AJP - Renal Physiology, Vol 252, Issue 2 283-F290, Copyright © 1987 by American Physiological Society
Two renal alpha 2-adrenergic receptor sites revealed by p-aminoclonidine binding
B. Sripanidkulchai, R. Dawson, S. Oparil and J. M. Wyss
[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.
