Recent advances in calcium transport by the kidney.

Abstract

In the normal adult human, net intestinal calcium absorption and renal calcium excretion are similar in steady-state conditions. 7 Approximately 15 mg/kg (1,000 mg) of calcium are ingested in food each day; 80% of ingested calcium or approximately 11 mg/kg appears in the feces (Fig 1). The fraction of calcium absorbed by the intestine is approximately 30 % to 40 % of amount present in the lumen. Some calcium (the endogenous fecal calcium) is excreted into the intestine via intestinal and pancreatic secretions. Thus, the net calcium absorbed is the difference between the total amount actually absorbed and the amount excreted via endogenous fecal calcium. The calcium absorbed (4 mg/kg) enters the plasma and extracellular fluid (ECF) space. It equilibrates between the ECF and labile and stable bone pools and, to a smaller extent, between ECF and intracellular calcium pools. In a state of calcium balance, the amounts of calcium entering via the intestinal absorption route and the amounts excreted via the kidney are the same. The amount of calcium filtered by the glomeruli is approximately 10 g/d. Because only a fraction of this (1% to 3 %) appears in the urine, it follows that a considerable amount of calcium is reabsorbed in the tubules. Calcium balance is maintained through the action of parathyroid hormone and 1,25-dihydroxyvitamin D3 (1,25-[OH],D,) acting on target tissues such as the intestine, bone and kidney.8,9 The interactions of parathyroid hormone and the vitamin D endocrine system in response to changes in serum calcium concentrations are shown in Fig 2. This review does not attempt to exhaustively cover all aspects of calcium handling by the kidney but will discuss some recent findings concerning the nature of calcium pumps (Ca2+-pump) and exchangers and their distribution in different parts of the nephron. For more exhaustive reviews of the renal handling of calcium by different parts of the kidney, the reader is referred to several excellent recent reviews.‘O-14