Srikant C B, Heisler S
Endocrinology. 1985 Jul;117(1):271-8. doi: 10.1210/endo-117-1-271.
Somatostatin-14 (S-14) acts via specific receptors to inhibit basal as well as hormone- and forskolin-stimulated ACTH secretion in tumor cells (AtT-20/D16-16) of mouse anterior pituitary. In addition S-14 inhibits the stimulated but not basal cAMP accumulation. The potency of somatostatin-28 (S-28) for regulating these processes in these tumor cells has not been reported. In this study we have investigated the relationship between receptor-binding affinities of S-14 and S-28 and their biopotency in these cells. Membrane receptors for S-14 characterized using [125I-Tyr11]S-14 as the radioligand [maximum binding capacity (Bmax) = 1.28 +/- 0.1 pmol/mg; dissociation constant (Kd) = 1.1 +/- 0.04 nM] bound S-28 with 3-fold greater affinity than S-14. Binding sites quantitated using an S-28 analog [Leu8, D-Trp22, 125I-Tyr25]S-28 as radioligand (Bmax = 1.18 +/- 0.15 pmol/mg; Kd = 0.08 +/- 0.06 nM) also exhibited greater affinity for S-28 than S-14. Forskolin-stimulated cAMP accumulation and ACTH secretion in these cells were inhibited to a greater extent (4- and 9-fold, respectively) by S-28 than S-14. Preincubation of the cells with S-14 and S-28 (10(-7) M) resulted in a marked decrease (36% and 71%, respectively) of S-14 receptor concentration. Coincubation of the cells with both S-14 and S-28 led to 56% decrease in S-14 receptor binding. The responsiveness of the cells to forskolin stimulation of ACTH secretion and cAMP accumulation was significantly enhanced by preincubation with S-14 (10(-7) M) whereas the responsiveness to forskolin was completely abolished by preincubation with S-28. Simultaneous exposure of the cells to both S-14 and S-28 resulted in a partial reversal of the inhibiting effect of S-28 on forskolin-stimulated cAMP accumulation in these cells but did not result in a partial reversal of the inhibitory effect of S-28 on forskolin-stimulated ACTH secretion in these cells. These results demonstrate that S-28 is more potent than S-14 in AtT-20/D16-16 cells, its greater potency arising from its greater affinity for binding to S-14 receptors. The differential effects of these peptides after preincubation on the responsiveness of ACTH secretion and cAMP accumulation in these cells to forskolin stimulation suggests the possibility of existence of distinct S-14 and S-28 receptors, but these could not be identified by direct binding experiments using the S-14 and S-28 analogs employed in the study as radioligands.(ABSTRACT TRUNCATED AT 400 WORDS)
生长抑素 - 14（S - 14）通过特异性受体发挥作用，抑制小鼠垂体前叶肿瘤细胞（AtT - 20/D16 - 16）的基础促肾上腺皮质激素（ACTH）分泌以及激素和福斯可林刺激的ACTH分泌。此外，S - 14抑制刺激后的而非基础的环磷酸腺苷（cAMP）积累。生长抑素 - 28（S - 28）在这些肿瘤细胞中调节这些过程的效能尚未见报道。在本研究中，我们研究了S - 14和S - 28的受体结合亲和力与其在这些细胞中的生物活性之间的关系。以[125I - Tyr11]S - 14作为放射性配体表征的S - 14膜受体[最大结合容量（Bmax）= 1.28±0.1 pmol/mg；解离常数（Kd）= 1.1±0.04 nM]与S - 28的结合亲和力比与S - 14的高3倍。用S - 28类似物[Leu8，D - Trp22，125I - Tyr25]S - 28作为放射性配体定量的结合位点（Bmax = 1.18±0.15 pmol/mg；Kd = 0.08±0.06 nM）对S - 28的亲和力也高于S - 14。在这些细胞中，S - 28比S - 14更能抑制福斯可林刺激的cAMP积累和ACTH分泌（分别为4倍和9倍）。细胞与S - 14和S - 28（10^(-7) M）预孵育导致S - 14受体浓度显著降低（分别为36%和71%）。细胞与S - 14和S - 28共同孵育导致S - 14受体结合减少56%。用S - 14（10^(-7) M）预孵育可显著增强细胞对福斯可林刺激的ACTH分泌和cAMP积累的反应性，而用S - 28预孵育则完全消除了对福斯可林的反应性。细胞同时暴露于S - 14和S - 28导致S - 28对这些细胞中福斯可林刺激的cAMP积累的抑制作用部分逆转，但并未导致S - 28对这些细胞中福斯可林刺激的ACTH分泌的抑制作用部分逆转。这些结果表明，在AtT - 20/D16 - 16细胞中，S - 28比S - 14更有效，其更高的效能源于其与S - 14受体结合的更高亲和力。这些肽预孵育后对这些细胞中ACTH分泌和cAMP积累对福斯可林刺激的反应性的不同影响表明可能存在不同的S - 14和S - 28受体，但使用本研究中所用的S - 14和S - 28类似物作为放射性配体的直接结合实验未能鉴定出它们。（摘要截于400字）
