Skeletal Metabolism/Turnover

Bone turnover Skeletal turnover Skeletal metabolism Bone remodelling... 

There is no such clear cut difTcrcnlialiun as metamorphosis in the mammal, but development is an extremely complex process and has been shown to depend upon the presence of adequate amounts of thyroid hormones. Deficient development, especially of the central nervous system, is marked in ehildren suffering from thyroid deficiency early in life, ansi this inadequacy cannot be overcome completely by medication commenced after the first few weeks. In the adult, thyroxine is important in the maintenance of energy turnover in most of the tissues of the body, such as the heart, skeletal muscle, liver, and kidney, Other physiological functions, most notably brain aclivity and reproduction, are also dependent upon thyroxine, although the metabolic rales of the tissues concerned in these functions do not seem to be altered. 

Q3 The thyroid hormones thyroxine and triiodothyronine have many metabolic effects. In adults they increase metabolic rate, oxygen and calorie consumption, stimulate carbohydrate metabolism and turnover of protein, deplete fat stores and increase catabolism of free fatty acids. Thyroid hormones stimulate heart rate and force and increase pulmonary ventilation, gastrointestinal motility and central nervous system (CNS) activity. Actions on the heart can result in an increased incidence of dysrhythmias. Thyroid hormones are critical for the normal growth and development of the infant, particularly in respect of skeletal growth and maturation of the CNS. 

Immunoreactive parathyroid hormone concentrations may be increased by anticonvulsants, while bone mineral content is reduced. Hypocalcemia and osteopenia can occur, despite normal serum concentrations of active vitamin D metabolites, suggesting that they may be independent of drug effects on vitamin D metabolism. Bone biopsies have shown increased osteoid but normal calcification front formation, accelerated rate of mineralization, and reduced mineralization lag time, suggesting increased skeletal turnover rather than osteomalacia (96). The risk of age-related fractures in drug-treated epileptic patients is not greatly increased (97). 

Renal osteodystrophy (ROD)—The condition resulting from sustained metabolic changes that occur with chronic kidney disease including secondary hyperparathyroidism, hyperphosphatemia, hypocalcemia, and vitamin D deficiency. The skeletal complications associated with ROD include osteitis fibrosa cystica (high bone turnover disease), osteomalacia (low bone turnover disease), adynamic bone disease, and mixed bone disorders. 

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). 

Adipose tissue triglycerides are mobilized as their split products — fatty acids and glycerol. The identification of free fatty acids as the main form of transport of fat from adipose tissue has opened a completely new scope to our understanding of energy metabolism. The elevation of this minor plasma constituent to the rank of a major metabolite was made acceptable by the detection of its extremely rapid turnover. Half life times of approximately two minutes were reported for plasma free fatty acids. Free fatty acids are burnt in many organs and can supply a considerable portion of the metabohc energy of heart and skeletal muscle. A detailed discussion on the metabolism of free fatty acids has been published (Fredrickson and Gordon 1958). 

Newman, E., Heslin, M.J., Wolf, R.F., Rsters, RW, and Brennan, M.F., The effect of systemic hyperinsulinemia with concomitant amino acid infusion on skeletal muscle protein turnover in the human forearm. Metabolism, 43, 70, 1994. 

The magnitude of these metabolic pathways is different in the various mammalian species. In man, in steady-state conditions, cholesterol turnover is approximately 0-8-1-2 g/day. Total exchangeable cholesterol in man is about 60 g, and one-third may be attributed to a rapid turning-over pool (A) (including liver, plasma, erythrocytes and, possibly, intestine) the remainder to a slower pool (B) including, notably, skeletal muscle, skin and adipose tissue cholesterol Pool A receives dietary cholesterol and supplies most of the cholesterol for degradation and excretion both compartments synthesize the sterol and interexchange it... 

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