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 rate of ion secretion on entry to fresh water. Killifish (Fundulus heteroclitus) transferred from full-strength sea water to fresh water showed large reductions 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 applied on the basolateral side of opercular epithelia. This hypotonic shock reversibly reduced the short-circuit current (I(sc), equivalent to the rate of secretion of Cl(-)) in a dose-dependent fashion, with a 40 mosmol kg(-)(1) hypotonic 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(sc) was accompanied by reductions in epithelial conductance (G(t)) but no significant change in transepithelial potential (V(t)). The hypotonic inhibition was apparently not Ca(2+)-mediated because Ca(2+)-depleted salines, thapsigargin and ionomycin all failed to block the reduction in I(sc) produced by hypotonic shock. The inhibition was not mediated via a reduction in intracellular cyclic AMP level because cyclic AMP levels, measured by radioimmunoassay, were unchanged by hypotonic shock and by 1.0 micromol l(-)(1) clonidine (which inhibits I(sc) by changing intracellular [Ca(2+)]) but were increased markedly by 1.0 micromol l(-)(1) isoproterenol, a positive control. The protein tyrosine kinase inhibitor genistein (100 micromol l(-)(1)), but not its inactive analogue daidzein, inhibited I(sc) in normal osmolality but produced a stimulation of I(sc) after hypotonic shock (and after clonidine treatment). The inhibitory 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 tyrosine phosphorylation in cell-swelling-induced inhibition of Cl(-) secretion when euryhaline teleosts adapt to fresh water.