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

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.

Endothelin modulates the cardiovascular effects of clonidine in the rat

Clonidine decreases mean arterial pressure (MAP) by acting as an α(2)-adrenergic receptor (AR) agonist in the central nervous system; it also acts on peripheral α-ARs to produce vasoconstriction. Endothelin (ET) has been shown to modulate the action of ARs. The present study was conducted to determine the involvement of ET in cardiovascular effects of clonidine. Intravenous administration of clonidine (10, 30 and 90μgkg(-1)) produced a dose-dependent decrease in MAP and heart rate (HR). Treatment with ET-1 (100, 300 and 900ngkg(-1)) significantly attenuated clonidine (10μgkg(-1)) induced fall in MAP and HR. Rats treated with ET-1 (900ngkg(-1)) showed an increase in MAP and HR after clonidine administration compared to untreated rats, while ET(A/B) antagonist, TAK-044 (1mgkg(-1)) and ET(A) antagonist, BMS-182874 (9mgkg(-1)) potentiated the hypotensive effect of clonidine. ET(B) receptor agonist, IRL-1620 (5μgkg(-1)) produced significant attenuation of clonidine induced fall in MAP and HR, while ET(B) receptor antagonist, BQ-788 (0.3mgkg(-1)), potentiated the hypotensive effect of clonidine. Prazosin (0.1mgkg(-1)) completely blocked ET-1 induced changes in cardiovascular effects of clonidine. Clonidine-induced contraction of rat abdominal aortic ring was potentiated by ET-1, which was completely blocked by prazosin. Clonidine produced an increase in ET(A) receptor expression in the brain and abdominal aorta while ET(B) receptors were not affected. It is concluded that ET enhances the responsiveness of vascular ARs to the constrictor effect of clonidine and ET antagonists potentiate the hypotensive effect of clonidine suggesting that a combination of ET antagonist with clonidine may be a useful option to treat hypertension.

alpha(1)-Adrenergic receptor subtype function in fetal and adult cerebral arteries

In the developing fetus, cerebral artery (CA) contractility demonstrates significant functional differences from that of the adult. This may be a consequence of differential activities of alpha(1)-adrenergic receptor (alpha(1)-AR) subtypes. Thus we tested the hypothesis that maturational differences in adrenergic-mediated CA contractility are, in part, a consequence of differential expression and/or activities of alpha(1)-AR subtypes. In CA from fetal ( approximately 140 days) and nonpregnant adult sheep, we used wire myography and imaging, with simultaneous measurement of tension and intracellular Ca(2+) concentration ([Ca(2+)](i)), radioimmunoassay, and Western immunoblots to examine phenylephrine (Phe)-induced contractile responses. The alpha(1A)-AR antagonists (5-MU and WB-4101) completely inhibited Phe-induced contraction in adult but not fetal CA; however, [Ca(2+)](i) increase was reduced significantly in both age groups. The alpha(1D)-AR antagonist (BMY-7378) blocked both Phe-induced contractions and Ca(2+) responses to a significantly greater extent in adult compared with fetal CA. In both age groups, inhibition of alpha(1A)-AR and alpha(1B)-AR, but not alpha(1D)-AR, significantly reduced inositol 1,4,5-trisphosphate responses to Phe. Western immunoblots demonstrated that the alpha(1)-AR subtype expression was only approximately 20% in fetal CA compared with the adult. Moreover, in fetal CA, the alpha(1D)-AR was expressed significantly greater than the other two subtypes. Also, in fetal but not adult CA, Phe induced a significant increase in activated ERK1/2; this increase in phosphorylated ERK was blocked by alpha(1B)-AR (CEC) and alpha(1D)-AR (BMY-7378) inhibitors, but not by alpha(1A)-AR inhibitors (5-MU or WB-4101). In conclusion, in the fetal CA, alpha(1B)-AR and alpha(1D)-AR subtypes play a key role in contractile response as well as in ERK activation. We speculate that in fetal CA alpha(1B)-AR and alpha(1D)-AR subtypes may be a critical factor associated with cerebrovascular growth and function.

