Abstract
1. The effect on pregnant mouse of removing external Na was investigated by substituting sucrose, Tris, choline, and Li for Na. The membrane potential as well as the spike potential were not noticeably changed by a reduction of external Na to half (69mM Na) in Krebs solution. Although a reduction of Na to 15.5mM produced a slight depolarization, the amplitude of the action potentials was little affected. Reducing external Na to less than 5mM or to zero produced a prompt and sustained depolarization to-25mV, with cessation of spike activity and contracture. These observations are contrary to those expected from the Goldman equation.
2. Spike potentials were evoked by conditioning hyperpolarization for about 20 min in a Na-free solution. These then gradually diminished and eventually disappeared in 40min. Spontaneous spikes, which were abolished in Na- and Ca-free solution, reappeared upon subsequent addition of Ca (2.5mM).
3. The relationship between the amplitude of the action potential and the membrane potential, which was displaced by external current application, was investigated in normal and in low Na solutions. The relationship was S-shaped, with the amplitude of evoked action potentials decreasing at high membrane potentials. Its implication was discussed.
4. It was speculated that reduction of external Na inhibited the extrusion of Ca from the cell, hence producing a contracture in Na-free solution. Removal of external Na would therefore depolarize the membrane by changing the distribution of Ca across it. Alternatively, reduction of intracellular Na in Na-free solution may depress the electrogenic Na pump.
