Acinar cells, thyroid

Iodide transport from the blood into the thyroid acinar cell is accomplished through an energy-requiring, iodide-trapping mechanism that is poorly defined but may involve the Na, K -ATPase coupled to a cotransporter for Na and iodide in the plasma membrane of the acinar cell. 

The iodide concentrating or trapping process present in the plasma membrane of thyroid acinar cells creates iodide levels within the thyroid cell that are several hundredfold greater than those in the blood, depending on the current size of the total body iodide pool and the present need for new hormone synthesis. 

The oxidation of intracellular iodide is catalyzed by thyroid peroxidase (located at the apical border of the thyroid acinar cell) in what may be a two-electron oxidation step forming 1 (iodinium ion), lodinium ion may react with a tyrosine residue in the protein thyroglobulin to form a tyrosine quinoid and then a 3 -monoiodotyrosine (MIT) residue. It has been suggested that a second iodide is added to the ring by similar mechanisms to form a 3,5-diiodotyrosine (DIT) residue. Because iodide is added to these organic compounds, iodination is also referred to as the organification of iodide. ... 

The release of T3 and T4 from thyroglobulin is controlled by thyroid-stimulating hormone (TSH) from the anterior pituitary. TSH stimulates the endocytosis of thyroglobulin to form endocytic vesicles within the thyroid acinar cells (see Fig. 43.10). Lysosomes fuse with these vesicles, and lysosomal proteases hydrolyze thyroglobulin, releasing free T4 and T3 into the blood in a 10 1 ratio. In various tissues, T4 is deiodinated, forming T3, which is the active form of the hormone. 

The predominant mechanism of action of TSH is mediated by binding of TSH to its specific receptor on the plasma membrane of the thyroid acinar cell, leading to an increase in the concentration of cytosolic cAMP. Recent evidence indicates, however, that TSH also increases the cellular levels of inositol trisphosphate and diacylglycerol, causing a rise in cytosolic within the thyroid cell. 

Notes-. Experimental results showing the increase in weight of thyroids and increase in acinar cell height in response to a decrease in iodine intake, even in hypophysectomized rats (Chapman, 1941). Identifies a statistically significant difference with respect to the corresponding control group on iodine supplements. 

The distribution of HMFGPs is such that many, but not all, non-neoplastic human epithelial cells express at least one member of this protein family. Exceptions include the gastrointestinal surface epithelium, endocer-vical epithelium, prostatic acinar epithelium, epididymis, germ cells, hepatocytes, adrenal cortical cells, rete testis, squamous cells of the epidermis, and thyroid follicular epithelium. 

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