Abstract
1) The electrical activity of bullfrog ventricle was investigated with glass microelectrodes after the muscles had been made to accumulate Na ions in exchange for K ions.
2) The Na-loaded muscles, when immersed in recovery solutions, showed a membrane hyperpolarization of which magnitude and time course were dependent on [K] 0. Ouabain diminished this hyperpolarization. The hyperpolarization was attributed to the activity of the Na pump.
3) The Na-loaded muscles had an action potential with a markedly depressed plateau. The overshoot was also diminished. The depressed action potential gradually recovered, with changes in the membrane potential in recovery solution.
4) The recovery of the action potential was faster the higher [K] 0, not influenced by chloride removal, but inhibited by ouabain. Changes in [Ca] 0, of soaking solution for Na-loading had little effect.
5) It was concluded that the depressed plateau potential seen in Naloaded muscles was due to the increased [Na] 1, and possible mechanisms relating [Na] 1 to the plateau were discussed.
