Sodium-Sulfate Symport by Aplysia californica Gut

George A Gerencser,Randy Levin

doi:10.2108/zsj.17.579

Abstract

Sulfate transport across plasma membranes has been described in a wide variety of organisms and cell types including gastrointestinal epithelia. Sulfate transport can be coupled to proton, sodium symport or antiport processes involving chloride or bicarbonate. It had previously been observed in Aplysia gut that sulfate was actively absorbed. To understand the mechanism for this transport, short-circuited Aplysia californica gut was used. Bidirectional transepithelial fluxes of both sodium and sulfate were measured to see whether there was interaction between the fluxes. The net mucosal-to-serosal flux of Na(+) was enhanced by the presence of sulfate and it was abolished by the presence of serosal ouabain. Similarly, the net mucosal-to-serosal flux of sulfate was dependent upon the presence of Na(+) and was abolished by the presence of serosal ouabain. Theophylline, DIDS and bumetanide, added to either side, had no effect on transepithelial potential difference or short-circuit current in the Aplysia gut bathed in a Na2SO4 seawater medium. However, mucosal thiosulfate inhibited the net mucosal-to-serosal fluxes of both sulfate and Na(+) and the thiosulfate-sensitive Na(+) flux to that of sulfate was 2:1. These results suggest the presence of a Na-SO4 symporter in the mucosal membrane of the Aplysia californica foregut absorptive cell.

Highlights

Gastrointestinal and renal transport of the divalent anion sulfate across epithelial apical membranes has been investigated in various vertebrate groups including mammals (Ahearn and Murer, 1984; Pritchard, 1987), teleost fish (Renfro and Pritchard, 1982, 1983) and the domestic chicken (Renfro et al, 1987)
A proton-stimulated sulfate/ chloride exchanger has recently been described in apical membranes of lobster (Homarus americanus) hepatopancreatic epithelial cells (Cattey et al, 1992), while an oxalate/ sulfate antiporter has been described in the basolateral membranes of the same cells of lobster hepatopancreas (Gerencser et al, 1995)
In the current investigation we presented suggestive evidence for the existence of a carrier-mediated Na+-sulfate symport located in the apical membrane of Aplysia californica foregut epithelium

Summary

Introduction

Gastrointestinal and renal transport of the divalent anion sulfate across epithelial apical membranes has been investigated in various vertebrate groups including mammals (Ahearn and Murer, 1984; Pritchard, 1987), teleost fish (Renfro and Pritchard, 1982, 1983) and the domestic chicken (Renfro et al, 1987). A number of mechanisms for brush-border carriermediated sulfate transport across epithelial membranes have been proposed and include sodium-sulfate cotransport (Ahearn and Murer, 1984; Lucke et al, 1979), anion exchange (Renfro and Pritchard, 1982; Taylor et al, 1987) and pH gradient-dependent transfer (Schron et al, 1985). These processes contribute to transepithelial regulation of sulfate levels, and may affect acid-base balance and plasma osmolarity. This transport mechanism may contribute, in part, in maintaining sulfate homeostasis by Aplysia

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