Voltage-gated Currents in Neonatal Mouse Cardiomyocytes

Single whole cell patch electrode voltage clamp experiments were performed on neonatal mouse ventricular myocytes (NMVMs) using conventional procedures with an Axon Instruments Axopatch 1D or 200B patch clamp amplifier, Digidata 1320A or 1440 A/D converter, and pClamp8.2 or 10.1 software (Molecular Devices, San Jose, CA, USA). Transient (peak) and steady state outward potassium (I_K,to and I_K,ss) currents were recorded during 1 sec voltage steps from a holding potential (V_h) of −100 to +60 mV in 10 mV increments. Voltage-gated sodium currents (I_Na) were elicited from a V_h of −120 mV during voltage steps from −90 to +50 mV in 5 mV increments for 150 ms using reduced NaCl solutions. For the I_Na inactivation protocol, V was −120 mV and the prepulse voltage increased from −130 to −30 mV in +5-mV increments for 150 ms followed by a 30-ms activation step to −40 mV [13 (link)]. Late I_Na protocol was measured as the average current from 100 to 150 ms of the activating voltage steps or were recorded during a ramp from −80 to +10 mV in 0.1 mV/4 ms steps from a V_h of −60 mV [13 (link),28 (link)].