Na⁺ and Ca²⁺ Pumps in the Gills, Epipodites and Branchiostegites of the European Lobster Homarus Gammarus: Effects of Dilute Sea Water

Crude homogenates and plasma-membrane-enriched fractions were prepared from the epithelium of the gills, epipodites and branchiostegites of intermoult European lobsters Homarus gammarus, and Na⁺/K⁺-ATPase, Ca²⁺-ATPase and Na⁺/Ca²⁺ exchange activities were quantified in these tissues. Lobsters were kept in sea water (salinity 35 ‰) or were adapted to dilute sea water (22.1 ‰). The lobster hyperregulates haemolymph osmolarity and Ca²⁺ levels in both media. Homogenates of the podobranchs, arthrobranchs and pleurobranchs had comparable Na⁺/K⁺-ATPase specific activities, and mean activities increased significantly for all three types of gills when the animals were kept in dilute sea water. In the epipodites and branchiostegites, Na⁺/K⁺-ATPase specific activities exceeded those in the gills, and exposure to dilute sea water greatly enhanced these activities. In sea water, 80 % of the total Na⁺/K⁺-ATPase activity is associated with the gills and epipodites (each tissue containing 40 %) and 20 % with the branchiostegites; in dilute sea water, the gills contained approximately 25 %, the epipodites 40 % and the branchiostegites approximately 35 % of the total activity, indicating the relative importance of the epipodites and branchiostegites for ionic hyperregulation in dilute media. In plasma membrane vesicles isolated from the gills, epipodites and branchiostegites, Ca²⁺ transport driven by ATP and by a Na⁺ gradient was demonstrated. Exposure to dilute sea water enhanced Na⁺/Ca²⁺ exchange and Ca²⁺-ATPase activities in the epipodites and branchiostegites; in the gills, however, Ca²⁺ transport activities decreased. The role of these tissues and enzymes in Na⁺ and Ca²⁺ handling by the lobster is discussed.