Subtypes of functional alpha1-adrenoceptor

In this review, subtypes of functional alpha1-adrenoceptor are discussed. These are cell membrane receptors, belonging to the seven-transmembrane-spanning G-protein-linked family of receptors, which respond to the physiological agonist noradrenaline. alpha1-Adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A fourth alpha1-adrenoceptor, the alpha1L-, represents a functional phenotype of the alpha1A-adrenoceptor. alpha1-Adrenoceptor subtype knock-out mice have refined our knowledge of the functions of alpha-adrenoceptor subtypes, particuarly as subtype-selective agonists and antagonists are not available for all subtypes. alpha1-Adrenoceptors function as stimulatory receptors involved particularly in smooth muscle contraction, especially contraction of vascular smooth muscle, both in local vasoconstriction and in the control of blood pressure and temperature, and contraction of the prostate and bladder neck. Central actions are now being elucidated.

Inhibitory effect of fentanyl on phenylephrine-induced contraction of the rat aorta

PURPOSE

Fentanyl was reported to inhibit the alpha(1)-adrenoceptor agonist-induced contraction. The goal of this in vitro study was to identify the alpha(1)-adrenoceptor subtype primarily involved in the fentanyl-induced attenuation of phenylephrine-induced contraction in isolated endothelium-denuded rat aorta.

MATERIALS AND METHODS

Aortic rings were suspended in order to record isometric tension. Concentration-response curves for phenylephrine (10(-9) to 10(-5) M) were generated in the presence or absence of one of the following drugs: fentanyl (3 x 10(-7), 10(-6), 3 x10(-6) M), 5-methylurapidil (3 x10(-8), 10(-7), 3 x 10(-7) M), chloroethylclonidine (10(-5) M) and BMY 7378 (3 x 10(-9), 10(-8), 3 x 10(-8) M). Phenylephrine concentration-response curves were generated in the presence or absence of fentanyl in rings pretreated with either 3 x10(-9) M prazosin, 10(-9) M 5-methylurapidil or 3 x 10(-9) M BMY 7378.

RESULTS

Fentanyl (10(-6), 3 x 10(-6) M) attenuated phenylephrine-induced contraction in the rat aorta. 5-Methylurapidil and BMY 7378 produced a parallel rightward shift in the phenylephrine concentration-response curve. The pA(2) values for 5-methylurapidil and BMY 7378 were estimated to be 7.71 +/- 0.15 and 8.99 +/- 0.24, respectively. Fentanyl (10(-6) M) attenuated phenylephrine-induced contraction in rings pretreated with 10(-9) M 5-methylurapidil, but did not alter the rings when pretreated with 3 x 10(-9) M BMY 7378. Pretreatment of the rings with chloroethylclonidine showed a 72.9 +/- 2.3% reduction in phenylephrine-induced maximal contraction.

CONCLUSION

The results suggest that fentanyl attenuates phenylephrine-induced contraction by inhibiting the pathway involved in the alpha(1D)-adrenoceptor-mediated contraction of the rat aorta.

