Functional evidence equating the pharmacologically-defined alpha 1A- and cloned alpha 1C-adrenoceptor: studies in the isolated perfused kidney of rat

The Pharmacological Effects of Phenylephrine are Indirect, Mediated by Noradrenaline Release from the Cytoplasm

Phenylephrine (PE) is a canonical α₁-adrenoceptor-selective agonist. However, unexpected effects of PE have been observed in preclinical and clinical studies, that cannot be easily explained by its actions on α₁-adrenoceptors. The probability of the involvement of α₂- and β-adrenoceptors in the effect of PE has been raised. In addition, our earlier study observed that PE released noradrenaline (NA) in a [Ca²⁺]_o-independent manner. To elucidate this issue, we have investigated the effects of PE on [³H]NA release and α₁-mediated smooth muscle contractions in the mouse vas deferens (MVD) as ex vivo preparation. The release experiments were designed to assess the effects of PE at the presynaptic terminal, whereas smooth muscle isometric contractions in response to electrical field stimulation were used to measure PE effect postsynaptically. Our results show that PE at concentrations between 0.3 and 30 µM significantly enhanced the resting release of [³H]NA in a [Ca²⁺]_o-independent manner. In addition, prazosin did not affect the release of NA evoked by PE. On the contrary, PE-evoked smooth muscle contractions were inhibited by prazosin administration indicating the α₁-adrenoceptor-mediated effect. When the function of the NA transporter (NAT) was attenuated with nisoxetine, PE failed to release NA and the contractions were reduced by approximately 88%. The remaining part proved to be prazosin-sensitive. The present work supports the substantial indirect effect of PE which relays on the cytoplasmic release of NA, which might explain the reported side effects for PE.

The pharmacology of α1-adrenoceptor subtypes

This review examines the functions of α₁-adrenoceptor subtypes, particularly in terms of contraction of smooth muscle. There are 3 subtypes of α₁-adrenoceptor, α_1A- α_1B- and α_1D-adrenoceptors. Evidence is presented that the postulated α_1L-adrenoceptor is simply the native α_1A-adrenoceptor at which prazosin has low potency. In most isolated tissue studies, smooth muscle contractions to exogenous agonists are mediated particularly by α_1A-, with a lesser role for α_1D-adrenoceptors, but α_1B-adrenoceptors are clearly involved in contractions of some tissues, for example, the spleen. However, nerve-evoked responses are the most crucial physiologically, so that these studies of exogenous agonists may overestimate the importance of α_1A-adrenoceptors. The major α₁-adrenoceptors involved in blood pressure control by sympathetic nerves are the α_1D- and the α_1A-adrenoceptors, mediating peripheral vasoconstrictor actions. As noradrenaline has high potency at α_1D-adrenceptors, these receptors mediate the fastest response and seem to be targets for neurally released noradrenaline especially to low frequency stimulation, with α_1A-adrenoceptors being more important at high frequencies of stimulation. This is true in rodent vas deferens and may be true in vasopressor nerves controlling peripheral resistance and tissue blood flow. The α_lA-adrenoceptor may act mainly through Ca²⁺ entry through L-type channels, whereas the α_1D-adrenoceptor may act mainly through T-type channels and exhaustable Ca²⁺ stores. α₁-Adrenoceptors may also act through non-G-protein linked second messenger systems. In many tissues, multiple subtypes of α-adrenoceptor are present, and this may be regarded as the norm rather than exception, although one receptor subtype is usually predominant.

α(1D)-Adrenoceptor regulates the vasopressor action of α(1A)-adrenoceptor in mesenteric vascular bed of α(1D)-adrenoceptor knockout mice

1 The pressor action of the α(1A)-adrenoceptor (α(1A)-AR) agonist A61603 (N-[5-(4,5-dihydro-1H-imidazol-2-yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl] methanesulfonamide) and the α(1)-ARs agonist phenylephrine and their blockade by selective α(1)-ARs antagonists in the isolated mesenteric vascular bed of wild-type (WT) mice and α(1D)-AR knockout (KO α(1D)-AR) mice were evaluated. 2 The apparent potency of A61603 to increase the perfusion pressure in the mesenteric vascular bed of WT and KO α(1D)-AR mice is 86 and 138 times the affinity of phenylephrine, respectively. 3 A61603 also enhanced the perfusion pressure by ≈1.7 fold in the mesenteric vascular bed of WT mice compared with KO α(1D)-AR mice. 4 Because of its high affinity, low concentrations of the α(1A)-AR selective antagonist RS100329 (5-methyl-3-[3-[4-[2-(2,2,2,-trifluoroethoxy) phenyl]-1-piperazinyl] propyl]-2,4-(1H)-pyrimidinedione) shifted the agonist concentration-response curves to the right in the mesenteric vascular bed of WT and KO α(1D)-AR mice. 5 The α(1D)-AR selective antagonist BMY7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5] decane-7,9-dione) did not modify the A61603 or the phenylephrine-induced pressor effect. 6 The α(1B/D)-ARs alkylating antagonist chloroethylclonidine (CEC) shifted the agonist concentration-response curves to the right and decreased the maximum phenylephrine-induced vascular contraction in KO α(1D)-AR mice when compared to WT mice; however, CEC only slightly modified the contraction induced by A61603. 7 The results indicate that the isolated mesenteric vascular bed of WT and KO α(1D)-AR mice expresses α(1A)-AR, that the pressor action of α(1A)-AR is up-regulated for α(1D)-AR in WT mice and suggest an important role of α(1B)-AR in the vascular pressure evoked by phenylephrine in KO α(1D)-AR mice.

Interaction between alpha(1)- and alpha(2)-adrenoreceptors contributes to enhanced constrictor effects of norepinephrine in mesenteric veins compared to arteries

Mesenteric veins are more sensitive than arteries to the constrictor effects of sympathetic nerve stimulation and alpha-adrenoceptor agonists. We tested the hypothesis that alpha(1)- and alpha(2)-adrenoceptors interact to enhance adrenergic reactivity of mesenteric veins. We studied neurogenic and agonist-induced constrictions of mesenteric veins and arteries in vitro. Norepinephrine concentration-response curves were left-shifted in veins compared to arteries. UK 14,304 (0.01-1 microM, alpha(2)-adrenoceptor receptor agonist) did not constrict arteries or veins but enhanced constrictions and Ca(2+) signals mediated by alpha(1)-adrenoceptor stimulation in veins. Yohimbine (alpha(2)-adrenoceptor receptor antagonist) and MK912 (alpha(2C)-adrenoceptor receptor antagonist), but not alpha(2A)- or alpha(2B)-adrenoceptor antagonists, produced rightward shifts in norepinephrine concentration-response curves in veins. Pharmacological studies revealed that alpha(1D)-adrenoceptors mediate venous constrictions. Norepinephrine responses in veins from alpha(2C)-adrenoceptor knock-out (KO) mice were not different from wild type veins. Yohimbine inhibited norepinephrine constrictions in alpha(2C)-adrenoceptor KO veins suggesting that there is upregulation of other alpha(2)-adrenoceptors in alpha(2C)-KO mice. These data indicate that alpha(1D)- and alpha(2C)-adrenoceptors interact in veins but not in arteries. This interaction enhances venous adrenergic reactivity. Mesenteric vein-specific alpha(2)-adrenoceptor linked Ca(2+) and perhaps other signaling pathways account for enhanced venous adrenergic reactivity.

Functional subtypes of renal alpha1-adrenoceptor in diabetic and non-diabetic 2K1C Goldblatt renovascular hypertension

AIM

This study investigates the subtypes of the alpha1-adrenoceptor mediating the adrenergically-induced renal vasoconstrictor responses in streptozotocin-induced diabetic and non-diabetic 2-kidney one clip (2K1C) Goldblatt hypertensive rats.

METHODS

The renal blood flow responses to renal nerve stimulation, noradrenaline, phenylephrine, and methoxamine were measured in the absence and presence of nitrendipine, 5-methylurapidil, chloroethylclonidine and BMY 7378.

RESULTS

The renal vasoconstrictor responses were markedly attenuated by nitrendipine and 5- methylurapidil in the diabetic rats (all P< 0.05). In the non-diabetic rats, these responses were markedly attenuated by nitrendipine, 5-methylurapidil, and BMY 7378 (all P< 0.05). In both experimental groups, chloroethylclonidine markedly accentuated the renal vasoconstrictions caused by all the adrenergic stimuli (all P< 0.05).

CONCLUSION

These observations indicate that alpha 1A-adrenoceptor subtypes play a major role in mediating adrenergically-induced renal vasoconstriction in the diabetic 2K1C Goldblatt hypertensive rats. In the non-diabetic 2K1C Goldblatt hypertensive rats, contributions of alpha 1A and alpha 1D-adrenoceptor subtypes were proposed. Apart from post-synaptic alpha 1-adrenoceptors, both in the diabetic and non-diabetic 2K1C Goldblatt hypertensive rats, the potential involvement of presynaptic alpha 1- adrenoceptors is also suggested.

Analysis of the pharmacokinetic/pharmacodynamic relationship of a small molecule CXCR3 antagonist, NBI-74330, using a murine CXCR3 internalization assay

BACKGROUND AND PURPOSE

Pharmacokinetic/pharmacodynamic (PK/PD) models are necessary to relate the degree of drug exposure in vivo to target blockade and pharmacological efficacy. This manuscript describes a murine agonist-induced CXCR3 receptor internalization assay and demonstrates its utility for PK/PD analyses.

EXPERIMENTAL APPROACH

Activated murine DO11.10 cells were incubated with agonist in the presence or absence of a CXCR3 antagonist and changes in surface CXCR3 expression were detected by flow cytometry. For PK/PD analysis, mice were dosed with a small molecule CXCR3 antagonist, NBI-74330, (100 mg kg(-1)) orally or subcutaneously and plasma samples taken at specified timepoints for the CXCR3 internalization assay.

KEY RESULTS

Surface CXCR3 expression was specifically decreased in response to CXCL9, CXCL10 and CXCL11. CXCL11 was the most potent CXCR3 agonist in buffer (pA50=9.23+/-0.26) and the pA50 for CXCL11 was unaltered in murine plasma (pA50=9.17+/-0.15). The affinity of a small molecule CXCR3 antagonist, NBI-74330, was obtained in the absence or presence of plasma (buffer pA2 value: 7.84+/-0.14; plasma pKB) value 6.36+/-0.01). Administration of NBI-74330 to mice resulted in the formation of an N-oxide metabolite, also an antagonist of CXCR3. Both antagonists were detectable up to 7 h post oral dose and 24 h post subcutaneous dose. Measurement of CXCR3 internalization demonstrated significant antagonism of this response ex vivo, 24 h following subcutaneous administration of NBI-74330.

