Magnesium Flashcards
What percentage of total body magnesium is present in the extracellular space in humans?
1%. The other 99% is present in bone (67%), muscle (20%), and other soft tissues (11%).
What is the major store for magnesium in dogs, and how does this differ from humans?
Major magnesium store in dogs is from bone. In humans, current estimates suggest that only 15% of these stores are considered to be exchangeable with the extracellular fluid. It appears that bone, muscle, and red blood cell stores of magnesium are very slow to liberate magnesium to the extracellular pool ,and that soft tissues are much more able to liberate magnesium to the extracellular space in humans.
Where is magnesium absorbed in the body?
Predominently in the ileum via an unsaturable paracellular route (through tight junctions) and a saturable active transcellular route. Magnesium is also absorbed in the jejunum and colon.
How does water and salt absorption in the GI tract affect magnesium absorption?
Solvent drag created by sodium and water reabsorption will result in transepithelial movement of magnesium and other ions.
True/false: Magnesium is similarly reabsorbed in large amounts like other major cations in the proximal tubule.
False. Approximately 80% of total serum magnesium is filtered at the level of the glomerulus. Only 10-15% of this filtered load is reabsorbed in the proximal tubule, which is in stark contrast to most other major cations that have a much higher percentage of reabsorption in the proximal tubule.
In the kidney, where is the majority of filtered magnesium reabsorbed?
Approximately 60% to 70% of filtered magnesium is reabsorbed in the cortical thick ascending limb of the loop of Henle.
What is the primary channel in the cortical thick ascending limb of the loop of Henle that allows for paracellular magnesium reabsorption, and what other cation is reabsorbed with magnesium here?
PCLN-1 or claudin-16 is the primary divalent cation channel permitting paracellular movement of magnesium and calcium in the thick ascending limb.
List 3 hormones that can increase magnesium reabsorption in the kidneys and 3 factors that can decrease magnesium reabsorption in the kidneys.
Parathyroid hormone, calcitonin, glucagon, antidiuretic hormone, aldosterone, and insulin all act to increase magnesium absorption from the lumen. Prostaglandin E2, hypokalemia, hypophosphatemia, and acidosis can all act to decrease magnesium absorption.
True/false: The distal convoluted tubule is the most distal segment of the nephron where magnesium reabsorption/secretion can be altered.
True. There does not appear to be any ability to further reabsorb or secrete magnesium in nephron segments distal to the DCT, thus the final concentration of magnesium in the urine is principally determined by the DCT. The DCT normally reabsorbs approximately10% to 15% of the filtered magnesium. When necessary it can be very efficient at reabsorbing magnesium, reabsorbing as much as 70% to 80% of the magnesium that is delivered from the thick ascending limb.
How is magnesium reabsorbed in the distal convoluted tubule?
Reabsorption of magnesium in the DCT appears to occur only through active transcellular routes. Passive paracellular transport does not appear to occur to any significant degree. As a result, the absorption of magnesium via this route is an energy-requiring saturable process.
