with or representing the imidazoline receptor expressed in human brain.

Platelet I 1-imidazoline binding sites are decreased by two dissimilar antidepressant agents in depressed patients

Previous studies have indicated that there may be a dysregulation of alpha 2-adrenoceptors and imidazoline receptors in depression. This study compares the effects of chronic antidepressant treatment with a serotonin reuptake inhibitor (fluoxetine) versus a noradrenaline reuptake inhibitor (desipramine) on the binding parameters of the platelet imidazoline binding site (subtype I1) and of the platelet alpha 2-adrenoceptor in depressed patients. After 6 weeks of treatment with either antidepressant, platelet I1 binding sites became normalized (i.e. downregulated). A negative correlation was obtained between plasma epinephrine concentrations and platelet alpha 2-adrenoceptor Bmax values within the samples, but no correlation was obtained between any plasma catecholamine and a platelet I1 binding parameter. An additional finding was the increased affinity of alpha 2-adrenoceptors for p125I-clonidine in untreated depressed patients compared to healthy subjects. Because of the density of platelet I1 binding sites was downregulated by both of the antidepressants, we postulate that a decrease in platelet I1 binding site density may be related to an improved state from depression that these antidepressants produce.

Imidazoline binding sites in Huntington's and Parkinson's disease putamen

Binding of [ 3H]2-(2-benzofuranyl)-2-imidazoline ([ 3H]BFI) to the imidazoline I 2 receptor was determined in putamen taken post mortem from patients with two extrapyramidal motor disorders, Parkinson's and Huntington's diseases, and age-matched control subjects. No deficit of binding was apparent in Parkinson's disease, indicating that the receptors are not present on nigrostriatal terminals. A significant loss (by 56%) in imidazoline I 2 receptor binding was observed in Huntington's disease, consistent with the receptors being sited on degenerating neurons.

The elusive family of imidazoline binding sites

Various imidazoline and guanidinium derivatives elicit diverse cellular responses in peripheral and nervous tissues that are often difficult to attribute to known receptor signalling systems. Biochemical, functional and clinical evidence suggests that some activities of these compounds may be related to their action on defined imidazoline binding sites, which have been recently characterized. Unexpectedly, and of particular significance, recent data indicate that two members of the family of imidazoline binding sites are identical to the A and B isoforms of monoamine oxidase. In this article, Angelo Parini and colleagues summarize the evidence for the characterization and location of imidazoline binding sites, and speculate on the clinical implications of compounds acting on these sites.

Clotrimazole and efaroxan inhibit red cell Gardos channel independently of imidazoline I 1 and I 2 binding sites

In the present report, we investigated the potential involvement of imidazoline I 1 and I 2 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 μM). Binding studies with [ 3H]idazoxan and [ 3H] p-aminoclonidine did not reveal the expression of I 1 and I 2 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-I 1/non-I 2’ binding site.

Imidazoline/Guanidinium Binding Domains on Monoamine Oxidases: RELATIONSHIP TO SUBTYPES OF IMIDAZOLINE-BINDING PROTEINS AND TISSUE-SPECIFIC INTERACTION OF IMIDAZOLINE LIGANDS WITH MONOAMINE OXIDASE B

Pharmacologically active compounds with an imidazoline and/or guanidinium moiety are recognized with high affinity by a family of membrane-bound proteins collectively known as imidazoline binding sites or imidazoline/guanidinium receptive sites. Two such receptive sites may correspond to imidazoline binding domains identified on the A and B isoforms of monoamine oxidase (MAO), but the detection of monoamine oxidase isoforms in multiple tissues contrasts with the restricted expression of imidazoline-binding proteins. To address these issues, we determined the relationship between monoamine oxidase isoforms and subtypes of imidazoline-binding proteins in human tissues known to express one or both isoforms of MAO. 2-(3-Azido-4-[125I]iodophenoxy)methylimidazoline ([125I]A-ZIPI), a photoaffinity adduct that selectively labels imidazoline-binding proteins, photolabeled an M(r) = approximately 59,000 peptide in liver and an M(r) = approximately 63,000 peptide in placenta, consistent with the M(r) of the MAO isoforms identified by immunoblots in these tissues. The photolabeled species in liver was immunoprecipitated with MAO-B selective antibodies, whereas the photolabeled species in placenta was immunoprecipitated by MAO-A selective antibodies consistent with the isoform of MAO predominantly expressed in these tissues. The imidazoline/guanidinium ligands interact with the enzyme at a site distinct from the substrate recognition domain, and the immunoprecipitated peptides in liver and placenta display distinct ligand recognition properties consistent with those reported for subtypes of imidazoline binding sites. However, the imidazoline binding domain was not detected in platelet membrane preparations containing amounts of MAO-B equivalent to those in the photolabeled liver membranes indicating that recognition of this domain is tissue-restricted. Restricted access to the imidazoline binding domain on platelet MAO-B was not altered by membrane washing with 500 mM KCl or by solubilization and partial purification of the enzyme suggesting that there are distinct subpopulations of MAO. Identification of a binding domain on MAO that recognizes this class of pharmacologically active compounds suggests a novel mechanism for regulation of substrate oxidation/selectivity or that the enzyme may subserve an as yet undefined function.

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)

3H]-RS-45041-190: a selective high-affinity radioligand for I2 imidazoline receptors.

