Hi Monika,

Different cell types respond to FCCP differently due to the binding efficiency, which is why the highest concentration does not mean the maximum response. What you can try to do first is to characterise the primary cardiomyocytes with a six point titration, ranging from 0.0125-0.2 uM. In this range, hopefully, you should be able to find a concentration that gives you the maximum respiration.

I hope that is helpful, if you need more details, please do feel free to ask.