There is an inverse relationship between intracellular concentration of P(i) ([P(i)]i) in the kidney and maximum velocity (Vmax) of Na(+)-P(i) cotransport in brush-border membrane vesicles both in P(i)-deprived and growing animals. However, at any given [P(i)]i, the Vmax is substantially higher in growing than in P(i)-deprived animals. This suggests that growth and P(i) depletion act on P(i) transport via different mechanisms. We tested this hypothesis by measuring the nuclear magnetic resonance-visible phosphate and the Vmax of Na(+)-P(i) cotransport in proximal tubule-like cells [opossum kidney (OK) cells] cultured in vitro. OK cells incubated in 1 mM extracellular P(i) had a [P(i)]i of 1.1 +/- 0.2 mM and a P(i) uptake of 1.47 +/- 0.06 nmol/mg in 5 min. Exposure of OK cells to P(i)-free medium decreased [P(i)]i by 80 +/- 7% (P < 0.01) and stimulated P(i) transport by 34 +/- 7% (P < 0.05). Exposure of OK cells to 10(-8) M insulin-like growth factor I (IGF-I) increased P(i) transport by 25 +/- 8% (P < 0.05) but did not affect [P(i)]i. The stimulation of Vmax produced by IGF-I was additive to that due to P(i) restriction. In addition, P(i) deprivation decreased the phosphomonoesters by 0.66 +/- 0.04-fold (P < 0.05) and increased the phosphodiesters by 2.5 +/- 0.5-fold (P < 0.01). Treatment with IGF-I increased both the phosphomonoesters (1.2 +/- 0.1-fold) and the phosphodiesters (4.1 +/- 0.6-fold). These results support the assumption that low P(i) supply and IGF-I stimulate Na(+)-P(i) cotransport by independent mechanisms.
Animals Carrier Proteins