Sodium and chloride fluxes, as well as transbranchial potentials (TBP) were studied in isolated perfused gill filaments of the crab Carcinus mediterraneus. Experiments were carried out in media that were either hyposmotic to the perfusion solution (asymmetrical conditions) or isosmotic (symmetrical conditions). Fluxes were found to be diffusional in gills under asymmetrical conditions; amiloride induced an inhibitory effect on influxes, without affecting TBP. Under symmetrical conditions, TBP was −7.6±2.3mV, suggesting that the electrogenic ion pump contributes significantly to the development of TBP. Immediately after addition of 2.5 × 10−4 moll−1 amiloride to the external solution, sodium influxes were reduced to 31% of those in the control group, and TBP was significantly hyperpolarized from −7.6 to −14.8 mV. The absence of Ca2+ under symmetrical conditions diminished TBP hyperpolarization. Half-maximal inhibition of sodium influxes by amiloride was at 7 × 10−5 moll−1. This low amiloride affinity is typical of low resistance leaky epithelia. Sodium transport is discussed as an amiloride-affected influx, probably as a Na/H antiport.

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