Characteristics of the alpha2/beta-adrenoceptor-coupled adenylate cyclase system and their relationship with adrenergic responsiveness in hamster fat cells from different anatomical sites

Various studies have shown that the lipolytic response of white adipocytes to catecholamines was dependent on the anatomical origin of these cells. To provide a biological explanation for this phenomenon, we compared hamster white adipocytes, from femoral subcutaneous and epididymal fat, for their lipolytic activities, cAMP responses and adrenoceptor-coupled adenylate cyclase system. Basal and maximal lipolytic responses to the beta-adrenergic (isoproterenol) and the mixed alpha 2/beta-adrenergic (epinephrine) agonists were lower in femoral subcutaneous cells than in epididymal cells, but the alpha 2-adrenergic antilipolytic response to 5-bromo-6-(2-imidazolin-2-ylamino)quinoxaline bi-tartate (UK14304) was slightly greater in femoral subcutaneous fat cells than in epididymal fat cells. Identical results were observed for cAMP responses, except for the alpha 2-adrenergic inhibitory response which was identical in both fat deposits. Adrenoceptors studies revealed higher density of inhibitory alpha 2-adrenoceptors 2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline ([3H]RX821002-binding sites) in femoral subcutaneous fat cells than in epididymal fat cells, but identical density of stimulatory beta-adrenoceptors (125I-cyanopindolol-binding sites) and similar subdivision into beta-adrenoceptor subtypes in both adipose deposits. Finally, the level of the alpha-subunits of the stimulatory and inhibitors guanine-nucleotide-binding regulatory proteins, as well as the adenylate cyclase catalytic activity were 40-50% lower in femoral subcutaneous fat cell membranes than in epididymal fat cell membranes. These results suggest that the differences in cAMP and lipolytic responses to catecholamines between epididymal and femoral subcutaneous adipocytes result at least in part from site-related differences in the adenylate cyclase system rather than in the adrenoceptor status.

Characterization of imidazoline-guanidinium receptive sites in renal medulla from human kidney

Previous studies showed that alpha 2-adrenergic receptors and imidazoline-guanidinium receptive sites (IGRS) are colocalized in rabbit and human renal proximal tubule. In the present study we investigated the localization of these two binding sites in the renal medulla from human kidney. Binding studies performed with [3H]idazoxan (IGRS ligand) and [3H]rauwolscine (alpha 2-adrenergic ligand) showed that, in membrane preparations from renal medulla, the density of IGRS was 3.6-fold higher than that of alpha 2-adrenergic receptors (134 +/- 7 v 37 +/- 5 fmol/mg protein, respectively). These data indicate that imidazoline, guanidinium, and oxazoline derivatives could induce their therapeutic effects through the interaction with IGRS and/or alpha 2-adrenergic receptors located not only in the renal proximal tubule but also in other segments of the nephron.

Imidazoline binding sites in fat cells. Localization and pharmacologic differentiation from alpha 2-adrenergic receptors

Studies were carried out to define the cellular distribution and pharmacologic properties of the nonadrenergic [3H]idazoxan binding sites in white fat cells from various species (the term "nonadrenergic [3H]idazoxan binding sites" [NAIBS] has been retained pending a final decision about their name). NAIBS having quite similar binding properties were found in human, rat, hamster, rabbit, and dog fat cells. NAIBS are located both in plasma membranes (25 to 35%) and in intracellular membranes corresponding to the crude mitochondrial fraction of adipose tissue (65 to 75% of the total number of NAIBS found in the particulate fraction), while alpha 2-adrenergic receptors are exclusively located on the plasma membrane. NAIBS have no affinity for catecholamines but have a noticeable affinity for some imidazolinic and guanidinic derivatives. NAIBS are different from the "imidazoline receptors" which bind [3H]para-aminoclonidine in central nervous system areas. A comparative structure affinity study was performed to delineate the respective affinities of various imidazoline derivatives towards NAIBS and alpha 2-adrenergic receptors and to try to find selective ligands for both sites. Binding properties of the two sites were clearly different. Moreover, chronic administration of idazoxan and RX821002 to adult rabbits (4 mg/kg, subcutaneously for 7 days), resulted in an up-regulation of alpha 2-adrenoceptors in fat cells while the number of NAIBS was unaffected by the treatment. There is no clear demonstration of an interaction between NAIBS and G-proteins, adenylate cyclase, or lipolytic function in fat cells. Further studies are required to define their putative role in fat cells.