CONCLUSIONS AND IMPLICATIONS

The CXCR3 receptor internalization assay provides a robust method for determining agonist potency orders, antagonist affinity estimates and PK/PD analyses, which discriminate between dosing regimens for the CXCR3 antagonist NBI-74330.

Alpha1A-adrenoceptors predominate in the control of blood pressure in mouse mesenteric vascular bed

1 The pressor action of the alpha1A-adrenoceptor agonist, A61603 (N-[5-(4,5-dihydro-1H-imidazol-2-yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl] methanesulfonamide) or the alpha1-adrenoceptor agonist phenylephrine, and their blockade by selective alpha1-adrenoceptor antagonists in the mouse isolated mesenteric vascular bed were evaluated. 2 A61603 showed a approximately 235-fold higher potency in elevating perfusion pressure in mesenteric bed compared to phenylephrine. 3 The alpha1A-adrenoceptor selective antagonist RS 100329 (5-methyl-3-[3-[4-[2-(2,2,2,-trifluoroethoxy) phenyl]-1-piperazinyl] propyl]-2,4-(1H)-pyrimidinedione), displaced with high affinity agonist concentration-response curves to the right in a concentration-dependent manner. 4 The alpha1D-adrenoceptor selective antagonist BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5] decane-7,9-dione), did not displace A61603 nor did it block the phenylephrine-induced pressor response. 5 The alpha1B/D-adrenoceptor alkylating antagonist chloroethylclonidine (CEC), caused a rightward shift of the phenylephrine concentration-response curve and reduced its maximum response; however, CEC only slightly modified A61603 evoked contraction. 6 The results indicate that the isolated mouse mesenteric vascular bed expresses alpha1A-adrenoceptors and suggest a very discrete role for 1B-adrenoceptors.

“Phenotypic” pharmacology: The influence of cellular environment on G protein-coupled receptor antagonist and inverse agonist pharmacology

A central dogma of G protein-coupled receptor (GPCR) pharmacology has been the concept that unlike agonists, antagonist ligands display equivalent affinities for a given receptor, regardless of the cellular environment in which the affinity is assayed. Indeed, the widespread use of antagonist pharmacology in the classification of receptor expression profiles in vivo has relied upon this 'antagonist assumption'. However, emerging evidence suggests that the same gene-product may exhibit different antagonist pharmacological profiles, depending upon the cellular context in which it is expressed-so-called 'phenotypic' profiles. In this commentary, we review the evidence relating to some specific examples, focusing on adrenergic and muscarinic acetylcholine receptor systems, where GPCR antagonist/inverse agonist pharmacology has been demonstrated to be cell- or tissue-dependent, before going on to examine some of the ways in which the cellular environment might modulate receptor pharmacology. In the majority of cases, the cellular factors responsible for generating phenotypic profiles are unknown, but there is substantial evidence that factors, including post-transcriptional modifications, receptor oligomerization and constitutive receptor activity, can influence GPCR pharmacology and these concepts are discussed in relation to antagonist phenotypic profiles. A better molecular understanding of the impact of cell background on GPCR antagonist pharmacology is likely to provide previously unrealized opportunities to achieve greater specificity in new drug discovery candidates.

Reduced alpha adrenergic mediated contraction of renal preglomerular blood vessels as a function of gender and aging

As human males age, a decline in baroreflex-mediated elevation of blood pressure occurs due, at least in part, to a reduction in alpha-1 adrenergic vasoconstrictor function. Alpha adrenergic constriction is mediated by guanosine triphosphate binding Protein (G Protein) coupled signaling pathways. Alpha-1 A/C, B, and D adrenergic receptor expressions, measured by GeneChip array, are not reduced during aging in renal blood vessels of male or female rats. Alpha-1 A GeneChip expression is greater, at all ages studied, in females than in males. Prazosin binding by alpha-1 adrenergic receptors is greater in young adult female rats than in young adult male rats; however, it is reduced with aging in both male and female rats. G alpha q GeneChip expression declines while expression of adrenergic receptor kinase (GRK2) and tyrosine phosphatases (TyrP) increase with aging in male rats. The declines in alpha-1 adrenergic receptor binding and G alpha q expression and also the increases in GRK2 and TyrP expression likely relate to the age-related decline of vasoconstriction in male rats. The information that the expression of alpha-1 A adrenergic receptors is greater in female rats and (GRK2) expression does not increase during aging could relate to the gender differences in vasoconstrictor function with aging. Gene therapy to ameliorate the age-related decline in renal function could possibly reduce the need for renal dialysis. Signaling pathways such as those reviewed herein may provide an outline of the molecular pathways needed to move toward successful renal gene therapy for aging individuals.

Evidence for involvement of alpha1D-adrenoceptors in contraction of femoral resistance arteries using knockout mice

The role of alpha(1D)-adrenoceptors in vasoconstrictor responses to noradrenaline in mouse femoral resistance arteries was investigated using wire myography in alpha(1D)-adrenoceptor knockout (alpha(1D)-KO) and wild-type (WT) mice of the same genetic background.alpha(1D)-KO mice were 2.5-fold less sensitive than WTs to exogenous noradrenaline and BMY 7378 was significantly less potent against noradrenaline in alpha(1D)-KO mice than in WTs, showing a minor contribution of alpha(1D)-adrenoceptors in response to noradrenaline. Prazosin and 5-methyl-urapidil were equally effective against noradrenaline in alpha(1D)-KO and WT mice. Chloroethylclonidine produced a significantly greater attenuation of the response to noradrenaline in alpha(1D)-KO mice than in WTs. Responses to electrical field stimulation (EFS), at 2-20 Hz for 10 s and 0.09 ms pulse width were significantly smaller overall in alpha(1D)-KOs than in WTs although no significant differences were seen at the different frequencies.BMY 7378 produced significantly greater inhibition of responses at 2 and 5 Hz than at higher frequencies in WTs. In alpha(1D)-KOs, this greater sensitivity to BMY 7378 at lower frequencies was not apparent, confirming that the effect of BMY 7378 was due to blockade of alpha(1D)-adrenoceptors. Prazosin and 5-methyl-urapidil had similar inhibitory effects on responses to EFS in alpha(1D)-KO and WT mice. Chloroethylclonidine inhibited responses to EFS to a significantly greater extent in alpha(1D)-KO mice. The present study with alpha(1D)-KO mice shows that alpha(1D)-adrenoceptors contribute to vasoconstrictor responses to exogenous and neurally released noradrenaline in femoral resistance arteries.

Effects of Thiol Chelation on α1-Adrenoceptor-Induced Vasoconstriction In Vivo

The aims of this study were to determine whether systemic injections of the lipophobic thiol chelator, para-hydroxymercurobenzoic acid (PHMBA) would reduce the vasoconstrictor responses elicited by the alpha1-adrenoceptor agonist, phenylephrine, in urethane-anesthetized rats by chelation of thiol residues in alpha1-adrenoceptors in vascular smooth muscle rather than voltage-sensitive Ca(2+)-channels (Ca(2+)VERSUS-channels). The magnitudes and durations of the vasoconstrictor responses elicited by phenylephrine were markedly reduced after the injections of PHMBA. In contrast, the maximal phenylephrine-induced responses were not affected whereas the durations of these responses were markedly attenuated after injection of the Ca(2+)VERSUS-channel blocker, nifedipine. Nifedipine elicited pronounced and sustained falls in mean arterial blood pressure and vascular resistances in PHMBA-treated rats. Moreover, the vasodilator actions of the nitric oxide-donor, sodium nitroprusside were minimally attenuated by PHMBA whereas they were markedly attenuated by nifedipine. These findings support evidence that the vasoconstrictor responses due to activation of alpha1-adrenoceptors are initiated by mobilization of intracellular pools of Ca(2+) whereas they are sustained by opening of Ca(2+)VERSUS-channels. These findings also suggest that PHMBA diminishes the vasoconstrictor effects of phenylephrine by chelation of thiol residues in alpha1-adrenoceptors rather than by blockade of Ca(2+)VERSUS-channels, and that chelation of these thiol residues prevents agonist occupation and/or activation of these receptors and subsequent mobilization of intracellular pools of Ca(2+).

Potential role of nitration and oxidation reactions in the effects of peroxynitrite on the function of β-adrenoceptor sub-types in the rat

This study examined the hemodynamic responses elicited by the beta-adrenoceptor agonist, isoproterenol (1 and 10 microg/kg, i.v.) before and after administration of (i) peroxynitrite (10 x 10 micromol/kg, i.v.), (ii) the thiol chelator, para-hydroxymercurobenzoic acid (pHMBA, 75 micromol/kg, i.v.), and (iii) the electron acceptor, nitroblue tetrazolium (NBT, 10 micromol/kg, i.v.) in pentobarbital-anesthetized rats. The tachycardia elicited by the lower dose of isoproterenol was diminished whereas the tachycardia elicited by the higher dose was not attenuated after administration of peroxynitrite. The falls in hindquarter and renal vascular resistances elicited by both doses of isoproterenol were substantially diminished whereas the isoproterenol-induced falls in mesenteric vascular resistance were not changed after administration of peroxynitrite. All of the isoproterenol-induced responses were markedly attenuated after administration of pHMBA or NBT. These findings suggest that the oxidation and/or nitration of beta-adrenoceptors impair the ability of isoproterenol to bind to and/or activate these G protein-coupled receptors. beta1-, beta2- and beta3-adrenoceptors contain extracellular cysteine residues susceptible to oxidation (i.e., disulfide-bridge formation) whereas only the beta1- and beta2-adrenoceptors contain extracellular tyrosine residues susceptible to nitration. These findings also suggest that sustained impairment of beta1- and beta2-adrenoceptor function by peroxynitrite is due to nitration of extracellular tyrosine residues in these receptors. By analogy, beta3-adrenoceptors may not be permanently affected by peroxynitrite because these receptors are devoid of extracellular tyrosine residues.

