Thyroxine secretion by isolated hog thyroid cells: a cyclic AMP independent pathway.

The release of 131I-labeled thyroxine (T4) from isolated hog thyroid cells was increased 1.5--2-fold by thyrotropin (TSH). Dibutyryl cyclic AMP failed to reproduce this TSH action. In this in vitro system another cell activity, T4 synthesis, was stimulated in an essentially identical fashion by TSH and dibutyryl cyclic AMP (time course of action, dose-response relationship). 3-Isobutyl-1-methylxanthine (IBMX), 0.5 mM, did not alter the basal [131I]T4 release whereas it enhanced the [131I]T4 synthesis. TSH, 60 MU/ml, increased the intracellular cyclic AMP concentration 3-4-fold. Chlorpromazine (5 X 10(-4)M) abolished the TSH stimulation of cyclic AMP accumulation but did not alter the TSH-induced increase in [131I]T4 secretion. It is concluded that the TSH action on [131I]T4 secretion by isolated thyroid cells is not mediated by the adenylate cyclase-cylic AMP system.