Polar ionic currents around embryos of Lymnaea stagnalis during gastrulation and organogenesis

Embryos of Lymnaea stagnalis generate ionic currents which can be measured with the vibrating probe. Here we investigated the presence and origin of the currents during late embryonic development. During gastrulation the current pattern correlates with the animal-vegetal polarity and during organogenesis it is correlated to the newly formed antero-posterior axis. The origin of the ionic currents was studied by inhibition of the Na+/K(+)-pump with ouabain and by enzyme-cytochemical detection of the Ca(2+)-pump. Ouabain treatment resulted in a reduced current density around the embryo, indicating that the Na+/K(+)-pump contributes significantly to the net current. The Ca(2+)-pump was found to be localized in the vegetal blastomeres during gastrulation and in the larval kidney during organogenesis. It seems likely that this Ca(2+)-pump renders only a minor contribution to the net current in late embryonic development. Ionic currents have now been described in Lymnaea from the uncleaved egg up to the juvenile snail. During this period the overall current pattern changes only twice, demonstrating that the voltage gradient generated by the embryo remains stable during prolonged periods in development.