Alpha 1 adrenergic receptor control of renal blood vessels during aging

Aging humans and rats have a reduced renal vascular constriction response to stress, change in posture, or exercise. In this study, renal interlobar arteries from 9- (intermediate age) to 15-month-old (aging) male Wistar rats constricted less to alpha-adrenergic agonists than those of 4-month-old (young adult) rats. The reduced contraction to A61603 (alpha 1 A agonist) was similar to that to norepinephrine and phenylephrine. Therefore, it appears that the reduction in constriction is primarily related to alpha 1 A receptor stimulation. GeneChip microarray hybridization analysis of the interlobar arteries with the RAE 230A GeneChip indicated that there were no significant differences in gene expression for alpha 1 A/C, 1B, or 1D receptors between 4-month-old (young adult) and 1-year-old (aging) male Wistar rats. Competitive binding experiments (prazosin) revealed that maximal binding (Bmax, fmol/mg protein) of the alpha 1 receptors of interlobar arteries was reduced 25% by 10 months of age and 50% by 18+ months of age. Alpha 1 receptor-induced arterial constriction and prazosin binding were both down-regulated. The loss of receptor-initiated constriction likely includes down-regulation of maximum agonist binding by alpha 1 adrenergic receptors.Key words: kidney, stress, blood flow, male vs. female, GeneChip array, prazosin.

The contribution of adrenoceptor subtype(s) in the renal vasculature of diabetic spontaneously hypertensive rats

1. Diabetes and hypertension are both associated with an increased risk of renal disease and are associated with neuropathies, which can cause defective autonomic control of major organs including the kidney. This study aimed to examine the alpha(1)-adrenoceptor subtype(s) involved in mediating adrenergically induced renal vasoconstriction in a rat model of diabetes and hypertension. 2. Male spontaneously hypertensive rats (SHR), 220-280 g, were anaesthetized with sodium pentobarbitone 7-day poststreptozotocin (55 mg x kg(-1) i.p.) treatment. The reductions in renal blood flow (RBF) induced by increasing frequencies of electrical renal nerve stimulation (RNS), close intrarenal bolus doses of noradrenaline (NA), phenylephrine (PE) or methoxamine were determined before and after administration of nitrendipine (Nit), 5-methylurapidil (5-MeU), chloroethylclonidine (CEC) and BMY 7378. 3. In the nondiabetic SHR group, mean arterial pressure (MAP) was 146+/-6 mmHg, RBF was 28.0+/-1.4 ml x min(-1) x kg(-1) and blood glucose was 112.3+/-4.7 mg x dl(-1), and in the diabetic SHR Group, MAP was 144+/-3 mmHg, RBF 26.9+/-1.3 ml(-1) min x kg(-1) and blood glucose 316.2+/-10.5 mg x dl(-1). Nit, 5-MeU and BMY 7378 blunted all the adrenergically induced renal vasoconstrictor responses in SHR and diabetic SHR by 25-35% (all P<0.05), but in diabetic rats the responses induced by RNS and NA treated with 5-MeU were not changed. By contrast, during the administration of CEC, vasoconstrictor responses to all agonists were enhanced by 20-25% (all P<0.05) in both the SHR and diabetic SHR. 4. These findings suggest that alpha(1A) and alpha(1D)-adrenoceptor subtypes contribute in mediating the adrenergically induced constriction of the renal vasculature in both the SHR and diabetic SHR. There was also an indication of a greater contribution of presynaptic adrenoceptors, that is, alpha(1B)-, and/or alpha(2)-subtypes.

Alpha1-adrenoceptor subtypes involved in vasoconstrictor responses to exogenous and neurally released noradrenaline in rat femoral resistance arteries

1. The alpha(1)-adrenoceptor subtypes involved in responses to exogenous and neurally released noradrenaline in rat femoral resistance arteries were characterised using a small vessel myograph, with antagonists prazosin (nonsubtype selective), 5-methyl-urapidil (alpha(1A)-selective), BMY 7378 (alpha(1D)-selective) and the alkylating agent chloroethylclonidine (preferential for alpha(1B)-). 2. Prazosin and 5-methyl-urapidil produced rightward shifts of the exogenous noradrenaline concentration - response curve (CRC) with pA(2) values of 9.2 and 9.1 respectively, in agreement with the presence of alpha(1A)-adrenoceptors. BMY 7378 (1 microm) shifted the noradrenaline CRC with an apparent pK(B) of 6.7, in agreement with the presence of alpha(1A)-, but not alpha(1D)-, adrenoceptors. Chloroethylclonidine at 1 microm had no effect and at 10 microm produced only a small reduction (c. 20%) in the maximum response to noradrenaline, indicating little, if any, contribution from alpha(1B)-adrenoceptors. 3. Responses of the rat femoral resistance arteries to electrical field stimulation (EFS) at 5-30 Hz for 10 s and 0.05 ms pulse width were principally due to alpha(1)-adrenoceptor stimulation. Prazosin and 5-methyl-urapidil inhibited EFS-mediated responses with pIC(50)s of 9.3 and 8.2, respectively, consistent with the alpha(1A)-adrenoceptor being the predominant subtype. Responses to EFS at 10-30 Hz were relatively insensitive to BMY 7378 (pIC(50), 6.5-6.7), while responses to 5 Hz were inhibited with a significantly higher pIC(50) of 8.02, suggesting the contribution of alpha(1D)-adrenoceptors. Chloroethylclonidine had no effect on responses to EFS, ruling out the contribution of an alpha(1B)-subtype. In the presence of cocaine, the predominant subtype involved in responses to EFS was the alpha(1A)-adrenoceptor, with a contribution from alpha(1D)-adrenoceptors at low frequency, as seen in the absence of cocaine. However, there was also a significant increase in the sensitivity to BMY 7378 at higher frequencies, suggesting that a further small alpha(1D)-adrenoceptor component may be uncovered in the presence of cocaine. 5. The present study has shown a predominant role of the alpha(1A)-adrenoceptor in contractions due to exogenous noradrenaline and to neurally released noradrenaline in rat femoral resistance arteries. alpha(1D)-Adrenoceptors are not involved in responses to exogenous noradrenaline but appear to be activated by neurally released noradrenaline at a low frequency of stimulation and at higher frequencies in the presence of neuronal-uptake blockade.

Pharmacological characteristics of Ro 115-1240, a selective alpha1A/1L-adrenoceptor partial agonist: a potential therapy for stress urinary incontinence

OBJECTIVE

To describe the preclinical pharmacology of Ro 115-1240, a peripherally acting selective alpha1A/1L-adrenoceptor (AR) partial agonist, compared with the alpha1A/1L-AR full agonist amidephrine, as AR agonists have some utility in the treatment of stress urinary incontinence (SUI) but are limited by undesirable cardiovascular and central nervous system side-effects.

RESULTS

In radioligand-binding studies Ro 115-1240 had greater affinity for alpha1A than for alpha1B and alpha1D subtypes. The potency and intrinsic activity of amidephrine and Ro 115-1240 relative to noradrenaline were determined in native and cell-based assays using human recombinant alpha1-ARs; they acted as selective alpha1A/1L-AR full and partial agonists, respectively. In anaesthetized micropigs and rabbits, amidephrine and Ro 115-1240 produced non-selective, dose-dependent increases in intraurethral and arterial blood pressures but the magnitude of the pressure increases evoked by Ro 115-1240 were about a third of those with amidephrine. In conscious micropigs both agents produced dose-dependent increases in urethral tension. Again, the magnitude of the urethral response to Ro 115-1240 was about a third of that with amidephrine. More importantly, only amidephrine produced dose-dependent increases in blood pressure and decreases in heart rate. Ro 115-1240 produced a maximum increase in urethral tension with no effect on blood pressure or heart rate.

CONCLUSION

These results show that by combining selectivity for the alpha1A/1L-AR subtype with a reduction in intrinsic agonist efficacy, Ro 115-1240 has reduced haemodynamic effects while retaining to some degree the contractile effects on urethral smooth muscle. These studies indicate that Ro 115-1240 may be useful as a novel treatment for SUI.

Pharmacological characterization of unique prazosin-binding sites in human kidney

In human kidney, we found unique prazosin-binding sites that were insensitive to phentolamine and were thus unlikely to be alpha(1)-adrenoceptors. As the binding of [(3)H]prazosin to phentolamine-insensitive sites was prevented by 100 microM guanabenz, the insensitive sites were evaluated by subtracting [(3)H]prazosin binding in the presence of 100 microM guanabenz from that in the presence of 10 microM phentolamine. [(3)H]Prazosin bound to the phentolamine-insensitive sites monophasically with a high affinity (pK(d); 9.1+/-0.08, n=8), and the B(max) value (814+/-204 fmol mg(-1) protein, n=8) was more than ten times that of the phentolamine-sensitive alpha(1)-adrenoceptor (pK(d)=9.9+/-0.13, B(max)=66+/-23 fmol mg(-1) protein, n=7). The phentolamine-insensitive sites in human kidney were highly sensitive to other quinazoline derivatives such as terazosin and doxazosin. However, other alpha(1)-adrenoceptor antagonists (tamsulosin, WB4101 and corynanthine) did not inhibit the binding at a range of concentrations that generally exhibit alpha(1)-adrenoceptor antagonism, and noradrenaline, rauwolscine and propranolol were without effect on the [(3)H]prazosin binding. On the other hand, ligands for the renal Na(+)-transporter (amiloride and triamterene) and for imidazoline recognition sites (guanabenz, guanfacine and agmatine) displaced the binding of [(3)H]prazosin to phentolamine-insensitive sites at micromolar concentrations. Photoaffinity labeling with [(125)I]iodoarylazidoprazosin showed phentolamine-insensitive labeling at around 100 kDa, a molecular size larger than that of human alpha(1a)- and alpha(1b)-adrenoceptors expressed in 293 cells (50-60 and 70-80 kDa, respectively) on electrophoresis. In contrast, there was no detectable phentolamine-insensitive binding site but were phentolamine-sensitive alpha(1)-adrenoceptors in human liver (pK(d)=10.0+/-0.06, B(max)=44+/-6 fmol mg(-1) protein, n=3). Phentolamine-insensitive prazosin binding sites were also detected in rabbit kidney (approximately 50% of specific binding sites) but were minor in rat kidney (less than 20%). In conclusion, there are unique prazosin-binding sites in human kidney, the pharmacological profiles of which were distinct from those of known adrenoceptors.

