Comparison of in vivo effects of insulin-like growth factors I and II and of growth hormone in hypophysectomized rats.

Pure human IGF I (43 and 103 micrograms/day) and IGF II (131 micrograms/day) were infused into hypophysectomized rats during 6 days by means of sc implanted minipumps. Their effects on several growth indices were compared with those of various doses of sc infused human growth hormone. Growth hormone infusion produced a dose-dependent rise of endogenous rat IGF from 39 (without growth hormone) to 86 microU equivalents/ml (with 400 mU hGH/day) as determined by a competitive protein binding assay with a human IGF standard. In rats receiving the two doses of IGF I, total serum IGF levels rose to 83 and 99 microU equivalents/ml, respectively, in those receiving the IGF II dose the total serum IGF level rose to 146 microU equivalents/ml. These increases corresponded to steady state levels of 168 and 286 ng/ml of immunoreactive insulin-like growth factor (IR-IGF) I and 320 ng/ml of IR-IGF II. IGF I, but not IGF II led to an increase in body weight similar to that induced by the low doses of hGH (12.5 and 25 mU, respectively). The rise of endogenous rat IGF as well as the infused human IGF I and II caused a widening of the tibial epiphysis and an increase of the [3H]thymidine incorporation into costal cartilage. With respect to these two indices IGF II was clearly less potent that IGF I. When expressed in microU equivalents of the protein binding assay, endogenous rat IGF induced by hGH appeared to be relatively more effective than infused human IGF I or II.(ABSTRACT TRUNCATED AT 250 WORDS)