Electrogenic 2Na+/H+ Antiport in Echinoderm Gastrointestinal Epithelium

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Abstract
Purified brush-border membrane vesicles (BBMV) of starfish (Pycnopodia helianthoides) pyloric caecal epithelium were prepared by a magnesium precipitation technique in order to compare the properties of Na+/H+ exchange in this invertebrate tissue with those of an apparently unique recently described crustacean electrogenic antiporter. In starfish BBMV 22Na uptake was markedly enhanced by an outwardly directed pH gradient and membrane potential (inside negative) compared to control short-circuited vesicles. External amiloride abolished the stimulatory capacity of the proton gradient and membrane potential as driving forces for sodium transport. Sodium influx, in the presence of an outwardly directed proton gradient, was a sigmoidal function of [Na+]o and yielded a Hill coefficient of 2.6, suggesting that more than one sodium ion was exchanged with each internal proton during the exchange event. Two additional findings were used to establish the number of external Na+ binding sites and the transport stoichiometry of the starfish antiporter. First, amiloride acted as a competitive inhibitor of Na+ binding to two external sites with markedly dissimilar apparent amiloride affinities (Kil=28 μmoll−1; Kil=1650 μmoll−1). Second, a static head flux ratio analysis resulted in a 2Na+/H+ exchange stoichiometry where a balance of driving forces (e.g. no net Na+ flux) was attained with a combination of a 10:1 Na+ gradient and a 100:1 H+ gradient. Results suggest that the electrogenic 2Na+/H+ exchanger previously characterized for crustacean epithelia also occurs in echinoderm cells and may be a widely distributed invertebrate antiporter.