Insulin-like growth factor II (IGF-II) mRNA expression during skeletal muscle development of double-muscled and normal bovine foetuses

We investigated the IGF-II gene expression in developing Semitendinosus mus cle in foetal normal and double-muscled cattle. Samples from normal and dou ble-muscled foetuses ranging from 90 to 210 d post-conception were collecte d and total RNA extracted. Northern blot analysis was performed using the h uman IGF-II cDNA probe. Five IGF-II transcripts, 5.1, 4.4, 3.7, 2.6 and 1.7 kb, were detected in muscle samples. Throughout gestation, all transcripts , except for the 5.1 kb one, decreased similarly in both genetic types. In double-muscled foetuses, the amount of the 5.1 kb transcript was higher tha n those of the other transcripts and its expression remained stable through out the gestational stages analysed. These results indicated that the regul ation of IGF-II gene transcription was distinct in both genetic types. The IGF-II foetal plasma concentrations increased throughout gestation. In bovi ne foetuses, the first muscle cell differentiation was concomitant with a h igh autocrine IGF-II mRNA expression and low plasma IGF-II levels in both g enetic types. The second step of muscle cell differentiation was associated with high IGF-II plasma concentrations and the autocrine expression of IGF -II was reduced. (C) Inra/Elsevier, Paris.