EXPRESSION OF MYOSIN HEAVY-CHAIN AND OF MYOGENIC REGULATORY FACTOR GENES IN FAST OR SLOW RABBIT MUSCLE SATELLITE CELL-CULTURES

We investigated the myogenic properties of rabbit fast or slow muscle satellite cells during their differentiation in culture, with a partic ular attention to the expression of myosin heavy chain and myogenic re gulatory factor genes. Satellite cells were isolated from Semimembrano sus proprius (slow-twitch muscle; 100% type I fibres) and Semimembrano sus accessorius (fast-twitch muscle; almost 100% type II fibres) muscl es of 3-month-old rabbits. Satellite cells in culture possess differen t behaviours according to their origin. Cells isolated from slow muscl e proliferate faster, fuse earlier into more numerous myotubes and mat ure more rapidly into striated contractile fibres than do cells isolat ed from fast muscle. This pattern of proliferation and differentiation is also seen in the expression of myogenic regulatory factor genes. M yf5 is detected in both fast or slow 6-day-old cell cultures, when sat ellite cells are in the exponential stage of proliferation. MyoD and m yogenin are subsequently detected in slow satellite cell cultures, but their expression in fast cell cultures is delayed by 2 and 4 days res pectively. MRF4 is detected in both types of cultures when they contai n striated and contractile myofibres. Muscle-specific myosin heavy cha ins are expressed earlier in slow satellite cell cultures. No adult my osin heavy chain isoforms are detected in fast cell cultures for 13 da ys, whereas cultures from slow cells express neonatal, adult slow and adult fast myosin heavy chain isoforms at that time. In both fast and slow satellite cell cultures containing striated contractile fibres, n eonatal and adult myosin heavy chain isoforms are coexpressed. However , cultures made from satellite cells derived from slow muscles express the slow myosin heavy chain isoform, in addition to the neonatal and the fast isoforms. These results are further supported by the expressi on of the mRNA encoding the adult myosin heavy chain isoforms. These d ata provide further evidence for the existence of satellite cell diver sity between two rabbit muscles of different fibre-type composition, a nd also suggest the existence of differently preprogrammed satellite c ells.