Metallothionein and cortisol receptor expression in gills of Atlantic salmon, Salmo salar, exposed to dietary cadmium

Commercial fish feeds may contain significant levels of cadmium (Cd). However, little is known about the effects of dietary cadmium on fish organs, especially gills, the key osmoregulatory organ. We therefore studied the effects of dietary cadmium on metallothionein (MT) and cortisol receptor (GR) immunoreactivity in the branchial epithelium of the Atlantic salmon (Salmo salar). Cadmium was daily administered via food at 0.2mg (control), 5mg (low dose) and 125 mg (high dose) Cd per kilogram dry pellet weight. Fish were sampled after four and eight weeks. After both four and eight weeks, plasma cadmium concentration had increased significantly only in fish fed the high cadmium dose. Plasma calcium, sodium, chloride and cortisol levels were not affected. In the controls, most MT was colocated with the chloride cell marker, Na(+)/K(+)-ATPase, but some MT was present in pavement and respiratory cells. GR expression was found in chloride, pavement, respiratory and undifferentiated cells in all fish groups, but cadmium accumulation and a marked stimulation of MT expression were seen only in the chloride cells in the gills of fish fed the high cadmium dose. Cadmium treatment did not alter GR expression. When the double staining technique for MT and GR was applied, a marked heterogeneity became apparent in the chloride, pavement and respiratory cells of both groups of cadmium-treated fish and in the control fish. Some fish showed double staining, others stained only for one of the antibodies, whereas other cells were negative for both. We conclude that cadmium entering the gut also enters the gills, where it accumulates in chloride cells and stimulates MT expression.