Antiviral triterpenes fromPrunella vulgaris

Two triterpenes 1 and 2 with antiviral activity againstHerpes simplex virus type 1in vitro were isolated fromPrunella vulgaris. Each compound caused a significant reduction in viral cytopathic effect whenvero cells were exposed to them for 72 hours after viral challenge. They were identified asbetulinic acid(1) and 2α, 3α-dihydroxyurs-12-en-28-oic acid(2) on the basis of their spectroscopic properties. The antiviral activity of them was estimated as EC₅₀=30 μg/ml(1) and 8 μg/ml(2), respectively by plaque reduction assay.     

Aromatic compounds and triterpenoidal saponins fromClematis koreana var.umbrosa

From the methanolic extract of aerial parts ofClematis koreana var.umbrosa, one new triterpenoidal saponin, 3-O-β-D-xylopyranosyl(1–3)-α-L-arabinopyranosyl oleanolic acid 28-O-α-L-rhamnopyranosyl(1–4)-β-D-glucopyranosyl(1–6)-β-D-glucopyranosyl ester, along with five known aromatic compounds and two known triterpenoidal saponins were isolated.     

Sesquiterpene components from the flower buds ofMagnolia fargesii

From the Chinese crude drugshin-i, the flower buds ofMagnolia fargesii, four sesquiterpene, oplopanone (1), oplodiol (2), homalomenol A (3) and 1β,4β,7α-trihydroxyeudesmane (4) were isolated. These structures were elucidated and the¹³C-NMR chemical, shifts of these compounds were revised by means of various 2D-NMR techniques.     

Triterpenoic acids of Prunella vulgaris var. lilacina and their cytotoxic activities In Vitro

The column chromatographic separation of the MeOH extract from the aerial parts of Prunella vulgaris var. lilacina Nakai led to the isolation of fifteen triterpenoic acids (2–6, 9–13, 16–20), four flavonoids (14, 21–23), four phenolics (7, 8, 15, 24), and a diterpene (1). Their structures were determined by spectroscopic methods to be trans-phytol (1), oleanic acid (2) ursolic acid (3), 2α,3α,19α-trihydroxyurs-12en-28oic acid (4), 2α,3α-dihydroxyurs-12en-28oic acid (5), maslinic acid (6), caffeic acid (7), phydroxy cinnamic acid (8), 2α,3α,19α,23-tetrahydroxyurs-12en-28oic acid (9), 2α,3α,23-trihydroxyurs-12en-28oic acid (10), 2α,3β-dihydroxyurs-12en-28oic acid (11), 2α,3β,24-trihydroxyolea-12en-28oic acid (12), (12R, 13S)-2α,3α,24,trihydroxy-12,13-cyclo-taraxer-14-en-28oic acid (13), quercertin 3-O-β-D-glucopyranoside (14), rosmarinic acid (15), 2α,3α,24-trihydroxyurs-12,20(30)-dien-28oic acid (16), 2α,3α,24-trihydroxyolea-12en-28oic acid (17), 2α,3β,19α,24-tetrahydroxyurs-12en-28oic acid 28-O-Dglucopyranoside (18), 2α,3α,19α,24-tetrahydroxyurs-12en-28oic acid 28-O-D-glucopyranoside (19), prunvuloside A (20), kaempferol 3-O-α-L-rhamnopyranosyl(1→6)-β-D-glucopranoside (21), kaempferol 3-O-β-D-glucopyranoside (22), quercertin 3-O-α-L-rhamnopyranosyl(1→6)-β-D-glucopyranoside (23), and 2-hydroxy-3-(3’,4’-dihydroxyphenly)propanoic acid (24). Compounds 1, 8–12, 17, 21, 23, and 24 were isolated from this plant source for the first time. The isolated compounds were evaluated for their cytotoxicity against A549, SK-OV-3, SK-MEL-2, and HCT15 cells in vitro using the sulforhodamin B bioassay (SRB) method. Compound 3 exhibited moderate cytotoxic activity against A549, SK-OV-3, SK-MEL-2, and HCT15 cells, with ED₅₀ values of 3.71, 3.65, 13.62, and 5.44 μM, respectively.     

