Inhibitory effect of somatostatin on cAMP accumulation and calcitonin secretion in C-cells: involvement of pertussis toxin-sensitive G-proteins.

The effect of somatostatin on cAMP accumulation and calcitonin secretion in C-cells of the rat medullary thyroid carcinoma cell line rMTC 6-23 was investigated. Intracellular cAMP accumulation as well as calcitonin secretion could be dose-dependently stimulated by rat growth hormone releasing factor (rGRF). The long-acting somatostatin analogue octreotide inhibited rGRF-stimulated cAMP accumulation and calcitonin secretion dose dependently but failed to block 8-bromo-cAMP-stimulated calcitonin secretion. The inhibitory effect of octreotide on rGRF-induced calcitonin secretion was partially abolished by pretreating the cells with pertussis toxin. The octreotide effect was not due to changes in the degradation of cAMP, as it was similarly seen in the presence of isobutylmethylxanthine. Thus we conclude that pertussis toxin-sensitive G-proteins are involved in the cAMP-mediated regulation of calcitonin secretion in C-cells.