TRANSEPITHELIAL POTENTIAL DIFFERENCES AND SODIUM FLUXES IN ISOLATED-PERFUSED GILLS OF THE MANGROVE CRAB UCIDES CORDATUS

Marked differences in transepithelial potential difference (TEP) and u nidirectional sodium fluxes were observed in isolated, perfused poster ior gills 5 and 6 of the hyper-hyporegulating crab Ucides cordatus. Gi lls were perfused with a saline which closely matched the composition of the crab's haemolymph, and bathed with 9, 17, 26 and 34 parts per t housand seawater (SW). In 26 parts per thousand SW, TEPs were small an d similar in magnitude and sign for both gills, but in all other salin ities TEPs were very different, exhibiting opposite polarities and dif ferent responses to the changes in external salinity. Calculated relat ive ionic permeabilities, P-Na:P-Cl, showed that gills 5 were more per meable to sodium than gills 6, while the latter were chloride permeabl e. In gills 5 the TEP can be regarded as a diffusion potential. This i s not the case in gill 6 where ion extrusion probably takes place. Inf lux rates in gill 5 were larger than efflux rates in all salinities an d ranged from 726.93 +/- 103.52 to 1111.47 +/- 135.55 mu M Na+ g(-1) h (-1). Sodium efflux varied from 337.49 +/- 56.42 to 689.03 +/- 108.87 mu M Na+ g(-1) h(-1). In gill 6 influx rates were orders of magnitude smaller (3.67 +/- 0.25 to 8.45 +/- 0.79 mu M Na+ g(-1) h(-1)), while e fflux rates varied from 87.24 +/- 20.25 to 223.27 +/- 19.38 mu M Na+ g (-1) h(-1). Flux-ratio analysis showed active uptake of Na+ in gill 5 and active extrusion in gills 6, suggesting functional differences in adjacent gills of this hyper-hyporegulating crab. (C) 1998 Elsevier Sc ience Inc. All rights reserved.