Somatostatin Receptors in Human Growth Hormone and Prolactin- Secreting Pituitary Adenomas

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Abstract
[125I-Tyr]Somatostatin ([125I-Tyr]SRIH) binding was found in 11 GH-secreting pituitary adenomas [Kd = 0.46 ± 0.15 (±SE) nM; maximum binding, 165 ± 35 fmol/mg protein). This binding was specific, since it was displaced by somatostatin- 14 (SRIH-14), N-Tyr-SRIH-14, and SRIH-28. In contrast, a number of peptides and drugs not structurally related to SRIH, such as bombesin, dopamine, LHRH, met-enkephalin, naloxone, neurotensin, secretin, substance P, TRH, or vasoactive intestinal peptide, did not affect [125I-Tyr]SRIH binding. [125I-Tyr] SRIH specific binding also was found in PRL-secreting pituitary adenomas. The kinetic characteristics of the specific binding were similar to those of GH-secreting adenomas. However, maximal binding was one quarter that of GH-secreting adenomas (37 ± 9 fmol/mg protein). In contrast, nonsecreting (chromophobe) tumors were devoid of any specific binding. Finally, in acromegaly, the density of [125I-Tyr]SRIH-binding sites in the adenomas was negatively correlated with plasma GH levels before surgery (r = −0.80). This suggests that somatostatinergic control is involved in GH secretion in acromegalic patients.

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