Srikant C B, Patel Y C
Endocrinology. 1985 May;116(5):1717-23. doi: 10.1210/endo-116-5-1717.
Specific receptors for tetradecapeptide somatostatin (S-14) in rat adrenal cortical membranes were quantitated by direct binding studies using [125I-Tyr11]S-14. Competitive inhibition of this radioligand by S-14 showed that these receptors constitute a single class of high affinity binding sites [dissociation constant (Kd) = 1.08 nM and maximum binding capacity (Bmax) = 0.35 pmol/mg membrane protein]. Structural analogs of S-14 with halogenated Trp8 moiety exhibited 4- to 46-fold greater binding affinity than S-14, [D-F5-Trp8]S-14 being the most potent. [Tyr11]S-14 and [des-Ala1]S-14 bound to these receptors with reduced affinity whereas [Phe4]S-14 exhibited 1.5-fold greater affinity than S-14. Somatostatin-28 (S-28) and S-14 were equipotent, whereas the N-terminal fragments of S-28 [S-28(1-14) and S-28(1-12)] were inactive. High affinity binding sites were also quantitated using a radioligand prepared from the tyrosinated S-28 analog, [Leu8, D-Trp22, Tyr25]S-28 (Kd = 1.2 nM; Bmax = 0.21 pmol/mg membrane protein). Both S-14 and S-28 exhibited comparable relative potencies for inhibiting the specific binding of this radioligand and [125I-Tyr11]S-14. Extracts of whole adrenal or the adrenal medulla and cortex contained very low levels of S-14-like immunoreactivity (2.4 pg/mg protein). These studies confirm the presence of specific receptors for S-14 in the adrenal cortex and suggest that 1) with respect to S-14 biological activity, Trp8-modified S-14 analogs should be more potent than S-14, S-28 equipotent with S-14, and N-terminal fragments of S-28 inactive in this tissue. 2) Direct binding studies using radioiodinated [Tyr11]S-14 and [Leu8,D-Trp22, Tyr25]S-28 appear to quantitate the same receptor sites in adrenocortical tissue. 3) The ligand specificity of the adrenocortical S-14 receptor differs from that previously reported for the pituitary and brain providing further evidence for the heterogeneity of the S-14 receptor. 4) In view of the very low concentrations of endogenous S-14-like immunoreactivity, the adrenal actions of S-14 and S-28 are probably mediated through an endocrine mechanism.
采用[125I-Tyr11]S-14通过直接结合研究对大鼠肾上腺皮质膜中十四肽生长抑素（S-14）的特异性受体进行定量。S-14对这种放射性配体的竞争性抑制表明，这些受体构成了一类单一的高亲和力结合位点[解离常数（Kd）=1.08 nM，最大结合容量（Bmax）=0.35 pmol/mg膜蛋白]。带有卤代Trp8部分的S-14结构类似物表现出比S-14高4至46倍的结合亲和力，[D-F5-Trp8]S-14最为有效。[Tyr11]S-14和[des-Ala1]S-14与这些受体的结合亲和力降低，而[Phe4]S-14的亲和力比S-14高1.5倍。生长抑素-28（S-28）和S-14效力相当，而S-28的N端片段[S-28(1-14)和S-28(1-12)]无活性。还使用由酪氨酸化的S-28类似物[Leu8, D-Trp22, Tyr25]S-28制备的放射性配体对高亲和力结合位点进行了定量（Kd = 1.2 nM；Bmax = 0.21 pmol/mg膜蛋白）。S-14和S-28在抑制这种放射性配体和[125I-Tyr11]S-14的特异性结合方面表现出相当的相对效力。整个肾上腺或肾上腺髓质和皮质的提取物中生长抑素-14样免疫反应性水平非常低（2.4 pg/mg蛋白）。这些研究证实了肾上腺皮质中存在S-14的特异性受体，并表明：1）就S-14的生物活性而言，Trp8修饰的S-14类似物应比S-14更有效，S-28与S-14效力相当，S-28的N端片段在该组织中无活性。2）使用放射性碘化的[Tyr11]S-14和[Leu8,D-Trp22, Tyr25]S-28进行的直接结合研究似乎对肾上腺皮质组织中的相同受体位点进行了定量。3）肾上腺皮质S-14受体的配体特异性与先前报道的垂体和脑的不同，这为S-14受体的异质性提供了进一步证据。4）鉴于内源性生长抑素-14样免疫反应性浓度非常低，S-14和S-28的肾上腺作用可能是通过内分泌机制介导的。
