SCHOLNICK, D.A.; BARABAS, A.E.*; Eckerd College; Eckerd College: Effects of chloride on glucose regulation in decapod crustaceans

Animals must maintain blood glucose concentrations within a narrow range in order to maintain normal cellular function. In most species, the export of cellular glucose depends on the enzyme system glucose-6-phosphatase (G-6-Pase), which catalyzes the final step in the production of new glucose (gluconeogenesis) and the breakdown of glycogen (glycogenolysis). Although much is known about glucose regulation in vertebrates, the effectors that regulate glucose release in the hepatopancreas of decapod crustaceans remain largely unexplored. This study examined the influence of chloride ion concentrations on G-6-Pase activity in three decapod crustaceans that regularly experience ion disequilibrium due to exposure to variable ion environments. G-6-Pase activity and kinetics were determined in the ion-regulating stone crab Menippe mercenaria and blue crab Callinectes sapidus, and compared to the ion-conforming spider crab Libinia dubia. The sensitivity of G-6-Pase to increasing chloride was determined from crabs acclimated to 20 and 45 ppt seawater. Increasing chloride concentrations stimulated hepatopancreas G-6-Pase activity in all three species examined. Average G-6-Pase activity increased over two times in stone crabs and over three times in spider crabs exposed to 500 mM NaCl. These results are in stark contrast to vertebrate liver G-6-Pase activity that has previously been shown to decline linearly when exposed to increasing concentrations of chloride. Hepatopancreas G-6-Pase activity and chloride sensitivity was reduced by about half in crabs acclimated to 45 ppt. These results suggest that glucose export in marine crustaceans is stimulated and regulated by chloride. Chloride activation of G-6-Pase may be an important mechanism for meeting metabolic demand in crustaceans living in variable ion environments.