Gastrointestinal tract vanadium

Probably the vanadium compound to which people are most likely to be exposed is vanadium pentoxide, V205. Exposure normally occurs via the respiratory route, and the pulmonary system is the most likely to suffer from vanadium toxicity. Bronchitis and bronchial pneumonia are the most common pathological effects of exposure skin and eye irritation may also occur. Severe exposure can also adversely affect the gastrointestinal tract, kidneys, and nervous system. 

Since vanadium is poorly absorbed in the gastrointestinal tract, a large percentage of vanadium is excreted unabsorbed in the feces in rats following oral exposure. More than 80% of the administered dose of ammonium metavanadate accumulated in the feces after 6 days (Patterson et al. 1986). After 2 weeks of exposure, 59.1 18.8% of sodium metavanadate was found in the feces (Bogden et al. 1982). However, the principal route of excretion of the small absorbed portion of vanadium is through the kidney in animals. 

Reproductive Toxicity. No data exist on reproductive effects on humans from exposure to vanadium by any exposure route. One animal study shows that vanadium did not affect reproductive parameters in rats following oral exposure (Domingo et al. 1986). Since vanadium is poorly absorbed from the gastrointestinal tract (Conklin et al. 1982 Roschin et al. 1980) and skin (WHO 1988), exposure by these routes is unlikely to be a health risk in humans. Toxicokinetic studies in humans (Schroeder et al. 1963) and reliable studies in animals (Edel and Sabbioni 1988) do not indicate that the reproductive system accumulates vanadium. Humans are most likely to be exposed to vanadium in the air, but the reproductive system does not appear to be a sensitive target of vanadium toxicity. Further studies would not appear to be particularly useful. 

The (metabolic) pathways of dietary vanadium, such as vanadate [H2V04], can be expressed as illustrated in Scheme 5.1 after oral uptake, vanadate reaches the gastrointestinal tract, where it is partially reduced and precipitated to vanadyl (VO ) hydroxides, which are excreted with the faeces. Another portion is absorbed and circulated in the blood, where it undergoes redox speciation and complexation by the serum proteins transferrin and albumin. Vanadate and vanadyl are finally incorporated into cells, mainly those of the liver, spleen and kidney. Excretion is achieved via the urine. Part of the vanadium is taken up by bones, where the mean retention time is comparatively long. 

Animal studies generally support the concept that vanadium is poorly absorbed (< 10%). A significant amount of vanadium absorption occurs in the upper gastrointestinal tract. Most ingested vanadium probably is transformed to VO in the stomach and passes into the duodenum in this form. In-vitro studies have suggested that vanadate can enter cells via the phosphate or other anion transport system, and this may be why V is absorbed three to five times more effectively than V. ... 

The first reports of vanadium poisoning in humans described rather widespread systemic effects consisting of polycythemia, followed by red blood cell destruction and anemia, loss of appetite, pallor and emaciation, albumin and blood in the urine, gastrointestinal disorders, nervous complaints and cough, sometimes severe enough to cause hemoptysis. More recent reports describe symptoms that are restricted to the conjunctivae and respiratory system. No evidence is found for disturbances of the gastrointestinal tract, kidneys, blood, or central nervous system. 

Vanadium can be absorbed through the respiratory and gastrointestinal tracts, although the gastrointestinal route is the most studied. Vanadium occurs in the diet, or as a pharmacological agent, mainly as vanadyl or vanadate. Vanadate is transformed into vanadyl in the stomach [67], which primarily is the absorbed form. Most studies indicate that relatively little (<5%) ingested vanadium is absorbed and the remainder is excreted in the feces [68-74]. However, as with all substances, other dietary components probably affect absorption. The main route of excretion of absorbed vanadium is in the urine with lesser amounts lost in the feces, probably via the bile [74-77]. 

In the human gastrointestinal tract, only 0.1-1% of vanadium contained in the diet is resorbed. For some laboratory animals, however, fairly high resorption of vanadium from the diet with a predominance of casein and carbohydrates has been observed. Vanadium resorbed in the form of metavanadate ions (VOj") containing V is reduced by glutathione in the blood to VO + ions that contain V and form 2 1 metal-protein complexes with ferritin and transferrin. Transferrin then obviously provides the distribution of vanadium in the tissues. Vanadium is also present in a metallopro-tein (metaUoporphyrin) called haemovanadin. Excessive intake of vanadium is excreted in the urine. 

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