Early changes of gene expression in fast-twitch m. Tibialis anterior of rat during chronic low-frequency stimulation

Chronic low frequency stimulation elicits in mammalian skeletal muscles fast-to-slow fibre type conversions. To identify early changes in gene expression total RNA from 12-h stimulated Tibialis anterior (TA) rat-muscles were analysed by differential display RT-PCR. Among the signals of differentially expressed mRNAs, the cDNAs of both muscle LIM protein (MLP) and parathyroid hormone-related protein (PTHrP) were identified. Both mRNAs were hardly detectable in unstimulated, but gave a strong signal in 12-h stimulated TA.MLP was previously described as an important factor for muscle differentiation but has not yet been identified in the adult skeletal muscle. In this study MLP is detected by immunoblotting as a strong band in the heart, in slow-twitch Soleus muscle and in 8-d stimulated fast twitch TA, but not in the contralateral TA. The weak immunohistochemical staining of 8 of muscle fibres in unstimulated TA increased to about 18 strong staining after 4d CLFS. MLP expression, initially restricted two type I and type IIA fibres, is induced after 4 days in these fibres and has spread to all fibres after 8 days of CLFS. A role of an elevated MLP content for muscle fibres with an increased contractile activity is being discussed.The mRNA of PTHrP is transiently up-regulated by 70 - 80 fold after 12h CLFS as compared to the unstimulated TA muscle, but returns to basal levels with ongoing stimulation after 2 days. No differences in mRNA levels between Soleus and TA were detected. In all muscles under study PTHrP was exclusively located in blood vessels of both control and stimulated muscles. Moreover, PTHrP immunohistochemistry revealed enhanced capillarisation in 8-day-stimulated muscles. In view of its vasodilative effect, the upregulation of PTHrP could be an important step related to enhanced capillarisation in response to increased contractile activity