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

α-Adrenoceptor assays

α-Adrenoceptors mediate responses to activation of both peripheral sympathetic nerves and central noradrenergic neurons. They also serve as autoreceptors that modulate the release of norepinephrine (NE) and other neurotransmitters. There are two major classes of α-adrenoceptors, the α(1)- and α(2). Each class is subdivided into three subtypes: α(1A), α(1B), α(1D), and α(2A), α(2B), α(2C). Described in this unit are in vitro isolated tissue methods used to study α-adrenoceptor functions and to identify novel ligands for these receptors. Detailed protocols describing use of isolated tissues to study the various α(1)- and α(2)-adrenoceptor subtypes are provided.

Discovery of LASSBio-772, a 1,3-benzodioxole N-phenylpiperazine derivative with potent alpha 1A/D-adrenergic receptor blocking properties

We described herein the discovery of 1-(2-(benzo[d] [1,3]dioxol-6-yl)ethyl)-4-(2-methoxyphenyl) piperazine (LASSBio-772), as a novel potent and selective alpha 1A/1D adrenoceptor (AR) antagonist selected after screening of functionalized N-phenylpiperazine derivatives in phenylephrine-induced vasoconstriction of rabbit aorta rings. The affinity of LASSBio-772 for alpha 1A and alpha 1B AR subtypes was determined through displacement of [(3)H]prazosin binding. We obtained Ki values of 0.14 nM for the alpha 1A-AR, similar to that displayed by tamsulosin (K(i) = 0.13 nM) and 5.55 nM for the alpha 1B-AR, representing a 40-fold higher affinity for alpha 1A-AR. LASSBio-772 also presented high affinity (K(B) = 0.025 nM) for the alpha 1D-AR subtype in the functional rat aorta assay, showing to be equipotent to tamsulosin (K(B) = 0.017 nM).

Identification of the alpha1L-adrenoceptor in rat cerebral cortex and possible relationship between alpha1L- and alpha1A-adrenoceptors

BACKGROUND AND PURPOSE

In addition to alpha1A, alpha1B and alpha1D-adrenoceptors (ARs), putative alpha1L-ARs with a low affinity for prazosin have been proposed. The purpose of the present study was to identify the alpha1A-AR and clarify its pharmacological profile using a radioligand binding assay.

EXPERIMENTAL APPROACH

Binding experiments with [3H]-silodosin and [3H]-prazosin were performed in intact tissue segments and crude membrane preparations of rat cerebral cortex. Intact tissue binding assays were also conducted in rat tail artery.

KEY RESULTS

[3H]-silodosin at subnanomolar concentrations specifically bound to intact tissue segments and membrane preparations of rat cerebral cortex at the same density (approximately 150 fmol mg(-1) total tissue protein). The binding sites in intact segments consisted of alpha1A and alpha1L-ARs that had different affinities for prazosin, while the binding sites in membranes showed an alpha1A-AR-like profile having single high affinity for prazosin. [3H]-prazosin also bound at subnanomolar concentrations to alpha1A and alpha1B-ARs but not alpha1L-ARs in cerebral cortex; the binding densities being approximately 200 and 290 fmol mg(-1) protein in the segments and the membranes, respectively. In the segments of tail artery, [3H]-silodosin only recognized alpha1A-ARs, whereas [3H]-prazosin bound to alpha1A and alpha1B-ARs.

CONCLUSIONS AND IMPLICATIONS

The present study clearly reveals the presence of alpha1L-ARs as a pharmacologically distinct entity from alpha1A and alpha1B-ARs in intact tissue segments of rat cerebral cortex but not tail artery. However, the alpha1L-ARs disappeared after tissue homogenization, suggesting their decomposition and/or their pharmacological profile changes to that of alpha1A-ARs.

