Mechanism of the cardiovascular activity of laudanosine: comparison with papaverine and other benzylisoquinolines

Synthesis of 1-benzylisoindoline and 1-benzyl-tetrahydroisoquinoline through nucleophilic addition of organozinc reagents to N,O-acetals

A new approach to access 1-benzylisoindoline and 1-benzyl-tetrahydroisoquinoline has been developed through nucleophilic addition of organozinc reagents to N,O-acetals. A number of substituted organozinc reagents were amenable for this transformation, and the desired products were obtained with excellent yields. Moreover, Sc(OTf)3 proved to be an effective catalyst for the formation of 1-benzylisoindoline and 1-benzyl-tetrahydroisoquinoline using such nucleophilic addition.

Therapeutic potentials of natural compounds acting on cyclic nucleotide phosphodiesterase families

Intracellular cyclic AMP and/or cyclic GMP are characterized in the 1960th. These second messengers, hydrolysed specifically by cyclic nucleotide phosphodiesterase (PDE), play a major role in intracellular signalling. Natural products have been a rich source of drug discovery, Theophylline and Methylxanthine originated from tea leaves used for asthma treatment, whereas, Papaverine, a natural isoquinolein originated from Papaver somniferum traditionally used in impotency, altogether as caffeine where firstly described as PDE-inhibiting compounds. Since that time, the knowledge in PDE field has been drastically increased, allowing the design and development of new therapeutic drugs acting against different pathologies in the nanomolar range. During this period some natural compounds have been identified as PDE inhibitors and used in that context to investigate their therapeutic potential effects. The aim of this literature review is to point out the reported data and demonstrating the contribution of natural characterized molecules as PDE inhibitors in various pathologies that can open new fields of research for drug discovery, notably in epigenetic regulation.

Enantioselective Oxidative Aerobic Dealkylation of N-Ethyl Benzylisoquinolines by Employing the Berberine Bridge Enzyme

N-Dealkylation methods are well described for organic chemistry and the reaction is known in nature and drug metabolism; however, to our knowledge, enantioselective N-dealkylation has not been yet reported. In this study, exclusively the (S)-enantiomers of racemic N-ethyl tertiary amines (1-benzyl-N-ethyl-1,2,3,4-tetrahydroisoquinolines) were dealkylated to give the corresponding secondary (S)-amines in an enantioselective fashion at the expense of molecular oxygen. The reaction is catalyzed by the berberine bridge enzyme, which is known for CC bond formation. The dealkylation was demonstrated on a 100 mg scale and gave optically pure dealkylated products (ee>99 %).

Naturally occurring plant isoquinoline N-oxide alkaloids: their pharmacological and SAR activities

The present review describes research on novel natural isoquinoline alkaloids and their N-oxides isolated from different plant species. More than 200 biological active compounds have shown confirmed antimicrobial, antibacterial, antitumor, and other activities. The structures, origins, and reported biological activities of a selection of isoquinoline N-oxides alkaloids are reviewed. With the computer program PASS some additional SAR (structure-activity relationship) activities are also predicted, which point toward new possible applications of these compounds. This review emphasizes the role of isoquinoline N-oxides alkaloids as an important source of leads for drug discovery.

Deracemisation of benzylisoquinoline alkaloids employing monoamine oxidase variants

Deracemisation of benzylisoquinoline alkaloids was performed employing a recently developed variant of monoamine oxidase from Aspergillus niger (MAO-N variant D11).

Controlling stereoselectivity by enzymatic and chemical means to access enantiomerically pure (1S,3R)-1-benzyl-2,3-dimethyl-1,2,3,4-tetrahydroisoquinoline derivatives

A chemoenzymatic strategy for the synthesis of enantiomerically pure novel alkaloids (1S,3R)-1-benzyl-2,3-dimethyl-1,2,3,4-tetrahydroisoquinolines is presented. The key steps are the biocatalytic stereoselective reductive amination of substituted 1-phenylpropan-2-one derivatives to yield chiral amines employing microbial ω-transaminases, and the diastereoselective reduction of a Bischler–Napieralski imine intermediate by catalytic hydrogenation in the presence of palladium on charcoal, leading exclusively to the desired cis-isomer.

