An endogenous, non-catecholamine clonidine antagonist increases mean arterial blood pressure

Non-Central Influences of α2-Adrenergic and Imidazoline Agonist Interactions in Isolated ardiomyocytes Cardiac Cells

Aim: to investigate the functional interaction of α2-adrenergic and imidazoline receptors recently identified on the sarcolemma of isolated cardiomyocytes for regulation of the intracellular calcium and the production of the signal molecule of nitric oxide (NO).Materials and methods:experiments were performed on isolated left ventricular cardiomyocytes of Wistar rats. Potential-dependent Ca2+-currents were measured from the whole-cell by the patch-clamp method in “perforated-patch” configuration. The intracellular calcium and the production of nitric oxide were estimated from the changes in fluorescence intensity of the Ca2+-specific and NO-sensitive dyes at fluorescent or confocal microscope.Results:It has been shown that α2‑adrenergic and imidazoline receptor agonists inhibit L-type Ca2+-currents by themselves, but their effects do not develop against each other’s background. The blockade of key effector molecules: protein kinase B (Akt kinase) for α2‑adrenergic receptors, and protein kinase C for imidazoline receptors causes the action of agonists to become additive. Both the selective α2‑agonist, guanabenz, and the specific agonist of the first type imidazoline receptors, rilmenidine, show an additional inhibition of Ca2+-currents against the basal background already reduced by the activation of one of the two receptor systems. Wherein rilmenidine increases the level of free  Ca2+in the cytosol, and guanabenz, on the contrary, decreases it. The action of guanabenz does not develop against the background of rilmenidine, although it, in turn, effectively increases the intracellular level of calcium in guanabenz-pretreated cardiac cells. Activation of α2‑adrenergic receptors leads to significant stimulation of the endothelial isoform of NO-synthase, and as a result to an increase in the NO level. Activation of imidazoline receptors itself does not affect NO synthesis but it prevents the production of NO induced by α2‑agonists.Conclusion:obtained data make it possible to formulate a number of useful recommendations for clinical practice, and also to clarify the non-central peripheral effects arising from the activation of α2‑adrenergic or imidazoline systems under conditions of endogenous hyperactivation on of the two systems.

Synergism of myocardial β-adrenoceptor blockade and I1-imidazoline receptor-driven signaling: Kinase-phosphatase switching

Recently identified imidazoline receptors of the first type (I₁Rs) on the cardiomyocyte's sarcolemma open a new field in calcium signaling research. In particular, it is interesting to investigate their functional interaction with other well-known systems, such as β-adrenergic receptors. Here we investigated the effects of I₁Rs activation on L-type voltage-gated Ca²⁺-currents under catecholaminergic stress induced by the application of β-agonist, isoproterenol. Pharmacological agonist of I₁Rs (I₁-agonist), rilmenidine, and the putative endogenous I₁-ligand, agmatine, have been shown to effectively reduce Ca²⁺-currents potentiated by isoproterenol. Inhibitory analysis shows that the ability to suppress voltage-gated Ca²⁺-currents by rilmenidine and agmatine is fully preserved in the presence of the protein kinase A blocker (PKA), which indicates a PKA-independent mechanism of their action. The blockade of NO synthase isoforms with 7NI does not affect the intrinsic effects of agmatine and rilmenidine, which suggests NO-independent signaling pathways triggered by I₁Rs. A nonspecific serine/threonine protein phosphatase (STPP) inhibitor, calyculin A, abrogates effects of rilmenidine or agmatine on the isoproterenol-induced Ca²⁺-currents. Direct measurements of phosphatase activity in the myocardial tissues showed that activation of the I₁Rs leads to stimulation of STPP, which could be responsible for the I₁-agonist influences. Obtained data clarify peripheral effects that occur during activation of the I₁Rs under endogenous catecholaminergic stress, and can be used in clinical practice for more precise control of heart contractility in some cardiovascular pathologies.

Agmatine Reduces Lipopolysaccharide-Mediated Oxidant Response via Activating PI3K/Akt Pathway and Up-Regulating Nrf2 and HO-1 Expression in Macrophages

Macrophages are key responders of inflammation and are closely related with oxidative stress. Activated macrophages can enhance oxygen depletion, which causes an overproduction of reactive oxygen species (ROS) and leads to further excessive inflammatory response and tissue damage. Agmatine, an endogenous metabolite of L-arginine, has recently been shown to have neuroprotective effects based on its antioxidant properties. However, the antioxidant effects of agmatine in peripheral tissues and cells, especially macrophages, remain unclear. In this study we explored the role of agmatine in mediating antioxidant effects in RAW 264.7 cells and studied its antioxidant mechanism. Our data demonstrate that agmatine is an activator of Nrf2 signaling that markedly enhances Nrf2 nuclear translocation, increases nuclear Nrf2 protein level, up-regulates the expression of the Nrf2 downstream effector HO-1, and attenuates ROS generation induced by Lipopolysaccharide (LPS). We further demonstrated that the agmatine-induced activation of Nrf2 is likely through the PI3K/Akt pathway. LY294002, a specific PI3K/Akt inhibitor, abolished agmatine-induced HO-1 up-regulation and ROS suppression significantly. Inhibiting HO-1 pathway significantly attenuated the antioxidant effect of agmatine which the products of HO-1 enzymatic activity contributed to. Furthermore, the common membrane receptors of agmatine were evaluated, revealing that α2-adrenoceptor, I1-imidazoline receptor or I2-imidazoline receptor are not required by the antioxidant properties of agmatine. Taken together, our findings revealed that agmatine has antioxidant activity against LPS-induced ROS accumulation in RAW 264.7 cells involving HO-1 expression induced by Nrf2 via PI3K/Akt pathway activation.

