Expression of myosin heavy and light chains changes during pregnancy in the rat uterus.

We investigated the in vivo expression of myosin heavy chains (MHC) and myosin light chains (MLC) in the rat uterus during pregnancy and post partum (p. p.). According to their antigenic reactivity, we observed two smooth-muscle-specific MHC (SM-MHC) of 204 kDa and 200 kDa (SM1 and SM2 respectively) and one non-muscle-specific MHC band of 198 kDa (NM-MHC). Adult virgin female rats expressed SM1 and NM-MHC (65/35) but no SM2. During the first pregnancy NM-MHC expression decreased in favour of the SM1 form, the SM1/NM-MHC ratio being 82/12 after 20 days of pregnancy. SM2 is newly expressed in the first p. p. state, the SM1/SM2/NM-MHC ratio being 58/28/14 between 3 and 33 days p. p. During the second pregnancy, starting 34 days p. p., both SM2 and NM-MHC expression decreased, the SM1/SM2/NM-MHC ratio being 85/0/15 after 20 days of pregnancy. Two protein bands of approximately 154 kDa and 145 kDa (sodium dodecylsulphate electrophoresis), designated SMP1 and SMP2 respectively, with the same immunoreactivity as the SM-MHC were observed in vivo in the pregnant but not in the non-pregnant uterus. In addition, a Ca(2+)-independent protease, which uses MHC as substrate, is expressed in the pregnant but not in the non-pregnant rat uterus. Two isoforms of the 17-kDa MLC (LC17a and LC17b) exist in the rat uterus. Expression of the LC17a isoform increased during the first pregnancy from 46% in virgin rats to 65% in uteri of rats 20 days pregnant.(ABSTRACT TRUNCATED AT 250 WORDS)