Regional Distribution of ?2A-and ?2B-Adrenoceptor Subtypes in Postmortem Human Brain

The newly available and highly selective radiolabeled antagonist [3H]RX 821002 was used to examine the distribution of alpha 2 adrenoceptors in human brain. High densities of alpha 2 adrenoceptors were found in the hippocampus, frontal cortex, thalamus, amygdala, pons, and medulla oblongata. Intermediate densities were observed in the striatum (nucleus accumbens, nucleus caudatus, and putamen), globus pallidus, and substantia nigra. The KD values for [3H]RX 821002 were similar in all regions (ranging from 2.8 to 7.5 nM). On the basis of their different affinities for prazosin and oxymetazoline, the alpha 2 adrenoceptors have been divided into alpha 2A and alpha 2B subtypes. To examine the alpha 2A/alpha 2B-adrenoceptor ratio in the different brain regions, we performed oxymetazoline and prazosin/[3H]RX 821002 competition binding experiments. In frontal cortex membranes, the competition curves with prazosin were steep, indicating a single class of binding sites, whereas the competition curves with oxymetazoline were shallow and fitted by computer best to a two-site model. However, in the presence of GTP, the high-affinity sites for oxymetazoline were partially converted into low-affinity sites, indicating that this agonist interacts with high- and low-affinity states of the alpha 2 adrenoceptors. This implies that oxymetazoline is not very suitable for discriminating the alpha 2A- and alpha 2B-receptor subtypes in radioligand binding studies. Therefore, prazosin/[3H]RX 821002 competition binding experiments were used to investigate the distribution of the alpha 2-adrenoceptor subtypes in human brain. The alpha 2A-receptor subtype was detected in all brain regions examined. In contrast, alpha 2B receptors were only observed in striatum and globus pallidus.

Contribution of an α1-adrenergic receptor subtype to the expression of the “ventral tegmental area syndrome”

Bilateral electrolytic lesions of the rat ventral tegmental area, a mesencephalic structure containing the cell bodies of ascending dopaminergic neurons, induce a behavioural syndrome characterized by a permanent locomotor hyperactivity. Acute intraperitoneal injections of prazosin, an alpha 1-adrenergic receptor antagonist, at a dose (0.5 mg/kg) which does not affect locomotor activities of control animals, abolished locomotor hyperactivities of lesioned rats. Antagonists of other monoaminergic receptors (propranolol, ritanserin, yohimbine), and also another antagonist of alpha 1-adrenergic receptors, 2-(2',6'-dimenthoxyphenoxyethyl)-aminomethyl-1,4-benzodioxan (WB4101) were ineffective. Comparisons of autoradiograms of brain slices incubated in the presence of 1 nM [3H]prazosin or 10 nM [3H]WB4101 indicated clear topographical differences. [3H]Prazosin labelling is present in the septum and in layer III of the cerebral cortex but absent in the striatum. [3H]WB 4101 labelling is diffuse in the superficial layers of the cerebral cortex and present in the striatum. In addition, intraperitoneal injection of WB4101 displaces, only weakly, [3H]prazosin binding in layer III of the cerebral cortex (-18%) while it decreases by 50% [3H]prazosin binding in the more superficial cortical layers. These observations strongly suggest that the binding site labelled by [3H]prazosin is different from alpha 1A- and alpha 1B-adrenergic receptor subtypes labelled by [3H]WB4101. Finally, it is proposed that the prazosin-induced blockade of the locomotor hyperactivity exhibited by ventral tegmental area lesioned animals is linked to the previously demonstrated regulatory role of noradrenergic neurons on cortical dopamine transmission.

