TRANSPORT OF MAGNESIUM THROUGH MEMBRANES

The basis of electrochemical and biochemical compartments in man is th e energy dependent formation and maintenance of membrane gradients bet ween cells, insterstitial space and other body spaces. Different conce ntrations of, for example, electrolytes in those compartments can only be maintained by active and passive transport mechanisms. With regard to magnesium a concentration gradient has been observed between bones , cells and extracellular liquid. In mammalia, four active transport m echanisms exist for magnesium: In the intestinal mucosa, magnesium is absorbed by an active, energy dependent mechanism from ingested food. The active reabsorption of magnesium in the kidney is controlled by PT H, vasopressin, calcitonin and glucagon. In the placenta, magnesium is actively transferred to the umbilical cord by a protein, which is rel ated to PTH. Finally, magnesium is actively transported from the plasm a to the cerebrospinal fluid (CSF), so that an increase of a concentra tion of 30 % of magnesium is maintained in the CSF. No other active tr ansport mechanisms for magnesium in man are known. The concentration o f free magnesium not bound to other molecules in extracellular fluids and in the cytosol is located in comparable dimensions. Obviously, the concentration gradient between magnesium in cells and in the plasma i s maintained by the variety and number of magnesium binding molecules existing in cells, such as phophate derivatives rich in energy and pro teins. In comparison, numerous data exit for the magnesium efflux from cell to the extracellular fluid while the efflux system for magnesium in the erythrocytes has been characterized extensively.