NMR-VISIBLE INTRACELLULAR P-I AND PHOSPHOESTERS DURING REGULATION OF NA+-P-I COTRANSPORT IN OPOSSUM KIDNEY-CELLS

There is an inverse relationship between intracellular concentration of P-i ([P-i](i)) in the kidney and maximum velocity (V-max) 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 V-max 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 V-max 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 V-max 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.