Characterization of alpha 2-adrenergic receptors on rat pinealocytes.

alpha 2-Adrenergic receptors in rat pineal membranes were characterized using p-[125I]iodoclonidine, a highly selective, high specific activity ligand. Binding was rapid (association constant rate = 0.0462 nM/min-1) and reversible after the addition of phentolamine (apparent dissociation rate constant = 0.04 min-1). Saturation experiments indicate the presence of a single class of noncooperative binding sites, with an equilibrium binding constant (Kd) of 1.1 +/- 0.3 nM and a binding capacity (Bmax) of 69 +/- 9 fmol/mg protein. Analysis of the relative potency of selected adrenoreceptor agonists and antagonists in competition studies with p-[125I]iodoclonidine indicates that the ligand is binding to a member of the family of alpha 2-adrenergic receptors that has a high affinity for oxymetazoline, phentolamine, and (-)norepinephrine and a low affinity for prazosin, similar to the recently described alpha 2-adrenergic receptor present in the bovine pineal gland, classified as belonging to the newly described alpha 2D-adrenergic receptor subtype. Rat pineal alpha 2-adrenergic receptors were unaltered after nerve endings degenerated. This observation and the recent finding that alpha 2-adrenergic agonists potentiate N6,2'-O-dibutyryl-cAMP or isobutylmethylxanthine stimulation of arylalkylamine N-acetyltransferase in the rat pineal gland establish that alpha 2D-like adrenergic receptors are located on pinealocytes.