Calcium ion triggers rapid morphological oscillation of chloride cells in the mudskipper, <Emphasis Type="Italic">Periophthalmus modestus</Emphasis>

Euryhaline teleosts rapidly regulate their ion flux at the chloride cells on entry to different salinities. The external trigger(s) for the rapid opening and closing of the chloride cell apical surface, the site of salt secretion, were examined in the skin of the mudskipper. With DASPEI (a mitochondrial probe) and Concanavalin-A (an apical surface marker of chloride cells), chloride cells were classified into two groups: those in contact and those not in contact with the external water. The fraction of chloride cells in contact with the water increased dramatically 1.5 h after transfer to seawater, and similarly after transfer to 10 mmol x l(-1) CaCl(2) solution. In comparison, transfer to 1.1 mol x l(-1) mannitol, 0.5 mol x l(-1) NaCl or 50 mmol x l(-1) MgCl(2) resulted in increases of only 40-60% of those in the seawater-transfer group. After return of the fish from 10 mmol x l(-1) CaCl(2) to freshwater, the cells in contact with the water decreased. Environmental Ca(2+) is the trigger for the morphological oscillation of chloride cell apical surface, presumably through modifications in Ca(2+) flux.