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First published online January 12, 2004
Journal of Experimental Biology 207, 571-578 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00495
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Electrogenic proton-regulated oxalate/chloride exchange by lobster hepatopancreatic brush-border membrane vesicles

George A. Gerencser1,*, Frank Robbins2, Jianliang Zhang2 and Gregory A. Ahearn3

1 Department of Physiology and Functional Genomics, College of Medicine, University of Florida, Gainesville, FL 32610, USA
2 Department of Medicine, College of Medicine, University of Florida, Gainesville, FL 32610, USA
3 Department of Biology, College of Arts & Sciences, University of North Florida, Jacksonville, FL 32224, USA

* Author for correspondence (e-mail: gag{at}phys.med.ufl.edu)

Accepted 6 October 2003

The transport of [14C]oxalate (Ox2–) by epithelial brush-border membrane vesicles (BBMV) of lobster (Homarus americanus) hepatopancreas, formed by a magnesium precipitation technique, was stimulated by an outward Cl– gradient (in > out). By contrast, Ox2– uptake was not enhanced by an inward Na+ or K+ transmembrane gradient. Generation of an inside-positive membrane potential by K+ in the presence of valinomycin stimulated Ox2–/Cl– exchange, while an inside-negative membrane potential generated by K+ efflux in the presence of valinomycin inhibited this process. Neither Ox2–/Ox2– nor Ox2–/SO42– transport exchange were affected by alterations of transmembrane potential. An inwardly directed proton gradient, or the presence of low bilateral pH, enhanced Ox2–/Cl– exchange, yet the H+ gradient alone could not stimulate Ox2– uptake in Cl–-equilibrated BBMV or in vesicles lacking internal Cl–. The stilbenes 4-acetamido-4'-isothiocyanotostilbene-2,2'-disulfonic acid (SITS) and 4,4'-diisothiocyano-2,2'-disulfonic stilbene (DIDS) strongly inhibited Ox2–/Cl– exchange. Oxalate influx occurred by a combination of carrier-mediated transfer, exhibiting Michaelis–Menten kinetics, and nonsaturable `apparent diffusion'. Apparent kinetic constants for Ox2–/Cl– exchange were Kt=0.20 mmol l–1 and Jmax=1.03 nmol l–1 mg–1 protein 7 s–1. 36Cl– influx into oxalate-loaded BBMV was stimulated by an inside-negative transmembrane potential compared with short-circuited vesicles. These results suggest that Ox2–/Cl– exchange in crustacean hepatopancreatic BBMV occurred by an electrogenic carrier mechanism exhibiting a 1:1 flux ratio that was modulated by an external proton-sensitive regulatory site.

Key words: brush-border membrane vesicle, BBMV, oxalate, ion transport, hepatopancreas, electrogenic carrier mechanism, lobster, Homarus americanus


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© The Company of Biologists Ltd 2004