Recent advances in the identification of a 1- and a 2-adrenoceptor subtypes: therapeutic implications

The cloning of multiple subtypes of both alpha1- and alpha2-adrenoceptors has renewed interest in the therapeutic application of agents interacting with these receptors. Effort has primarily been directed towards the design of uroselective alpha1-adrenoceptor antagonists for the treatment of benign prostatic hyperplasia (BPH). Evidence is accumulating for the involvement of a novel alpha1-adrenoceptor, designated as alpha1L-adrenoceptor, in alpha1-adrenoceptor-mediated smooth muscle contraction in prostatic and other urogenital tissues. While several antagonists showing a high degree of uroselectivity in animal models have been identified, their clinical superiority over the currently available alpha1-adrenoceptor antagonists has not yet been demonstrated. It is possible that the interaction with alpha1-adrenoceptors, as yet uncharacterised subtypes, at non-prostatic sites contributes to the therapeutic activity of this drug class in BPH. The alpha1-adrenoceptor subtypes involved in the control of vascular tone are currently being evaluated, and the profile of interaction with the various alpha1-adrenoceptor subtypes may play a key role in the efficacy of cardiovascular drugs such as carvedilol. Alpha2-adrenoceptor agonists are now being employed for a variety of therapeutic applications, most involving actions on receptors within the central nervous system (CNS). These agents are useful in the treatment of hypertension, glaucoma, opiate withdrawal and attention deficit hyperactivity disorder (ADHD), and as analgesics and adjuncts to general anaesthesia. While subtype selectivity has not yet been applied to the design of new alpha2-adrenoceptor agonists for these applications, recent gene mutation/knock-out experiments have identified the alpha2-subtypes involved in some of these actions, and optimisation of a therapeutic profile may be possible. Furthermore, the design of agents combining affinities for multiple adrenoceptor subtypes, or the combination of a specific adrenoceptor affinity profile with another pharmacological action, may offer advantages over molecules selective for an individual adrenoceptor subtype.

Neuropeptide Y selectively potentiates alpha1-adrenoceptor-mediated contraction through Y1 receptor subtype in rat femoral artery

The aim of this study was to investigate the synergism between neuropeptide Y and other vasoconstrictors (phenylephrine and serotonin) and which neuropeptide Y receptor subtype is responsible for the neuropeptide Y-induced potentiation. Exogenous neuropeptide Y (10 nM) potentiated alpha1-adrenoceptor-mediated (PE-induced) contraction in rat femoral artery permissively without its direct action, but not in the thoracic aorta. In contrast, neuropeptide Y produced no change in serotonin-induced contraction in both arteries. Increasing concentrations of neuropeptide Y caused dose-dependent potentiation of the phenylephrine-induced contraction in the femoral artery. This potentiation was blocked by a selective neuropeptide Y-Y1 receptor antagonist, BIBP3226 [(R)-N2-(diphenylacetyl)-N-[4-hydroxyphenyl)methyl]-argininamide] (1 microM). Semiquantitative reverse transcriptase polymerase chain reaction showed the selective expression of neuropeptide Y-Y1 receptor mRNA in the femoral artery. These findings indicated that the neuropeptide Y-induced selective potentiation of alpha1-adrenoceptor-mediated contraction is mediated through neuropeptide Y-Y1 receptor in rat femoral artery.

Functional characterization of alpha 1-adrenoceptor subtypes in vascular tissues using different experimental approaches: a comparative study

1. The alpha(1)-adrenergic responses of rat aorta and tail artery have been analysed measuring the contractility and the inositol phosphate (IP) formation induced by noradrenaline. Three antagonists, prazosin, 5-methylurapidil (alpha(1A) selective) and BMY 7378 (alpha(1D) selective) have been used in different experimental procedures. 2. Noradrenaline possesses a greater potency inducing contraction and IP accumulation in aorta (pEC(50)-contraction=7.32+/-0.04; pEC(50)-IPs=6.03+/-0.08) than in the tail artery (pEC(50)-contraction=5.71+/-0.07; pEC(50)-IPs=5.51+/-0.10). Although the maximum contraction was similar in both tissues (E(max)-tail=619.1+/-55.6 mg; E(max)-aorta-698.2+/-40.8 mg), there were marked differences in the ability of these tissues to generate intracellular second messengers the tail artery being more efficient (E(max)-tail=1060+/-147%; E(max)-aorta=108.1+/-16.9%). 3. Concentration response curves of noradrenaline in presence of antagonist together with concentration inhibition curves for antagonists added before (CICb) or after (CICa) noradrenaline-induced maximal response in Ca(2+)-containing or Ca(2+)-free medium have been performed. A comparative analysis of the different procedures as well as the mathematical approaches used in each case to calculate the antagonist potencies, were completed. 4. The CICa was the simplest method to characterize the predominant alpha(1)-adrenoceptor subtype involved in the functional response of a tissue. 5. In aorta, where constitutively active alpha(1D)-adrenoeptors are present, the use of different experimental procedures evidenced a complex equilibrium between alpha(1D)- and alpha(1A)-adrenoceptor subtypes. 6. The appropriate management of LiCl in IP accumulation studies allowed us to reproduce the different experimental procedures performed in contractile experiments giving more technical possibilities to this methodology.

