Renal Vitamin

It is possible that lead s interference with heme synthesis may underlie the effects on vitamin D metabolism. Evidence that lead affects heme synthesis in the kidney was presented in the section on hematological effects. In addition, apparent thresholds for the effects of lead on renal vitamin D metabolism and for erythrocyte protoporphyrin accumulation are similar. 

Vitamin A homeostasis was altered in rats that were exposed to 100 mg/kg/day (only tested dose) of PCB 169 in the diet for 77 days (Bank et al. 1989). Effects included significantly decreased hepatic vitamin A, increased renal vitamin A, increased serum retinol, decreased plasma clearance and half-time of injected retinol (i.e., intravenously administered [ Hjretinol-labeled retinol binding protein-transthyretin complex), decreased hepatic and increased renal uptake uptake of injected retinol, and increased urinary and fecal excretion of injected retinol. 

The vitamin D3 metabolite la,25-dihydroxycholecalciferol is a lifesaving drug in treatment of defective bone formation due to renal failure. Retrosynthetic analysis (E.G. Baggjolint, 1982) revealed the obvious precursors shown below, a (2-cyclohexylideneethyl)diphenylphosphine oxide (A) and an octahydro-4f/-inden-4-one (B), to be connected in a Wittig-Homer reaction (cf. section 1.5). 

Parathyroid hormone, a polypeptide of 83 amino acid residues, mol wt 9500, is produced by the parathyroid glands. Release of PTH is activated by a decrease of blood Ca " to below normal levels. PTH increases blood Ca " concentration by increasing resorption of bone, renal reabsorption of calcium, and absorption of calcium from the intestine. A cAMP mechanism is also involved in the action of PTH. Parathyroid hormone induces formation of 1-hydroxylase in the kidney, requited in formation of the active metabolite of vitamin D (see Vitamins, vitamin d). 

Phosphorus Disorders. Phosphoms nutrient deficiency can lead to rickets, osteomalacia, and osteoporosis, whereas an excess can produce hypocalcemia. Faulty utilisation of phosphoms results in rickets, osteomalacia, osteoporosis, and Paget s disease, and renal or vitamin D-resistant rickets. 

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). 

Although it is being found that vitamin D metaboUtes play a role ia many different biological functions, metaboHsm primarily occurs to maintain the calcium homeostasis of the body. When calcium semm levels fall below the normal range, 1 a,25-dihydroxy-vitainin is made when calcium levels are at or above this level, 24,25-dihydroxycholecalciferol is made, and 1 a-hydroxylase activity is discontiaued. The calcium homeostasis mechanism iavolves a hypocalcemic stimulus, which iaduces the secretion of parathyroid hormone. This causes phosphate diuresis ia the kidney, which stimulates the 1 a-hydroxylase activity and causes the hydroxylation of 25-hydroxy-vitamin D to 1 a,25-dihydroxycholecalciferol. Parathyroid hormone and 1,25-dihydroxycholecalciferol act at the bone site cooperatively to stimulate calcium mobilization from the bone (see Hormones). Calcium blood levels are also iafluenced by the effects of the metaboUte on intestinal absorption and renal resorption. 

Chronic renal disease Hypophosphatemic vitamin D-resistant rickets Vitamin D-dependent rickets... 

In the treatment of diseases where the metaboUtes are not being deUvered to the system, synthetic metaboUtes or active analogues have been successfully adrninistered. Vitamin metaboUtes have been successfully used for treatment of milk fever ia catde, turkey leg weakness, plaque psoriasis, and osteoporosis and renal osteodystrophy ia humans. Many of these clinical studies are outlined ia References 6, 16, 40, 51, and 141. The vitamin D receptor complex is a member of the gene superfamily of transcriptional activators, and 1,25 dihydroxy vitamin D is thus supportive of selective cell differentiation. In addition to mineral homeostasis mediated ia the iatestiae, kidney, and bone, the metaboUte acts on the immune system, P-ceUs of the pancreas (iasulin secretion), cerebellum, and hypothalamus. 