Evidence that Drug Binding to Non-Adrenergic [3H]-Idazoxan Binding Sites (I-Receptors) Occurs to Interacting or Interconvertible Affinity Forms of the Receptor

Characterization of [3H]idazoxan binding to guinea pig kidney membranes showed that approximately 90% bound to nonadrenergic I-receptors and approximately 10% to alpha 2-adrenoceptors. I-Receptors could be studied separately by including 3 microM rauwolscine to the assay. During these conditions 22 different compounds out of 29 (including imidazoline and guanidinium compounds) generated biphasic or shallow competion curves (Hill coefficients down to 0.57), for which computer modelling suggested that drugs bound to two sites with different affinities. However, the proportion of sites varied considerably depending on which drug was used as competitor; the variation being from approximately 50/50% to approximately 8/92% and analysis of variance clearly indicated that the variation of proportions of sites could be attributed to an effect induced by the drugs which indicated that the sites were dynamically formed or modulated by the presence of the drug. A few drugs (guanabenz, (-)-medetomidine, phentolamine, clemastin, prazosin and idazoxan itself) yielded steep uniphasic curves (Hill coefficients near unity) which were resolved only into one site fits. One drug (detomidine) yielded supersteep competition curves (Hill coefficient 1.29), the data being reminiscent of positive cooperativity. In another set of experiments attempts were made to block one of the affinity forms of the I-receptor with histamine, a compound which had grossly different affinities for the two I-receptor sites, and competition curves were then obtained using other drugs which also seemed to distinguish between the two sites. Computer analysis of experimentally obtained as well as simulate,d computer generated, competition curves clearly showed that the kidney I-receptors did not exist in two independent non-interacting forms. Instead the data suggested that the I-receptor was composed of interacting or interconverting receptor entities with different affinities for drugs. Monovalent cations such as Cs+ or NH4+ were found to be powerful inhibitors of [3H]idazoxan binding to the kidney I-receptors, the ions decreasing both the apparent Bmax and affinity of the ligand for the receptor. The cations also decreased the affinities of UK-14,304 for both the high and low affinity forms of the I-receptor, but the apparent proportions of sites were not at all affected by the ions. Moreover, ligand binding to kidney I-receptor were not at all affected by non-hydrolyzable guanine nucleotides. It is suggested that the binding data reflects functional properties of the I-receptor protein.

Characterization of brain imidazoline receptors in normotensive and hypertensive rats: differential regulation by chronic imidazoline drug treatment

The binding of [3H]idazoxan in the presence of l-epinephrine was used to characterize and quantitate imidazoline receptors in the brain of spontaneously hypertensive (SHR), normotensive Wistar-Kyoto (WKY) and Sprague-Dawley (SD) rats before and after chronic imidazoline drug treatment. In the cerebral cortex of WKY and SHR rats, the rank order of potency of imidazoli(di)ne drugs (cirazoline greater than idazoxan greater than naphazoline greater than clonidine much greater than RX821002) competing with [3H]idazoxan showed the specificity for an imidazoline receptor which also appeared heterogeneous in nature. In SHR rats, the density of imidazoline receptors (hypothalamus greater than medulla oblongata greater than cerebral cortex) and proportion of high- and low-affinity sites for the receptor were not different from those in WKY and SD rats, suggesting that the receptor itself is not altered in hypertension. However, chronic treatment with idazoxan and cirazoline (10 and 1 mg/kg, i.p., every 12 h for 7 days) consistently increased (about 35%) the density of imidazoline receptors in the brain of WKY and SD, but not in SHR rats. A similar treatment with RX821002, the 2-methoxy analog of idazoxan, which is a highly selective alpha-2 adrenoceptor antagonist, did not increase the density of brain imidazoline receptors. Moreover, the up-regulation of these receptors induced by cirazoline was still present after alkylation of the alpha-2 adrenoceptors with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline. The lack of regulation by idazoxan and cirazoline of the density of imidazoline receptors in the brain of SHR rats suggests the existence of a relevant abnormality in the adaptive process of these receptors in this genetic model of hypertension.

Differential localization of α2-adrenergic receptor subtypes in brain

The pharmacological identification and characterization of subtypes of alpha 2-adrenergic receptors have been confirmed by molecular biological investigations. Using receptor autoradiographic techniques, it has been possible to show regions of the brain where alpha 2 agonist binding ([3H]para-aminoclonidine) is preferentially labeling the presumed guaninenucleotide-sensitive, high-affinity conformations of the alpha 2 receptor. Careful examination of autoradiograms generated using the tritiated antagonists yohimbine, idazoxan, and rauwolscine also indicates some disparity in the regions occupied by these radiolabeled ligands. Inhibition of [3H]rauwolscine binding with the subtype selective compounds, ARC-239, or oxymetazoline demonstrates that there are discrete regions of the brain where one receptor subtype predominates over the other. These studies indicate that previous investigations utilizing the agonist para-aminoclonidine as the ligand for obtaining labeling of alpha 2 receptors have overlooked some regions of binding due to the subtype selectivity of this ligand. A more complete localization of alpha 2-adrenergic receptors can be obtained using the tritiated antagonist rauwolscine, and the differential distribution of at least two subtypes of the alpha 2 receptor can be obtained by selective inhibition of this binding.

