The different modes of action of thyrotropin and prostaglandin E1 on cyclic adenosine 3',5'-monophosphate synthesis in human thyroid, as studied by sequential stimulations.

PGE1 was equally effective in increasing 3H-cyclic AMP in normal and in toxic thyroids, whereas TSH was less effective but over a longer time in the toxic thyroids. Stimulation by a large second dose of TSH could not be elicited after prior stimulation by large doses of TSH. Similar results were obtained with regard to the effect of PGE1. However, stimulation by a large dose of PGE1 was still effective after the slices became refractory to TSH. Similarly, stimulation by a large dose of TSH was still effective after the slices became refractory to PGE1. It is suggested that the site and/or mode of action of TSH is quite different from that of PGE1.