The Acid–Base Balance of the Outer Mantle Epithelium of Anodonta Cygnea

ABSTRACT Under short-circuit conditions the outer mantle epithelium of Anodonta cygnea is known to produce an acidification of the solution bathing the shell side and an alkalinization of the solution bathing the haemolymph side. At steady state, the rates of secretion of acid and base were numerically equal to the simultaneously measured short-circuit current, expressed in the same units. The rates of acid and base secretion, and the short-circuit current, showed close similarity in the reductions caused by anoxia, diamox, DIDS (from haemolymph side), DNP and iodoacetamide. The short-circuit current (Isc) was sensitive to the concentration of CO2, bicarbonate or protons in the solution on the shell side. The short-circuit current was insensitive to vanadate or oligomycin, was slowly inhibited by DCCD added under anoxia to the shell side, and was almost completely inhibited within seconds by TBTO (shell side) which also caused a 40% reduction in transepithelial conductance. It is suggested that Isc is due to a Cl−/HCO3−exchange shunted by a Cl− recirculation across the basolateral membrane and to the operation of an electrogenic proton pump located in the apical membrane.