During extrusion of the first polar body in eggs of Lymnaea stagnalis and Bithynia tentaculata a localized Ca2+ /Mg2+ ATPase activity was detected, using Ando's enzyme-cytochemical method for electron microscopy [Ando et al. (1981) Acta Histochem Cytochem 14:705-726]. The enzyme activity was distributed in a polar fashion, along the cytoplasmic face of the plasma membrane. In the eggs of Lymnaea it was found only in the vegetal hemisphere, whereas in Bithynia eggs it was localized both in the vegetal hemisphere and at the animal pole. This pattern of enzyme activity corresponds to the polar pattern of transcellular ionic currents measured with the vibrating probe, which we showed to be partially carried or regulated by calcium [Zivkovic and Dohmen (1989) Biol Bull (Woods Hole) 176 (Suppl):103-109]. The characteristics of the ATPase were studied using a variety of approaches such as ion and substrate depletions and substitutions, addition of specific inhibitors of ATPase activity, treatment with EDTA/EGTA and electron energy-loss spectrometry. The results indicate that, in Lymnaea, there are at least two enzymatic entities. The first one is a Ca2+ /Mg2+ ATPase localized along the membrane and in the cortex of the vegetal hemisphere. The second one is a Ca2+-stimulated ATPase (calcium pump of the plasma membrane) localized in a small region of the membrane at the vegetal pole. We speculate that in the eggs of Lymnaea and Bithynia a functional relationship exists between the plasma-membrane-associated ATPase activity and the transcellular ionic currents measured in the same region.

Ca2+/Mg2+ ATPase, Calcium, Ionic currents, Molluscan eggs, Polarity
dx.doi.org/10.1007/BF01681486, hdl.handle.net/1765/64672
Roux's Archives of Developmental Biology
Department of Pathology

Zivkovic, D, Créton, R, Zwaan, G, de Bruijn, W.C, & Dohmen, M.R. (1990). Polar localization of plasma membrane Ca2+/Mg2+ ATPase correlates with the pattern of steady ionic currents in eggs of Lymnaea stagnalis and Bithynia tentaculata (Mollusca). Roux's Archives of Developmental Biology, 199(3), 134–145. doi:10.1007/BF01681486