Characterization of alpha1-adrenoceptor-mediated contraction in the mouse thoracic aorta

In the mouse thoracic aorta, noradrenaline, adrenaline, phenylephrine and methoxamine behaved as full agonists. The pA(2) values for 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4,5]decane-7,9-dione dihydrochloride (BMY 7378) against each agonist were in good agreement with the generally accepted affinity value of alpha(1D)-adrenoceptors. 5-Methylurapidil, 2-[2,6-dimethoxyphenoxyethyl]aminomethyl-1,4-benzodioxane hydrochloride (WB 4101) and prazosin inhibited the contraction in response to noradrenaline. A significant correlation was obtained between the antagonist affinities in mouse thoracic aorta and those of native alpha(1D)-adrenoceptors in rat thoracic aorta or with those of cloned alpha(1d)-adrenoceptors, but not with those for either alpha(1a)- or alpha(1b)-adrenoceptors. Buspirone behaved as a partial agonist in mouse thoracic aorta, the contraction of which was antagonized by BMY 7378 with a pA(2) value (8.49) consistent with that found against noradrenaline (8.43). Clonidine acted as a partial agonist (pD(2)=5.94). The pK(p) value for clonidine against noradrenaline was similar to the pD(2) value for clonidine. The apparent pK(B) value for BMY 7378 against clonidine was similar to the pA(2) value against other full agonists used in the present study. These results suggest that the alpha(1D)-adrenoceptor subtype exists, and that the full agonists and the partial agonists evoke the contraction mediated through the alpha(1D)-adrenoceptor in mouse thoracic aorta.

Pharmacokinetic-pharmacodynamic analysis of the EEG effect of alfentanil in rats following beta-funaltrexamine-induced mu-opioid receptor "knockdown" in vivo

PURPOSE

The objective of this investigation was to determine the influence of pre-treatment with the irreversible mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA) on the pharmacokinetic-pharmacodynamic (PK/PD) relationship of alfentanil in rats.

METHODS

The PK/PD correlation of alfentanil (2 mg x kg(-1) intravenously in 20 min) was determined in chronically instrumented rats using amplitudes in the 0.5-4.5 Hz frequency band of the EEG as pharmacodynamic endpoint. Beta-FNA was administered intravenously (10 mg x kg(-1)) either 35 min or 24 h prior to the PK/PD experiments.

RESULTS

Pre-treatment with beta-FNA had no influence on the pharmacokinetics of alfentanil. The in vivo concentration-EEG effect relationships, however, were steeper and shifted towards higher concentrations with no difference between the 35-min and the 24-h pre-treatment groups. Analysis of the data on basis of the operational model agonism revealed that the observed changes could be explained by a 70-80% reduction in alfentanil efficacy in beta-FNA pre-treated rats. This is consistent with results from an in vitro receptor bioassay showing a 40-60% reduction in the number of specific mu-opioid binding sites in the brain.

CONCLUSIONS

This investigation confirms the validity of a previously postulated mechanism-based PK/PD model for the effect of synthetic opiates in rats.