Effects of subunit selective nACh receptors on operant ethanol self-administration and relapse-like ethanol-drinking behavior

Rationale and objectives  The sensitivity to ethanol central effects is partially determined by the subunit composition of brain nicotinic acetylcholine receptors (nAChRs). Thus, the effects of intraventral tegmental area (VTA) administration of the nicotinic subunit-specific antagonist, α-conotoxin MII (αCtxMII, α₃β₂*, β₃*, α₆*), were compared to those of systemic mecamylamine (MEC, an allosteric negative modulator of the nAChR), dihydro-β-erythroidine (DHβE, α₄β₂*), and methyllycaconitine (MLA, α₇*) to elucidate involvement of different subunits of nAChRs in operant ethanol self-administration and relapse-like activation of ethanol consumption after ethanol deprivation in rats. Methods  The effects of drugs were studied in rats trained for operant oral self-administration of ethanol (FR = 1). For ethanol deprivation, trained animals were subjected to a period of alcohol deprivation for 10 days. αCtxMII was given directly into the VTA through implanted permanent intracranial cannulae, whereas MEC, DHβE, and MLA were administered systemically. Results  αCtxMII reduced operant ethanol self-administration and blocked the deprivation-induced relapse-like ethanol consumption. MEC reduced operant ethanol self-administration and inhibited the deprivation-induced increase in alcohol consumption. DHβE did not alter ethanol self-administration in the lower-dose range but inhibited ethanol intake at a higher dose (4 mg/kg), although this effect might have been nonspecific. MLA failed to block self-administration of ethanol and relapse-like drinking after deprivation. Conclusions  Our results indicate that nAChRs are involved in the modulation of operant alcohol self-administration and relapse-like alcohol drinking behavior in rats. Our observations support the working hypothesis that systemically active selective ligands for nAChR α₃β₂*, β₃, and/or α₆* receptor subunits might be of therapeutic value for the treatment of alcoholism.     

Pharmacological properties of a wide array of ergolines at functional alpha1-adrenoceptor subtypes

Ergot alkaloids act as (partial) agonists or antagonists at serotonergic, dopaminergic and α-adrenergic receptors. In contrast to their affinity at serotonergic (5-HT) and dopaminergic receptor subtypes, only limited information is available concerning their interaction with α-adrenoceptor subtypes. This especially holds true for native α-adrenoceptors. Therefore, we studied the pharmacological profile of 25 ergolines at α_1A-, α_1B- and α_1D-adrenoceptors in vascular rings or strips of rat and guinea pig endowed with these receptors. Contractile responses were studied by measurement of isometric tension changes in rat tail artery (α_1A, α_1B), guinea pig spleen (α_1B) and rat thoracic aorta (α_1D). The anti-migraine drugs ergotamine and dihydroergotamine, the anti-parkinsonian drug lisuride and the anti-hyperprolactinemic drug terguride behaved as antagonists or low-efficacy partial agonists at all three α₁-adrenoceptor subtypes with nanomolar receptor affinity. Derivatives of these drugs showed lower affinity at α₁-adrenoceptors than the parent compounds. Each individual ergoline derivative tested showed low discriminatory capability at the subtypes, α_1A, α_1B, α_1D. A low discriminatory power between the subtypes (α_1A, α_1B, α_1D) seems to be a class effect of the ergolines. The nanomolar affinities of ergotamine, dihydroergotamine and lisuride for α₁-adrenoceptors may affect their effectiveness as anti-migraine and anti-parkinsonian drugs, respectively.     

Noradrenergic vasoconstriction of pig prostatic small arteries

The current study investigated the distribution of adrenergic nerves and the action induced by noradrenaline (NA) in pig prostatic small arteries. Noradrenergic innervation was visualized using an antibody against dopamine-beta-hydroxylase (DBH), and the NA effect was studied in small arterial rings mounted in microvascular myographs for isometric force recordings. DBH-immunoreactive nerve fibers were located at the adventitia and the adventitia-media border of the vascular wall. Electrical field stimulation (EFS, 1-32 Hz) evoked frequency-dependent contractions that were reduced by guanethidine and prazosin (adrenergic neurotransmission and α₁-adrenoceptors blockers, respectively) and by the α₂-adrenoceptor agonist UK 14,304. The α₂-adrenoceptor antagonist rauwolscine reversed the UK 14,304-produced inhibition. NA produced endothelium-independent contractions that were antagonized with low estimated affinities and Schild slopes different from unity by prazosin and the α_1A-adrenoceptor antagonist N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-α-α-dimethyl-1H-indole-3-ethanamine (RS 17053). The α_1A-adrenoceptor antagonist 5-methyl-3-[3-[4-[2-(2,2,2,-trifluoroethoxy) phenyl]-1-piperazinyl]propyl]-2,4-(1H)-pyrimidinedione (RS 100329), which also displays high affinity for α_1L-adrenoceptors, and the α_1L-adrenoceptor antagonist tamsulosin, which also has high affinity for α_1A- and α_1D-adrenoceptors, induced rightward shifts with high affinity of the contraction-response curve to NA. The α_1D-adrenoceptor antagonist 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]8-azaspiro[4,5]decane-7,9-dione dihydrochloride (BMY 7378) failed to modify the NA contractions that were inhibited by extracellular Ca²⁺ removal and by voltage-activated (L-type) Ca²⁺ channel blockade. These data suggest that pig prostatic resistance arteries have a rich noradrenergic innervation; and NA, whose release is modulated by prejunctional α₂-adrenoceptors, evokes contraction mainly through activation of muscle α_1L-adrenoceptors coupled to extracellular Ca²⁺ entry via voltage (L-type)- and non-voltage-activated Ca²⁺ channels.     

