1 We have examined the potencies of a series of a2-adrenoceptor antagonists in functional studies of prejunctional x2-adrenoceptors in rat atrium and vas deferens, and compared potencies with affinities for the a2A-ligand binding site of human platelet and the a2B-site of rat kidney. 49, 12 and 7 times higher potency in rat atrium, respectively. ARC 239 was also 17 times more potent in rat atrium than rat vas deferens when EC30 values were compared. 5 The correlation of affinity for the a2A-site of human platelet was better with prejunctional potency. in rat vas deferens than rat atrium. 6 The correlation of affinity for the CX2B-site of rat kidney was better with prejunctional potency in rat atrium than rat vas deferens. 7 It is concluded that prejunctional a2-adrenoceptors of rat vas deferens and rat atrium differ, and these receptors may resemble the Oe2A-and a2B-ligand binding sites, respectively.

No abstract

In the present investigation, the rationale for the design, synthesis, and biological evaluation of potent inhibitors of neuronal Na+ channels is described. N,N'-diaryl- and N-aryl-N-aralkylguanidine templates were locked in conformations mimicking the permissible conformations of the flexible diarylguanidinium ion (AS+, AA+, SS+). The resulting set of constrained guanidines termed "lockamers" (cyclophane, quinazoline, aminopyrimidazolines, aminoimidazolines, azocino- and tetrahydroquinolinocarboximidamides) was examined for neuronal Na+ channel blockade properties. Inhibition of [14C]guanidinium ion influx in CHO cells expressing type IIA Na+ channels showed that the aminopyrimidazoline 9b and aminoimidazoline 9d, compounds proposed to lock the N,N'-diarylguanidinium in an SS+ conformation, were the most potent Na+ channel blockers with IC50's of 0.06 microM, a value 17 times lower than that of the parent flexible compound 18d. The rest of the restricted analogues with 4-p-alkyl substituents retained potency with IC50 values ranging between 0.46 and 2.9 microM. Evaluation in a synaptosomal 45Ca2+ influx assay showed that 9b did not exhibit high selectivity for neuronal Na+ vs Ca2+ channels. The retention of significant neuronal Na+ blockade in all types of semirigid conformers gives evidence for a multiple mode of binding in this class of compounds and can possibly be attributed to a poor structural specificity of the site(s) of action. Compound 9b was also found to be the most active compound in vivo based on the high level of inhibition of seizures exhibited in the DBA/2 mouse model. The pKa value of 9b indicates that 9b binds to the channel in its protonated form, and log D vs pH measurements suggest that ion-pair partitioning contributes to membrane transport. This compound stands out as an interesting lead for further development of neurotherapeutic agents.

1 We have compared prejunctional x2-adrenoceptors in rat and guinea-pig vas deferens and rat and guinea-pig atria with postjunctional a2-adrenoceptors in human saphenous vein and human platelets employing the antagonists yohimbine and SK&F 104078 and other a2-adrenoceptor antagonists. 2 Yohimbine was approximately 10 times more potent prejuunctionally than SK&F 104078 at antagonizing the inhibition by the x2-adrenoceptor agonist xylazine of stimulation-evoked contractions in rat and guinea-pig vas deferens, and at increasing stimulation-evoked release of tritium in rat and guinea-pig atria pre-incubated with [3H]-noradrenaline. 3 Yohimbine was approximately 10 times more potent postjunctionally than SK&F 104078 at antagonizing contractions to noradrenaline in human saphenous vein and at displacing [3H]-yohimbine binding in human platelet membranes. 4 For the antagonists yohimbine, SK&F 104078, prazosin, phentolamine, CH 38083 and urapidil, there was a significant correlation between prejunctional potency in rat vas deferens atrium and postjunctional potency in human platelet, although the correlation was improved by the omission of prazosin. 5 We have no evidence for differences between functional pre-and postjunctional ac2-adrenoceptors in the periphery, although these functional receptors may differ from the ligand binding site in the human platelet.

We have investigated the proposed postjunctional selectivity of the alpha 2-adrenoceptor antagonist SK & F 104078 employing the pithed rat, rat isolated atrium and human saphenous vein preparations. In the pithed rat, SK & F 104078 (5 mg kg-1) produced a 20-fold shift in the prejunctional ID50 (concentration of agonist producing 50% inhibition of the cardioacceleration to a single stimulus) and a 3-fold shift in the postjunctional ED50 (concentration producing 50% of maximum rise in diastolic blood pressure) of the alpha 2-adrenoceptor agonist xylazine. However, the alpha 2-adrenoceptor antagonist yohimbine also showed apparent selectivity for prejunctional receptors in the pitched rat. In the rat isolated atrium, yohimbine was approximately 10 times more potent than SK & F 104078 at enhancing the stimulation-evoked release of tritium in tissues preincubated with (3H)-noradrenaline. In the human saphenous vein, yohimbine and SK & F 104078 had pA2 values of 7.40 and 6.33, respectively, against contractions to noradrenaline, so that yohimbine was again approximately ten times more potent than SK & F 104078. In conclusion, SK & F 104078 behaved like yohimbine in its relative potencies at pre- and postjunctional alpha 2-adrenoceptors both in vivo and in vitro, so that we fail to find any selectivity of SK & F 104078 for postjunctional alpha 2-adrenoceptors.