Methyl mercury inhibits short-circuit current and Cl− influx across isolated epipodite of European lobster (Homarus gammarus)

The effect of methyl mercuric chloride (MeHg) on short-circuit current (ISC) was studied in the isolated perfused epipodite preparation from the branchial chamber of European lobster (Homarus gammarus) acclimated to dilute seawater. When applied at the apical surface, 0.2, 1.0 and 3.0 μM MeHg depressed ISC by a 26%, 81% and 98%, respectively. The half-maximal inhibitory concentration (IC50) of apically applied MeHg was 0.6 μM. Basolaterally added MeHg (3.0 μM) had no effect on ISC, whereas addition of the specific Na+,K+-ATPase inhibitor ouabain (1.5 mM) reduced ISC by ∼ 90%. Ouabain effects were reversible, and ISC fully recovered upon removal of ouabain. The MeHg-induced block of ISC was partially reversed by the reducing agent, 1,4-dithiothreitol, suggesting that the formation of S–Hg–S bridges is important in the inhibitory mechanism. A significant reduction of ISC and conductance occurred when low Na+ and Cl− salines were substituted. Furthermore, in the low Na+ saline, JClA → B fluxes were reduced by about 50%. In the highly conductive epipodite epithelium, coupling of Na+ and Cl− fluxes was suggested. The effects of MeHg on ISC in the lobster epipodite are attributed to inhibition of an apical Cl− influx.