Alterations in myometrial stress during ovine pregnancy and the
puerperium.
Ipson, Merle A., Charles R. Rosenfeld, Ronald R. Magness, and Kristine
E. Kamm.
Departments of Pediatrics and Physiology, The University of Texas
Southwestern Medical Center at Dallas, Dallas, Texas 75235
APStracts 3:0089R, 1996.
Substantial alterations occur in female reproductive tissues in order
to ensure the successful outcome of and recovery from pregnancy.
Although sheep have been widely used to study several aspects of
pregnancy, little information is available regarding alterations in
myometrial function. We therefore characterized the alterations that
occur in ovine myometrial stress generating capacity and examined
mechanisms that might account for these changes. Length-force
relations were determined for longitudinal myometrial strips from
nonpregnant (n=6), pregnant (n=11, 67-140 d gestation), and
postpartum (n=6) ewes. Active stress (force per cross-sectional area)
was calculated at optimal length for maximal force (Fo) as determined
from length-force relations. Stimulation by 65 mM KCl resulted in
3.5-times greater stress in strips from late-pregnant vs nonpregnant
ewes, 1.20+/-0.16 vs 0.34+/-0.04 x 105 N/m2 (+/-SEM, p<0.05),
respectively. Responses returned to values seen in strips from
nonpregnant ewes within 2 wk postpartum. Increases in stress were not
associated with differences in the phosphorylated myosin light chain
fraction or the amount of smooth muscle bundles. Although basal
prostacyclin production was 15-fold greater in myometrium from
nonpregnant versus pregnant ewes (222+/-28 vs 14.9+/-2.0 pg/mg wet
weight x hr), cyclooxygenase inhibition did not potentiate stress
responses in strips from nonpregnant animals. However, smooth muscle
contents of actin (26.0+/-1.8 vs 19.1+/-2.2 [mu]g/mg wet weight) and
myosin heavy chain (5.5+/-0.4 vs 2.0+/-0.3 [mu]g/mg wet weight) were
greater (p<0.04) in myometrium from late-pregnant versus
nonpregnant ewes. Myometrial growth during ovine pregnancy is
associated with reversibly augmented contractile properties which
appear to primarily reflect increased cellular contents of
contractile proteins.
Received 7 June 1995; accepted in final form 23 February 1996.
APS Manuscript Number R348-5.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 20 March 96