The Uptake of Ethinyl-Estradiol and Cortisol From Water by Mussels (Mytilus spp.)

Previous toxicokinetic studies have shown that mussels (Mytilus spp.) can readily absorb the three main mammalian sex steroids, estradiol (E(2)), testosterone (T) and progesterone (P) from water. They also have a strong ability to store E(2) and the 5α-reduced metabolites of T and P in the form of fatty acid esters. These esters were shown to have half-lives that were measured in weeks (i.e. they were not subject to fast depuration). The present study looked at the toxicokinetic profile of two other common steroids that are found in water, the potent synthetic oestrogen, (ethinyl-estradiol) (EE(2;) one of the two components of ‘the pill’), and cortisol, a natural stress steroid in vertebrates. In the first three hours of uptake, tritiated EE(2) was found to be taken up at a similar rate to tritiated E(2). However, the levels in the water plateaued sooner than E(2). The ability of the animals to both esterify and sulphate EE(2) was found to be much lower than E(2), but nevertheless did still take place. After 24 h of exposure, the majority of radiolabelled EE(2) in the animals was present in the form of free steroid, contrary to E(2,) which was esterified. This metabolism was reflected in a much lower half-life (of only 15 h for EE(2) in the mussels as opposed to 8 days for E(2) and >10 days for T and P). Intriguingly, hardly any cortisol (in fact none at all in one of the experiments) was absorbed by the mussels. The implications of this finding in both toxicokinetic profiling and evolutionary significance (why cortisol might have evolved as a stress steroid in bony fishes) are discussed.