MULTIPLE MODULATION OF NA-DEPENDENT P(I) UPTAKE BY CELLULAR CA IN MDCK CELLS

The events accounting for the adaptation of the sodium-dependent phosp hate cotransport (Na-P(i)) to phosphate deprivation other than genomic regulation remain unknown. The involvement of changes in intracellula r calcium concentration was investigated in Madin-Darby canine kidney (MDCK) cells. Calcium concentration was decreased by 15 h of phosphate deprivation (-24 to -35%) or low-calcium medium (calcium deprivation) (-45%), or -(N,N'-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB8) (-32%). Calcium deprivation stimulated Na-P(i) (2-fold at 1 h and up t o 15 h) by increasing the affinity for phosphate. Combined calcium and phosphate deprivation had more than additive effects on phosphate upt ake. The effect of a 15-h calcium deprivation, but not of a 2-h one, w as dependent on gene transcription and protein synthesis. TMB8 stimula ted phosphate uptake similarly to phosphate deprivation (increase in m aximum velocity dependent on gene transcription). The ionophore A23187 decreased basal Na-P(i) as well as its stimulation by phosphate or ca lcium deprivation or by TMB8. Calcium deprivation stimulated (3.2-fold increase) the sodium-coupled alanine transport, whereas phosphate dep rivation and TMB8 did not. We conclude that 1) phosphate deprivation d ecreases intracellular calcium concentration, 2) low intracellular cal cium concentration is instrumental in the stimulation by prolonged cal cium or phosphate deprivation of Na-P(i), and 3) phosphate or calcium deprivation modulates Na-P(i) through different cellular pathways.