AJP - Endocrinology and Metabolism, Vol 262, Issue 4 E389-E393, Copyright © 1992 by American Physiological Society
Insulin-like growth factor I stimulates sodium-phosphate cotransport in the rat osteoblastic cell line UMR-106-01
P. I. Campbell, D. K. Breedlove and S. A. Kempson
Department of Physiology and Biophysics, Indiana University School of Medicine, Indianapolis 46202-5120.
The effect of recombinant insulin-like growth factor I (IGF-I) on Pi 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(+)-Pi cotransport. A 30-70% increase in Na(+)-dependent Pi 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(+)-Pi
uptake, because Na(+)-independent Pi uptake and Na(+)-alanine cotransport
were unaffected by IGF-I. A reversal of IGF-I induced stimulation of
Na(+)-Pi 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 (Vmax) of Na(+)-Pi cotransport. The Vmax was
5.22 +/- 0.47 vs. 3.33 +/- 0.45 nmol Pi.mg protein-1.10 min-1 in confluent
monolayers exposed to 10(-7) M IGF-I and vehicle alone, respectively, for 3
h (P less than 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 Pi uptake in
osteoblastic cells. The effect is characterized by an increase in Vmax and
is not dependent on de novo protein synthesis. The mechanism remains to be
determined.