Alpha(1)-adrenoceptor subtypes mediating contractions of the rat mesenteric artery

The alpha(1)-adrenoceptor subtype(s) mediating contractions of the rat mesenteric artery were investigated using the agonists methoxamine, cirazoline, P7480 (N-(4-pyridinyl)-1H-indol-1-amine) and subtype-selective antagonists including BMY 7378 (8-(-2(-4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-8-azaspiro(4, 5)decane-7,9,dione dihydrochloride). pA(2) or apparent pK(B) values of antagonists against methoxamine contractions correlated best with its pK(i) values at the cloned alpha(1b)-(0.88), with cirazoline, antagonists affinities correlated equally well with those at alpha(1a)-(0.79) or the alpha(1b)-(0.81) while with P7480 antagonist affinities correlated best with the alpha(1d)-adrenoceptor subtype (0.94). The low affinity estimate for 5-methylurapidil (7.5) against the alpha(1a)-selective cirazoline suggests an alpha(1A)-subtype mediating contraction is unlikely. Shallow Schild plot slopes of subtype selective antagonists against all three agonists are consistent with heterogeneity of alpha(1)-adrenoceptors. P7480 (putative alpha(1D)-adrenoceptor-selective) acts primarily at this subtype and at another which is more likely to be an alpha(1B)- than an alpha(1A)-adrenoceptor. The results with both agonists and antagonists are consistent with contractions of the rat mesenteric artery being mediated via the alpha(1D)- and possibly alpha(1B)-adrenoceptor.

Microphysiometric analysis of human alpha1a-adrenoceptor expressed in Chinese hamster ovary cells

1. The human recombinant alpha1a-adrenoceptor (AR) has been stably expressed in Chinese hamster ovary cells. Four stable clones, aH4, aH5, aH6 and aH7, expressing 30, 370, 940 and 2900 fmol AR mg(-1) protein, respectively, have been employed to characterize this AR subtype using radioligand binding and microphysiometry to measure extracellular acidification rates. 2. Noradrenaline (NA) gave concentration-dependent responses in microphysiometry with increasing extracellular acidification rates. The potency of NA increased as the receptor density increased; pEC50 values of NA for the clones aH4, aH5, aH6 and aH7 were 6.9, 7.5, 7.8 and 8.1, respectively. This increase of potency according to receptor density indicates the presence of spare receptor for NA. Methoxamine, phenylephrine, oxymetazoline and clonidine also gave concentration-dependent responses with various intrinsic activities. 3. Antagonists shifted concentration-response curves for NA rightward in a concentration-dependent manner. Schild analysis revealed that the affinity profile of this AR subtype to antagonists in the clone aH7 had a typical pattern for the alpha1a-AR; high affinity for prazosin and WB 4101, and low affinity for BMY7378 (pA2=9.5, 9.8 and 7.3, respectively). This profile is similar in the case of the clone aH4. These affinities were in good agreement with those obtained in binding experiments. 4. These results have demonstrated that (1) classical receptor theory can be applied in microphysiometry, and (2) microphysiometry is a useful tool to investigate the pharmacological characterization of alpha1a-AR.

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.

Analysis of asymmetry of agonist concentration-effect curves

We have developed a fitting procedure, based on nonlinear mixed effect modelling and original work by Richards (1959, J Exp Botany 10, 290-300), to describe the degree of asymmetry of concentration-effect E/[A] curves and analysed the shape of E/[A] curves obtained with alpha1-adrenoceptor agonists in rat aorta. The four-parameter Richards model provided a significantly better fit of the data than the standard logistic/Hill model for all ligands investigated, which implies that E/[A] curves were asymmetrical. With the exception of ST 587, the asymmetry parameter (delta) tended toward zero and the Richards model could be replaced without significant loss of goodness-of-fit by the three-parameter, asymmetrical Gompertz model. The alpha1-adrenoceptor antagonist, prazosin (10 nM), had no effect on the asymmetry of the noradrenaline E/[A] curve but significantly increased the slope at the point of inflection. In contrast, pretreatment with the irreversible antagonist, phenoxybenzamine (60 nM), produced a shift of the delta estimate for noradrenaline from zero to unity, indicating a change from an asymmetrical to a symmetrical curve. Therefore, detailed statistical analysis of E/[A] curve asymmetry demonstrates that alpha1-adrenoceptors in rat aorta do not operate as a homogenous one-receptor-one-transducer system. This conclusion could not have been reached by either an analysis with the standard logistic/Hill model or visual inspection of experimental data. Overall, the curve-fitting analysis developed in this study provides a quantitative and sensitive measure of asymmetry and a novel method for the objective discrimination of agonist action on the basis of curve shape. The method is generally applicable to other pharmacological assays and provides a new tool in receptor classification studies.

