INSULIN-LIKE GROWTH FACTOR-I STIMULATES SODIUM-PHOSPHATE COTRANSPORT IN THE RAT OSTEOBLASTIC CELL-LINE UMR-106-01

The effect of recombinant insulin-like growth factor I (IGF-I) on P(i) uptake by a rat osteoblast-like cell line (UMR-106-01) in culture was investigated. IGF-I (10(-6)-10(-8) M) caused a dose-related stimulation of Na+-P(i) cotransport. A 30-70% increase in Na+-dependent P(i) uptake over control values was observed after 1- to 5-h exposure of these cells to 10(-7) M IGF-I. The increase was detected within 45 min, in contrast to the slower action of insulin. This effect of IGF-I was specific for Na+-P(i) uptake, because Na+-independent P(i) uptake and Na+-alanine cotransport were unaffected by IGF-I. A reversal of IGF-I induced stimulation of Na+-P(i) cotransport was observed within 1 h of removal of the hormone. Kinetic analysis of the IGF-I effect indicates a significant change only in the apparent maximum velocity (V(max)) of Na+-P(i) cotransport. The V(max) was 5.22 +/- 0.47 vs. 3.33 +/- 0.45 nmol P(i).mg protein-1.10 min-1 in confluent monolayers exposed to 10(-7) M IGF-I and vehicle alone, respectively, for 3 h (P < 0.05, group t test). Blocking de novo protein synthesis with cycloheximide had no effect on this stimulatory effect of IGF-I. These observations indicate that IGF-I specifically stimulates P(i) uptake in osteoblastic cells. The effect is characterized by an increase in V(max) and is not dependent on de novo protein synthesis. The mechanism remains to be determined.