Figure 516

Postulated mechanism of the Ca transport pathway shared by PTH and 1,25(OH)2D3. Cyclic adenosine monophosphate (cAMP) generated by PTH stimulation leads to increased influx of Ca into the apical dihydropyridine-sensitive Ca channel. There also is increased activity of the basolateral Na-Ca exchanger and, perhaps, of the plasma membrane-associated Ca-adenosine triphosphatase (PMCA), which can rapidly extrude the increased intracellular free Ca (Ca2+). Calcitriol (1,25(OH)2D3), by way of the vitamin D receptor (VDR), stimulates transcription of calbindin D28k (CaBP2g) and calbindin D9k (CaBPg). CaBP28 increases apical uptake of Ca by both the dihydropyridine- and thiazide-sensitive Ca channels by decreasing the concentration of unbound free Ca2+ and facilitates Ca movement to the basolateral membrane. CaBP9 stimulates PMCA activity, which increases extrusion of Ca by the cell. Similar hormonally induced mechanisms of Ca transport are believed to exist throughout the cortical thick ascending limb, the DCT, and the connecting tubule (CNT) [6]. ATP—adenosine triphosphate; Na+—ionized sodium.

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