Schaad N C, Klein D C
Section on Neuroendocrinology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892.
Endocrinology. 1992 May;130(5):2804-10. doi: 10.1210/endo.130.5.1315259.
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.
使用高选择性、高比活性配体对-[¹²⁵I]碘可乐定对大鼠松果体膜中的α₂-肾上腺素能受体进行了表征。结合迅速（缔合常数速率 = 0.0462 nM/min⁻¹），加入酚妥拉明后可逆（表观解离速率常数 = 0.04 min⁻¹）。饱和实验表明存在一类单一的非协同结合位点，平衡结合常数（Kd）为1.1±0.3 nM，结合容量（Bmax）为69±9 fmol/mg蛋白质。在与对-[¹²⁵I]碘可乐定的竞争研究中对选定的肾上腺素能激动剂和拮抗剂的相对效力进行分析表明，该配体与α₂-肾上腺素能受体家族的一个成员结合，该成员对羟甲唑啉、酚妥拉明和（-）去甲肾上腺素具有高亲和力，对哌唑嗪具有低亲和力，类似于最近在牛松果体中描述的α₂-肾上腺素能受体，归类为新描述的α₂D-肾上腺素能受体亚型。神经末梢退化后，大鼠松果体α₂-肾上腺素能受体未发生改变。这一观察结果以及最近发现α₂-肾上腺素能激动剂可增强大鼠松果体中N⁶,2'-O-二丁酰-cAMP或异丁基甲基黄嘌呤对芳基烷基胺N-乙酰转移酶的刺激作用，证实α₂D样肾上腺素能受体位于松果体细胞上。
