ANTIPORT-DRIVEN SULFATE SECRETION IN AN INVERTEBRATE EPITHELIUM

A novel invertebrate gastrointestinal transport mechanism has been sho wn to couple chloride/sulfate exchange in an electrogenic fashion. In the lobster, Homarus americanus, the hepatopancreas, or digestive glan d, exists as an outpocketing of the digestive tract, representing a si ngle cell layer separating the gut lumen and an open circulatory syste m comprised of hemolymph. Investigations utilizing independently prepa red brush-border and basolateral membrane vesicles revealed discrete a ntiport systems which possess the capacity to bring about a transcellu lar secretion of sulfate. The luminal antiport system functions as a h igh affinity, one-to-one chloride-sulfate exchanger that is stimulated by an increase in luminal hydrogen ion concentration. Such a system w ould take advantage of the high chloride concentration of ingested sea water, as well as the high proton concentrations generated during dige stion, which further suggests a potential regulation by resident sodiu m-proton exchangers. Exchange of one chloride for one divalent sulfate ion provides the driving force for electrogenic vectorial translocati on. The basolateral antiport system was found to be electroneutral in nature, responsive to gradients of the dicarboxylic anion oxalate, whi le lacking in proton stimulation. No evidence of sodium-sulfate cotran sport, commonly reported for the brush border of vertebrate renal and intestinal epithelia, was observed in either membrane preparation. The two antiporters together can account for the low hemolymph to seawate r sulfate levels previously described in decapod crustaceans. A secret ory pathway for sulfate based upon electrogenic chloride-antiport may appear among invertebrates partly in response to digestion taking plac e in a seawater environment. (C) 1996 Wiley-Liss, Inc.