The osteoglossid gill: correlations of structure, function, and metabolism with transition to air breathing

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Abstract
The gills of aruana and Arapiama, closely related water- and air-breathing osteoglossids commonly found living in similar environments in the Amazon, were compared in terms of cell composition, structure, and metabolic organization. Enzymes selected to represent different metabolic functions, including glycolysis, pentose shunt, gluconeogenesis, the Krebs cycle, hydrogen shuttling, and ammonia release, were found in the gills of both species. However, the aruana gill had higher activities of key glycolytic enzymes, such as pyruvate kinase and phosphofructokinase, but lower potential for gluconeogenesis (lower fructose diphosphatase: phosphofructokinase ratios), and lower activities of enzymes in oxidative metabolism (citrate synthase, malate dehydrogenase, aspartate aminotransferase, and glutamate dehydrogenase) and in the pentose shunt (glucose-6-phosphate dehydrogenase). Gill lactate dehydrogenase activities were quite high in both species, while hexokinase activities were low; coupled with other studies, these data were taken to mean that lactate is a major source of carbon and energy for the gill. In Arapaima adjustments in cell composition, ultrastructure, and structural organization develop the potential for close control of the regional distribution of cardiac output flowing through the gill. It was speculated that use of this potential control of blood flow would have a major effect (a) on the uptake of blood metabolites such as lactate, (b) on CO2 release, and (c) on ion regulation by the abundant chloride cell population of the Arapaima gill.

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