Phosphate, absorption excretion

Vitamin D. Vitamin D is a steroidlike hormone that can be obtained from dietary sources or synthesized in the skin from cholesterol derivatives in the presence of ultraviolet light. Vitamin D produces several metabolites that are important in bone mineral homeostasis.27,31 In general, vitamin D derivatives such as 1,25 dihydroxyvitamin D3 increase serum calcium and phosphate levels by increasing intestinal calcium and phosphate absorption and by decreasing renal calcium and phosphate excretion.27,46... 

Although, owing to the wide distribution of vitamin Bg in nature, clinical deficiency symptoms are seldom observed, there is little doubt that pyridoxine is essential in human nutrition. Pyridoxine is absorbed from the gastrointestinal tract and is converted to the active form pyri-doxal phosphate. Absorption is decreased in gastrointestinal diseases and also in subjects taking isoniazid (3). It is excreted in the urine as 4-pyridoxic acid (2). The metabolism of vitamin Bg in human beings has been investigated (56). 

Patel VK, Emmett M, Santa Ana C, Fordtran JS. Pathogenesis of nephrocalcinosis after sodium phosphate catharsis to prepare for colonoscopy Intestinal phosphate absorption and its effect on urine mineral and electrolyte excretion. Fluman Pathol 2007 38 193-4. 

Thus, a complex relationship exists among serum Ca + and phosphate, PTH, and vitamin D and its metabolites. Release of PTH in response to low serum Ca + directly mobilizes calcium from bone and increases synthesis of 1, 25-(0H)2D, which in turn mobilizes skeletal Ca + and causes increased intestinal calcium absorption. These effects raise the serum Ca level sufficiently to reduce PTH secretion. The effect of PTH on the kidneys occurs within minutes, whereas the effects of PTH on bone and (indirectly) on intestine take hours and days, respectively. An increase in serum phosphate acts in a way qualitatively similar to that of hypocalcemia to release PTH, increase excretion of phosphate in the proximal tubules, and decrease intestinal phosphate absorption. These events are mediated predominantly by the decrease in serum calcium that accompanies a rise in phosphate concentration. In addition, phosphate may inhibit 25-(OH)D-la-hydroxylase. 

Effects on Kidney In the kidney, PTH enhances the efficiency of Ca reabsorption, inhibits tubular reabsorption of phosphate, and stimulates conversion of 25-OHD to calcitriol (Figure 61-3, see below). As a result, filtered Ca + is avidly retained and its plasma concentration increases, whereas phosphate is excreted and its plasma concentration falls. Newly synthesized calcitriol interacts with specific high-affinity vitamin D receptors (VDRs) in the intestine to increase the efficiency of calcium absorption, thereby also increasing the plasma Ca concentration. 

PTH increases serum calcium levels by two other mechanisms. PTH increases the synthesis of the active form of vitamin D, 1,25-dihyroxycholecalciferol, in the kidney, which in turn stimulates production of calcium binding protein. Calcium binding protein enhances calcium phosphate absorption from the gut lumen. PTH also inhibits renal calcium excretion, while promoting phosphate excretion, causing a small increase in serum calcium levels. 

Fosrenol decreases phosphate absorption in the intestines and is excreted in the feces. 

The amount of each element required in daily dietary intake varies with the individual bioavailabihty of the mineral nutrient. BioavailabiUty depends both on body need as deterrnined by absorption and excretion patterns of the element and by general solubiUty, and on the absence of substances that may cause formation of iasoluble products, eg, calcium phosphate, Ca2(P0 2- some cases, additional requirements exist either for transport of substances or for uptake or binding. For example, calcium-binding proteias are iavolved ia calcium transport an intrinsic factor is needed for vitamin cobalt,... 

Hydroxy vitamin D pools ia the blood and is transported on DBF to the kidney, where further hydroxylation takes place at C-1 or C-24 ia response to calcium levels. l-Hydroxylation occurs primarily ia the kidney mitochondria and is cataly2ed by a mixed-function monooxygenase with a specific cytochrome P-450 (52,179,180). 1 a- and 24-Hydroxylation of 25-hydroxycholecalciferol has also been shown to take place ia the placenta of pregnant mammals and ia bone cells, as well as ia the epidermis. Low phosphate levels also stimulate 1,25-dihydtoxycholecalciferol production, which ia turn stimulates intestinal calcium as well as phosphoms absorption. It also mobilizes these minerals from bone and decreases their kidney excretion. Together with PTH, calcitriol also stimulates renal reabsorption of the calcium and phosphoms by the proximal tubules (51,141,181—183). 

