Alpha 1-adrenoceptor subtype(s) mediating noradrenaline-induced contractions of the guinea-pig aorta

Affinity profile at alpha(1)- and alpha(2)-adrenoceptor subtypes and in vitro cardiovascular actions of (+)-boldine

The present study examines the functional and binding affinities of the aporphine alkaloid, (+)-boldine, at different alpha(1)- and alpha(2)-adrenoceptor subtypes, namely, alpha(1A) (rat vas deferens and kidney) and its L-like state (rabbit spleen), alpha(1B) (guinea pig spleen, mouse spleen and rabbit aorta), alpha(1D) (rat aorta and pulmonary artery), at possible subtypes of prejunctional alpha(2)-adrenoceptors in rat and rabbit vas deferens and rat atrium, alpha(2D) in guinea pig ileum, cloned human alpha(1)-adrenoceptor subtypes A, B and D and alpha(2)-adrenoceptor subtypes A, B and C as well as rat alpha(2D)-adrenoceptors. Additionally, we investigated its Ca(2+) channel antagonism in vascular and cardiac preparations. (+)-Boldine had higher affinity at alpha(1)-adrenoceptor subtype A (pA(2)=7.46, pK(i)=7.21) compared with its L-like state (pA(2)=5.63) or subtype B (pA(2)=5.98- 6.12, pK(i)=5.79) and subtype D (pA(2)=6.18-6.37, pK(i)=6.09). Its affinities at alpha(2)-adrenoceptors in rat and rabbit vas deferens and rat atrium (pA(2)=6.02, 6.36, 6.06, respectively) were identical, but lower at guinea pig ileum alpha(2D)-adrenoceptors (pA(2)=4.38). (+)-Boldine displayed nearly undistinguishable affinity at cloned human alpha(2)-adrenoceptor subtypes A, B and C (pK(i)=6.26, 5.79 and 6.35, respectively), whereas its affinity at rat alpha(2D)-adrenoceptors was low (pK(i)=4.70). In perfused rat kidney, (+)-boldine inhibited K(+)-evoked vasoconstriction at doses 70-fold higher than diltiazem. In guinea pig Langendorff heart, (+)-boldine (10(-5) - 2 x 10(-4) M) was equieffective in increasing coronary flow and in depressing cardiac force, while lower concentrations already depressed heart rate. In papillary muscles from guinea pig, (+)-boldine (10(-6) - 10(-5) M) mainly prolonged the duration of action potential at levels >30% of repolarization. These data reveal that (+)-boldine, except for its moderate selectivity (15 to 25-fold) for alpha(1A)-adrenoceptors, does not discriminate between the alpha(1)-adrenoceptor subtypes B and D and alpha(2)-adrenoceptor subtypes A, B and C, at which the drug consistently displays micromolar affinity. In vascular and cardiac preparations, (+)-boldine, although being at least 50-fold weaker than diltiazem, shows Ca(2+) channel antagonistic properties but no specificity for coronary dilatation relative to cardiodepression.

Buspirone functionally discriminates tissues endowed with alpha1-adrenoceptor subtypes A, B, D and L

The affinity for functional alpha1-adrenoceptor subtypes of buspirone in comparison with its close structural analogs and selective alpha1D-adrenoceptor antagonists, BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]dec ane-7,9-dione) and MDL 73005EF (8-[2-(1,4-benzodioxan-2-ylmethylamino)ethyl]-8-azaspiro+ ++[4.5]decane-7,9-dione), was determined, namely at subtype A in rat vas deferens and perfused kidney, at subtype B in guinea-pig and mouse spleen, at subtype L in rabbit spleen, and at subtype D in rat aorta and pulmonary artery against noradrenaline-evoked contractions. BMY 7378 and MDL 73005EF were confirmed as 30- and 20-fold selective antagonists, respectively, for alpha1D- over both alpha1A- and alpha1B-adrenoceptors. Buspirone was a weak antagonist without intrinsic activity at alpha1A-adrenoceptors in rat vas deferens (pA2 = 6.12), at alpha1B-adrenoceptors in guinea-pig and mouse spleen (pA2 = 5.54 and 5.59) and at alpha1L-adrenoceptors in rabbit spleen (pA2 = 4.99), but caused partial vasoconstriction in rat kidney that was attenuable by the subtype D-selective adrenoceptor antagonist BMY 7378, but hardly by the subtype A-selective adrenoceptor antagonist B8805-033 ((+/-)-1,3,5-trimethyl-6-[[3-[4-((2,3-dihydro-2-hydroxymethyl)-1,4-be nzodioxin-5-yl)-1-piperazinyl]propyl]amino]-2,4(1H,3H)-pyrimidinedion e), confirming the additional presence of alpha1D-adrenoceptors mediating rat renal vasoconstriction. Buspirone behaved as a partial agonist at alpha1D-adrenoceptors in rat aorta (pD2 = 6.77, intrinsic activity (i.a.)= 0.40) and pulmonary artery (pD2 = 7.16, i.a. = 0.59). With buspirone as agonist in these tissues, the pA2 values of subtype-discriminating antagonists were consistent with their alpha1D-adrenoceptor affinity determined in rat aorta against noradrenaline and with published binding data on cloned alpha1d-adrenoceptors. The results provide pharmacological evidence that (1) in functional preparations for the A subtype, like rat vas deferens and perfused kidney, for the B subtype, like guinea-pig and mouse spleen, and for the L subtype, like rabbit spleen, buspirone is a weak antagonist without intrinsic activity, but (2) behaves as a partial agonist in rat aorta and pulmonary artery as models for the D subtype and (3) detects an additional vasoconstrictor alpha1D-adrenoceptor in rat kidney. Buspirone, like its close analogs BMY 7378 and MDL 73005EF, thus might also be a useful tool for functionally discriminating alpha1D- from alpha1A-, alpha1B- and alpha1L-adrenoceptors in various tissues.

Evidence of the existence of alpha 1B-adrenoceptors on hepatocytes of domestic fowl (Gallus domesticus)

In liver cell membranes of laying hens alpha 1-adrenergic receptors were characterized using 3H-prazosin. Specific binding was saturable and indicative of a single class of high-affinity binding sites with a KD of 0.40 +/- 16 nM and a Bmax of 390 +/- 38 fmol/mg membrane protein. Preincubation of membranes with chlorethylclonidine significantly diminished the binding of 3H-prazosin (Bmax = 99 +/- 14 fmol/mg membrane protein). Furthermore, competition experiments revealed a rank order of prazosin >> phentolamine = oxymetazoline >> WB 4101, with Ki values of 0.71 +/- 0.62, 20 +/- 6, 32 +/- 13, and 119 +/- 48 nM, respectively. These results suggest the existence of alpha 1B-adrenergic receptors in hen liver cell membranes.

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.