Changes in the cyclic nucleotides (cCMP, cAMP, and cGMP) of rat thyroid induced by prolonged administration of LATS and TSH.

Abstract

Abstract. It has recently been suggested that cyclic CMP (cCMP) has a physiological role in cell function. In this study, we examined the content of cCMP, cAMP & cGMP and each hydrolyzing enzyme (phosphodiesterase, (PDE)) in rat thyroid during administration of LATS and TSH for 7 days (Exp. 1), and also in the thyroid of rats given methylthiouracil (MTU) for 4 months (Exp. 2). Each cyclic nucleotide was determined by RIA, and PDE activity was determined according to Kuo's method. Exp. 1: the wet weight of thyroid in the LATS and TSH group increased 3 and 2 times respectively, from the control. Both the LATS and TSH groups showed increased plasma T4 and T3 levels. In both groups the cAMP content per wet weight of thyroid was increased slightly, but cGMP content did not change. In contrast, the cCMP content decreased markedly. Both the LATS and TSH groups showed an increase of cAMP-PDE activity in the thyroid, and no change of cGMP-PDE, while cCMP-PDE decreased markedly. In all groups levels of these 3 cyclic nucleotides and PDE activities in plasma were unaffected. Exp. 2: in the MTU group the thyroid weight increased 4.5 times over the control, and plasma T4 and T3 levels were extremely low. The cAMP content in whole thyroid gland and also per mg wet weight was increased markedly, but cGMP content did not change. cCMP content decreased significantly. The plasma cAMP level increased slightly, but the plasma cGMP and cCMP levels did not change. The increase of cAMP content and cAMP-PDE activity, and conversely the decrease of cCMP content and cCMP-PDE in the thyroid by LATS and TSH stimulation, suggest that cCMP may play a physiologically important role within the cells, similar to cAMP in the mechanism of hormone action.