NEURAL MODULATION OF SALT SECRETION IN TELEOST OPERCULAR EPITHELIUM BY ALPHA(2)-ADRENERGIC RECEPTORS AND INOSITOL 1,4,5-TRISPHOSPHATE

Opercular epithelia from seawater-adapted killifish (Fundulus heterocl itus) were dissected with the nerve intact, mounted in Ussing-style me mbrane chambers and bathed in symmetrical saline solutions, Nerve stim ulation rapidly inhibited transepithelial current (a measure of Cl- se cretion 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 ef fect was blocked in a dose-dependent manner by yohimbine, but not by p ropranolol, atropine or tubocurarine, indicating mediation by alpha(2) -adrenergic receptors, The effect was also present, but significantly diminished, in opercular membranes from animals that had been transfer red to sea water for 48 h (18+/-8.6 % inhibition, N=14). The resting c urrent and the effect were absent in membranes from freshwater-adapted animals. The addition of clonidine (1.0 mu mol l(-1) serosal side) st arted to inhibit Cl- current after 40-60 s; immediately before this, a t 30 s, there was a significant rise (P<0.05, N=14) in tissue inositol 1,4,5,-trisphosphate (InsP(3)) level, but no change at later times, c ompared with LiCl-treated control membranes and measured by radiolabel ed receptor assay, The results indicate that seawater-adapted killifis h can decrease their Cl- secretion rate through the action of the symp athetic nervous system, a response appropriate for the entry of estuar ine fish to fresh water, and that the effect is mediated by alpha(2)-a drenoceptors via InsP(3). The results imply that euryhaline fish enter ing fresh water can undergo an autonomic reflex reduction in salt secr etion that does not require a stress response.