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Journal of Experimental Biology 158,495-507 (1991)
Published by Company of Biologists 1991

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

GREGORY A. AHEARN ¹ and PIERETTE FRANCO ¹

¹ Department of Zoology, 2538 The Mall, University of Hawaii at Manoa, Honolulu, HI 96822, USA

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 (K_il=28 µmoll^-1; K_il=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.

Key words: Na⁺/H⁺ exchange, antiport, electrogenic, epithelium, brush-border membrane uesicles, stoichiometry, pyloric caeca, echinoderm, starfish, Pycnopodia helianthoides

Accepted on March 7, 1991

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