Sodium dependency of active chloride transport across isolated fish skin (Gillichthys mirabilis).

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Abstract
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, G. mirabilis. 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. Thiocyanate inhibits short-circuit current with an ED50 of 6.4 .times. 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. Ouabain (10-5 M) reduces Cl- efflux and net flux by 56 and 86%, respectively, indicating that the Cl- transport requires Na,K-ATPase. Subsequent addition of thiocyanate to ouabain-treated skins reduces Cl- efflux, net flux and short-circuit current, suggesting that the 2 agents operate at different sites involved in Cl- transport. 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. The addition of NaCl to the mucosal side, which mimics the in vivo sea water condition, increases Cl- influx, transepithelial potential and decreases tissue resistance. The net flux (secretion) of Cl- with hypertonic saline on the mucosal side (0.51 .+-. 0.06 .mu.eq/cm2.cntdot.h) demonstrates that the skin could secrete Cl- in vivo. Na+ fluxes across the skin are passive, as the observed flux ratio (efflux/influx) is similar to that predicted by the Ussing-Teorell equation under both closed- and open-circuit conditions. The permeability ratio (PNa:PC1) is .apprx. 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. Cl- secretion by Gillichthys skin is probably secondary active transport involving Na,K-ATPase and serosal Na+.