MAGNESIUMDEFICIENCY INANIMALS
Experimental magnesium deficiency has been
produced in many animal species. Many of the data
pertain to work on rats, but magnesium deficiency has
frequently been observed in sheep and cattle, species in
which the syndrome develops spontaneously.
In 1932 Kruse, Orent and McCollum70 induced
magnesium deficiency in' weanling rats by feeding a
diet containing 0.09 mEq per kilogram of the element.
In these animals hyperemia and progressive
neuromuscular irritability developed, culminating in
generalized and sometimes fatal seizures.
These early experiments have been confirmed and
extended by many workers. Hypomagnesemia, accompanied by diminution in red cell, muscle, bone and
total carcass magnesium1'1"'71'73 occurs.
Magneskim deficiency has a profound influence on
other metals and metabolites. In adult rats hypercalcemia, decrease in potassium and increase in
sodium in muscle and liver and azotemia have been
reported.71'74'75 Soft tissue potassium depletion and
sodium increase have been correlated with loss of
cellular noncollagen nitrogen, indicating cellular
atrophy.75 Available evidence suggests that with sufficient
magnesium depletion the pumping mechanisms that
maintain normal intracellular Na+ and K+ gradients
falter, resulting in these changes.
The mechanism of hypercalcemia in these circumstances is elusive. Although it is tempting to invoke
increased parathyroid function, there is no concomitant
hypophosphatemia71 or decrease in bone calcium. 75 The
azotemia is also unexplained. Glomeru-lar filtration
rates were not determined in these studies. Among the
multiple pathologic lesions nephrocalcinosis, myocardial
necrosis and calcification and lipid deposition in the
aorta have been most striking.'•"'•72-74'7(i'77
Hypomagnesemic tetany occurs spontaneously in
calves fed for a long time on a pure milk diet. 1'ln'78'711 A
gradual reduction of serum magnesium results in
hyperirritability and tetany, the latter appearing at
concentrations below 1 mEq per liter. Defective
absorption of magnesium has been documented as this
diet is maintained.78'79 Magnesium supplements prevent
the disease.1'8"
It should be noted that milk is a relatively poor
dietary source oi magnesium. Fresh whole cow's m i l k
contains 11.5 mEq per kilogram, whereas human milk
contains 1.9 mEq per kilogram. Most meats, grains and
raw green vegetables have magnes ium in excess of 20
mEq per kilogram.81'82
A second and more puzzling form of magnesium
deficiency known as "grass tetany" occurs in adult
luctating sheep and cattle.K:i Neuromuscular irritability,
tetany and convulsions develop in the affected animals,
sometimes within two days, when
the element. Again, magnesium administration can
reverse or prevent the disease.83"85 The acute hypomagnesemia is thought to be related to peculiar
conditions of their forage, and may be due to fertilization with ammonium salts. Severe hypomag-nesemia
will develop in a few days in lactating cows given
artificial rations low in magnesium unless magnesium
supplements are provided.
The regulation of serum magnesium under these
circumstances appears critically dependent on the daily
intake, a phenomenon not found in other species.
Magnesium content of grass from heavily fertilized
fields does not differ from that ot normal pastures. The
protein content of grass from fields topdressed with
ammonium salts is increased, however, and this has
been implicated as a factor causing grass tetany.8fi
In view of the absence of magnesium depletion it
appears that there is a failure to mobilize tissue stores
under these circumstances. In animals the serum
magnesium concentration at which tetany appears varies
with the nutritional status. Thus, when sheep are made
magnesium deficient, then treated by magnesium
administration and allowed to become deficient a
second time, tetany occurs only after the serum
magnesium has fallen considerably below the level at
which it occurred the first time.* 7 This observation has
important implications for human magnesium
deficiency since the onset of symptoms cannot be
correlated directly with the concentration of magnesium
in serum.
(To be continued)
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