Endocrine Effects Of Tributyltin Compounds
Drs. Reinhard Länge and Hermann Schweinfurth
Since the early eighties the development of male sexual characteristics (the so-called imposex condition) has been reported for females of some marine neogastropod snails. This phenomenon has been primarily attributed to the contamination of coastal areas with tributyltin (TBT) as described in Smith (1981) and Bryan et al. (1987). Species inhabiting rocky shores, as well as soft bottoms of peripheral seas, are reported to be affected (Fioroni et al. 1991, Ten Hallers-Tjabbes et al. 1996). At higher concentrations of TBT, Bryan et al. (1986) reported reduced reproduction in field populations of Nucella lapillus, while other species showed no similar effect.
The mechanism of toxicity in N. lapillus is described by Spooner and Goad (1990) as a result of increased testosterone level in treated female snails. Bettin et al. (1996) postulated TBT inhibition of Cytochrome-P450-aromatase, which normally metabolizes testosterone to 17b -estradiol in females. An alternative hypothesis is neurotoxic effects of TBT resulting in increased testosterone production in female snails (Féral and Le Gall 1983).
Based on the published studies, a hormonal (androgen) effect of TBT is assumed in this group of marine snails. The effect is definitively concentration dependent. On the basis of an extensive two-generation study with N. lapillus, it can be demonstrated that imposex is induced in adult, female N. lapillus at TBT concentrations below 2 ng/L (Davies et al. 1997). Although Davies et al. did not report the concentration causing reproductive effects, a NOEC of 8 ng/L and a LOEC of 32 ng/L has been determined for reproduction as the relevant parameter for sustainability of populations (Harding et al. 1996).
Areas close to two commercial harbours affected by dock activities showed TBT concentrations leading to a decrease in N. lapillus reproduction in laboratory tests. In spite of this, TBT concentrations in open seas used by ocean-going vessels are, in general, below the level that causes a decrease in reproduction of N. lapillus (Länge 1996).
Endocrine effects of TBT are not reported for other wildlife organisms. There is no conclusive evidence for an endocrine effect on the chambering of the Pacific oyster Crassostrea gigas associated with higher TBT concentrations found in marinas (Alzieu et al. 1986, Thain and Waldcock 1986). As a consequence of international legislation (the ban on TBT-based antifouling paints in Europe for use on ships < 25m), TBT concentrations in marinas are well below the level that induce chambering in C. gigas.
Different parameters are examined in laboratory animals (e.g., rats of different strains) which should indicate any alterations in the endocrine system response during treatment with TBT. Only at high dosages in food were effects found. Adrenal gland weight and hypophysis, as well as histological changes of the pituitary gland and certain hormone levels like insulin, thyroxin, TSH (thyroid stimulating hormone) and LH (lutenisating hormone) (Funahashi et al. 1980, Krajnc et al. 1984). As the effect of TBT on the immune system is a more significantly sensitive endpoint (IPCS 1990), the determined but weak endocrine effects on mammals have no significance on the establishment of limit values in risk assessment
In particular, a two-generation study in rats did not give any evidence of disturbance in fertility or reproductivity (Biodynamics 1990).
Therefore, hormonal effects of TBT in marine snails are a singular phenomenon. Very low concentrations may induce malformations of the female genitals, however a reduction in reproduction ability is proven only for N. lapillus. TBT concentrations which have effects on the reproduction in laboratory tests could only be found near harbour areas but not in the open sea.
Other groups of organisms, including mammals, are not expected to be affected by endocrine disruption of TBT exposure which is significant for risk assessment procedures.
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