Biocatalytic organic synthesis of optically pure (S)-scoulerine and berbine and benzylisoquinoline alkaloids

A chemoenzymatic approach for the asymmetric total synthesis of the title compounds is described that employs an enantioselective oxidative C-C bond formation catalyzed by berberine bridge enzyme (BBE) in the asymmetric key step. This unique reaction yielded enantiomerically pure (R)-benzylisoquinoline derivatives and (S)-berbines such as the natural product (S)-scoulerine, a sedative and muscle relaxing agent. The racemic substrates rac-1 required for the biotransformation were prepared in 4-8 linear steps using either a Bischler-Napieralski cyclization or a C1-Cα alkylation approach. The chemoenzymatic synthesis was applied to the preparation of fourteen enantiomerically pure alkaloids, including the natural products (S)-scoulerine and (R)-reticuline, and gave overall yields of up to 20% over 5-9 linear steps.

(+)-Nantenine isolated from Nandina domestica Thunb. inhibits adrenergic pressor responses in pithed rats

The effects of (+)-nantenine on various pressor responses, recently reported exerting competitive antagonistic activity at the alpha1-adrenoceptor/5-hydroxytryptamine (5-HT)2A receptor, were examined in vivo. (+)-Nantenine (0.03-3 mg/kg) caused a dose-dependent inhibition of the pressor response to phenylephrine (alpha1-adrenoceptor agonist) or 5-HT (5-HT receptor agonist) in both anesthetized and pithed rats. The pressor response to UK 14304 (5-Bromo-N-[2-imidazolin-2-yl]-6-quinoxalinamine) (an alpha2-adrenoceptor agonist) was inhibited by (+)-nantenine (0.003-3 mg/kg) in pithed rats in a dose-dependent manner without affecting the angiotensin II-induced pressor response in anesthetized rats. The pressor response to sympathetic nerve stimulation was also inhibited by (+)-nantenine (0.3-3 mg/kg) in a dose-dependent manner. (+)-Nantenine (3 mg/kg) facilitated the norepinephrine release induced by sympathetic nerve stimulation in pithed rats. In the guinea pig vas deferens, the initial component of contractions induced by electrical field stimulation was enhanced by (+)-nantenine (1-30 microM) in a concentration-dependent manner, while the later component was inhibited by it. These data suggest that (+)-nantenine has antagonistic activities on alpha1-adrenoceptors, alpha2-adrenoceptors and 5-HT2A receptors in pithed rats.

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.

(+/-)-Domesticine, a novel and selective alpha1D-adrenoceptor antagonist in animal tissues and human alpha 1-adrenoceptors

The pharmacological profile of (+/-)-domesticine, a novel alpha(1)-adrenoceptor antagonist, was examined in animal tissues and Chinese hamster ovary (CHO) cells expressing cloned human alpha(1)-adrenoceptor subtypes and compared with the properties of BMY-7378 ([8-(2-[4-(2-methoxy-phenyl)-1-piperazinyl]ethyl)-8-azaspirol [4.5]decane-7,9-dione dihydrochloride], the prototypical alpha(1D)-adrenoceptor antagonist. Both (+/-)-domesticine and BMY-7378 were more potent in inhibiting the phenylephrine-induced contraction in rat thoracic aorta than tail artery or spleen. The selectivity of (+/-)-domesticine to inhibit phenylephrine-induced contraction in rat thoracic aorta was 32- and 17-fold higher than that in tail artery and spleen, respectively, while that of BMY-7378 it was 125- and 11-fold, respectively. The functional affinity profiles of these compounds for the alpha(1)-adrenoceptor subtypes in animal tissues were consistent with the respective binding affinity profiles in cloned human alpha(1)-adrenoceptor subtypes. (+/-)-Domesticine displayed a 34- and 9-fold higher selectivity for alpha(1d)-adrenoceptor than for alpha(1a)- and alpha(1b)-adrenoceptor, respectively, while BMY-7378 showed a selectivity for alpha(1d)-adrenoceptor of 102-fold higher than that of alpha(1a)-adrenoceptor and 21-fold higher than that of alpha(1b)-adrenoceptor. Interestingly, in [3H]8-OH-DPAT (8-hidroxy-2-(di-n-propyl-amino)tetraline hidrobromide) binding to 5-HT(1A) receptors of rat cerebral cortex, (+/-)-domesticine showed a 183-fold higher selectivity for alpha(1D)-adrenoceptor relative to 5-HT(1A) receptor, whereas BMY-7378 displayed a similar affinity at this receptor with respect to the alpha(1D)-adrenoceptor (0.89-fold). Both compounds, however, showed a weak affinity for 5-HT(2A)/5-HT(2C) receptors in rat frontal cortex. These results suggest that (+/-)-domesticine is more potent for alpha(1D)-adrenoceptor than for alpha(1A)- or alpha(1B)-adrenoceptor subtypes and it is highly selective compared to 5-HT(1A) and other receptors.

