Kinetics and magnitude of metallothionein induction by Cd, Cu, Hg and Zn in European flounder — calibration for environmental monitoring

Abstract

The inductive response of metallothionein (MT) synthesis to heavy-metal exposure in many Phyla is well documented; however, before this response can be used and interpreted as an environmental biomarker of exposure, the kinetics and dose responses must be known. We have carried out a series of experiments where we have studied these parameters of MT mRNA and MT protein induction in different tissues (liver, kidney, gills) of European flounder after intraperitoneal (ip) injection of Cd (1–320 μg/kg), Cu (25–200 μg/kg), Hg (235–100 μg/kg) and Zn (1–20 mg/kg). Basal hepatic levels of 320 μg MT/g wet tissue for the wild caught flounder used were five times higher than in fish from a pristine area and 10–20 times higher than in plaice or turbot obtained from pristine areas. Transient increases in renal MT mRNA levels were only observed 4–10 days post injection (p.i.) of Cu or Hg and there were no significant increases in MT protein levels in any tissue. Doses of >5 mg/kg Zn induced MT mRNA levels three- to four-fold in all tissues 2–4 days p.i. but only produced a transient elevation in MT levels at 4 days p.i. There was no measurable response to low doses of Cd, doses of >80 μg/kg Cd had no significant effect on hepatic MT levels but caused a significant elevation in gill and kidney levels 10 and 21 days p.i. MT mRNA levels in gill and kidney were induced four- to five-fold 24 h p.i. of Cd (and remained elevated for >10 days) whilst a response was only observed in liver (two-fold) 4 and 10 days p.i. These laboratory results, obtained with a wild population exhibiting moderately elevated basal MT levels indicate that the response to metal exposure is too small and too highly dependent upon the metal and exposure time to provide a robust biomarker of contaminant effect. This supports our previous observation of analyses of flounders from the southern North Sea and Dutch estuaries and would indicate that caging studies with ‘ultra-clean’ animals would be required for any monitoring studies.