Potential anthelmintics: polyphenols from the tea plant <Emphasis Type="Italic">Camellia sinensis</Emphasis> L. are lethally toxic to <Emphasis Type="Italic">Caenorhabditis elegans</Emphasis>

A novel gallate of tannin, (−)-epigallocatechin-(2β→O→7′,4β→8′)-epicatechin-3′-O-gallate (8), together with (−)-epicatechin-3-O-gallate (4), (−)-epigallocatechin (5), (−)-epigallocatechin-3-O-gallate (6), and (+)-gallocatechin-(4α→8′)-epigallocatechin (7), were isolated from the tea plant Camellia sinensis (L.) O. Kuntze var. sinensis (cv., Yabukita). The structure of 8, including stereochemistry, was elucidated by spectroscopic methods and hydrolysis. The compounds, along with commercially available pyrogallol (1), (+)-catechin (2), and (−)-epicatechin (3), were examined for toxicity towards egg-bearing adults of Caenorhabditis elegans. The anthelmintic mebendazole (9) was used as a positive control. Neither 2 nor 3 were toxic but the other compounds were toxic in the descending order 8, 7 ≈ 6, 9, 4, 5, 1. The LC₅₀ (96 h) values of 8 and 9 were evaluated as 49 and 334 μmol L⁻¹, respectively. These data show that many green tea polyphenols may be potential anthelmintics.     

The effect ofN-alkyloxycarbonyl group on the anticonvulsant activities ofN-alkyloxycarbonyl-α-aminoglutarimides

In connection with the development of new anticonvulsant agents with a broad spectrum, we reported thatN-Cbz-α-aminoglutarimides, combining common structures of other anticonvulsants such as N-CO-C-N and cyclic imides in a single molecule, showed significant anticonvulsant activities in the MES (maximal electroshock seizure) and PTZ (pentylenetetrazole induced seizure) tests. In these studies, a series of (R) and (S)N-alkyloxycarbonyl-α-aminoglutarimides7a∼7e and8a∼8e, which were substituted with various alkyloxycarbonyl group instead ofCbz group, were prepared from the corresponding (R) and (S)N-Cbz-glutamic acid3 and4, and were evaluated with their anticonvulsant activities against the MES and PTZ tests, including neurotoxicity, in order to define the effect ofN-alkyloxycarbonyl group on the anticonvulsant activities ofN-alkyloxycarbonyl-α-aminoglutarimides. Among them, (S)N-4-nitrobenzyloxycarbonyl-α-amino-N-methylglutarimide8e was the most active in MES (ED₅₀=35.6 mg/kg, Pl=2.7) and PTZ tests (ED₅₀=15.6, Pl=6.1). Interestingly, (R) and (S)N-4-nitrobenzyloxycarbonyl-α-amino-N-methylglutarimide7e and8e and (R)N-phenoxycarbonyl-α-amino-N-methylglutrimide7d showed significant anticonvulsant activities in both the MES and PTZ tests and other compounds showed anticonvulsant activities in only the PTZ test. In addition, it was found that their anticonvulsant activities were dependent on their stereochemistries andN-substituted alkyloxycarbonyl groups.     

