Fibre-type specificity of interleukin-6 gene transcription during muscle contraction in rat: association with calcineurin activity

In this study, we quantified the transcription of the interleukin-6 (IL-6) gene in individual fibres and the associated changes in calcineurin activity assessed at the cellular level during prolonged muscle contraction. Individual myofibres were isolated from plantaris and soleus muscles of rats at the end of an exhaustive running exercise test (n=10), categorized according to their myosin heavy chain isoform content, and compared to those of resting rats (n=10). Using real-time PCR analysis in individual fibres, a marked rise in IL-6 transcript levels occurred in type I and IIa fibres at the end of exercise (P<0.05). Transcription of the gene encoding for the modulatory calcineurin-interacting protein-1 (MCIP-1), a sensitive indicator of calcineurin activity, also mainly increased in type I and IIa fibres (P<0.05). Moreover, a slight increase in MCIP-1 mRNA levels was observed in type IIx (P<0.05). Fibre types determined by immunohistochemistry were qualitatively examined for glycogen content using periodic acid-Shiff staining, and no direct relationship was found, at the cellular level, between glycogen content, fibre-type and IL-6 transcription. Our data clearly suggest that IL-6 gene transcription was mainly observed in early recruited myofibres and that contraction-induced IL-6 transcription could be associated with enhanced calcineurin activity.