Sodium Dependency of Active Chloride Transport Across Isolated Fish Skin (Gillichthys Mirabilis)

1. The effects of thiocyanate, ouabain, ion-substituted Ringer solution and electrochemical gradients on Na+ and Cl- transport were examined using the isolated skin of the marine teleost, Gillichthys mirabilis. 2. Bilateral replacement of Na+ with choline in the bathing solutions reduces net Cl- flux by 93%, indicating that active Cl- transport by the skin is Na-dependent. 3. Thiocyanate inhibits short-circuit current with an ED50 of 6.4 x 10(-4)M, and, at 10(-2)M, decreases Cl-efflux, influx, net flux and short-circuit current by 68, 33, 74 and 81%, respectively. 4. Ouabain (10(-5)M) reduces Cl- efflux and net flux by 56 and 86%, respectively, indicating that the Cl- transport requires Na,K-ATPase. 5. Subsequent addition of thiocyanate to ouabain-treated skin reduces Cl- efflux, net flux and short-circuit current, suggesting that the two agents operate at different sites involved in Cl- transport. 6. Unilateral substitution of gluconate for Cl- on the serosal side does not affect Cl- influx, indicating that Cl- passive transport is via Fickean diffusion, not Cl-Cl exchange diffusion. 7. The addition of NaCl to the mucosal side, which mimics the in vivo sea-water condition, increases Cl- influx and transepithelial potential and decreases tissue resistance. The net flux (secretion) of Cl- with hypertonic saline on the mucosal side (0.51 +/- 0.06 muequiv/cm2 . hr) demonstrates that the skin could secrete Cl- in vivo. 8. Na+ fluxes across the skin are passive, as the observed flux ration (efflux/influx) is similar to that predicted by the Ussing-Teorell equation under both closed- and open-circuit conditions. 9. The permeability ratio (PNa:PCl) in approximately 5.4:1.0, indicating that the skin is more permeable to Na+, and that at least part of the serosa-positive transepithelial potential may be a Na+ diffusion potential. 10. The results suggest that Cl- secretion by Gillichthys skin is secondary active transport involving Na,K-ATPase and serosal Na+.