Heterogeneity between soluble human and rabbit splenic alpha 2-adrenoceptors.

The pharmacological and biochemical characteristics of soluble alpha 2-adrenoceptors were investigated to determine whether differences observed in membranes were maintained in solution and to probe the nature of any such differences. alpha 2-Adrenoceptors were solubilized from purified plasma membrane preparations of human and rabbit spleen using digitonin. [3H]yohimbine bound to one population of alpha 2-adrenoceptors in the preparations with dissociation constants of 2.4 nM and 7.8 nM respectively. The pharmacological profile of the alpha 2-adrenoceptors has been examined. Upon solubilization the affinity of the alpha 2-adrenoceptors for yohimbine was unchanged. In contrast, the potency of idazoxan and RX 811066 were increased, whereas the potency for prazosin (human only), phentolamine and WY 26392 was decreased 2-3-fold. The potency of the agonists oxymetazoline, UK 14304 and adrenaline were all reduced upon solubilization of alpha 2-adrenoceptors. The selectivity of yohimbine, idazoxan, RX 811066 and WY 26392 for human rather than rabbit alpha 2-adrenoceptors was maintained in solution. Possible sources of heterogeneity between human and rabbit alpha 2-adrenoceptors were investigated. The protein structure was probed by comparing the susceptibility of the receptors to inactivation by sulphydryl modifying agents. No differences were observed in the potency of N-ethylmaleimide or p-chloromercuribenzoate to inactivate the receptor. The carbohydrate component of the receptors was investigated using agarose-linked lectins. Rabbit splenic alpha 2-adrenoceptors had a lower affinity for the lectins wheatgerm agglutinin (Triticum vulgaris) and soybean (Glycine max) which bind the sugars N-acetyl d-glucosamine and N-acetyl d-galactosamine respectively. These findings suggest that heterogeneity of the alpha 2-adrenoceptor derives from its structural characteristics rather than its environment in the membrane.