Journal of Clinical Endocrinology & Metabolism, Vol 53, 1117-1122, Copyright © 1981 by Endocrine Society

ARTICLES

Molecular heterogeneity and biological activity of immunoreactive somatostatin in medullary carcinoma of the thyroid

S Saito, H Saito, M Matsumura, K Ishimaru and T Sano

Two cases of somatostatin (SRIF)-producing medullary carcinoma of the thyroid are presented. The plasma levels of SRIF and calcitonin changed in accordance with the clinical course before and after resection of the tumor. On chromatographic analysis of tumor extracts derived from primary and metastatic tumors of medullary thyroid carcinoma, at least two components of immunoreactive SRIF (IR-SRIF) of large molecular size were found, besides a component corresponding to the tetradecapeptide (SRIF 14); one of these seemed to have almost the same molecular weight as the octacosapeptide (SRIF 28). Most of the IR-SRIF in one patient's plasma was eluted in the same position as tetradecapeptide SRIF. The biological activity of SRIF recovered from the tumor extract was assessed by measuring activity for inhibiting GH release using dispersed rat anterior pituitary cells. GH release from the cells into the incubation medium tended to decrease at a concentration of 1 x 10(- 9) M IR-SRIF derived from the tumor (3000 +/- 482 ng/10(5) cells . 3 h) compared with the control value (4288 +/- 567 ng/10(5) cells . 3 h). A significant decrease was observed at concentrations of 1 x 10(-8) M (1880 +/- 403 ng/10(5) cells . 3 h) and 5 x 10(-8) M (938 +/- 262 ng/10(5) cells . 3 h). Responses to equivalent amounts of synthetic SRIF 14 were similar. This suggests that IR-SRIF in the tumor has almost the same biological activity as synthetic tetradecapeptide SRIF.