EXPRESSION OF LACTATE-DEHYDROGENASE, MYOSIN HEAVY-CHAIN AND MYOGENIC REGULATORY FACTOR GENES IN RABBIT EMBRYONIC MUSCLE-CELL CULTURES

The expression of myogenic regulatory factors (MRFs), lactate dehydrog enase (LDH) and myosin heavy chains (MyHC), as markers of myogenesis, metabolism and contractility respectively, were investigated during di fferentiation of rabbit embryonic muscle cells in primary culture. Myf 5, MyoD and myogenin mRNAs were abundantly expressed at day 1 of cultu re. The expression of Myf5 and MyoD mRNA transcripts decreased sharply as myoblasts fused and differentiated into myotubes, whilst myogenin mRNA was maintained throughout the duration of the culture. Ln contras t, MRF4 mRNA was weakly expressed on day 1 of culture, its expression increased slightly as myoblasts fused and reached a maximum level in 7 -day-old cultures containing striated myofibres. The specific activity of LDH increased linearly during myoblast proliferation and fusion. I n 7-day-old cultures, LDH-M mRNA (dominant in glycolytic muscles) and LDH-H mRNA (predominant in perinatal and oxidative muscles) represente d 38% and 62% of total LDH mRNA respectively. At this stage, immunocyt ochemical staining with perinatal and adult-type MyHC antibodies showe d that embryonic and perinatal MyHC isoforms were expressed in all myo tubes, while few of them were stained by type I MyHC antibody. However , none of them expressed adult type II MyHC. The latter results were f urther supported by RT-PCR analysis of adult-type MyHC mRNA which show ed that only the type I MyHC mRNA transcript was expressed. These data were in agreement with those reported in vivo on perinatal rabbit mus cles. They differed from those obtained on cultured satellite cells is olated from adult rabbit fast-twitch or slow-twitch muscles which did not express embryonic MyHC, and instead expressed fast- or slow-type M yHC according to their muscle origin. Taken together, these results fu rther suggest that myogenic mononucleated cells express different prop erties in vitro according to their developmental origin as well. as pr operties related to those of the muscles from which they were isolated . (C) Chapman & Hall Ltd.