Electrogenic 2 Na+/1 H+ exchange in crustaceans

Area of Science:

• * Biochemistry and Physiology
• * Comparative Animal Physiology
• * Membrane Transport Mechanisms

Background:

• * The hepatopancreas is a vital organ for digestion and nutrient absorption in crustaceans.
• * Understanding ion transport mechanisms, particularly sodium-hydrogen exchange, is crucial for comprehending crustacean physiology and adaptation.

Purpose of the Study:

• * To investigate the characteristics of the sodium-hydrogen (Na/H) exchanger in the hepatopancreatic brush border membrane vesicles of the freshwater prawn (Macrobrachium rosenbergii) and the marine lobster (Homarus americanus).
• * To determine the kinetic properties, ion dependencies, and transport stoichiometry of the Na/H exchanger in these species.

Main Methods:

• * Utilized 22Na uptake assays in isolated membrane vesicles.
• * Employed kinetic analyses, including sigmoidal function fitting and substrate inhibition studies.
• * Applied electrophysiological techniques like equilibrium shift experiments and Static Head analysis to determine transport stoichiometry and mechanism.

Main Results:

• * Identified an amiloride-sensitive, chloride-independent Na/H exchanger stimulated by a transmembrane H gradient.
• * Observed sigmoidal kinetics for sodium influx, suggesting cooperative binding, with distinct half-saturation constants (KNa) indicating adaptation to environmental salinity (82.2 mM for prawn, 280.1 mM for lobster).
• * Demonstrated electrogenic 2 Na/1 H exchange with specific binding site characteristics and transport stoichiometries.

Conclusions:

• * The characterized Na/H exchangers are electrogenic and possess distinct kinetic and binding properties, likely adapted to the salinity of their respective environments.
• * These exchangers are thermodynamically capable of generating gastric acidification essential for digestive processes in both prawns and lobsters.