PTH has a dual effect on bone cells, depending on the temporal mode of administration given intermittently, PTH stimulates osteoblast activity and leads to substantial increases in bone density. In contrast, when given (or secreted) continuously, PTH stimulates osteoclast-mediated bone resorption and suppresses osteoblast activity. Further to its direct effects on bone cells, PTH also enhances renal calcium re-absorption and phosphate clearance, as well as renal synthesis of 1,25-dihydroxy vitamin D. Both PTH and 1,25-dihydroxyvitamin D act synergistically on bone to increase serum calcium levels and are closely involved in the regulation of the calcium/phosphate balance. The anabolic effects of PTH on osteoblasts are probably both direct and indirect via growth factors such as IGF-1 and TGF 3. The multiple signal transduction... 

The salicylates are used cautiously in patients witii hepatic or renal disease, preexisting hypoprotiirombine-mia, or vitamin K deficiency and during lactation. The dragp are also used with caution in patients with gastrointestinal irritation such as peptic ulcers and in patients with mild diabetes or gout. 

Alcohol dextrose solutions are used cautiously in patients with hepatic and renal impairment, vitamin deficiency (may cause or potentate vitamin deficiency),... 

Primary hyperparathyroidism occurs as a result of hyperplasia or the occurrence of adenoma. Secondary hyperparathyroidism may result from renal failure because of the associated phosphate retention, resistance to the metabolic actions of PTH, or impaired vitamin D metabolism. The last-mentioned factor is primarily responsible for the development of osteomalacia. Muscle symptoms are much more common in patients with osteomalacia than in primary hyperparathyroidism. Muscle biopsy has revealed disseminated atrophy, sometimes confined to type 2 fibers, but in other cases involving both fiber types. Clinical features of osteomalacic myopathy are proximal limb weakness and associated bone pain the condition responds well to treatment with vitamin D. 

Camitine deficiency can occur particularly in the newborn—and especially in preterm infants—owing to inadequate biosynthesis or renal leakage. Losses can also occur in hemodialysis. This suggests a vitamin-fike dietary requirement for carnitine in some individuals. Symptoms of deficiency include hypoglycemia, which is a consequence of impaired fatty acid oxidation and hpid accumulation with muscular weakness. Treatment is by oral supplementation with carnitine. 

There are numerous abnormalities of cysteine metabolism. Cystine, lysine, arginine, and ornithine are excreted in cystine-lysinuria (cystinuria), a defect in renal reabsorption. Apart from cystine calculi, cystinuria is benign. The mixed disulfide of L-cysteine and L-homocysteine (Figure 30-9) excreted by cystinuric patients is more soluble than cystine and reduces formation of cystine calculi. Several metabolic defects result in vitamin Bg-responsive or -unresponsive ho-mocystinurias. Defective carrier-mediated transport of cystine results in cystinosis (cystine storage disease) with deposition of cystine crystals in tissues and early mortality from acute renal failure. Despite... 

H)2-D3 is a weak agonist and must be modified by hydroxylation at position Cj for full biologic activity. This is accomplished in mitochondria of the renal proximal convoluted tubule by a three-component monooxygenase reaction that requires NADPFl, Mg, molecular oxygen, and at least three enzymes (1) a flavoprotein, renal ferredoxin reductase (2) an iron sulfur protein, renal ferredoxin and (3) cytochrome P450. This system produces l,25(OH)2-D3, which is the most potent namrally occurring metabolite of vitamin D. 

Renal osteodystrophy stems from disruptions in calcium, phosphorus, and vitamin D homeostasis through the interaction with the parathyroid hormone. 

Water-soluble vitamins removed by hemodialysis (HD) contribute to malnutrition and vitamin deficiency syndromes. Patients receiving HD often require replacement of water-soluble vitamins to prevent adverse effects. The vitamins that may require replacement are ascorbic acid, thiamine, biotin, folic acid, riboflavin, and pyridoxine. Patients receiving HD should receive a multivitamin B complex with vitamin C supplement, but should not take supplements that include fat-soluble vitamins, such as vitamins A, E, or K, which can accumulate in patients with renal failure. 

