Characterization of Metabolic Carbon Flow in Hepatocytes Isolated from Thermally Acclimated Killifish Fundulus heteroclitus

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Abstract
Hepatocytes isolated from temperature-acclimated Fundulus heteroclitus were used in determinations of respiratory rate, maximal activities of regulatory enzymes, and evolution of from labeled glucose and palmitic acid. Oxygen consumption was relatively independent of acclimation temperature between 25 C and 15 C but exhibited greater thermal sensitivity between 15 C and 5 C acclimation temperatures. When determined at the temperature of acclimation, evolution of from labeled palmitate increased from 5 C to 15 C and decreased from 15 C to 25 C. Rates of release from labeled glucose increased slightly between 5 C and 15 C and more dramatically between 15 C and 25 C. The ratios from glucose were the same for each acclimation temperature, indicating that the relative participation of the pentose shunt in carbohydrate oxidation remained unchanged with acclimation. The results of labeled substrate utilization experiments indicate that substantial changes in the relative contributions of carbohydrate and lipid to aerobic energy metabolism accompany temperature acclimation in F. heteroclitus. Of enzymes studied, glucose-6-phosphate dehydrogenase, pyruvate kinase, and cytochrome oxidase exhibited compensatory changes in activity. Maximal activities of the glycolytic enzymes phosphofructokinase and pyruvate kinase did not demonstrate changes corresponding to patterns of labeled glucose utilization, indicating that modulation of pathway flux during temperature acclimation may be achieved by means other than alterations in enzyme quantity. Reorganization of metabolic carbon flow in Fundulus hepatocytes may reflect adaptive strategies for freezing avoidance via serum hyperglycemia and seasonal variations in reproductive effort and food resource availability.

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