Tamoxifen inhibits the myoprotective action of oestrogen at sexual maturity in laying hens.

Previous studies have demonstrated a growth associated myopathy in modern broiler chickens reflected in an elevated plasma activity of the intracellular muscle enzyme creatine kinase (CK). In broilers onset of ovarian steroid secretion or administration of estradiol were shown to exert an apparent myoprotective effect possibly mediated by occupation of the oestrogen receptor (ER). It is not known if such mechanisms exist in other birds or if muscle membrane integrity is enhanced at the beginning of lay in strains less predisposed to spontaneous myopathy than the rapidly growing broiler lines. Fourteen 23 week old white leghorn hens in full lay were divided into two groups. One group received 4mgkg−1 of tamoxifen per day for 4 days whilst the other group received vehicle. Blood samples were obtained before the first injection and then at three day intervals for a total period of five weeks. Plasma CK activity was influenced by both treatment and time (p<0.001). Peak activity occurred in the tamoxifen group at the end of treatment (p<0.05) and was maintained until day 8. Hepatic egg yolk precursor synthesis was reduced significantly by the anti-oestrogen (p<0.05) until day 29 and egg production was inhibited within 3-5 days an effect which persisted for 4 to 5 weeks. The anti-oestrogen markedly elevated plasma CK activity in laying birds suggesting that ER modulation may influence maintenance of muscle membrane integrity and the development of myopathy in normal laying hens at sexual maturity. It is suggested that estradiol controls the synthesis of intracellular components involved in the regulation of sarcoplasmic calcium concentration thereby exerting a myoprotective effect. These findings may have important implications for the maintenance of muscle function during egg production when increased calcium demand, intake and mobilisation may constitute a threat to intracellular calcium homeostasis and the consequent induction of myopathy.