The Experience of Pregnancy Induces Variations in the Myometrial Response to Oxytocin, PGE2 and PGF2α: Observations in Non‐pregnant and Pregnant Rat Tissue

Abstract

ObjectiveDuring pregnancy, hormonal and mechanical stimuli induce profound changes in the uterus. Involution reverses these changes during the postpartum period. Epidemiological studies have shown that parity is associated with increased intensity of postpartum contractions and risk for uterine atony, and decreased incidence in dysmenorrhea and fibroids. Thus, we hypothesize that an epigenetic memory of pregnancy is preserved in in the myometrium, the smooth muscle component of the uterus. To test this hypothesis, we conducted motility studies on myometrial strips from non‐pregnant (NP) and pregnant Sprague‐Dawley (Charles River) rats (P).MethodsVirgin (V) and proven breeders (PB) female non‐pregnant 18 week old rats were euthanized during proestrus. First time (Preg1) and second time (Preg2) pregnant female 18 week old rats were euthanized on day 20 of a 22 day pregnancy. The uterine horns were extracted, dissected longitudinally, and the content and lining of the uterus were removed. One horn was dissected into six 3×10 mm strips, which were then suspended in a tissue bath containing Krebs solution (in mM, NaCl 117, KCl 4.7, NaHCO3 25, MgCl2 1.2, KH2PO4 1.2, CaCl2 2.5, glucose 11, pH= 7.4) at 37°C and bubbled with 95% O2 + 5% CO2. Separate dose response curves to the endogenous uterotonics oxytocin, PGE2, and PGF2α were generated by incremental additions of each drug to the tissue bath (10E‐10 to 10E‐5 M in all cases). Contractility was expressed as area under the curve (AUC) normalized to AUC in the presence of 40 mM KCl (AUC/AUC KCl). Data are mean ± SE.ResultsIn non‐pregnant tissue (12 rats/group), the affinity of oxytocin for its receptor did not change significantly (P<0.05) between V and PB, but the maximal response to the drug was higher in PB (Max for V= 0.756 ± 0.001; Max for PB= 0.905 ± 0.002 AUC/AUC KCl, P<0.001). In P tissue, EC50 was similar in Preg1 and Preg2 and significantly higher than in NP myometrium (logEC50 in NP PB= −8.44 ± 0.02; logEC50 in P Preg2= −8.9 ± 0.2 M, P=0.03). Maximal response was similar in Pre1 and Preg2 tissue, but significantly reduced compared to PB tissue (Max for Pre1= 0.57 ± 0.02; Max for PB= 0.59 ± 0.03 AUC/AUC KC, P<0.001). For PGE2, EC50 and Max were not significantly different in V and PB. However, in pregnancy, Preg2 tissue displayed higher affinity for PGE2 than Preg1 tissue (logEC50 in Preg1= −6.1 ± 0.01; logEC50 in Preg2= −6.3 ± 0.01 M, P=0.037), while maximal response remained unchanged. Dose response to PGF2 in NP myometrium revealed both higher affinity (P=0.022) and higher maximal response (P=0.02) in V than in PB (logEC50 in V= −6.49 ± 0.07; logEC50 in PB= −6.2 ± 0.1 M; Max for V= 0.44 ± 0.02; Max for PB= 0.33 ± 0.02 AUC/AUC KCl). This trend was partially reversed in P myometrium (logEC50 in Preg1= −5.72 ± 0.08; logEC50 in Preg2= −5.77 ± 0.08 M, P>0.05; Max for Preg1= 0.30 ± 0.01; Max for Preg2= 0.35 ± 0.02 AUC/AUC KCl, P<0.05).ConclusionsOur results confirm that the myometrial response to endogenous uterotonic agents is altered after the first pregnancy. Understanding the mechanisms underlying these differences is an important first step toward explaining the impact of pregnancy in subsequent uterine function.Support or Funding InformationThis study was funded by a MWU intramural grant to MP.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.