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Freshwater-adapted killifish (Fundulus heteroclitus) were transferred directly from soft fresh water to full-strength sea water for periods of 1 h, 3 h, 8 h and 1, 2, 7, 14 and 30 days. Controls were transferred to fresh water for 24 h. Measured variables included: blood [Na+], osmolality, glucose and cortisol levels, basal and stimulated rates of ion transport and permeability of in vitro opercular epithelium, gill Na+/K+-ATPase and citrate synthase activity and chloride cell ultrastructure. These data were compared with previously published killifish cystic fibrosis transmembrane conductance regulator (kfCFTR) expression in the gills measured over a similar time course. Plasma cortisol levels peaked at 1 h, coincident with a rise in plasma [Na+]. At 8 h after transfer to sea water, a time at which previous work has shown kfCFTR expression to be elevated, blood osmolality and [Na+] were high, and cortisol levels and opercular membrane short-circuit current (Isc; a measure of Cl- secretion rate) were low. The 24 h group, which showed the highest level of kfCFTR expression, had the highest plasma [Na+] and osmolality, elevated plasma cortisol levels, significantly lower opercular membrane resistance, an increased opercular membrane ion secretion rate and collapsed tubule inclusions in mitochondria-rich cells, but no change in gill Na+/K+-ATPase and citrate synthase activity or plasma glucose levels. Apparently, killifish have a rapid (<1 h) cortisol response to salinity coupled to subsequent (8–48 h) expression of kfCFTR anion channel proteins in existing mitochondria-rich cells that convert transport from ion uptake to ion secretion.

Opercular epithelia from seawater-adapted killifish (Fundulus heteroclitus) were dissected with the nerve intact, mounted in Ussing-style membrane chambers and bathed in symmetrical saline solutions. Nerve stimulation rapidly inhibited transepithelial current (a measure of Cl- secretion rate) by 27.3+/-3.3 % (N=22), and the effect could be sustained for more than 10 min using intermittent pulse trains at 10 Hz. The effect was blocked in a dose-dependent manner by yohimbine, but not by propranolol, atropine or tubocurarine, indicating mediation by <IMG src="/images/symbols/&agr ;.gif" WIDTH= "9" HEIGHT="12" ALIGN="BOTTOM" NATURALSIZEFLAG="3">2-adrenergic receptors. The effect was also present, but significantly diminished, in opercular membranes from animals that had been transferred to sea water for 48 h (18+/-8.6 % inhibition, N=14). The resting current and the effect were absent in membranes from freshwater-adapted animals. The addition of clonidine (1.0 micromol l-1 serosal side) started to inhibit Cl- current after 40-60 s; immediately before this, at 30 s, there was a significant rise (P<0.05, N=14) in tissue inositol 1,4,5, -trisphosphate (InsP3) level, but no change at later times, compared with LiCl-treated control membranes and measured by radiolabeled receptor assay. The results indicate that seawater-adapted killifish can decrease their Cl- secretion rate through the action of the sympathetic nervous system, a response appropriate for the entry of estuarine fish to fresh water, and that the effect is mediated by <IMG src="/images/symbols/&agr ;.gif" WIDTH="9" HEIGHT="12" ALIGN="BOTTOM" NATURALSIZEFLAG="3">2-adrenoceptors via InsP3. The results imply that euryhaline fish entering fresh water can undergo an autonomic reflex reduction in salt secretion that does not require a stress response.