The insulin-secreting cell line, RINm5F, expresses an alpha-2D adrenoceptor and nonadrenergic idazoxan-binding sites

The pharmacological properties of alpha-2 adrenoceptors and the existence of nonadrenergic idazoxan-binding sites (NAIBS) were investigated in the insulin-secreting cell-line, RINm5F, using [3H]RX821002 and [3H]idazoxan. Analysis of [3H]RX821002 saturation isotherms revealed the presence of a single class of binding sites (Bmax = 47.5 +/- 3.5 fmol/mg protein) having high affinity (Kd = 1.26 +/- 0.18 nM). Inhibition of [3H]RX821002 binding by adrenergic compounds showed that the labeled sites displayed the properties expected for an alpha-2 adrenoceptor. Based on competition data with drugs having alpha-2 adrenoceptor subtype selectivity, the receptor from RINm5F is neither an alpha-2B nor an alpha-2C. It resembles the alpha-2A, but deviates from this subtype because of a weak affinity for yohimbine and rauwolscine. In this respect, RINm5F alpha-2 adrenoceptor is identical to the receptor previously described in rat intestinal mucosa and corresponds to a fourth subtype: alpha-2D. Agonist inhibition curves were better fitted by a two-site model and indicated that about half of the receptor population was under a high-affinity state corresponding to G protein-coupled receptors. [32P]ADP-ribosylation with pertussis toxin and immunodetection with specific antibodies permitted the identification of three distinct G proteins: Gi2, Gi3 and G0. Binding experiments with [3H]idazoxan showed that this imidazoline labeled two types of sites corresponding to alpha-2 adrenoceptors and NAIBS. Analysis of saturation isotherms under binding conditions allowing to discriminate between the two site populations indicated that the density of NAIBS (44 +/- 2 fmol/mg protein) was fairly identical to that of alpha-2 adrenoceptors. The pharmacological properties of NAIBS, as assessed by determining the relative affinity of imidazolinic and nonimidazolinic compounds, reasonably matched that reported in other tissues. Taken together, these data make the RINm5F cell-line 1) the first model in permanent culture known as expressing an alpha-2 adrenoceptor of the alpha-2D subtype; 2) a good system for studying in vitro the respective role of alpha-2 adrenoceptors and NAIBS in the regulation of insulin secretion by beta cells.

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.

Long-term treatment with different calcium- and calmodulin-antagonists induces changes in rat brain alpha-adrenoceptors

1. The binding characteristics (Bmax and Kd) of the alpha-adrenoceptor radioligand [3H] WB4101 in crude membrane fraction (fraction P2) from cerebral cortex were studied after 13-day oral treatment of male Wistar rats with the Ca(2+)-antagonists nifedipine (20 mg/kg), verapamil (50 mg/kg), flunarizine (10 mg/kg) and with the calmodulin-antagonist trifluoperazine (TFP) (3 mg/kg). 2. A significant reduction of the binding sites (Bmax) for [3H] WB4101 was established after the three Ca(2+)-antagonists as well as after TFP treatment. 3. Different changes in the affinity constant (Kd) of brain adrenoceptors were observed depending on the type of the Ca2+ or CaM-antagonist used: nifedipine did not change the Kd value, verapamil and TFP decreased whereas flunarizine increased the Kd value. 4. Relationships between Ca ions and alpha-adrenoceptor functions are suggested.