Disopyr mide. Disopyramide phosphate, a phenylacetamide analogue, is a racemic mixture. The dmg can be adininistered po or iv and is useful in the treatment of ventricular and supraventricular arrhythmias (1,2). After po administration, absorption is rapid and nearly complete (83%). Binding to plasma protein is concentration-dependent (35—95%), but at therapeutic concentrations of 2—4 lg/mL, about 50% is protein-bound. Peak plasma concentrations are achieved in 0.5—3 h. The dmg is metabolized in the fiver to a mono-AJ-dealkylated product that has antiarrhythmic activity. The elimination half-life of the dmg is 4—10 h. About 80% of the dose is excreted by the kidneys, 50% is unchanged and 50% as metabolites 15% is excreted into the bile (1,2). 

Phosphate-Binding Agents When serum phosphorus levels cannot be controlled by restriction of dietary intake, phosphate-binding agents are used to bind dietary phosphate in the GI tract to form an insoluble complex that is excreted in the feces. Phosphorus absorption is decreased, thereby... 

Abou-Donia MB, Nomeir AA, Bower JH, et al. 1990b. Absorption, distribution, excretion, and metabolism of a single oral dose of [14C] tri-ort/zo-cresyl phosphate (TOCP) in the male rat. Toxicology 65 61-74. 

Gatz. 1992a. Intravenous and dermal absorption, distribution, and excretion of 14C-tributyl phosphate in Yucatan minipigs Part I. MRI Project No. 9526-F(02). 

The a ns wer is a. (Hardman, pp 1525-1528.) Pa r a thyroid ho r m o ne is synthesized by and released from the parathyroid gland increased synthesis of PTI1 is a response to low serum Ca concentrations. Resorption and mobilization of Ca and phosphate from bone are increased in response to elevated PTI1 concentrations. Replacement of body stores of Ca is enhanced by the capacity of PTH to promote increased absorption of Ca by the small intestine in concert with vitamin D, which is the primary factor that enhances intestinal Ca absorption. Parathyroid hormone also causes an increased renal tubular reabsorption of Ca and excretion of phosphate. As a consequence of these effects, the extracellular Ca concentration becomes elevated. 

Various factors may be associated with variations in calcium needs differences in vitamin D supply, differences in absorption and excretion, differences in activity of the parathyroid glands, differences in steroid hormone production, differences in thyroid function, differences in phosphate supply and utilization. 10 These we will not discuss, although these considerations may make it possible, in individual cases, to circumvent extra needs for calcium by removing the basis for the augmented need. We are here concerned primarily with the fact that individual people, under prevalent conditions, require amounts of calcium which may vary from individual to individual by a factor of 5. 

T. L. Morton, E. A. Murrill, J. M. Cannon, J. L. Skaptason, E. C. Bisinger, V. Reddy, R. N. Gatz, Absorption, Distribution, Metabolism and Excretion of 14C-Tributyl Phosphate in Sprague-Dawley Rats Following Dermal, Oral and Intravenous Administration , in Proceedings of the Fifth North American ISSX Meeting , Tucson, Arizona, USA, October 17-21, 1993, p. 196. 

In two cases of moderate intoxication from mevinphos, urinary excretion of dimethylphos-phate (a metabolite of mevinphos) was almost complete 50 hours after exposure/ Although a number of other organophosphorus pesticides also yield dimethyl phosphate, the presence of significant amounts of this metabolite in the urine may be useful in estimating the absorption of mevinphos. 

Mechanism of Action An antacid that reduces gastric acid by binding with phosphate in the intestine, and then is excreted as aluminum carbonate in feces. Aluminum carbonate may increase the absorption of calcium due to decreased serum phosphate levels. The drug also has astringent and adsorbent properties. Therapeutic Effect Neutralizes or increases gastric pH reduces phosphates in urine, preventing formation of phosphate urinary stones reduces serum phosphate levels decreases fluidity of stools. 

Calcium and phosphate enter the body from the intestine. The average American diet provides 600-1000 mg of calcium per day, of which approximately 100-250 mg is absorbed. This figure represents net absorption, because both absorption (principally in the duodenum and upper jejunum) and secretion (principally in the ileum) occur. The amount of phosphorus in the American diet is about the same as that of calcium. However, the efficiency of absorption (principally in the jejunum) is greater, ranging from 70% to 90%, depending on intake. In the steady state, renal excretion of calcium and phosphate balances intestinal absorption. In general, over 98% of filtered calcium and 85% of filtered phosphate is reabsorbed by the kidney. The movement of calcium and phosphate across the intestinal and renal epithelia is closely regulated. Intrinsic disease of the intestine (eg, nontropical sprue) or kidney (eg, chronic renal failure) disrupts bone mineral homeostasis. 

---