Imidazoline-guanidinium and alpha 2-adrenergic binding sites in basolateral membranes from human kidney

In the present study, we used [3H]idazoxan and [3H]rauwolscine to characterize the imidazoline-guanidinium receptive site (IGRS) and alpha 2-adrenoceptors in the human renal proximal tubule, respectively. In purified basolateral membranes, 11-fold enriched in Na(+)-K+ ATPase. [3H]idazoxan and [3H]rauwolscine binding was twofold higher than in homogenates ([3H]idazoxan: 87 +/- 19 vs. 45 +/- 23.3 fmol/mg protein, P less than 0.05; [3H]rauwolscine: 56.4 +/- 21.4 vs. 25.2 +/- 7.3 fmol/mg protein, P less than 0.01). In competition studies performed at saturating concentration of [3H]idazoxan (15 NM), specific binding was competed for by epinephrine and rauwolscine only by 10-15% but was completely inhibited by imidazoline and guanidinium compounds. Thus, in human renal proximal tubule. [3H]idazoxan mainly binds to an IGRS. The highest density of alpha 2-adrenoceptors in basolateral membranes and of IGRS in partially purified membrane preparations, suggests that these two binding sites have a different subcellular localization. When compared to the rabbit renal IGRS, the human [3H]idazoxan binding site displays different affinities for guanabenz, rilmenidine, clonidine, amiloride and its derivatives that persist after membrane solubilization. In contrast, the human and rabbit renal IGRS share similar regulatory properties such as the sensitivity to K+ and the insensitivity to Na+, divalent cations and 5'-guanylylimidodiphosphate (Gpp(NH)p). In conclusion, we demonstrated that, in the human renal proximal tubule, alpha 2-adrenoceptors are mainly located in basolateral membranes while IGRS appear to be associated with another cell compartment. As indicated by their common interaction with imidazoline and guanidinium derivatives and by similar regulatory properties, human and rabbit IGRS belong to the same family of membrane proteins.(ABSTRACT TRUNCATED AT 250 WORDS)