Action potential of amphibian single auricular muscle fiber: a dual response

Abstract

The intracellular recording of a single auricle fiber of frog or toad shows a spike-dip-plateau contour, suggesting two depolarizations are involved and two deflections comprise the action potential contour. Low oxygen reduces the plateau phase. Acetylcholine applied locally by microejection (or by vagal stimulation) acts specifically to cause plateau phase to be diminished or abolished and simultaneously affects the mechanical response in a like manner. The Q₁₀ of the spike exposed alone by the acetylcholine effect is significantly different (1.2) from the Q₁₀ value of the normal prolonged response (2.4) or the mechanogram (2.5). The evidence supports the idea that two ion carrier systems exist in the excitable membrane: one which reacts rapidly, the other more slowly. In cardiac muscle the fast one causes the spike depolarization, which itself is adequate to trigger the slow response. The slow system must react not specifically with potassium but with sodium also and causes the prolonged depolarization or plateau. Recovery is probably due to slow inactivation of this second process.