Effects of Ca2+ and calmodulin on contraction of chemically skinned muscle fibers from pregnant rat uteri.

The effects of Ca2+ and calmodulin on contraction of saponin-treated (chemically skinned) uterine smooth muscle fibers of pregnant rats were examined. Ca2+ sensitivity, defined as the pCa required for half maximum activation of force production, was found to change with the progress of pregnancy; low in the early and middle stages and high in the later stages of pregnancy. The overall change of Ca2+ sensitivity was about pCa 1.5 during the period of pregnancy. The effect of calmodulin on contraction was also found to be dependent on the stages of pregnancy. Calmodulin was effective on the augmentation of the tension rather than the change in Ca2+ sensitivity, and this augmentation was large in the early and middle stages of pregnancy. The amount of calmodulin, which eluted out of uterine muscle cells during saponin treatment, was large in the early and middle stages of pregnancy. The results indicate that the contractile response of the uterine muscle cells during the period of pregnancy seems to be controlled by both the changes in Ca2+ sensitivity and in the amount of free calmodulin in uterine muscle cells.