Autoradiographic distribution of alpha 2 adrenoceptors, NAIBS, and 5-HT1A receptors in human brain using [3H]idazoxan and [3H]rauwolscine

The regional distribution of [3H]idazoxan and [3H]rauwolscine was studied autoradiographically in human brain. [3H]Idazoxan binds with high affinity to alpha 2 adrenoceptors as well as to non-adrenergic sites (NAIBS). [3H]Rauwolscine, besides binding to alpha 2 adrenoceptors, also binds to 5-HT1A receptors. Both radioligands labelled the same population of alpha 2 adrenoceptors, defined as the epinephrine-displaceable binding component. The highest densities of alpha 2 adrenoceptors occur in the leptomeninges, cerebral cortex and claustrum; lower densities were visualised in the basal ganglia, thalamus, pons, substantia nigra, cerebellum and medulla oblongata; no alpha 2 adrenoceptors were detected in amygdala and nucleus ruber. NAIBS were present in all the examined brain areas, with the highest densities found in the basal ganglia and substantia nigra. The finding that certain brain regions, such as the amygdala, contained NAIBS but no detectable alpha 2 adrenoceptors, suggests that the binding sites are independent from each other. The regional distribution of 5-HT1A receptors labelled by [3H]rauwolscine is in agreement with previous studies using [3H]8-OH-DPAT.

Behavioral and receptor binding analysis of the alpha 2-adrenergic agonist, 5-bromo-6 [2-imidazoline-2-yl amino] quinoxaline (UK-14304): evidence for cognitive enhancement at an alpha 2-adrenoceptor subtype

The ability of the alpha 2-agonists clonidine, B-HT920 (6-allyl-2-amino-5,6,7,8-tetrohydro-4H-thiazolo-[4,5-d]-azepine) and guanfacine to improve memory in aged monkeys has been related to their affinity to bind at a proposed rauwolscine-insensitive (Ri) subtype of alpha 2-adrenergic receptor, while their hypotensive and sedating effects have been related to affinity at a rauwolscine-sensitive site (Rs) (Arnsten et al., 1988). The present study examined the alpha 2-agonist UK-14304 (5-bromo-6 [2-imidazoline-2-yl amino] quinoxaline) for its binding characteristics in tissue from the brain of the rat and for its behavioral effects in aged monkeys. The drug UK-14304 was found to have slightly higher affinity for the Ri than the Rs site (Ki values of 138 and 245 nM, respectively), but was not as selective as the alpha 2-agonist guanfacine (Ki values of 23 and 340 nM, respectively). Consistent with this binding profile, very small doses of UK-14304 (0.00017-0.17 micrograms/kg) produced a reliable but modest improvement in memory in the aged monkeys (average improvement of 16.7% +/- 2.6% following an optimal dose). No hypotensive or sedating side effects were observed at these small doses. However, hypotension and sedation emerged rapidly when the dose was raised above 1.7 micrograms/kg and at the largest doses tested (50.0-100.0 micrograms/kg), hypotension was severe (systolic pressure below 70 mm Hg) and the animals were too sedated to complete cognitive testing. The separation between doses that improved memory and those that produced hypotension and sedation was not as great for UK-14304 as it was for guanfacine, consistent with the greater selectivity of guanfacine for the Ri site. These results offer a fourth example whereby the ability of an alpha 2-agonist to improve cognitive function, without side effects, could be related to the relative affinities for the Ri and Rs sites.

Clonidine-displacing substance from bovine brain binds to imidazoline receptors and releases catecholamines in adrenal chromaffin cells

Identification of nonadrenergic binding sites for clonidine and related imidazolines in brain and peripheral tissues and partial purification of an endogenous ligand for these sites have led to the postulation of a novel transmitter/receptor system. The receptors seem to be present in adrenal medulla and to regulate chromaffin cell function. The present study was undertaken to test the ability of the putative endogenous ligand clonidine-displacing substance (CDS) to displace [3H]idazoxan binding to adrenal chromaffin cell membranes and to release catecholamines from cultured chromaffin cells. CDS potently displaces [3H]idazoxan binding to chromaffin cell membranes, with an IC50 of 5 units. The displacement of [3H]idazoxan binding by CDS was not modified by guanosine 5'-(beta, gamma-imido)triphosphate, suggesting that the imidazoline binding sites may not be GTP-binding protein-coupled receptors. CDS produced a large release of catecholamines from chromaffin cells, and the release was partially blocked by cobalt, a calcium channel blocker. The calcium-dependent release reached a plateau above 5 units of CDS, with a maximal response at 15 min. It is concluded that endogenous CDS, prepared from brain, regulates the secretion of catecholamines from adrenal chromaffin cells, probably by activating imidazole receptors.

