Calcium turnover

Calcium turnover in the bone is a highly dynamic process the bone serves as a reservoir for circulating and intracellular calcium. 

Total calcium turnover Is five to ten times greater in the newborn than in the adult This is reflected in the fact that normalized calcium intake is 5-10 times higher in the newborn than in the adult although the fraction absorbed (Va/Vi) is similar A second observation is that in all five newborns more calcium is lost through the endogenous fecal route than by urinary excretion The reverse is true for typical adults Finally, the is in positive balance for both bone and total organism calcium in contrast to the typical adult ... 

Much has been written about the value of urinary hydroxyproline elevations in separating bone formation from bone resorption. Correlations have been attempted clinically between urinary hydroxyproline, on the one hand, and serum alkaline phosphatase, serum and urinary calcium, calculations of calcium turnover such as the bone formation rate and bone resorption rate, bone histology, and bone radiology. Most authors feel that the level of urinary hydroxyproline refiects bone destruction (B6, B16, D8, D9, Gl, K26, L8, SI 6) more than bone formation (H9, K20). This does not seem to be a very useful argument clinically, because bone formation and destruction are usually occurring simultaneously in... 

K26. Klein, L., Lafferty, F. W., Pearson, O. H., and Curtiss, P. H., Jr., Correlation of urinary hydroxyproline, serum alkaline phosphatase and skeletal calcium turnover. Metab., Clin. Expil. 13, 272-284 (1964). 

Much has been written about the favorable effects of androgenic-anabolic agents on calcium balance, for which they have been widely used. A recent report combined both calcium balance and radioactive calcium turnover studies (25). It again showed the favorable effects of the steroids, which these authors attributed to a decrease in resorption of bone. 

The most important manifestation of hyperfluoric status is dental and skeletal fluorosis. In regions of volcanic activity, and in some arid zones and areas where phosphorites and apatites are mined and processed, agricultural animals and population often suffer from endemic and civilization-related fluorosis that mostly affects the teeth and skeleton. There arises the problem of reproduction and maintenance of dairy cattle (Ermakov etal. 1998, Ermakov 2001). In toxic quantities, fluorides impair the metabolism of calcium and synthesis of bone collagen by stimulating bone accretion, and speed up bone resorption and total calcium turnover in the body (Susheela and Mukher-jee 1981). 

The bone Is composed of two distinct tissue structures cortical (compact) bone, and trabecular (cancellous) bone (3). Eighty percent of the skeleton is composed of cortical bone (e.g., long bones such as the humerus, radius, and ulna) (4,5), which is a relatively dense tissue (80-90% calcified) (4) that provides structure and support (3). Bone marrow cavities, flat bones, and the ends of long bones are all composed of trabecular bone, which Is considerably more porous (5-20% calcified) (4,5). To maintain healthy, well-mineralized bone, a continuous process of bone resorption (loss of ionic calcium from bone) and formation occurs along the bone surface. Cortical bone Is remodeled at the rate of 3% per year, whereas 25% of trabecular bone, which has considerably higher surface area, is remodeled annually (3). In terms of calcium turnover in bone, approximately 500 mg are removed and replaced on a daily basis. 

Phosphorus. Eighty-five percent of the phosphoms, the second most abundant element in the human body, is located in bones and teeth (24,35). Whereas there is constant exchange of calcium and phosphoms between bones and blood, there is very Httle turnover in teeth (25). The Ca P ratio in bones is constant at about 2 1. Every tissue and cell contains phosphoms, generally as a salt or ester of mono-, di-, or tribasic phosphoric acid, as phosphoHpids, or as phosphorylated sugars (24). Phosphoms is involved in a large number and wide variety of metaboHc functions. Examples are carbohydrate metaboHsm (36,37), adenosine triphosphate (ATP) from fatty acid metaboHsm (38), and oxidative phosphorylation (36,39). Common food sources rich in phosphoms are Hsted in Table 5 (see also Phosphorus compounds). 

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. 

Vasodilators dilate or relax the smooth muscles of the vasculatures directly or iadirecfly by releasiag endogenous vasodepressors or antagoni2iag the endogenous vasopressors or vasopressor systems (200,243,244). Vasodilators may iaterfere with the entry, iatraceUular release, and utili2ation of calcium, the activation of the proteia kinase C system, cGMP formation, and EDRF turnover. 

Harrison et al., 1998 10 week old OVX rats low calcium diet treatment starts 2 weeks after OVX Soy protein with isoflavones (6 mg/d) for 4 weeks Reduced bone loss (higher bone weight, higher calcium content) but no decrease of markers of bone turnover for the soy group... 