Identification of α-1L Adrenoceptor in Rabbit Ear Artery

The alpha-1L adrenoceptor (AR) was identified in rabbit ear artery by both functional and ligand binding studies. In functional studies using arterial rings, the contractile response to NS-49 [(R)-(-)-3'-(2-amino-1-hydroxyethyl)-4'-fluorometh-anesulfonanilide hydrochloride] (alpha-1A and alpha-1L AR-selective agonist) was competitively antagonized with low affinities by prazosin, RS-17053 [N-[2-(2-cyclopropylmethoxyphenoxy) ethyl]-5-chloro-alpha,alpha-dimethyl-1H-indole-3-ethamine hydrochloride], and 5-methylurapidil but with high affinities by tamsulosin and KMD-3213 [(-)-1-(3-hydroxypropyl)-5-[(2R)-2-([2-[(2,2,2-trifluoroethoxy)phenoxy]ethyl]amino)propyl]-2,3-dihydro-1H-indole-7-carboxamide]. In contrast, the response to noradrenaline (nonselective alpha-1 AR agonist) was inhibited noncompetitively by these antagonists (except 5-methylurapidil) with Schild slopes different from unity. These results suggest that the response to NS-49 was mediated predominantly via alpha-1L ARs, whereas the response to noradrenaline was produced through two distinct alpha-1 AR subtypes (presumably alpha-1B and alpha-1L ARs). In binding studies with intact segments of rabbit ear artery, [3H]KMD-3213 bound with high affinity (pKD=9.7) to alpha-1 ARs, which were subdivided by prazosin, RS-17053, and 5-methylurapidil into two subtypes (alpha-1A and alpha-1L ARs). In contrast, [3H]prazosin binding sites in ear artery segments (pKD = 9.8) were identified as alpha-1A and alpha-1B ARs. In conventional binding studies using isolated rabbit ear artery microsomal membranes, [3H]KMD-3213 binding sites were identified as alpha-1A ARs with high affinities for prazosin, RS-17053, and 5-methylurapidil. Our study indicates that an alpha-1L AR having a unique pharmacological profile coexists with alpha-1A and alpha-1B ARs in rabbit ear artery and can be identified either functionally or by binding studies using intact tissues but not microsomal membrane preparations.

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.

Pharmacological characterization of alpha1-adrenoceptor subtypes in the bovine tail artery

Ioudina, M. V., Dyer, D. C. Pharmacological characterization of alpha1-adrenoceptor subtypes in the bovine tail artery. J. vet. Pharmacol. Therap. 25, 363-369. The purpose of this study was to identify the alpha1-adrenoreceptor subtypes present in the bovine tail artery which mediate contractions to adrenergic agonists. A61603, an alpha1A-selective agonist, was more potent compared with norepinephrine and phenylephrine. The pKA value of A61603 was 6.93 +/- 0.19 microM (n=6). Antagonists, BMY 7378, WB 4101 and 5-methylurapidil, caused a parallel shift to the right of the concentration-response curve to A61603 with pA2 values of 6.62, 9.27 and 8.86, respectively. Prazosin, BMY 7378 and WB 4101 inhibited phenylephrine induced contraction with pA2 values of 9.47, 7.17 and 9.73, respectively. The pA2 values obtained for 5-methylurapidil, WB 4101, BMY 7378 and prazosin against alpha1-adrenoceptor agonists were significantly correlated with pKi values (Zhu, Zhang & Han, 1997, Eur. J. Pharmacol.329, 55-61) for the cloned alpha1a-adrenoceptor but not with the cloned alpha1b- or alpha1d-adrenoceptor. BMY 7378, a selective alpha1D-adrenoceptor antagonist, was significantly more potent against the nonsubtype selective agonist phenylephrine than to A61603. Chloroethylclonidine (50 microM for 10 min) did not affect contractile responses to A61603, but caused a significant inhibition of contractile responses to phenylephrine. In conclusion, it appears that alpha1A- and alpha1D-adrenoceptors play a role in adrenergic mediated contraction in the bovine tail artery.

