Replacement of troponin-I in slow-twitch skeletal muscle alters the effects of the calcium sensitizer EMD 53998.

We extracted troponin-I (TnI) from skinned rat and rabbit soleus muscle fibres using a modification of the method described by Strauss et al. (FEBS Lett 310:229-234, 1992) for replacement of TnI in cardiac preparations. Incubation of soleus muscle fibres with 10 mmol/l vanadate virtually completely abolished the Ca2+dependence of force. Immunoblot analysis revealed that more than 80% of TnI had been extracted from the preparations. The Ca2+dependence of force was restored by incubation with a complex of cardiac TnI (cTnI) and troponin-C (cTnC). We examined the effects of the Ca2+-sensitizing compound EMD 53998 on isometric tension in native porcine cardiac and rabbit soleus skinned fibres as well as soleus in which the endogenous slow skeletal TnI (ssTnI) had been replaced by cTnI (soleus-cTnI). It was found that 10 micromol/l EMD 53998 in native soleus increased maximum Ca2+-activated force to 120+/-1.4% of control. In soleus-cTnI fibres, maximum force was increased to only 105+/-0.9%, which was similar to the effect observed in cardiac muscle (108+/-0.6%). In cardiac muscle, 10 micromol/l EMD 53998 induced a leftward shift of the pCa-tension relation by 0.65 log units. In native soleus, DeltapCa was only 0.40. Again, the effect of EMD 53998 on soleus-cTnI (DeltapCa=0.56) more closely resembled the response found in cardiac muscle than that observed in native soleus muscle. The apparent TnI-isoform dependence of the effects elicited by EMD 53998 suggests that its actions are modulated by the regulatory proteins of the thin filament.