Electrogenic 2 Na+/1 H+ exchange in crustanceans

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Abstract
Summary Hepatopancreatic brush border membrane vesicles of the freshwater prawn,Macrobrachium rosenbergii and the marine lobster,Homarus americanus exhibited22Na uptake which was Cl-independent, amiloride sensitive, and stimulated by a transmembrane H gradient (H i >H o ). Sodium influx by vesicles of both species were sigmoidal functions of [Na] o , yielding Hill coefficients that were not significantly different (P>0.5) than 2.0. Estimations of half-saturation constants (K Na) were 82.2mm (prawn) and 280.1mm (lobster), suggesting a possible adaptation of this transporter to environmental salinity.Trans-stimulation andcis-inhibition experiments involving variable [H] suggested that the exchangers in both species possessed single internal cation binding sites (pK 6.5–6.7) and two external cation binding sites (prawn, pK 4.0 and 5.7; lobster pK 3.5 and 6.1). Similarcis inhibition studies using amiloride as a competitive inhibitor of Na uptake supported the occurrence of dual external sites (prawn,K i 50 and 1520 μm; lobsterK i 9 and 340 μm). Electrogenic Na/H exchange by vesicles from both crustaceans was demonstrated using equilibrium shift experiments where a transmembrane potential was used as the only driving force for the transport event. Transport stoichiometries of the antiporters were determined using Static Head analysis where driving forces for cation transfer were balanced using a 10∶1 Na gradient, a 100∶1 H gradient, and a stoichiometry of 2.0. These electrogenic 2 Na/1 H exchangers appear thermodynamically capable of generating sufficient gastric acidification for organismic digestive activities.