Modulation of stress by imidazoline binding sites: implications for psychiatric disorders

In this review, we present evidence for the involvement of imidazoline binding sites (IBS) in modulating responses to stress, through central control of monoaminergic and hypothalamo-pituitary-adrenal (HPA) axis activity. Pharmacological and physiological evidence is presented for differential effects of different IBS subtypes on serotoninergic and catecholaminergic pathways involved in control of basal and stress-stimulated HPA axis activity. IBS ligands can modulate behavioural and neuroendocrine responses in animal models of stress, depression and anxiety, and a body of evidence exists for alterations in central IBS expression in psychiatric patients, which can be normalised partially or fully by treatment with antidepressants. Dysfunction in monoaminergic systems and the HPA axis under basal and stress-induced activation has been extensively reported in psychiatric illnesses. On the basis of the literature, we suggest a potential therapeutic role for selective IBS ligands in the treatment of depression and anxiety disorders.

Activation of protein kinase B/Akt and endothelial nitric oxide synthase mediates agmatine-induced endothelium-dependent relaxation

The ability of agmatine, formed from L-arginine by the enzyme arginine decarboxylase (ADC), to modulate vasomotor function in rat aorta was investigated in the present study. Agmatine-mediated modulation of vasomotor tone was studied in organ chambers, protein expression quantified by Western blot analysis and cyclic guanosine 5'-monophosphate (cGMP) levels measured by radioimmunoassay. Agmatine (10(-10) to 10(-3) M) produced concentration-dependent relaxations (82+/-5%) in phenylephrine-contracted endothelium intact rat aorta. Relaxations to agmatine were diminished on denudation of endothelium and nitric oxide synthase (NOS) inhibition by L-Nomega-nitro arginine or soluble guanylate cyclase inhibition by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (P<0.001) abolished agmatine-mediated relaxations, while relaxations were insensitive to inducible NOS inhibition by 1400W. Agmatine-treated aorta demonstrated increased protein expression of phosphorylated S473-Akt and phosphorylated S1177-endothelial nitric oxide synthase (eNOS), and elevated the levels of cyclic GMP (P<0.01). Agmatine-mediated potentiation of relaxations and elevation of cGMP levels was sensitive to phosphatidylinositol 3'-kinase inhibitor, wortmannin. Relaxations to agmatine were also affected by pre-treatment with tetraethylammonium (P<0.01) or apamin (P<0.05), and were not affected by charybdotoxin. Relaxations to agmatine were partially affected by pre-treatment of aortic rings with barium chloride (P<0.05), and glybenclamide (P<0.05). Results obtained suggest that agmatine activates protein kinase B/Akt to phosphorylate eNOS and elevate cyclic GMP levels to produce vasodilatation of aorta. Agmatine-mediated relaxations in rat aorta seems to be mediated mainly by endothelial NO-mediated activation of small conductance Ca2+-activated K+ channels, and partly by ATP-sensitive and inward rectifying K+ channels.

Imidazoline receptors: new targets for antihyperglycaemic drugs

In both animal models of Type 2 diabetes and in man, it has been evident for many years that certain imidazoline drugs can stimulate insulin secretion and improve glycaemia. This suggests that they may be useful new reagents for use in the management of Type 2 diabetes. However, despite their promise, no imidazoline compound has yet come into clinical use as an effective therapeutic agent in diabetes. This should not be taken as evidence of a flaw in the basic hypothesis, but derives, in part, from continuing ignorance about the molecular characteristics of imidazoline binding proteins, and the precise structure-activity relationships of their ligands. In this review, the mode of action of antihyperglycaemic imidazoline compounds is considered, and the possibility discussed that these agents may interact with a unique subtype of imidazoline binding site associated with ATP-sensitive potassium channels. The functional consequences of this interaction are summarised together with evidence that the binding site may actually lie within the channel complex. Additional data implicating the participation of alpha2-adrenoceptors in some actions of imidazolines are evaluated, and examples of relevant drugs having encouraging therapeutic profiles are highlighted. The possibility that some anti-diabetic imidazoline reagents may exert extra-pancreatic effects is also considered. Overall, the article aims to highlight important developments within the field but also draws attention to those areas where controversy remains.

Antihypertensive drugs interacting with central imidazoline (I1)-receptors

Central imidazoline (I(1))-receptors have been recognised as targets of a new class of centrally acting antihypertensives. The stimulation of these I(1)-receptors induces peripheral sympatho-inhibition and a reduction of (elevated) blood pressure. Moxonidine and rilmenidine are the prototypes of this new class of centrally acting antihypertensives. These imidazoline receptor stimulants are effective antihypertensives with a haemodynamic profile which is attractive from a pathophysiological point of view. Since both moxonidine and rilmenidine have a much weaker affinity for central (2)-adrenoceptors than classic centrally acting drugs, for example, clonidine and alpha-methyl-DOPA, the side-effects profile of the I(1)-receptor stimulants is significantly better. The imidazoline (I(1))-receptor stimulants are the subject of the current survey. They appear to offer the possibility of developing centrally acting antihypertensives with the same attractive haemodynamic characteristics as the classic alpha(2)-adrenoceptor stimulants, but with clearly better tolerability. Their potential use in the treatment of congestive heart failure and the metabolic syndrome is subject to clinical investigation.

Clonidine toxicity revisited

The incidence of clonidine overdose is increasing, yet there is a paucity of new information regarding treatment options for clonidine toxicity. Reported treatment approaches vary widely, demonstrating the lack of science on which current treatment is based. Available research needs to be reassessed. Neurotransmitters, receptors, endogenous opioids, and baseline sympathetic tone determine the clinical response to clonidine as well as the potential response to drug therapy following clonidine overdose. This article reviews aspects of clonidine toxicity that need to be further investigated. Multicenter research trials will be required to evaluate new treatment options.

