Imidazoline-guanidinium receptive site in renal proximal tubule: asymmetric distribution, regulation by cations and interaction with an endogenous clonidine displacing substance.
In the present report we have used [3H]idazoxan to characterize the rabbit renal imidazoline preferring site by defining its plasmalemma distribution, its regulation by cations and the type of interaction with the clonidine displacing substance (CDS), a putative endogenous ligand for the imidazoline receptor. The density of [3H]idazoxan binding sites was 12-fold higher in purified basolateral membranes than in brush-border membranes (maximal binding activity, 566 +/- 118 vs. 46 +/- 2 fmol/mg of protein). In basolateral membranes, [3H]idazoxan binding was inhibited not only by imidazoline compounds but also by guanidinium analogs such as guanabenz, amiloride, 5-(M-ethyl-N-isopropyl)amiloride and phenamylamiloride. Amiloride had no effect on the dissociation rate of [3H]idazoxan, suggesting a direct interaction of this molecule with the ligand binding site. [3H]Idazoxan binding was 80% inhibited by 150 mM K+ or Rb+. The effect of K+ appeared to occur through the interaction with an allosteric site in as much as both the apparent dissociation constant and the dissociation rate of [3H]idazoxan were increased in the presence of 75 mM K+. CDS inhibited [3H]idazoxan binding with a half-maximal effective concentration of 2 U/250 microliters. The competitive nature of CDS effect was indicated by the increase in the apparent dissociation constant of [3H]idazoxan (Kd from 3 +/- 0.3 to 8.5 +/- 0.2 nM, P less than .01) in the presence of CDS. In conclusion, our findings showed that the imidazoline-guanidinium receptive site is located mainly in the basolateral side of the tubular cell, recognizes CDS and is regulated by K+.(ABSTRACT TRUNCATED AT 250 WORDS)
Coupry
I
I
Institut National de la Santé et de la Recherche Médicale U7/UA 318 Centre National de la Recherche Scientifique, Department of Pharmacology, Hôpital Necker, Paris, France.
Atlas
D
D
Podevin
R A
RA
Uzielli
I
I
Parini
A
A
D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites; D003000:Clonidine / Q000037:antagonists & inhibitors* / Q000378:metabolism / Q000031:analogs & derivatives; D004146:Dioxanes / Q000378:metabolism*; D004147:Dioxins / Q000378:metabolism*; D019329:Idazoxan; D054755:Imidazoline Receptors; D066298:In Vitro Techniques; D007687:Kidney Tubules, Proximal / Q000032:analysis*; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D011188:Potassium / Q000494:pharmacology; D011817:Rabbits; D011942:Receptors, Adrenergic, alpha / Q000032:analysis / Q000187:drug effects; D011955:Receptors, Drug / Q000032:analysis*; D015016:Yohimbine / Q000378:metabolism
D004146:Dioxanes; D004147:Dioxins; D054755:Imidazoline Receptors; D011942:Receptors, Adrenergic, alpha; D011955:Receptors, Drug; D015016:Yohimbine; D000584:Amiloride; C055140:clonidine-displacing substance; D008274:Magnesium; D003000:Clonidine; D011188:Potassium; D019329:Idazoxan