Effects of hypo- or hyperosmotic stress on gluconeogenesis, phosphoenolpyruvate carboxykinase activity, and gene expression in jaw muscle of the crab Chasmagnathus granulata:: seasonal differences

In its habitat, Chasmagnathus granulata is exposed to many different environmental challenges according to the season of the year. For this reason, the investigation of the participation of the gluconeogenic pathway in the acclimation to hypo- and hyperosmotic conditions in summer and winter was considered interesting. By comparing the gluconeogenesis capacity, phosphoenolpyruvate carboxykinase (PEPCK) activity, and mRNA PEPCK gene expression values obtained in control crabs during summer and winter, we found two opposite tendencies: a decrease in winter and an increase in summer. The present results show that, in the. C. granulata jaw muscle, PEPCK activity is divided almost equally between the cytosol and mitochondria in winter crabs. This distribution is different from the one found in muscle from summer crabs, in which most of the PEPCK activity (85%) takes place in the mitochondria. The data reported here show that the natural light/dark cycle typical for winter regulated the proportion of PEPCK activity in the cytosolic and mitochondrial fractions at the transcriptional level, with a marked decrease in the mitochondrial PEPCK activity and, as a result, in the gluconeogenic capability. The gluconeogenic activity decreased 48% after 24 It of hyperosmotic stress in summer. Furthermore, this treatment reverted the proportion of PEPCK activity in cytosolic and mitochondrial fractions: it decreased in the mitochondrial fraction and increased in the cytosolic one. However, at 72 It of hyperosmotic shock, the incorporation of label from alanine into glucose increased 45% as compared to a 24-h group. Hence, it is possible that the increase in gluconeogenic capacity after 72 It of hyperosmotic stress is due to the enhanced PEPCK gene expression at 24 h of osmotic stress. The present study shows that the rises in the incorporation of C-14-alanine into glucose and in the mitochondrial and cytosolic activities in the jaw muscle of C granitlata after 24 h of acclimation to a dilute media in summer did not require an increase in PEPCK gene expression. During hypoosmotic shock in winter, the gluconeogenenic capacity and the PEPCK activity are still remarkably low, and the PEPCK gene expression remains undetectable. The muscle gluconeogenesis seems to be one of the pathways implicated in the metabolic adjustment during hypo- and hyperosmotic shock in C. granulata. On the other hand, the present study highlights the importance of seasonal environmental differences in determining the development of metabolic patterns. (c) 2004 Published by Elsevier B.V.