Myosin heavy chain fibre type composition in foals: analyses at the mRNA and protein level.

An optimal developed musculoskeletal system is vital for the performance of the horse. Previously, we showed that in the m. gluteus medius from adult untrained horses, identical mRNA and protein expression patterns were found in the majority of fibres. However, co-expression of IIa and IId/x myosin heavy chain (MyHC) was substantially more common at the protein than at the mRNA level, suggesting a transcriptionally controlled fine-tuning of these 2 genes.To analyse the MyHC transcripts and proteins (including the cardiac alpha isoform) in the same muscle during post natal development when the muscle is adapting to movement and load.Biopsies were taken from the m. gluteus medius of 2 Dutch Warmblood foals at 0, 2, 4, 22 and 48 weeks of age. mRNA was compared to protein expression on a fibre-to-fibre basis using in situ hybridisation and immunofluorescence. The MyHC slow (I), alpha, IIa and IId/x isoforms were analysed.At all ages the expression of the mRNA and protein MyHC isoforms was almost identical. Surprisingly, coexpression of the IIad isoform was also detected at the mRNA level especially early in life. The transcript of the alpha isoform was only detectable at young age, indicating silencing of the gene around birth.During the first year of life, MyHCs are continuously adapting at the mRNA and protein level. Additionally, the regulation of hybrid fibres is different from that in adult fibres.We postulate that interfering in this process by e.g. early training will be levelled out by the maturation of the muscle.