Membrane currents underlying delayed rectification and pace‐maker activity in frog atrial muscle

A double sucrose gap method has been used to polarize and voltage clamp frog atrial muscle strips.2. In response to steady depolarizing currents, normally quiescent strips often show pace-maker activity, and long lasting depolarization occurs when the current is terminated.3. Voltage clamp experiments reveal the presence of two current components underlying delayed rectification.4. The first of these components has a time constant which varies with potential and is approximately 500 msec at -90 mV. Its reversal potential usually lies between -70 and -40 mV and has always been found to be positive to the resting potential of normally quiescent fibres.5. The time constant of the second component is extremely slow (tau unk 5 sec at -90 mV). Its reversal potential is much more positive than that of the faster component.6. The results confirm the presence of a component of inward current which is insensitive to tetrodotoxin (TTX), having an activation threshold about 20 mV positive to the sodium threshold. This current differs from the two components underlying delayed rectification both in its greater speed of activation and in showing inactivation. The inactivation of this TTX-insensitive current is also a fairly rapid process.7. It is suggested that pace-making in sino-atrial muscle may depend upon the deactivation of the faster component of delayed rectification and that the TTX-insensitive inward current is also involved.