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The Journal of Experimental Biology 204, 2911-2919 (2001)
© 2001 The Company of Biologists Limited

BAFILOMYCIN A₁ AT NANOMOLAR CONCENTRATIONS SATURABLY INHIBITS A PORTION OF TURTLE BLADDER ACIDIFICATION CURRENT

STEVEN J. YOUMANS^* and CATHERINE R. BARRY

Department of Physiology, New York College of Osteopathic Medicine, New York Institute of Technology, Old Westbury, Long Island, NY 11568-8000, USA
^* Author for correspondence (e-mail: syoumans{at}iris.nyit.edu )

Accepted May 17, 2001

An earlier report indicated that acid secretion in turtle urinary bladder is driven by an unusual vacuolar H⁺-ATPase and that the ATPase accounts for essentially all acid secreted. These results, however, are difficult to reconcile with the acid transporters currently ascribed to the renal collecting duct. Here, we re-examine the effect of bafilomycin A₁, an inhibitor of vacuolar (V-type) H⁺-ATPases, on acid secretion by intact isolated bladders from Pseudemys scripta turtles. Serosal-side bafilomycin had no effect on the transepithelial acidification current (AC). In the mucosal solution, bafilomycin inhibited the AC, with inhibition developing over the range 0.1-10 nmol l^-1, with a sigmoidal dose—response curve, and an IC₅₀ of 0.47 nmol l^-1. At saturation, approximately 70 % of H⁺ secretion was inhibited. The remaining 30 % could be abolished by 30 µmol l^-1 Sch-28080, which is a level that in other systems is known to inhibit H⁺/K⁺-ATPase transport activity specifically and essentially completely. When the order of addition was reversed (Sch-28080 first), there was no change in the magnitude of the effect produced by either inhibitor, and the two together again eliminated the AC. The data indicate that baseline acid secretion in intact bladders is due (i) in part to a highly bafilomycin-sensitive process, with sensitivity typical of vacuolar H⁺ ATPases; and (ii) in part to a more bafilomycin-resistant process that is sensitive to Sch-28080.

Key words: turtle, Pseudemys scripta, urinary bladder, acid secretion, bafilomycin A₁, vacuolar H⁺ ATPase, Sch-28080

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