| Abstract: | Somatostatin-like immunoreactivity (SRIF-LI) has previously been demonstrated immunohistochemically in rat thyroid parafollicular cells. Studies were therefore performed to determine whether SRIF-LI was present in a transplantable medullary carcinoma of the thyroid (MCT) in the WAG/Rij strain of rat. SRIF-LI was found in MCT in significantly higher concentrations than in normal thyroid tissue. Thyroid and MCT SRIF-LI showed parallelism with the synthetic SRIF and RIA displacement curves and coeluted with synthetic SRIF on immunoaffinity chromatography. On gel filtration, thyroid SRIF-LI and the major peak of MCT SRIF-LI coeluted with synthetic SRIF. SRIF-LI of a larger molecular size was also present in the MCT. MCT and thyroid SRIF-LI coeluted with synthetic SRIF on high pressure liquid chromatography. MCT SRIF-LI purified by affinity chromatography was equipotent to synthetic SRIF in inhibiting dibutyryl cAMP-stimulated GH release by rat pituitary cells in monolayer culture. Serum SRIF-LI was elevated in tumor-bearing rats and showed characteristics similar to those of MCT SRIF-LI and synthetic SRIF on affinity and high pressure liquid chromatography (HPLC). Tumor-bearing rats showed diminished secretion of insulin after orally administered glucose and impaired secretion of GH in response to pentobarbital compared to normal control rats. The results indicate that SRIF-LI is produced in excessive quantities by a transplantable rat MCT and impairs the secretion of GH and insulin. The immunological, chromatographic, and biological properties of MCT SRIF-LI suggest that it is indistinguishable from synthetic SRIF. |
