Sodium-phosphate symport by Aplysia californica gut.

George A Gerencser,Randy Levin,Jianliang Zhang

doi:10.2108/zsj.19.163

Abstract

Phosphate transport across plasma membranes has been described in a wide variety of organisms and cell types including gastrointestinal epithelia. Phosphate transport across apical membranes of vertebrate gastrointestinal epithelia requires sodium; whereas, its transport across the basolateral membrane requires antiport processes involving primarily chloride or bicarbonate. To decipher the phosphate transport mechanism in the foregut apical membrane of the mollusc, Aplysia californica, in vitro short-circuited Aplysia californica gut was used. Bidirectional transepithelial fluxes of both sodium and phosphate 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 phosphate and it was abolished by the presence of serosal ouabain. Similarly, the net mucosal-to-serosal flux of phosphate was dependent upon the presence of Na+ and was abolished by the presence of serosal ouabain. Theophylline, DIDS and bumetande, added to either side, had no effect on transepithelial difference or short-circuit current in the Aplysia gut bathed in a Na2HPO4 seawater medium. However, mucosal arsenate inhibited the net mucosal-to-serosal fluxes of both phosphate and Na+ and the arsenate-sensitive Na+ flux to that of phosphate was 2:1. These results suggest the presence of a Na-PO4 symporter in the mucosal membrane of the Aplysia californica foregut absorptive cell.

Highlights

Gastrointestinal and renal transport of the anion phosphate across epithelial apical membranes has been investigated in various vertebrate groups including: mammals such as rabbit (Murer et al, 1983) and rat (Berner et al, 1976); avian such as chicken (Matsumoto et al, 1980), and other lower vertebrates (Danisi and Murer, 1991)
Studies with intact vertebrate tissue preparations have documented that transepithelial inorganic phosphate (Pi) transport against an electrochemical potential difference in the small intestine is dependent on the presence of sodium (Na+) (Fuchs and Peterlik, 1980)
In the current investigation we presented suggestive evidence for the existence of a carrier-mediated Na+- Pi symport located in the apical membrane of Aplysia californica foregut epithelium

Summary

Introduction

Gastrointestinal and renal transport of the anion phosphate across epithelial apical membranes has been investigated in various vertebrate groups including: mammals such as rabbit (Murer et al, 1983) and rat (Berner et al, 1976); avian such as chicken (Matsumoto et al, 1980), and other lower vertebrates (Danisi and Murer, 1991). Studies with intact vertebrate tissue preparations have documented that transepithelial inorganic phosphate (Pi) transport against an electrochemical potential difference in the small intestine is dependent on the presence of sodium (Na+) (Fuchs and Peterlik, 1980) In vertebrates, this process can contribute to the transepithelial regulation of Pi levels, and may affect acid-base balance and plasma osmolarity. The present sudy uses isolated foregut from Aplysia californica to chracterize a Na+/Pi symporter that is located in the mucosal membrane of the gut cells and is inhibited by arsenate and ouabain. This transport mechanism may contribute, in part, to the maintenance of Pi homeostasis by Aplysia

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