Iodide secretion

Sodium Iodide Symporter (NIS) in Gastric Mucosa Gastric Iodide Secretion... 

Studies on bovines showed a recirculation of iodide, and this was suggested to be an important iodide-conserving mechanism (Miller et ai, 1975b). The functional role of iodide secretion into the gastric lumen has, however, remained elusive. 

Iodide secretion into the gastric lumen is mediated by NIS. 

Because investigators were chiefly interested in the behavior of in tracer amounts, no one made a systematic study of iodide secretion over a wide range on plasma concentrations. In fact, one pair of investigators said that unlike thyroid accumulation of iodide, gastric secretion of iodide is not depressed by administration of stable iodide. The data in Figure 1-14 were available to contradict them. 

Brown-Grant K, Gumming JD, Haigh AL, et al. The effect of intravenous infusion of histamine upon iodide secreted by the stomach of the anaesthetized dog. J Physiol Land 168 1 IP-12P, 1963. 

Only the small amounts of T and T that are free in the circulation can be metabolized. The main route is deiodination of T to T and i-T, and from these to other inactive thyronines (21). Most of the Hberated iodide is reabsorbed in the kidney. Another route is the formation of glucuronide and sulfate conjugates at the 4 -OH in the Hver. These are then secreted in the bile and excreted in the feces as free phenols after hydrolysis in the lower gut. 

In addition, Pfister and coworkers investigated 3-hydroxyflavone-6-carboxylic acids as histamine induced gastric secretion inhibitors. After condensing 3-acetyl-4-hydroxybenzoic acid (45) with a variety of aldehydes 46 to deliver the chalcones 47, these purified chalcones were then subjected to the standard AFO conditions to afford flavonols 48 in 51-80% yield. Subsequent alkylation of 48 with methyl iodide or isopropyl iodide followed by saponification of the corresponding esters gave the target compounds. 

This clearly indicates that the antimicrobial activity of the stomach acid is effected by a range of dietary compounds and may be regulated by both salivary (N03, thiothyanate, iodide) and stomach (e.g. ascorbic acid) secretions. 

Thyroxine synthesis begins when iodide (I-) is transferred from the blood stream to the thyroid follicle cell by an active ATP-driven membrane pump mechanism this process is stimulated by cAMP following TSH stimulation of the gland. Iodide is transported through the follicular cell and secreted into the lumen of the follicle where it is oxidized to iodine and incorporated in to tyrosine residues by the enzyme thyroid peroxidase (TPO). 

Breast milk During lactation human mammary tissue expresses the sodium iodide symporter [260], and thus significant transfer of perchlorate into human milk is likely. The presence of micrograms per liter concentrations of perchlorate in milk collected fi om US women [233] confirms lactation as a relevant perchlorate excretion path. If lactating women are secreting perchlorate in milk, then urine-based estimates of total perchlorate exposure for these individuals are likely to be lower than actual [242]. 

Clinical applications include preoperative suppression of thyroid secretion according to Plummer with Lugol s solution (5% iodine + 10% potassium iodide, 50-100 mg iodine/d for a maximum of 10 d). In thyrotoxic crisis, LugoTs solution is given together with thioamides and p-blockers. Adverse effects allergies contraindications iodine-induced thyrotoxicosis. 

Mucous airway obstruction. Mu-colytics, such as acetylcysteine, split disulfide bonds in mucus, hence reduce its viscosity and promote clearing of bronchial mucus. Other expectorants (e.g., hot beverages, potassium iodide, and ipecac) stimulate production of watery mucus. Acetylcysteine is indicated in cystic fibrosis patients and inhaled as an aerosol. Whether mucolytics are indicated in the common cold and whether expectorants like bromohexine or am-broxole effectively lower viscosity of bronchial secretions may be questioned. 

