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 muscle in foetal normal and double-muscled cattle. Samples from normal and double-muscled foetuses ranging from 90 to 210 d post-conception were collected and total RNA extracted. Northern blot analysis was performed using the human 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 than those of the other transcripts and its expression remained stable throughout the gestational stages analysed. These results indicated that the regulation of IGF-II gene transcription was distinct in both genetic types. The IGF-II foetal plasma concentrations increased throughout gestation. In bovine foetuses, the first muscle cell differentiation was concomitant with a high autocrine IGF-II mRNA expression and low plasma IGF-II levels in both genetic 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.