Electroconvulsive shock increases alpha 1b- but not alpha 1a-adrenoceptor binding sites in rat cerebral cortex

Repeated administration of electroconvulsive shock (ECS) increases [3H]prazosin binding to alpha 1-adrenoceptors in rat cerebral cortex. In contrast, [3H]WB4101 binding in cortex has been reported to be unchanged after ECS. [3H]Prazosin labels two alpha 1-adrenoceptor subtypes, termed alpha 1a and alpha 1b, whereas [3H]WB4101 labels the alpha 1a subtype preferentially. The purpose of this study was to determine whether ECS increases one or both alpha 1-adrenoceptor subtypes in rat cerebral cortex. We found that treatment of rats with ECS once daily for 10-12 days increased [3H]prazosin binding in cortex by about 25% but did not significantly alter [3H]WB4101 binding to alpha 1-adrenoceptors. Measurement of alpha 1a and alpha 1b receptors by competition analysis of the selective alpha 1a antagonist 5-methylurapidil against [3H]prazosin and measurement of [3H]prazosin binding in homogenates preincubated with chlorethylclonidine, which alkylates alpha 1b binding sites, also indicated that the ECS-induced increase in alpha 1-adrenoceptors is confined to the alpha 1b subtype. In contrast to its effect on [3H]prazosin binding, ECS did not increase phosphoinositide hydrolysis as measured by [3H]inositol 1-phosphate accumulation in slices of rat cerebral cortex stimulated by either norepinephrine or phenylephrine. The failure of ECS to increase [3H]inositol 1-phosphate accumulation stimulated by phenylephrine, which is a partial agonist for this response, suggests that spare receptors do not account for the apparent absence of effect of ECS on alpha 1-adrenoceptor-mediated phosphoinositide hydrolysis.

Repeated idazoxan increases brain imidazoline receptors in normotensive (WKY) but not in hypertensive (SHR) rats

The specific binding of [3H]idazoxan in the presence of 10(-6) M (-)-adrenaline was used to evaluate the density of imidazoline receptors in the brain of spontaneously hypertensive (SHR) rats and sex- and age-matched normotensive Wistar-Kyoto (WKY) rats. In SHR rats the density of imidazoline receptors (cerebral cortex, hypothalamus, and medulla oblongata) was not different from that in normotensive (WKY) rats. However, repeated treatment with idazoxan consistently increased (23-80%) the density of imidazoline receptors in the various brain regions of WKY rats but not in SHR rats. In normotensive Sprague-Dawley rats, repeated treatment with the imidazoline drugs idazoxan and cirazoline also increased (33-37%) the density of imidazoline receptors in the cerebral cortex. The lack of regulation by idazoxan of the density of imidazoline receptors in the brain of SHR rats might reflect the existence of a relevant abnormality of these receptors in this genetic model of hypertension.

Interaction of subtype-selective antagonists with alpha 1-adrenergic receptor binding sites in rat tissues

(+)-Niguldipine inhibited specific 125I-BE 2254 binding more potently in membrane preparations from rat tissues enriched in the alpha 1A subtype (hippocampus and vas deferens) than those with the alpha 1B subtype (liver and spleen). Inhibition curves for (+)-niguldipine were better fit by a two-site model in most tissues, although Kl values for each site varied markedly between tissues. The potency of this lipophilic drug was highly dependent on tissue concentration, probably accounting for most of this variability. Pretreatment of membranes with chloroethylclonidine (CEC) to inactivate the alpha 1B subtype did not completely eliminate the low affinity sites for (+)-niguldipine, particularly in heart. Saturation analysis showed that (+)-niguldipine competitively inhibited both alpha 1A and alpha 1B subtypes. However, substantial non-competitive inhibition was also observed in several tissues. Analysis of inhibition curves for 5-methylurapidil gave similar proportions of alpha 1A and alpha 1B receptor sites as were calculated for (+)-niguldipine in various tissues. Although (+)-niguldipine and 5-methylurapidil revealed variable proportions of low affinity sites in CEC-pretreated hippocampus and heart, this was not observed with inhibition curves for WB 4101 and phentolamine. These results are generally consistent with the previously defined alpha 1A and alpha 1B subtypes. 5-Methylurapidil currently appears to be the best antagonist for discriminating these subtypes; (+)-niguldipine shows similar selectivity but is complicated by a high lipophilicity. However, the persistence of low affinity sites for 5-methylurapidil and (+)-niguldipine after CEC pretreatment and the noncompetitive effects of (+)-niguldipine in some tissues raise the possibility of an additional subtype(s) of alpha 1-adrenergic receptors in rat tissues.

