1 In the present study, the properties of glaucine (an aporphine structurally related to papaverine) were compared with those of papaverine, diltiazem, nifedipine and prazosin. The work includes functional studies on rat isolated aorta contracted with noradrenaline, caffeine or KCl, and a determination of the affinity of glaucine at calcium channel binding sites of x-adrenoceptors, by use of [3H]-( + )-cis-diltiazem, [3H]-nitrendipine and [3H]-prazosin binding to cerebral cortical membranes. The effects of glaucine on the different molecular forms of cyclic nucleotide phosphodiesterases (PDE) isolated from bovine aorta were also determined. 2 Contraction evoked by noradrenaline (1 gLM) or depolarizing solution (60 mM KCl) were inhibited in a concentration-dependent manner by all the compounds tested. As expected, prazosin showed a greater selectivity of action on NA-induced contraction, whereas nifedipine and diltiazem appeared more potent on KCl-induced contraction. Glaucine had a greater potency on the contraction elicited by noradrenaline whereas papaverine acted non specifically.3 In Ca2'-free solution, prazosin (0.1 pM) and glaucine (0.1 mM) inhibited the contraction evoked by NA; diltiazem (0.1 mM) diminished this contraction whereas nifedipine (1 pM) had no effect. Preincubation of tissues with glaucine, diltiazem, nifedipine and prazosin did not modify the contractile response induced by caffeine. In contrast, papaverine (0.1 mM) significantly inhibited the contractions evoked by NA or caffeine in Ca2"-free medium. 5 This study confirms the presence of four phosphodiesterase (PDE) activities in bovine aorta: a calmodulin-activated PDE (CaM-PDE type I) which hydrolyzed preferentially guanosine 3':5'-cyclic monophosphate (cyclic GMP); a cyclic GMP selective form (cGMP-PDE type V); and two low Km adenosine 3':5'-cyclic monophosphate (cyclic AMP) PDEs that are insensitive to the stimulatory effect of CaM, one of which was inhibited by cyclic GMP (CGI-PDE, type III) and the other by rolipram (cAMP-PDE, type IV). Glaucine selectively inhibits one of the two forms of Ca2+-independent low Km cAMP-PDE, the type IV. In contrast, papaverine exerts a non-selective inhibitory effect upon all PDE forms.6 The present work provides evidence that glaucine, a benzyltetrahydroisoquinoline alkaloid, has interesting properties as an al-adrenoceptor antagonist, calcium entry blocker (through the benzothiazepine recognition site in the calcium channel) and as a selective inhibitor of the rolipram-sensitive cAMP-PDE, type IV PDE.

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.

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.

1 In the present study we assessed the activity of antioquine, a bisbenzyltetrahydroisoquinoline alkaloid isolated from Pseudoxandra sclerocarpa, by examining its effects on 3 Paradoxically, at the highest concentration tested (300 gM) antioquine induced a contractile response of similar magnitude in the presence and absence of extracellular calcium, at 37°C. This activity was greatly attenuated at 25°C. Antioquine-induced contractions were not inhibited by prazosin (0.1 JIM), nifedipine (1 gM) or diltiazem (100 gM). On the contrary, prazosin and nifedipine slightly increased the contractions in the presence of extracellular calcium. Papaverine (100 jiM) partially inhibited the contractile response to antioquine both in the presence and absence of extracellular calcium. 4 At 25°C, in Ca2+-free solution, antioquine (300 jiM) did not modify the contractile response (phasic and tonic) evoked by noradrenaline, but increased the phasic contraction induced by caffeine. At 37°C, the contraction elicited by antioquine made it impossible to observe the noradrenaline-induced one. Antioquine showed affinity for the [3H]-prazosin binding site and for the [3H]-(+)-cis-diltiazembinding site of the Ca2+-channel receptor complex, but had no effect at the dihydropyridine binding site in rat cerebral cortex.6 Antioquine weakly inhibited some PDE forms isolated from bovine aorta: a CaM-PDE (PDE I) which preferentially hydrolyzes cyclic GMP and is activated by calmodulin, and a rolipram-sensitive cyclic AMP-PDE (PDE IV) which hydrolyzed cyclic AMP. Antioquine did not exert any inhibitory effect on the other forms of PDE, a cyclic GMP selective form (PDE V) and a low Km cyclic AMP-PDE that is inhibited by cyclic GMP (CGI-PDE, PDE III). 7 The present work provides evidence that antioquine has properties both as a calcium entry blocker (possibly through the benzothiazepine recognition site in the calcium channel) and as a contractile agent.Its mechanism of action as a contractile agent is not related to Ca2+-entry and is hypothetically similar to that of calyculin-A or okadaic acid. The possible involvement of a-adrenoceptors in this paradoxical effect cannot be excluded. The rigidity of the molecule provides an interesting model for analyzing this contractile mechanism and the intracellular processes involved.

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