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 (I-Na) 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 I-Na with near-physiological ion concentrations and temperature is technically challenging due to the large amplitude and rapidly activating nature of I-Na, 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 I-Na properties under physiological conditions. Principal Findings: We studied I-Na 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 I-Na during a depolarizing VC step or maximal upstroke velocity, dV/dt(max), during VC/CC served as an indicator of available I-Na. In HEK cells, biophysical properties of I-Na, 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 I-Na in left ventricular myocytes isolated from control or failing rabbit hearts. Conclusions: Our results demonstrate that the alternating VC/CC technique is a valuable experimental tool for I-Na measurements under close-to-physiological conditions in cardiac myocytes.