Transepithelial potential differences and Na(+) flux in isolated perfused gills of the crab Chasmagnathus granulatus (Grapsidae) acclimated to hyper- and hypo-salinity.

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, respectively. 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.

Active osmoregulatory ion uptake across the pleopods of the isopod Idotea baltica (Pallas): electrophysiological measurements on isolated split endo- and exopodites mounted in a micro-ussing chamber.

The mechanism of active, osmoregulatory ion uptake was investigated in the pleopods of the marine isopod Idotea baltica (Pallas). Using isolated split half-podites of isopods acclimated to brackish water (20 salinity) mounted in a micro-Ussing chamber and symmetrically superfused with identical haemolymph-like salines, a mean short-circuit current I(sc) of -445 microA cm(-)(2) was measured in endopodites 3-5, corresponding to an inwardly directed transcellular movement of negative charge. Application of ouabain (5 mmol l(-)(1)) to the basolateral superfusate resulted in the almost total abolition of the I(sc) (reduced from -531 to -47 microA cm(-)(2)), suggesting that the Na(+)/K(+)-ATPase is the driving force for active, electrogenic uptake of NaCl. In contrast, mean I(sc) values close to zero were found in preparations of all exopodites and in endopodites 1 and 2. The specific activities of Na(+)/K(+)-ATPase corresponded with these results. Specific activities were highest in posterior endopodites 3-5 and depended on ambient salinity. In all other rami, the activities were much lower and independent of ambient salinity. Activities in posterior endopodites 3-5 were lowest in isopods acclimated to 30 salinity (2-4 micromol P(i )mg(-)(1 )protein h(-)(1)), increased in individuals kept in 20 salinity (8.4 micromol P(i )mg(-)(1 )protein h(-)(1)) and were highest in isopods acclimated to 15 salinity (18.2 micromol P(i )mg(-)(1 )protein h(-)(1)). When specimens were transferred from 30 to 40 salinity, Na(+)/K(+)-ATPase activity increased in the posterior endopodites. The electrophysiological and Na(+)/K(+)-ATPase activity measurements show that active electrogenic ion transport in this species occurs almost exclusively in posterior endopodites 3-5. The endopodite of the fifth pleopod of I. baltica exhibited a microscopic structure remarkably similar to that described for the lamellae of the phyllobranchiae of brachyurans. It is composed of two opposed epithelial monolayers of ionocytes, each covered by cuticle. Bundles of pillar cells are located within the ionocyte layers, which are separated by a fenestrated lamellar septum of connective tissue. The results obtained in this study indicate that endopodites 3-5 play the main role in osmoregulatory ion uptake of the isopod I. baltica. Moreover, the Na(+)/K(+)-ATPase is the only driving force behind active electrogenic ion uptake across the epithelial cells.