Differences between the α₂‐adrenoceptor in rat submaxillary gland and the α_2A‐ and α_2B‐adrenoceptor subtypes

Abstract

1. The alpha 2-adrenoceptors of rat submaxillary gland, labelled with [3H]-rauwolscine, were characterized by use of a range of subtype selective ligands and were compared to rabbit spleen alpha 2A-adrenoceptors and rat kidney alpha 2B-adrenoceptors. 2. In rat submaxillary gland, [3H]-rauwolscine labelled an apparently homogeneous population of binding sites with relatively low affinity (Kd = 11.65 nM) compared to the affinity in rat kidney (Kd = 2.18 nM) and rabbit spleen (Kd = 4.64 nM). 3. In competition studies using 16 ligands the alpha 2-adrenoceptors in rat submaxillary gland appeared to differ from both the alpha 2A-adrenoceptor of rabbit spleen (r = 0.62) and also the alpha 2B-adrenoceptor of rat kidney (r = 0.28). 4. The affinity data obtained with benoxathian, imiloxan and WB 4101 indicated the presence of an alpha 2B-adrenoceptor in rat submaxillary gland. However, data for chlorpromaxine, oxymetazoline, spiroxatrine and xylometazoline indicated that submaxillary gland alpha 2-adrenoceptors were of the alpha 2A subtype. The affinity estimate for prazosin in rat submaxillary gland was intermediate between its affinity at the alpha 2A- and alpha 2B-adrenoceptors while affinity estimates for idazoxan and phentolamine in rat submaxillary gland were greater than those obtained at either the alpha 2A- or alpha 2B-adrenoceptor. 5. These data indicate that rat submaxillary gland alpha 2-adrenoceptors differ from the alpha 2A- and alpha 2B-adrenoceptors found in rabbit spleen and rat kidney, respectively.