Alpha-adrenoceptor subtypes

Different studies have led to our present knowledge of the membrane receptors responsible for mediating the responses to the endogenous catecholamines. These receptors were initially differentiated into alpha - and beta-adrenoceptors. Alpha-adrenoceptors mediate most excitatory functions, and were in turn differentiated in the 1970s into alpha(1)- and alpha(2)-adrenoceptors. The alpha(1)-adrenoceptor type usually mediates responses in the effector organ. The alpha(2)-adrenoceptor type is located presynaptically and regulates the release of the neurotransmitter but it is also present in postsynaptical locations. Both alpha-adrenoceptors are important for the control of vascular tone, but we now know that neither alpha(1)- nor alpha(2)-adrenoceptors constitute homogeneous groups. Each alpha-adrenoceptor type can be subdivided into different subtypes and in this review we have turned our attention to these. The alpha(1)- and the alpha(2)-adrenoceptor subtypes were previously defined pharmacologically by functional and binding studies, and later they were also isolated and identified using cloning methods. In fact, the study of alpha-adrenoceptors was revolutionized by the techniques of molecular biology which permitted us to establish the present classification. The present classification of alpha(1)-adrenoceptors stands as follows: alpha(1A)-adrenoceptor subtype (cloned alpha(1c) and redesignated alpha(1a/c)), alpha(1B)-adrenoceptor subtype (cloned alpha(1b)) and alpha(1D)-adrenoceptor subtype (cloned alpha(1d) and redesignated alpha(1a/d)). It has not been easy to establish the distribution of these alpha(1)-adrenoceptor subtypes in the various organs and tissues, or to define the functional response mediated by each one in the different species studied. Nevertheless it seems that the alpha(1A)-adrenoceptor subtype is more implicated in the maintenance of vascular basal tone and of arterial blood pressure in conscious animals, and the alpha(1B)-adrenoceptor subtype participates more in responses to exogenous agonists. It has also been observed that the expression of the alpha(1B)-adrenoceptor subtype can be modified in pathological situations and particular attention has been paid to the regulation of expression of this receptor. The present classification of alpha(2)-adrenoceptors stands as follows: alpha(2A/D)-adrenoceptor subtype (today it is accepted that the alpha(2A)-adrenoceptor subtype and the alpha(2D)-adrenoceptor subtype are the same receptor but they were identified in different species: the alpha(2A) in human and the alpha(2D) in rat); alpha(2B)-adrenoceptor subtype (cloned alpha(2b)) and alpha(2C)-adrenoceptor subtype (cloned alpha(2c)). Today we know that the alpha(2A/D)- and alpha(2B)-adrenoceptor subtypes in particular control arterial contraction, and that the alpha(2C)-adrenoceptor subtype is responsible above all for venous vasoconstriction. We also know that the alpha(2 A/D)-adrenoceptor subtype fundamentally mediates the central effects of the alpha(2)-adrenoceptor agonists. Despite the validity of the above-mentioned classification of the alpha(1)- and alpha(2)-adrenoceptors, it seems clear that the contractions of a large number of tissues including smooth muscle are mediated by more than one alpha-adrenoceptor subtype. Moreover, few ligands recognise only one alpha-adrenoceptor subtype and the lack of specifity in the different drugs for each one limits their administration in vivo and their therapeutic use.

Alpha 1-adrenergic receptor regulation: basic science and clinical implications

Adrenergic receptors (ARs) are members of the G-protein-coupled receptor family, which includes alpha 1ARs, alpha 2ARs, beta 1ARs, beta 2ARs, beta 3ARs, adenosine, muscarinic, angiotensin, endothelin receptors, and many others that are responsible for a large variety of physiologic effects through G-protein coupling. This review focuses on alpha 1ARs and their regulation at both the mRNA and protein levels. Currently, three alpha 1AR subtypes have been characterized both pharmacologically and at the gene level: alpha 1aAR, alpha 1bAR, and alpha 1dAR. These are expressed in a species- and tissue-dependent manner. Mutagenesis approaches have been extremely valuable in the identification of key residues that govern alpha 1AR ligand binding and signaling. These studies reveal that alpha 1ARs have evolved an exquisitely sensitive regulation of their activity in which any disruption of the native structure has profound effects on subsequent function and effector coupling. Significant advances have also been made in the elucidation of signaling pathway components, resulting in the identification of novel pathways that can lead to pathologic conditions. Specific topics include mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and G-protein-coupled receptor cross-talk pathways. Within this context, recent studies identifying underlying transcriptional mechanisms involved in the regulation of the alpha 1AR subtypes are also discussed. Finally, given the potentially important role of alpha 1ARs in the vasculature, as well as in the pathology of many diseases, such as myocardial hypertrophy and benign prostatic hyperplasia, the clinical relevance of alpha 1AR distribution, pharmacology, and therapeutic intervention is reviewed.

