Effect of fasting on glycogen metabolism and activities of glycolytic and gluconeogenic enzymes in the mudskipper Boleophthalmus boddaerti

Abstract

During starvation, muscle glycogen in Boleophthalmus boddaerti was utilized preferentially over liver glycogen. In the first 10 days of fasting, the ratio of the active 'a' form of glycogen phosphorylase to total phosphorylase present in the liver was small. During this period, the active 'I' form of glycogen synthetase increased in the same tissue. In the muscle, the phosphorylase 'a' activity declined during the first 7 days and increased thereafter while the total glycogen synthetase activity showed a drastic decline during the first 13 days of fasting. The glycogen level in the liver and muscle of mudskippers starved for 21 days increased after refeeding. After 6 and 12 h refeeding, liver glycogen level was 8·5±2·3 and 6·9±4·5 mg·g wet wt 1, respectively, as compared to 5·8±1·6 mg·g wet wt 1 in unfed fish. Muscle glycogen level after 6 and 12 h refeeding was 0·96±0·76 and 0·82±0·50 mg·g wet wt 1, respectively, as opposed to 0·21±0·12 mg·g wet wt 1 in the 21-days fasted fish. At the same time, activities of glycogen phosphorylase in the muscle and liver increased while the active 'I' form of glycogen synthetase showed higher activity in the liver. Since glycogen was resynthesized upon refeeding, this eliminated the possibility that glycogen depletion during starvation was due to stress or physical exhaustion after handling by the investigator. Throughout the experimental starvation period, the body weight of the mudskipper decreased, with a maximum of 12% weight loss after 21 days. Liver lipid reserves were utilized at the onset of fasting but were thereafter resynthesized. Muscle proteins were also metabolized as the fish were visibly thinner. However, no apparent change in protein content expressed as per gram wet weight was detected as the tissue hydration state was maintained constant. The increased degradation of liver and muscle reserves was coupled to an increase in the activities of key gluconeogenic enzymes in the liver (G6Pase, FDPase, PEPCK, MDH and PC). The increase in glucose synthesis was possibly necessary to counteract hypoglycemia brought about by starvation in B. boddaerti. © 1989.