Abstract
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)
JPET articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|