Contractile actions of imidazoline alpha-adrenoceptor agonists and effects of noncompetitive alpha1-adrenoceptor antagonists in human vas deferens

The contractile actions of imidazoline alpha-adrenoceptor agonists were investigated in human vas deferens longitudinal and circular muscle. The effects of phenoxybenzamine were studied in comparison to dibenamine and SZL-49 (4-amino-6,7-dimethoxy-2-quinazolinyl-4-(2-bicyclo[2,2,2]octa-2,5-dienylcarbonyl-2-piperazine), an alkylating prazosin analogue that discriminates between alpha(1H)- and alpha(1L)-adrenoceptor subtypes. The imidazoline alpha-adrenoceptor agonist, A-61603 (N-[5-(4,5-dihydro-1H-imidazol-2yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl]methanesulfonamide hydrobromide), was a potent agonist (pD(2); longitudinal muscle 6.9, circular muscle 6.4) and cirazoline a partial agonist (pD(2); longitudinal muscle 6.1, circular muscle 5.1). Oxymetazoline was less effective, indanidine and clonidine were ineffective. SZL-49 produced a differential inhibition of contractions evoked by A-61603 in circular (alpha(1H)) compared to longitudinal (alpha(1L)) muscle and phenoxybenzamine had the opposite effect. Dibenamine inhibited the contractions comparably in both muscle types and analyses of its partial alkylation of receptors yielded identical estimates of equilibrium dissociation constant (pK(d)) for A-61603 in longitudinal (5.82) and circular (5.84) muscle. Receptor occupancy-response relationships revealed that whilst the muscle types are not different in receptor reserves for A-61603, contraction to the potent imidazoline is more efficiently coupled in longitudinal than in circular muscle. This underlies the markedly different responsiveness of the muscle types to cirazoline or oxymetazoline (alpha-adrenoceptor agonists with lower efficacies relative to A-61603). The differential inhibitory actions of phenoxybenzamine and SZL-49 are discussed.

Discrimination by SZL49 between contractions evoked by noradrenaline in longitudinal and circular muscle of human vas deferens

The effects of irreversible alpha1-adrenoceptor antagonists, SZL-49 (an alkylating analogue of prazosin), dibenamine and benextramine on contractions to noradrenaline (NA) in longitudinal and circular muscle of human epididymal vas deferens were investigated. Competitive alpha1-adrenoceptor antagonists were also used to further characterize the alpha1-adrenoceptor subtype stimulated by NA in longitudinal and circular muscle. NA evoked concentration-dependent contractions of both muscle types (pD2; 5.4 and 5.2 respectively). The contraction of circular muscle was comparatively more sensitive than that of longitudinal muscle to pretreatment with SZL-49. In contrast, dibenamine or benextramine produced comparable effects in both muscle types. The relationship between receptor occupancy and contraction in either longitudinal or circular muscle was nonlinear, with half-maximal response requiring similar receptor occupancy (longitudinal muscle 14%, circular muscle 16%). Maximal response in both muscle types occurred with little or no receptor reserve (<10%). The competitive alpha1-adrenoceptor antagonists produced dextral shifts of the dose-response curves to NA in longitudinal and circular muscle. The inhibitory potencies, estimated from the apparent pKB values were significantly different in longitudinal and circular muscle respectively for either WB 4101 (pKB, 8.6 and 9.5) or RS-17053 (pKB, 7.1 and 9.0) but not for Rec 15/2739 (pKB, 9.2 and 9.8) or HV 723 (pKB, 8.3 and 8.4). In conclusion, the potency profile of the competitive alpha1-adrenoceptor antagonists and the lack of different receptor reserves for NA in the muscle types suggest that the discriminatory effects of SZL-49 is primarily due to a predominance of the alpha1L-adrenoceptor subtype in longitudinal muscle and alpha1A-subtype in circular muscle.

Functional characterization of alpha(1)-adrenoceptor subtypes in human skeletal muscle resistance arteries

alpha(1)-adrenoceptor subtypes in human skeletal muscle resistance arteries were characterized using agonists noradrenaline (non-selective) and A61603 (alpha(1A)-selective), the antagonists prazosin (non-selective), 5-methyl-urapidil (alpha(1A)-selective) and BMY7378 (alpha(1D)-selective) and the alkylating agent chloroethylclonidine (preferential for alpha(1B)). Small arteries were obtained from the non-ischaemic skeletal muscle of limbs amputated for critical limb ischaemia and isometric tension recorded using wire myography. Prazosin antagonized responses to noradrenaline with a pA(2) value of 9.18, consistent with the presence of alpha(1)-adrenoceptors, although the Schild slope (1.32) was significantly different from unity. 5-Methyl-urapidil competitively antagonized responses to noradrenaline with a pK(B) value of 8.48 and a Schild slope of 0.99, consistent with the presence of alpha(1A)-adrenoceptors. In the presence of 300 nM 5-methyl-urapidil, noradrenaline exhibited biphasic concentration response curves, indicating the presence of a minor population of a 5-methyl-urapidil-resistant subtype. Contractile responses to noradrenaline were not affected by 1 microM chloroethylclonidine suggesting the absence of alpha(1B)-adrenoceptors. Maximum responses to noradrenaline and A61603 were reduced to a similar extent by 10 microM chloroethylclonidine, suggesting an effect of chloroethylclonidine at alpha(1A)-adrenoceptors at the higher concentration. BMY7378 (10 and 100 nM) had no effect on responses to noradrenaline. BMY7378 (1 microM) poorly shifted the potency of noradrenaline giving a pA(2) of 6.52. These results rule out the presence of the alpha(1D)-subtype. These results show that contractile responses to noradrenaline in human skeletal muscle resistance arteries are predominantly mediated by the alpha(1A)-adrenoceptor subtype with a minor population of an unknown alpha(1)-adrenoceptor subtype.

The alpha(1A)-adrenoceptor subtype mediates contraction in rat femoral resistance arteries

In this study, alpha(1)-adrenoceptor subtypes were characterised in rat femoral resistance arteries mounted on a small vessel myograph. A-61603 was found to be more potent than noradrenaline and phenylephrine in these arteries. Brimonidine (UK 14304) could not evoke any contractile responses and the sensitivity to noradrenaline and phenylephrine was not affected by (8aR,12aS,13aS)-5,8,8a,9,10,11,12,12a,13a-decahydro-3-methoxy-12-(ethylsulphonyl)-6H-isoquino[2,1-g][1,6]-naphthyridine (RS 79948), ruling out the presence of alpha(2)-adrenoceptors. Prazosin, 5-methyl-urapidil and 2-([2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4-benzodioxane (WB 4101) produced rightward shifts in the sensitivity to noradrenaline, giving pA(2) values of 9.6, 9.4 and 10.4, respectively, in agreement with the presence of alpha(1A)-adrenoceptors. (8-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY 7378; 1 microM) produced a small shift in the sensitivity of noradrenaline giving a pK(B) of 7.2. In the presence of 300 nM 5-methyl-urapidil, sensitivity to noradrenaline was not further shifted by 1 microM BMY 7378. Responses to noradrenaline were unaffected by the alpha(1B)-adrenoceptor alkylating agent chloroethylclonidine (1 microM). These results suggest alpha(1A)-adrenoceptors mediate contractile responses to noradrenaline in rat femoral resistance arteries.

Effect of methoxamine on maximum urethral pressure in women with genuine stress incontinence: a placebo-controlled, double-blind crossover study

The aim of the study was to evaluate the potential role for a selective alpha1-adrenoceptor agonist in the treatment of urinary stress incontinence. A randomised, double-blind, placebo-controlled, crossover study design was employed. Half log incremental doses of intravenous methoxamine or placebo (saline) were administered to a group of women with genuine stress incontinence while measuring maximum urethral pressure (MUP), blood pressure, heart rate, and symptomatic side effects. Methoxamine evoked non-significant increases in MUP and diastolic blood pressure but caused a significant rise in systolic blood pressure and significant fall in heart rate at maximum dosage. Systemic side effects including piloerection, headache, and cold extremities were experienced in all subjects. The results indicate that the clinical usefulness of direct, peripherally acting sub-type-selective alpha1-adrenoceptor agonists in the medical treatment of stress incontinence may be limited by associated piloerection and cardiovascular side effects.

Failure of AH11110A to functionally discriminate between alpha(1)-adrenoceptor subtypes A, B and D or between alpha(1)- and alpha(2)-adrenoceptors

The potency of the putatively alpha(1B)-adrenoceptor selective drug, 1-[biphenyl-2-yloxy]-4-imino-4-piperidin-1-yl-butan-2-ol (AH11110A), to antagonize contraction upon stimulation of alpha(1A)-adrenoceptors in rat vas deferens and rat perfused kidney, alpha(1B)-adrenoceptors in guinea-pig spleen, mouse spleen and rabbit aorta, and alpha(1D)-adrenoceptors in rat aorta and pulmonary artery was evaluated and compared to that of a number of subtype-discriminating antagonists. N-[3-[4-(2-Methoxyphenyl)-1-piperazinyl]propyl]-3-methyl-4-oxo-2-phenyl-4H-1-benzopyran-8-carboxamide (Rec 15/2739) and (+/-)-1,3,5-trimethyl-6-[[3-[4-((2,3-dihydro-2-hydroxymethyl)-1,4-benzodioxin-5-yl)-1-piperazinyl]propyl]amino]-2,4(1H,3H)-pyrimidinedione (B8805-033) were confirmed as selective for alpha(1A)-adrenoceptors, 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY 7378), 8-[2-(1,4-benzodioxan-2-ylmethylamino)ethyl]-8-azaspiro[4.5]decane-7,9-dione (MDL 73005EF), and cystazosin were found to be selective for alpha(1D)-adrenoceptors, whereas spiperone was weakly selective for alpha(1B)-over alpha(1A)-adrenoceptors. However, from the functional affinity profile obtained for AH11110A at alpha(1A)-adrenoceptors (pA(2)=6.41 in rat vas deferens), alpha(1B)-adrenoceptors (pA(2)=5.40-6.54) and alpha(1D)-adrenoceptors (pA(2)=5.47-5.48), the affinity and presumed selectivity previously obtained for AH11110A in radioligand binding studies at native alpha(1B)- and cloned alpha(1b)-adrenoceptors (pK(i)=7.10-7.73) could not be confirmed. Additionally, AH11110A enhanced the general contractility of rat vas deferens, produced a bell-shaped dose-response curve of vasodilation in perfused rat kidney, and its antagonism in most other tissues was not simply competitive. The affinity of AH11110A for prejunctional alpha(2)-adrenoceptors in rabbit vas deferens (pA(2)=5.44) was not much lower than that displayed for alpha(1)-adrenoceptor subtypes, revealing that AH11110A, besides alpha(1)-adrenoceptors, also interacts with alpha(2)-adrenoceptors, and thus may be unsuitable for alpha-adrenoceptor subtype characterization, at least in smooth muscle containing functional studies.

