Calcium-dependent activation of glucose-6-phosphate dehydrogenase by 1,25-dihydroxycholecalciferol in the guinea pig distal convoluted tubule.

The effect of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) on glucose-6-phosphate dehydrogenase (G6PD) activity in the distal convoluted tubule of a vitamin D-depleted guinea pig was determined using quantitative cytochemistry. When kidney segments were incubated with 1,25(OH)2D3 (0, 0.15, 0.30, 1.5 or 3.0 nM) during the initial 5 h maintenance culture, G6PD activity at each steroid concentration decreased gradually to reach its stable basal level, which was higher in proportion to the increasing concentration of 1,25(OH)2D3. 1,25(OH)2D3-induced activity of this enzyme was completely abolished by cycloheximide (35 microM). In the presence of 1.5 nM 1,25(OH)2D3, at 0.15 mM calcium, increasing concentrations of EGTA from 0.1 to 2.0 mM caused a dose-dependent reduction in the enzyme activity and abolished it to the cycloheximide-treated level at 2 mM, whereas in the absence of 1,25(OH)2D3, the enzyme activity remained unchanged regardless of the concentration of calcium. These results indicate that G6PD is activated by 1,25(OH)2D3 via new protein synthesis, and that the extracellular calcium plays a crucial role in the regulation of this enzyme activity.