Multiple actions of glaucine on cyclic nucleotide phosphodiesterases, α1‐adrenoceptor and benzothiazepine binding site at the calcium channel

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Abstract
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 α-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 μm) 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 μm) and glaucine (0.1 mm) inhibited the contraction evoked by NA; diltiazem (0.1 mm) diminished this contraction whereas nifedipine (1 μm) 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. 4 Glaucine and papaverine show affinity at the [3H]-prazosin binding site and at the benzothiazepine binding site of the Ca2+-channel receptor complex, but have no effect at the dihydropyridine binding site in rat cerebral cortex. Glaucine exerts some selectivity as an inhibitor of [3H]-prazosin binding as opposed to [3H]-(+)-cis-diltiazem binding while papaverine appears to have approximately equal affinity in this respect. 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 α1-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.

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