alpha 1D-adrenoceptors contribute to the neurogenic vasopressor response in pithed rats

The aim of the present study was to assess the role of vascular alpha 1D-adrenoceptors in the sympathetic vasopressor response in vivo. Specifically, we evaluated the effect of a selective alpha 1D-adrenoceptor antagonist, BMY 7378 (8-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-8-azaspiro(4,5)dec ane-7,9- dione 2HCl), on the vasopressor response induced by preganglionic (T7-T9) sympathetic stimulation in the pithed rat. The vasopressor response was dose-dependently sensitive to inhibition by intravenous BMY 7378 (0.1, 0.31, 1 and 3.1 mg/kg), doses of 1 and 3.1 mg/kg being equally effective. Like BMY 7378, 5-methylurapidil (0.1, 0.31, 1 and 3.1 mg/kg) antagonized the vasopressor response to spinal stimulation; doses of 1 and 3.1 mg/kg were also equally effective. In combination experiments, BMY 7378 (1 mg/kg, i.v.) and the alpha 1A-adrenoceptor antagonist, 5-methylurapidil (1 mg/kg, i.v.), showed an additive effect. The present results demonstrate that the alpha 1D-adrenoceptor subtype plays an important role in the pressor response to sympathetic nerve stimulation in the pithed rat, and confirm the participation of the alpha 1A-adrenoceptor subtype in the same response.

Functional characterisation of the pharmacological profile of the putative alpha1B-adrenoceptor antagonist, (+)-cyclazosin

We studied the functional pharmacological profile of (+)-cyclazosin, which has been characterised as a selective, high-affinity (pKi = 9.68) alpha1B-adrenoceptor ligand in binding experiments with rat liver membranes. The pKa/pA2 values for antagonism of contractions mediated via alpha1A/L-adrenoceptors of rat small mesenteric artery, alpha1B-adrenoceptors of rat aorta and alpha1B-adrenoceptors of rat spleen were 7.78 +/- 0.04, 6.86 +/- 0.07 and 7.96 +/- 0.08, respectively. Furthermore, in mouse spleen, which is also regarded as an alpha1B-adrenoceptor preparation, (+)-cyclazosin displayed low potency and did not act as a competitive antagonist. Thus, in contrast with results obtained in radioligand binding experiments, (+)-cyclazosin does not behave as a selective alpha1B-adrenoceptor antagonist in functional tissues. Whether this discrepancy has consequences for the classification of alpha1-adrenoceptors requires further investigation.

Characterization of receptors mediating contraction of the rat isolated small mesenteric artery and aorta to arginine vasopressin and oxytocin

1. The exact nature of the receptor subtype(s) involved in the action of arg-vasopressin (AVP) on the rat aorta and small mesenteric artery (SMA) is controversial. Therefore, we have studied the effects of the selective V1A receptor antagonists, OPC 21268 and SR 49059, and the oxytocin (OT) receptor antagonist, atosiban, on the AVP- and OT-induced contractions of the two vessels. 2. AVP and OT displayed similar intrinsic activities in the rat aorta and SMA, but AVP was approximately 130 fold and approximately 500 fold more potent than OT, respectively. In the rat aorta, Hill slopes (nH) were similar for OT and AVP. However, in rat SMA, the OT concentration-effect (E/[A]) curve was significantly steeper than the AVP E/[A] curve (nH, = 3.3+/-0.20, 2.3+/-0.15; P<0.001). 3. In the aorta OPC 21268, SR 49059 and atosiban competitively antagonized the AVP and OT E/[A] curves. Except for atosiban and SR 49059 against AVP, competitive antagonism was also observed in the SMA. Atosiban caused concentration-dependent steepening of the AVP E/[A] curve, whereas SR 49059 decreased the upper asymptote. 4. Schild analysis yielded affinities indicative of V1A receptor involvement in both vessels: pKB/ pA2=9.20 9.48, 7.56 7.71 and 6.19 6.48 for SR 49059, OPC 21268 and atosiban, respectively. 5. Neither AVP nor OT relaxed U46619 pre-contracted aorta or SMA in the presence of SR 49059, suggesting no interference of a vasodilatory component. 6. Despite predominant involvement of V1A receptors in both vessels, the different Hill slopes of AVP and OT E/[A] curves as well as the steepening of the AVP E/[A] curves by atosiban are indicative of receptor heterogeneity in the rat SMA.

