Na+/H+-exchangers differentially contribute to midgut fluid sodium and proton concentration in the sea urchin larva

Regulation of ionic composition and pH is a requisite of all digestive systems in the animal kingdom. Larval stages of the marine superphylum ambulacraria, including echinoderms and hemichordates, were demonstrated to have highly alkaline conditions in their midgut with the underlying epithelial transport mechanisms being largely unknown. Using ion-selective microelectrodes, the present study demonstrated that pluteus larvae of the purple sea urchin have highly alkaline pH (pH ∼9) and low [Na+] ( ̴120 mM) in their midgut fluids, compared to the ionic composition of the surrounding sea water. We pharmacologically investigated the role of Na+/H+-exchangers in intracellular pH regulation and midgut proton and sodium maintenance using the NHE inhibitor 5-(n-ethyl-n-isopropyl)amiloride (EIPA). Basolateral EIPA application decreased midgut pH while luminal application, via micro-injections increased midgut [Na+], without affecting pH. Immunohistochemical analysis demonstrated a luminal localization of NHE-2 (SpSlc9a2) in the midgut epithelium. Specific knockdown of spslc9a2 using vivo morpholinos led to an increase in midgut [Na+] without affecting pH. Acute acidification experiments in combination with qPCR analysis and measurements of midgut pH and [Na+] identified two other NHE isoforms, Spslc9a7 and SpSlc9a8 that potentially contribute to the regulation of [Na+] and pH in midgut fluids. This work provides new insights to ion regulatory mechanisms in the midgut epithelium of sea urchin larvae. The involvement of NHEs in regulating pH and Na+ balance in midgut fluids shows conserved features to insect and vertebrate digestive systems and may contribute to the ability of sea urchin larvae to cope with changes in seawater pH.