Physiological characterization of 2-week regenerated soleus muscle fibres

The functional properties of 14-day regenerated rat soleus muscle was investigated in chemically skinned fibres. Regeneration was induced by myotoxic injury with bupivacaine. Myofibrillar and sarcoplasmic reticulum properties of single fibres were correlated to the expression of specific protein isoforms. The maximal specific tension of regenerating fibres was not different from that of controls, despite the smaller CSA (about 45%) of regenerating fibres. The 14-day regenerating soleus fibres expressed type 1 or 1+2A myosin heavy chain isoforms in addition to residual embryonic and/or neonatal isoforms. The regenerating fibres showed a significant right shift of pCa-tension relationships and a higher pCa threshold. SDS-PAGE analysis showed regenerated fibres containing the slow troponin C isoform as well as fibres with both the slow and fast isoform. The presence of the fast isoform was correlated to the higher pCa threshold of fibres. The caffeine threshold concentration of sarcoplasmic reticulum Ca2+ release was significantly higher in regenerated than in control fibres. Consistent with the lower sensitivity to caffeine of RyR-3, regenerated muscles expressed a significant higher level of this RyR isoform. The amount of Ca2+ released at maximally-activating caffeine concentration (20 mM) was higher than in controls. Moreover, sarcoplasmic reticulum Ca2+ capacity, measured by a light-scattering method, was higher in regenerated fibres than in controls. In contrast, Western blot analysis in the whole muscle did not show appreciable differences in SERCA isoform expression between regenerated and control muscles, suggesting that the large sarcoplasmic reticulum Ca2+ capacity of regenerating fibres is likely due to a larger volume. Consistently, the rate of Ca2+ uptake, was not different in 14-day regenerating fibres with respect to control. Finally, the regenerated muscle shows a larger amount of the cardiac isoform of DHPR with respect to the control.