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

Nitric oxide as a signal in thyroid

It is now well established that agonist activation of the PIP2/calcium cascade in the thyroid results in the enhancement of cGMP accumulation presumably by activation of the soluble guanylate cyclase. In many tissues the physiological signal controlling soluble guanylate cyclase is nitric oxide (NO) and its synthesis from arginine is controlled by the intracellular Ca2+. In this report we show results that suggest that NO may be the intermediate of the cGMP response to the activation of the PIP2/calcium cascade. In dog thyroid slices, incubation with carbamylcholine or A23187 increases significantly free intracellular Ca2+ levels and the cGMP content of the slices. NG-Monomethyl-L-arginine (NMMA), a competitive inhibitor of arginine for nitric oxide synthase, inhibited these cGMP responses but not the action of sodium nitroprusside which activates soluble guanylate cyclase directly. The inhibition was relieved by arginine. Methylene blue, which blocks the activation of soluble guanylate cyclase by NO, also decreased the three stimulatory effects. NMMA and methylene blue also decreased the basal levels of cGMP. NO may therefore be an important autocrine and paracrine factor in thyroid.

Early adaptation of thyrotropin and thyroglobulin secretion to experimentally decreased iodine supply in man

Five healthy male volunteers (aged 25 to 28 years) were studied both after 4 weeks of treatment with 200 micrograms iodine/d orally (PO) and following experimental iodine depletion by treatment with 3 x 300 mg perchlorate/d PO over a 4-week period, in an attempt to better define the early adaptive responses to an alteration in iodine supply in thyroid function. Intrathyroidal iodine, serum triiodothyronine (T3), free T3 (FT3), thyroxine (T4), free T4 (FT4), reverse T3 (rT3), thyroxine-binding globulin (TBG), thyroglobulin (Tg), and thyrotropin (TSH) levels (10-minute sampling over 24 hours) were measured at the end of iodine administration and at the end of perchlorate treatment. Thyroid volume was determined by sonography, and iodine content was determined by fluorescence scintigraphy. TSH pulses were analyzed by computer-assisted programs. Comparing both experimental situations, perchlorate treatment significantly reduced intrathyroidal iodine concentration (4.0 +/- 1.3 to 3.0 +/- 1.2 nmol/mL, P less than .05), but thyroid volume and total serum T4, T3, FT3, and TBG levels were not altered. Mean 24-hour serum TSH levels (1.8 +/- 0.3 to 1.0 +/- 0.3 mU/L, P less than .001), amount of TSH secreted/pulse (0.5 +/- 0.1 to 0.3 +/- 0.1 mU/L, P less than .001), and FT4 levels (15.7 +/- 1.7 to 14.3 +/- 1.4 pmol/L, P less than .005) were significantly diminished, whereas Tg levels (18.6 +/- 10.0 to 35.1 +/- 14.0 ng/mL, P less than .01) were significantly increased. Thyroid-specific antibodies were normal and were not altered by treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Decrease of prostaglandin I2 binding sites in thyroid cancer

The properties of specific prostaglandin I2 (prostacyclin, PGI2) binding sites in normal thyroid tissue have been characterised. Tissue samples obtained intraoperatively from patients with 'cold' solitary thyroid nodules (as preoperatively selected by thyroid gland scintigraphy, thyroid gland ultrasonography and Papanicolaou cytology following fine needle aspiration of the nodule area) have been used for thyroid membrane preparation. Employing [3H]iloprost, a chemically stable PGI2-analogue as a radioligand, saturation experiments for comparative binding studies have been attempted. Scatchard analysis of the binding data obtained for normal thyroid parenchyma distant to the nodule area revealed heterogeneity of the [3H]iloprost sites exhibiting a high-affinity binding capacity (Bmax) of 613.2 +/- 130.4 fmol mg-1 membrane protein and a low-affinity binding capacity of 5.1 +/- 1.6 pmol mg-1 membrane protein. The equilibrium dissociation constant (Kd) amounted to 18.9 +/- 8.9 nM and to 131.5 +/- 39.2 nM, respectively. Scatchard analysis of the binding data obtained for benign thyroid adenoma indicated significant lower binding capacities exhibiting a Bmax of 325.8 +/- 110.0 fmol mg-1 membrane protein (Kd: 31.0 +/- 7.5 nM) for the high-affinity sites and of 3.9 +/- 2.5 pmol mg-1 membrane protein (Kd: 364.9 +/- 183.6) for the low affinity sites. In cancer tissue a selective loss of the low affinity sites and a significant diminution of the high-affinity sites was observed: in well differentiated cancer the high-affinity sites showed a Bmax of 299.7 +/- 46.0 fmol mg-1 membrane protein (Kd: 38.9 +/- 7.3 nM), in anaplastic cancer, less differentiated papillar and follicular cancers of 180.6 +/- 25.1 fmol mg-1 membrane protein (Kd: 54.6 +/- 16.7 nM). Well differentiated papillar and follicular cancers did not differ from each other.(ABSTRACT TRUNCATED AT 250 WORDS)

Arachidonate metabolism in the mouse thyroid implication of the lipoxygenase pathway in thyrotropin action

The present study of compares the effects of various inhibitors of arachidonate metabolism on mouse thyroid cyclo-oxygenase and lipoxygenase activities and thyrotropin-augmented cyclic-AMP accumulation. Mouse thyroid homogenate converts [1-14C]- arachidonate to several products of the cyclo-oxygenase pathway as well as one major product of the lipoxygenase pathway, 12-L-hydroxyeicosatetraenoic acid (12-Hete). Prostaglandin (PG) formation in thyroid homogenates is inhibited by 1-10 microM indomethacin and etya. 12-HETE accumulation is reduced by 91%, 83% and 20% by 5 microM ETYA, 15-HETE, and indomethacin, respectively. Thyrotropin-stimulated cyclic-AMP accumulation, measured in whole thyroid lobes by radioimmunoassay, is reduced by 45% and 73% by 50 microM and 100 microM ETYA, respectively; indomethacin is without effect at these concentrations. 15-HETE reduces thyrotropin-augmented cyclic-AMP accumulation by 57% and 100 microM. In product inhibition studies, 10 microM 12-HETE reduced the formation of radiolabeled 12-HETE by 20%. 10 microM PGE2, PGF2 alpha or PGD2 had no effect on [1-14C]-PG formation. 12-HETE, however, reduced PG synthesis by 76% at 10 microM. This is the first report implicating the arachidonate lipoxygenase pathway in thyrotropin action at the level of cyclic-AMP regulation. Additionally, our finding that 12-HETE inhibits prostaglandin synthesis suggests that the cyclo-oxygenase and lipoxygenase pathways in the mouse thyroid may be highly integrated.

Novel transformations of arachidonic acid by the rat thyroid in vitro

The transformation of arachidonic acid by the rat thyroid in vitro has been investigated. At least two metabolites have been partially characterized: they differed from known metabolites of arachidonic acid in terms of retention volume in liquid chromatography, ultraviolet spectrophotometry and pharmacology (formation not inhibited by indomethacin and enhanced by eicosatetraynoic acid). The analysis by chemical ionization mass spectrometry suggested that these metabolites might be diketo-monohydroxy- and monoketo-dihydroxy-compounds. The conversion of arachidonic acid into these compounds was stimulated by ionophore A23187, decreased by the peroxidase inhibitor methimazole and potentiated by iodide, suggesting that this pathway is under the control of Ca2+ and of a peroxidase product.