Srikant C B, Patel Y C
J Biol Chem. 1986 Jun 15;261(17):7690-6.
The binding of somatostatin-14 (S-14) to rat pancreatic acinar cell membranes was characterized using [125I-Tyr11]S-14 as the radioligand. Maximum binding was observed at pH 7.4 and was Ca2+-dependent. Such Ca2+ dependence of S-14 receptor binding was not observed in other tissues. Scatchard analysis of the competitive inhibition by S-14 of [125I-Tyr11]S-14 binding revealed a single class of high affinity sites (Kd = 0.5 +/- 0.07 nM) with a binding capacity (Bmax) of 266 +/- 22 fmol/mg of protein. [D-Trp8]S-14 and structural analogs with halogenated Trp moiety exhibited 2-32-fold greater binding affinity than S-14, [D-F5-Trp8]S-14 being the most potent. [Tyr11]S-14 was equipotent with S-14. The affinity of somatostatin-28 for binding to these receptors was 50% of that of S-14. Cholecystokinin octapeptide (CCK-8) inhibited the binding of [125I-Tyr11]S-14, but its inhibition curve was not parallel to that of S-14. In the presence of 1 nM CCK-8, the Bmax of S-14 receptors was reduced to 150 +/- 17 fmol/mg of protein. Dibutyryl cyclic GMP, a CCK receptor antagonist, partially reversed the inhibitory action of CCK-8, suggesting that CCK receptors mediate the inhibition of S-14 receptor binding. GDP, GTP, and guanyl-5'-yl imidodiphosphate inhibit S-14 receptor binding in this tissue. The inhibition was shown to be due to decrease in binding capacity and not due to change in affinity. Specifically bound [125I-Tyr11]S-14 cross-linked to the S-14 receptors was found associated with three proteins of approximate Mr = 200,000, 80,000, and 70,000 which could be detected under both reducing and nonreducing conditions. Finally, pancreatic acinar cell S-14 receptors were shown to be down-regulated by persistent hypersomatostatinemia 1 week after streptozotocin-induced diabetes characterized by decreased Bmax (105 +/- 13 fmol/mg of protein) without any change in affinity. We conclude that pancreatic acinar cell membrane S-14 receptors require Ca2+ for maximal binding and thus differ from S-14 receptors in other tissues, S-14 receptors in this tissue also exhibit selective ligand specificities, these receptors are regulated by CCK-8 and guanine nucleotides, three receptor proteins of apparent Mr = 200,000, 80,000, and 70,000 specifically bind S-14, and (v) these receptors are regulated by S-14 in vivo as evidenced by decreased binding in streptozotocin diabetic rats characterized by hypersomatostatinemia.
以[125I - 酪氨酰11]生长抑素 - 14（S - 14）作为放射性配体，对生长抑素 - 14（S - 14）与大鼠胰腺腺泡细胞膜的结合特性进行了研究。在pH 7.4时观察到最大结合，且其为钙离子依赖性。在其他组织中未观察到S - 14受体结合的这种钙离子依赖性。对S - 14竞争性抑制[125I - 酪氨酰11]S - 14结合进行Scatchard分析，结果显示存在一类单一的高亲和力位点（解离常数Kd = 0.5±0.07 nM），结合容量（最大结合量Bmax）为266±22 fmol/mg蛋白质。[D - 色氨酰8]S - 14以及带有卤化色氨酸部分的结构类似物与S - 14相比，表现出高2 - 32倍的结合亲和力，[D - F5 - 色氨酰8]S - 14的活性最强。[酪氨酰11]S - 14与S - 14的活性相当。生长抑素 - 28与这些受体结合的亲和力为S - 14的50%。胆囊收缩素八肽（CCK - 8）抑制[125I - 酪氨酰11]S - 14的结合，但其抑制曲线与S - 14的抑制曲线不平行。在1 nM CCK - 8存在的情况下，S - 14受体的最大结合量降至150±17 fmol/mg蛋白质。二丁酰环鸟苷酸，一种CCK受体拮抗剂，部分逆转了CCK - 8的抑制作用，提示CCK受体介导了对S - 14受体结合的抑制。GDP、GTP和鸟苷 - 5'- 基 - 亚氨基二磷酸抑制该组织中S - 14受体的结合。这种抑制作用表现为结合容量降低而非亲和力改变。与S - 14受体交联的特异性结合的[125I - 酪氨酰11]S - 14与三种蛋白质相关，其表观分子量约为200,000、80,000和70,000，在还原和非还原条件下均能检测到。最后，链脲佐菌素诱导的糖尿病导致持续性高生长抑素血症1周后，胰腺腺泡细胞S - 14受体表现为下调，其特征为最大结合量降低（105±13 fmol/mg蛋白质），而亲和力无任何变化。我们得出结论，胰腺腺泡细胞膜S - 14受体需要钙离子以实现最大结合，因此与其他组织中的S - 14受体不同；该组织中的S - 14受体还表现出选择性配体特异性；这些受体受CCK - 8和鸟嘌呤核苷酸调节；三种表观分子量约为200,000、80,000和70,000的受体蛋白特异性结合S - 14；并且（v）如链脲佐菌素诱导的糖尿病大鼠中以高生长抑素血症为特征的结合减少所证明，这些受体在体内受S - 14调节。