In vitro studies on L-771,688 (SNAP 6383), a new potent and selective alpha1A-adrenoceptor antagonist

L-771,688 (SNAP 6383, methyl(4S)-4-(3, 4-difluorophenyl)-6-[(methyloxy)methyl]-2-oxo-3-[(¿3-[4-(2-pyridin yl)-1-piperidinyl]propyl¿amino)carbonyl]-1,2,3, 4-tetrahydro-5-pyrimidine carboxylate) had high affinity (Ki less than or = 1 nM) for [3H]prazosin binding to cloned human, rat and dog alpha1A-adrenoceptors and high selectivity (>500-fold) over alpha1B and alpha1D-adrenoceptors. [3H]Prazosin / (+/-)-beta-[125I]-4-hydroxy-phenyl)-ethyl-aminomethylteralone ([125I]HEAT) binding studies in human and animal tissues known to contain alpha1A and non-alpha1A-adrenoceptors further demonstrated the potency and alpha1A-subtype selectivity of L-771,688. [3H]L-771,688 binding studies at the cloned human alpha1A-adrenoceptors and in rat tissues indicated that specific [3H]L-771,688 binding was saturable and of high affinity (Kd=43-90 pM) and represented binding to the pharmacologically relevant alpha1A-adrenoceptors. L-771,688 antagonized norepinephrine-induced inositol-phosphate responses in cloned human alpha1A-adrenoceptors, as well as phenylephrine or A-61603 (N-[5-4,5-dihydro-1H-imidazol-2yl)-2-hydroxy-5,6,7, 8-terahydro-naphthlen-1-yl] methanesulfonamide hydrobromide) induced contraction in isolated rat, dog and human prostate, human and monkey bladder neck and rat caudal artery with apparent Kb values of 0.02-0.28 nM. In contrast, the contraction of rat aorta induced by norepinephrine was resistant to L-771,688. These data indicate that L-771,688 is a highly selective alpha1A-adrenoceptor antagonist.

Cloning of rabbit alpha(1b)-adrenoceptor and pharmacological comparison of alpha(1a)-, alpha(1b)- and alpha(1d)-adrenoceptors in rabbit

We have isolated a cDNA clone of the rabbit alpha(1b)-adrenoceptor which has an open reading frame of 1557 nucleotides encoding a protein of 518 amino acids. The sequence shows higher identity to those of hamster, human, and rat alpha(1b)-adrenoceptors than to those of rabbit alpha(1a)- and alpha(1d)-adrenoceptors. The pharmacological binding properties of this clone expressed in Cos-7 cells showed a characteristic profile as alpha(1b)-adrenoceptor; high affinity for prazosin (pK(i)=10.3), relatively high affinity for tamsulosin (9.5) and low affinity for (-)-(R)-1-(3-hydroxypropyl)-5-[2-[[2-[2-(2,2, 2-trifluoroethoxy)phenoxy]ethyl]amino]propyl]indoline-7-carboxamid e (KMD3213) (8.5), 2-(2,6-dimethoxy-phenoxyethyl)-aminomethyl-1, 4-benzodioxane hydrochloride (WB4101) (8.7), and 8-[2-[4-(2-methoxy-phenyl)-L-piperazinyl]-8-azaspiro[4,5]decane-7, 9-dione dihydrochloride (BMY7378) (7.3). We have compared the levels of mRNA expression of three alpha(1)-adrenoceptor subtypes in rabbit tissues using the competitive reverse transcription/polymerase chain reaction (RT/PCR) assay. In most rabbit tissues except heart, alpha(1a)-adrenoceptor mRNA was expressed 10 folds more than the other two subtypes. However, binding experiments with [3H]prazosin and [3H]KMD3213 in rabbit tissues revealed a poor relationship between binding density and mRNA level. Especially, alpha(1b) binding sites were exclusively predominant in spleen, whereas the alpha(1b) subtype was minor at the mRNA level. These results indicate a high identity of structural and pharmacological profiles of three distinct alpha(1)-adrenoceptor subtypes between rabbit and other species, but there are species differences in their distribution.

Buspirone functionally discriminates tissues endowed with alpha1-adrenoceptor subtypes A, B, D and L

The affinity for functional alpha1-adrenoceptor subtypes of buspirone in comparison with its close structural analogs and selective alpha1D-adrenoceptor antagonists, BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]dec ane-7,9-dione) and MDL 73005EF (8-[2-(1,4-benzodioxan-2-ylmethylamino)ethyl]-8-azaspiro+ ++[4.5]decane-7,9-dione), was determined, namely at subtype A in rat vas deferens and perfused kidney, at subtype B in guinea-pig and mouse spleen, at subtype L in rabbit spleen, and at subtype D in rat aorta and pulmonary artery against noradrenaline-evoked contractions. BMY 7378 and MDL 73005EF were confirmed as 30- and 20-fold selective antagonists, respectively, for alpha1D- over both alpha1A- and alpha1B-adrenoceptors. Buspirone was a weak antagonist without intrinsic activity at alpha1A-adrenoceptors in rat vas deferens (pA2 = 6.12), at alpha1B-adrenoceptors in guinea-pig and mouse spleen (pA2 = 5.54 and 5.59) and at alpha1L-adrenoceptors in rabbit spleen (pA2 = 4.99), but caused partial vasoconstriction in rat kidney that was attenuable by the subtype D-selective adrenoceptor antagonist BMY 7378, but hardly by the subtype A-selective adrenoceptor antagonist B8805-033 ((+/-)-1,3,5-trimethyl-6-[[3-[4-((2,3-dihydro-2-hydroxymethyl)-1,4-be nzodioxin-5-yl)-1-piperazinyl]propyl]amino]-2,4(1H,3H)-pyrimidinedion e), confirming the additional presence of alpha1D-adrenoceptors mediating rat renal vasoconstriction. Buspirone behaved as a partial agonist at alpha1D-adrenoceptors in rat aorta (pD2 = 6.77, intrinsic activity (i.a.)= 0.40) and pulmonary artery (pD2 = 7.16, i.a. = 0.59). With buspirone as agonist in these tissues, the pA2 values of subtype-discriminating antagonists were consistent with their alpha1D-adrenoceptor affinity determined in rat aorta against noradrenaline and with published binding data on cloned alpha1d-adrenoceptors. The results provide pharmacological evidence that (1) in functional preparations for the A subtype, like rat vas deferens and perfused kidney, for the B subtype, like guinea-pig and mouse spleen, and for the L subtype, like rabbit spleen, buspirone is a weak antagonist without intrinsic activity, but (2) behaves as a partial agonist in rat aorta and pulmonary artery as models for the D subtype and (3) detects an additional vasoconstrictor alpha1D-adrenoceptor in rat kidney. Buspirone, like its close analogs BMY 7378 and MDL 73005EF, thus might also be a useful tool for functionally discriminating alpha1D- from alpha1A-, alpha1B- and alpha1L-adrenoceptors in various tissues.

[3H]AL-5848 ([3H]9beta-(+)-Fluprostenol). Carboxylic acid of travoprost (AL-6221), a novel FP prostaglandin to study the pharmacology and autoradiographic localization of the FP receptor

AL-5848 (5Z,13E)-(9 S,11R,15S)-9,11,15-trihydroxy-5,13-prostadienoic acid) is the carboxylic acid of travoprost (AL-6221), a single (+)-isomer of (+/-)-fluprostenol, an FP-class prostaglandin agonist which lowers intraocular pressure. We have prepared a radioligand from this selective prostaglandin and demonstrated its utility for studying the pharmacology and autoradiographic location of the FP-receptor. Specific [3H]AL-5848 binding (84% of total) was linearly related to bovine corpus luteum tissue concentration and reached equilibrium within 275 min at 23 degrees C. Scatchard analysis of saturation isotherms indicated interaction of [3H]AL-5848 with a single class of high-affinity (dissociation constant, Kd, = 33.8+/-2.9 nM, n = 4) and saturable (Bmax = 37.3+/-3.0 pmol (g wet weight tissue)(-1)) FP receptor-binding sites in bovine corpus luteum. Specific [3H]AL-5848 binding was potently inhibited by the FP-receptor ligands 16-phenoxyPGF2alpha (inhibition constant Ki = 17.3 nM); cloprostenol (Ki = 56.8 nM); 17-phenyl PGF2alpha (Ki = 87.0 nM); AL-5848 (Ki = 52.1 nM); PGF2alpha (Ki = 195 nM); PHXA85 (Ki = 223 nM); (n = 3-11) but very weakly by PGD2, ZK118182, BW245C, PGE2, PGI2 and U-46619. The pharmacology of specific [3H]AL-5848 binding correlated well with the pharmacology of [3H]PGF2alpha binding in the bovine corpus luteum preparation (r = 0.98, n = 14, P<0.0001) and also with functional responses in Swiss 3T3 and rat vascular smooth muscle cells (A7r5) (r = 0.96) expressing FP receptors. Autoradiographic studies revealed high levels of specific FP-receptor binding with [3H]AL-5848 on granulosa cells in the bovine corpus luteum sections, and on longitudinal ciliary muscle, the ciliary process, the iris sphincter and the retina in eye sections from man. These studies show [3H]AL-5848 to be a high-affinity agonist radioligand capable of selectively labelling the FP prostaglandin receptor.

In vitro alpha1-adrenoceptor pharmacology of Ro 70-0004 and RS-100329, novel alpha1A-adrenoceptor selective antagonists

It has been hypothesized that in patients with benign prostatic hyperplasia, selective antagonism of the alpha1A-adrenoceptor-mediated contraction of lower urinary tract tissues may, via a selective relief of outlet obstruction, lead to an improvement in symptoms. The present study describes the alpha1-adrenoceptor (alpha1-AR) subtype selectivities of two novel alpha1-AR antagonists, Ro 70-0004 (aka RS-100975) and a structurally-related compound RS-100329, and compares them with those of prazosin and tamsulosin. Radioligand binding and second-messenger studies in intact CHO-K1 cells expressing human cloned alpha1A-, alpha1B- and alpha1D-AR showed nanomolar affinity and significant alpha1A-AR subtype selectivity for both Ro 70-0004 (pKi 8.9: 60 and 50 fold selectivity) and RS-100329 (pKi 9.6: 126 and 50 fold selectivity) over the alpha1B- and alpha1D-AR subtypes respectively. In contrast, prazosin and tamsulosin showed little subtype selectivity. Noradrenaline-induced contractions of human lower urinary tract (LUT) tissues or rabbit bladder neck were competitively antagonized by Ro 70-0004 (pA2 8.8 and 8.9), RS-100329 (pA2 9.2 and 9.2), tamsulosin (pA2 10.4 and 9.8) and prazosin (pA2 8.7 and 8.3 respectively). Affinity estimates for tamsulosin and prazosin in antagonizing alpha1-AR-mediated contractions of human renal artery (HRA) and rat aorta (RA) were similar to those observed in LUT tissues, whereas Ro 70-0004 and RS-100329 were approximately 100 fold less potent (pA2 values of 6.8/6.8 and 7.3/7.9 in HRA/RA respectively). The alpha1A-AR subtype selectivity of Ro 70-0004 and RS-100329, demonstrated in both cloned and native systems, should allow for an evaluation of the clinical utility of a 'uroselective' agent for the treatment of symptoms associated with benign prostatic hyperplasia.

