Investigations of prejunctional alpha 2-adrenoceptors in rat atrium, vas deferens and submandibular gland.

We have examined the effects of a series of alpha 2-adrenoceptor antagonists on the stimulation-evoked release of tritium from rat atrium, vas deferens and submandibular gland pre-incubated with [3H]noradrenaline, and correlated these potencies with affinities for the alpha 2A-ligand binding site of human platelet and the alpha 2B-ligand binding site of rat kidney. The alpha 2B-selective adrenoceptor antagonists prazosin and ARC 239 showed significantly higher, and the alpha 2A-selective antagonist BRL 44408 showed significantly lower, potency in atrium than in vas deferens and submandibular gland. Yohimbine and BDF 8933 failed to distinguish between prejunctional alpha 2-adrenoceptors or between ligand binding sites. It is concluded that the prejunctional alpha 2-adrenoceptor of rat atrium resembles the alpha 2B-ligand binding site, and differs from the prejunctional alpha 2-adrenoceptors of rat vas deferens and submandibular gland, which resemble the alpha 2A-ligand binding site.