Changes in FGF and FGF receptor expression in low-frequency-stimulated ratmuscles and rat satellite cell cultures

This study compares effects of chronic electrical stimulation on the expres sion levels of FGF-1, FGF-2 and their receptors (FGFR1, FGFR4) in rat tibia lis anterior (TA) muscle of hypothyroid rat, as well as in satellite cell c ultures derived from normal rat TA and soleus (SOL) muscles. In 5-day (5-d) -stimulated hypothyroid TA muscle, FGF-1 and FGF-2 mRNA levels were threefo ld elevated over control. FGFR1 and FGFR4 mRNAs were twofold and 1.5-fold e levated, respectively. In longer stimulated muscles, FGF-1 and FGFR4 mRNAs returned to basal levels, whereas FGF-2 mRNA remained elevated. FGFR1 mRNA decreased to control levels in 10-d stimulated muscles, but increased again after 20 davs of stimulation. SOL and TA-derived satellite cell cultures w ere stimulated for 5 days. At this time point changes in myosin heavy chain isoforms were detectable consisting of increases in MHCI mRNA and decrease s in MHCIIb and MHCIId mRNA. The comparison between 5-d-stimulated hypothyr oid TA muscle and 5-d-stimulated TA- and SOL-derived satellite cell culture s revealed differences in the expression of FGF-1 and FGF-2, but similar ex pression levels of FGFR1 and FGFR4. Even though FGF-1 and FGF-2 mRNAs were elevated in the satellite cell cultures, their increases were less pronounc ed than in the stimulated hypothyroid muscle. Taking into consideration tha t skeletal muscle contains muscle fibres and various non-muscle tissues, e. g. blood vessels, these results suggest that the latter contribute to the o bserved increases in FGF-1 and FGF-2 expression in stimulated muscle.