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 i nformation is available regarding alterations in myometrial function. We therefore characterized the alterations that occur in ovine myometr ial stress-generating capacity and examined mechanisms that might acco unt for these changes. Length-force relations were determined for long itudinal myometrial strips from nonpregnant (n = 6), pregnant (n = 11; 67-140 days gestation), and postpartum (n = 6) ewes. Active stress (f orce per cross-sectional area) was calculated at optimal length for ma ximal force as determined from length-force relations. Stimulation by 65 mM KCl resulted in 3.5 times greater stress in strips from late-pre gnant vs. nonpregnant ewes, 1.20 +/- 0.16 vs. 0.34 +/- 0.04 x 10(5) N/ m(2) (+/- SE; P < 0.05), respectively. Responses returned to values se en in strips from nonpregnant ewes within 2 wk postpartum Increases in stress were not associated with differences in the phosphorylated myo sin light-chain fraction or the amount of smooth muscle bundles. Altho ugh basal prostacyclin production was 15-fold greater in myometrium fr om nonpregnant vs. pregnant ewes (222 +/- 28 vs. 14.9 +/- 2.0 pg . mg wet wt(-1). h(-1)), cyclooxygenase inhibition did not potentiate stres s 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 wt) and myosin heavy chain (5.5 +/- 0.4 vs. 2.0 +/- 0.3 mu g/mg wet wt) were g reater (P < 0.04) in myometrium from late-pregnant vs. nonpregnant ewe s. Myometrial growth during ovine pregnancy is associated with reversi bly augmented contractile properties that appear to primarily reflect increased cellular contents of contractile proteins.