Presence of Insulin-Like Growth Factors and Their Binding Proteins in Rat Milk

Abstract

The insulin-like growth factors (IGF’s) are potent mitogens in a variety of mammalian cell types in vitro and in vivo (1–3). Because of the rapid growth rate during the perinatal period, recent interest has focussed on the IGF’s as possible modulators of fetal and neonatal somatic and organ growth. Significant concentrations of IGF-1 and IGF-2 have been observed in fetal blood in a variety of mammalian species (4–6). In addition, recent demonstration of active IGF synthesis using probes for IGF mRNA in fetal and neonatal tissues has led credence to this hypothesis. Active synthesis and expression of Type 1 and 2 IGF receptors has been documented for a wide variety of cell types in several animal species in fetal life as well (7–8). Interestingly, although the production of IGF receptors in developing gastrointestinal (GI) tissues is high (9), expression of IGF specific mRNA is low in these tissues in comparison to that found in other organs such as liver and brain (7, 17) Since, in the neonate, GI tissues represent a rapidly dividing and differentiating cell mass, the possibility exists that exogenously derived IGF’s might play a role in postnatal GI growth. IGF’s are also found in most biological fluids tightly bound to at least several molecular weight species of binding proteins (BP’s). Whether these BP’s act to carry the IGF’s to distant sites, fulfilling an endocrine function; act as a reservoir of IGF; or act to protect IGF from proteolytic degradation is unclear at present.