TUMOR AND PLASMA SOMATOSTATIN-LIKE IMMUNOREACTIVITY IN TRANSPLANTABLE RAT MEDULLARY-THYROID CARCINOMA

Abstract

Sixteen series of calcitonin-producing transplantable rat medullary thyroid carcinoma were evaluated for somatostatin-like immunoreactivity. Each of the series contained detectable levels of both peptides. Immunoreactive somatostatin varied from < 1 ng/mg of protein to almost 500 ng/mg of protein. The range of immunoreactive calcitonin was 0.3-30 .mu.g/mg of protein. Although somatostatin-like immunoreactivity was always less than that of calcitonin, the levels in certain series were as high as those found in neural or endocrine tissues used for in vitro studies of somatostatin elaboration. No significant correlation was found between tissue levels of these 2 peptides. Two tumor lines were generated by initiation of tumor growth with cells from primary monolayer cultures. Levels of immunoreactive calcitonin and somatostatin significantly differed from those of the parent lines, which were maintained by serial passage of tissue fragments only. Plasma somatostatin-like immunoreactivity assessed in 2 tumor series with high (149 ng/mg of protein) and low (1.5 ng/mg of protein) tissue levels was 3100 and 50 pg/ml, respectively. Gel filtration chromatography of tissue and blood extracts showed a predominant peak (> 90%) of immunoreactive somatostatin eluting at the position of the native hormone. Three other peaks were resolved in the tissue extract with estimated MW of 14,000, 8700 and 5000. The high level of somatostatin-like immunoreactivity and the presence of multiple large forms suggest that certain tumor lines will prove valuable for studies of somatostatin biosynthesis and secretion.