An investigation on the inactivation process of sodium currents in single frog ventricular cells.

Abstract

The characteristics of sodium currents (INa) in single frog ventricular cells were studied with the oil gap method. This method improves time-and space-control of the membrane potential under the voltage clamp, thereby making possible accurate analysis of fast events of INa. In this preparation the threshold of INa was about -60 mV and the reversal potential was 58 mV, which is close to the value calculated by the Nernst equation for sodium ions. Because the instantaneous current-voltage (I V) relationship is linear, the ease of permeation of sodium ions through Na + channels is well expressed by the chord conductance. The falling phase of INa and the time course of recovery from inactivation follow a time course of single exponential function. The time constants for on-and off-processes of inactivation at the same membrane potential are very close to each other, indicating only a single state of inactivation. Though almost all properties of INa were well described by Hodgkin-Huxley's model, a clear delay of onset of inactivation was demonstrated by two-pulse experiment. In this report the modified kinetics scheme was proposed which can account for both a delay of onset of inactivation development and a falling phase of INa that follows a single exponential time course.

Exponential activation of the cardiac Na+ current in single guinea‐pig ventricular cells.

“…The fact that inactivation was negligible at -90 mV agrees well with the result in multicellular preparations: the fmax studies (Weidmann, 1955), and 274 CARDIAC Na+ CURRENT ACTIVATION the classical studies using the sucrose-gap method or the two-microelectrode method (Haas, Kern, Einwichter & Tarr, 1971;Colatsky, 1980). The application of the oilgap method to frog cardiac cells (Miyoshi & Seyama, 1989) also failed to show the voltage-shift of inactivation. We believe that the voltage dependence of the gating kinetics measured using the oil-gap method is relevant to physiological conditions.…”

“…Also, the Na+ current recorded by the oil-gap method does not show the voltage shift of the kinetics Miyoshi & Seyama, 1989), which was frequently observed using different methods (Cachelin, De Peyer, Kokubun & Reuter, 1983;Kunze, Lacerda, Wilson & Brown, 1985;Makielski, Sheets, Hanck, January & Fozzard, 1987;Follmer, Ten Eick & Yeh, 1987;Kimitsuki, Mitsuiye & Noma, 1990). In this study we could establish stable sealing resistances in the gigaohm range across the oil gap, and compensate more than 90 % of Rs.…”

State‐dependent action of grayanotoxin I on Na+ channels in frog ventricular myocytes

“…To facilitate comparison with the integrated channel conductance T oNa , these current records were divided by the driving force for Na + ions. Although the interval (100 ms) between prepulses was long enough for Na + channels to recover from fast inactivation (Miyoshi et al 1988), peak amplitude of I Na was gradually attenuated (upper most and lower most records in Fig. 8 B) .…”