Abstract
The air-breathing singhi catfish (Heteropneustes fossilis) is frequently being challenged by different environmental insults such as hyper-ammonia, dehydration and osmotic stresses in their natural habitats throughout the year. The present study investigated the effect of hyperosmotic stress, due to exposure to hypertonic environment (300 mM mannitol) for 14 days, on gluconeogenesis in this catfish. In situ exposure to hypertonic environment led to significant stimulation of gluconeogenic fluxes from the perfused liver after 7 days of exposure, followed by further increase after 14 days in presence of three different potential gluconeogenic substrates (lactate, pyruvate and glutamate). Environmental hypertonicity also caused a significant increase of activities of key gluconeogenic enzymes, namely phosphoenolpyruvate carboxykinase, fructose 1, 6-bisphosphatase and glucose 6-phosphatase by about 2-6 fold in liver, and 3-6 fold in kidney tissues. This was accompanied by more abundance of enzyme proteins by about 1.8–3.7 fold and mRNAs by about 2.2–5.2 fold in both the tissues with a maximum increase after 14 days of exposure. Hence, the increase in activities of key gluconeogenic enzymes under hypertonic stress appeared to be as a result of transcriptional regulation of genes. Immunocytochemical analysis further confirmed the tissue specific localized expression of these enzymes in both the tissues with the possibility of expressing more in the same localized places. The induction of gluconeogenesis during exposure to environmental hypertonicity possibly occurs as a consequence of changes in hydration status/cell volume of different cell types. Thus, these adaptational strategies related to gluconeogenesis that are observed in this catfish under hypertonic stress probably help in maintaining glucose homeostasis and also for a proper energy supply to support metabolic demands mainly for ion transport and other altered metabolic processes under various environmental hypertonic stress-related insults.
Highlights
Gluconeogenesis from lactate, pyruvate and amino acids is important for the maintenance of circulating glucose level during strenuous [1] and fasting conditions in vertebrates [2]
Gluconeogenesis occurs at relatively higher rates [6,7,8,9,10], and is thought to be a key process in maintaining glucose homeostasis [11], especially in carnivorous fish that have high protein and low carbohydrate diets [12]
Even though most of the enzymes involved in glucose metabolism have been detected in fish, the regulation of carbohydrate metabolism differs in some aspects from that of mammals [14]
Summary
Gluconeogenesis from lactate, pyruvate and amino acids is important for the maintenance of circulating glucose level during strenuous [1] and fasting conditions in vertebrates [2]. Gluconeogenesis occurs at relatively higher rates [6,7,8,9,10], and is thought to be a key process in maintaining glucose homeostasis [11], especially in carnivorous fish that have high protein and low carbohydrate diets [12]. Carbohydrate may be used for short term responses in acute stress situations as a last resort in fish [13]. Even though most of the enzymes involved in glucose metabolism have been detected in fish, the regulation of carbohydrate metabolism differs in some aspects from that of mammals [14]. The regulation of hepatic glucose metabolism in teleost fish is reported to be influenced by different stressful conditions, such as low dietary carbohydrates and changes in hepatocellular hydration status [15,16,17]
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