Comparison of Effects of Various Types of NA and 5-HT Agonists on Transmission from Group II Muscle Afferents in the Cat

A number of noradrenaline and serotonin agonists were tested to investigate which of them replicate the depressive actions of monoamines on transmission from group II muscle afferents in the cat spinal cord. The agonists were applied ionophoretically at the two sites at which maximal monosynaptic focal field potentials are evoked from group II afferents-in the intermediate zone and the dorsal horn of the 4th and 5th lumbar segments. Their effects were estimated from changes in the amplitude of the field potentials. The compounds tested fell into three categories according to the site at which they depressed transmission from group II afferents: one category with highly selective actions in the intermediate zone, a second category with similarly selective actions in the dorsal horn, and a third category with non-selective actions. Drugs in the first category included three noradrenaline agonists (tizanidine, B-HT 933 and clonidine), included in the second were five serotonin agonists (8-OH-DPAT, 5-methoxytryptamine, alpha-methyl serotonin, DOI and 2-methyl-serotonin), and in the third two noradrenaline agonists (phenylephrine and isoproterenol) and two serotonin agonists (RU 24969 and 5-carboxamidotryptamine). Field potentials evoked by group I afferents remained unaffected by all but one compound (8-OH-DPAT). Effects of one noradrenaline agonist and one serotonin agonist (tizanidine and 5-methoxytryptamine) were also tested on responses of single extracellularly recorded neurons. Tizanidine depressed responses induced by stimulation of group II afferents in intermediate zone interneurons, but not in dorsal horn neurons, while 5-methoxytryptamine depressed activation of the latter. Tizanidine had no effect on responses evoked by group I afferents, either in intermediate zone interneurons or in the dorsal spino-cerebellar tract neurons of Clarke's column. It is hypothesized that noradrenaline and serotonin released by descending monoaminergic neurons differ in the potency with which they depress transmission from group II afferents to different functional types of neuron. The results suggest that this depression may involve different membrane receptors at different locations, primarily alpha2 adrenoceptors in the intermediate zone/ventral horn and 5-HT1A serotonin receptors in the dorsal horn.

Agmatine, an endogenous ligand at imidazoline binding sites, does not antagonize the clonidine-mediated blood pressure reaction

Since agmatine has been identified as a clonidine displacing substance (CDS), the aim of this study was to investigate whether agmatine can mimic CDS-induced cardiovascular reactions in organ bath experiments, pithed spontaneously hypertensive rats (SHR) and anaesthetized SHR. Intravenously-administered agmatine significantly reduced the blood pressure and heart rate of anaesthetized SHR at doses higher than 1 and 3 mg kg(-1), respectively. These effects are probably mediated via central mechanisms, since there was an approximate 8 fold rightward shift of the dose-response curve in the pithed SHR (indicating a weakened cardiovascular effect). Moreover, in organ bath experiments, agmatine failed to alter the contractility of intact or endothelium-denuded aortal rings. When agmatine was administered i.c.v. to anaesthetized SHR, blood pressure was increased without any alteration of heart rate, whereas blood pressure was unchanged and heart rate was increased after injection into the 4th brain ventricle. This suggests that haemodynamic reaction patterns after central application are related to distinct influences on central cardiovascular mechanisms. Agmatine reduces noradrenaline release in pithed SHR while alpha(2)-adrenoceptors are irreversibly blocked with phenoxybenzamine, but not while I(1)-binding sites are selectively blocked with AGN192403. This suggests that agmatine may modulate noradrenaline release in the same way that clonidine does, i.e. via imidazoline binding sites; this involves a reduction in sympathetic tone which in turn reduces blood pressure and heart rate. Finally, CDS-like cardiovascular activity appears not to be due to agmatine, since (i) blood pressure in anaesthetized SHR is decreased by agmatine and clonidine, and (ii) agmatine did not antagonize the blood pressure reaction to clonidine in pithed or anaesthetized SHR.

Imidazoline receptors in cardiovascular and metabolic diseases

The site of the hypotensive action of imidazoline compounds, such as clonidine, was first identified within the rostroventrolateral part of the brainstem. Afterwards, it was shown that imidazolines reduced blood pressure when applied in this area, whereas no catecholamine was capable of such an effect. These data led us to suggest the existence of receptors specific for imidazolines different from the alpha-adrenergic receptors. Soon after, the existence of imidazoline binding sites (IBS) was reported in the brain and in a variety of peripheral tissues including pancreatic gland and kidney. As expected, these specific binding sites do not bind the catecholamines. The IBS are classified in two groups: the I1 type, sensitive to clonidine and idazoxan; and the I2 type, sensitive to idazoxan and largely insensitive to clonidine. Imidazoline receptors were shown to be involved in several physiological regulations and pathological processes such as hypertension, diabetes mellitus and some mood disorders. Evidence for their implication in the nervous regulation of blood pressure and in the insulin secretion control will be presented. The hypotensive effects of clonidine-like drugs involve imidazoline receptors (I1Rs), while their most frequent side-effects only involve alpha2-adrenergic receptors. A new class of centrally acting antihypertensive drugs selective for I1Rs is now available. At hypotensive doses, these drugs are devoid of significant side effects. It was shown that the good acceptability of these drugs is likely due to their selectivity for I1Rs.

Imidazoline receptors: a challenge

The hypotensive effect of imidazoline-like drugs (IMs) directly injected into the rostroventrolateral part of the brainstem (NRL/RVLM) was shown to involve non-adrenergic imidazoline specific receptors (IRs). Some IMs caused hypotension when injected there, irrespective of their affinity and selectivity for any alpha-adrenoceptor subtype. Compounds, such as LNP 509, S 23515, S 23757 or benazoline with very high selectivities for IRs over alpha 2-adrenoceptors (A2Rs), became available recently. Some of these compounds (LNP 509, S 23515) caused hypotension when injected alone into the NRL/RVLM region. Nevertheless, high selectivity for IRs will not predict by its own the capability of IMs to elicit hypotension as some of these substances behaved as antagonists towards the hypotensive effects of the latter. As far as hybrid drugs, i.e., with mixed binding profiles (I1/alpha 2), were concerned, a significant correlation has been reported between their central hypotensive effect and their affinity for IRs. Imidazoline antagonists, such as idazoxan, were repeatedly shown to competitively prevent and reverse the centrally induced hypotensive effect of IMs. The sole stimulation of A2Rs within the NRL/RVLM region was not sufficient to decrease blood pressure as much as IMs did, as shown by the lack of significant blood pressure lowering effect of alpha-methylnoradrenaline (alpha-MNA). No correlation was observed between affinity of IMs for A2Rs and their central hypotensive effects. It is also noticeable that yohimbine, an A2Rs antagonist, was repeatedly shown to abolish the hypotensive effect of hybrids but usually in a non-competitive manner. Mutation of A2Rs was shown to prevent the hypotensive effects of centrally acting drugs. It is concluded that (i) drugs highly selective for I1Rs over A2Rs can reduce blood pressure by their own; (ii) the central hypotensive effect of IMs needs implication of IRs and appears to be facilitated by additional activation of A2Rs; and (iii) this effect requires intact A2Rs along the sympathetic pathways.