Role of alpha-1 adrenoceptor subtypes mediating constriction of the rabbit ear thermoregulatory microvasculature

An acute in vivo preparation of the microvasculature of the rabbit ear was used to evaluate the functional role of alpha1 (alpha1)-adrenoceptor subtypes in thermoregulatory microcirculation. The effect of alpha1-adrenoceptor subtype blockade on phenylephrine-induced vasoconstriction was assessed with the alpha1A, alpha1B, and alpha1D-adrenoceptor-selective antagonists 5-methyl-urapidil (10(-8) M), chloroethylclonidine (10(-5) M), and 8-[2-[4(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol[4.5]deca ne-7,9-dione dihydrochloride (BMY7378) (10(-6) M), respectively. The results demonstrated that pretreatment of the ear microvasculature with 5-methyl-urapidil or BMY7378 shifted the phenylephrine concentration-response curve rightward and significantly changed the log of the phenylephrine concentration, causing half-maximum stimulation (EC50) in arterioles (p < 0.05). BMY7378 shifted the phenylephrine concentration-response curve of the arteriovenous anastomoses about 100-fold rightward (p < 0.05). All three alpha1-adrenoceptor antagonists eliminated the vasoconstrictive effects of phenylephrine on venules. The results indicate that the ear microvasculature has a heterogenous distribution of alpha1-adrenoceptor subtypes. The alpha1A and alpha1D-adrenoceptor subtypes appear to have a greater influence on constrictive function in arterioles, whereas the alpha1D-adrenoceptor is the dominant constrictor of arteriovenous anastomoses. In general, the alpha1-adrenoceptor does not play a major vasoconstrictor role in venules. Chloroethylclonidine, an irreversible alpha1B-adrenoceptor antagonist, induced contractile responses in the ear microvasculature, probably due to its alpha2-adrenoceptor agonist effects. This study extended our understanding of the adrenergic receptor control mechanisms of a cutaneous thermoregulatory end organ characterized by two parallel perfusion circuits providing nutritional and thermoregulatory functions.

Effects of indoramin in rat vas deferens and aorta: concomitant alpha1-adrenoceptor and neuronal uptake blockade

1. The actions of the alpha1-adrenoceptor antagonist indoramin have been examined against the contractions induced by noradrenaline in the rat vas deferens and aorta taking into account a putative neuronal uptake blocking activity of this antagonist which could result in self-cancelling actions. 2. Indoramin behaved as a simple competitive antagonist of the contractions induced by noradrenaline in the vas deferens and aorta yielding pA2 values of 7.38+/-0.05 (slope=0.98+/-0.03) and 6.78+/-0.14 (slope=1.08+/-0.06), respectively. 3. When the experiments were repeated in the presence of cocaine (6 microM) the potency (pA2) of indoramin in antagonizing the contractions of the vas deferens to noradrenaline was increased to 8.72+/-0.07 (slope=1.10+/-0.05) while its potency remained unchanged in the aorta (pA2=6.69+/-0.12; slope=1.04+/-0.05). 4. In denervated vas deferens, indoramin antagonized the contractions to noradrenaline with a potency similar to that found in the presence of cocaine (8.79+/-0.07; slope=1.09+/-0.06). 5. It is suggested that indoramin blocks alpha1-adrenoceptors and neuronal uptake in rat vas deferens resulting in Schild plots with slopes not different from unity even in the absence of selective inhibition of neuronal uptake. As a major consequence of this double mechanism of action, the pA2 values for this antagonist are underestimated when calculated in situations where the neuronal uptake is active, yielding spurious pKB values.

