Selectivity of rilmenidine for I1-imidazoline-binding sites in rabbit proximal tubule cells

Imidazoline and imidazoline-like compounds may elicit their pharmacological activities through the interaction with three membrane proteins: alpha 2-adrenergic receptors (alpha 2-AR), I1-binding sites (I1BS), and I2-binding sites (I2BS). We have recently shown that these three proteins are co-expressed in the renal proximal tubule cells, where they could mediate the renal effects of imidazoline and structurally related antihypertensive drugs such as clonidine and rilmenidine. To identify the receptor involved in regulation of the tubular effects of clonidine and rilmenidine, we performed binding studies on isolated cells from rabbit kidney proximal tubule using [3H]idazoxan, an I1/I2 ligand, and [3H]rauwolscine, a selective alpha 2-adrenergic antagonist. Competition studies of [3H]idazoxan binding showed that rilmenidine and clonidine interact with both I1BS and I2BS. The comparison of inhibition constants (rilmenidine: Ki for I1BS. 7.1 +/- 3.5 nM; Ki for I2BS, 5,189 +/- 1,816 nM; clonidine: Ki for I1BS, 58.2 +/- 17.3 nM; Ki for I2BS, 4,179 +/- 2,633 nM) demonstrated that rilmenidine and clonidine are 731-and 72-fold, respectively, more selective for I1BS than for I2BS. In addition, in competition experiments with [3H]-rauwolscine, rilmenidine poorly interacted with alpha 2-AR (Ki 2,440 +/- 322 nM), whereas clonidine displayed an affinity (Ki, 32 +/- 12 nM) close to that observed for I1BS. Taken together, these data show that although clonidine is not able to discriminate alpha 2-AR from I1BS, rilmenidine selectively binds only to I1BS. This suggests that the renal effects of rilmenidine are related to its selective interaction with this class of binding site.