Active chloride transport in the in vitro opercular skin of a teleost (Fundulus heteroclitus), a gill‐like epithelium rich in chloride cells
Authors: Degnan Kevin J. , Karnaky Karl J. , Zadunaisky Jose A. 
Published: December 2014

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 × 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 Ω.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 Ω.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 Ω.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.