Page 226 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

216 ELECTROLYTE DISORDERS

identified and described, the influence of other factors completely explained and is thus an area where further
upon magnesium transport in DCT cells has been more study is needed. The classic tetanic presentation of post-
completely studied. Throughcomplexintracellularsignal- parturient hypocalcemia in dogs is markedly different
ing pathways, numerous hormones play a role in from the paretic presentation of cattle. In addition, the
regulating magnesium transport. Parathyroid hormone, tetanic presentation of hypomagnesemia or “grass tetany”
glucagon, antidiuretic hormone, insulin, aldosterone, in cattle adds further evidence to suggest that neuromus-
1,25-dihydroxycholecalciferol, prostaglandin E 2 ,ß- cular transmission is affected by complex interactions
adrenergic agonists, and increased sympathetic stimula- between calcium and magnesium. A case report of a lactat-
tion via the renal nerve have all been shown to play a role ingbitchwithsignificanthypocalcemiaandhypomagnese-
in increasing magnesium conservation. 37,190 The intracel- mia that presented with paresisrather than tetany adds fuel
lular signaling pathways that occur following activation of to the speculation that the type of presentation is depen-
the receptor by any of these hormones/factors are com- dent on numerous electrolyte factors and interactions
plexandinterrelated.Asaresult,theroleofeachindividual among them. 11 A follow-up retrospective study of serum
factor, particularly amid the “noise” of multiple simulta- electrolyte concentrations in 27 bitches with eclampsia or
neous influences, is difficult to predict. Regardless, how- post-parturient hypocalcemia revealed that 12 (44%) were
ever, the sheer number of regulatory signals being sent concurrently hypomagnesemic, further suggesting that
to this portion ofthe nephron underscores the importance that magnesium concentrations may play a role in the
of magnesium balance within the organism. pathophysiology of eclampsia in the dog and that magne-
Like the gut and the thick ascending limb, the DCT sium concentrations should be assessed in bitches with
also possesses significant concentrations of the CASR. eclampsia. 12
Further study of this receptor has confirmed separate Undoubtedly, lactation does play a role in gut and
binding sites for both calcium and magnesium. 37,169 renal handling of magnesium. Elevation of parathyroid
Thus, the receptor may be able to sense the extracellular hormone, in addition to calcium concentration, most
concentrations of both ions and send independent signals likely plays a role in magnesium conservation during lac-
to intracellular regulators (preferentially for calcium), tation to supply the mammary glands with sufficient mag-
allowing for completely separate control of magnesium nesium. The pathophysiology of calcium and magnesium
and calcium balance. 169 Once activated with magnesium, handling during lactation and the exact triggers that
however, the CASR sends a strong inhibitory signal via G result in eclampsia will require further study.
proteins, which negate the positive influence of peptide
and steroid hormones and vitamin D 3 on magnesium MANIFESTATIONS OF
transport. 37 How it accomplishes this is not yet clear. Pos- MAGNESIUM DEFICIENCY
sible sites of action include the purported basolateral
sodium-magnesium countertransporter or gating of the Alargebodyofresearchispresentinthescientificliterature
TRMP6 or TRPM7 ion channels. 37 evaluating the effects of magnesium deficiency induced in
The influence of other electrolyte and acid-base animalmodels.Rodentmodelshavebeenmostcommonly
abnormalities also can exert a powerful negative effect used, but early literature also evaluated the effects of mag-
on magnesium transport in the DCT. As in the loop of nesium deficient diets in young growing dogs. Several of
Henle, hypokalemia, hypophosphatemia, and acidosis these early canine studies showed dramatic changes that
all decrease magnesium reabsorption. 37 The precise occurred within 2 weeks of introduction of the diet with
mechanism of how these factors influence magnesium increasing severity until 5 to 7 weeks when the animals
transport is not known, and it is likely that each has a were euthanized. 16,25,26,117,171 Poor growth rates and
different mechanism. Their combined effect can there- numerous dermatologic and soft tissue problems, such
fore be additive because they influence magnesium as dry brittle hair and nails, peripheral vasodilation and
transport in different ways. swelling, and splaying of the paws, were identified in the
early weeks of these trials. After 5 to 7 weeks, however,
EFFECTS OF LACTATION neuromuscular signs, including seizure activity, were
The nutrient composition of both dog and cat milk has noted and postulated to contribute to the death of these
been recently studied. 1,2 Magnesium content of both animals. 117,171 In two studies, significant myocardial
cat and dog milk appears to be modestly higher during necrosis with associated fibrosis and calcification were also
the first 2 to 3 days of lactation, or in the colostrum of noted after euthanasia. 160,178 Results of these trials
both of these species, then tends to remain at a constant assisted researchers in understanding magnesium’s
level throughout the remainder of the lactation period. 1,2 physiologic role in the organism. As technology has
This is in sharp contrast to calcium levels, which rise improved, so too has our understanding of the central role
consistently throughout lactation. 1,2 magnesiumplaysinthemaintenanceofhealthyorganisms.
The contrasting presentation of postparturient hypo- Magnesium’s importance to living organisms can be
4
calcemia among species is an area that has not been traced through the early development of life. Primeval