Alpha1A-adrenergic receptors mediate vasoconstriction of the isolated spiral modiolar artery in vitro

Several lines of evidence suggest that cochlear blood flow is under the control of the sympathetic nervous system and that this control is mediated via alpha-adrenergic receptors. The goal of the present study was to determine whether alpha-adrenergic receptors mediate vasoconstriction of the spiral modiolar artery and, if so, to determine which subtype dominates this response. Vascular diameter was measured with video microscopy in the isolated superfused spiral modiolar artery in vitro. The diameter of the spiral modiolar artery under control conditions was 61 +/- 2 microm (n = 60). Spontaneous vasomotion was observed in most specimens. Addition of norepinephrine to the superfusate caused a phasic vasoconstriction and an increase in the amplitude of vasomotion. These effects were limited to the vicinity of arteriolar branch points of the spiral modiolar artery. Norepinephrine-induced vasoconstriction occurred with EC50 of (1.9 +/- 0.4) x 10(-5) M (n = 44) and the vascular diameter was maximally reduced by a factor of 0.87 +/- 0.01 (n = 29). Neither the phasic nature nor the EC50 of the norepinephrine-induced vasoconstrictions was altered in the presence of the beta2-adrenergic receptor antagonist 10(-5) M ICI118551 or the nitric oxide synthase inhibitor 10(-4) M NOARG. In contrast, the alpha2-adrenergic receptor antagonist 10(-7) M yohimbine and the alpha1-adrenergic receptor antagonist 10(-9) and 10(-8) M prazosin caused a significant shift in the dose-response curve. The affinity constants (K(DB)) for yohimbine and prazosin were (5+/-2) x 10(-8) M (n=4) and (2.0+/-0.7) x 10(-10) M (n=18), respectively. The alpha1A-adrenergic receptor antagonist 10(-8) M 5-methyl urapidil and the alpha1D-adrenergic receptors antagonist 5 x 10(-6) M BMY7378 caused a significant shift in the dose-response curve. The K(DB) values for 5-methyl urapidil and for BMY7378 were (2.7 +/- 0.7) x 10(-10) M (n = 8) and (4.4 +/- 2.7) x 10(-7) M (n = 8), respectively. Further, total RNA was isolated from microdissected spiral modiolar arteries and the presence of transcripts for alpha1-adrenergic receptor subtypes was determined by reverse transcription polymerase chain reaction (RT-PCR). Primers specific for gerbil alpha1-adrenergic receptor subtypes were developed using RNA from rat and gerbil brain. Analysis of RNA extracted from the spiral modiolar artery revealed RT-PCR products of the appropriate size for the alpha1A-adrenergic receptor, however, no evidence for the alpha1B- and alpha1D-adrenergic receptor was found. Further, analysis of RNA extracted from blood, which was a contaminant of the microdissected spiral modiolar arteries, revealed no RT-PCR products. Sequence analysis of the RT-PCR product of the alpha1A-adrenergic receptor from the spiral modiolar artery confirmed its identity. Identity between the 175 nt gerbil sequence fragment and the known rat, mouse and human alpha1A-adrenergic receptor sequences was 90.9, 92.0 and 85.2%, respectively. These observations demonstrate that the spiral modiolar artery contains alpha1A-adrenergic receptors which mediate vasoconstriction at branch points.

