Sodium-dependent phosphate transport inhibited by parathyroid hormone and cyclic AMP stimulation in an opossum kidney cell line

In the present study we investigated the characteristics of the transport of inorganic phosphate (Pi) in an opossum kidney cell line endowed with parathyroid hormone (PTH) receptors. In confluent epithelial cell culture, a Na-dependent Pi transport (NaPiT) was identified. Preincubation for 1 h with bovine (b)PTH(1-34) at 10(-7) M inhibited the NaPiT from 2.76 +/- 0.11 to 1.08 +/- 0.10 nmol/mg protein X 2 min-1 (p less than 0.001). This inhibition was already expressed 5 min after exposure to 10(-7) M bPTH. It was associated with a 4-fold increase in cellular cyclic AMP. The NaPiT was significantly inhibited at 10(-9) M bPTH, a hormonal concentration which stimulated the cellular cyclic AMP by only 30%. Kinetic analysis of the NaPiT inhibition by 10(-7) M bPTH revealed a decrease in Vmax (from 4.14 +/- 0.32 to 2.41 +/- 0.14 nmol/mg protein X 2 min-1) with no change in Km (0.093 +/- 0.016 versus 0.094 +/- 0.012 mM). The effect of bPTH on NaPiT was not associated with a change in the Na-dependent glucose methylglucopyranoside transport also present in the opossum kidney cell line. The inhibitory influence of bPTH on NaPiT was not affected by blockage of new protein synthesis by cycloheximide. Stimulation of cyclic AMP production by 10(-5) M forskolin, 10 micrograms/ml cholera toxin, 10(-5) M prostaglandin E2 or addition of 10(-5) M dibutyryl cyclic AMP mimicked the PTH-induced reduction in NaPiT. In conclusion, the present study indicates that the opossum epithelial cell line is endowed with a Na-dependent Pi transport system which is selectively inhibited by PTH and agents which increase cyclic AMP production.