Identification of a V-type proton pump in the outer mantle epithelium of Anodonta cygnea

The dissected outer mantle epithelium (OME) of Anodonta cygnea, when mounted in Ussing type chambers, generated a spontaneous potential difference of 22.0±12.6 mV and, when short-circuited, a positive current (I_sc) of 30.0±11 μA/cm² towards the shell side and a conductance of 1.1±0.4 mS/cm². When in contact with the shell side, Bafilomycin A₁ and Concanamycin A, specific inhibitors of V-proton pumps, induced 90% inhibition of I_sc in 5 and 35 min, respectively. They had no effect in the current from the hemolymph side. Both drugs induced a dramatic fall in conductance from the shell side. Tributyltin oxide (TBTO) inhibited 90% of the I_sc in 3 min and induced a fall in conductance only from the shell side. Two observations suggest a direct effect on the proton pump: it was effective only from the shell side and the time course of the effect was identical to that of Bafilomicyn A₁. Dicyclohexycarbodiimide (DCCD) and N-ethylmaleimide (NEM) inhibited I_sc from both sides but very slowly and there was a delay of the effect from the hemolymph side in relation to the shell side. Taken together, the results suggest the presence of a V-type proton pump located at the apical (shell side) barrier of the OME.