1517 1518 Metabolic alkalosis increases renal Ca2+ reabsorption (i.e., decreases excretion) at the level of the DCT, probably by relieving the inhibition of apical TRPV5/TRPV6 Ca2+ channels by H+. This effect is independent of PTH status as well as of Na+ transport. The latter observation suggests that factors such as solvent drag and diffusion are not involved. 1519 1520 1521 ## Phosphate Depletion 1522 1523 Chronic phosphate depletion impairs Ca2+ reabsorption at both proximal and distal sites. The effects are independent of the decrease in PTH secretion that follows phosphate depletion; the basis for the effect is otherwise unknown. 1524 1525 1526 1527 *TABLE 36-8 Components of Total Plasma Magnesium* 1528 1529 1530

	mg/dL	mM	% OF TOTAL
Ionized Mg²⁺	1.3	0.56	62
Diffusible magnesium complexes	0.1	0.06	7
Nondiffusible (protein-bound) magnesium	0.6	0.28	31
Total magnesium	2.0	0.90	100

1531 1532 diuretics. Clinical manifestations of Mg2+ depletion include neurological disturbances such as tetany (especially when associated with hypocalcemia), cardiac arrhythmias, and in-creased peripheral vascular resistance. Conversely, increased Mg2+ intake may lower blood pressure and may reduce the incidence of hypertension. Severe hypermagnesemia, which may occur following excessive intake or renal failure, results in a toxic syndrome involving nausea, hyporeflexia, respira-tory insufficiency, and cardiac arrest. 1533 1534 Normally, 5% or less of the filtered magnesium load appears in the urine (Fig. 36-17A). In contrast to the pre-dominantly "proximal" reabsorption pattern of the major components of the glomerular filtrate, Mg2+ reabsorption occurs mainly along the TAL. 1535 1536 1537 ![A diagram illustrates a nephron with labeled sections like PCT, TAL, and DCT, showing fluid flow and reabsorption percentages.](https://cdn.orris.care/cdss_images/887cd71e220547f66c3fa0f267146da4206840df2d72d3a116fe4292ee02e88c.png) 1538 1539 1540 ![A diagram illustrates a tubular structure with labeled components and arrows depicting ion movement and diffusion.](https://cdn.orris.care/cdss_images/a422eb98e9af925a31080089ef7b991369a178882dff5801b08964657b45f478.png) 1541 1542 *Figure 36-17 Magnesium handling by the kidney. In A, the numbered yellow boxes indicate the fraction of the filtered load that various nephron segments reabsorb. The green boxes indicate the fraction of the filtered load that remains in the lumen at various sites. The values in the boxes are approximations. CCT, cortical collecting tubule; ICT, initial collecting tubule; OMCD, outer medullary collecting duct; PCT, proximal convo-luted tubule; PST, proximal straight tubule.* 1543 1544 1545 1546 Normally, the proximal tubule reabsorbs only $\sim15\%$ of the filtered magnesium load (see Fig. 36-17B). Water reabsorption along the proximal tubule causes luminal [Mg$^{2+}$] to double compared with the value in Bowman's space, thereby establishing a favorable Mg$^{2+}$ electrochemical gradient for passive, paracellular Mg$ [... Output truncated due to length. 4617 tokens/13719 characters removed ...]