Characterization of a basolateral electroneutral Na+/H+ antiporter in Atlantic lobster (Homarus americanus) hepatopancreatic epithelial vesicles

Purified basolateral membrane vesicles (BLMVs) were prepared from Atlantic lobster (Homarus americanus) hepatopancreas using a Percoll density gradient technique. Enrichments of the Na+/K+-ATPase and alkaline phosphatase activities of these vesicles were 15.4- and 1.2-fold, respectively. The presence of amiloride-sensitive Na+/H+ exchange was demonstrated. Contrary to electrogenic 2Na+/1H+ exchange on apical membranes from the same tissue, kinetic studies of Na+ transport by these basolateral membranes indicate an electroneutral antiport with a Km of 28±1.7 mmol l-1 and a Jmax of 1.74±0.13 µmol mg-1 min-1. Amiloride interacted at a single binding site (Ki=39 µmol l-1) and external Li+ was shown to be an effective competitive inhibitor of the exchange process (Ki=493 µmol l-1). The presence of a membrane-potential-sensitive, Na+-accepting ion channel was also demonstrated. The basolateral Na+/H+ exchanger physiologically resembles members of the NHE family of Na+/H+ antiporters described in vertebrates and departs from the apical electrogenic system previously described in lobster. Whether or not the basolateral Na+/H+ antiporter is an NHE isoform remains to be determined.