Calcium transport in the nephron.

Ionized and complexed calcium are filtered at the glomerulus and more than 95% of the filtered load is reabsorbed along the length of the nephron. In the proximal convoluted tubule calcium is absorbed in proportion to sodium and water, suggesting a passive mechanism. The high permeability of this segment is compatible with passive transport, but evidence for active transport has been advanced. A role for Ca2+-ATPase and/or for a Ca2+/Na+ antiport has also been proposed. The straight portion of the proximal tubule appears to transport calcium actively but little is known about the mechanism and regulation of calcium absorption in this segment. Both passive and active transport of calcium in the thick ascending limb have been demonstrated, and heterogeneity in the function of medullary and cortical segments has been proposed. Definite evidence has been advanced for avid active calcium absorption in the distal convoluted tubule. Both chlorothiazide and parathyroid hormone enhance the transport of calcium in this segment. The granular portion of the collecting tubule resembles in its properties and function the distal convoluted tubule. The light portion, however, is incapable of transporting calcium. The distal tubule and collecting tubule may be the final regulators of urinary excretion of calcium but much more data are required before this view can be adopted.