The influence of the wall tension on the contractile responses of arteries

The present study aimed at determining the influence of tension on the responses of arterial smooth muscle to noradrenaline, phenylephrine and angiotensin II. Concentration-response curves to these agonists were obtained in the rat aorta, at two levels of tension: 3 g (29.4 mN) and 0.5 g (4.9 mN). The results obtained show that the maximal responses to the agonists used (in percent of the maximal response to noradrenaline) were significantly larger (P < 0.05) at 3 g than at 0.5 g: 113% versus 66%, respectively, for noradrenaline; 95% versus 59%, respectively, for phenylephrine; 60% versus 24%, respectively, for angiotensin II. In the presence of gadolinium (100 mumol/L)--a mechanogated ion channel blocker--the responses to noradrenaline at 3 g were still significantly larger than responses at 0.5 g: 103% versus 67%, respectively. The compound H-7 (20 mumol/L)--a protein kinase C inhibitor--caused a marked decrease in the maximal responses to noradrenaline at both levels of tension, the responses being reduced to 44% at 3 g and to 20% at 0.5 g. Isradipine (1 mumol/L)--a calcium channel blocker--caused a slight decrease in the responses to noradrenaline at both levels of tension, the responses being reduced to 86% (3 g) and to 50% (0.5 g). In endothelium-free arterial rings, the responses to noradrenaline at 3 g and 0.5 g were also significantly different: 118% versus 80%, respectively. It is concluded that the tension of the arterial wall is a major factor influencing the effects of vasoconstrictor agents; however the mechanisms underlying this supersensitivity at higher tension remain unknown.

Subtypes of functional alpha1- and alpha2-adrenoceptors

In this review, subtypes of functional alpha1- and alpha2-adrenoceptors are discussed. These are cell membrane receptors, belonging to the seven transmembrane spanning G-protein-linked family of receptors, which respond to the physiological agonists noradrenaline and adrenaline. Alpha1-adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A 4th alpha1-adrenoceptor, the alpha1L-, has been postulated to mediate contractions in some tissues, but its relationship to cloned receptors remains to be established. Alpha2-adrenoceptors can be divided into alpha2A-, alpha2B- and alpha2C-adrenoceptors, all of which mediate contractile responses. Prejunctional inhibitory alpha2-adrenoceptors are predominantly of the alpha2A-adrenoceptor subtype (the alpha2D-adrenoceptor is a species orthologue), although alpha2C-adrenoceptors may also occur prejunctionally. Although alpha2-adrenoceptors are linked to inhibition of adenylate cyclase, this may not be the primary signal in causing smooth muscle contraction; likewise, prejunctional inhibitory actions probably involve restriction of Ca2+ entry or opening of K+ channels. Receptor knock-out mice are beginning to refine our knowledge of the functions of alpha-adrenoceptor subtypes.

Alpha-adrenoceptor interaction of tetrandrine and isotetrandrine in the rat: functional and binding assays

