Daily intramuscular injection of cortisol (4 mg kg−1body weight) in rainbow trout,Salmo gairdneri, for 10 days caused significant increases in the number and individual apical surface area of gill chloride cells per mm2of filament epithelium. Concomitantly, whole body influxes of sodium (Na+) and chloride (Cl−) increased. Acute (3 h) intra-arterial infusion of cortisol did not affect whole body Na+or Cl−influx. A significant correlation was observed between both Na+and Cl−influxes and the fractional apical surface area of filament chloride cells in control, sham (saline-injected) and experimental (cortisol-injected) fish. The chloride cells displayed similar ultrastructural modifications in trout undergoing cortisol treatment as in trout transferred to ion-deficient water. These findings suggest the existence of structure/function relationships in which branchial chloride cell morphology is an important determinant of Na+and Cl−transport capacity. We conclude that chronic cortisol treatment enhances whole body Na+and Cl−influxes by promoting proliferation of branchial chloride cells. The results of correlation analysis indicate that the chloride cell is an important site of NaCl uptake in freshwater ra
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