Role of high zinc levels in the stress defense of common carp

We studied the link between high zinc levels and the extreme stress tolerance of common carp. Fish under stress showed much higher plasma cortisol levels than controls. Stress or cortisol injection induced large changes in zinc levels in the common carp but not in grass carp, silver carp or tilapia. The effect of 5 days of anoxia and 4 subsequent days of recovery on cortisol and zinc contents in the common carp was investigated. Elevated plasma cortisol resulting from anoxia was correlated with decreased zinc in digestive tract tissue and increased zinc in the head kidney. Zinc was mobilized in the common carp while under stress. Changes in cortisol and zinc contents were reversible during the subsequent recovery from anoxia. Under stress, protein-bound zinc levels increased in the head kidney cell nuclei of common carp as cortisol increased. Zinc and cortisol were bound to the same protein, which was bound to DNA. The protein is likely a glucocorticoid receptor. An increase in immature red blood cells in stressed common carp was observed. Zinc was involved in the stress erythropoiesis response. Zinc may play an important role in stress defense in the common carp via the glucocorticoid receptor.