Myosin light chain kinase and cyclic AMP-dependent protein kinase activities and calmodulin levels in placental and non-placental regions of the rabbit myometrium.

Myosin light chain kinase activity in the placental region of the rabbit myometrium on day 28 of gestation was 4.7 +/- 0.1 (mean +/- S.E.M.) nmol/min per mg protein, which was significantly higher than that (3.6 +/- 0.1 nmol/min per mg protein) in the non-placental region. The amount of calmodulin in the placental region was 4.2 +/- 0.1 micrograms/mg protein, which was significantly higher than that (3.2 +/- 0.1 micrograms/mg protein) in the non-placental region. In contrast, cyclic AMP-dependent protein kinase activities showed no difference between the two regions. These findings suggest that calcium- and calmodulin-dependent protein phosphorylation is activated mainly in the placental region, and uterine contractions can occur more strongly in this part than in the non-placental region. Such enzymatic phenomena may be related to the mechanism whereby the placenta separates from the myometrium after delivery of the fetus.