J. exp. Biol. 150, 159-169 (1990) 159 Printed in Great Britain © The Company of Biologists Limited 1990 THE ACID-BASE BALANCE OF THE OUTER MANTLE EPITHELIUM OF ANODONTA CYGNEA BY J. MACHADO Institute de Ciencias Biomedicas Abel Salazar, Oporto, Portugal K. G. FERREIRA, H. G. FERREIRA AND P. L. FERNANDES Laboratorio de Fisiologia, Institute Gulbenkian de Ciencia, Oeiras, Portugal Accepted 11 January 1990 Summary 1. 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. 2. 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. 3. 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. 4. The short-circuit current (7^) was sensitive to the concentration of CO 2 , bicarbonate or protons in the solution on the shell side. 5. 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. 6. It is suggested that I K is due to a Cr/HCO 3 ~exchange shunted by a Cl~ recirculation across the basolateral membrane and to the operation of an electrogenic proton pump located in the apical membrane. Introduction The outer mantle epithelium (OME) of Anodonta cygnea produces a spon- taneous electrical potential in vitro (Istin and Kirschner, 1968). Under short-circuit conditions it generates a current (7^) which, when expressed as a molecular flux, has a magnitude the same as that of the simultaneously measured [ 14 C]bicarbonate net flux from shell to haemolymph side (Coimbra et al. 1988). Both the net flux of bicarbonate and 7^ are abolished by 4-acetamido-4'-isothiocyamatostilbene-2,2'- disulphonic acid (SITS) and 4,4-diisothiocyanostilbene 2,2-disulphonic acid (DIDS). The current is blocked by iodoacetamide, partially blocked by diamox ^ e y words: Anodonta, mantle, acid-base balance.