Microvascular protective activity in rabbits of triterpenes from <Emphasis Type="Italic">Hylocereus undatus</Emphasis>

Two new pentacyclic triterpene taraxast-20-ene-3α-ol (1) and taraxast-12,20(30)-dien-3α-ol (2) were isolated from chloroform extract of leaves of Hylocereus undatus. These triterpenes showed protective activity against the increase of skin vascular permeability in rabbits. The protective effect, measured as the reduction in leakage of Evans blue, was of 53.5% and 70.1% for 1 and 2, respectively, after peritoneal treatment at a dose of 50 mg/kg. Comparison was made between the action of the compounds and a protective microvascular drug known as troxerutin (64.5%) at the same doses.     

A new ceramide from <Emphasis Type="Italic">Ramaria botrytis</Emphasis> (Pers.) Ricken

A new ceramide, (2S,2′R,3R,4E,8E)-N-2′-hydroxyoctadecanoyl-2-amino-9-methyl-4,8-heptadecadiene-1,3-diol (1), was isolated together with four known sterols, 5α,6α-epoxy-3β-hydroxy-(22E)-ergosta-8(14),22-dien-7-one (2), ergosterol peroxide (3), cerevisterol (4) and 9α-hydroxycerevisterol (5), from the fruiting bodies of Ramaria botrytis (Pers.) Ricken (Ramariaceae). The structure of the new compound was elucidated based on spectral data.     

Ginsenoside Rf2, a new dammarane glycoside from Korean red ginseng (Panax ginseng)

A new dammarane glycoside named ginsenoside Rf₂ has been isolated from Korean red ginseng (Panax ginseng) and its chemical structure has been elucidated as 6-O-[α-L-rhamnopyranosyl (1→2) β-D-glucopyranosyl]dammarane-3β, 6α, 12β, 20(R), 25-pentol by chemical and spectral methods.     

Studies on the constituents from the fruits of <Emphasis Type="Italic">Phaleria macrocarpa</Emphasis>

From the fruits of Phaleria macrocarpa, icariside C₃ (1), phalerin (2), and mangiferin (3) were isolated and their structures were identified on the basis of spectroscopic data. Icariside C₃ (1) showed a slow vasorelaxant activity against noradrenaline-induced contraction of isolated rat aorta. The structure of phalerin (2) was revised as 2,4′,6-trihydroxy-4-methoxybenzophenone-2-O-β-d-glucoside.     

Anti-oxidative and inhibitory activities on nitric oxide (NO) and prostaglandin E<Subscript>2</Subscript> (COX-2) production of flavonoids from seeds of <Emphasis Type="Italic">Prunus tomentosa</Emphasis> Thunberg

Chemical investigation of the 80% Me₂CO extract from the seeds of Prunus tomentosa led to the isolation and identification of six flavonoids: kaempferol (1), kaempferol 3-O-α-L-rhamnopyranoside (2; afzelin), kaempferol 3-O-β-D-(6-acetyl)-glucopyranosyl(1→4)-α-L-rhamnopyranoside (3; multiflorin A), kaempferol 3-O-β-D-glucopyranosyl(1→4)-α-L-rhamnopyranoside (4; multiflorin B), quercetin 3-O-α-L-rhamnopyranoside (5; quercitrin), and quercetin 3-O-β-D-glucopyranosyl (1→4)-α-L-rhamnopyranoside (6; multinoside A). Anti-oxidative and inhibitory activities on nitric oxide (NO) and prostaglandin E₂ production in interferon-γ (INF-γ) and lipopolysaccharide (LPS)-activated RAW 264.7 cells in vitro (COX-2) of the isolated compounds were evaluated. Compounds 1, 5, and 6 exhibited potent anti-oxidative activity in the DPPH radical scavenging assay with IC₅₀ values of 57.2, 59.4, and 54.3 μg/mL respectively. The positive control, ascorbic acid, had an IC₅₀ of 55.5 μg/mL. Compounds 1, 5, and 6 also reduced COX-2 levels in a dose dependent manner with IC₅₀ values of 10.2, 8.7, and 9.6 μg/mL respectively, with the positive control, indomethacin, having an IC₅₀ of 5.1 μg/mL. All six compounds inhibited NO production in a dose dependent manner with IC₅₀ values of 35.1, 42.8, 40.0, 44.8, 43.7, and 43.9 μg/mL respectively, while the positive control, L-NMMA, had an IC₅₀ of 42.1 μg/mL.     

