Chapter 8 - Disorders of magnesium Flashcards
What is the percentage of magnesium stored intracellularly?
Approximately 99%
What are the 2 body systems magnesium plays an important role in maintaining normal homeostasis for?
Cardiovascular and neuromuscular systems
Where is magnesium distributed in the body (organs)?
Approximately two thirds (67%) of body magnesium is stored in the bone with calcium and phosphorus, 20% is found in muscle tissue, and 11% in other soft tissues not including muscle
Which ion is the most affected by changes in albumin concentration? Calcium or magnesium?
Unlike calcium, which is approximately 40% protein bound, magnesium is only 20% to 30% bound to protein and so is less affected by changes in albumin concentration
What are the driving forces for paracellular magnesium movement from the intestinal lumen to the blood?
- Transepithelial magnesium concentration gradient
- Transepithelial voltage gradient formed by salt and water absorption
- Permeability of the tight junctions to magnesium
What is the unique character of the ions channel involved in the active transcellular magnesium movement from the gut?
These two proteins (TRMP6 and TRMP7) are unique because they are the only known ion channels that combine a protein channel with an intracellular protein kinase or enzyme
T/F: The percentage of magnesium absorbed depends primarily on the dietary concentration of magnesium
T
What is the percentage of magnesium reabsorbed by the proximal tubule?
10 - 15% (in sharp contrast to most other major cations, where at least 60% of reabsorption occurs in the proximal tubule)
T/F: The reabsorption of Mg by the proximal tubules is saturable
F: The reab- sorption process in this segment of the nephron appears to occur via passive and unsaturable mechanisms
In which segment is the majority of magnesium reabsorbed from the kidneys?
Approximately 60% to 70% of filtered magnesium is reabsorbed in the cortical thick ascending limb of the loop of Henle
Name few hormones that increase magnesium absorption from the lumen in the kidneys.
Parathyroid hormone, calcitonin, glucagon, antidiuretic hormone, aldosterone, and insulin
Name few conditions that decrease magnesium absorption from the lumen in the kidneys.
Prostaglandin E2, hypokalemia, hypophosphatemia, and acidosis
The distal convoluted tubule determine the final magnesium excretion. Is reabsorption of magnesium passive, active or both in this segment?
Only active, through the unique transient receptor protein TRPM6
T/F: Bitches with ecclampsia also tend to be hypomagnesemic
T, magnesium blood concentrations may play a role in the pathophysiology of eclampsia in the dog (high concentration in the milk, especially during the first 2-3 days)
What are disease conditions reportedly associated with hypomagnesemia in dogs?
GDV, parvoviral enteritis, CKCS MMVD, PLE
What are disease conditions reportedly associated with hypomagnesemia in cats?
DM, DKA, renal transplant, CRF
What are the mechanisms of magnesium deficiency in hospitalized ill patients?
- lack of dietary intake
- excessive loss through GI tract due to diarrhea
- through kidneys due to excessive diuresis
What are the mechanisms of development of hypokalemia associated with magnesium deficit?
- simultaneous occurrence of intracellular potassium loss (Mg is a cofactor for most ATPase pumps. Reduced Na-K-ATPase pump will lead to net loss of potassium outside the cell and a net gain of sodium in the cell)
- decreased ability for potassium to reenter the cell (decreased function of Na-K-Cl cotransport, thus decreasing potassium reentry into the cell)
- concentration of Na-K pumps decreases in the cell membrane in response to prevent potassium leak from the cell through potassium channels that further compounds potassium reentry into the cell
- Mg act from both within and outside the cell to prevent potassium leak from the cell through potassium channels
What are the most likely origin of the concurrent deficiency of Ca and Mg?
- Loss through kidneys
2. Decreased liberation from bone stores
What are the two most important divalent cations?
Ca, Mg
What is the proposed mechanism of severe magnesium deficit leading hypoparathyroidism?
Mg’s role as a cofactor in the production of the intracellular signaling molecule cAMP.
T/F: Mg has been shown to block NMDA receptors within the peripheral nervous system.
F: it’s central nervous system.
T/F: Some researchers speculated that NMDA receptor blockade by Mg deficit may play a role in bronchial smooth muscle relaxation.
True
T/F: Animal models evaluating cardiac effects of Mg deficiency have also shown an increased susceptibility to ischemic and reperfusion injury, indicating that Mg also has a protective antioxidant effect.
True
List common arrhythmias documented in humans in which magnesium deficiency has been implicated as a cause of, or contributing to the severity
atrial fibrillation, SVT, torsades de pointes, ventricular ectopy, ventricular tachycardia, toxic digitalis arrhythmias
Simply explain magnesium’s effect on the peripheral vasculature (vasoconstriction, vasodilation)
higher intracellular concentration of magnesium- vasodilatory effect, lower intracellular concentration of magnesium- vasoconstricting effect
What is the initiating factor in muscle contraction in both smooth and cardiac muscle?
intracellular calcium, released from the sarcoplasmic reticulum or entering the cell from the extracellular space
What molecule rapidly shunts intracellular calcium back into the sarcoplasmic reticulum after the contraction cycle is complete?
Ca2+ ATPase, and Mg is a cofactor
T/F: Ionized Mg appears to equilibrate rapidly across the cell membrane, so extracelluar iMg may be reflective of intracellular stores.
True
What percentage of total body Mg stores does iMg represent?
0.2-0.3%
Name 2 pathologies in humans that can affect redistribution of Mg.
Acute pancreatitis, myocardial infarction
Name 2 modalities to assess physiologic Mg handling.
Assessment of renal Mg handling, testing Mg retention.
What is the best assay for diagnosis of hypomagnesemia?
There is no consensus
T/F: A normal iMg helps to rule out hypomagnesemia.
False
T/F: A normal total Mg helps to rule out hypomagnesemia.
False
What veterinary population is at risk for Mg deficit?
The hospitalized dogs and cats, esp those that have been anorexic for several days.
Patients with what electrolyte imbalances in spite of appropriate supplementation should be evaluated for Mg deficit?
HypoK and HypoCa
What ECC patients are frequently ID’d with Mg deficit?
Pt’s in heart failure with concurrent ventricular arrhythmias and being medications on loop diuretics/ digitalis; Pts with DM, esp DKA
Where does the dose range for Mg come from?
Extrapolated from human medicine and tested empirically
In what isotonic fluid would Mg not be compatible?
LRS- due to the Ca
What is the most common pathology associated with hyperMg?
Renal insufficiency, post renal azotemia
T/F: HyperMg with concurrent clinical signs occurs often.
False, clinically significant hyperMg is rarely documented in the literature.
