Ion movements associated with chorion elevation during egg laying in trout (Oncorhynchus mykiss) in fresh water

Within 1 min of transfer from coelomic fluid to fresh water, eggs of rainbow trout (Oncorhynchus mykiss) underwent a transient loss of Na⁺ and K⁺ coupled with an elevation of the chorionic envelope. Both mechanisms were blocked by adding a monovalent cation Li⁺ or K⁺ (140 mmol·l^-1) to the fresh water, but the divalent ion Mg²⁺ (100 mmol MgCl₂·l^-1) or elevating the osmotic pressure to 300 mOsmol·l^-1 with glycine had no inhibitory effect. The blocking of Na⁺ loss occurred at external monovalent cation (LiCl) concentrations above 70 mmol·l^-1. A 20-s exposure of eggs to fresh water was sufficient to trigger Na⁺ loss and chorion elevation, even when the eggs were subsequently transferred to fresh water containing 140 mmol LiCl·l^-1. Eggs placed in a medium containing 140 mmol LiCl·l^-1 and 2 mmol Ca(NO₃)₂·l^-1 showed chorion elevation and associated Na⁺ loss after addition of calcium ionophore (20 mol·l^-1 A.23187). This activation by calcium ionophore was supressed in a Ca²⁺-free medium containing 5 mmol EGTA·l^-1.