The action of 1S,1'S-tetrandrine, a bisbenzyltetrahydroisoquinoline alkaloid, on alpha1-adrenoceptors has been compared with that of its isomer 1R,1'S-isotetrandrine. The work includes binding assays to analyse the affinity of these products for the [3H]prazosin binding site of rat cerebral cortical membranes and functional studies on rat isolated aorta to examine the effects of both alkaloids on intracellular calcium processes related or not to alpha-adrenoceptor activation. A radioligand receptor-binding study showed that both compounds interacted with the alpha1-adrenoceptors displacing [3H]prazosin from the specific binding site. The Ki values (inhibition constants) were 0.69+/-0.12 and 1.6+/-0.4 microM for tetrandrine and isotetrandrine, respectively. The functional studies showed that both alkaloids concentration-dependently inhibited noradrenaline-induced contraction in Ca2+-free solution (IC50 values, i.e. the concentrations needed to induce 50% inhibition, were 252.8 and 174.9 microM for tetrandrine and isotetrandrine, respectively), the spontaneous contractile response elicited by extracellular calcium after depletion of noradrenaline-sensitive intracellular stores (increase in resting tone; IC50 values 11.6 and 19.6 microM for tetrandrine and isotetrandrine, respectively) and the refilling of intracellular Ca2+ stores sensitive to noradrenaline (IC50 values 7.4 and 14.9 microM for tetrandrine and isotetrandrine, respectively). The results show that tetrandrine and isotetrandrine interact with alpha1-adrenoceptors by displacing the [3H]prazosin binding site and that both compounds inhibit mainly the Ca2+-dependent process and have less action on alpha1-adrenoceptors. Tetrandrine is more potent than isotetrandrine.

Characterization of alpha1-adrenoceptor subtypes mediating contractions to phenylephrine in rat thoracic aorta, mesenteric artery and pulmonary artery

1. The subtype of alpha1-adrenoceptor mediating contractions to phenylephrine of the rat thoracic aorta, mesenteric artery and pulmonary artery were investigated by use of antagonists which show selectivity between the cloned alpha1-adrenoceptor subtypes in binding studies. 2. Cumulative concentration-contraction curves for phenylephrine were competitively antagonized in the rat thoracic aorta by prazosin (pA2 9.9), WB4101 (pA2 9.6), 5-methylurapidil (pA2 8.1), benoxathian (pA2 9.2) and indoramin (pA2 7.4). These compounds were also competitive antagonists in the mesenteric and pulmonary arteries (except for 5-methylurapidil in the pulmonary artery), (prazosin pA2 9.9 and 9.7; WB4101 pA2 9.8 and 9.6; 5-methylurapidil pA2 7.9 and pK(B) estimate 8.0; benoxathian pA2 8.8 and 9.3; indoramin pA2 7.2 and 7.5, respectively). 3. RS 17053 was not a competitive antagonist in any blood vessel as Schild plot slopes were greater than unity. The pK(B) estimates for RS 17053 were 7.1 in aorta, 7.0 in the mesenteric artery and 7.7 in the pulmonary artery. 4. The alpha1D-subtype selective antagonist BMY 7378 appeared to be non-competitive with shallow Schild plot slopes. The data were better fitted with two lines in all tissues, with Schild plot slopes that were no longer different from unity, except in the pulmonary artery. The higher affinity site for BMY 7378 in the aorta had a pA2 of 9.0, while it was 8.8 and 8.9 in the mesenteric and pulmonary arteries, respectively. 5. MDL73005EF acted in a non-competitive manner in all three blood vessels, with shallow Schild plot slopes. The pK(B) estimates for MDL73005EF were 8.4 in aorta, 7.5 in the mesenteric artery and 8.0 in the pulmonary artery. 6. In all three blood vessels the functionally determined antagonist affinity estimates correlated best with published pKi values for their displacement of [3H]-prazosin binding on membranes expressing cloned alpha1d-adrenoceptors compared with alpha1a- or alpha1b-adrenoceptors. The antagonist affinity estimates in the aorta, mesenteric and pulmonary arteries correlated highly with their previously published pA2 values in rat aorta (alpha1D) and less well with those for alpha1A- and alpha1B-adrenoceptors mediating contraction of the rat epididymal vas deferens and rat spleen, respectively. 7. The results of this study suggest that the contraction to phenylephrine of the rat thoracic aorta, mesenteric artery and pulmonary artery are mediated in part via the alpha1D-subtype of adrenoceptor. The data for both BMY 7378 and MDL73005EF in all three blood vessels are consistent with receptor heterogeneity. However, the identity of the second site is unclear.