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 se rum from DM fetuses stimulated proliferation of L6 myoblasts to a grea ter extent than serum from NM fetuses. Although the exact role of insu lin-like growth factor-II (IGF-II) in the regulation of fetal myogenes is is unknown, it has been shown to serve as a autocrine-acting growth factor during terminal differentiation of myoblasts in mitogen-deplet ed culture media. Delay of IGF-II expression may alter the ultimate nu mber of muscle fibers formed during fetal development. To investigate this, forty-seven skeletal muscle and twenty-nine liver samples were c ollected from NM fetuses representing fetuses grouped by crown-rump le ngths (CRL) of < 20, 21-30, 31-40, 41-50, 51-60, 61-70 or > 70 cm. Twe lve DM fetuses representing 20, 40, 50, and 85 cm were compared to NM groups with the same CRL. Total RNA preparations from these samples we re subjected to northern and dot blot analysis using rat IGF-II and be ta 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 increa sed (P<0.05) in both DM and NM fetuses with CRL. Mean concentrations o f 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 5 3 cm CRL, DM muscle IGF-II was greater (P<0.05) than that of NM fetuse s. 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.