1,25(OH) 2D 3 only affects long-term levels of plasma Ca 2+ but not the rapid minute-to-minute plasma Ca 2+ homeostasis in the rat

Abstract

Results from our lab have shown previously that parathyroid hormone (PTH) is not the key factor in the rapid regulation of plasma Ca 2+. The possible role of 1,25(OH) 2D 3 in the rapid minute-to-minute regulation of plasma Ca 2+, as addressed by a possible rapid non-genomic action of 1,25(OH) 2D 3, was therefore studied in vivo in rats. The rapid calcemic recovery from induction of hypocalcemia by a brief EGTA infusion was examined in vitamin D-depleted rats with intact parathyroid glands and in vitamin D depleted rats 1 h after parathyroidectomy (PTX). The influence of different levels of plasma 1,25(OH) 2D 3 on the rapid calcemic recovery from hypocalcemia was examined in PTX rats treated with 1,25(OH) 2D 3 for two days at two different doses of 0.2 μg/day, 0.05 μg/day or vehicle, and in PTX rats being BNX for two days, as well. Additionally, the long-term effect of 1,25(OH) 2D 3 on plasma Ca 2+ homeostasis was examined. Plasma Ca 2+ recovered significantly ( P < 0.05) 10 min after discontinuing EGTA in vitamin D-depleted rats with or without parathyroid glands. Plasma Ca 2+ increased significantly ( P < 0.05) and at the same rate after induction of hypocalcemia in PTX rats with different levels of plasma 1,25(OH) 2D 3. The final levels of plasma Ca 2+ obtained were set by 1,25(OH) 2D 3 in a dose-related manner. 1,25(OH) 2D 3 did not affect the rapid calcemic recovery from EGTA induced hypocalcemia, but only had an effect on the long-term plasma Ca 2+ homeostasis in the rat.