Upon stimulation with high K+, oxytocin, prostaglandin E2, prostaglandin F2α or carbachol, myometrium isolated from pregnant rats (21 days after pregnancy) developed 2-3 times greater isometric force than that from non-pregnant rats (estrus). High K+increased the level of myosin light chain (MLC) phosphorylation to a similar extent in these tissues, and therefore pregnant myometrium developed greater contraction than non- pregnant myometrium at a given MLC phosphorylation. In the permeabilized muscle with α-toxin, Ca2+(0.1-10 μM) induced greater contraction in pregnant myometrium than in non-pregnant myometrium. Ca2+sensitivity was not altered after pregnancy. MLC kinase and phosphatase activities did not differ significantly between pregnant and non-pregnant myometria. Stimulation with 10 μM Ca2+and 1 μM calyculin-A elicited similar magnitudes of contractions in the permeabilized muscles isolated from non-pregnant and pregnant rats. SDS-PAGE showed that the percentage of the content of MLC was not altered between these preparations, although actin content increased after pregnancy. These results suggest that the stress generating capacity of myometrium is increased after pregnancy without changing the MLC phosphorylation step. The equal capacity of force generation after the maximum phosphorylation by Ca2+and phosphatase inhibitor suggests that a MLC phosphorylation-independent mechanism is responsible for the development of greater force in the pregnant myometrium.
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