Long-term effect of norepinephrine on iodide uptake in FRTL-5 cells

Role of hypothyroidism and associated pathways in pregnancy and infertility: Clinical insights

Thyroid disorders are the most common endocrine problems in women. In most of the cases, thyroid can lead to infertility or miscarriages. The etiology of infertility is multifactorial with thyroid disorders as the most common presenting factor, hypothyroidism in particular. Infertility in women can lead to emotional and psychological stress. The prevalence of hypothyroidism during pregnancy is estimated to be 0.3%–0.5%. Hypothyroidism and hyperthyroidism can result in menstrual irregularities and anovulatory cycles, thus affecting the fertility. There is a significant high prolactin (PRL) level in infertile women with hypothyroidism when compared to euthyroid patients, indicating the relation between hypothyroidism and hyperprolactinemia. The amount of thyrotropin releasing hormone (TRH) from the hypothalamus is markedly increased by inhibition of pyroglutamyl peptidase II, the enzyme catalyzing TRH. The increased TRH in hypothyroidism causes increased thyroid-stimulating hormone and PRL secretion by pituitary, leading to infertility and galactorrhea. In recent years, a neuropeptide called kisspeptin, encoded by Kiss1 gene, a potent stimulus for GnRH secretion, has been recognized, which suggests a future direction of treatment with kisspeptin and benefits the fertility induction among hyperprolactinemic infertile patients. Untreated hypothyroidism during pregnancy can lead to subfertility, fetal deaths, premature deliveries, and abortions. Therefore, women planning for pregnancy and infertile women should be assessed for thyroid hormones and serum PRL.

Central regulation of hypothalamic-pituitary-thyroid axis under physiological and pathophysiological conditions

TRH is a tripeptide amide that functions as a neurotransmitter but also serves as a neurohormone that has a critical role in the central regulation of the hypothalamic-pituitary-thyroid axis. Hypophysiotropic TRH neurons involved in this neuroendocrine process are located in the hypothalamic paraventricular nucleus and secrete TRH into the pericapillary space of the external zone of the median eminence for conveyance to anterior pituitary thyrotrophs. Under basal conditions, the activity of hypophysiotropic TRH neurons is regulated by the negative feedback effects of thyroid hormone to ensure stable, circulating, thyroid hormone concentrations, a mechanism that involves complex interactions between hypophysiotropic TRH neurons and the vascular system, cerebrospinal fluid, and specialized glial cells called tanycytes. Hypophysiotropic TRH neurons also integrate other humoral and neuronal inputs that can alter the setpoint for negative feedback regulation by thyroid hormone. This mechanism facilitates adaptation of the organism to changing environmental conditions, including the shortage of food and a cold environment. The thyroid axis is also affected by other adverse conditions such as infection, but the central mechanisms mediating suppression of hypophysiotropic TRH may be pathophysiological. In this review, we discuss current knowledge about the mechanisms that contribute to the regulation of hypophysiotropic TRH neurons under physiological and pathophysiological conditions.

Long-term effect of norepinephrine on thyroglobulin gene expression in FRTL-5 cells

Many types of evidence support a role of the sympathetic nervous system in the regulation of thyroid function, although there is no general consensus on the type of influence that catecholamines exert. Depending on the experimental approach, epinephrine and norepinephrine (NE) can stimulate, inhibit, or fail to act on thyroid function. The aim of this study was to determine the effect of NE on thyroglobulin (Tg) synthesis and gene expression in FRTL-5 cells. Tg content, measured by immunoprecipitation with a specific antibody, showed that NE caused a 45% inhibition of thyrotropin (TSH) effect. The content of Tg mRNA was analyzed by Northern blot, the relative inhibition in total Tg mRNA levels from NE-treated cells, compared to TSH alone, ran parallel with inhibition in Tg content, while total RNA did not change after incubation with NE. There was no alteration in Tg mRNA stability by NE. When plasmids harboring different sequences of Tg promoter fused to the CAT reporter gene were transfected into FRTL-5 cells, TSH treatment stimulated promoter activity while NE diminished this effect by 43%-55%. Northern blots were performed to analyze mRNA for thyroid transcription factors (TTF1, TTF2, Pax8), and no significant changes were observed with the different treatments. In conclusion these results suggest that NE inhibits Tg synthesis at the transcriptional level.

The protein kinase C pathway inhibits iodide uptake by calf thyroid cells via sodium potassium-adenosine triphosphatase

The effect of the phorbol esther phorbol myristate acetate (PMA) on iodide uptake was studied in primary cultures of calf thyroid cells. PMA caused a dose- and time-dependent inhibition of thyrotropin (TSH), forskolin, and db-cAMP stimulation, indicating an effect distal to both TSH receptor and cAMP generation. No action was found on iodide efflux, indicating a selective inhibition of iodide uptake. This inhibition was observed even after 5 minutes of incubation, thus excluding a possible genomic action. Bisindolmaleimide (BS), a specific inhibitor of the protein kinase C (PKC) pathway, reverted the effect of PMA. A similar degree of inhibition of the Na+/K+ adenosine triphosphatase (ATPase) and iodide uptake by PMA was found, thus suggesting a link between both parameters. These results indicate that the PKC pathway inhibits thyroid iodide uptake by an action distal to cAMP generation and probably because of a decrease in Na+/K+-ATPase activity.