Causes of hypocalcemia include hypoparathyroidism, hypomagnesemia, alcoholism, hyperphosphatemia, blood product infusion (due to chelation by the citrate buffers), chronic renal failure, vitamin D deficiency, acute pancreatitis, alkalosis, and hypoalbuminemia. Medications that cause hypocalcemia include phosphate replacement products, loop diuretics, phenytoin (Dilantin, available as generic), pheno-barbital (available as generic), corticosteroids, aminoglycoside antibiotics, and acetazolamide (available as generic).34,39,42... 

Around 99% of calcium is contained in the bones, whereas the other 1% resides in the extracellular fluid. Of this extracellular calcium, approximately 40% is bound to albumin, and the remainder is in the ionized, physiologically active form. Normal calcium levels are maintained by three primary factors parathyroid hormone, 1,25-dihydroxyvitamin D, and calcitonin. Parathyroid hormone increases renal tubular calcium resorption and promotes bone resorption. The active form of vitamin D, 1,25-dihydroxyvitamin D, regulates absorption of calcium from the GI tract. Calcitonin serves as an inhibitory factor by suppressing osteoclast activity and stimulating calcium deposition into the bones. 

The water-soluble and fat-soluble vitamins in the parenteral multivitamin mix are essential cofactors for numerous biochemical reactions and metabolic processes. Parenteral multivitamins are added daily to the PN. Patients with chronic renal failure are at risk for vitamin A accumulation and potential toxicity. Serum vitamin A concentrations should be measured in patients with renal failure when vitamin A accumulation is a concern. Previously, vitamin K was administered either daily or once weekly because intravenous multivitamin formulations did not contain vitamin K. However, manufacturers have reformulated their parenteral multivitamin products to provide 150 meg of vitamin K in accordance with FDA recommendations. There is a parenteral multivitamin formulation available without vitamin K (e.g., for patients who require warfarin therapy), but standard compounding of PN formulations should include a parenteral multivitamin that contains vitamin K unless otherwise clinically indicated. 

Renal osteodystrophy Altered bone turnover that results from sustained metabolic conditions that occur in chronic kidney disease, including secondary hyperparathyroidism, hyperphosphatemia, hypocalcemia, and vitamin D deficiency. 

Secondary hyperparathyroidism Increased secretion of parathyroid hormone from the parathyroid glands caused by hyperphosphatemia, hypocalcemia, and vitamin D deficiency that result from decreased kidney function. It can lead to bone disease (renal osteodystrophy). 

Effects on Vitamin D Metabolism. Lead interferes with the conversion of vitamin D to its hormonal form, 1,25-dihydroxyvitamin D. This conversion takes place via hydroxylation to 25-hydroxyvitamin D in the liver followed by 1-hydroxylation in the mitochondria of the renal tubule by a complex cytochrome P-450 system (Mahaffey et al. 1982 Rosen and Chesney 1983). Evidence for this effect comes primarily from studies of children with high lead exposure. 

The possible mechanism kidney-induced hypertension is discussed in Section 2.4.2, Mechanisms of Toxicity. Lead appears to affect vitamin D metabolism in renal tubule cells, such that circulating levels of the vitamin D hormone, 1,25-dihydroxyvitamin D, are reduced. This effect is discussed later in this section under Other Systemic Effects. 

Because the vitamin D-endocrine system is responsible in large part for the maintenance of extra- and intracellular calcium homeostasis, it is reasonable to conclude that the interference of lead with renal... 

Health effects that have been associated with lead exposures during infancy or childhood include, anemia (Schwartz et al. 1990) (and related disorders of heme synthesis), neurological impairment (e.g., encephalopathy), renal alterations, and colic (Chisolm 1962, 1965 Chisolm and Harrison 1956), and impaired metabolism of vitamin D (Mahaffey et al. 1982 Rosen and Chesney 1983). Death from encephalopathy may occur with PbB levels 125 pg/dL. In addition to the above effects, the following health effects have been associated with lead exposures either in utero, during infancy or during... 

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