MAGNESIUM TRANSPORT IN FRESH-WATER TELEOSTS

The magnesium handling of freshwater teleost fish is discussed, with a n emphasis on the role of branchial, intestinal and renal transport. I n response to the eminent threat of constant diffusive losses of miner als such as magnesium, freshwater fish have developed efficient mechan isms for magnesium homeostasis, Magnesium losses are overcome by the u ptake of magnesium from the food, making the intestine an important ro ute for magnesium uptake. Some evidence suggests that intestinal magne sium uptake in fish is a regulated, cellular process. The ambient wate r is an additional magnesium source for fish, implicating the gills as a secondary route for magnesium uptake. Certainly, in some species, d irect uptake from the water? probably via branchial routes. ameliorate s the effects of a low-magnesium diet. The hard tissues, representing over 50% of the total body magnesium pool, form a reservoir from which magnesium can be recruited to perform its functions in the cellular m etabolism of soft tissues such as muscle. In fish, as in terrestrial v ertebrates, the balance of a variety of elements becomes disturbed whe n the magnesium homeostasis of the soft tissues is disrupted. However, fish appear to be less sensitive than terrestrial vertebrates to thes e perturbations. Magnesium is reabsorbed in the kidneys to minimise lo sses. For renal cells, part of a cellular pathway has been elucidated that would allow absorptive magnesium transport (a magnesium conductiv e pathway in renal brush-border membranes). In some euryhaline teleost s, the kidneys appear to switch instantaneously to rapid magnesium sec retion upon magnesium loading, a response common to marine fish that a re threatened by diffusive magnesium entry. This enigmatic mechanism u nderlies the capacity of some euryhaline species to acclimate rapidly to sea water. Despite the progress made over the last decade, much of the cellular and molecular basis of magnesium transport in the gills, intestine and kidneys remains obscure. The application of fluorescent, radioactive and molecular probes, same of which have only recently be come available, may yield rapid progress in the field of magnesium res earch.