Structure-activity relationship studies with (+/-)-nantenine derivatives for alpha1-adrenoceptor antagonist activity

A series of (+/-)-nantenine derivatives of the natural aporphine alkaloids was synthesized and examined for a blocking action on alpha1-adrenoceptors in rat aorta and A10-cells. The potency of these derivatives was compared with that of an aporphine-related compounds (+)-boldine, an alpha1-adrenoceptor antagonist. Among nine (+/-)-nantenine derivatives having different substituents at N-6, C-1, or C-4 of the aporphine skeleton, (+/-)-domesticine had the most powerful alpha1-adrenoceptor-blocking action. The order of pA2 values was (+/-)-domesticine (8.06+/-0.06)>(+/-)-nordomesticine (7.34+/-0.03)>(+/-)-nantenine (7.03+/-0.03)>(+)-boldine (6.91+/-0.02)>other derivatives. Study of the structure-activity relationships showed that the replacement of a methoxy moiety at C-1 position of (plus minus)-nantenine with a hydroxyl group increased affinity for the receptor. In contrast, replacement of a methyl group with a hydrogen atom or an ethyl group at N-6 position in the (+/-)-nantenine structure decreased affinity for the receptor. These results suggest that a hydroxyl group at the C-1 position and a methyl group at the N-6 position in the (+/-)-nantenine structure are essential for the enhancement of affinity for the alpha1-adrenoceptor.

Influence of hypertension on tetrahydropapaveroline-induced vasorelaxation in rat thoracic aorta

Tetrahydropapaveroline (THP), a condensation product of ethanol-derived acetaldehyde, has been shown to elicit a vasorelaxant response in rat thoracic aorta. This study examined the influence of hypertension on the THP-induced vasorelaxant responsiveness. Ring segments of thoracic aorta were isolated from Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) and isometric tension development was measured with a force transducer. In aorta, with or without intact endothelium, the contractile responses to potassium chloride (0-120 mM) were comparable between the WKY and the SHR groups. Hypertension did not affect the vasoconstrictive response to norepinephrine (0-10 microM) in vessels with intact endothelium, whereas it attenuated the norepinephrine-induced response in vessels without endothelium. THP (0.1-100 microM) elicited endothelium-intact as well as -denuded vasorelaxation in aorta from both WKY and SHR groups. Interestingly, the THP-induced endothelium-dependent vasorelaxation was significantly enhanced, whereas the THP-induced endothelium-independent vasorelaxation was not affected by hypertension. These data indicate that the THP-induced endothelium-dependent vasorelaxant response is altered by the hypertensive state.

Mechanisms involved in the vasorelaxant effect of (-)-stepholidine in rat mesenteric small arteries

The purpose of the present investigation was to clarify whether the hypotensive action of the protoberberine alkaloid, and dopamine receptor antagonist, (-)-stepholidine, can be ascribed to an effect on peripheral small arteries. For this purpose isolated mesenteric small arteries were suspended in microvascular myographs for isometric tension recording. Relaxations mediated by dopamine D1 receptors were antagonized by (-)-stepholidine. (-)-Stepholidine inhibited in a concentration-dependent manner the contractile responses evoked by noradrenaline (10(-6) M), but not the contractile responses evoked by depolarizing solution (KCl, 60 mM) or 9,11-dideoxy-11alpha,9alpha-epoxymethano prostaglandin F2alpha (U46619, 10(-7) M). Mechanical endothelial cell removal, blockade of K+ channels, muscarinic receptors or adrenoceptors did not influence the inhibitory effect of (-)-stepholidine on the contractile response evoked with noradrenaline in the segments. (-)-Stepholidine caused rightward shifts of the concentration-response curves for noradrenaline and phenylephrine. The pA2 values for (-)-stepholidine were 6.05 and 5.94 against noradrenaline and phenylephrine, respectively. Electrical field stimulation induced prazosin-sensitive frequency-dependent contractions in mesenteric small arteries. These contractions were significantly inhibited by 10(-6) and 10(-5) M (-)-stepholidine. In membranes from the rat cerebral cortex labelled with [3H]prazosin, (-)-stepholidine (10(-7)-10(-4) M) completely inhibited the specific binding of the ligand with a pKi of 5.6. The present investigation suggests the inhibitory effect of (-)-stepholidine on the alpha1-adrenoceptor-mediated contractions induced by exogenously added and nerve-released noradrenaline in peripheral small arteries might contribute to a hypotensive effect of the drug.

