The Characterization of Circular and Longitudinal Uterine Smooth Muscle Function from Non‐Pregnant and Late‐Pregnant Rats
Authors: Mejia Raqueliz E , Leovic Michael , Hoxha Brikena , Ion Elena , Vallejo‐Elias Johana , Jones Carleton “Buck” , Struthers Jason , Eckman Delrae M. 
Published: June 2021

Objective/IntroductionThe rodent uterus is a hollow organ that increases approximately 10 fold in size with pregnancy. Myometrial smooth muscle orients longitudinally and circularly allowing flexibility in size and shape during pregnancy. To date most studies have focused on changes in longitudinal smooth muscle with pregnancy. Here we studied how circular smooth muscle of the uterus changes with pregnancy in comparison with longitudinal muscle.MethodsStudies were performed in age‐matched (~77days) non‐pregnant (NP, n=5) and late‐pregnant (LP, d21g±1d, n=4) Sprague‐Dawley rats. Uterine horns from NP and LP rats were rapidly removed, placed in 4°C PSS, and cleaned of all connective tissue. Uterine tissue was cut (ovary to uterine body) allowing both longitudinal muscle (LM) and circular muscle (CM) strips (width: 1.5mm, length: ~4mmt) to be isolated. Muscle strips were mounted in vertical organ chamber baths containing PSS and maintained @ 37°C. After ~45 min. equilibration, strips were challenged with 60mM [K+]o. Tissues not responding to 60mM [K+]o were excluded. Strips were assessed using one of two protocols. Protocol 1: Optimal stretch (L0) ‐ determined by comparing spontaneous contractile activity (e.g., contraction amplitude, area under the curve (AUC), and frequency), along with 60mM [K+]o ‐induced contraction (contraction amplitude and AUC) at 250mg, 500mg, and 1000mg resting tension. Protocol 2: Passive mechanical characteristics ‐ Stress vs strain (SvS) relationships were assessed in CM and LM strips. Tissue equilibrated in Ca2+ free PSS to eliminate spontaneous contractility. Tissue was then stretched in 500 μm steps until tissue failure.ResultsOur data indicates pregnancy significantly increases K+‐induced constriction amplitude, and K+‐induced AUC of CM (P<0.05, NP n=5; LP n=4). Interestingly, neither pregnancy status nor resting tension significantly altered spontaneous contractile frequency, amplitude or contractile AUC in CM or LM. Finally, SvS values were significantly different between CM and LM in both NP and LP (P<0.05).Summary/ConclusionOur preliminary SvS data suggests that CM is “stiffer” than LM in both NP and LP rats. Furthermore, SvS values of CM and LM from LP rats has a significant rightward shift compared to NP suggesting greater compliance in pregnancy. Finally, the effect of pregnancy on spontaneous contractile activity and K+‐induced constrictions appear to be directed at CM vs LM, suggesting that CM remodeling may play a key role in healthy pregnancy. These studies will serve as the foundation for future studies assessing rodent models of complicated pregnancies.Support or Funding InformationFunding: Biomedical Sciences Program (RM, JVE, DME) and Biomedical Sciences Start‐up Funds (DME)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.