Degradation of dioxane, tetrahydrofuran and other cyclic ethers by an environmental Rhodococcus strain

By enrichment and isolation techniques bacterial strains with the capacity to grow on aliphatic cyclic ethers (dioxane, tetrahydrofuran, 1,3-dioxolane) have been isolated. Six strains that degrade tetrahydrofuran were classified as belonging to the genus Rhodococcus. One of two strains that degrade dioxane instead of or in combination with tetrahydrofuran was further characterized and a hypothetical catabolic pathway comprising an initial 2-hydroxylation and several oxidation steps is postulated.

Identification of an imidazoline-guanidinium receptive site in mitochondria from rabbit cerebral cortex

In the present report, we used [3H]idazoxan to characterize imidazoline-guanidinium receptive sites (IGRS) in mitochondria from rabbit cerebral cortex. When compared to the starting homogenate, [3H]idazoxan binding was higher (1.161 +/- 0.159 vs. 0.102 +/- 0.024 pmol/mg of protein) in a membrane fraction 6-fold enriched in cytochrome oxidase activity, a specific marker for mitochondria. In addition, the enrichment of [3H]idazoxan binding sites positively correlates with cytochrome oxidase activity in different membrane preparations (r = 0.977, P less than 0.001). In competition studies, [3H]idazoxan binding was completely inhibited by imidazoline and guanidinium derivatives but not affected by 10 microM epinephrine. Taken together, these data show the localization of IGRS in the mitochondria from rabbit cerebral cortex.

The effects of idazoxan and other alpha 2-adrenoceptor antagonists on food and water intake in the rat

1. Idazoxan (1, 3, 10 mg kg-1, i.p.) produced a significant increase in food and water intake in freely feeding rats during the daylight phase. 2. The more selective and specific alpha 2-adrenoceptor antagonists, RX811059 (0.3, 1, 3 mg kg-1, i.p.) and RX821002 (0.3, 1, 3 mg kg-1, i.p.), did not produce hyperphagia in rats, however, the highest dose produced a significant increase in water intake. 3. The peripherally acting alpha 2-adrenoceptor antagonist, L-659,066 (1, 3, 10 mg kg-1, i.p.), did not affect food intake in the 4 h following injection, but the highest dose (10 mg kg-1), produced a large increase in water intake. 4. These results indicate that alpha 2-adrenoceptor antagonists may increase water intake by a peripherally mediated mechanism. 5. The lack of effect RX811059 and RX821002 on food intake contrasts with the large dose-related increases induced by idazoxan and suggests that the hyperphagic effects of idazoxan are not due to alpha 2-adrenoceptor blockade but may instead reflect its affinity for a non-adrenoceptor site, a property not shared by the other alpha 2-antagonists.

Impairment of glucostatic, adrenergic and serotoninergic feeding parallels the lack of glucoprivic signals in the golden hamster

The administration of 2-deoxyglucose (2-DG) to several animal species, including humans, results in reduction of cellular glucose availability which evokes sympathoadrenal activation, hyperglycemia and stimulation of food intake. We have investigated the effects in the hamster of several drugs which are known to stimulate food intake and induce hyperglycemic response in other species. Golden hamsters pretreated with either 2-DG (0.5 g/kg IP), the alpha-2 adrenoceptor agonist UK-14304 (0.3 mg/kg IP) or the 5-HT1A selective agonist 8-OH-DPAT (0.03 mg/kg IP), have a significant hyperglycemic response, which is similar to the response in mice or rats. However, neither 2-DG, UK-14304 nor 8-OH-DPAT were capable of stimulating food intake in these hamsters. Previous studies in rats and mice demonstrated that hyperglycemic conditions result in activation of a hypothalamic anorectic recognition site, labeled with [3H]mazindol, as well as alpha-2 adrenoceptors, labeled with [3H]idazoxan. No such activation of [3H]mazindol nor [3H]idazoxan binding was observed in the hypothalamus of hamsters treated with 2-DG, despite a normal glycemic response. Thus, in this species an uncoupling between feeding responses and glucoprivic signals may represent a lack of ischymetric regulation of feeding.

