1. Cultured monkey kidney cells (JTC-12) have a Na+-dependent phosphate (Pi) transport system with characteristics similar to that of the renal proximal tubule. Na+-dependent Pi uptake in JTC-12 cells is affected by Pi concentrations in the culture medium. In this investigation, further characterization of this phenomenon was carried out.

2. Lowering the concentration of extracellular Pi (3.0 mmol/l to 0.3 mmol/l) induced an increase in Na+-dependent Pi uptake compared with that in control cells maintained in 3.0 mmol/l Pi whereas Na+-dependent transport of hexose and amino acid was not altered. This response was first evident at 4 h after the extracellular Pi concentration was reduced and slowly developed over the subsequent 24 h.

3. Kinetic analysis showed an increase in the Vmax without a change in the apparent Km for Pi in cells cultured in the low Pi concentration compared with control cells.

4. The response of Pi uptake was only partially prevented by cycloheximide, suggesting that both protein synthesis-dependent and -independent mechanisms are involved in the development of the response. Insulin, which has a stimulatory effect on Pi uptake in JTC-12 cells, did not affect this response.

5. These data indicate that JTC-12 cells respond to changes in extracellular Pi concentration by changing the Na+-dependent Pi uptake system. This response has a number of properties typical of the phenomenon of adaptation of renal Pi transport in vivo to dietary phosphorus load.

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