Is isosmotic MgSO4 isotonic with Condylactis gigantea tentacles? Comparison of their ionosmotic response to H2O–diluted seawater before and after exposure to isosmotic MgSO4–sodium seawater mixtures

Tentacles of Condylactis gigantea were exposed successively to H2O–NaASW dilutions, isosmotic 1 mol/l MgSO4–NaASW mixtures and returned to H2O–NaASW dilutions to explore the possible isotonicity of isosmotic MgSO4 and the reversibility of the effects of this solution on the ionosmotic characteristics of the tentacles. Total tissue water responded as predicted by van''t Hoff''s osmotic relation to H2O–diluted seawater but was not clearly affected by the admixture of seawater with isosmotic MgSO4, suggesting that this solution may act as an isotonic medium. However, total tissue water, potassium, chloride and sodium contents, as well chloride and sodium spaces were lower in MgSO4–NaASW mixtures than in H2O–diluted seawater as a consequence of the loss of these electrolytes and presumably other osmolytes accompanied by an osmotic water loss. The loss of osmolytes did not appear to be compensated by MgSO4 uptake by the tentacles with a consequent reduction of the cell compartment. Therefore, in MgSO4–NaASW mixtures, inulin space approached total tissue water, resulting in considerable uncertainty in the estimation cell volume. When the tentacles were re-exposed to H2O–diluted seawater, tissue ionic contents and ionic and inulin spaces did not revert completely to the values obtained previous to exposure to isosmotic MgSO4. Therefore, although 1 mol/l MgSO4 solution was isosmotic with artificial seawater, it could not efficiently replace the latter as an isotonic medium. [Copyright &y& Elsevier]