Hormone regulation, calcium

In addition to these hormonal regulators, calcium and phosphate themselves, other ions such as sodium and fluoride, and a variety of drugs (bisphosphonates, plicamycin, and diuretics) also alter calcium and phosphate homeostasis. 

How does calcium work its wonders on blood pressure This gets a bit technical. The mineral reduces the concentration of parathyroid hormone in the blood that hormone regulates calcium metabolism. In turn, that might lower calcium concentrations in the body s cells and slow the calcium from entering the arteries. Calcium in the arteries affects the tone of the vessels, thus potentially leading to higher blood pressure as the arteries stiffen. 

Vitamin D is produced endogenously by exposure of skin to sunlight, and is absorbed from foods containing or supplemented with vitamin D. Vitamin D is metabolized to its biologically active form, 1,25-dihydroxyvitamin D (1,25[OH]2D), a hormone regulating calcium and phosphate metabolism. Deficiency of vitamin D results in impaired formation of bone, because of a mineralizing defect, producing rickets in children and osteomalacia in adults. 

Parathyroid hormone along with vitamin D and calcitonin (a polypeptide hormone) regulate calcium metabolism. 

Parathyroid Glands Small glands, usually located within the thyroid gland, which produce parathyroid hormone this hormone regulates calcium levels in the bloodstream and bones. 

Three hormones regulate turnover of calcium in the body (22). 1,25-Dihydroxycholecalciferol is a steroid derivative made by the combined action of the skin, Hver, and kidneys, or furnished by dietary factors with vitamin D activity. The apparent action of this compound is to promote the transcription of genes for proteins that faciUtate transport of calcium and phosphate ions through the plasma membrane. Parathormone (PTH) is a polypeptide hormone secreted by the parathyroid gland, in response to a fall in extracellular Ca(Il). It acts on bones and kidneys in concert with 1,25-dihydroxycholecalciferol to stimulate resorption of bone and reabsorption of calcium from the glomerular filtrate. Calcitonin, the third hormone, is a polypeptide secreted by the thyroid gland in response to a rise in blood Ca(Il) concentration. Its production leads to an increase in bone deposition, increased loss of calcium and phosphate in the urine, and inhibition of the synthesis of 1,25-dihydroxycholecalciferol. 

Vitamin A (retinol), present in carnivorous diets, and the provitamin (P-carotene), found in plants, form retinaldehyde, utilized in vision, and retinoic acid, which acts in the control of gene expression. Vitamin D is a steroid prohormone yielding the active hormone derivative calcitriol, which regulates calcium and phosphate metaboUsm. Vitamin D deficiency leads to rickets and osteomalacia. 

Parathyroid hormone (PTH) regulates calcium levels in blood and bone remodeling. The activation domain of that 84-amino acid polypeptide locates around the N-terminal (1-34 amino acids). Parathyroid hormone receptor is a typical G-protein coupled receptor, which is coupled to both adenyl cyclase/cAMP and PLCy/IP3/cytosolic Ca2+ intracellular signaling pathways. In order to identify the structural elements involved in the peptide hormone binding and signal initiation, Chorev et al. employed a photoaffinity scanning approach. The N-terminal amino acids were successively deleted or modified and the new N-terminus was replaced for photoreactive Bpa. The most active peptide ana-... 

The parathyroid hormone content of blood has not been studied sufficiently to yield any data with regard to variation. The functioning of the glands is so closely related to other factors which regulate calcium and phosphorus metabolism that it is impossible to assign differences in these areas to variation in parathyroid function. The variation of the calcium (and phosphorus) in the blood has been noted (p. 55), and this variation, of course, may be due in a substantial degree to differences in parathyroid functioning. 

The main inorganic components of the urine are the cations Na"", C, Ca "", Mg and NH4 and the anions Cl , S04 , and HP04 , as well as traces of other ions. In total, Na"" and Cl represent about two-thirds of all the electrolytes in the final urine. Calcium and magnesium occur in the feces in even larger quantities. The amounts of the various inorganic components of the urine also depend on the composition of the diet. For example, in acidosis there can be a marked increase in the excretion of ammonia (see p. 326). Excretion of Na C, and phosphate via the kidneys is subject to hormonal regulation (see p. 330). 

Vitamin D3, also called cholecalciferol, is normally formed in the skin from 7-dehydrocholesterol in a photochemical reaction driven by the UV component of sunlight (Fig. 10-20). Vitamin D3 is not itself biologically active, but it is converted by enzymes in the liver and kidney to 1,25-dihydroxycholecalciferol, a hormone that regulates calcium uptake in the intestine and calcium levels in kidney and bone. Deficiency of vitamin D... 

Vitamin D. The term vitamin D refers to a group of seco-steroids that possess a common conjugated triene system of double bonds. Vitamin I), (10a) and vitamin D, (10b) are the best-known examples (Fig. 2). Vitamin D (10a) is found primarily in vertebrates, whereas vitamin 11 (10b) is found primarily in plants. The term vitamin is a misnomer. Vitamin I) is a prohormonc that is converted into physiologically active form, primarily 1.25-dihydroxy vitamin D3 (11), by successive hydroxylalions in the liver and kidney. This active form is part of a hormonal system that regulates calcium and phosphate metabolism in the target tissues. 

PTH works with two other primary hormones— calcitonin and vitamin D—in regulating calcium homeostasis. These three hormones, as well as several other endocrine factors, are all involved in controlling calcium levels for various physiologic needs. How these hormones interact in controlling normal bone formation and resorption is of particular interest to rehabilitation specialists. Regulation of bone mineral homeostasis and the principal hormones involved in this process are presented in the following section. 

Consequently, bone is a rather dynamic tissue that is constantly undergoing changes in mineral content and internal structure. The balance between bone resorption and formation is controlled by the complex interaction of local and systemic factors. In particular, several hormones regulate bone formation and help maintain adequate plasma calcium levels. The primary hormones involved in regulating bone mineral homeostasis are described below. 

Vitamin D regulates calcium and phosphorus absorption and deposition and serum alkaline phosphatase levels. The recommended daily allowance is 5 /xg, increasing to 10 to 15 /xg in older age.109 Vitamin D3 is synthesized under UVB irradiation in the skin where it is stored and released into the circulation in a complex with the vitamin D binding protein. In liver it is hydroxylated to 25(OH)-cholecalciferol, the hormonal precursor, followed by another hydroxylation step in the... 

The fact that vitamin D3 toxicity results from primarily uncontrolled intestinal calcium absorption suggests that it is dietary calcium and not vitamin D3 that exacerbates the hypervitaminosis D3 toxicity effect [119]. This was tested by the interaction of excess vitamin D3 and calcium restriction [113]. Rats fed a calcium-deficient diet and given 25,000 IU of vitamin D3 three dmes/week for 2.5 weeks did not succumb to overt hypervitaminosis D3. Simple calcium restriction increased intestinal but not renal 24-OHase activity, presumably because of the absence of parathyroid hormone regulation in the intestine [113]. Coupled with vitamin D3, excess intestinal 24-OHase increased several fold more. However, when dietary calcium was adequate, vitamin D3 excess increased intestinal 24-OHase activity only slightly because of a suppressive mechanism regulated in part by increased blood calcitonin [120],... 

The parathyroid hormone (PTH), calcitonin, and vitamin D regulate calcium. 

In this chapter, after an overview of bone and mineral metabolism, we discuss the clinical chemistry of calcium, phosphate, and magnesium the hormones regulating these minerals the major disorders of bone and the clinical use of markers of bone formation and degradation. A goal of the chapter is to provide, in one place, a view of these highly interrelated topics. 

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