Presynaptic α2A/D-autoreceptors in the brain cortex of Cercopithecus aethiops

The existence of presynaptic α₂-autoreceptors, and the subtype to which the receptors belong, were studied in the brain cortex from two African green monkeys (Cercopithecus aethiops). Slices of the brain cortex were preincubated with ³H-noradrenaline. The slices were then superfused with medium containing desipramine (1 μM) and stimulated electrically with trains of 72 pulses, 3 Hz. Concentration-response curves of nine α-adrenoceptor antagonists were determined. All antagonists enhanced the stimulation-evoked overflow of tritium, i.e. the release of noradrenaline, indicating the existence of presynaptic α₂-autoreceptors. The EC _30% values of the antagonists (concentrations that increased the evoked overflow of tritium by 30%) were taken as a measure of affinity for the autoreceptors. The subclassification was based on a comparison of these affinity estimates with affinities for α_2A, α_2B, α_2C and α_2D radioligand binding sites and for previously classified α_2A- and α_2D-autoreceptors. The comparison indicates that the α₂-autoreceptors in the brain cortex of Cercopithecus aethiops belong to the genetic α_2A/D and not the α_2B or α_2C subtype. In their pharmacological properties, the autoreceptors are closest to the pharmacological α_2D subtype. The results support the notion that α₂-autoreceptors belong at least predominantly to the α_2A/D subtype of α₂-adrenoceptors. Received: 7 October 1996 / Accepted: 22 November 1996     

Saponins from the fructus ofKochia scoparia

Two new triterpenoidal saponins,B(1) andC(2) were isolated from the fruccus ofKochia scoparia. On the basis of chemico-spectral evidences, the structures of1 and2 were elucidated as oleanolic acid 3-O-β-D-ribopyranosyl-(1→2)-β-D-glucuronopyranoside and 3-O-β-D-xylopyranosyl-(1→3)-β-D-glucuronopyranosyl-olean-12-en-28-O-β-D-glucopyranosyl ester, respectively.     

Prejunctional α2A-autoreceptors in the human gastric and ileocolic arteries

This study was undertaken to determine the subtype of prejunctional α₂-autoreceptors in human blood vessels. Segments of gastric and ileocolic arteries were incubated with [³H]noradrenaline and subsequently perifused with modified Krebs-Henseleit solution containing cocaine (12 μM). Five periods of electrical stimulation (S₁–S₅) were applied (1 Hz, 1 ms, 50 V for 1 min). Concentration-response curves for the facilitatory effect of eight α-adrenoceptor antagonists [rauwolscine, 2-[2-(2-methoxy-1,4-benzodioxanyl)] imidazoline (RX821002), yohimbine, phentolamine, idazoxan, 2-(2’,6’-dimethoxyphenoxyethyl)-aminomethyl-1,4-benzodioxan (WB4101), spiroxatrine and prazosin] were determined. All antagonists enhanced the stimulation-evoked overflow of tritium, indicating the existence of α₂-autoreceptors. The EC _30% values of the antagonists (concentrations that increased the evoked overflow of tritium by 30%) were taken as a measure of affinity to the autoreceptors. Correlations between the pEC _30% values obtained in the present study and the pK _i values of the same antagonists at cloned human α_2A-, α_2B-, α_2C-adrenoceptors expressed in Chinese hamster lung cells and at α_2D-adrenoceptors in the rat submaxillary gland or the bovine pineal gland showed that the α₂-autoreceptors in the human gastric and ileocolic arteries resemble most closely the α_2A-subtype. Received: 6 April 1998 / Accepted: 4 May 1998     

α<Subscript>1</Subscript>-Adrenergic and α<Subscript>2</Subscript>-adrenergic balance in the dorsal pons and gross behavioral activity of mice in a novel environment

Rationale Central α₁- and α₂-adrenoceptors in a number of different brain regions are known to have opposing actions on gross behavioral activity, with the former stimulating and the latter inhibiting activity. Therefore, blockade of α₁-receptors may induce inactivity by leading to unopposed α₂ activity.Objective The aim of this study was to test if central blockade of α₂-receptor function restores behavioral activity in α₁-receptor-blocked mice.Methods Dose-response studies were undertaken on the effects of α₁- and α₂-agonists and antagonists microinjected into the dorsal pons on gross behavioral activity in a novel cage test.Results The behavioral inactivity resulting from blockade of α₁-receptors in the pons with the antagonist, terazosin, was reversed by either a low dose of an α₂-antagonist, atipamezole, or a low dose of an α₂-agonist, dexmedetomidine, but was exacerbated by a high dose of the α₂-agonist.Conclusion The results support the hypothesis that blockade of α₁-receptors in the dorsal pons of mice produces inactivity by causing unopposed activity of α₂-receptors. This condition may be relevant to inactive states seen after stress or during depressive illness.     