Extraction of active clonidine-displacing substance from bovine lung and comparison with clonidine-displacing substance extracted from other tissues

Crude methanolic clonidine-displacing substance (CDS) extracted from bovine lung competed for radioligand binding from alpha2-adrenoceptors and I2-sites present in rat brain membranes, and from I1-sites present in rat brain and kidney membranes. There was no difference in the competition of [3H]clonidine binding to alpha2-adrenoceptors present in either rat or rabbit brain membranes by the crude CDS extract and therefore either tissue could be used to estimate the number of units of CDS present in extracts. Further purification by reverse phase high performance liquid chromatography (RP-HPLC), with UV detection, of extracts obtained from bovine lung, brain and rat brain exhibited similar three-peak profiles, previously reported. Corresponding fractions competed for radioligand binding to alpha2-adrenoceptors present in rat brain membranes, eluting between 19 and 23 min, which corresponded with the middle peak of the three-peaks. Therefore, we propose the CDS-like material eluting from all these tissues to be similar. Interestingly, CDS extracted from bovine adrenal glands under the same conditions showed a similar three-peak profile, but did not repeat the displacement of binding just at 19-23 min, but at every time point after 4 min. This suggests this tissue could represent a source of CDS in this species.

Imidazoline receptors: from discovery to antihypertensive therapy (facts and doubts)

The hypothesis and indirect evidence of imidazoline receptors has been promoted since some 15 years ago and it gave a substantial impetus for research in this field, resulting in a better understanding of neuronal and cardiovascular regulatory processes. The nomenclature of the imidazoline receptors has been accepted by international forums but no direct proof for the existence of these receptors has been published. Authors summarise the most important available data, including facts and doubts as far as the discovery, characterisation, and function of imidazoline receptors and their subtypes, the differences between imidazoline receptors and alpha-2 adrenoceptors, and also on their participation in regulatory processes.

Modulation of MAO activity by imidazoline and guanidine derivatives

I2-binding sites (I2-BS) are attributed to be a regulative site on monoamine oxidase (MAO). The in vivo and in vitro effects of various imidazoline and guanidine derivatives on MAO activity and on mitochondrial respiration were studied. Substances with high affinity for I2-BS (antazoline, idazoxan, and cirazoline: IC50 = 20.3, 33.8, and 43.4 microM) had a stronger inhibitory effect on MAO activity than did I1-ligands (efaroxan, rilmenidine, clonidine, and moxonidine: IC50 = 277, 801, 1,224, and > 10,000 microM). Substances with the highest inhibitory effects were BDF8082 (IC50 = 1.7 microM) and 2-(2-benzofuranyl)-2-imidazoline (BFI; IC50 = 4.0 microM). The enzyme is inhibited noncompetitively and is reversible, because its activity is completely or partially restored after dialysis. Agmatine, the putative endogenous ligand for IBS, also decreased MAO activity (IC50 = 168 microM), whereas its precursor, L-arginine, and its metabolite, putrescine, had no effects. In vitro inhibition of MAO and mitochondrial respiration by the IBS-ligands tested could not be correlated, suggesting no link between the function of the inner and outer mitochondrial membrane. MAO activity in vivo was significantly reduced only by pargyline (-95%), BDF8082 (-68%), BFI (-43%), and 1-(m-chlorophenyl)-biguanide (-28%). Catecholamine content of livers obtained from animals treated with different IBS-ligands was consequently increased. In conclusion, the strong inhibitory effects of I2 selective imidazoline ligands confirm the existence of I2-BS as a regulatory site on MAO.

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.

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.

Modulation of catecholamine release by alpha 2-adrenoceptors and I1-imidazoline receptors in rat brain

The physiological and pharmacological effects of imidazoli(di)ne derivatives, such as clonidine, have been related not only to the interaction with alpha 2-adrenoceptors but also to their activity on non-adrenoceptor sites termed imidazoline receptors. The modulation of catecholamine release by imidazoline drugs was studied by monitoring extracellular levels of norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) with microdialysis in cingulate cortex of rats, with or without irreversible alpha 2-adrenoceptor blockade. NE and DA levels were in the 1 nM range whereas DOPAC and HIVA levels were approximately equal to 100 nM. NE and DA levels were increased when the uptake blocker desipramine (1 microM) or KCl (100 mM) were added to the perfusion medium. Clonidine induced a dose-dependent (0.3-1.2 mg/kg i.p.) decrease in NE (max 61%) and DA (max 40+) levels that was reversed by the alpha 2-adrenoceptor antagonist RX821002. After alpha 2-adrenoceptor irreversible blockade with the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), [3H]clonidine binding to alpha 2-adrenoceptors was reduced by 94 +/- 1%. Under such conditions, clonidine elicited a paradoxical dose-dependent (0.6-2.4 mg/kg i.p.) increase of NE (max 56%) without modifications in DA, DOPAC and HVA levels. The stimulatory effect of clonidine was prevented by the imidazoline receptor antagonist idazoxan (10 mg/kg i.p.) but not by RX821002 (5 mg/kg i.p.). In rats pretreated with EEDQ, cirazoline (I1/I2-imidazoline receptor agonist), moxonidine (I1-imidazoline receptor agonist), but not guanabenz (I2-imidazoline receptor agonist) (1.2-2.4 mg/kg i.p.) elicited an increase of NE levels in a similar manner to clonidine (11-82%). Idazoxan also abolished these responses to cirazoline or moxonidine. In contrast to systemic administration, local perfusion of clonidine (10-100 microM) through the microdialysis probe under alpha 2-adrenoceptor alkylating conditions, did not modify extracellular levels of NE and DA suggesting an indirect mechanism. The results demonstrate that clonidine and related imidazoli(di)ne drugs are able not only to inhibit NE release in rat cerebral cortex involving an alpha 2-adrenoceptor mechanism, but also to induce a paradoxical NE release through an indirect extracortical mechanism. The findings evidence that the indirect modulation of NE levels by imidazoline drugs is mainly due to a functional activity on I1-imidazoline receptors.

