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Abstract

In metabolism Mg plays an important role because it affects many cellular enzymes in the body. The energy metabolism and protein synthesis are dependent on Mg as an enzyme activator, Mg is also involved in the synthesis of RNA and DNA. Absorption of Mg primarily takes place over the ruminal epithelium in ruminants. The main storage of Mg is in the bones but that can only be reached if the cow is in great need of calcium or phosphorus, it's a hormonal regulated process that induces catabolism of bones. Free and available sources of Mg are found in soft tissues and in the extra cellular fluids that includes cerebrospinal fluids and blood. Free Mg in serum is present in two forms, either bound to protein or free and ionized.

The majority of absorption takes place through the ruminal epithelium by trans-cellular and para-cellular systems. Every epithelium membrane has a potential difference across the membrane. The para-cellular system is a passive transport based on Mg travelling through tight junctions or through the intracellular surface between cells. The diffusion occurs mainly when the rumen contains high concentrations of Mg in the rumen. There is an active transport of Mg across the membrane in the trans-cellular system, which occurs by the exchange of ions across the membrane, thereby maintaining the electrochemical gradient across the membrane. For the trans-cellular and para-cellular systems to function is requires that Mg is dissolved in the ruminal fluid.

There are several factors that affect the absorption of Mg. High levels of K impairs digestibility of Mg, witch makes it harder for Mg to cross the epithelium. High concentrations of K in the ruminal fluid depolarize lumen membrane, resulting in that the driving forces for Mg transport over the membrane is reduced, which decrease Mg absorption. Another factor affecting the absorption of Mg is whether there is a lack of Na. There is an equilibrium between Na and K, so if there is a lack of Na, this means in turn that there is an increased concentration of K in the ruminal fluid that inhibits Mg absorption. The relationship between forage and concentrates can affect how well the Mg can be absorbed. A diet, which consists largely of cereals and limited quantities of forage, can lead to high production of acid in the gastrointestinal area. A high proportion of acid in the rumen lowers pH, that is positive for Mg absorption because pH regulate solubility and how well Mg can absorb over the membrane. Generally, Mg absorption is sensitive to differences in the balance of ions and pH changes in the rumen.

When ruminants are not getting enough of Mg they may develop a disease state called grass tetany that lead to metabolic disorders. Characteristic of the disease is that the animal is grinding teeth, have increased salivation and stop eating which may cause problems in the fermentation process and milk production. If the cows have a serious deficiency of Mg, they can also suffer from muscle cramps, which at worst can lead to death.