Bk. Kim et al., INCREASED CONTRACTILITY OF RAT UTERINE SMOOTH-MUSCLE AT THE END OF PREGNANCY, Comparative biochemistry and physiology. Part A, Molecular & integrative physiology, 121(2), 1998, pp. 165-173
Upon stimulation with high K+, oxytocin, prostaglandin E-2, prostaglan
din F(2)alpha or carbachol, myometrium isolated from pregnant rats (21
days after pregnancy) developed 2-3 times greater isometric force tha
n 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 contract
ion than non-pregnant myometrium at a given MLC phosphorylation. In th
e permeabilized muscle with oc-toxin, Ca2+ (0.1-10 mu M) induced great
er contraction in pregnant myometrium than in non-pregnant myometrium.
Ca2+ sensitivity was not altered after pregnancy. MLC kinase and phos
phatase activities did not differ significantly between pregnant and n
on-pregnant myometria. Stimulation with 10 mu M Ca2+ and 1 mu M calycu
lin-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 the
se preparations, although actin content increased after pregnancy. The
se results suggest that the stress generating capacity of myometrium i
s increased after pregnancy without changing the MLC phosphorylation s
tep. The equal capacity of force generation after the maximum phosphor
ylation by Ca2+ and phosphatase inhibitor suggests that a MLC phosphor
ylation-independent mechanism is responsible for the development of gr
eater force in the pregnant myometrium. (C) 1998 Elsevier Science Inc.
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