An antiserum to idazoxan recognizes an immunoreactive substance in human serum and cerebral spinal fluid which is not agmatine

A polyclonal antibody was generated in rabbits to an idazoxan-albumin antigen. The anti-idazoxan antiserum had high affinity for unconjugated 3H-idazoxan (Kd of 19.8 nM) in a radio-immunoassay (RIA). Of various drugs and native molecules only idazoxan potently (Ki of 24 nM) inhibited 3H-idazoxan binding to the anti-idazoxan antibody. A few drugs weakly inhibited 3H-idazoxan binding (IC50 > 605 microM) with rank order of UK 14304 > guanabenz > cirazoline > amiloride > naphazoline. Neither agmatine, an endogenous clonidine displacing substance (CDS), catecholamines or imidazoles inhibited the binding of 3H-idazoxan to the anti-idazoxan antibody. The anti-idazoxan RIA was 4-6 fold more sensitive than an antibody to para-amino clonidine. The CDS detected by ligand displacement from bovine brain dose-dependently inhibited 3H-idazoxan binding. This immunoreactive (ir-) CDS activity was present in human (0.9-4.1 U/ml) and rat sera (1-2 U/ml) and in the cerebro-spinal fluid of eight patients with serious disease of the central nervous system, but not in controls. We conclude: (1) an anti-idazoxan RIA is a sensitive, selective and clinically applicable RIA for measuring ir-CDS; (2) ir-CDS is not agmatine; (3) CDS represents a family of endogenous ligands for imidazoline receptors including ir-CDS and agmatine.

The relationship between the adrenoceptor and nonadrenoceptor-mediated effects of imidazoline- and imidazole-containing compounds

This article brings together work on imidazoline or imidazole-containing compounds concerned with the pharmacology of alpha-adrenoceptors, principally on smooth muscle, to illustrate how imidazolines have contributed to the subclassification of alpha-adrenoceptors and how, against this background, attempts have been made to use this knowledge to uncover "nonadrenoceptor"-mediated biological effects of previously uncharacterized compounds, notably imidazole-containing dipeptides and "clonidine displacing substance" (CDS). Recent data are included on (1) the pharmacology of UK-14304, (2) nonadrenoceptor actions of phentolamine, (3) the pharmacology of tissue extracts containing imidazole-containing dipeptides and CDS activity, and (4) ligand binding data at I1 and I2 sites.

Endogenous ligands of imidazoline receptors: classic and immunoreactive clonidine-displacing substance and agmatine

1. There are several endogenous ligands that bind to I-receptors of both the I1 and I2 subclass. These include: (a) classic CDS, a partially purified entity isolated by the criteria that it displaces binding ligands to alpha 2- and I-receptors; (b) immunoreactive (ir)-CDS, a moiety that binds to antibodies raised against clonidine, para-amino-clonidine, or idazoxan; and (c) agmatine. 2. Classic-CDS, not yet defined structurally, binds to I1, I2, and alpha 2-adrenergic receptors, is neither a peptide nor a catecholamine, and has purportedly a molecular weight of 588 Da. By ligand binding assays, it was found in brain, serum, CSF, and placenta and in a neural-glial cell line. Partially purified classic CDS is bioactive. Like clonidine, it contracts aorta and vas deferens and inhibits platelet aggregation, effects largely attributable to agonism at alpha 2-adrenergic receptors. Unlike clonidine, it contracts rat gastric fundus and releases catecholamines from chromaffin cells, effects attributable to actions at I-receptors. Injected into the RVL, classic CDS alters arterial pressure, but the direction of change of pressure has differed between groups of investigators. However, in the absence of structure, it is possible that ligand binding and bioactivity may be attributable to different molecules. 3. Ir-CDS, also of unknown structure, is a material(s) that binds to antibodies raised against clonidine, PAC, or idazoxan. Ir-CDS, measured by radioimmunoassay, is unevenly distributed in brain with highest concentrations in the hypothalamus, midbrain, and dorsal medulla. It is contained in the gastric fundus, adrenal gland, heart, kidney, and serum in amounts substantially higher than found in brain. Ir-CDS may be elevated in the serum of some patients with hypertension and in the CSF of patients with structural brain disease. The concentration of ir-CDS and bioactivity on gastric fundus directly correlates, suggesting that it may share similarities with classic-CDS. However, until the structure of classic and ir-CDS is determined, the possibility that ligand binding and antibody recognition are properties of different molecules must be considered. 4. Agmatine (decarboxylated arginine) is the only endogenous molecule that, like CDS, binds to alpha 2- and I-receptors of both classes. It and its biosynthetic enzyme arginine decarboxylase are present in brain, and agmatine is widely distributed throughout the body. However, the distribution of agmatine and ir-CDS differs, whereas the biological actions of agmatine do not mimic those of classic CDS. Its presence raises the possibility of an alternative pathway for polyamine biosynthesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular and physiological properties of clonidine-displacing substance

An endogenous small molecular mass compound, termed clonidine-displacing substance (CDS), has been isolated and purified from bovine brain. The estimated level of CDS in bovine brain is 400-1,000 units/wet brain, with 1 unit of activity calculated to be approximately 1-2 ng. It is present in human serum, urine, and cerebrospinal fluid. The isolation procedure consists of initial aqueous and methanolic extractions followed by a series of HPLC chromatography steps (reverse phase and TSK sizing columns). The reverse-phase chromatography of CDS extracted under identical conditions from bovine brain and human serum show similar retention times. The final chromatography step gives a single active peak with a distinct ultraviolet spectrum, a single molecular peak m/z 587.8 +/- 2 in plasma desorption mass spectrometry (PDMS), and a unique pharmacological and physiological profile. Clonidine-displacing substance does not partition into organic solvents and it is ninhydrin and fluorescamine negative. All of these molecular properties clearly distinguish CDS from agmatine, an endogenous 130-dalton compound of far greater abundance which displays lower affinity for p-aminoclonidine-labeled sites in rat brain membranes. The ultraviolet spectrum of CDS consists of two aromatic peaks at 224 and 276 nm, whereas agmatine is an aliphatic substance with no ultraviolet absorbance. Like many antihypertensive drugs of the guanidine and imidazoline family of compounds, CDS recognizes alpha 2-adrenergic receptors, clonidine sites (IR-I1), and imidazoline sites (IR-I2). A good correlation exists between the affinities of various imidazoline/guanidine type ligands for IR-I2 in both human placenta and rat liver membranes which can be accurately determined because both tissues lack IR-I1 and alpha 2-adrenergic receptors. There is no correlation in the affinities of these ligands for IR-I2 of human-placental versus alpha 2-adrenergic receptors of human platelets. By uncovering the role of CDS in the central nervous system we will be able to understand the coupling of IRs to neurotransmission and, in turn, to changes in arterial pressure.

