Characterization of [3H]Clonidine Binding to Two Sites in Calf Brain Membranes

Abstract

[3H]clonidine evidently binds to 2 distinct receptor sites in calf cortex membranes. At 23.degree. C, binding was obtained at a low-affinity site (Kd = 5.4 nM) and a high-affinity site (Kd = 1.1 nM). At 0.degree. C, selective binding occurred to the low-affinity site only. At this temperature, it was possible to evaluate the interaction of drugs with the low-affinity receptor directly. Competition with the high-affinity receptor could be ascertained by generating displacement curves representing the differential between specific binding values obtained at 23 and 0.degree. C. Guanine nucleotides selectively decreased binding to the high-affinity site without apparent influence on the low-affinity [3H]clonidine binding. The activities of various pharmacological agents at the low- and high-affinity clonidine receptors are discussed and compared with WB-4101 binding data.