Role of seawater concentration and major ions in oxygen consumption rate of isolated gills of the shore carb Carcinus mediterraneus Csrn

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In the posterior gills isolated from the shore crab Carcinus mediterraneus, the effects of seawater concentration, ion substitution and ion transport inhibitors on oxygen consumption was studied. Oxygen consumption rate (VO 2) of the excised gills depended on the seawater concentration of the adaptation medium of intact crabs. The VO 2 measured in the gills isolated from the crabs acclimated to dilute seawater (DSW) was higher than VO 2 in the gills excised from the crabs living in ordinary seawater. In the gills isolated from seawater-acclimated crabs, and transferred to dilute medium the VO 2 was not increased but, rather, slightly decreased. A specific inhibitor of the enzyme Na + K + -ATPase ouabain (2.5 × 10 −3 mol 1 −1)_reduced oxygen consumption rate by 30%. Moreover, in the K +-free saline, the VO 2 was decreased by 40% suggesting the key role of the enzyme in oxidative requirements for ionic regulatory purposes. In Ca 2+-free saline, containing 0.1 mmol 1 −1 EGTA VO 2 was markedly decreased. Significant differences between the specific activity of Na + K + -ATPase in homogenates and membrane vesicles isolated from gills acclimated to seawater and DSW were found. In DSW-acclimated crabs, the specific activity of gill homogenate and membrane vesicles Na + K + -ATPase was increased, respectively by 71.6 and 122.9%, compared to the gills isolated from seawater-acclimated crabs. Addition of NaCl salts in solution preparated by isoosmotic replacement of Na + by N-methyl-D-glucamine (NMDG) steadily increased O 2 consumption. The curve was similar to an enzyme catalyzed reaction, K m amounted to 16.0 mmol Na 1 −1 and V max to 465 μl O 2 h −1 per gram gill (w.w.). The inhibitor of H +-ATPase, N-ethylmaleimide (NEM), reduced O 2 consumption; I 50 (50% reduced oxygen consumption from control) was attained at 5 × 10 −5 mol 1 −1 NEM. The results are discussed in terms of energetic demands of the gills in ion regulation.