Pharmacokinetics of rilmenidine in patients with chronic renal insufficiency and in hemodialysis patients

The pharmacokinetics of rilmenidine (1 mg orally) was studied in 3 groups of patients with stable chronic renal insufficiency. This was an open, single-blind study following a single administration, and after 15 days of treatment. Group 1 included 11 patients with a creatinine clearance between 15 and 80 mL/min. Group 2 included 17 patients with a creatinine clearance < 15 mL/min. Group III included 10 hemodialysis patients. In patients with chronic renal failure, total plasma clearance and renal clearance of rilmenidine decreased; terminal half-life was 30-42 hours, which is clearly longer than previous values achieved in healthy volunteers. After repeated administration (1 mg daily in group 1, 1 mg every other day in group 2, 1 mg at the end of each dialysis session in group 3), the area under the curve was significantly increased, corresponding to drug accumulation. The steady state was reached after 6 days in patients in group 1 and after 8 days in patients in group 2. The pharmacokinetics of rilmenidine was linear since the terminal elimination half-life and renal clearance were not significantly different after single and repeated administration of rilmenidine. A positive correlation was found between rilmenidine total plasma clearance and creatinine clearance, and between rilmenidine renal clearance and creatinine clearance. Mean rilmenidine hemodialysance was 85 mL/min, that is, 26% of the rilmenidine renal clearance value achieved in healthy volunteers (330 mL/min). Thus, the following dosage schedule can be proposed. In patients whose creatinine clearance ranges between 15 and 80 mL/min, a 1 mg dose every day can be recommended.(ABSTRACT TRUNCATED AT 250 WORDS)

Historic aspects in the identification of the I1 receptor and the pharmacology of imidazolines

The central nervous system is involved in the control of arterial blood pressure. Stimulation of central alpha 2-adrenoceptors in the nucleus tractus solitarii (NTS) decreases sympathetic outflow, resulting in a fall in arterial blood pressure. One of the first antihypertensive substances with actions on the alpha 2-adrenoceptors of the NTS was alpha-methylnoradrenaline. Later on the imidazoline clonidine was developed for which numerous effects, mediated by alpha 2-adrenoceptors, in the CNS could be demonstrated. Since the centrally acting alpha 2-adrenoceptor agonists possess severe side effects, the development of more specific and selective centrally acting imidazolines resulted in the derivatives moxonidine and rilmenidine. The effects of the "second-generation imidazolines" could not be fully understood as alpha 2-adrenoceptor agonists. In the meantime, the rostral ventrolateral medulla (RVLM) has been identified as the site of action of the imidazolines and an I1-imidazoline binding site was characterized in this region. For the antihypertensive action of the imidazolines, agonism at the I1-imidazoline subtype seems to be responsible. In addition, an acid- and heat-stable endogenous substance, called clonidine displacing substance (CDS), was reported to bind at the putative I receptor. In 1992 a receptor protein for I receptors (70 kD) could be separated that is different from that of alpha 2-adrenoceptors. However, up to now we are still lacking the amino-acid sequence of the I receptor and its second messenger system.

A circulating substance cross-reacting with antiimidazoline antibodies. Detection in serum in relation to essential hypertension

It has been shown in various mammal species that clonidine, a well known centrally acting hypotensive agent, acts through the activation of imidazoline receptors (IRs) in the nucleus reticularis lateralis (NRL) of the brainstem. Specific binding sites sensitive to imidazolines and insensitive to catecholamines have been detected in rat and bovine, as well as human brains. An endogenous ligand, other than catecholamines, should exist for these IRs. Such a ligand could play a role in the pathophysiology of human essential hypertension. Therefore, we developed two RIAs with polyclonal and monoclonal anticlonidine antibodies. These antibodies presented specificity spectra similar to that of the IRs: they bound imidazolines and not catecholamines at all. These RIAs were used to detect imidazoline-like immunoreactivity in the human serum. Immunoreactive substance was measured in 26 normotensive subjects' sera, and specificity of interaction between antibodies and sera was verified. None of the known endogenous substances tested so far were able to interact with the two antibodies. Immunoreactivity in 32 essential hypertensive patients' sera proved higher in approximately 30% of cases. Values of immunoreactivity positively correlated with the mean arterial pressure values. This study demonstrates the existence of an "imidazoline-like" immunoreactive substance in the human serum with high levels in some hypertensive patients.

Detection and measurement of an endogenous clonidine-displacing substance in human cerebrospinal fluid

Clonidine-displacing substance (CDS) is a novel endogenous ligand for clonidine receptors previously detected in bovine brain and human serum. We examined for the first time whether CDS can be detected and measured in human cerebrospinal fluid (CSF). Using the [3H]clonidine displacement assay, we found that CDS could be identified and quantified in each of the CSF samples obtained from 81 patients with various neurological disorders. Mean level of CDS in CSF was 4.66 units/ml. Exceedingly high levels were observed in the CSF of patients with neoplastic meningitis (mean, 36.75 units/ml) and stroke (mean, 19.5 units/ml) (P < 0.0001). No correlation was found between CDS levels in CSF and age, gender, CSF protein or number of cells. CDS levels in CSF were higher than those in the serum (P < 0.01). We conclude that CDS is present and can be measured in human CSF. High CDS levels in CSF from patients with leptomeningeal metastases may serve as a tumor marker for malignant infiltration of the meninges. Additional studies in stroke patients will determine whether this endogenous ligand plays a role in the pathogenesis of cerebral ischemia.

