Control of epithelial Cl- secretion by basolateral osmolality in the euryhaline teleost Fundulus heteroclitus

Euryhaline teleost fish adapt rapidly to salinity change and reduce their r ate of ion secretion on entry to fresh water. Killifish (Fundulus heterocli tus) transferred from full-strength sea water to fresh water showed large r eductions in plasma [Na+] and osmolality at 6 h which were corrected by 24 h. To mimic this in vitro, a hypotonic shock of 20-70 mosmol kg(-1) was app lied on the basolateral side of opercular epithelia. This hypotonic shock r eversibly reduced the short-circuit current (I-sc, equivalent to the rate o f secretion of Cl-) in a dose-dependent fashion, with a 40 mosmol kg(-1) hy potonic shock reducing I-sc by 58+/-4.6% in 40 min. Similar reductions in [ NaCl], but with added mannitol to maintain osmolality, were without effect, indicating that the effect was purely osmotic. Hypotonic inhibition of I-s c was accompanied by reductions in epithelial conductance (G(t)) but no sig nificant change in transepithelial potential (V-t). The hypotonic inhibitio n was apparently not Ca2+-mediated because Ca2+-depleted salines, thapsigar gin and ionomycin all failed to block the reduction in I-sc produced by hyp otonic shock. The inhibition was not mediated via a reduction in intracellu lar cyclic AMP level because cyclic AMP levels, measured by radioimmunoassa y, were unchanged by hypotonic shock and by 1.0 mu mol l(-1) clonidine (whi ch inhibits I-sc by changing intracellular [Ca2+]) but were increased marke dly by 1.0 mu mol l(-1) isoproterenol, a positive control. The protein tyro sine kinase inhibitor genistein (100 mu mol l(-1)), but not its inactive an alogue daidzein, inhibited I-sc in normal osmolality but produced a stimula tion of I-sc after hypotonic shock (and after clonidine treatment). The inh ibitory effects of genistein and hypotonicity were not additive, suggesting that the same portion of the I-sc was inhibited by both treatments. These data are consistent with a model for Cl- transport regulation involving tyr osine phosphorylation in cell-swelling-induced inhibition of Cl- secretion when euryhaline teleosts adapt to fresh water.