Regulation of thyroxine release in the thyroid by protein kinase C

Previous studies suggested that the inhibition of thyroxine (T4) release by alpha1-adrenoceptor and muscarinic receptor stimulation results in activated protein kinase C (PKC) from mouse and guinea pig thyroids. In the present study, the effect of carbachol, methoxamine, phorbol myristate acetate (PMA), and R59022 on the release of T4 from the mouse, rat, and guinea pig thyroids was compared to clarify the role of PKC in the regulation of the release of T4. The thyroids were incubated in the medium containing the test agents, samples of the medium were assayed for T4 by EIA kits. Forskolin, an adenylate cyclase activator, chlorophenylthio-cAMP sodium, a membrane permeable analoge of cAMP, and isobutyl-methylxanthine, a phospho-diesterase inhibitor, like TSH (thyroid stimulating hormone), enhaced the release of T4 from the mouse, rat, and guinea pig thyroids. Methoxamine, an alpha1-adrenoceptor agonist, inhibited the TSH-stimulated release of T4 in mouse, but not rat and guinea pig thyroids. In contrast, carbachol, a muscarinic receptor agonist, inhibited the release of T4 in guinea pig, but not mouse and rat thyroids. These inhibition were reversed by prazosin, an alpha1-adrenoceptor antagonist or atropine, a muscarinic antagonist or M1- and M3- muscarinic antagonists, in mouse or guinea pig thyroids. In addition, staurosporine, a PKC inhibitor, reversed methoxamine or carbachol inhibition of TSH stimulation. Furthermore, PMA, a PKC activator, and R59022, a diacylglycerol (DAG) kinase inhibitor, inhibited the TSH-stimulated release of T4 in mouse, rat, and guinea pig thyroids. These inhibition were blocked by staurosporine. these findings suggest that the activation of receptor or DAG inhibits TSH-stimulated T4 release through a PKC-dependent mechanism in thyroid gland.