Mysterious Ca²⁺-independent muscular contraction: déjà vu

The permeabilized cells and muscle fibres technique allows one to study the functional properties of mitochondria without their isolation, thus preserving all of the contacts with cellular structures, mostly the cytoskeleton, to study the whole mitochondrial population in the cell in their natural surroundings and it is increasingly being used in both experimental and clinical studies. The functional parameters (affinity for ADP in regulation of respiration) of mitochondria in permeabilized myocytes or myocardial fibres are very different from those in isolated mitochondria in vitro. In the present study, we have analysed the data showing the dependence of this parameter upon the muscle contractile state. Most remarkable is the effect of recently described Ca²⁺-independent contraction of permeabilized muscle fibres induced by elevated temperatures (30–37°C). We show that very similar strong spontaneous Ca²⁺-independent contraction can be produced by proteolytic treatment of permeabilized muscle fibres that result in a disorganization of mitochondrial arrangement, leading to a significant increase in affinity for ADP. These data show that Ca²⁺-insensitive contraction may be related to the destruction of cytoskeleton structures by intracellular proteases. Therefore the use of their inhibitors is strongly advised at the permeabilization step with careful washing of fibres or cells afterwards. A possible physiologically relevant relationship between Ca²⁺-regulated ATP-dependent contraction and mitochondrial functional parameters is also discussed.