1,25 Dihydroxyvitamin D increases hepatocyte cytosolic calcium levels. A potential regulator of vitamin D-25-hydroxylase.

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Abstract
1,25 dihydroxyvitamin D (1,25(OH)2D) has been demonstrated to inhibit hepatic 25 hydroxyvitamin D (25 OHD) production. Changes in cytosolic calcium have been shown to regulate cellular processes. Using the fluorescent dye Quin 2, we have investigated the effects of 1,25(OH)2D and 24,25(OH)2D on cytosolic calcium levels in hepatocytes. 1,25(OH)2D exposure for 5 min increases cytosolic calcium levels by 24% at a concentration of 100 pg/ml, 39% at a concentration of 1 ng/ml, and 50% at a concentration of 2 ng/ml. The latter increment occurs in both the presence and absence of extracellular calcium, indicating that 1,25(OH)2D is mobilizing intracellular calcium pools. 24,25(OH)2D, 10 ng/ml, does not increase cytosolic calcium levels while the calcium ionophore A23187, 3 microM, increases levels by 52%. Calcium inhibits hepatic 25 OHD synthesis in liver homogenates in a dose-dependent fashion, which can be prevented by chelation of calcium with EGTA. 1,25(OH)2D and A23187 decrease hepatocyte 25 OHD synthesis. The inhibitory effect of A23187 can be prevented by chelation of extracellular calcium. The data demonstrate that 1,25(OH)2D increases hepatocyte cytosolic calcium, and that these increments in cytosolic calcium may regulate some of the hepatic actions of the vitamin D metabolite.