GROWTH HORMONE-INDEPENDENT EXPRESSION OF INSULIN-LIKE GROWTH-FACTOR-IMESSENGER-RIBONUCLEIC-ACID IN EXTRAHEPATIC TISSUES OF THE CHICKEN

The sex-linked dwarf (SLD) chicken, which lacks GH receptor (GHR), and its normal littermates provide a useful experimental system to invest igate OH-dependent cellular responses. The GH dependence of insulin-li ke growth factor I (IGF-I)expression in tissues was examined in SLD an d normal chickens of the Gifu 20 strain. Four weeks after hatching, th e most abundant expression of IGF-I messenger RNA (mRNA) was observed in liver of normal chickens, whereas no IGF-I mRNA expression was dete cted in that organ of dwarf chickens. On the contrary, in extrahepatic tissues such as spleen, lung, brain, kidney, heart, intestine, thymus , and muscle, IGF-I mRNA expression was equally observed in normal and GHR-lacking dwarf chickens. In the testis, expression of IGF-I mRNA w as enhanced by about 5-fold in dwarf chickens, showing an expression l evel comparable to that in normal liver. On day 16 in the embryonic st age, IGF-I mRNA was expressed in muscle, brain, eye, heart, and lung i n both normal and SLD chick embryos. However, no IGF-I mRNA expression was observed in liver or kidney of normal and dwarf chick embryos. Th ese results suggest that in chicken, IGF-I mRNA is expressed in liver in a GH-dependent manner after hatching, whereas in other tissues, mRN A expression is independent of GH and GHR before and after hatching, e xcept for testis, in which GH seems to inhibit IGF-I mRNA expression.