The most dramatic consequence of sHPT is alterations in bone turnover and the development of ROD. Other complications of CKD can also promote ROD. Metabolic acidosis decreases bone formation and aluminum toxicity causes aluminum uptake into bone in place of calcium, weakening the bone structure. The pathogenesis of sHPT and ROD are depicted in Fig. 23-5. 

Parathyroidectomy is a treatment of last resort for sHPT, but should be considered in patients with persistently elevated iPTH levels above 800 pg/mL (800 ng/L) that is refractory to medical therapy to lower serum calcium and/or phosphorus levels.39 A portion or all of the parathyroid tissue may be removed, and in some cases a portion of the parathyroid tissue may be transplanted into another site, usually the forearm. Bone turnover can be disrupted in patients undergoing parathyroidectomy whereby bone production outweighs bone resorption. The syndrome, known as hungry bone syndrome, is characterized by excessive uptake of calcium, phosphorus, and magnesium for bone production, leading to hypocalcemia, hypophosphatemia, and hypomagnesemia. Serum ionized calcium levels should be monitored frequently (every 4 to 6 hours for the first 48 to 72 hours) in patients receiving a parathyroidectomy. Calcium supplementation is usually necessary, administered IV initially, then orally (with vitamin D supplementation) once normal calcium levels are attained for several weeks to months after the procedure. 

Thiazide diuretics decrease urinary calcium excretion and may decrease bone turnover. However, their effects on bone mineral density and fracture rates have not been studied in controlled trials. Thiazide diuretics are not recommended solely for potential beneficial effects in osteoporosis. 

Marcus, R., Agents affecting calcification and bone turnover calcium, phosphate, parathyroid hormone, vitamin D, calcitonin, and other compounds, in Goodman and Gilman s The Pharmacological Basis of Therapeutics, 9th ed., Hardman, J.G. and Limbird, L.E., Eds., McGraw-Hill, New York, 1996, chap. 61. 

In a study with 40 healthy men and women, average age 63.7 years, who were randomized to either an alkali diet (meat plus fruits and vegetables) or an acid diet (meat plus cereal grains) (Jajoo and others 2006), altering the renal net acid excretion over a period of 60 days affected several biochemical markers of bone turnover and calcium excretion. The acidity of the diet had a significant effect on increasing NTX, a urinary marker of bone breakdown, and increasing the amount of calcium excreted in the urine. 

Lin P-H, Ginty F, Appel L, Aickin M, Bohannon A, Gamero P, Barcaly D and Svetkey L. 2003. The DASH diet and sodium reduction improve markers of bone turnover and calcium metabolism in adults. 

HRP C contains two different types of metal center (i.e., iron(III) protoporphyrin IX-heme group and two calcium atoms) that are fundamental for the integrity of the enzyme. The heme group is attached to the enzyme at His 170 by a coordinate bond between the histidine side-chain NE2 atom and the heme iron atom. The second axial coordination site is unoccupied in the resting state of the enzyme but available to hydrogen peroxide during enzyme turnover. Small molecules such as carbon monoxide, cyanide, fluoride, and azide bind to the heme iron atom at this distal site, giving six-coordinated PX complexes. 

A few enzymes, such as the previously mentioned CNP, are believed to be fairly specific for myelin/oligodendro-cytes. There is much more in the CNS than in peripheral nerve, suggesting some function more specialized to the CNS. In addition, a unique pH 7.2 cholesterol ester hydrolase is also enriched in myelin. On the other hand, there are many enzymes that are not myelin-specific but appear to be intrinsic to myelin and not contaminants. These include cAMP-stimulated kinase, calcium/calmodulin-dependent kinase, protein kinase C, a neutral protease activity and phosphoprotein phosphatases. The protein kinase C and phosphatase activities are presumed to be responsible for the rapid turnover of MBP phosphate groups, and the PLP acylation enzyme activity is also intrinsic to myelin. 

Age-related osteoporosis occurs mainly because of hormone, calcium, and vitamin D deficiencies leading to accelerated bone turnover and reduced osteoblast formation. 

One can see that for calcium, potassium, and silicon, biogeochemical turnover is within the limits of 10-30 kg/ha per year. The turnover for magnesium, phosphorus, manganese, sulfur, and aluminum is less than 10 kg/ha per year. These values are about 1 kg/ha per year for iron and sodium. These values can characterize the safety limits of exposure to the given species. 

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