Calcitonin gene-related peptide and calcitonin secretion from a human medullary thyroid carcinoma cell line: effects of ionomycin, phorbol ester and forskolin

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Abstract
Calcitonin gene-related peptide (CGRP) and calcitonin are secreted together from medullary thyroid carcinoma (MTC) cell. Interactions of cytosolic free calcium concentration (Cai2+) and the protein kinase C and A pathways on the secretion of immunoreactive CGRP and calcitonin have been investigated in a human MTC cell line. Ionomycin (10 .mu.mo1/1) raised the concentration of Cai2+, concomitant with a transient stimulation of the secretion of CGRP and calcitonin. 12-O-tetradecanoylphorbol-13-acetate (TPA; 16 nmo1/1) did not affect the concentration of Cai2+, but caused a gradual rise of the secretion of CGRP and calcitonin. Combined addition of 10 .mu.mo1 ionomycin/1 and 16 nmol TPA/1 resulted in additive stimulation of CGRP and calcitonin secretory responses. Forskolin (10 .mu.mo1/1) alone did not change the concentration of Cai2+, marginally enhanced (P > 0 .cntdot. 1) the release of CGRP and calitonin and increased by 23-fold the cellular levels of cyclic AMP (cAMP). Ionomycin and TPA did not change cellular cAMP. Forskolin synergistically enhanced (P < 0 .cntdot. 01) the ionomycin-induced early phase as well as the TPA-induced late phase of the CGRP and calcitonin secretory responses. In conclusion, increased concentrations of Cai2+ together with protein kinase C and A activation mediate the secretion of CGRP and calcitonin in MTC cells.

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