Investigation of the subtypes of alpha2-adrenoceptor mediating prejunctional inhibition in rat atrium and cerebral cortex.

We have investigated the subtype of alpha2-adrenoceptor mediating prejunctional inhibition of neurotransmission in rat atrium in comparison with the alpha2-adrenoceptor mediating prejunctional inhibition in rat cerebral cortex. In rat atrium and cerebral cortex, prejunctional alpha2-adrenoceptors were investigated in terms of the ability of alpha2-adrenoceptor antagonists to increase the stimulation-evoked overflow of tritium in tissues pre-incubated with [3H]-noradrenaline. The relatively non-selective alpha2-adrenoceptor antagonist yohimbine and the alpha2D-adrenoceptor selective antagonist BRL 44408 had potencies in rat atrium which were similar to their potencies in rat cerebral cortex. The antagonists ARC 239, HV 723, WB 4101, prazosin, chlorpromazine and abanoquil, which have low affinity for alpha2D-adrenoceptors, significantly increased stimulation-evoked overflow at lower concentrations in rat atrium than rat cerebral cortex. Antagonist potency at prejunctional alpha2-adrenoceptors was correlated with antagonist affinity at alpha2-adrenoceptor ligand binding sites in membranes of rat kidney (alpha2B) and submandibular gland (alpha2D), and human recombinant alpha2C-adrenoceptors labelled with [3H]yohimbine. The correlation between ligand binding sites and the functional receptor in the rat cerebral cortex was significant only for the alpha2D-adrenoceptor ligand binding site (r=0.87, n=8, P<0.01) as compared to the alpha2B-adrenoceptor (r=0.32, n.s.) or alpha2C-adrenoceptor (r=0.12, n.s.) ligand binding sites. The correlation between ligand binding sites and the functional receptor in the rat atrium was not significant for any ligand binding site, with r=0.64, 0.68 and 0.67 for the alpha2D-, the alpha2B- and the alpha2C-adrenoceptor ligand binding sites, respectively. It is concluded that the functional prejunctional alpha2-adrenoceptor of rat cerebral cortex closely resembles the alpha2D-adrenoceptor ligand binding site of rat submandibular gland, but the rat atrium may contain two subypes of prejunctional alpha2-adrenoceptor, alpha2D and another subtype, possibly alpha2B or alpha2C.