Intracellular sodium and the differentiation of amphibian embryonic neurones

1. Experiments have been done to examine the mechanism of the inhibition of neural differentiation produced by inhibiting the sodium pump with cardiac glycosides during the mid-neural fold stages of development of the amphibian embryo. Neural differentiation was assessed quantitatively by counting the number of neurones that undergo primary differentiation in tissue culture, as a proportion of the total number of differentiated cells.2. Inhibition of the sodium pump by lowering the extracellular potassium concentration ([K(o)]) to 0 during the mid-neural fold stages inhibited neural differentiation.3. Raising the extracellular calcium concentration to 10 mM during treatment with strophanthidin protected differentiating neurones from the effects of the sodium pump inhibitor. Lowering [Ca](o) to 0.05 mM potentiated the effect of low doses of glycoside.4. In the presence of high extracellular calcium and 5 x 10(-6) M-strophanthidin the membrane potential of neural plate cells remained close to the levels recorded at the beginning of neurulation; the normal increase in resting potential was not restored.5. Addition of 10 mM-Sr(2+) to the bathing medium also protected nerve cells against the inhibition produced by strophanthidin; Sr(2+) was less effective than Ca(2+).6. Addition of either 10 mM-Mg(2+) or Mn(2+) had no effect on the inhibition of differentiation produced by strophanthidin.7. Addition of Mn(2+) along with high Ca(2+) prevented calcium from exerting its protective effect.8. The eyes of embryos treated with high Ca(2+) together with strophanthidin during neurulation and then allowed to grow into tadpoles developed normally. When Mn(2+) was present together with Ca(2+) and strophanthidin the eyes were disrupted similarly to those of embryos treated with strophanthidin alone.9. Replacement of extracellular sodium with equimolar amounts of choline or lithium prevented the cardiac glycoside from inhibiting neural differentiaion.10. The protection afforded by lowering [Na](o) was abolished when [Ca](o) was simultaneously lowered to 0.05 mM.11. Tadpoles from embryos treated with low extracellular sodium together with strophanthidin during neurulation had normal eyes compared to those treated with strophanthidin alone.12. Measurement of the intracellular sodium concentration ([Na](i)) with sodium-sensitive micro-electrodes put [Na](i) at about 30 mM before the neural folds lift. As the sodium pump is activated (stages 14(1/2)-15)[Na](i) in the neural plate falls; by the end of the mid-neural fold stage it is less than 10 mM.13. Addition of 5 x 10(-6)M-strophanthidin to the bathing fluid before activation of the sodium pump prevented the fall in [Na](i); in embryos where [Na](i) had begun to drop strophanthidin produced a rise to about 30 mM.14. When 10 mM-calcium was present along with strophanthidin [Na](i) fell to about 17 mM during neurulation, despite inhibition of the sodium pump.15. It is concluded that it is unlikely that either abolition of the normal increase in resting potential or a fall in gap junction permeability is responsible for the reduction in neural differentiation produced by blocking the sodium pump during neurulation.16. The results are consistent with the view that strophanthidin achieves its effect by preventing the fall in [Na](i) that occurs during normal neurulation because of activation of the sodium pump. They are discussed in the light of this suggestion.