Serum-induced myoblast proliferation and gene expression during the development of double muscled and normal cattle

Double muscled (DM) cattle possess more muscle fibers than normal beef or dairy (NM) cattle. Insulin-like growth factor-II (IGF-II) has been implicated in the regulation of myogenesis. Insulin-like growth factor binding protein 2 (IGF-BP2) regulates the actions of IGF-II. These, along with other growth factors in serum, may modulate development of bovine fetuses. Twelve DM and 60 NM fetuses were utilized to test the effect of fetal serum on muscle cell proliferation and to determine the expression of IGF-II and IGF-BP2 in the liver and muscle of bovine fetuses. Crown-rump length (CRL) was used to determine fetal age. Incorporation of 3H-thymidine by myoblast cultures was assessed using media containing serum from DM and NM fetuses grouped by CRL of $\le25$, 26-50, 51-75 or $\u3e$75 cm. Thymidine incorporation was 56%, 41%, and 41% greater (P $\u3c$.05) with serum from DM fetuses than that of NM fetuses at CRL of $\le$25, 26-50, and 51-75 cm, respectively. Apparent muscle fiber number of the semitendinosus muscle was 30% greater for DM fetuses than NM fetuses at term (P $\u3c$.05). Skeletal muscle and liver samples were taken from DM fetuses and NM fetuses representing 10 cm CRL growth increments during gestation. Total RNA preparations from these samples were subjected to northern and dot blot analysis using rat IGF-II, IGF-BP2 and human skeletal muscle alpha-actin cDNAs. Expression of skeletal alpha-actin in muscle increased (P $\u3c$.05) with age in both NM and DM fetuses, but DM fetuses expressed greater (P $\u3c$.05) amounts of alpha-actin than NM fetuses near term. Relative abundance of muscle IGF-II mRNA decreased (P $\u3c$.001) with age in DM and NM fetuses. Muscle IGF-II expression was less (P $\u3c$.05) at 20 cm CRL, but greater (P $\u3c$.05) at 40 cm CRL in DM than NM fetuses. Insulin-like growth factor II and IGF-BP2 expression in the liver increased (P $\u3c$.05) with age in DM and NM fetuses. Expression of IGF-BP2 was greater (P $\u3c$.05) near term in DM fetuses than NM fetuses. These results show DM fetuses have delayed expression of muscle IGF-II during early development and have greater blood-borne growth factor activity than NM fetuses. These factors may play a role in the development of bovine muscle cell hyperplasia