I am trying to record some Nav1.5 current from adult rat ventricular cardiomyocytes in the cell attached configuration. I am working at 37C with extracellular (bath) isotonic potassium in order to have the resting potential at 0 mV. In the pipette I'm using regular Tyrode (140 mM NaCl) + 5 uM Nifedipine. Regarding the voltage protocol, from an holding potential of +100 mV, I'm applying a ladder of 5 mV steps from +100 mV to -60 mV. I'm not compensating for the fast transient of the pipette (fast and slow pipette capacitance compensation turned OFF). The pipette resistance is between 1.3 and 1.6 MOhm and I can reach a giga-seal around 1.3 Giga Ohm.

I'm expecting to see some outward sodium current but I see nothing... When I apply the voltage protocol I see a leak holding current between 50 and 100 pA and absolutely no peak at the voltage steps.

Does anyone has some ideas about what's going on? How can I solve this problem?

(my only idea is that the holding leak current is a symptom of a loose giga-seal that is lost upon depolarization of the pipette. Maybe the membrane is not clean and I'm not actually patching the membrane but some film around the cell).