A comment on: 'Fish glucocorticoid receptor with splicing variants in the DNA binding domain' by Jiro Takeo, Jun-ichiro Hata, Chisako Segawa, Haruhiko Toyohara, Shinya Yamashita, FEBS Letters 389 (1996) 244-248

Pathway-specific response to cortisol in the metabolism of catfish

Cortisol produced biochemical pathway-specific effects on metabolic enzymes and other macromolecules in the freshwater catfish, Clarias batrachus. Injection of cortisol increased 1.6-fold activity of citrate synthase (CS) in brain, liver and skeletal muscle of the fish over vehicle-injected control, while administration of metyrapone (a cortisol synthesis inhibitor) reduced CS activity by 52%. Cortisol treatment of metyrapone-treated fish induced CS activity by approximately 2.5-fold, which was blocked after administration of actinomycin D or cycloheximide. This shows de novo synthesis of CS to enhance aerobic capacity of fish. In contrast the activities of glucose-6-phosphate dehydrogenase (G6-PDH) and lactate dehydrogenase (LDH) increased in response to metyrapone and decreased after administration of cortisol in all the three tissues. The cortisol-mediated decrease in G6-PDH and LDH activities reflects reduction in biosynthetic and anaerobic capacity of fish. Administration of metyrapone significantly increased RNA/DNA ratio and protein but cortisol decreased these macromolecular contents in brain, liver and skeletal muscle. It shows cortisol-induced decrease in protein synthesis capacity of fish. The present study suggests that cortisol-induces catabolic and aerobic but inhibits anabolic and anaerobic processes in freshwater catfish. The cortisol-dependent metabolic responses may also be associated with the permissive effect of cortisol on other hormone(s) in fish.

A regulatory role for cortisol in muscle glycogen metabolism in rainbow trout Oncorhynchus mykiss Walbaum

To test the hypothesis that cortisol has a regulatory role in fish muscle glycogenesis post-exercise, rainbow trout were treated 1 h prior to exercise with either saline (control) or metyrapone (2-methyl-1, 2-di-3-pyridyl-1-propanone) to block cortisol synthesis. Following exercise (time 0), half of the metyrapone-treated fish received a single injection of cortisol, to mimic the post-exercise rise usually observed. Muscle glycogen and the relative activities of glycogen phosphorylase a (Phos a) and glycogen synthase I (GSase I), regulatory enzymes for glycogen resynthesis, were monitored 4 h post-exercise. Metyrapone treatment succeeded in blocking the post-exercise rise in plasma cortisol (17+/-2 vs 118+/-13 ng ml(-1) in controls at time 0), and cortisol injection resulted in a larger and more prolonged cortisol increase than in controls (159+/-22 vs 121+/-14 ng ml(-1) in controls at 1 h). Muscle glycogen was completely restored in the metyrapone-treated fish within 2 h after exercise (8.3+/-0.6 vs 8+/-0.7 micromol g(-1) pre-exercise), only partially restored in control fish at 4 h (5.4+/-01.4 vs 8.8+/-1.3 micromol g(-1) pre-exercise), and not at all in cortisol-treated fish (1.0+/-0.5 micromol g(-1) at 4 h). The rapid glycogen resynthesis in the metyrapone-treated fish was associated with a more rapid inactivation of Phos a and stimulation of GSase I compared to controls. In cortisol-treated fish, Phos a activity persisted throughout 4 h post-exercise; there was also a significant stimulation of GSase I activity. As a consequence of dual activation of Phos a and GSase I, glycogen cycling probably occurred, thus preventing net synthesis. This explains why the post-exercise elevation of cortisol inhibits net glycogen synthesis in trout muscle.

Metabolic zonation in teleost gastrointestinal tract. Effects of fasting and cortisol in tilapia

Activities of several metabolic enzymes show distinct patterns of zonation along the intestinal tract of tilapia (Oreochromis niloticus), rainbow trout (Oncorhynchus mykiss) and copper rockfish (Sebastes caurinus). Zonation is species and enzyme specific, with different metabolic activities concentrated in specific areas, and few generalizations can be made. The rockfish show the smallest degree of zonation, with highest activities in the third quarter of the intestine, and shallow gradients to either side, and a general upswing in activity towards the distal end. In the trout, mitochondrial enzyme activities (citrate synthase, glutamate dehydrogenase, malate dehydrogenase) are highest in the pyloric caeca and decrease along the length of the small intestine. This pattern is accentuated for malic enzyme and glucose 6-phosphate dehydrogenase. These enzymes drop precipitously in activity after the first few sections of the small intestine, while other NADP-linked dehydrogenases (isocitrate dehydrogenase, and 6-phosphogluconate dehydrogenase) show moderate activity in pyloric caeca and peak toward the distal section of the small intestine. In tilapia, glutamate dehydrogenase shows a similar decrease as in trout, but citrate synthase peaks towards the distal sections. NADP-dependent dehydrogenases reveal distinct patterns, peaking in different sections of the intestine-malic enzyme in the proximal midsection, glucose 6-phosphate dehydrogenase in the distal mid-section, and isocitrate dehydrogenase in the anal section. Enzyme activities in the stomach of trout and tilapia also show zonation, with the midsection generally displaying the highest activities. A 5-day treatment of tilapia with an intraperitoneal cortisol deposit (25 mg kg(-1) wet mass) drastically alters metabolic performance along the gut in enzyme specific patterns, generally increasing enzyme activities in site-specific arrangements. Cortisol treatment also leads to the expected increases in activities of phosphoenolpyruvate carboxykinase, pyruvate kinase and aspartate aminotransferase in liver, but not in kidney. Aspartate aminotransferase is the only enzyme in brain significantly increased by cortisol treatment. Short-term food deprivation changes enzyme patterns, often resembling those observed after cortisol administration. We conclude that brain, liver and intestinal amino acid metabolism is an important target for cortisol action in fish and that metabolic zonation is a key factor to be reckoned with when analyzing physiological phenomena in the fish intestine.