Insulin-like Growth Factor I Gene Expression during Parr-Smolt Transformation of Coho Salmon

The status of insulin-like growth factor I (IGF-I) mRNA in the liver and gill of coho salmon (Oncorhynchus kisutch) during the parr-smolt transformation (smoltification) was examined in relation to changes in plasma levels of thyroxine, cortisol, growth hormone and somatolactin. Smoltification coincided with silvering and an increase in gill Na /K -ATPase activity occurred during mid-April and May [22]. The levels of IGF-I mRNA in the liver rose steadily from the first sampling date in February to maximum levels in April. Increases in IGF-I mRNA in the gill occurred later in smoltication. Plasma cortisol, thyroxine and growth hormone increased during the latter half of April and May. Plasma somatolactin declined in April. These results demonstrate that IGF-I gene expression in both liver and gill increased during smoltification and suggest that IGF-I expression in both organs may be involved in salmonid smoltification.