ONTOGENY OF THE PORCINE INSULIN-LIKE GROWTH-FACTOR SYSTEM

The developmental expression of the individual components of the insul in-like growth factor (IGF) system in pigs was examined. Serum IGF-I a nd IGF-binding protein-3 (IGFBP-3) levels were low during fetal life a nd increased during postnatal development. Levels of mRNAs encoding th ese proteins were not greater for liver than for nonhepatic tissues (s keletal muscle, lung, kidney) and did not increase during the postnata l period, whereas hepatic growth hormone (GH) receptor mRNA expression was increased postnatally. Serum IGF-II levels exceeded IGF-I levels at all developmental stages examined and both exhibited postnatal incr eases. IGF-II mRNA abundance, in contrast, was high in the fetal tissu es with the exception of lung and declined during the perinatal transi tion. Hepatic IGFBP-2 mRNA and serum IGFBP-2 levels increased during t he latter half of gestation and then declined postnatally. The levels in muscle and liver of type I IGF receptors and the corresponding mRNA s also exhibited postnatal decreases. The discordance of changes in he patic IGF-I and IGF-II mRNA abundance with serum IGF levels during the postnatal period does not support the concept that liver is the prima ry endocrine source of IGFs in the young pig. The postnatal increases in serum IGF levels may reflect decreased plasma clearance rates of th ese peptides which may be related to the transition in IGFBP type from IGFBP-2 to IGFBP-3 in blood and the reduced tissue expression of IGF receptors.