Figure 514

Renal handling of calcium (Ca). Ca is filtered at the glomerulus, with the ultrafilterable fraction (UFca) of plasma Ca entering the proximal tubule (PT). Within the proximal convoluted tubule (PCT) and the proximal straight tubule (PST), isosmotic reabsorption of Ca occurs such that at the end of the PST the UF^ to TFCa ratio is about 1.1 and 60% to 70% of the filtered Ca has been reabsorbed. Passive paracellular pathways account for about 80% of Ca reabsorption in this segment of the nephron, with the remaining 20% dependent on active transcellular Ca movement. No reabsorption of Ca occurs within the thin segment of the loop of Henle. Ca is reabsorbed in small amounts within the medullary segment of the thick ascending limb (MAL) of the loop of Henle and calcitonin (CT) stimulates Ca reabsorption here. However, the cortical segments (cTAL) reabsorb about 20% of the initially filtered load of Ca. Under normal conditions, most of the Ca reabsorption in the cTAL is passive and paracellular, owing to the favorable electrochemical gradient. Active transcellular Ca transport can be stimulated by both parathyroid hormone (PTH) and 1,25-dihydroxy-vitamin D3 (1,25(OH)2D3) in the cTAL. In the early distal convoluted tubule (DCT), thiazide-activated Ca transport occurs. The DCT is the primary site in the nephron at which Ca reabsorption is regulated by PTH and 1,25(OH)2D3. Active transcellular Ca transport must account for Ca reabsorption in the DCT, because the transepithelial voltage becomes negative, which would not favor passive movement of Ca out of the tubular lumen. About 10% of the filtered Ca is reabsorbed in the DCT, with another 3% to 10% of filtered Ca reabsorbed in the connecting tubule (CNT) by way of mechanisms similar to those in the DCT [1,2,6, 7,18]. ATPase—adenosine triphosphatase; CaBP-D—Ca-binding protein D; DT—distal tubule; VDR—vitamin D receptor. (Adapted from Kumar [1].)

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