title: Evidence for calcium-dependent control of 1,25-dihydroxyvitamin D3 production by rat kidney proximal tubules.
abstract: The role of calcium in the parathyroid hormone-mediated increase in 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) production was evaluated using isolated proximal tubules from rats fed a low calcium diet (0.002% Ca) for 14 days. Tubules were prepared by collagenase digestion and centrifugation through Percoll. Tubules from rats fed a low calcium diet produced 1,25-(OH)2D3 at rates 10 times that of tubules from rats fed normal calcium diet (1.2% Ca). In vitro 1,25-(OH)2D3 biosynthesis was highly dependent upon extracellular calcium with inhibition in the absence of medium calcium and maximal production at 0.25 mM medium calcium (0.9 +/- 0.25 versus 15.1 +/- 2.3 nmol/mg protein/5 min, p less than 0.03). Inhibition of 1,25-(OH)2D3 production was partly due to depressed ATP content (0 versus 1.2 mM calcium, 6.8 +/- 0.6 versus 12.7 +/- 0.6 nmol/mg protein, p less than 0.006). EGTA reduced 1,25-(OH)2D3 synthesis and total cell calcium and ATP production. Ruthenium red blocked the inhibitory effects of EGTA on 1,25-(OH)2D3 production. Barium (1.0 mM) inhibited 1,25-(OH)2D3 production (7.2 +/- 0.5 versus 3.4 +/- 0.3, p less than 0.001) without altering ATP production. The calcium ionophore A23187 increased 1,25-(OH)2D3 production in a calcium-dependent manner. It is concluded that parathyroid hormone-mediated increases in 1,25-(OH)2D3 production, as during low calcium diet, require extracellular calcium. Extracellular calcium maintains mitochondrial calcium at optimal concentrations for normal ATP production, a requirement for 25-hydroxyvitamin D3-1-hydroxylase (25-OH-D3-1-hydroxylase) activity. Inhibition of 25-OH-D3-1-hydroxylase activity by barium without an alteration of ATP suggests calcium may also control 1,25-(OH)2D3 production independent of its effects on oxidative phosphorylation, perhaps through a direct interaction with one or more components of the 25-OH-D3-1-hydroxylase.
mesh_terms: D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001464:Barium / Q000494:pharmacology; D000001:Calcimycin / Q000494:pharmacology; D002117:Calcitriol / Q000096:biosynthesis*; D002118:Calcium / Q000378:metabolism*; D002136:Calcium, Dietary / Q000008:administration & dosage; D002352:Carrier Proteins; D004533:Egtazic Acid / Q000494:pharmacology; D007473:Ion Channels; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism; D024101:Mitochondrial Proteins; D051381:Rats; D012430:Ruthenium Red / Q000494:pharmacology; D000071256:Uncoupling Protein 1
chemical_list: D002136:Calcium, Dietary; D002352:Carrier Proteins; D007473:Ion Channels; D008565:Membrane Proteins; D024101:Mitochondrial Proteins; D000071256:Uncoupling Protein 1; D012430:Ruthenium Red; D001464:Barium; D000001:Calcimycin; D004533:Egtazic Acid; D000255:Adenosine Triphosphate; D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase; D002117:Calcitriol; D002118:Calcium