[Interaction of rilmenidine with renal imidazoline-guanidine sites]

Several studies have suggested that clonidine, guanfacine and rilmenidine decrease systemic blood pressure by stimulating central alpha 2-adrenergic receptors. However, we have shown that these molecules interact not only with alpha 2-adrenergic but also a new type of "non catecholamine" receptor in rabbit and human renal proximal tubules. This receptor, which we have called the imidazoline-guanidium receptor site (IGRS) seems to be pharmacologically, biochemically and fractionally distinct from alpha 2-adrenergic receptors. In order to determine the relative affinity of rilmenidine for these two types of receptor, we studied its capacity to inhibit the liaison of (H3)-idazoxan, a ligand with a high affinity for the IGRS, and of (H3)-rauwolscine, a ligand selective for alpha 2-adrenergic receptors in the rabbit kidney. The results based on the apparent constants of inhibition (Ki) of the two radioligands [231 +/- 34 nM for (H3)-idazoxan and 2440 +/- 322 nM for (H3)-rauwolscine] showed that the selectivity of rilmenidine was 10 times greater for IGRS than for alpha 2-adrenergic receptors. This preferential activity on IGRS was confirmed by studies of the influx of Na22 into isolated renal proximal tubule cells of the rabbit. They showed that rilmenidine, in contrast to catecholamines, inhibited the transport of Na22 into the renal cells. In conclusion, the data from our studies shows that rilmenidine interacts with renal IGRS and inhibits cellular transport of sodium by a mechanism other than the stimulation of alpha 2-adrenergic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)