Calcium hormonal regulation

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. 

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). 

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. 

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. 

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],... 

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. 

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. 

Bone contains nearly all of the calcium (99%), most of the phosphate (85%), and much of the magnesium (55%) of the body. The concentrations of calcium, phosphate, and magnesium in plasma are dependent on the net effect of bone mineral deposition and resorption, intestinal absorption, and renal excretion. PTH and 1,25-dihydroxyvitamin D are the principal hormones regulating these three processes. 

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. 

Fig. 10.12 Combined actions of parathyroid hormone, calcitriol, and calcitonin on the calcium ion concentration of blood plasma (From Figure 13-11 in Biochemistry a case-oriented approach, 4th Ed. 1983. Authors Montgomery R, Dryer RL, Conway, TW and Spector AA. Chap. 13, Hormonal regulation of metabolism, page 675. The C.V. Mosby Co., St Louis, MO. Copyright Elsevier (2009))...

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

The parathyroid hormone regulates ionic calcium homeostasis in extracellular fluids by a direct action on the skeleton. It also promotes the renal excretion of phosphorus and produces other physiological effects which are not obviously related to its calcemic action. 

V. HORMONAL REGULATION OF THE SMOOTH MUSCLE CALCIUM CHANNEL... 

Michigami, X, H. Yamato, H. Suzuki, Y. Nagai-Itagaki, K. Sato, and K. Ozono (2001). Conflicting actions of parathyroid hormone-related protein and serum calcium as regulators of 25-hydroxyvitamin Dj-la-hydroxylase expression in a nude rat model of humoral hypercalcemia of malignancy. J. Endocrinol. 171, 249-257. 

Eckermann-Ross, C. 2008. Hormonal regulation and calcium metabolism in rabbits. Veterinary Climes of North America Exotic Animal Practice 11 139-152. 

Schoemnakers, 1., R. C. Nap, J. A. Mol, and H. A. Hazewinkel. 1999. Calcium metabohsm An overview of its hormonal regulation and interrelation with skeletal integrity. Veterinary Quarterly 21 147-153. 

Similarly, approximately 500 to 600 g of phosphate are present, 85% of which Is found in the bone. The normal plasma concentration of calcium Is approximately 4.5 to 5.7 mEq/L, 50% of which Is protein bound. The remainder of the calcium is either complexed to corresponding counterions (46%) or exists in its ionized form (4%). It Is only the ionized form of calcium that is tightly hormonally regulated (varies less than 5-10%) (1,2). Because serum calcium concentrations fluctuate, so do the plasma levels of the hormones associated with calcium homeostasis. Serum phosphorous levels vary with age, diet, and hormonal status. The most common... 

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