Insulin-like growth factor-II expression in developing skeletal muscle of double muscled and normal cattle

Double muscled (DM) cattle possess nearly 40% more muscle fibers than normal muscled (NM) beef or dairy cattle. Previous work showed that serum from DM fetuses stimulated proliferation of L6 myoblasts to a greater extent than serum from NM fetuses. Although the exact role of insulin-like growth factor-II (IGF-II) in the regulation of fetal myogenesis is unknown, it has been shown to serve as a autocrine-acting growth factor during terminal differentiation of myoblasts in mitogen-depleted culture media. Delay of IGF-II expression may alter the ultimate number of muscle fibers formed during fetal development. To investigate this, forty-seven skeletal muscle and twenty-nine liver samples were collected from NM fetuses representing fetuses grouped by crown-rump lengths (CRL) of < 20, 21-30, 31-40, 41-50, 51-60, 61-70 or > 70 cm. Twelve DM fetuses representing 20, 40, 50, and 85 cm were compared to NM groups with the same CRL. Total RNA preparations from these samples were subjected to northern and dot blot analysis using rat IGF-II and beta actin cDNAs and a human 28S rRNA oligomer. IGF-II transcripts of 4.5, 3.6, 2.75, 2.5, and 1.15 kilobases (kb) were detected in liver and muscle RNA from both DM and NM fetuses. Liver IGF-II expression increased (P < 0.05) in both DM and NM fetuses with CRL. Mean concentrations of muscle IGF-II mRNA initially increased (P < 0.05), then decreased (P < 0.05) with CRL in DM and NM fetuses. Muscle IGF-II mRNA was greater (P < 0.05) for NM fetuses compared to DM fetuses at 20 cm CRL, whereas at 53 cm CRL, DM muscle IGF-II was greater (P < 0.05) than that of NM fetuses. These results show that the maximum expression of muscle IGF-II is delayed in DM fetuses compared to NM fetuses. This delayed expression may play an explicit role in controlling myogenesis in the development of double muscle cattle.