1. RS-45041-190 (4-chloro-2-(imidazolin-2-yl)isoindoline) is an I2 imidazoline receptor ligand with the highest affinity and selectivity so far described; [3H]-RS-45041-190 has a tritium atom attached to the 7-position on the isoindoline ring. 2. [3H]-RS-45041-190 binding to rat kidney membranes was saturable (Bmax = 223.1 +/- 18.4 fmol mg-1 protein) and of high affinity (Kd = 2.71 +/- 0.59 nM). Kinetic studies revealed that the binding was rapid and reversible, with [3H]-RS-45041-190 interacting with two sites or two affinity states. 3. Competition studies showed that 60-70% of [3H]-RS-45041-190 binding (1 nM) was specifically to imidazoline binding sites of the I2 subtype, characterized by high affinity for idazoxan (pIC50 7.85 +/- 0.03) and cirazoline (pIC50 8.16 +/- 0.05). The remaining 30-40% was displaced specifically by the monoamine oxidase A inhibitors, clorgyline and pargyline. 4. alpha 1- and alpha 2-adrenoceptor, I1 imidazoline, histamine, 5-hydroxytryptamine or dopamine receptor ligands had low affinity suggesting that [3H]-RS-45041-190 did not label receptors of these classes. 5. In autoradiography studies, [3H]-RS-45041-190 labelled discrete regions of rat brain corresponding to the distribution of I2 subtypes, notably the subfornical organ, arcuate nucleus, interpeduncular nucleus, medial habenular nucleus and lateral mammillary nucleus, and additional sites in the locus coeruleus, dorsal raphe and dorsomedial hypothalamic nucleus. 6. [3H]-RS-45041-190 therefore labels I2 receptors with high affinity, and an additional site which has high affinity for some monoamine oxidase inhibitors.

Upregulation of imidazoline receptors in the medulla oblongata accounts for the enhanced hypotensive effect of clonidine in aortic barodenervated rats

The present study tested the hypothesis that an upregulation of the imidazoline receptor in the rostral ventrolateral medulla (RVLM) of aortic barodenervated (ABD) rats may account for the enhanced hypotensive effect of clonidine. In vitro autoradiographic radioligand binding studies were utilized to investigate the binding characteristics of imidazoline receptors in the RVLM and nucleus tractus solitarius (NTS), areas that play critical roles in cardiovascular regulation and elicitation of clonidine responses. ABD but not sham operation (SO) caused immediate and significant ( P < 0.05) increases in mean arterial pressure (MAP) and heart rate (HR) and an impairment of the baroreflex-mediated HR response (baroreflex sensitivity, BRS). Two days after ABD, these parameters, except BRS, subsided to near-control (SO) levels. Intracisternal (i.c.) administration of clonidine (0.1 μg) elicited a 3-fold greater decrease in BP of conscious ABD compared with SO rats (−20.3 ± 2.6 vs. −7.4 ± 0.9 mmHg) thus demonstrating the ability of ABD to enhance centrally-mediated hypotensive responses. Autoradiographic visualization of brain sections obtained from separate groups of ABD and SO rats 48 h after surgery preincubated with [ 3H]idazoxan (2.5–3.5 nM) showed that [ 3H]idazoxan binding in RVLM, middle NTM (mTNS) and rostral NTS (rNTS) was saturable and of high affinity. Uneven distribution of imidazoline binding sites was evident since in control (SO) rats, Scatchard analysis of binding data revealed similar densities ( B max) of [ 3H]idazoxan binding sites in the RVLM and mNTS versus significantly higher density in the rNTS. In ABD rats, the binding dissociation constant ( K d) was significantly decreased in both the RVLM (8.1 ± 3.1 vs. 21.4 ± 5.0 nM) and rNTS (12.3 ± 1.3 vs. 18.6 ± 3.1 nM) compared with SO rats while the B max was not affected. This finding suggests an increased receptor affinity in the RVLM and rNTS of barodenervated rats. The mNTS of ABD rats exhibited significant increases in the B max (861 ± 96 vs. 570 ± 87 fmol/mg protein) compared with values of SO rats but the receptor affinity was not affected. It is concluded that: (i) aortic baroreceptors exert a tonic inhibitory influence on central imidazoline receptor function; and (ii) the enhanced hypotensive effect of clonidine in conscious ABD rats may be accounted for by the increased affinity of the medullary imidazoline receptors particularly in the RVLM.

Differential characteristics and localisation of [ 3H]oxazoline and [ 3H]imidazoline binding sites in rat kidney

Comparative autoradiography revealed that the imidazolines [ 3H] p-aminoclonidine and [ 3H]idazoxan labelled high densities of α 2A-adrenoceptor sites in the inner medulla and inner stripe of the outer medulla of rat kidney. In contrast, the oxazoline [ 3H]rilmenidine labelled a high density of non-adrenergic sites in the cortex and outer stripe of the outer medulla, a lower density in the inner stripe and inner medulla and a low density of α 2A-adrenoceptor sites in inner medulla. The existence of novel, non-adrenergic oxazoline sites of potential functional importance in rat kidney has important implications for the classification of imidazoline receptors.

Inhibition of red cell Ca(2+)-activated K+ transport by clotrimazole does not take place via imidazoline binding sites.

Inhibition of red cell Ca(2+)-activated K+ transport by clotrimazole does not take place via imidazoline binding sites.