Hormone treatments and pregnancy alter myosin light chain kinase and calmodulin levels in rabbit myometrium

Myosin light chain kinase activity and the amount of calmodulin were measured in rabbit myometrium after the rabbits had been treated with oestrogen, progesterone and human chorionic gonadotrophin, and also during pregnancy. Injections of oestrogen and progesterone produced significant increases of of 2.8- and 2.1-fold in the enzyme activity/mg protein and of 1.4- and 1.2-fold in the calmodulin content/mg protein respectively. The increases seen with oestrogen treatment were inhibited by cycloheximide administration, suggesting that protein synthesis is involved in the process. Calmodulin increased about twofold in the particulate fraction under conditions of oestrogen and progesterone treatments and the raised level of calmodulin detected in the cytosol fraction was not due to translocation from the particulate fraction. During pregnancy the specific enzyme activity significantly increased in comparison with the values seen in samples from the non-treated rabbits. These findings suggest that myosin light chain kinase and calmodulin in the myometrium are under hormonal regulation and may play a role in expulsion of the fetus at the latter phase of pregnancy.