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) an d myosin light chains (MLC) in the rat uterus during pregnancy and pos t partum (p. p.). According to their antigenic reactivity, we observed two smooth-muscle-specific MHC (SM-MHC) of 204 kDa and 200 kDa (SM1 a nd 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 fav our 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 SM2/S M2/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 expressi on 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 (sod ium dodecylsulphate electrophoresis), designated SMP1 and SMP2 respect ively, with the same immunoreactivity as the SM-MHC were observed in v ivo in the pregnant but not in the non-pregnant uterus. In addition, a Ca2+-independent protease, which uses MHC as substrate, is expressed in the pregnant but not in the nonpregnant 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. During the first p. p. state LC17a expression decreased to 42% at 19 days p. p. then r emained constant up to 33 days p. p. Again, during the second pregnanc y LC17a expression increased to the same extent as observed in the fir st pregnancy. Phosphorylation of the 20-kDa MLC forms of skinned uteru s fibres derived from 6-days- and 20-days-pregnant rats (first and sec ond pregnancy) increased upon Ca2+ activation but the Phosphorylation levels were the same in both groups. As extrapolated from the force/ve locity relation, the maximal shortening velocity (V(max)) of chemicall y skinned uteri from 6-days pregnant-rats was 0.104 muscle length (ML) s-1 and increased up to 0.201 ML s-1 in uteri from 20-days-pregnant a nimals (first pregnancy). V(max) remained high 2 days p. p. (0.2 ML s- 1) and decreased to 0.09 ML s-1 19 days p. p. During the second pregna ncy V(max) was 0.1 ML s-1 for 6-days-pregnant uteri and increased up t o 0.21 ML s-1 after 20 days of pregnancy. Our results suggest a correl ation between LC17a expression and V(max) rather than between MHC and V(max).