title: The role of cytosolic calcium in chronic adaptation to phosphate depletion in opossum kidney cells.
abstract: Chronic dietary phosphate restriction is associated with up-regulation of sodium-dependent phosphate (Na/Pi) cotransport by renal proximal tubular epithelial cells in association with increases in Na/Pi cotransporter mRNA and protein. We investigated whether changes in cytosolic calcium mediate this adaptive response in opossum kidney cells, a continuous line of renal epithelial cells. After 24 h of phosphate depletion, steady-state cytosolic calcium levels were increased; this increase was observed at physiologic levels of phosphate restriction and was prevented by the calcium channel blocker verapamil. Chronic phosphate depletion was also associated with parallel increases in Na/Pi cotransport activity, Na/Pi cotransporter mRNA, and Na/Pi cotransporter protein, all of which were blocked in verapamil-treated cells. Actinomycin D, at a dose that prevented the increase in NaPi-4 mRNA during phosphate depletion, also prevented the increase in Na/Pi cotransport activity. Incubation with the calcium ionophore ionomycin or A23187 reproduced the increase in Na/Pi cotransporter mRNA in phosphate-replete cells. Conversely, chelation of cytosolic calcium by quin-2/AM prevented the increase in Na/Pi cotransporter mRNA in phosphate-depleted cells. The effect of an increase in cytosolic calcium was specific for the Na/Pi cotransporter as mRNA levels for the sodium-dependent glucose transporter were not affected. Our observations suggest that chronic phosphate restriction increases steady-state cytosolic calcium, which, in turn, increases transcription of Na/Pi cotransporter mRNA, thereby stimulating Na/Pi cotransport activity.
authors.affiliation: Nephrology Research and Training Center, University of Alabama at Birmingham, 35294, USA.
authors.forename: S
authors.initials: S
authors.lastname: Saxena
authors.forename: M
authors.initials: M
authors.lastname: Allon
mesh_terms: D000818:Animals; D001483:Base Sequence; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002352:Carrier Proteins / Q000096:biosynthesis*; D002460:Cell Line; D016895:Culture Media, Serum-Free; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D017931:DNA Primers; D003609:Dactinomycin / Q000494:pharmacology; D004848:Epithelium; D015870:Gene Expression / Q000187:drug effects; D007668:Kidney / Q000378:metabolism*; D008969:Molecular Sequence Data; D009893:Opossums; D010710:Phosphates / Q000378:metabolism*; D016133:Polymerase Chain Reaction; D012333:RNA, Messenger / Q000032:analysis / Q000096:biosynthesis; D050608:Sodium-Phosphate Cotransporter Proteins; D027981:Symporters; D014700:Verapamil / Q000494:pharmacology
chemical_list: D002121:Calcium Channel Blockers; D002352:Carrier Proteins; D016895:Culture Media, Serum-Free; D017931:DNA Primers; D010710:Phosphates; D012333:RNA, Messenger; D050608:Sodium-Phosphate Cotransporter Proteins; D027981:Symporters; D003609:Dactinomycin; D014700:Verapamil; D002118:Calcium