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Journal of Experimental Biology 59,543-564 (1973)
Published by Company of Biologists 1973

The Ionic Properties of the Capsular Fluid bathing Embryos of Lymnaea Stagnalis and Biomphalaria Sudanica (Mollusca: Pulmonata)

H. H. TAYLOR ¹

¹ Department of Zoology, University of Newcastle upto Tyne; Department of Zoology, University of Canterbury, Christchurch, New Zealand

1. The cations sodium, potassium, calcium and magnesium are always at a higher concentration in the capsular fluid (perivitelline fluid) of Lymnaea stagnalis and Biomphalaria sudanica than in the bathing medium. The concentration of each ion is a complex function of the concentrations of all ions in the medium and of pH.

2. Typical values for the ratio of capsular concentration to medium concentration for Lymnaea capsules isolated in slightly alkaline water of average hardness are 2·4-5 for monovalent and 18-40 for divalent cations.

3. Lowering the pH of the medium from 8·5 to 4·0 reduces the capsular concentrations of monovalent ions by about 45% and of divalent ions by about 85%.

4. Consideration of the potential difference between the capsular fluid and the medium, and comparison of the monovalent and divalent ion ratios, indicates that a Donnan equilibrium exists across the capsular membrane.

5. On dilution of the medium the above ratios may rise more than tenfold and the potential difference may rise from about 23 mV to above 60 mV (inside negative). It is concluded that the capsular fluid/membrane system buffers the internal cation activities against changes in external cation activities, particularly in the case of calcium.

6. It is suggested that the ‘calcium-buffer’ may be important in maintaining ion-uptake mechanisms near saturation, in reducing the permeability of the embryo to salts and water, in maintaining cellular adhesion and in morphogenesis.

7. The fluxes of ²²Na⁺ and ⁴⁵Ca²⁺ across the capsular membrane have half-times of40-100 sec and about 300 sec respectively. The membrane offers little resistance to the free diffusion of these ions.

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