Mechanism of chronic hypocalciuria with chlorthalidone: reduced calcium absorption.

Chlorthalidone, like other benzothiadiazides, lowers urine calcium excretion chronically. If intestinal calcium absorption did not fall or bone accretion did not increase, serum calcium and the filtered load of calcium would increase and urine calcium would return to pretreatment levels. To determine whether overall intestinal calcium absorption fell, we fed chlorthalidone (5 mg X kg body wt-1 X 24 h-1) to 10 adult male rats eating 15 g/day of a 0.6% calcium diet. Compared with 10 control rats, chlorthalidone reduced urine calcium [2.1 +/- 0.1 (SE) vs. 5.8 +/- 0.5 mg/6 days; P less than 0.001]. Fecal calcium rose (307 +/- 9 vs. 257 +/- 12; P less than 0.005) because percent intestinal calcium absorption fell (41 +/- 2 vs. 52 +/- 2; P less than 0.002). Twenty other rats given the same diet were injected subcutaneously with 1,25(OH)2D3 (50 ng/day). In these rats, chlorthalidone reduced urine calcium (23 +/- 3 vs. 59 +/- 3; P less than 0.001) and percent intestinal calcium absorption (60 +/- 1 vs. 66 +/- 1; P less than 0.01). With or without 1,25(OH)2D3, chronic administration of chlorthalidone reduces intestinal calcium absorption, and this reduction seems to be the mechanism that permits urine calcium excretion to remain low.