Inhibition by iodide of the cholinergic stimulation of prostaglandin synthesis in dog thyroid

Inhibition of H2O2 production by iodoaldehydes in cultured dog thyroid cells

2-Iodohexadecanal (IHDA) has been identified as a major thyroid iodolipid which can be formed upon addition of iodine to the vinyl ether group of plasmalogens (Pereira et al., 1990). In order to test whether IHDA plays a role in the thyroid autoregulation by iodide, we have investigated its effects on the production of H2O2 by cultured dog thyroid cells. IHDA inhibited the formation of H2O2 in dog thyroid cells stimulated by carbamylcholine (CCHOL). In the presence of BSA, which potentiated its action, the effect of IHDA was maximal after 2 h and had an IC50 around 5 microM. The effect of IHDA was not decreased by methimazole, which abolished the inhibition by iodide. IHDA also inhibited the stimulatory effect of bradykinin, but had only a marginal effect on the production of H2O2 induced by ionomycin or phorbol 12-myristate 13-acetate (PMA). The accumulation of inositol phosphates in CCHOL-stimulated thyroid cells was decreased by IHDA. As evaluated by measurements of 51Cr release and [3H]thymidine incorporation into DNA, IHDA had no adverse effect on thyroid cell viability. Several analogs of IHDA, of which the synthesis is described, have been tested for their inhibitory activity. This allowed the identification of two major structural features required for the biological activity: the carbonyl group at C1 and an halogen atom at C2, with iodine conferring a greater activity than bromine, while chlorine and fluorine were inactive. In conclusion, IHDA inhibits the production of H2O2 in CCHOL-stimulated dog thyroid cells by decreasing the phospholipase C cascade activity. This effect involves both the aldehyde function and the iodine atom. These results suggest that IHDA might be the mediator of some of the regulatory actions of iodide on the thyroid gland.

Methimazole increases thyroid-specific mRNA concentration in human thyroid cells and FRTL-5 cells

Methimazole (1-methyl-2-mercaptoimidazole; MMI) increases thyroglobulin mRNA and thyroid peroxidase mRNA concentration in human thyroid cells and in FRTL-5 cells. MMI (1-10,000 microM) gives a dose-dependent increase of thyroglobulin concentration in the medium of human thyroid cells and FRTL-5 cells. The stimulation by MMI has no effect on the TSH-induced cAMP production and occurs in the presence or absence of thyrotropin (TSH). TSH increases the thyroglobulin and thyroid peroxidase mRNA synthesis in human thyroid cells and FRTL-5 cells. The accumulation of thyroglobulin in the medium has an optimum at 100 microU TSH/ml in FRTL-5 cells. This optimum can also be found in most human thyroid cell cultures.

Interaction of VIPergic and cholinergic receptors in human thyroid cell

The thyroid tissue is innervated by cholinergic and VIPergic nerves. The present study investigated the possible interactions of cholinergic agents with VIP-induced cAMP accumulation and thyroid hormone release in vitro. Carbamylcholine (Cch), acting through the muscarinic receptor increases cellular cGMP content in cultured human thyroid cells incubated with a phosphodiesterase inhibitor. Cch (10 microM) inhibits cellular cAMP accumulation and thyroxine (T4) release induced by vasoactive intestinal peptide (VIP), with or without a phosphodiesterase inhibitor. Cch (10 microM) inhibits 8-bromo-cAMP-induced T4 release from human thyroid slices. 8-Bromo-cGMP inhibits VIP-induced T4 release from human thyroid slices, only in cells incubated without the phosphodiesterase inhibitor. The results indicate that interactions between VIPergic and cholinergic receptors may be of importance in human thyroid cell.

Cholinergic and VIPergic effects on thyroid hormone secretion in the mouse

The thyroid gland is known to harbor cholinergic and VIPergic nerves. In the present study, the influences of cholinergic stimulation by carbachol, cholinergic blockade by methylatropine and stimulation with various VIP sequences on basal, TSH-induced and VIP-induced thyroid hormone section were investigated in vivo in mice. The mice were pretreated with 125I and thyroxine; the subsequent release of 125I is an estimation of thyroid hormone secretion. It was found that basal radioiodine secretion was inhibited by both carbachol and methylatropine. Furthermore, TSH-induced radioiodine secretion was inhibited already by a low dose of carbachol. Moreover, a high dose of carbachol could inhibit VIP-induced radioiodine secretion. Methylatropine did not influence TSH- or VIP-stimulated radioiodine secretion, but counteracted the inhibitory action of carbachol on TSH- and VIP-induced radioiodine release. In addition, contrary to VIP, six various synthesized VIP fragments had no effect on basal or stimulated radioiodine release. It is concluded that basal thyroid hormone secretion is inhibited by both cholinergic activation and blockade. Furthermore, TSH-induced thyroid hormone secretion is more sensitive to inhibition with cholinergic stimulation than is VIP-induced thyroid hormone secretion. In addition, the VIP stimulation of thyroid hormone secretion seems to require the full VIP sequence.

