Hypervitaminosis A and Calcium-Regulating Hormones in the Rat

The effect of vitamin A on calcium-regulating hormones was studied in rats. A single oral dose of 30 mg retinol equivalents (RE) given to adult rats caused no change to serum biologically active parathyroid hormone (bioactive-PTH) concen trations. Bioactive-PTH secretion from rat thyroparathyroid gland complexes was not significantly altered after in vitro incubation with 1.18 x IO'6 M retinol. Chronically intoxicated rats given 15 mg RE 3 times a week for 6 wk, showed higher osteoclast numbers and lower osteoid than controls. Serum bioactive-PTH was not detectable and serum 25-hydroxyvitamin D (25-OHD) (25.2 ±12.5 nmol/L) was significantly (P < 0.03) lower than controls (43.3 ±3.1). In acutely intoxicated rats (60 mg RE/d for 2 d), serum bioactive-PTH levels were significantly lower (0.02 ±0.05 ng/ml, P < 0.03) than in control animals (0.14 ±0.08). Lower doses of vitamin A, 7.5 mg RE 3 times a week for 3 wk, suppressed serum bioactive-PTH to undetectable levels but had no significant effect on serum 25-OHD. Serum calcium and 25-OHD levels were significantly lower in vitamin D-intoxicated rats given 7.5 mg RE 3 times a week (ca. 3.16 ±0.19 mmol/L; 25-OHD 599.7 ±110.6 nmol/L) than vitamin D-intoxi cated controls (3.42 ±0.17; 789.3 ±17.7). These results suggest that hypervitaminosis A can alter the metabolism of calcium-regulating hormones. J. Nutr. 116: 578-587, 1986. INDEXING