The current voltage relationship of the delayed outward current in the heart of the frog (Rana esculenta) and the tortoise (Testudo germani).

Steady state and non steady state I-V relationships of the resting membrane and delayed rectifying membrane were analysed by applying trapezoid voltage clamp pulses on atrial fibres isolated from the frog and the tortoise. The fibres were disposed in a perfusion chamber for double sucrose gap. In both types of preparation, a component of delayed rectification current was identified which was activated following a comparable time course. The amplitude of the delayed rectification current, when expressed either as normalized to the calculated membrane capacity or to the initial background current, is significantly larger in the frog than in the tortoise. Full activation of the delayed rectifying system can be demonstrated in the tortoise, while in the frog this process is presumably complicated by simultaneous accumulation of K ions in the extracellular region. The difference in magnitude of the delayed outward current has no influence on the duration of action potential which is recorded over the sucrose gap.