Similarity of central and peripheral alpha-1 adrenoceptors in rat and rabbit

In order to examine species and tissue differences in alpha 1 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 (pKD) ranging from 9.43 to 10.20. The rank orders of affinities of three competing antagonists (prazosin much greater than idazoxan greater than rauwolscine) and five agonists (cirazoline greater than clonidine approximately equal to (-)-norepinephrine greater than (-)-phenylephrine greater than (+)-norepinephrine) were typical of alpha 1 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 (greater than .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 alpha 1 receptors of the rat and rabbit, with small differences observed between central and peripheral sites in both species.