Similarity of Central and Peripheral Alpha-1 Adrenoceptors in Rat and Rabbit

Abstract

In order to examine species and tissue differences in α₁ adrenoceptors, binding experiments were performed using ³H-prazosin and membrane homogenates of central nervous and peripheral tissues of rabbit (cortex and spleen), and rat (cortex, spleen, and liver). Saturation studies indicated one binding site for ³H-prazosin, with apparent log molar dissociation constants (pK_D) ranging from 9.43 to 10.20. The rank orders of affinities of three competing antagonists (prazosin ≫ idazoxan > rauwolscine) and five agonists (cirazoline > clonidine ∼ (-)-norepinephrine > (-)-phenylephrine > (+)-norepinephrine) were typical of α₁ receptors in all tissues. There were small but significant differences in the mean affinities of rauwolscine, idazoxan and cirazoline among the five tissues. No significant differences in pseudo-Hill coefficients were observed among tissues, although agonist binding curves were shallow (.7 to.85) and prazosin competition curves were significantly steeper (>.85). Guanine nucleotide did not affect the position or slope of the (-)-norepinephrine competition profile in rat cortex. These results demonstrate a qualitative similarity among central and peripheral α₁ receptors of the rat and rabbit, with small differences observed between central and peripheral sites in both species.