Effects of visnagin on cyclic nucleotide phosphodiesterases and their role in its inhibitory effects on vascular smooth muscle contraction

1. Visnagin relaxed aortae previously contracted by noradrenaline. This effect was unalterated by endothelium removal and potentiated, at high concentrations, by the previous incubation with sodium nitroprusside. 2. Visnagin weakly inhibited the hydrolytic activity of the cyclic nucleotide phosphodiesterase (PDE) isozymes (PDE5, PDE4, PDE3, cyclic GMP activated PDE2 and PDE1). 3. The present results indicate an involvement of PDE inhibition in the relaxant effect of visnagin at high concentration (>5x10(-5) M).

The 5-HT and alpha-adrenoceptor antagonist effect of four benzylisoquinoline alkaloids on rat aorta

The action of four benzylisoquinoline alkaloids (two aporphines-glaucine and apomorphine, a benzylisoquinoline-papaverine and a bisbenzyltetrahydroisoquinoline-antioquine) on 5-HT-induced contraction in rat thoracic aorta has been examined and compared with that of the control drugs: ketanserin, nifedipine, prazosin and phentolamine. The relaxant action on 5-HT-induced contraction was contrasted with that on the contraction induced by noradrenaline and KCl. The results obtained with control drugs show that ketanserin has clear selectivity for 5-HT receptors, whereas prazosin and phentolamine have high selectivity for the alpha1-adrenoceptor and nifedipine seems to have a more potent effect on KCl-induced contraction than on that induced by 5-HT or noradrenaline. The contraction evoked by 5-HT (10 microM) was inhibited in a concentration-dependent manner by all the alkaloids. The order of potency was: papaverine = glaucine > apomorphine > antioquine. Papaverine had a non-specific relaxant action on 5-HT-, noradrenaline- and KCl-induced contraction, antioquine had a weak relaxant action on the agonist assays, and glaucine and apomorphine inhibited noradrenaline- and 5-HT-induced contraction more potently than they inhibited the K+-depolarized response. These results indicate that the aporphines assayed, S-glaucine and R-aporphine, had selective action against agonist (noradrenaline or 5-HT)-induced contraction rather than against KCl-depolarization of rat aorta. In contrast papaverine, a benzylisoquinoline alkaloid, relaxes all agents used non-selectively as could be expected from the lack of specificity that characterizes this alkaloid.

Relationships between structure and vascular activity in a series of benzylisoquinolines

1. In the present work, the properties of 3-methyl isoquinoline, 3,4-dihydropapaverine, tetrahydropapaverine and tetrahydropapaveroline were compared with those of papaverine and laudanosine. The work includes functional studies on rat isolated aorta contracted with noradrenaline, caffeine or KCl, and a determination of the affinity of the compounds for alpha1-adrenoceptors and calcium channel binding sites, with [3H]-prazosin, [3H]-nitrendipine and [3H]-(+)-cis-diltiazem binding to rat cerebral cortical membranes. The effects of papaverine derivatives on the different molecular forms of cyclic nucleotide phosphodiesterases (PDE) isolated from bovine aorta were also determined. 2. The three papaverine derivatives show greater affinity than papaverine for the [3H]-prazosin binding site. They are therefore more selective as inhibitors of [3H]-prazosin binding as opposed to [3H]-(+)-cis-diltiazem, while papaverine appears to have approximately equal affinity for both. [3H]-nitrendipine binding was not affected by either papaverine or papaverine derivatives in concentrations up to 100 microM. 3-Methylisoquinoline had no effect on any of the binding sites assayed. 3. Contractions evoked by noradrenaline (1 microM) in rat aorta were inhibited in a concentration-dependent manner by 3,4-dihydropapaverine, tetrahydropapaverine and with a lower potency, by tetrahydropapaveroline. In Ca2+-free solution, tetrahydropapaverine and to a lesser extent, tetrahydropapaveroline, inhibited the noradrenaline (1 microM) evoked contraction in a concentration-dependent manner and did not modify the phasic contractile response evoked by caffeine (10 mM). This suggests that these alkaloids do not act at the intracellular level, unlike papaverine which inhibits the contractile response to caffeine and noradrenaline. 4. Inositol phosphates formation induced by noradrenaline (1 microM) in rat aorta was inhibited by tetrahydropapaverine (100 microM) and tetrahydropapaveroline (300 microM), thus suggesting that alpha1D-adrenoceptors are coupled to phosphoinositide metabolism in rat aorta. 5. Unlike papaverine, which has a significant effect on all the PDE isoforms, the three alkaloids assayed did not have an inhibitory effect on the different forms of PDE isolated from bovine aorta. 6. These results provide evidence that papaverine derivatives with a partially or totally reduced isoquinoline ring have a greater affinity for alpha1-adrenoceptors and a lower affinity for benzothiazepine sites in the Ca2+-channel than papaverine. This structural feature also implies a loss of the inhibitory activity on PDE isoforms. The planarity of the isoquinoline ring (papaverine) impairs the interaction with the alpha1-adrenoceptor site and facilitates it with the Ca2+-channels and PDEs, whereas the more flexible tetrahydroisoquinoline ring increases the binding to alpha1-adrenoceptors.