A new triterpenoid from <Emphasis Type="Italic">Panax ginseng</Emphasis> exhibits cytotoxicity through p53 and the caspase signaling pathway in the HepG2 cell line

A new triterpenoid, 20(R),22(ξ),24(S)-dammar-25(26)-ene-3β,6α,12β,20,22,24-hexanol (1), and three known triterpenoids, β-D-glucopyranoside,(3β,12β)-12,20-dihydroxydammar-24-en-3-yl,6-acetate (2), 20(R)-ginsenoside Rg₃ (3), and 20(R)-ginsenoside Rh₂ (4), were isolated from the leaves of Panax ginseng. Their structures were determined by chemical analysis and spectral methods (IR, 1D and 2D NMR, HR-ESI-MS). Compounds 1–4 were exhibited various degrees of cytotoxicity in the human hepatoma cell line, HepG2. Compound 1 had the highest cytotoxic potency, with an IC₅₀ value of 20.1 μM, by stimulating p53-mediated cell cycle arrest at the G1 to S phase transition, leading to apoptosis via activation of the caspase signaling pathway.     

A re-investigation of questionable subclassifications of presynaptic α2-autoreceptors: rat vena cava, rat atria, human kidney and guinea-pig urethra

It has been suggested that at least the majority of mammalian presynaptic α₂-autoreceptors belong to the genetic α_2A/D-adrenoceptor subtype. The aim of the present study was to re-examine the α₂-autoreceptors in tissues in which previous assignments conflicted with this α_2A/D rule: in the rat vena cava and rat heart atria, where the autoreceptors were classified as α_2B or similar to, but not identical with, α_2D, and in the human kidney, where they were classified as α_2C. Also re-examined were the autoreceptors in the guinea-pig urethra, where they were suggested to be α_2A, in agreement with the rule, but in contrast to indications that the α_2A/D-adrenoceptor of the guinea pig possesses α_2D pharmacological properties. Tissue pieces were preincubated with ³H-noradrenaline and then superfused and stimulated electrically under autoinhibition-free or almost autoinhibition-free conditions. The K _d values of up to 14 antagonists (including the partial agonist oxymetazoline) against the release-inhibiting effect of the α₂ agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) were determined. EC ₅₀ between 6.3 and 13.2 nM. All antagonists (except prazosin in one case) shifted the concentration-inhibition curve of UK 14,304 to the right. Comparison of the K _d values thus obtained with K _d values at known α₂ subtypes indicated that the autoreceptors in the rat vena cava, rat atria and the guinea-pig urethra were α_2D and those in the human kidney α_2A. For example, the pK _d values of the antagonists in the rat vena cava, in rat atria and in the guinea-pig urethra were closely correlated with pK _d values at the prototypic α_2D radioligand binding sites in the bovine pineal gland (r = 0.96, P < 0.001; r = 0.92, P < 0.01; and r = 0.95; P < 0.001) and with the pK _d values at the α_2D-autoreceptors of guinea-pig atria (r = 0.99, P < 0.001; r = 0.95, P < 0.001; and r = 0.98, P < 0.001). The pK _d values at the autoreceptors in rat vena cava, rat atria and guinea-pig urethra were not significantly or more loosely correlated with pK _d values at α_2A, α_2B and α_2C binding sites and α_2A-autoreceptors. On the other hand, the pK _d values of the antagonists in the human kidney were closely correlated with pK _d values at the prototypic α_2A radioligand binding sites in HT29 cells (r = 0.95; P < 0.001) and with pK _d values at the α_2A-autoreceptors of the pig brain cortex (r = 0.97; P < 0.001), but were not significantly or more loosely correlated with pK _d values at α_2B, α_2C and α_2D binding sites and α_2D-autoreceptors. 2D , those in the human kidney α_2A, and those in the guinea-pig urethra equally α_2D. All, therefore, conform to the rule that α₂-autoreceptors belong at least predominantly to the genetic α_2A/D subtype of the α₂-adrenoceptor. The apparent paradox of an α_2A-autoreceptor in the urethra of the guinea pig, a species in which the genetic α_2A/D-adrenoceptor otherwise has α_2D pharmacological properties, is removed. Received: 30 May 1997 / Accepted: 1 September 1997