Investigation of alpha1-adrenoceptor subtypes mediating vasoconstriction in rabbit cutaneous resistance arteries

1. Cutaneous resistance arteries (c.r.a.) (internal diameter=240.94+/-5.42 microm, n=67/25 (number arteries/number animals)) from New Zealand white rabbits were mounted in wire myographs and a normalization procedure followed. 2. Cumulative concentration-response curves (CCRCs) were constructed for the alpha-adrenoceptor agonists noradrenaline (NA), (R)A61603 and phenylephrine (PE) in the presence of cocaine (3 microM), propranolol (1 microM) and corticosterone (10 microM). The effects of competitive alpha1-adrenoceptor antagonists, prazosin, WB4101, 5-methyl-urapidil, HV723, BMY7378 and the irreversible alpha1B selective compound chloroethylclonidine (CEC) were examined versus the potency and maximum response of the c.r.a.s to noradrenaline. 3. The high potency of A-61603 relative to PE has been shown to differentiate both functional and binding site alpha1A- or alpha1B-adrenoceptors from alpha1D-adrenoceptors: A-61603 was 944 times more potent than phenylephrine (at EC50) suggesting the presence of a functional alpha1A or alpha1B as opposed to an alpha1D-subtype. 4. Exposure to chloroethylclonidine (CEC; 100 microM) decreased the maximum response to noradrenaline but did not significantly change noradrenaline sensitivity indicating that a substantial part of noradrenaline-induced vasoconstriction in rabbit cutaneous arteries is CEC-insensitive. 5. The potencies of prazosin (pA2=9.14) and WB4101 (pA2=9.30) indicate the involvement of prazosin-sensitive functional alpha1-adrenoceptors. The slopes of corresponding Schild plots for prazosin and WB4101 did not include negative unity which implies the possible involvement of more than one functional alpha1-adrenoceptor subtype in noradrenaline-induced vasoconstriction in rabbit cutaneous resistance arteries. In contrast to this, in the case of 5-methyl-urapidil and HV723, the Schild plot slope parameters were not significantly different from negative unity over the range of concentrations used; the low pA2 value for 5-methylurapidil (7.27) suggests the non-involvement of an alpha1A- or an alpha1D-adrenoceptor; the low pA2 value for HV723 (8.47) was similar to that against responses postulated as alpha1L. 6. We conclude that rabbit cutaneous resistance arteries express a prazosin-sensitive functional alpha1-adrenoceptor resembling the alpha1B and another low affinity site for prazosin which on the basis of the functional antagonism produced by HV723 most closely resembles the alpha1L-adrenoceptor; the low pA2 value for HV723 (8.47) is similar to that against responses postulated as alpha1L.

The antipsychotic drug sertindole is a specific inhibitor of alpha1A-adrenoceptors in rat mesenteric small arteries

We have investigated the adrenergic antagonist effect of the antipsychotic sertindole in rat resistance vessels and in membranes of HEK293 cells transfected with alpha1-adrenoceptors. Segments of rat mesenteric small arteries or rat aorta were mounted on a myograph for isometric tension recording. In mesenteric small arteries, specific alpha1A-adrenoceptor antagonists (5-methyl urapidil and WB-4101 (2-(2,6-dimethoxyphenoxyethyl) aminomethyl-1,4-benzodioxane)) inhibited phenylephrine responses with high affinity (pA2 9.1 and 9.5, respectively). Chlorethylclonidine (alpha1B- and alpha1D-adrenoceptor antagonist) and BMY7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro [4,5] decane-7,9-dione dihydrochloride, alpha1D-adrenoceptor antagonist) had little effect. This indicated that the adrenoceptor subtype in the mesenteric small arteries was of the alpha1A subtype. Sertindole inhibited the phenylephrine response of mesenteric small arteries (pA2 9.0), but had little effect on the phenylephrine response of aorta (which lacks alpha1A-adrenoceptors). The specific action of sertindole on alpha1A-adrenoceptors was supported by experiments with membranes of HEK293 (human embryonic kidney) cells transfected with the alpha1A-, alpha1B- and alpha1D-adrenoceptors. Here, with concurrent incubation, sertindole showed selective competitive inhibition of BE2254 (2-beta(4-hydroxyphenyl)-ethylaminomethyl)-tetralone) binding to alpha1A-adrenoceptors (Ki 8.9), compared to alpha1B-adrenoceptors (Ki 7.1) and alpha1D-adrenoceptors (Ki 6.8). Despite the apparent competitive action sertindole, it was not possible to wash out its antagonist effect within 6 h in the functional phenylephrine concentration-response experiments. Furthermore, in the membranes of HEK293 cells transfected with the alpha1A-adrenoceptors, binding of [125I]BE2254 was reduced by 45% following preincubation with sertindole (1 nM). We conclude that the alpha-adrenoceptors of rat mesenteric small arteries are of the alpha1A-type, and that sertindole is a specific pseudo-irreversible competitive antagonist of this adrenoceptor subtype.

