title: Mechanism and regulation of vitamin B(6) uptake by renal tubular epithelia: studies with cultured OK cells.
abstract: The kidneys play an important role in regulating vitamin B(6) body homeostasis, but limited information exists regarding the mechanism of pyridoxine uptake by renal epithelial cells, and no study exists on its regulation. To address these issues, we used the renal opossum-derived tubular epithelial (opossum kidney; OK) cells and found pyridoxine uptake to 1) be temperature and energy dependent, 2) be pH dependent, with a higher uptake at alkaline or neutral buffer pH compared with acidic pH, 3) be Na(+) independent, 4) involve a saturable component (apparent Michaelis- Menten constant of 2.40 +/- 0.23 microM), 5) be inhibited by structural analogs, and 6) be amiloride sensitive. Maintaining OK cells in a vitamin B(6)-deficient growth medium (for 48 h) led to a significant upregulation of pyridoxine uptake. This upregulation was found to be specific for pyridoxine, inhibited by cyclohexamide and actinomycin D, reversible, and mediated via an increase in maximal velocity. Pretreating OK cells with modulates of a Ca(2+)/calmodulin-mediated pathway led to a significant downregulation in pyridoxine uptake via inhibition of maximal velocity. These results demonstrate that pyridoxine uptake by renal tubular epithelial OK cells is via a specialized pH-sensitive carrier-mediated mechanism. This mechanism appears to be regulated by extracellular vitamin B(6) levels and an intracellular Ca(2+)/calmodulin-mediated pathway.
authors.affiliation: Veterans Affairs Medical Center, Long Beach, California 90822, USA. hmsaid@uci.edu
authors.forename: Hamid M
authors.initials: HM
authors.lastname: Said
authors.forename: Alvaro
authors.initials: A
authors.lastname: Ortiz
authors.forename: Nike D
authors.initials: ND
authors.lastname: Vaziri
mesh_terms: D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000494:pharmacology; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000378:metabolism; D002147:Calmodulin / Q000378:metabolism; D002478:Cells, Cultured; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004140:Dinitrophenols / Q000494:pharmacology; D004232:Diuretics / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004847:Epithelial Cells / Q000166:cytology / Q000378:metabolism*; D005109:Extracellular Matrix / Q000378:metabolism; D005665:Furosemide / Q000494:pharmacology; D007093:Imidazoles / Q000494:pharmacology; D007461:Iodoacetates / Q000494:pharmacology; D007684:Kidney Tubules / Q000166:cytology / Q000378:metabolism*; D009893:Opossums; D011339:Probenecid / Q000494:pharmacology; D011736:Pyridoxine / Q000493:pharmacokinetics*; D019810:Sodium Azide / Q000494:pharmacology; D014316:Tritium; D014475:Uncoupling Agents / Q000494:pharmacology; D014528:Uricosuric Agents / Q000494:pharmacology
chemical_list: D002147:Calmodulin; D004140:Dinitrophenols; D004232:Diuretics; D004791:Enzyme Inhibitors; D007093:Imidazoles; D007461:Iodoacetates; D014475:Uncoupling Agents; D014528:Uricosuric Agents; D014316:Tritium; C031938:calmidazolium; D000584:Amiloride; D005665:Furosemide; D019810:Sodium Azide; D017868:Cyclic AMP-Dependent Protein Kinases; D011736:Pyridoxine; D011339:Probenecid; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; D002118:Calcium