Small signal impedance of heart cell membranes

The electrical impedance of seven-day ventricular embryonic chick heart cell membranes maintained in tissue culture was measured under voltage clamp using the two-microelectrode voltage-clamp technique. Small sinusoidal perturbations were added to the voltage-clamp potential and the amplitude and phase of the steady-state sinusoidal response in current was recorded as a function of mean clamp potential or perturbing frequency. The experimental results are compared with two models of excitability for heart: the MNT model (McAllister, Noble & Tsien, J. Physiol. (London) 251:1-59, (1975) and the BR model (Beeler & Reuter, J. Physiol. (London) 268:177-210, 1977). The small signal impedance of heart cell membranes, in theory and experiment, shows a resonance near 1 Hz and near the threshold potential. The effect of this resonance is to increase the effective length constant of the membrane for these conditions.