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.

The effect of S-(+)-boldine on the alpha 1-adrenoceptor of the guinea-pig aorta

1. The cardiovascular activity of S-(+)-boldine, an aporphine alkaloid structurally related to papaverine, was determined. The work includes functional studies on guinea-pig isolated aorta contracted with noradrenaline, caffeine, KCl or Ca2+, and on guinea-pig trachea contracted with acetylcholine or histamine. 2. S-(+)-boldine inhibited in a concentration-dependent manner the contractile response evoked by noradrenaline (10 microM) in guinea-pig aorta (IC50 = 1.4 +/- 0.2 microM) while the KCl depolarizing solution (60 mM)- or the Ca2+ (1 mM)-induced contractions were only partially affected by boldine up to 300 microM. In contrast, papaverine relaxed noradrenaline (NA), KCl or Ca2+ induced contractions showing similar IC50 values in all cases. S-(+)-boldine had a greater potency on the contraction elicited by NA whereas papaverine acted in a non-selective manner. 3. S-(+)-boldine was found to be an alpha 1-adrenoceptor blocking agent in guinea-pig aorta as revealed by its competitive antagonism of noradrenaline-induced vasoconstriction (pA2 = 5.64 +/- 0.08), and its potency was compared with that of prazosin (pA2 = 8.56 +/- 0.24), a known potent alpha 1-adrenoceptor antagonist. In contrast, papaverine caused rightward shifts of the NA concentration-response curves with depression of maximal response indicating that it acts as a non-competitive antagonist. 4. Contraction of guinea-pig aorta induced by caffeine (60 mM) in a Ca(2+)-containing Krebs solution was not affected by a 60 min incubation period with different doses of S-(+)-boldine (1-300 microM). Papaverine inhibited partially this caffeine-induced contraction at the maximal dose used (100 microM). 5. Inositol phosphates formation induced by noradrenaline (10 microM) in guinea-pig thoracic aorta was inhibited by S-(+)-boldine (30 microM) but not by papaverine (10 microM). 6. Contractions of guinea-pig trachea caused by acetylcholine (100 microM) or histamine (10 microM) were not modified by S-(+)-boldine (0.1-100 microM). 7. These results provide evidence that S-(+)-boldine, an aporphine alkaloid, has interesting properties as an alpha 1-adrenoceptor blocker in vascular smooth muscle, and acts as a competitive antagonist of the alpha 1-adrenoceptor present in the guinea pig aorta.

Functional evidence of inverse agonism in vascular smooth muscle

1. In the present study, depletion of internal Ca2+ stores sensitive to noradrenaline (1 microM) in rat aorta, is the signal for the entry of extracellular Ca2+, not only to refill the stores but also, in our experimental conditions, to activate the contractile proteins. This induces an increase in the resting tone that constitutes, the first functional evidence of this Ca2+ entry. 2. The fact that methoxamine (100 microM) reproduces the same processes as noradrenaline but clonidine (1 microM) does not, indicates that alpha(1)-adrenoceptor activation is related to the increase in the resting tone observed after depletion of adrenoceptor-sensitive internal Ca2+-stores. 3. Benoxathian and WB 4101 (alpha(1A)- and alpha(1D)-adrenoceptor antagonists) selectively inhibit, in a concentration-dependent manner, this mechanical response observed in absence of the agonist, which suggests that these agents can act as inverse agonists and provide a functional model for studying this phenomenon. Since chloroethylclonidine (100 microM) has no effect on this response, the participation of alpha(1B)-adrenoceptors can be ruled out. 4. Contractile responses to noradrenaline (1 microM) in Ca2+-free medium were selectively blocked by chloroethylclonidine. This suggests that the response to noradrenaline in Ca2+-free medium mainly depends on the activation of the alpha(1B)-adrenoceptor subtype.