Gill Na,K-ATPase in the spiny lobster Palinurus elephas and other marine osmoconformers. Adaptiveness of enzymes from osmoconformity to hyperregulation.

Haemolymph inorganic osmolyte changes and Na,K-ATPase activities in trichobranchiate and epipodite tissues were examined in the spiny lobster Palinurus elephas gradually acclimated from seawater (SW; 38 ppt, salinity; 1291 mOsmol/l) down to dilute seawater (DSW; 20 ppt, salinity; 679 mOsmol/l). During acclimation to DSW haemolymph was only transiently hypoosmotic, becoming isosmotic to the medium over a 24-h period of acclimation. Na,K-ATPase specific activities in homogenates of the trichobranchiate gills from SW- and DSW-acclimated spiny lobsters were in the range of 2-3 µmol Pi/h/mg protein and were not significantly different. It has also been confirmed for the marine stenohaline crustaceans Maja crispata and Dromia personata that gill Na,K-ATPase maintains the same level of specific activity in SW- and DSW-acclimated crabs. The saponin-treated fraction of Na,K-ATPase activity in trichobranchiate gills was 67-89% and epipodites 63-64% over the native homogenates' activity and no differences in enzyme activities upon saponin treatment between SW- and DSW-acclimated spiny lobsters were found. Recovery of 6% and enrichment factor (1.6) of Na,K-ATPase in partially purified plasma membrane fractions of epipodites was relatively low and not different in SW- and DSW-acclimated spiny lobsters. In the hemiepipodite, negative short-circuit current was in the range from -16.7 to -22.7 µA cm(-2) and conductance varied in the range of 205-290 mS cm(-2), values which were not significantly different in spiny lobsters residing in SW or DSW. Very high conductance suggests leakiness of the hemiepipodite epithelium-cuticular complex. In contrast to the group of euryhaline hyperosmoregulating Crustacea in which activation of the specific activity of Na,K-ATPase upon acclimation to dilute seawater occurs, in marine osmoconformers there is no activation of the enzyme in dilute seawater. Based on the literature data and our own results, we have reported a correlation coefficient of 0.65 between specific activity of Na,K-ATPase and the sodium gradient (mmol Na/l; haemolymph-seawater ) between 12 species of osmoconforming and osmoregulating Crustacea. During evolution, hyperosmoregulating Crustacea have achieved internal osmolyte gradients generated by Na,K-ATPase and lowering the gill surface permeability. However these adaptive characteristics are not present in marine osmoconforming Crustacea, restraining them to migrate in the brackish water habitats.