Epinephrine precursors: effects on the iodine and intermediary metabolism of isolated calf thyroid cells

Effects of dietary energy level on luteinizing hormone and adrenal function in the post partum beef cow

The effects of dietary energy and suckling on adrenal function and luteinizing hormone (LH) concentrations were investigated in primiparous postpartum cows. Ten heifers were assigned at calving to either high (22.8 Mcal/day) or low (15.2 Mcal/day) energy diets. Blood samples were collected every 15 minutes for 8 hours on 28, 42, and 56 days post partum. Calves were allowed to suckle ad libitum during sampling periods. Serum samples were analyzed by radioimmunoassay for LH and cortisol. Concentrations of catecholamines were quantified by reverse-phase HPLC. Body weights were decreased (P<0.01) by low energy intake. In addition, low energy diet cows had lower mean LH concentrations (0.97+/-0.09 vs 1.57+/-0.07 ng/ml), P<0.05) than high energy diet cows. Luteinizing hormone concentrations in high energy diet cows increased with days post partum, resulting in a treatment-by-time interaction (P<0.005). Treatment did not affect mean cortisol concentrations. However, within 15 minutes of suckling cortisol release was significantly above baseline in 77% of the observed suckling events. Dihydroxyphenylalanine (DOPA) increased in high energy diet cows compared with that of low energy diet cows (2,833+/-243 vs 1,294+/-243 pg/ml, P<0.01). Norepinephrine (NE) and 3,4-dihydroxyphenylacetic acid (DOPAC) were not influenced by treatment. Plasma NE decreased during the postpartum interval (P<0.005). These data suggest that reduced energy intake may prevent the increase in LH associated with increasing days post partum and alter adrenal function. In addition, spontaneous suckling events elicit a release of cortisol.

Activity of the hypothalamic-pituitary-thyroid axis during exposure to cold

It seems clear from the studies reviewed here that there is adequate evidence to support the concept of a biphasic response of the thyroid gland to cold as first postulated by Moll et al. (1972). The initial response to acute exposure to cold begins at the level of the hypothalamus as a result of either neural stimuli from skin and other areas and/or blood of somewhat lower than normal temperature reaching the hypothalamus (Andersson et al., 1963). As a result, the secretion of norepinephrine and/or dopamine may increase, and serotonin and/or somatostatin may decrease. The net result of these is an increase in the release of TRH from the hypothalamus. This, in turn, stimulates the cascade for the release of TSH from the anterior pituitary gland and thyroid hormone from the thyroid gland. Moll et al. (1972) postulated the lack of a feedback limb in this acute phase, and, indeed, this may be the case. It is possible, however, that certain hormones, such as somatostatin, norepinephrine, T3, and T4 could act in the capacity of feedback inhibitors. Additional experiments will be required to assess this possibility. The transitional link between the acute (less than 1 day) and chronic (greater than 1 day) phases of the response of the thyroid gland to cold could be T4 itself. An increase in the concentration of T4 in plasma has been reported to increase peripheral deiodination of T4 to T3 by kidneys and liver of rats. There are no studies at present to indicate that hepatic conjugation can be increased by elevation of plasma levels of T4 and T3. If it can, these responses would provide adequate reasons as to why peripheral metabolism of thyroid hormones increases during chronic exposure to cold. The time-course for these changes to occur needs to be studied in greater detail to establish the sequence of events following acute exposure to cold. The latter may also increase urinary excretion of T4 and T3 in man, but not the rat. This suggests that another aspect of exposure to cold needing additional study is measurement of the binding affinities of T4 and T3 for their transport proteins during exposure to cold as compared to affinities prior to exposure to cold. If binding affinities are reduced, the amount of free hormones would increase and, consequently the likelihood of being excreted into urine and conjugated by the liver would also increase.(ABSTRACT TRUNCATED AT 400 WORDS)

Biosynthesis of thyroid hormone: basic and clinical aspects

Thyroid hormone formation requires the coincident presence of peroxidase, H2O2, iodide, and acceptor protein at one anatomic locus in the cell. The peroxidase enzyme appears to be a protoporphyrin lX containing heme protein, with binding sites for both iodide and tyrosine. It is probable that both iodide and tyrosine are oxidized to free radical forms which unite to form iodotyrosine. The peroxidase is also involved through an uncertain mechanism in iodotyrosine coupling and probably in oxidation of sulfhydryl bonds in thyroglobulin. H2O2 may be supplied by microsomal NADPH-cytochrome c reductase or NADH-cytochrome b5 reductase. Other possible intracellular H2OI generating systems include monoamine oxidase and xanthine oxidase. The usual acceptor for iodide is thyroglobulin, which is currently believed to be iodinated within apical secretory vesicles at the cell border just prior to liberation into the colloid, or possibly after liberation into the colloid. Other soluble an insoluble proteins are also iodinated within the gland. The peroxidase is present in numerous cellular structures, but iodination activity occurs primarily, if not only, at the apical cell border. The controls of iodination are imperfectly known. Thyrotrophin modulation of iodide uptake, H2O2 generation, thyroglobulin synthesis, and peroxidase enzyme level obviously are the main regulations. Many of these actions are thought to involve mediation of adenyl cyclase and subsequent activation of intracellular phosphokinases. Antithyroid drugs of the thiocarbamide group are competitive inhibitors of iodination under some circumstances, but if much iodide is present, they react with the oxidized iodine intermediate and are irreversibly inactivated themselves. Clinical problems involving defective peroxidase function are among the most frequent hereditary defects of thyroid hormone formation. Recognized abnormalities include deficient peroxidase, abnormality in binding of the peroxidase apoprotein to its prosthetic group, and other less well-identified abnormalities in peroxidase structure and function. Peroxidase is typically elevated in thyroid tissue from patients with hyperthyroidism sometimes deficient in cold thyroid nodules, and frequently diminished in tissue from patients with Hashimoto's thyroiditis.