Atypical [ 3 H]Clonidine Binding Sites in Human Caudate and Platelets on Cryostat‐cut Sections

A bstract : Pharmacological characterization is described for a human imidazoline binding site (I‐site) labeled by [ 3 H]clonidine using standard autoradiographic method. Under conditions that mask a 2 ‐adrenergic sites, only a single high affinity site was observed in human caudate and blood platelet sections. Affinity constants (K i ) were highly correlated between the two tissues ( r 5 0.90, P 5 0.0003). This site is dissimilar to classical I 1 and I 2 sites, even though both tissues possess abundant I 1 and I 2 sites by filtration binding methods. It is suggested that the isotonic buffer conditions inherent to the procedure alter drug affinities to the classical I 1 site.