Characterisation of the functional alpha-adrenoceptor subtype in the isolated female pig urethra

The aim of the present study is to characterise the contraction-mediating functional alpha-adrenoceptor of the female pig urethra. Alpha-adrenoceptor reference agonists were used to contract the isolated female pig urethra. The relative intrinsic activity was noradrenaline (1.0), phenylephrine (0.91), methoxamine (0.74), (+/-)-3'-(2-amino-1-hydroxyethyl)-4'-fluoromethane-sulfonanilid e hydrochloride (NS-49) (0.68), oxymetazoline (0.60), dopamine (0.50), clonidine (0.43), midodrine (0.32), ephedrine (0.30), 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK 14,304) (0.11), and phenylpropanolamine (0.11). The 21 competitive antagonists used caused parallel rightward shifts in the alpha-adrenoceptor agonist concentration-response curves, giving linear Schild-plots with slopes not significantly different from unity, suggesting that contraction was mediated by a single receptor. The antagonist pK(B) values calculated were R(-)-tamsulosin (9.68), risperidone (9.19), 2-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-4,4-dimethyl-1,3(2H,4H)-+ ++isoquinolinedione (AR-C 239) (9.09), 2-([2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4-benzodioxane (WB-4101) (8.87), N-[3-[4-(2-methoxyphenyl)-1-piperazinyl]propyl]-3-methyl-4-oxo-2-phenyl- 4H-1-benzopyran-8-carboxamide monomethanesulfonate (Rec 15/2739/3) (8.81), 5-methylurapidil (8.59), prazosin (8.57), benoxathian (8.56), S(+)-tamsulosin (8.27), indoramin (8.11), doxazosin (7.96), alfuzosine (7.82), phentolamine (7.70), terazosin (7.52), spiperone (7.48), oxymetazoline (7.40), 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4,5]deca ne-7,9-dione dihydrochloride (BMY 7378) (7.05), corynanthine (6.98), rauwolscine (6.40), yohimbine (6.22), and N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha,alpha-dime thyl-1H-indole-3-ethanamine hydrochloride (RS 17053) (6.07). Correlation of subtype-selective antagonist pK(B) values was best with published values for the alpha1a/1A-adrenoceptor subtype. Therefore, the present results suggest that contraction of the female pig urethra is caused by activation of the alpha1A-adrenoceptor.

Distribution of alpha1-adrenoceptor subtype mRNAs in human renal cortex

OBJECTIVE

To determine the quantity and distribution of alpha1-adrenoceptor subtype mRNAs in human renal cortex.

MATERIALS AND METHODS

Specimens of renal cortex tissue were obtained at the time of radical nephrectomy or total nephroureterectomy from 46 patients (mean age 59.0 years, sd 14.7) with renal cell carcinoma, renal pelvic or ureteric tumour. Using the reverse-transcriptase polymerase chain reaction (RT-PCR), the RNase protection assay and in situ hybridization, the presentation, quantity and distribution of alpha1-adrenoceptor subtype mRNAs were determined.

RESULTS

Expression of the three alpha1-adrenoceptor subtype mRNAs (alpha1a, alpha1b and alpha1d) was confirmed in the arteries of the renal cortex (arciform, interlobular, arteriole), but among the three subtypes, the alpha1b was less apparent by in situ hybridization. Intense alpha1-mRNA staining was apparent especially in the smooth muscle of arterial walls. In both proximal and distal renal tubules, each of the alpha1-mRNAs was less marked in cytoplasm than in the arteries. In the glomeruli weak staining was detected in the endothelium but there was no obvious staining in the veins. RT-PCR showed all three subtypes of alpha1-adrenoceptor. The RNase protection assay showed that the predominant alpha1-adrenoceptor subtype mRNA in human renal cortex was alpha1a. However, the abundance of alpha1a-mRNA in human kidney was much less than in the prostate.

CONCLUSION

Three alpha1-adrenoceptor subtype mRNAs were recognized in human renal cortex and detected particularly in the smooth muscle of the arteries. There was more alpha1a-adrenoceptor subtype in human renal cortex than the other subtypes. It is not known how each subtype operates against adrenergic stimulation; further studies are needed to examine receptor density or receptor function.

Differential vascular alpha1-adrenoceptor antagonism by tamsulosin and terazosin

AIMS

In patients with lower urinary tract symptoms suggestive of benign prostatic obstruction the alpha1-adrenoceptor antagonist terazosin lowers blood pressure whereas only very small if any alterations were reported with the alpha1-adrenoceptor antagonist tamsulosin. Therefore, we have compared the vascular alpha1-adrenoceptor antagonism of tamsulosin and terazosin directly.

METHODS

Ten healthy subjects were investigated in a randomized, single-blind, three-way cross-over design and received a single dose of 0.4 mg tamsulosin, 5 mg terazosin or placebo on 3 study days at least 1 week apart. Before and 1, 3, 5, 7, 10 and 23.5 h after drug intake, alterations of diastolic blood pressure and other haemodynamic parameters in response to a graded infusion of the alpha1-adrenoceptor agonist phenylephrine were determined non-invasively.

RESULTS

At most time points tamsulosin inhibited phenylephrine-induced diastolic blood pressure elevations significantly less than terazosin (5 h time point: median difference in inhibition 35%, 95% CI: 18.7-50.3%). On the other hand, phenylephrine-induced changes of cardiac output, heart rate and stroke volume were similar during both active treatments.

CONCLUSIONS

In doses equi-effective for treatment of lower urinary tract symptoms tamsulosin causes less inhibition of vasoconstriction than terazosin.

Alterations of cardiac alpha 1-adrenoceptor subtypes in hypothyroid rats

1. Alterations in the cardiac alpha 1-adrenoceptor and its subtypes in hypothyroid rats were studied by radioligand binding assays and reverse transcription-polymerase chain reaction (RT-PCR). Hypothyroidism was created by feeding rats with 0.2% 2-thiouracil solution instead of daily drinking water for 20 days. 2. The density of cardiac alpha 1-adrenoceptors (Bmax) was increased from 67.5 +/- 4.3 fmol/mg in control rats to 81.1 +/- 7.2 fmol/mg (P < 0.05) in hypothyroid rats. 3. Compared with control rats, in hypothyroid rats the percentages of high-affinity sites for (+)-niguldipine and 5-methylurapidil were increased from 13.8 +/- 5.6 and 31.9 +/- 6.3%, respectively, to 24.9 +/- 7.3 and 45.5 +/- 2.4%, respectively (both P < 0.05), while those for BMY7378 were decreased from 37.2 +/- 8.9 to 23.8 +/- 8.4% (P < 0.05), respectively. The percentage of high-affinity sites for WB4101 was not significantly different in control and hypothyroid rats (43.3 +/- 9.1 and 39.4 +/- 3.6%, respectively). 4. Reverse transcription-PCR experiments revealed that the steady state levels of mRNA for alpha 1A- and alpha 1B-adrenoceptors were increased, while those for alpha 1D-adrenoceptor were decreased in the hearts of hypothyroid rats. 5. The concentration-contraction response curves for noradrenaline in the presence of a beta-adrenoceptor antagonist in control and hypothyroid rats showed that the maximal response was reduced from 344 +/- 58 to 200 +/- 23 mg, respectively (P < 0.05). 6. The data suggest that in hypothyroid rats the total number of cardiac alpha 1-adrenoceptors is increased. The change is subtype-selective, with levels of alpha 1A- and alpha 1B-adrenoceptors being increased and levels of alpha 1D-adrenoceptors being reduced. Furthermore, the positive inotropic response mediated by alpha 1-adrenoceptors is reduced in hypothyroid rats.

Pharmacological pleiotropism of the human recombinant alpha1A-adrenoceptor: implications for alpha1-adrenoceptor classification

1. Three fully-defined alpha1-adrenoceptors (alpha1A, alpha1B and alpha1D) have been established in pharmacological and molecular studies. A fourth alpha1-adrenoceptor, the putative alpha1L-adrenoceptor, has been defined in functional but not molecular studies, and has been proposed to mediate contraction of human lower urinary tract tissues; its relationship to the three fully characterized alpha1-adrenoceptors is not known. 2. In the present study, binding affinities were estimated by displacement of [3H]-prazosin in membrane homogenates of Chinese hamster ovary (CHO-K1) cells stably expressing the human alpha1A-, alpha1B- and alpha1D-adrenoceptors and were compared with affinity estimates obtained functionally in identical cells by measuring inhibition of noradrenaline (NA)-stimulated accumulation of [3H]-inositol phosphates. 3. For the alpha1A-adrenoceptor, binding studies revealed a pharmacological profile typical for the classically defined alpha1A-adrenoceptor, such that prazosin, RS-17053, WB 4101, 5-methylurapidil, Rec 15/2739 and S-niguldipine all displayed subnanomolar affinity. A different profile of affinity estimates was obtained in inositol phosphates accumulation studies: prazosin, WB 4101, 5-methylurapidil, RS-17053 and S-niguldipine showed 10 to 40 fold lower affinity than in membrane binding. However, affinity estimates were not 'frameshifted', as tamsulosin, indoramin and Rec 15/2739 yielded similar, high affinity estimates in binding and functional assays. 4. In contrast, results from human alpha1B- and alpha1D-adrenoceptors expressed in CHO-K1 cells gave antagonist affinity profiles in binding and functional assays that were essentially identical. 5. A concordance of affinity estimates from the functional (inositol phosphates accumulation) studies of the alpha1A-adrenoceptor in CHO-K1 cells was found with estimates published recently from contractile studies in human lower urinary tract tissues (putative alpha1L-adrenoceptor). These data show that upon functional pharmacological analysis, the cloned alpha1A-adrenoceptor displays pharmacological recognition properties consistent with those of the putative alpha1L-adrenoceptor. Why this profile differs from that obtained in membrane binding, and whether it explains the alpha1L-adrenoceptor pharmacology observed in many native tissues, requires further investigation.

