Calbindin expression in renal tubular epithelial cells. Altered sodium phosphate co-transport in association with cytoskeletal rearrangement.

Sodium-phosphate transport in the opossum kidney (OK) cell line was studied in an OK clonal cell line that was transfected with an episomal vector expressing high levels of rat calbindin (28 kDa). High level expression of calbindin buffered the influx of calcium induced by ionomycin by 53% and raised the basal intracellular calcium from 100 +/- 6 to 150 +/- 8 nM. The decrement in sodium phosphate uptake induced by parathyroid hormone or forskolin was identical in the two cell lines. However, phorbol esters (10(-10)-10(-7) M), which decreased sodium phosphate uptake in the parental OK line, increased it in the calbindin-expressing line. Similarly, the parental clone did not respond to phosphate deprivation, while the calbindin-expressing clone did increase phosphate uptake in response to phosphate deprivation. In the calbindin-expressing cells, phorbol 12-myristate 13-acetate or low phosphate medium, which increased phosphate transport, produced actin filament aggregation, dissociation of the myristoylated alanine-rich C kinase substrate protein from sub-apical actin, and membrane-associated tyrosine phosphate staining. Agonists that reduced sodium phosphate uptake (cAMP, parathyroid hormone) did not affect these cellular features. The cytoskeletal rearrangement, redistribution of the myristoylated alanine-rich C kinase substrate protein, and membrane tyrosine phosphorylation are suggested to be involved in the events by which phosphate transport is increased in this cell line.