Evidence for a bioactive clonidine-displacing substance in peripheral tissues and serum

Clonidine-displacing substance (CDS) from brain is biologically active in the kidney and stomach and on platelets. To determine whether CDS is contained in these and other peripheral tissues, homogenates of fresh brain, eight other organs and serum from rat were ultrafiltered (less than 10,000 mol. wt only), dried and extracted with methanol. Evaluation by radioimmunoassay (RIA) using antibodies to p-aminoclonidine showed that adrenal gland and gastric fundus (GF) contained significantly greater amounts of CDS-like radioimmunoactivity than brain; intermediate-to-low activity was present in heart, small intestine, serum, kidney and liver; lung and skeletal muscle values were near-background. RIA-positive extracts elicited well-correlated contractile activity in a GF smooth muscle bioassay; contractions persisted in the presence of antagonists of various transmitters and modulators, but were abolished by low concentrations of the calcium channel blocker verapamil. Serum levels of CDS were profoundly reduced following removal of the adrenal glands. We conclude that a CDS-like substance is present not only in brain as previously reported, but also in peripheral organs and in the circulation.

Production and characterization of an iminoimidazolidine specific monoclonal antibody using para-aminoclonidine as antigen

Para-aminoclonidine coupled to hemocyanin was used to produce mouse monoclonal antibodies directed against clonidine. The properties of one of these, called mFE7, secreted by a clone of hybrid myeloma, are described. This antibody displayed total crossreactivity with imidazolidines and no crossreactivity at all with catecholamines or other known naturally occurring substances tested. A liquid phase radioimmunoassay permitted the detection of immunoreactivity in human brain extracts. The mFE7 antibody could be useful for immunopurifying the endogenous ligand of Imidazolines Preferring Receptors (IPR) which are catecholamines insensitive.

The role of imidazoline receptors in blood pressure regulation

Using the ligands [3H] clonidine and [3H] idazoxan, nonadrenergic imidazoline preferring binding sites have been identified in a range of tissues from several species including man. These sites may represent a new family of receptors. An endogenous ligand and potential clonidine displacing substance has been identified. There is strong evidence for an involvement of the nonadrenergic imidazoline [3H] clonidine labelled sites in the nucleus reticularis lateralis in blood pressure regulation, and some evidence for a role in sodium regulation in the kidney for the [3H] idazoxan labelled sites. Some drugs which were previously thought to act via alpha 2-adrenoceptors, may mediate their effects in part via these imidazoline sites.

Human plasma-derived material with clonidine displacing substance (CDS)-like properties contracts the isolated rat aorta

1. The biological activity of a plasma-derived, clonidine displacing substance (CDS)-like material was tested on isolated rat aortic rings and compared to that of clonidine, an imidazoline with alpha 2-adrenoceptor agonist properties. 2. The CDS-like material was partially purified from expired human blood. This product inhibited in a dose-dependent manner the binding of [3H]-clonidine to rat brain membranes with a ki of 0.87 +/- 0.4 u ml-1, without affecting the binding of the alpha 1-antagonist, [3H]-WB4101. 3. When the CDS-like material (0.14-6 u ml-1) was applied to the bathing medium of isolated rat aortic rings, it caused dose-dependent contractions with an EC50 of 1.0 +/- 0.18 u ml-1. Clonidine also dose-dependently contracted rat aortic rings (EC50, 1.1 +/- 0.24 x 10(-7) M). The maximal tension developed in response to clonidine, however, was higher (1.37 +/- 0.15 g) compared to that developed in response to the CDS-like material (0.92 +/- 0.12 g). 4. Contractions induced by both CDS-like material and clonidine were antagonized by 5 x 10(-7) M rauwolscine, an alpha 2-adrenoceptor antagonist. Prazosin, an alpha 1-adrenoceptor antagonist, at 10(-8) M, greatly reduced contractions caused by clonidine while leaving those caused by CDS-like material unaffected. 5. The CDS-like material failed to alter the tension of intact or endothelium-denuded rat aortic rings which had been precontracted with methoxamine. Clonidine on the other hand, caused dose-dependent relaxations in intact, though not in denuded, precontracted rat aortic rings.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for clonidine presynaptically modulating amino acid release in the rostral ventral medulla: role in hypertension

Reports suggested that the predominant site of action for the antihypertensive effects of clonidine is the rostral ventrolateral medulla (RVL), the presumed tonic vasomotor center. This study examined whether clonidine directly interacts with nerve terminal alpha 2-adrenergic receptors in the RVL to inhibit the release of sympathoexcitatory transmitters like glutamate (Glu) and aspartate (Asp), and/or facilitate the release of sympathoinhibitory transmitters like gamma-aminobutyric acid (GABA). Release of GABA and Glu was measured from synaptosomes prepared from the rostral ventral medulla of spontaneously hypertensive rats (SHR), a genetic model of hypertension, and normotensive Wistar-Kyoto rats (WKY). Quantification of neurotransmitter release was performed by high-performance liquid chromatography. Depolarization with 35 mM K+ significantly increased by 58-110% the release of GABA, Glu and Asp; however, no strain differences were observed. In contrast, spontaneous release of GABA and Asp was significantly lower in SHR than that of WKY (-36 and -41%, respectively); this effect was not observed for Glu. Clonidine (1 and 10 microM) enhanced the spontaneous release of GABA (+44%), Asp (+50%) and Glu (+70%) in SHR, but not WKY; this effect was prevented by yohimbine (1 microM). These data, together with previous findings, support the presence of facilitory alpha 2-adrenergic receptors on nerve terminals of GABAergic, glutamatergic and aspartatergic neurons in the rostral ventral medulla. These findings also suggest the existence of another inhibitory transmitter that may mediate the actions of clonidine to decrease sympathetic outflow from the RVL.

Imidazoline receptors in rat liver cells: a novel receptor or a subtype of alpha 2-adrenoceptors?

