Abstract

The binding properties of a new radioligand, R(-)-5-[2-[[2[ethoxyring(n)-3H](o- ethoxyphenoxy)ethyl]amino]propyl]-2-methoxybenzenesulfonamide++ + HCl ([3H]YM617), were studied in membranes of the rat hippocampus and spleen. [3H]YM617 rapidly associated with its binding sites in both membranes and reached steady state by 20 min at 25 degrees C. The specific binding of [3H]YM617 appeared to be saturable, and Scatchard analysis revealed a linear plot, suggesting a single population of binding sites with a dissociation constant of 0.170 +/- 0.016 nM (n = 6) in the hippocampus and 0.195 +/- 0.036 (n = 4) in the spleen. The maximal binding sites in the hippocampus and spleen were 203.0 +/- 43.2 (n = 6) and 72.4 +/- 17.0 (n = 4) fmol/mg protein, respectively. Chlorethylclonidine (10(-5) M for 10 min) treatment reduced the Bmax values of [3H]YM617 and [3H]prazosin to a similar degree in the rat hippocampus (10-15%) and spleen (40-50%). Alpha adrenoceptor agonists and antagonists competed with [3H]YM617 for binding sites in the following order: YM617 > prazosin > WB4101 > bunazosin > 5-methylurapidil > S(+)-isomer of YM617 > phentolamine > yohimbine > norepinephrine = phenylephrine > methoxamine in the hippocampus, and prazosin > YM617 > bunazosin > WB4101 > 5-methylurapidil > phentolamine > S(+)-isomer of YM617 > yohimbine > norepinephrine > phenylephrine > methoxamine in the spleen. In the hippocampus, prazosin and bunazosin produced biphasic displacement of [3H]YM617, but not [3H]prazosin binding. In contrast, only monophasic curves were obtained against either radioligand in the spleen.(ABSTRACT TRUNCATED AT 250 WORDS)