Figure 1.
Activation of INa in HEK cells assessed with square-step voltage protocols in conventional VC experiments.
a, Top: Typical examples of Na+ current in response to depolarizing voltage steps from −140 mV. Bottom: Average fast and slow time constants of Na+ current inactivation. Note logarithmic ordinate scale. b, Average current-voltage relationship. c, Average steady-state activation. The solid line is the Boltzmann fit to the average data. d, Typical membrane depolarizations in response to a super- and subthreshold current pulse in a HEK cell during an alternating VC/CC experiment from a holding potential of −140 mV (top) and the first derivatives of the resulting membrane potential changes (dV/dt; bottom).
Figure 2.
‘Dynamic’ INa activation in HEK cells assessed with VC (a) and alternating VC/CC (b).
a, Typical example of Na+ current (bottom) during the upstroke phase of the ventricular action potential like waveform used as command potential in an action potential clamp experiment (top). b, Typical membrane depolarization from −85 mV (top) in response to a superthreshold current pulse and dV/dt (bottom). c, Average phase plane plots with current or dV/dt plotted against membrane potential. Note the similarity between the currrent densities of the VC and alternating VC/CC experiments. d, Average dynamic INa activation. Solid lines represent the Boltzmann fits to the average data.
Table 1.
Biophysical properties of INa at close-to-physiological conditions determined in the VC and VC/CC configurations in HEK cells and in rabbit ventricular myocytes.
Figure 3.
Voltage-dependence of INa inactivation in HEK cells assessed with VC (a) and alternating VC/CC (b).
a and b, Top: voltage clamp (a) and VC/CC (b) protocols. Bottom: typical currents (a) and dV/dt's (b) measured after a 1-s prepulse (P1) to membrane potentials between −110 and −65 mV. c, Average voltage-dependence of inactivation. Solid lines represent the Boltzmann fits to the average data.
Figure 4.
Recovery from INa inactivation in HEK cells assessed with VC (a) and alternating VC/CC (b).
a and b, Top: voltage clamp (a) and VC/CC (b) protocols with an interpulse interval of 1–1000 ms, as indicated. Bottom: typical examples of recovery from inactivation with an interpulse interval of 5 ms. c, Average recovery from inactivation. Inset, Average recovery from inactivation on a logarithmic time scale. Solid lines are double-exponential fits to the average data.
Figure 5.
Slow inactivation properties in HEK cells assessed with VC (A) and alternating VC/CC (B).
a and b, Top: voltage clamp (A) and VC/CC (B) protocols, with a conditioning prepulse interval (P1) of 10–1000 ms, as indicated, and a 30-ms interval to remove fast inactivation. Bottom: typical examples of slow inactivation with a conditioning prepulse of 1000 ms. c, Average development of slow inactivation. Note the logarithmic time scale. Solid lines are double-exponential fits to the average data.
Figure 6.
INa properties in control (CTRL) and heart failure (HF) rabbit ventricular myocytes assessed with alternating VC/CC, using the protocols shown in Figs. 1–5.
a, Typical examples of AP upstrokes (top) and their dV/dt's (bottom). b, Current-voltage relationships of the AP upstrokes. c, Dynamic INa activation curve with Boltzmann fits in solid lines. d, Steady-state voltage-dependence of inactivation with Boltzmann fits in solid lines. e, Recovery from inactivation with double-exponential fits in solid lines. f, Development of slow inactivation with double-exponential fits in solid lines.