The calcium ionophore A23187 is a potent stimulator of the vitamin D3-25 hydroxylase in hepatocytes isolated from normocalcaemic vitamin D-depleted rats.

The role played by 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and/or by calcium on the C-25 hydroxylation of vitamin D3 (D3) was studied in hepatocytes isolated from D-depleted rats which were divided into four treatment groups: Group 1 served as controls, Group 2 received calcium gluconate, Groups 3 and 4 were infused with 1,25(OH)2D3 at 7 and 65 pmol/24 h x 7 days respectively. The treatments normalized serum calcium in all but the controls which remained hypocalcaemic, while serum 1,25(OH)2D3 remained low in Groups 1 and 2 but increased to physiologic and supraphysiologic levels in Groups 3 and 4. The data show that basal D3-25 hydroxylase activities were not significantly affected by any of the treatments. Addition of CaCl2, EGTA, or Quin-2 in vitro revealed that relative to basal values, EGTA strongly inhibited the enzyme activity in all groups (P less than 0.0001), except in G 1; Quin-2 and CaCl2 had no significant effect on the activity of the enzyme in any of the groups. Addition of 1,25(OH)2D3 or A23187 in vitro in the presence of CaCl2 revealed that 1,25(OH)2D3 did not significantly affect enzyme activity, while A23187 was found to stimulate its activity in vitamin D-depleted animals, but most specifically in Group 2 (P less than 0.001); low serum calcium (Group 1) dampened (P less than 0.01), and 1,25(OH)2D3 treatment in vivo totally blunted (P less than 0.001) the response to A23187. The data suggest that 1,25(OH)2D3 supplementation in vivo has per se little or no effect on the basal D3-25 hydroxylase activity. The data show, however, that the magnitude of the response to various challenges in vitro is greatly influenced by the conditioning in vivo of the animals. They also show that A23187 can be a potent stimulator of the enzyme activity, which allowed us to demonstrate a significant reserve for the C-25 hydroxylation of D3 which is well expressed in hepatocytes obtained from D-depleted calcium-supplemented rats.