Bafilomycin-sensitive acid secretion by mantle epithelium of the freshwater clam, Unio complanatus.

This study reports the effects of a series of putative inhibitors of proton translocation on electrogenic acid secretion by the isolated shell-facing mantle epithelium of the freshwater clam, Unio complanatus. In this epithelium, adenosine 3',5'-cyclic monophosphate (cAMP)-dependent acid secretion is electrogenic and equivalent to the cAMP-dependent short-circuit current (Isc) when mounted in a Ussing-type flux chamber. Bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPases (V-type ATPases), inhibited cAMP-dependent electrogenic acid secretion, with an IC50 of 7 nM and a maximum effect at 100 nM. Inhibition of acid secretion by bafilomycin A1 occurred only when added to the shell side and was not reversed by repeated washings. Concomitant with the inhibition of cAMP-dependent Isc by bafilomycin A1, the electrical potential difference across the shell-facing membrane depolarized, and the conductance ratio of the apical to basolateral membranes decreased. The cAMP-dependent Isc and cAMP-dependent acid secretion also were inhibited by dicyclohexyl-carbodiimide, diethylstilbestrol, N-ethylmaleimide, and tributyltin. Amiloride, omeprazole, oligomycin, venturicidin, and ouabain had no effect on either cAMP-dependent Isc or cAMP-dependent acid secretion. The cAMP-dependent Isc and cAMP-dependent acid secretion were not affected by Na(+)-free or K(+)-free solutions. Finally, the cAMP-dependent Isc was strongly dependent on the pH of the shell-facing compartment, with 50% inhibition of the maximum cAMP-dependent Isc occurring at pH 6.0 and complete inhibition at pH 5.5.(ABSTRACT TRUNCATED AT 250 WORDS)