The effect of lowering external sodium on the intracellular sodium activity of crab muscle fibres.

1. Intracellular Na activity, aiNa, was continuously measured in crab (Carcinus maenas) muscle fibres using a recessed-tip Na+ -sensitive glass micro-electrode. Experiments could last up to several hours. AiNa remained stable during prolonged experiments. The mean resting aiNa was 8-4 +/- 0-02 mM (S.E. of mean for eighty-nine fibres) and the mean resting membrane potential was 65-3 mV +/- 0-3 (S.E. of mean for eighty-nine fibres). 2. Reducing [Na]o to 1/10 normal (maintaining ionic strength with equivalent amounts of either Li or Tris) caused a large and rapid fall of aiNa. There appeared to be two components of the effect, a fast and slow. The initial fast rate of decrease was about 3-5 m-mole/min decreasing to half this value in about 1 min. The rate of decrease of aiNa was not linearly related to aiNa. The size of the fast change of aiNa was related to the magnitude of the Na gradient across the membrane. 3. High concentrations (2 x 10-4m) of ouabain caused a very slow rise of aiNa by 1 or 2 mn/hr. This was equivalent to a net Na influx of between 1 and 10 p-mole/cmi. sec, depending on whether or not a correction was applied to account for the increased surface area of the fibre caused by the invaginating cleft system. 4. The response to low Nso was virtually insensitive to the removal of Ko or to prolonged reatment with high concentrations of ouabain (2 x 10-4 m; 100 min) and so could not readily be attributed to active Na/K pumping. 5. The response of aiNa to low Nao was reversibly inhibited by high concencentrations of Mn (50 mm) and by low concentrations of La (3-1 mm). La itself stimulated a rapid fall of aiNa in normal Nao.