Tissue Distribution of Insulin-Like Growth Factor I and II Messenger Ribonucleic Acid in the Adult Rat*

The insulin-like growth factors (IGFs) have both metabolic and growth-promoting activities in many cell and tissue types. Although the IGFs are present in serum, they are also thought to have important autocrine and paracrine functions. Using complementary DNA (cDNA) probes for rat IGF-I and mouse IGF-II, we have investigated the tissue distribution of messenger RNAs (mRNAs) for these growth factors in adult rats. IGF-I cDNA hybridized with three groups of transcripts, 7.0, 1.8 and 0.7-1.1 kilobases, which were detectable in all tissues examined, with liver demonstrating the highest level of expression. IGF-II cDNA also hybridized to a number of mRNAs, the most abundant of which was 4.0 kilobases. Of the tissues examined, IGF-II expression was highest in the brain, barely detectable in the liver, and undetectable under the conditions used, in lung, ovary, testes, and mammary gland. These studies support the notion of paracrine or autocrine function for IGF-I and demonstrate tissue-specific IGF-II expression in the adult rat.