Vascular actions of octreotide in the portal hypertensive rat

1. We have investigated the actions of the somatostatin analogue octreotide in the portal hypertensive Wistar rat in vivo and in rat small mesenteric artery and aorta in vitro. 2. In small mesenteric artery, octreotide (0.1-0.3 microM) failed to produce any direct contraction, nor did it affect contractions to noradrenaline (NA, 10 microM) or endothelium-dependent relaxations to acetylcholine. 3. In rat aorta, octreotide (0.3 microM) and somatostatin (1 microM) failed to affect contractions to NA (1 microM), or concentration-contractile response curves to NA. 4. In rat vas deferens, octreotide and somatostatin significantly reduced contractile responses to electrical stimulation with pD2 values (-log IC50) of 8.19 +/- 0.10 (n = 4) and 8.16 +/- 0.26 (n = 4), respectively. Hence, the lack of effect of these agents in aorta or mesenteric artery was not due to lack of efficacy or inappropriate choice of concentration. 5. In the anaesthetized portal hypertensive rat, intravenous injection of octreotide (1-100 microg kg[-1]) did not significantly affect systemic blood pressure, nor did it affect mesenteric vascular conductance as measured by laser doppler flow probes. However, octreotide (100 microg kg[-1]) significantly reduced vascular conductance to 74.2 +/- 7.7% of control (n = 6) in porto-systemic shunt vessels as measured by laser doppler flow probes. 6. Phenylephrine (1 microg kg[-1]) significantly raised blood pressure and significantly decreased vascular conductance in both mesenteric (66.6 +/- 3.7% of control) and porto-systemic shunt vessels (58.7 +/- 10.0% of control). 7. It was concluded that octreotide has selective effects on porto-systemic shunt vessles in vivo in the portal hypertensive rat.

On the reliability of affinity and efficacy estimates obtained by direct operational model fitting of agonist concentration-effect curves following irreversible receptor inactivation

Recently, Zernig and colleagues (1996) (J Pharmacol Toxicol Meth 35: 223-237) suggested that for the estimation of agonist affinity and efficacy, the method of simultaneously fitting of concentration-effect curves from control and irreversible antagonist-treated tissues to the operational model of agonism is superior to other analytical approaches. In the present study, we have evaluated the limitations of this simultaneous curve fitting method. Simulation studies showed that this method can be only employed with confidence when the upper asymptotes of the control curves display minimal variation between tissues, which makes its practical utility rather limited. The unreliability of the simultaneous fitting procedure was further underscored with the analysis of experimental data obtained from the interaction between noradrenaline and phenoxybenzamine in rat isolated aorta. The lack of robustness of the parameter estimates showed that under standard experimental conditions the outcomes of simultaneous model fitting are highly dependent on between-tissue variations of the upper asymptotes of the control curves and, therefore, may be unreliable. Therefore, whenever possible, a multiple curve design should be adopted, in which control and treated curves are obtained in one tissue and provide enough information for an independent estimation of affinity and efficacy that is free of intertissue differences.

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 action of idazoxan calls into question the reliability of the rat isolated small mesenteric artery assay as a predictor for alpha 1-adrenoceptor-mediated pressor activity

We have studied the effects of idazoxan in rat aorta and small mesenteric artery. In the aorta, idazoxan behaved as a partial agonist (pKA = 6.30). Prazosin produced rightward shift (pA2 = 9.88) and steepening of the idazoxan curve. In contrast, idazoxan had no effect of basal tension in the mesenteric artery, but shifted the noradrenaline curve to the right in a parallel manner (pA2 = 6.12). The selective alpha 1-adrenoceptor agonist, indanidine, also behaved as a partial agonist in the aorta and produced no significant contractions of the small mesenteric artery. Since idazoxan and indanidine have been reported to raise blood pressure in the pithed rat via an action at vascular alpha 1-adrenoceptors, these results call into question the reliability of the small mesenteric artery assay as a predictor for alpha 1-adrenoceptor-mediated pressor activity in vivo.