Calcium and phosphorus deficiency in rats: Effects on PTH and 1,25-dihydroxyvitamin D₃

Weanling male Holtzman rats were fed calcium-deficient, phosphorus-deficient, or control diets for 8 wk. Parathyroid hormone (PTH) was measured by radioimmunoassay, and 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) by a competitive binding assay. Rats fed the calcium-deficient diet (0.01% calcium, 0.6% phosphorus) became mildly hypocalcemic after 6 days. Serum calcium levels reached 5.5 ± 0.4 mg/dl (mean ± SD) in 5 wk (control 10.3 ± 0.4 mg/dl). PTH increased from 285 ± 112 to 3658 ± 428 pg/ml within 6 wk. Maximum serum levels of 1,25(OH)₂D₃ (111.8 ± 17.3 vs. control 11.4 ± 3.8 ng/dl) were reached at 3 wk and thereafter declined to 44.6 ± 14.0 ng/dl. In rats fed the phosphorus-deficient diet (0.6% calcium, 0.04% phosphorus), serum phosphorus fell within 24 hr from 9.1 ± 0.6 to 3.2 ± 0.1 mg/dl, recovered to 5.6 ± 0.4 mg/dl for 2-3 days, and then declined again. Serum calcium reached a maximum of 14.4 ± 0.4 mg/dl at day 2 (control 10.8 ± 0.5 mg/dl) and then slowly declined. PTH decreased within 24 hr from 243 ± 59 to 36 ± 0 pg/ml in phosphorus-depleted rats. Serum levels of 1,25(OH)₂D₃ increased within 24 hr and remained elevated after 6 wk of phosphorus deprivation (61.2 ± 11.7 ng/dl vs. control 18.3 ± 0.4 ng/dl).