Two renal alpha 2-adrenergic receptor sites revealed by p-aminoclonidine binding

[3H]p-aminoclonidine [3H]PAC, a specific alpha 2-adrenergic agonist, was used to characterize alpha 2-adrenoceptor binding in rat renal membranes. Rosenthal plots demonstrated two binding sites with Kds of approximately 1.7 and 14.2 nM and Bmaxs (maximum binding) of 47.3 and 218.8 fmol/mg protein for the high- and low-affinity sites, respectively. These characteristics were confirmed by estimate of Kd parameters based on association and dissociation experiments. Pseudo-Hill coefficients generated from drug inhibition experiments were all less than unity, suggesting differential binding at two alpha 2-adrenoceptor binding sites. Specific alpha 2-adrenergic agonists exhibited greater binding affinity to both sites than did nonspecific drugs, and all drugs displayed greater affinity for the high- than the low-affinity binding site. Both guanyl nucleotides and sodium chloride inhibited [3H]PAC binding more at the high-affinity than at the low-affinity site. Renal denervation resulted in significant upregulation of receptor density only at the high-affinity sites with no change in receptor affinity at either site, suggesting that a majority of the alpha 2-adrenoceptors in the kidney are postsynaptic. Thus all lines of evidence in this study indicate that two alpha 2-adrenoceptor binding sites exist in the rat kidney.