Recovery of Circular and Longitudinal Uterine Smooth Muscle Structure/Function during the Post‐Partum Period
Authors: Beard J. Megan , Mejia Raqueliz , Hoxha Brikena , Vallejo-Elias Johana , Cooper Kimbal E. , Eckman Delrae M. 
Published: April 2020

Objective/Introduction

Numerous studies have looked at uterine smooth muscle (USM) remodeling during pregnancy. Surprisingly, few studies have looked at the post‐partum period and the recovery of USM back to its pre‐pregnant state. Therefore, we designed a study in rodents to assess changes in USM structure and function of both circular and longitudinal smooth muscle (CM and LM respectively) during pregnancy and the post‐partum period.

Methods

Studies were performed in NP (n=4), LP (21±1d gestation, n=4), 3 days post‐partum (PP3, n=4), and 30 days post‐partum (PP30, n=4) Sprague Dawley rats. Uterine horns were rapidly removed, cleaned of connective tissue, cut longitudinally allowing both LM and CM strips (width: 2mm, length: 4±1mm) to be isolated. Muscle strips were mounted in vertical organ chamber baths containing Krebs and maintained @ 37°C. After 60 min. equilibration, strips were challenged with 60mM [K+]o for viability. Tissues not responding to 60mM [K+]o were excluded. USM strips were assessed using one of two protocols. Protocol 1: Optimal stretch (L0) ‐ determined by comparing spontaneous contractile ‐frequency (SCF) and ‐amplitude (SCA), along with 60mM [K+]o ‐induced contraction (contraction amplitude) in CM and LM at the following resting tensions (in g): 0.10, 0.25, 0.50, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0, and 6.0. Protocol 2: Passive mechanical characteristics ‐ stress vs strain (SvS) relationships were assessed in CM and LM strips. USM was equilibrated in Ca2+ free PSS to eliminate spontaneous contractility. Tissue strips were then stretched in 500 μm increments until tissue failure. Statistical comparisons were assessed in CM and LM between experimental groups (NP vs LP, NP vs PP3, and NP vs PP30). Data was considered statistically different at P<0.05 using a two‐way ANOVA.

Results

Our preliminary results suggest: SCF in CM and LM from NP rats was attenuated compared to CM and LM from LP, PP3, and PP30 (P<0.05). SCA was greater in LP CM vs NP CM, NP CM vs PP3 CM (P<0.05), but was similar in all other groups studied. K+‐induced constrictions were greater in LP‐CM vs NP‐CM, LP‐LM vs NP‐LM, and NP‐CM vs PP3‐CM (P<0.05), all other groups were similar. Finally, NP‐CM and NP‐LM SvS relationships were significantly shifted to the left of LP‐CM and LP‐LM, PP3‐CM and PP3‐LM, and PP30‐LM (P<0.05). There was no difference observed NP CM and PP30 CM.

Summary/Conclusion

To the best of our knowledge, this is the first study assessing USM recovery during the postpartum period. Our data suggest that SCAs and K+‐induced constrictions in CM and LM from PP3 and PP30 animals were similar to NP values. While SCF in CM and LM, along with SVS in CM required 30 days to recover to NP values. The only post‐partum value which remained significantly different after 30 days post‐partum was LM SvS values from PP30 rats. Molecular and histological biomarkers of USM structure/function are currently underway to identify potential mechanisms for these structural and functional changes.

Support or Funding Information

Biomedical Sciences Program (JMB, DME) and Biomedical Sciences Start‐up Funds (DME)