Somatostatin Elaboration by Monolayer Cell Cultures Derived from Transplantable Rat Medullary Thyroid Carcinoma: Synergistic Stimulatory Effects of Glucagon and Calcium*

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Abstract
Somatostatin (SRIF)-producing transplantable rat medullary thyroid carcinomas (MTCs) were used to establish primary monolayer cell cultures for studying SRIF secretion and production. Immunoreactive SRIF (iSRIF) and calcitonin (iCT) levels in the MTC cell cultures were 252 and 4900 ng/mg cell protein, respectively. The acute effects of calcium and glucagon on the elaboration of iSRIF and iCT were investigated in 3-h experiments. Basal secretion rates (mean ± SE, five experiments) for iSRIF and for iCT were 10.7 ± 2.5 and 297 ± 33 ng/mg cell protein, respectively. Calcium stimulated the secretion of iSRIF and iCT in a dose-dependent fashion. Maximal secretory effect of calcium was observed at 4-5 mM: iCT secretion increased 148% over basal, and iSRIF secretion increased 228% over basal. At 1 mM calcium, maximally effective doses of glucagon (1 μM) significantly increased iCT secretion (44% over basal) but not iSRIF secretion. At 5 mM calcium, as little as 10 nM glucagon had a significant stimulatory effect on secretion of both peptides. The largest stimulation was observed at 5 mM calcium and 1 im. glucagon; for iSRIF a 527% increase over basal and for iCT a 308% increase over basal were observed under this condition. At high calcium the stimulatory effect of glucagon on iSRIF secretion and on iCT secretion was significantly greater than at low calcium. In these 3-h experiments neither 5 mM calcium nor 1 JUM glucagon increased culture content (cells plus inedium) of iCT or iSRIF. However, 3-h treatment of the cell cultures with both agonists increased total culture content of iSRlF by 53% and iCT by 21%. The establishment of SRIF-producing cell cultures from transplantable rat MTCs and the identification of a synergistic secretagogue interaction for glucagon and calciumopen the way for systematic in vitro investigation of SRIF secretion and biosynthesis.