Re-evaluation of the action potential upstroke velocity as a measure of the Na+ current in cardiac myocytes at physiological conditions

Background: The SCN5A encoded sodium current (INa) generates the action potential (AP) upstroke and is a major determinant of AP characteristics and AP propagation in cardiac myocytes. Unfortunately, in cardiac myocytes, investigation of kinetic properties of INa with near-physiological ion concentrations and temperature is technically challenging due to the large amplitude and rapidly activating nature of INa, which may seriously hamper the quality of voltage control over the membrane. We hypothesized that the alternating voltage clamp-current clamp (VC/CC) technique might provide an alternative to traditional voltage clamp (VC) technique for the determination of INa properties under physiological conditions. Principal Findings: We studied INa under close-to-physiological conditions by VC technique in SCN5A cDNA-transfected HEK cells or by alternating VC/CC technique in both SCN5A cDNA-transfected HEK cells and rabbit left ventricular myocytes. In these experiments, peak INa during a depolarizing VC step or maximal upstroke velocity, dV/dtmax, during VC/CC served as an indicator of available INa. In HEK cells, biophysical properties of INa, including current density, voltage dependent (in)activation, development of inactivation, and recovery from inactivation, were highly similar in VC and VC/CC experiments. As an application of the VC/CC technique we studied INa in left ventricular myocytes isolated from control or failing rabbit hearts.