Differences between the α2-adrenoceptor in rat submaxillary gland and the α2A- and α2B-adrenoceptor subtypes

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Abstract
1 The α2-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 α2A-adrenoceptors and rat kidney α2B-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 α2-adrenoceptors in rat submaxillary gland appeared to differ from both the α2A-adrenoceptor of rabbit spleen (r = 0.62) and also the α2B-adrenoceptor of rat kidney (r = 0.28). 4 The affinity data obtained with benoxathian, imiloxan and WB 4101 indicated the presence of an α2B-adrenoceptor in rat submaxillary gland. However, data for chlorpromazine, oxymetazoline, spiroxatrine and xylometazoline indicated that submaxillary gland α2-adrenoceptors were of the α2A subtype. The affinity estimate for prazosin in rat submaxillary gland was intermediate between its affinity at the α2A- and α2B-adrenoceptors while affinity estimates for idazoxan and phentolamine in rat submaxillary gland were greater than those obtained at either the α2A- or α2B-adrenoceptor. 5 These data indicate that rat submaxillary gland α2-adrenoceptors differ from the α2A- and α2B-adrenoceptors found in rabbit spleen and rat kidney, respectively.