Effect of a 7-day treatment with idazoxan and its 2-methoxy derivative RX 821002 [correction of RX 821001] on alpha 2-adrenoceptors and non-adrenoceptor idazoxan binding sites in rabbits

1. The present study investigates the influence of a 7-day treatment with 2 mg kg-1, s.c., twice daily of RX 821002 (an alpha 2-adrenoceptor antagonist which binds only to alpha 2-adrenoceptors) or idazoxan (alpha 2-antagonist which binds to alpha 2-adrenoceptors and also to non-adrenoceptor idazoxan binding sites: NAIBS) on alpha 2-adrenoceptor (labelled with [3H]-RX 821002) and NAIBS (labelled with [3H]-idazoxan) number in three tissues (adipocytes, colocytes and platelets) in the rabbit. 2. Acute administration of RX 821002 or idazoxan increased plasma non-esterified fatty acids (NEFA) and catecholamine levels with no change in plasma glucose levels. 3. The 7-day treatment with RX 821002 or idazoxan failed to influence food intake, total body weight or perirenal adipose tissue weight. 4. RX 821002 and idazoxan increased the number of [3H]-RX 821002 binding sites in adipose tissue with no change in colocytes or platelets. 5. RX 821002 and idazoxan failed to modify [3H]-idazoxan binding sites on adipocytes and colocytes. No significant [3H]-idazoxan binding was detected on rabbit platelets. 6. The results show that a 7-day treatment with alpha 2-antagonists induces an up-regulation in adipocyte alpha 2-adrenoceptors. In contrast, this phenomenon does not involve all the tissues since colocytes and platelets escape the effects of alpha 2-antagonists. The data suggest a differential regulation of alpha 2-adrenoceptors according to their location. 7. The fact that NAIBS did not vary suggests that alpha 2-adrenoceptors and NAIBS are two different entities. Finally, since RX 821002 and idazoxan exert similar effects after either acute or chronic treatment, it is suggested that NAIBS are not involved in the control of catecholamine release or in NEFA or glucose metabolism.

Regulation of the alpha 2A-adrenergic receptor in the HT29 cell line. Effects of insulin and growth factors

The density of the alpha 2A-adrenergic receptor in the HT29 cell line, a human colonic adenocarcinoma, increases when the cells are placed in fetal calf serum (FCS)-free culture medium and decreases again, in a concentration-dependent manner, when they are re-exposed to FCS. In an attempt to identify the FCS components responsible for this phenomenon, we examined the effect of insulin and of various growth factors on receptor expression. Incubation of HT29 cells with insulin resulted in a time- and dose-dependent lowering of the alpha 2-adrenergic receptor number. The decrease of [3H] RX821002 binding sites after a 48-h period of treatment reached 70-75% with 170 nM insulin, and a half-maximal effect was observed at 2.6 nM. This value is in agreement with the EC50 of the hormone for stimulating the glycolytic activity of HT29 cells (8 nM) and is sufficiently low to indicate that the decrease of alpha 2-adrenergic receptor number is mediated through stimulation of insulin receptors. Direct quantification of [3H] UK14304 binding sites and the study of the inhibition of [3H]RX821002 binding by (-)-epinephrine indicated that the degree of receptor coupling to Gi protein was not affected when the receptor number was decreased by insulin treatment. The reduction in receptor number did result in an attenuation of the inhibitory effect of UK14304 on forskolin-induced cAMP accumulation in a manner which was consistent with the existence of a large population of spare receptors in untreated cells. The action of insulin is not due to an accelerated rate of receptor degradation and can be mimicked by other growth factors (epidermal growth factor and insulin-like growth factors I and II) acting through stimulation of tyrosine kinase receptors. RNase mapping experiments with a 0.35-kilobase riboprobe prepared from the human alpha 2 C10-adrenergic receptor gene demonstrated that the decrease of receptor number induced by the different treatments is a reflection of changes occurring at the level of its mRNA. The use of cycloheximide indicated that the effect of insulin on alpha 2-adrenergic receptor mRNA does not require protein synthesis. The half-life of the alpha 2-adrenergic receptor mRNA measured after the addition of actinomycin D was unchanged by insulin which suggests that a decrease in the transcription rate is the predominant factor responsible for the observed regulation of receptor expression.