We studied the transepithelial potential difference (TEPD) and (22)Na flux across isolated perfused gills (anterior pair 5 and posterior pairs 6-8) of the crab Chasmagnathus granulatus acclimated to either hypo- or hyper-osmotic conditions. The gills of crabs acclimated to low salinity, perfused and bathed with 10 per thousand saline solutions, produced the following TEPDs (hemolymph side with respect to bath side): 0.4+/-0.7, -10.2+/-1.6, -10.8+/-1.3 and -6.7+/-1.3 mV for gills 5, 6, 7 and 8,

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... espectively. Gills 6, 7 and 8 did not differ significantly. Reducing the saline concentration of bath and perfusate from 30 per thousand to 20 per thousand or 10 per thousand increased significantly the TEPDs of these gills. TEPDs of gill 6 (representative of posterior gills) were reduced by 69+/-5 % and 60+/-5 % after perfusion with ouabain or BaCl(2) (5 mmol l(-1) each), respectively. The same gill showed a net ouabain-sensitive Na(+) influx of 1150+/-290 microequiv g(-1) h(-1). Gill 6 of crabs acclimated to high salinity produced TEPDs of -1.5+/-0.1 and -1.3+/-0.09 mV after perfusion with 30 per thousand or 40 per thousand salines, respectively. Perfusion with ouabain or BaCl(2) reduced TEPDs by 76+/-7 % and 86+/-4 %, respectively. A net ouabain-sensitive Na(+) efflux of 2282+/-337 microequiv g(-1) h(-1) was recorded in gill 6 perfused with 38 per thousand saline.