ARE THE PREJUNCTIONAL ALPHA-2-ADRENOCEPTORS OF THE RAT VAS-DEFERENS AND SUBMANDIBULAR-GLAND OF THE ALPHA-2A-OR ALPHA-2D-SUBTYPE

We have investigated the effects of antagonists at functional prejunctional alpha-2-adrenoceptors of rat vas deferens and rat submandibular gland and compared potencies with affinities for the alpha-2A-, alpha-2B- and putative alpha-2D-ligand binding sites of human platelet, rat kidney, and rat submandibular gland, respectively. Prejunctional potency of antagonists was expressed as an EC30 (concentration producing a 30% increase in stimulation-evoked release of tritium in tissues pre-incubated with [H-3]noradrenaline) in rat vas deferens and submandibular gland, and also as a pA2 (antagonist concentration producing a 2-fold shift in the inhibition by the alpha-2-adrenoceptor agonist xylazine of the electrically-evoked isometric twitch) in rat vas deferens. The functional prejunctional alpha-2-adrenoceptors of rat vas deferens and submandibular gland are alpha-2A-like since there was a good correlation between affinity for the alpha-2A-ligand binding site in human platelet and prejunctional potency in rat vas deferens (r = 0.90, n = 7, P < 0.001), and submandibular gland (r = 0.84, n = 7, P < 0.05). However, there was a better correlation between affinity for the alpha-2-ligand binding site of rat submandibular gland and prejunctional potency in rat vas deferens (r = 0.95, n = 7, P < 0.001), and submandibular gland (r = 0.97, n = 7, P < 0.001). It is concluded that the prejunctional alpha-2-adrenoceptors of rat vas deferens and rat submandibular gland closely resemble the alpha-2-site of rat submandibular gland, which may be a species homologue of the human alpha-2A- or the putative alpha-2D-site.