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). Howev er, little is known about the effects of dietary cadmium on fish organs, es pecially gills, the key osmoregulatory organ. We therefore studied the effe cts of dietary cadmium on metallothionein (MT) and cortisol receptor (GR) i mmunoreactivity in the branchial epithelium of the Atlantic salmon (Salmo s alar). Cadmium was daily administered via food at 0.2 mg (control), 5 mg (l ow dose) and 125 mg thigh dose) Cd per kilogram dry pellet weight. Fish wer e sampled after four and eight weeks. After both Four and eight weeks, plas ma cadmium concentration had increased significantly only in fish fed the h igh cadmium dose. Plasma calcium, sodium, chloride and cortisol levels were not affected. In the controls, most MT was colocated with the chloride cel l marker, Na+/K+-ATPase. but some MT was present in pavement and respirator y cells. GR expression was found in chloride, pavement, respiratory and und ifferentiated cells in all fish groups, but cadmium accumulation and a mark ed 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 G R expression. When the double staining technique for MT and CR was applied, a marked heterogeneity became apparent in the chloride. pavement and respi ratory 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 anti bodies, whereas other cells were negative for both. We conclude that cadmiu m entering the gut also enters the gills, where it accumulates in chloride cells and stimulates MT expression. (C) 2001 Elsevier Science B.V. All righ ts reserved.