Chapter 8 - Disorders of magnesium

Question 1

What is the percentage of magnesium stored intracellularly?

Answer 1

Approximately 99%

Question 2

What are the 2 body systems magnesium plays an important role in maintaining normal homeostasis for?

Answer 2

Cardiovascular and neuromuscular systems

Question 3

Where is magnesium distributed in the body (organs)?

Answer 3

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

Question 4

Which ion is the most affected by changes in albumin concentration? Calcium or magnesium?

Answer 4

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

Question 5

What are the driving forces for paracellular magnesium movement from the intestinal lumen to the blood?

Answer 5

• Transepithelial magnesium concentration gradient
• Transepithelial voltage gradient formed by salt and water absorption
• Permeability of the tight junctions to magnesium

Question 6

What is the unique character of the ions channel involved in the active transcellular magnesium movement from the gut?

Answer 6

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

Question 7

T/F: The percentage of magnesium absorbed depends primarily on the dietary concentration of magnesium

Answer 7

T

Question 8

What is the percentage of magnesium reabsorbed by the proximal tubule?

Answer 8

10 - 15% (in sharp contrast to most other major cations, where at least 60% of reabsorption occurs in the proximal tubule)

Question 9

T/F: The reabsorption of Mg by the proximal tubules is saturable

Answer 9

F: The reab- sorption process in this segment of the nephron appears to occur via passive and unsaturable mechanisms

Question 10

In which segment is the majority of magnesium reabsorbed from the kidneys?

Answer 10

Approximately 60% to 70% of filtered magnesium is reabsorbed in the cortical thick ascending limb of the loop of Henle

Question 11

Name few hormones that increase magnesium absorption from the lumen in the kidneys.

Answer 11

Parathyroid hormone, calcitonin, glucagon, antidiuretic hormone, aldosterone, and insulin

Question 12

Name few conditions that decrease magnesium absorption from the lumen in the kidneys.

Answer 12

Prostaglandin E2, hypokalemia, hypophosphatemia, and acidosis

Question 13

The distal convoluted tubule determine the final magnesium excretion. Is reabsorption of magnesium passive, active or both in this segment?

Answer 13

Only active, through the unique transient receptor protein TRPM6

Question 14

T/F: Bitches with ecclampsia also tend to be hypomagnesemic

Answer 14

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)

Question 15

What are disease conditions reportedly associated with hypomagnesemia in dogs?

Answer 15

GDV, parvoviral enteritis, CKCS MMVD, PLE

Question 16

What are disease conditions reportedly associated with hypomagnesemia in cats?

Answer 16

DM, DKA, renal transplant, CRF

Question 17

What are the mechanisms of magnesium deficiency in hospitalized ill patients?

Answer 17

1. lack of dietary intake
2. excessive loss through GI tract due to diarrhea
3. through kidneys due to excessive diuresis

Question 18

What are the mechanisms of development of hypokalemia associated with magnesium deficit?

Answer 18

1. 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)
2. decreased ability for potassium to reenter the cell (decreased function of Na-K-Cl cotransport, thus decreasing potassium reentry into the cell)
3. 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
4. Mg act from both within and outside the cell to prevent potassium leak from the cell through potassium channels

Question 19

What are the most likely origin of the concurrent deficiency of Ca and Mg?

Answer 19

1. Loss through kidneys

2. Decreased liberation from bone stores

Question 20

What are the two most important divalent cations?

Answer 20

Ca, Mg

Question 21

What is the proposed mechanism of severe magnesium deficit leading hypoparathyroidism?

Answer 21

Mg’s role as a cofactor in the production of the intracellular signaling molecule cAMP.

Question 22

T/F: Mg has been shown to block NMDA receptors within the peripheral nervous system.

Answer 22

F: it’s central nervous system.

Question 23

T/F: Some researchers speculated that NMDA receptor blockade by Mg deficit may play a role in bronchial smooth muscle relaxation.

Answer 23

True

Question 24

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.

Answer 24

True

Question 25

List common arrhythmias documented in humans in which magnesium deficiency has been implicated as a cause of, or contributing to the severity

Answer 25

atrial fibrillation, SVT, torsades de pointes, ventricular ectopy, ventricular tachycardia, toxic digitalis arrhythmias

Question 26

Simply explain magnesium’s effect on the peripheral vasculature (vasoconstriction, vasodilation)

Answer 26

higher intracellular concentration of magnesium- vasodilatory effect, lower intracellular concentration of magnesium- vasoconstricting effect

Question 27

What is the initiating factor in muscle contraction in both smooth and cardiac muscle?

Answer 27

intracellular calcium, released from the sarcoplasmic reticulum or entering the cell from the extracellular space

Question 28

What molecule rapidly shunts intracellular calcium back into the sarcoplasmic reticulum after the contraction cycle is complete?

Answer 28

Ca2+ ATPase, and Mg is a cofactor

Question 29

T/F: Ionized Mg appears to equilibrate rapidly across the cell membrane, so extracelluar iMg may be reflective of intracellular stores.

Answer 29

True

Question 30

What percentage of total body Mg stores does iMg represent?

Answer 30

0.2-0.3%

Question 31

Name 2 pathologies in humans that can affect redistribution of Mg.

Answer 31

Acute pancreatitis, myocardial infarction

Question 32

Name 2 modalities to assess physiologic Mg handling.

Answer 32

Assessment of renal Mg handling, testing Mg retention.

Question 33

What is the best assay for diagnosis of hypomagnesemia?

Answer 33

There is no consensus

Question 34

T/F: A normal iMg helps to rule out hypomagnesemia.

Answer 34

False

Question 35

T/F: A normal total Mg helps to rule out hypomagnesemia.

Answer 35

False

Question 36

What veterinary population is at risk for Mg deficit?

Answer 36

The hospitalized dogs and cats, esp those that have been anorexic for several days.

Question 37

Patients with what electrolyte imbalances in spite of appropriate supplementation should be evaluated for Mg deficit?

Answer 37

HypoK and HypoCa

Question 38

What ECC patients are frequently ID’d with Mg deficit?

Answer 38

Pt’s in heart failure with concurrent ventricular arrhythmias and being medications on loop diuretics/ digitalis; Pts with DM, esp DKA

Question 39

Where does the dose range for Mg come from?

Answer 39

Extrapolated from human medicine and tested empirically

Question 40

In what isotonic fluid would Mg not be compatible?

Answer 40

LRS- due to the Ca

Question 41

What is the most common pathology associated with hyperMg?

Answer 41

Renal insufficiency, post renal azotemia

Question 42

T/F: HyperMg with concurrent clinical signs occurs often.

Answer 42

False, clinically significant hyperMg is rarely documented in the literature.