Age-related and tissue-specific levels of five forms of insulin-like growth factor mRNA in a teleost.

A reverse transcriptase/PCR assay was employed to amplify an internal portion of insulin-like growth factor I (IGF I) and IGF II cDNA from the RNA of juvenile rainbow trout (Oncorhynchus mykiss) in an effort to detect analogues to the multiple size forms of IGF mRNA known to exist in mammals. After cloning and sequencing the PCR products, 4 variant size forms of trout IGF I mRNA and 1 form of IGF II mRNA were evident. The full cDNA nucleotide sequence of 1 form of IGF I mRNA and IGF II mRNA have been previously reported; the full cDNA nucleotide sequence of a second form of IGF I mRNA and PCR-derived nucleotide sequences of the 2 additional IGF I mRNA forms are reported herein. We define the 4 forms of IGF I mRNA (IGF IEa-1, Ea-2, Ea-3, and Ea-4) as having E-domain peptide lengths of 35, 47, 62, and 74 predicted amino acid residues, respectively. RNAse protection assays were used to quantify the mRNA levels of each of the 5 IGFs in 8 tissues of both juvenile and adult trout, as well as in testes and ovaries of adult trout. The IGF mRNA levels, normalized with 18S rRNA levels, suggest (1) at least 1 form of IGF I and IGF II mRNA are present at both developmental stages in all tissues examined; (2) all 5 IGF mRNA forms are present at their highest levels in the liver (p < 0.05); and (3) adults have significantly higher IGF mRNA levels than juveniles in the liver (p = 0.0047).