Characterization of subtype of alpha1-adrenoceptor mediating vasoconstriction in perfused rat hind limb

The subtype of alpha1-adrenoceptor mediating the exogenous noradrenaline-induced vasopressor response in perfused rat hind limb was determined by functional measurements and radioligand binding assays. The potencies (pA2 values) of alpha1A-adrenoceptor-selective antagonists, RS-17053 (N-[2-(2-cyclopropylmethoxy-phenoxy) ethyl]-5-chloro-alpha,alpha-dimethyl-1H-indole-3-ethanamine hydrochloride), WB 4101 (2-(2,6-dimethoxyphenoxyethyl) aminomethyl-1,4 benzodioxane), 5-methyl-urapidil, and the alpha1D-adrenoceptor-selective antagonist, BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol[4.5]de cane-7,9-dione), to inhibit the noradrenaline-induced vasopressor response determined by Schild plot were 9.47 +/- 0.21, 9.48 +/- 0.19, 8.10 +/- 0.27 and 6.66 +/- 0.14, respectively, with no slope significantly different from unity. The affinities (K(i) values) of these antagonists were determined by displacement of 125I-BE 2254 (2-beta(4-hydroxyphenyl)-ethylaminomethyl)-tetralone) binding from the cloned alpha1a-, alpha1b-, alpha1d-adrenoceptor, stably expressed in human embryonic kidney (HEK) 293 cells. The pA2 values of the above antagonists correlated well with the binding K(i) values only for alphaIA-adrenoceptors (r = 0.93), but not for alpha1B-adrenoceptors (r = 0.51) and alpha1D-adrenoceptors (r = 0.13). The concentration-vasopressor response curve for noradrenaline was not significantly affected by pretreatment with 50 microM chloroethylclonidine for 30 min. The results suggest that only alpha1A-adrenoceptors mediate the noradrenaline-induced vasopressor response in perfused rat hind limb.

Alterations of mRNA levels of alpha 1-adrenoceptor subtypes with maturation and ageing in different rat blood vessels

1. Alterations of mRNA levels of alpha 1-adrenoceptor subtypes during maturation and ageing were determined by reverse transcription-polymerase chain reaction (RT-PCR) in aortae and renal, pulmonary and mesenteric arteries isolated from 3, 12 and 24-month-old rats. 2. The steady state levels for alpha 1A-, alpha 1B- and alpha 1D-adrenoceptors in aorta declined with maturation and ageing. In renal artery there was a decrease in mRNA for the alpha 1B-adrenoceptor in aged rats. However, in mesenteric and pulmonary arteries there were no changes in mRNA levels for the three subtypes of alpha 1-adrenoceptors as a result of maturation and ageing. 3. The results suggest that expression of alpha 1-adrenoceptors is changed heterogeneously in different blood vessels during maturation and ageing in rats.

Alpha 1D-adrenoceptors play little role in the positive inotropic action of phenylephrine

This study was designed to determine if the positive inotropic action of alpha 1-adrenergic stimulation in rat heart is mediated via alpha 1D-adrenoceptors. Isolated left atrial and papillary muscle were suspended in oxygenated Krebs-Henseleit buffer (37 degrees C) containing 1 microM nadolol and paced at 3.0 Hz. Isometric tension was continuously monitored. Cumulative concentration-response curves for phenylephrine were obtained in the presence and absence of BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol[4.5] decane-7,9-dione), a selective competitive alpha 1D-adrenoceptor antagonist. BMY 7378 at concentrations up to 30 nM did not significantly affect the positive inotropic response to phenylephrine. In contrast, as reported by other investigators, alpha 1D-adrenoceptor-selective concentrations of this antagonist (3 and 10 nM) did elicit a concentration-dependent right-ward shift in the vasoconstrictor response to phenylephrine in rat abdominal aorta. These data suggest that alpha 1D-adrenoceptors do not play a major role in the positive inotropic action of alpha-adrenoceptor stimulation in rat cardiac muscle.

Pharmacological characterization of an alpha 1A-adrenoceptor mediating contractile responses to noradrenaline in isolated caudal artery of rat

1. The alpha 1-adrenoceptor population mediating contraction of caudal artery of rat has been characterized by using quantitative receptor pharmacology. 2. Cumulative concentration-effect (E/[A]) curves to noradrenaline (NA) yielded a p[A]50 of 5.56 +/- 0.05 (n = 16). Prazosin caused concentration-dependent, parallel, dextral shifts of E/[A] curves to NA yielding a pKb of 8.9 (Schild regression slope = 1.0). RS-17053 (N-[2-(2-cyclopropyl methoxy phenoxy) ethyl]-5-chloro-alpha, alpha-dimethyl-1H-indole- 3-ethanamine hydrochloride; 10-100 nM), a selective alpha 1 A-adrenoceptor antagonist, produced non-parallel, biphasic, dextral shifts of E/[A] curves to NA, suggesting the involvement of more than one alpha 1-adrenoceptor subtype. Analysis of the high affinity component yielded an apparent pA2 value of 9.2 +/- 0.3. 3. A-61603, a selective agonist at alpha 1A adrenoceptors behaved as a full agonist relative to NA and yielded monophasic E/[A] curves with a p[A50] of 7.59 +/- 0.04 (n = 15). Pretreatment of tissues with chloroethylclonidine (CEC; 100 microM for 20 min, followed by 40 min washout), which preferentially alkylates alpha 1B- and alpha 1D-adrenoceptors, did not alter E/[A] curves to A-61603. Prazosin (3-300 nM) caused concentration-dependent, parallel, dextral shifts of E/[A] curves to A-61603 yielding a pA2 estimate of 9.2 +/- 0.2. 4. Experiments with alpha 1-adrenoceptor antagonists of varying subtype selectivities (RS-17053, SNAP 5089, tamsulosin, 5-methylurapidil, BMY 7378, HV 723 and REC 15/2739) revealed parallel dextral shifts of E/[A] curves to A-61603. Schild regression analyses yielded pA2 estimates of 9.2, 9.3, 11.2, 9.0, 6.3, 8.7 and 10.0 for RS-17053, SNAP 5089, tamsulosin, 5-methylurapidil, BMY 7378, HV 723 and REC 15/2739, respectively, although deviations from unit slope (possibly reflecting a secondary involvement of another alpha 1-adrenoceptor) hindered estimations of pKb for some antagonists. The antagonist affinity profile obtained reflects best that described for the alpha 1A-adrenoceptor. 5. In conclusion, caudal artery of rat contracts in response to NA via activation of at least two alpha 1-adrenoceptor subtypes. One of these subtypes displays the pharmacology of the alpha 1A-adrenoceptor, while the other remains to be defined. Use of the novel selective agonist, A-61603, allows for limited pharmacological isolation of the alpha 1A-adrenoceptor permitting characterization of the properties of selective antagonists.

Pharmacological classification of alpha 1-adrenoceptors mediating contractions of rabbit isolated ear artery: comparison with rat isolated thoracic aorta

1. The present study attempted to classify pharmacologically the alpha 1-adrenoceptor subtype(s) present in two isolated, vascular ring preparations, the rabbit ear artery and rat thoracic aorta. 2. In the ear artery, the agonist effects of phenylephrine were antagonized by 5-methyl urapidil (pA2 = 7.90; Schild slope = 0.85) and BMY 7378 (pA2 = 6.11; Schild slope = 0.80) but not in a simple competitive manner. The shallow Schild slopes are consistent with the activation of a heterogeneous receptor population. Indeed the 5-methyl urapidil data set could be fitted to a two-receptor model yielding a high antagonist affinity (pKBH) estimate of 7.85 and a low affinity (pKBL) estimate of 6.03. 3. The effects of clonidine in the ear artery were competitively antagonised by 5-methyl urapidil (pKB = 7.91) and BMY 7378 (pKB = 5.53). These data are consistent with contractions to clonidine being mediated by a single receptor subtype. 4. In the aorta, the effects of phenylephrine were antagonized by 5-methyl urapidil (pA2 = 7.95; Schild slope = 1.11) and BMY 7378 (pA2 = 9.08; Schild slope = 0.73). Neither data set was consistent with a simple competitive interaction. The BMY 7378 data suggested again, that phenylephrine was acting at a heterogeneous receptor population. Subsequent analysis by the two-receptor model yielded a high affinity (pKBH) estimate of 8.95 and a low affinity (pKBL) estimate of 7.00. 5. The alkylating agent, chloroethylclonidine (CEC) elicited concentration-dependent contractions in the ear artery with a potency (p[A]50) of 5.57. Pretreatment of this tissue with CEC (5 microM, 30 min incubation) had no effect on subsequent responses to phenylephrine. In contrast, in the aorta, CEC demonstrated no agonism but pretreatment with this agent (5 microM, 15 min incubation) caused a rightward shift and depression of subsequent phenylephrine concentration-effect curves. 6. The affinity of clonidine in the rabbit ear artery (pKA = 6.17) was found to be significantly different to its affinity in the rat thoracic aorta (pKA = 7.12) suggesting that this agonist activates different alpha 1-adrenoceptor subtypes in the two tissues. 7. These results suggest that heterogeneous populations of alpha 1-adrenoceptors are present in both tissues. In the ear artery, the profile of antagonist and agonist activity is most consistent with alpha 1A-adrenoceptors being the predominant receptor subtype. The second receptor population does not appear to correspond to any of the recognized alpha 1-adrenoceptor subtypes. In the aorta alpha 1D-adrenoceptors appear to predominate, with alpha 1A-adrenoceptors being the most likely candidate for the second receptor population.

