Abstract
The effects of various external media on changes in the transmembrane action potential of the rabbit atrium during repetitive stimulation after a long period of rest were studied.
1) The action potential elicited after the rest was characterized by a spike potential followed by a low-level plateau. Subsequent repetitive stimulation caused a progressive slowing down of the repolarization, an enhancement of the plateau and an increase in the amplitude of the action potential.
2) Sodium lack and TTX added solutions caused a considerable decrease in the initial rapid rise of a train of action potentials with little modification of the plateau. A gradual prolongation and an increase in the amplitude of the plateau during the repetitive stimulation were also observed with these solutions.
3) ACh abolished the plateau phase of the action potential without any modification of the initial spike potential, and suppressed changes in the action potential by repetitive stimulation remarkably. On the other hand, the plateau and the changes that were caused by repetitive stimulation were enhanced by the application of TEA.
4) Calcium-free and calcium-rich solutions had little effect on the first action potential after the rest. In the subsequent action potentials, however, some shortening of the plateau was caused in both solutions.
5) From the results, it was suggested that changes in the action potential by the repetitive stimulation could be explained by a decrease in the potassium conductance of the membrane in the sequence of stimulation.