An imidazoline/guanidine receptor has been characterized in rat liver cells. Binding of [3H]idazoxan, a selective benzodioxan antagonist, to imidazoline receptor on intact fresh hepatocytes (Bmax = 801 +/- 23 fmol/mg protein, Kd = 11 +/- 0.8 nM) and to liver membranes (Bmax = 400 +/- 38 fmol/mg protein, Kd = 10 +/- 2 nM) was saturable at 4 degrees C within 3.5 h and at 30 degrees C within 30 min, respectively. Rat lung membranes had more imidazoline sites (Bmax = 578 +/- 30 fmol/mg protein, Kd = 14 +/- 1.4 nM) than alpha 2-adrenoceptors (Bmax = 175.0 +/- 20.0 fmol/mg protein, Kd = 4.8 +/- 2.0 nM). We also screened other tissues for imidazoline sites; the ratio of adrenoceptors to total sites labeled with [3H]idazoxan displaced by cirazoline was lower in rat lung compared to rat brain and human platelets. The imidazoline receptor has common pharmacological properties with alpha 2-adrenoceptors, although it is not a subtype of the adrenoceptor, since it bound neither the endogenous agonists norepinephrine and epinephrine, nor the selective alpha 2-antagonists yohimbine and phentolamine. All guanidine type alpha 2-adrenoceptor drugs (e.g. guanbenz, guanoxan) and imidazolines (e.g., UK-14,304, naphazoline) competed with high affinity for the liver imidazoline receptor. The lack of effect by Gpp(NH)p, a non-hydrolysable GTP analogue, on the affinity of guanidine- and imidazoline-type ligands for liver imidazoline receptors suggests that the mode of action of these drugs at imidazoline receptors is different than at conventional alpha 2-adrenoceptors. Ionic changes were considered as a possible mechanism underlying the alpha 2-adrenoceptor effects in various cells. Opening of K+ channels by alpha 2-adrenoceptors agonists is a pathway which might be shared by imidazoline-type agonists at imidazoline sites. Indeed, 4-aminopyridine, a K+ channel blocker, inhibited the specific binding of [3H]idazoxan to liver cells with an IC50 of 0.34 +/- 0.07 mM a concentration which is effective in blocking K+ channels in neuronal cells. Similarly, Cs+ and NH4+ effectively interfered with [3H]idazoxan binding, suggesting a possible coupling of imidazoline sites to K+ gating. The endogenous ligand clonidine-displacing substance (CDS), which was isolated from bovine brain and which binds to alpha 2-adrenoceptors in brain membranes and human platelets competed with idazoxan at rat liver imidazoline receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Anti-imipramine antibodies recognize endogenous serotonin uptake and imipramine binding inhibitors

Calf brain and human platelet extracts purified by Bio-Gel P2 column chromatography contained substances that inhibited serotonin uptake and 3H-imipramine binding. Some of these endogenous substances were also recognized by rabbit antibodies produced against imipramine. The data suggest the possible existence of endogenous serotonin uptake modulators, which may possess a partial molecular structure similar to that identified by the antibodies.

Evidence for the involvement of imidazoline receptors in the central hypotensive effect of rilmenidine in the rabbit

1. Rilmenidine has recently been introduced as a new centrally-acting antihypertensive agent. We examined its cardiovascular effects after intracerebral injection to anaesthetized rabbits. Cumulative doses of rilmenidine injected intracisternally (1 to 300 micrograms kg-1) led to dose-dependent decreases in arterial blood pressure and heart rate. The effective doses of rilmenidine were lower when injected centrally than when injected intravenously. 2. Pretreatment with the same dose of yohimbine or idazoxan shifted the rilmenidine dose-response curves for its hypotensive and bradycardic effects to the right. Idazoxan, which has an imidazoline structure, proved to be a more active antagonist than yohimbine of rilmenidine centrally-mediated cardiovascular effects. 3. The dose-response curve for the central hypotensive effect of rilmenidine was also shifted to the right after pretreatment with a bovine brain extract. This extract contains the endogenous ligand of the imidazoline-preferring receptors which is not a catecholamine. 4. Rilmenidine, like clonidine, proved to be active when micro-injected into the rabbit nucleus reticularis lateralis region. 5. In conclusion, rilmenidine exhibited in the rabbit a central hypotensive effect which originated in the same area as where clonidine acts. Specific imidazoline-preferring receptors appear to be involved in this hypotensive effect.

Effect of imidazolines on Na+ transport and intracellular pH in renal proximal tubule cells

Recently, we characterized an imidazoline-guanidinium receptive site (IGRS) in the renal proximal tubule of rabbit kidney. Although recognized by a series of imidazoline and guanidinium alpha-2 adrenergic compounds, IGRS is insensitive to catecholamines and can be physically separated from alpha-2 adrenergic receptors after solubilization. In the present study, we investigated the effect of imidazoline derivatives on 22Na+ uptake and intracellular pH in isolated cells from rabbit renal proximal tubule. After 5 min of preincubation, idazoxan inhibited the total 22Na+ influx (-30%) in a dose-dependent manner, with a maximum effect at 10(-5) M. The effect of idazoxan was not competitive as shown by the decrease of the maximal velocity of 22Na+ entry (control: 3.80 +/- 0.42; idazoxan 10(-5) M: 3.23 +/- 0.33 nmol/30 s per mg protein, P less than 0.01). A series of imidazoline derivatives inhibited 22Na+ entry with an order of potency similar to that previously found for inhibition of [3H]idazoxan binding to IGRS (cirazoline greater than idazoxan greater than UK 14304 greater than rilmenidine much greater than cimetidine). The inhibition of 22Na+ uptake by these compounds does not appear to be related to interaction with alpha-adrenergic receptors since it was observed in the presence of saturating concentrations of the adrenergic antagonists rauwolscine (alpha-2) or prazosin (alpha-1). When tested on the regulation of intracellular pH by fluorimetric techniques, 10(-5) M cirazoline or idazoxan inhibited by 20% the velocity of the sodium-dependent H+ efflux in acidified cells (P less than 0.02). The concomitant inhibition of 22Na+ entry and of cell realkalinization suggests that imidazoline derivatives inhibit Na+/H(+)-exchanger. This effect could be mediated via the renal IGRS and intracellular second messengers that are not yet known.