Alterations in myometrial stress during ovine pregnancy and the puerperium

Substantial alterations occur in female reproductive tissues 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 days gestation), and postpartum (n = 6) ewes. Active stress (force per cross-sectional area) was calculated at optimal length for maximal force 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 (±SE; 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 vs. pregnant ewes (222 ± 28 vs. 14.9 ± 2.0 pg · mg wet wt^-1h^-1), 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 pg/mg wet wt) and myosin heavy chain (5.5 ± 0.4 vs. 2.0 ± 0.3 pg/mg wet wt) were greater (P < 0.04) in myometrium from late-pregnant vs. nonpregnant ewes. Myometrial growth during ovine pregnancy is associated with reversibly augmented contractile properties that appear to primarily reflect increased cellular contents of contractile proteins.