Further characterization of the iodide inhibitory effect on the cyclic AMP system in dog thyroid slices

Iodide inhibits cyclic AMP accumulation in the thyroid by a process which is prevented by inhibition of iodide uptake and of thyroid peroxidase. By a similar process, it also exerts other independent effects such as the enhancement of iodinated protein release. Iodide inhibited the stimulation of adenylate cyclase by prostaglandin E1, cholera toxin and forskolin. The action of iodide was not relieved by phosphodiesterase inhibitors and was not additive with the effect of norepinephrine or adenosine. Iodide did not decrease the cellular level of ATP. The data are compatible with an inhibition of adenylate cyclase beyond the level of the receptor, presumably at the level of the catalytic unit or its interaction with the positive transducing unit NS. The effect of iodide required TSH for its expression but not for its installation. It was decreased under all conditions in which iodide organification was decreased: decreased iodide or increased methimazole concentration, absence of calcium in the medium, etc. However, the relation between iodide binding to proteins and effect was not linear. The effect was not relieved by washing in the absence of iodide and in the presence of perchlorate, but it was partly reversible in the presence of methimazole propylthiouracyl or thiourea. It was not relieved by cooling to 20 degrees C and cytochalasin b, which block stimulated thyroglobulin hydrolysis and iodothyronine release, nor by actinomycin D, cycloheximide, puromycin, mepacrine or indomethacin. The data suggest that iodide binds to a saturable cell component by a reaction which is reversible only in the presence of thiol-containing drugs.

Negative control of norepinephrine on the thyroid cyclic AMP system

Negative control on the thyroid cyclic AMP system has been studied. The increase of cyclic AMP levels induced by TSH in dog thyroid slices and its consequent secretion were inhibited by norepinephrine. This effect was different from the previously described activation of cyclic AMP disposal by acetylcholine: it was not calcium-dependent, was observed in the presence of isobutyl methylxanthine and was not inhibited by atropine. The inhibitory action of norepinephrine was abolished by phentolamine but not by L-propranolol. Clonidine and epinephrine also markedly inhibited the elevation of cyclic AMP levels, but phenylephrine did not. The inhibitory effect of norepinephrine and clonidine was abolished by yohimbine but not by prazosin. These results suggest that the effect of adrenergic agents on dog thyroid follicular cells is mediated by alpha 2-receptors. Similar results were obtained on dog thyroid adenylate cyclase activity: norepinephrine diminished the activation of adenylate cyclase induced by TSH, in a sodium-dependent process. This inhibition was abolished by phentolamine and yohimbine, but not by L-propranolol and and prazosin. This shows that the negative alpha 2-adrenergic effect bears directly on adenylate cyclase.

Feeding supplemental iodine to mink: reproductive and histopathologic effects

Adult mink were fed various concentrations of supplemental iodine, ranging from 10 to 320 ppm, for 1 or 7 mo before breeding. Long-term, low-level (10-20 ppm) iodine supplementation was beneficial for both reproduction and lactation. Supplemental iodine in excess of 80 ppm, however, resulted in a reduction in the number of females that whelped, a decrease in litter size, and an increase in kit mortality. Thyroid glands of kits whelped and nursed by dams fed more than 20 ppm supplemental iodine, both short-term and long-term, showed hypertrophy marked by follicular cell hyperplasia and a decreased amount of colloid. Similar histopathologic lesions were observed in the thyroids of adults that received 80 ppm or more supplemental iodine; also observed were numerous lesions in the gallbladder.

In vitro effects of arachidonic acid and of inhibitors of its metabolism on the dog thyroid gland

Incubation of dog thyroid tissue with arachidonic acid (10 to 200 microM) led to the following events: --low conversion to prostaglandins E2 and F2 alpha: 0.07% and 0.02% per hour and 100 mg tissue, respectively --inhibition of the stimulatory effect of low concentrations of TSH on thyroid secretion: the secretory effect of supra-maximal concentrations of TSH and of dB-cAMP was unaffected --inhibition of the cyclic AMP accumulation induced by TSH: this effect was inhibited neither by indomethacin nor by ETYA; cyclic AMP accumulation in response to cholera toxin or PGE1 was unaffected --no effect on cyclic GMP level --stimulation of thyroid proteins iodination. ETYA, but not indomethacin, depressed the iodination of thyroid proteins in resting and stimulated tissue. These data show that arachidonic acid-or a metabolite-can modulate thyroid responsiveness to TSH and suggest that lipoxygenase-products of arachidonic acid metabolism could be involved in thyroid proteins iodination.