EVIDENCE OF A ROLE FOR GROWTH-HORMONE, BUT NOT FOR INSULIN-LIKE GROWTH FACTOR-I OR FACTOR-II IN THE GROWTH OF THE NEONATAL RAT

Neonatal rats were injected with antiserum raised against either insul in-like growth factor (IGF)-I, IGF-II, rat growth hormone (rGH) or som atostatin (SRIF) on each of days 2-5 of life: controls received normal sheep immunoglobulin. Plasma levels of rGH and IGF-I were measured by radioimmunoassay and growth rates recorded. Neonatal administration o f anti-rGH resulted in the suppression of plasma IGF-I levels at day 2 1 and of body weight pin compared with control animals from day 5 of a ge; relative growth velocity continued to diverge in the absence of an y further treatment. Immunoneutralization of IGF-I or of IGF-II had no effect on growth rates of rats at any time during the experiment and had no effect upon plasma rGH concentrations at day 21. However, at da y 7, plasma rGH was lower in anti-IGF-I-treated rats than in controls; in contrast, plasma rGH in anti-IGF-II-treated animals at day 7 was h igher than in controls. Plasma levels of IGF-I at 49 days of age were similar regardless of the neonatal immunization treatment received. An ti-SRIF treatment of neonatal rats was associated with elevated rGH le vels, but no significant stimulation of growth. These results indicate d that growth hormone, but not circulating IGF-I or IGF-II are essenti al for normal growth in the neonatal rat.