Influence of calcium on phosphate transport in cultured kidney epithelium

Renal epithelia cultured from opossum kidney (OK) cell line were exposed to high- and low-Ca media (HCaM and LCaM, respectively). Changing the extracellular Ca concentration from 2.0 mM to 0.02 mM stimulated sodium-dependent Pi transport (NaPiT) of OK epithelia. The effect was detectable after 1 h of LCaM exposure and reached a maximal value at 4 h (HCaM, 1.49 +/- 0.02; LCaM, 2.71 +/- 0.02 nmol.mg prot-1.4 min-1, P less than 0.001), this difference remaining constant up to 16 h. The Na-dependent transports of both alanine and methyl-alpha-D-glucopyranoside, a glucose analogue, were not affected by LCaM. Kinetic analysis of NaPiT revealed an increase in Vmax with no change in the apparent Km. The cellular adenosine 3',5'-cyclic monophosphate production was not affected by LCaM. In OK epithelia pretreated with either cycloheximide or actinomycin D the LCaM stimulatory effect was completely abolished. In the presence of 10(-9) to 10(-7) M bovine parathyroid hormone (1-34), the effect of LCaM on NaPiT was markedly blunted. In conclusion, this study in cultured kidney epithelia demonstrates that the renal NaPiT can be directly and selectively stimulated by lowering the extracellular Ca concentration. This effect required the de novo synthesis of proteins. The results also indicate that parathyroid hormone antagonizes the stimulatory effect of extracellular Ca on the renal transport of Pi.