In vivo effects of the insulin-like growth factors (IGFs) in the hypophysectomized rat: comparison with human growth hormone and the possible role of the specific IGF carrier proteins.

Insulin-like growth factors (IGF) I and II produce acute insulin-like effects or promote growth indices in hypophysectomized rats, depending on their mode of administration. Intravenous bolus injections of IGF I and II, like insulin, lower the blood sugar level and lead to a pronounced stimulation of glycogen synthesis in skeletal muscle. The two factors are equipotent in this respect. Continuous subcutaneous infusions of IGF I or II over six days have no hypoglycaemic effect. They mimic the effects of growth hormone, increasing the width of the tibial epiphysis and stimulating the thymidine-incorporating activity of costal cartilage in the absence of growth hormone. IGF I (identical to somatomedin C) is more potent than IGF II and causes a dose-dependent increase in body weight at concentrations where IGF II is still ineffective. The absence of acute insulin-like effects in long-term experiments is explained by the presence in serum of highly specific carrier proteins that interfere with the insulin-like properties of IGF I and II and restrict their capillary permeability. Growth hormone induces the formation of both IGF and the major IGF carrier protein in the liver. Thus, while IGFs mediate the actions of growth hormone on growth, growth hormone appears to modulate these actions via IGF carrier proteins.