Nitropolyzonamine (49) (Table IV) can be isolated as colorless crystals from the secretion of the millipede, P. rosalbum, by preparative gas chromatography or as its crystalline perchlorate from an ethereal solution of the crude defensive secretion. The structure and stereochemistry have been determined by an X-ray analysis of the base perchlorate 153). Racemic nitropolyzonamine (49) can be synthesized from polyzonimine (19). Treatment of polyzonimine (19) with 3-nitropropyl iodide gives a crystalline salt (344) which is readily cyclized to racemic nitropolyzonamine (49) in boiling pyridine (Scheme 37) 153). 

A second dietary trace element, selenium, is also essential for normal thyroid hormone metabohsm. Selenium in the form of selenocysteine is a required component for three enzymes that remove iodide from thyroid hormones. Deiodination is the major metabohc pathway by which T4 and T3 are cleared from the system. After secretion by the thyroid gland, T4 may be deiodinated to yield either T3 or the physiologically inactive reverse Tj (3,3, 5 -triiodothyronine, or rX3). T3 and rTj are further deiodinated to form less active metabolites. Selenium, like iodine, is deficient in many areas of the world. 

Thyroid epithelial cells synthesize and secrete T4 and T3 and make up the functional units of thyroid glandular tissue, the thyroid follicles. Thyroid follicles are hollow vesicles formed by a single layer of epithelial cells that are filled with colloid. T4,T3, and iodine are stored in the follicular colloid. T4 and T3 are derived from tyrosyl residues of the protein thyroglobulin (Tg). Thyroid follicular cells synthesize and secrete Tg into the follicular lumen. Thyroid follicular cells also remove iodide (I ) from the blood and concentrate it within the follicular lumen. Within the follicles, some of the tyrosyl residues of Tg are iodinated, and a few specific pairs of iodoty-rosyl residues may be coupled to form T4 and T3. Thus, T4, T3, and iodine (in the form of iodinated tyrosyl residues) are found within the peptide structure of the Tg that is stored in the follicular lumen. 

Lithium inhibits thyroidal incorporation of L into Tg, as well as the secretion of thyroid hormones, but it does not inhibit the activity of the Na+-I symporter or the accumulation of I within the thyroid. Lithium offers no particular advantage over drugs of the thionamide class but may be employed for temporary control of thyrotoxicosis in patients who are allergic to both thion-amides and iodide. 

The excretion of drugs through sweat and saliva is primarily dependent upon the diffusion of the non-ionized, lipophilic form of the drug across the epithelial cells of the glands. The compounds like lithium, potassium iodide and heavy metals are present in these secretions. 

Potassium salts (as iodide) act by both direct action and reflexly to increase the respiratory secretions and decrease its viscosity thus they are easy to expel out. Potassium iodide is generally used for cough associated with chronic bronchitis and asthma but it interferes with thyroid function tests, so it is dangerous in patients... 

Iodide. Relatively large dosages of iodide (exceeding 6 mg/d) cause a rapid and dramatic decrease in thyroid function.35 In sufficient amounts, iodide inhibits virtually all the steps involved in thyroid hormone biosynthesis. For instance, high iodide levels limit the uptake of iodide into thyroid follicle cells, inhibit the formation of T4 and T3, and decrease the secretion of the completed hormones from the thyroid cell. 

Iodide, ingested from food, water, or medication, is rapidly absorbed and enters an extracellular fluid pool. The thyroid gland removes about 75 ug a day from this pool for hormone secretion, and the balance is excreted in the urine. If iodide intake is increased, the fractional iodine uptake by the... 

Regulation of secretion Secretion of TSH by the anterior pituitary is stimulated by the hypothalamic TRH. Feedback inhibition of both TRH and TSH secretion occurs with high levels of circulating thyroid hormone or iodide. Most of the hormone (T3 and T4) is bound to thyroxine-binding globulin in the plasma. 

Correct answer = O. Propylthiouracil blocks the synthesis of the thyroid hormones, but does not affect the uptake of iodide, proteolytic cleavage of thyroglobulin, or the release of hormones from the thyroid gland. The thyroid hormones inhibit the secretion of thyroid-stimulating hormone from the anterior pituitary. ... 

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