Characterization and Visualization of Clonidine-sensitive Imidazole Sites in Rat Kidney Which Recognize Clonidine-displacing Substance

Abstract

In the ventrolateral medulla oblongata, clonidine binds not only to alpha 2-adrenergic receptors but also to a novel class of nonadrenergic sites that are specific for imidazoles. Since clonidine has direct actions on the kidney, we sought to determine whether imidazole binding sites could be detected in renal cell membranes. Adrenergic agents having an imidazole ring, like clonidine, completely displaced the specific binding of the high-affinity clonidine analog 3H-p-aminoclonidine (3H-PAC) to kidney membranes. Nonimidazole adrenergic agents inhibited 3H-PAC binding by only 75%, indicating that the remaining 25% of the sites were nonadrenergic. Cimetidine, an imidazole compound lacking adrenergic potency, showed a high affinity for approximately 25% of the sites. 3H-PAC binding to imidazole sites was high-affinity (KD = 11 +/- 3 nmol/L) and saturable (Bmax = 41 +/- 10 fmol/mg protein). Like clonidine, an endogenous clonidine-displacing substance (CDS) completely inhibited 3H-PAC binding to renal cortex membranes. Quantitative receptor autoradiography revealed that imidazole receptors exhibited a specific regional distribution within the kidney that was unique, and distinct from that of alpha 2-adrenergic receptors. We conclude that clonidine binds to specific imidazole sites in the renal cortex of the rat and that CDS may be the endogenous ligand at these sites. Thus, in addition to their role in central nervous system control of arterial pressure, imidazole receptors may be involved in the regulation of renal function.