A possible structural determinant of selectivity of boldine and derivatives for the alpha 1A-adrenoceptor subtype

1. The selectivity of action of boldine and the related aporphine alkaloids, predicentrine (9-O-methylboldine) and glaucine (2,9-O-dimethylboldine) and alpha 1-adrenoceptor subtypes was studied by examining [3H]-prazosin competition binding in rat cerebral cortex. WB 4101 and benoxathian were used as selective alpha 1A-adrenoceptor antagonists. 2. In the competition experiments [3H]-prazosin (0.2 nM) binding was inhibited by WB 4101 and benoxathian. The inhibition curves displayed shallow slopes which could be subdivided into high and low affinity components (pKi = 9.92 and 8.29 for WB 4101, 9.35 and 7.94 for benoxathian). The two antagonists recognized approximately 37% of the sites with high affinity from among the total [3H]-prazosin specific binding sites. 3. Boldine, predicentrine and glaucine also competed for [3H]-prazosin (0.2 nM) binding with shallow and biphasic curves recognizing 30-40% of the sites with high affinity. Drug affinities (pKi) at the high and low affinity sites were, 8.31 and 6.50, respectively, for boldine, 8.13 and 6.39 for predicentrine, and 7.12 and 5.92 for glaucine. The relative order of selectivity for alpha 1A-adrenoceptors was boldine (70 fold alpha 1A-selective) = predicentrine (60 fold, alpha 1A-selective) > glaucine (15 fold, alpha 1A-selective). 4. Pretreatment of rat cerebral cortex membranes with chloroethylclonidine (CEC, 10 microM) for 30 min at 37 degrees C followed by thorough washing out reduced specific [3H]-prazosin binding by approximately 70%. The CEC-insensitive [3H]-prazosin binding was inhibited by boldine monophasically (Hill slope = 0.93) with a single pKi value (7.76). 5. These results suggest that whereas the aporphine structure shared by these alkaloids is responsible for their selectively of action for the alpha 1A-adrenoceptor subtype in rat cerebral cortex, defined functional groups, namely the 2-hydroxy function, induces a significant increase in alpha 1A-subtype selectivity and affinity.

Analysis of the effects of alpha 1-adrenoceptor antagonists on noradrenaline-mediated contraction of rat small mesenteric artery

1. In this study, we examined the interaction between noradrenaline (NA) and phenylephrine (PE) with seven antagonists (prazosin, tamsulosin, phentolamine, WB-4101, 5-methylurapidil, spiperone and HV 723) in an attempt to characterize the alpha 1-adrenoceptor population of the rat isolated small mesenteric artery (SMA) preparation. 2. Six of the seven antagonists investigated produced concentration-dependent, parallel, rightward shift of the NA concentration-effect (E/[A]) curves. The exception was tamsulosin, which produced significant decrease of the upper asymptote. In the case of 5-methylurapidil and HV723, the Schild plot slope parameters were not significantly different from unity over the range of concentrations used. However, the Schild plot slopes obtained for the other antagonists were all significantly greater than unity, inconsistent with expectations for simple competitive antagonism. 3. HV723, prazosin and tamsulosin were also tested using PE as an agonist. All three antagonists produced concentration-dependent, parallel, rightward shifts of the PE curves and Schild analysis yielded slope parameters not significantly different from unity. The pKB estimates obtained for tamsulosin and prazosin were not significantly different from the pA2 values obtained when NA was used as agonist. In the case of HV723, the 95% confidence intervals for the pKB values yielded with NA and PE did not overlap (pKB = 8.80-9.13 and 8.15-8.77 for NA and PE, respectively). 4. In the absence of evidence to indicate that the steep Schild plots were due to failure to satisfy the basic criteria for quantitative analysis in a one-receptor system, we considered the possibility that the complexity was caused by an action of NA at inhibitory D1 receptors. The selective D1 receptor antagonists, SCH-23390 (10 nM), had no significant effect on the NA E/[A] control curve, but the apparent potency of 100 nM prazosin was reduced by approximately 3.5 fold. 5. This study indicates that the steep Schild plots obtained from the interaction between NA and alpha 1-adrenoceptor antagonists were due to the simultaneous activation of inhibitory D1 receptors by NA. Notwithstanding this complexity, our explanatory model of the system (see Appendix) suggests that the antagonist affinity values estimated in the absence of D1 receptor block were not significantly affected by this other action of NA. The low affinity estimate obtained for prazosin suggests that the pharmacologically-defined alpha IL-subtype operates in the SMA.

Analysis of the activity of alpha 1-adrenoceptor antagonists in rat aorta

1. In this study, the effect of seven alpha 1-adrenoceptor antagonists (tamsulosin, phentolamine, prazosin, WB-4101, 5-methylurapidil, spiperone and HV723) have been examined on the contractile response to noradrenaline (NA) and phenylephrine (PE) in rat isolated aorta. 2. NA and PE, when administered using a cumulative dosing schedule, both produced concentration-dependent contraction of aortic rings. It was possible to fit the individual concentration-effect (E/[A]) curve data to the Hill equation to provide estimates of the curve midpoint location (p[A]50 = 7.74 +/- 0.10 and 7.14 +/- 0.18), midpoint slope (nH = 0.82 +/- 0.03 and 0.99 +/- 0.10) and upper asymptote (alpha = 3.2 +/- 0.3 and 3.1 +/- 0.2 g) parameters for NA and PE, respectively. However, the Hill equation provided a better fit to the E/[A] curve data obtained with another contractile agent, 5-hydroxytryptamine (5-HT) (p[A50] = 6.09 +/- 0.08, nH = 1.49 +/- 0.09, alpha = 2.6 +/- 0.3 g), as judged by calculation of the mean sum of squares of the differences between the observed and predicted values. 3. All of the antagonists investigated produced concentration-dependent inhibition of the contractile responses of the aorta to NA and PE. Although no significant effects on the upper asymptotes of the E/[A] curves of any of the antagonists tested were detected, only tamsulosin and 5-methylurapidil did not have a significant effect on the slope (nH) of the NA and PE E/[A] curves. The other antagonists produced significant steepening of the curves obtained with NA and/or PE. 4. Notwithstanding the fact that one of the basic criteria for simple competitive antagonism at a single receptor class was not always satisfied, the individual log [A]50 values estimated in the absence and presence of antagonist within each experiment were fitted to the competitive model. The Schild plot slope parameters for the antagonism of NA and PE by phentolamine and HV723 were found to be significantly less than unity. The Schild plot slope parameters for the other antagonists were not significantly different from unity. 5. In the absence of evidence to suggest that the deviations from simple competitive antagonism were due to failure to satisfy basic experimental conditions for quantitative analysis, an attempt was made to see whether the data could be accounted for by an existing two-receptor model (Furchgott, 1981). The goodness-of-fit obtained with the two-receptor model was significantly better than that obtained with the one-receptor model. Furthermore, with the exception of the data obtained with phentolamine, the pKB estimates for the two receptors were independent of whether NA or PE was used as agonist. 6. To determine which alpha 1-adrenoceptor subtypes may be associated with those defined by the two receptor model, the mean pKB estimates obtained from the two-receptor model fit were compared with affinities measured by Laz et al. (1994) for rat cloned alpha 1-adrenoceptor subtypes expressed in COS-7 cells. The sum of squared differences of the data points from the line of identity was smallest for both pKB1 and pKB2 in the case of the alpha 1a/d-adrenoceptor (now referred to as alpha 1d-adrenoceptor; Hieble et al., 1995). Therefore, the complexity exposed in this study may be due to the expression of closely-related forms of the alpha 1d-adrenoceptor. However, relatively good matches were also found between pKB1 and alpha 1c and between pKB2 and alpha 1b. Therefore, on the basis of these data, it is not possible to rule out the involvement of all three alpha 1-adrenoceptors. The conflicting reports concerning the characteristics of the alpha 1-adrenoceptor population mediating contraction of the rat aorta may, at least in part, be due to the lack of highly selective ligands and to between-assay variation in the expression of multiple alpha 1-adrenoceptors.

In functional experiments, risperidone is selective, not for the B, but for the A subtype of alpha 1-adrenoceptors

The potency of the antipsychotic drug, risperidone, to antagonize alpha 1A-adrenoceptor-mediated contraction in rat vas deferens and vasoconstriction in rat perfused kidney, and alpha 1B-adrenoceptor-mediated contractions in spleen from guinea-pig and mouse was evaluated and compared to that of alpha 1-adrenoceptor subtype-discriminating antagonists. Prazosin was found to be unselective; 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), 5-methyl-urapidil, indoramin and (+)-niguldipine were confirmed as selective for the alpha 1A-adrenoceptor, whereas spiperone was weakly alpha 1B-selective. Risperidone was equipotent to prazosin at alpha 1A-adrenoceptors in rat vas deferens and kidney. However, at guinea-pig and mouse splenic alpha 1B-adrenoceptors, the affinity values of risperidone were 10-fold lower than those of prazosin. Thus, in functional experiments the presumed high selectivity of risperidone for the B subtype of alpha 1-adrenoceptors could not be confirmed, the drug instead appears to be moderately selective (10-fold) for the A subtype.

Vascular alpha-1 adrenergic receptor subtypes in the regulation of arterial pressure

Alpha 1 (alpha 1)-adrenoceptors can be found at numerous end organs in the autonomic nervous system, especially vascular smooth muscle. The tonic sympathetic activation of vascular alpha 1-adrenoceptors maintains vascular resistance and is vital to the regulation of arterial pressure. Recent evidence clearly demonstrates that alpha 1-adrenoceptors are a heterogenous class of receptors and that each subtype may subserve specific cardiovascular functions. Elucidation of the physiological role of each subtype in the regulation of vascular resistance and arterial pressure will enhance our understanding of the cardiovascular system and may facilitate the development of therapeutics with improved efficacy and tolerability.

Chronic manipulation of dietary salt modulates renal physiology and kidney dopamine receptor subtypes: functional and autoradiographic studies

1. Compared to rats maintained on the normal NaCl (0.33%) diet, animals maintained on the low NaCl (0%) diet for 4 weeks exhibited increased plasma aldosterone and chloride and decreased urinary sodium excretion. 2. Rats maintained on the high NaCl (8%) diet for 4 weeks showed increased systolic blood pressure, water intake, urine volume, sodium and dopamine excretion and decreased plasma aldosterone and glomerular filtration rate. 3. Administration of SCH 23390 (10 mg/kg, po), but not domperidone to the high salt diet rats attenuated the diuretic effect, indicating the involvement of DA1 rather than DA2 receptors. The dopamine decarboxylase inhibitor, carbidopa (30 mg/kg, i.p.), also reduced the high salt-induced diuresis. 4. Kidney sections from rats fed the low NaCl diet showed a 63-100% decrease (P < 0.001-0.02) in cortical and medullary DA1 and DA2 binding sites, while rats fed the high NaCl diet demonstrated only a 70% decrease (P < 0.01-0.02) in cortical DA1 binding, without affecting DA2 binding. 5. These data indicate that chronic modification of dietary salt profoundly affects the sodium, water and dopamine excretion and leads to selective modulation of renal dopamine receptor subtypes.