Figure 410

Magnesium (Mg) reabsorption in the cortical thick ascending limb (cTAL) of the loop of Henle. Most Mg reabsorption within the nephron occurs in the cTAL owing primarily to voltage-dependent Mg flux through the intercellular tight junction. Transcellular Mg movement occurs only in response to cellular metabolic needs. The sequence of events necessary to generate the lumen-positive electrochemical gradient that drives Mg reabsorption is as follows: 1) A basolateral sodium-potassium-adenosine triphosphatase (Na+-K+-ATPase) decreases intracellular sodium, generating an inside-negative electrical potential difference; 2) Intracellular K is extruded by an electroneutral K-Cl (chloride) cotrans-porter; 3) Cl is extruded by way of conductive pathways in the basolateral membrane; 4) The apical-luminal Na-2Cl-K (furosemide-sensitive) cotransport mechanism is driven by the inside-negative potential difference and decrease in intracellular Na; 5) Potassium is recycled back into the lumen by way of an apical K conductive channel; 6) Passage of approximately 2 Na molecules for every Cl molecule is allowed by the paracellular pathway (intercellular tight junction), which is cation permselective; 7) Mg reabsorption occurs passively, by way of intercellular channels, as it moves down its electrical gradient [1,2,6,7]. (Adapted from de Rouffignac and Quamme [1].)

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