Calcium is known primarily for its function as the main mineral component of bones. But calcium has other important functions, some of which are pertinent to asthma. The control of smooth muscle contraction is governed by changes in the intracellular concentration of calcium ions. (1) In the presence of calcium, ATPase is activated to hydrolyse ATP and provides an available energy source for muscle contraction.
Bones act as a calcium reservoir, supplying calcium when blood values decline and absorbing excesses when blood values are elevated above normal values. Calcium blood levels are generally independent of dietary intake, but may be altered by regulatory controls such as glandular malfunction or lack of vitamin D. (2)
Mg is a cofactor in over one hundred enzyme reactions in the body and as such is important for protein formation, DNA production and nerve conduction. Magnesium is also important in bone formation. Magnesium is normally conserved by the kidneys and intestinal mechanisms, allowing blood levels to remain stable through a wide range of dietary intakes. Magnesium assists in calcium uptake, but it also competes with dietary calcium for the same absorption site in the intestine. Excessive dietary calcium (or phosphorus) is much more likely to block magnesium absorption than vice- versa in the typical American diet. Magnesium absorption is also inhibited by excessive dietary fat, phosphate, lactose, phytates and oxalates which form insoluble compounds with magnesium. Magnesium is also lost in diarrhea, long term use of diuretics, excessive sugar intake and protein malnutrition. (3-6)
In the intracellular fluid compartment, magnesium is the second most abundant cation, after potassium. (7) The two cations are so strongly associated that that it is difficult to maintain cellular potassium levels during magnesium depletion , despite adequate potassium intake. Thus adequate body stores of magnesium and adequate repletion are necessary to prevent and effectively treat potassium loss. (8,9)
In the last half million years of evolution, the human body developed mechanisms to conserve sodium, a sometimes rare commodity and excrete potassium, the most common mineral in the primitive diet. The main regulator of this mechanism is the adrenal hormone, aldosterone. Potassium salts are excreted in the normal functioning of the kidney but in varying degrees of dehydration even more potassium is lost (in the urine) from the intracellular compartment as the body attempts to save remaining water along with sodium. Modern diets and lifestyles tend toward chronic elimination of potassium and its metabolic partner, magnesium.
When asthmatics are put on intravenous fluid repletion in an emergency room situation, potassium is the first component.
MAGNESIUM and SMOOTH MUSCLE RELAXATION
Magnesium is essential in muscle relaxation after contraction. A severe Mg deficiency causes neuromuscular symptoms such as tetany, an extreme and prolonged contraction of the muscles (10) Within the cells of striated and smooth muscles, magnesium is considered a natural calcium antagonist counteracting the adverse effects of excessive intracellular calcium. Excess magnesium blocks calcium entry, while low magnesium levels potentiate the actions of calcium. (11,12) Magnesium also plays a key role in the production of energy which is needed by the chest wall muscles and the diaphragm to perform the work of breathing. In a double blind study, individuals with low magnesium levels had an increase in the power of their respiratory muscles after receiving an intravenous infusion of magnesium. This effect was not seen in healthy individuals with normal magnesium levels.