Sulfate bicarbonate antiport by lobster hepatopancreatic basolateral membrane vesicles

Sulfate transport across plasma membranes has been described in a nide variety of organisms and cell types including gastrointestinal epithelia. Sulfate transport can be coupled to proton or sodium symport or antiport processes involving a variety of anions. It had been previously observed in lobster hepatopancreas that sulfate could be secreted, however, the mechanisms for this potential secretory process had not been fully described. Therefore, the present study was done to delineate one of the potential processes for this transport. Purified basolateral membrane vesicles (BLMV) were prepared from lobster hepatopancreas by osmotic disruption and discontinuous sucrose gradient centrifugation. Transport of (S-35) sulfate into BLMV was stimulated by an outwardly directed bicarbonate gradient compared with gluconate-loaded vesicles. An inside-positive membrane potential (valinomycin and K+) stimulated sulfate/bicarbonate exchange; whereas an inside-negative membrane potential was inhibitory. Sulfate/sulfate exchange was not affected by alterations of transmembrane potential difference. External protons stimulated sulfate/bicarbonate exchange although proton gradients had no effect on sulfate/bicarbonate exchange in the BLMV. The stilbenes, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), strongly inhibited sulfate/bicarbonate exchange. These results suggest that sulfate/bicarbonate exchange in hepatopancreatic BLMV occurred by an electrogenic carrier mechanism exhibiting a 1:1 flux ratio. The possible physiological role of this antiport process in sulfate transport across the crustacean hepatopancreas is discussed. J. Exp. Zool. 284:158-167, 1999. (C) 1999 Wiley-Liss, Inc.