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1. The opercular epithelium lining the inside of the gill chamber of the killifish, Fundulus heteroclitus, contains Cl^- cells, identical in fine structure to gill Cl^- cells, at the high density of 4 x 10⁵ cells/cm². This epithelium can be isolated, mounted in a Lucite chamber, and its ion transport properties studied with the short-circuit current technique.

2. The isolated opercular epithelia of seawater-adapted fish, when bathed on both sides with Ringer and gassed with 100% O₂, displayed a mean short-circuit current of 136.5 ± 11.1 µA/cm², a mean transepithelial potential difference of 18.7 ± 1.2 mV (blood side positive), and a mean transepithelial d.c. resistance of 173.7 ± 12.1 .cm² (mean ± S.E. of mean; n = 64).

3. The transepithelial potential difference across the opercular epithelia of seawater-adapted fish was dependent on both Na⁺ and Cl^- in the bathing solutions and increased linearly with increasing Cl^- concentrations with a slope of 28.3 ± 2.1 mV/tenfold concentration change. The short-circuit current was Na⁺ dependent and increased linearly with increasing Cl^- concentrations with no evidence of saturation kinetics below 142.5 m-equiv/l.

4. When the short-circuited epithelia of seawater-adapted fish, bathed on both sides with Ringer, was gassed with 100% O₂ the mean Cl^- blood side to seawater side flux was 211.7 ± 27.1 µA/cm² and the mean Cl^- seawater side to blood side flux was 48.9 ± 10.0 µA/cm². This resulted in a net Cl^- blood side to seawater side flux of 162.8 µA/cm² which was not statistically different (P > 0.70) from the mean short-circuit current of 158.6 ± 16.3 µA/cm² for these flux studies. The mean Na⁺ blood side to seawater side flux was 32.2 ± 3.3 µA/cm² and the mean Na⁺ seawater side to blood side flux was 34.8 ± 4.1 µA/cm², resulting in no significant (P > 0.20) net flux of this cation. Similar results were obtained with short-circuited epithelia of seawater-adapted fish when bathed on both sides with Ringer and gassed with 95% O₂/5% CO₂.

5. Ouabain (10^-5 M), furosemide (10^-3 M), thiocyanate (10^-2 M), adrenaline (10^-6 M), and anoxia (100% N₂) decreased the short-circuit current 92.7, 85.0, 45.3, 62.6, and 83.3% respectively. Theophylline (10^-4 M) stimulated the short-circuit current 54.9%. Increasing the HCO₃^- concentration in the bathing solutions had a stimulatory effect on the short-circuit current and the potential difference across epithelia from seawater-adapted fish.

6. The opercular epithelia of freshwater-adapted F. heteroclitus, when bathed on both sides with Ringer, displayed a mean short-circuit current of 94.1 ± 10.4 µA/cm², a mean transepithelial potential difference of 14.8 ± 1.9 mV (blood side positive), and a mean d.c. resistance of 169.0 ± 14.0 .cm² (mean ± S.E. of mean; n = 20). Isotope flux studies across these short-circuited epithelia revealed a net Cl^- blood side to freshwater side flux of 95.2 ± 16.1 µA/cm² and no significant net flux of Na⁺.

7. The opercular epithelia of 200% seawater-adapted F. heteroclitus, when bathed on both sides with Ringer, displayed a mean short-circuit current of 33.5 ± 8.5 µA/cm², a mean transepithelial potential difference of 10.5 ± 2.5 mV (blood side positive), and a mean transepithelial d.c. resistance of 440.7 ± 62.6 .cm² (mean ± S.E. of mean n = 18). Isotope flux studies across these short-circuited epithelia revealed a net Cl^- blood side to seawater side flux of 96.2 ± 51.5 µA/cm² and a net Na⁺ blood side to seawater side flux of 65.3 ± 28.6 µA/cm².

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