Study On The Stress Softening And Its Reversibility In The Esophagus Of Normal Rat And STZ Induced Diabetic Rat

For a long time,it has been generally accepted that stress softening?Mullins effect?of biological soft tissues caused by repeated cyclic loadings is irreversible.However,a recent study on rabbit detrusor has challenged this viewpoint by revealing that potassium chloride?KCl?stimulated tissue contraction can significantly restore the passive stiffness loss and stored energy loss of rabbit detrusor which caused by stress softening;meanwhile the concept of"adjustable passive biomechanical property"in biological soft tissue was proposed in this study.Esophagus is located at the initial segment of gastrointestinal tract,and is a tubular organ with motility.Because the peristalsis of esophagus and the movement of esophagus can constantly change the geometric characteristics of the esophageal wall and the distribution of stress and strain in esophagus,the biomechanical properties of esophagus are important factors in determining its normal physiological functions.Although the biomechanical properties of esophagus and the reconstruction of the esophageal biomechanical properties caused by growth,aging and diseases have been extensively studied by predecessors,the research on stress softening and its recovery characteristics of normal and diabetic esophagus are still blank.The peristalsis and the dilatation-contraction movement of esophageal wall can drive the transportation of food bolus in esophagus,and at the same time,make esophageal tissue tend to stress softening during repeated deformation.In this process,it is possible that the contraction of esophagus responding to the external stimulation can recover the passive stiffness loss and stored energy loss in esophageal tissues induced by previous stress softening,which possibly acts as a"self-protection mechanism"to maintain the normal physiological functions of esophagus.In addition,previous studies have shown that diabetes mellitus can lead to significant remodeling of esophageal biomechanical properties.Therefore,it can also be speculated that the remodeling of esophageal biomechanical properties caused by diabetes may have a significant impact on the stress softening and its recovery in esophagus,thus leading to physiological dysfunction of esophagus.Based on the above research status and hypotheses,in the research work of this thesis,the biomechanical experiments,numerical calculation,histological observation and immunohistochemical staining analytic method were comprehensively applied to a systematic study on the stress softening and its recovery characteristics of the intact esophagus,esophageal mucosa-submucosa layer and muscle layer in normal rats and STZ-induced diabetic rats.The main research contents and conclusions are as follows:1.Study on the stress softening and its recovery in the intact esophagus of normal ratIn the study of this part,the stress-softening characteristics of rat esophagus were revealed by experimental methods,and whether the contraction induced by KCl stimulation could recover the passive stiffness loss and stored energy loss of intact rat esophagus caused by stress softening were verified to explore the potential adjustable passive biomechanical properties of normal rat esophagus.The results showed that:1)during the inflation-deflation cyclic loadings,the stored energy loss and passive stiffness loss of the intact rat esophagus were mainly caused by the first cyclic loading at each maximum pressure level;2)Immersing in Krebs+solution(containing Ca²⁺)could not recover the stress softening induced stored energy loss and passive stiffness loss in intact rat esophagus,but could recover the stored energy loss caused by viscoelastic softening.3)After the activation with KCl in Krebs+solution,the stress softening induced stored energy loss and passive stiffness loss in intact rat esophagus were significantly restored.2.Study on the stress softening and its recovery in the layered esophagus of normal ratRat esophagus is a typical multi-layer tissue model.By careful microscopic manipulation,rat esophagus can be separated into intact muscle layer and mucosa-submucosa layer without apparent structural damages.Therefore,based on the results of the first part,the aim of the study in this part was to further investigate the stress softening characteristics of the mucosa-submucosa and muscle layers of normal rat esophagus and the effect of KCl activation on the stress softening recovery in the two separated rat esophageal layers.The results showed that:1)In the stratified state,comparing with the esophageal mucosal-submucosal layer,the esophageal muscle layer showed significantly less stiffness,but larger hysteresis loop area;2)KCl could stimulate the esophageal muscle layer to produce severe contraction with high amplitude and the mucosal-submucosal layer to produce tonic contraction with low amplitude;3)The distinction of contractile characteristics between the esophageal mucosa-submucosa layer and muscle layer during the activation of KCl could be related to the significant difference of the content and distribution of smooth muscle tissue in the two esophageal layers;4)After the activation with KCl in Krebs+solution,the stress softening induced passive stiffness loss and stored energy loss in normal rat esophageal mucosa-submucosa layer and muscle layer could be restored to varying degrees.3.Study on the effect of diabetes on the stress softening and its recovery in rat esophagusBased on the results of the first and second parts,in the study of this part,biomechanical experiments combined with immunohistochemical staining analysis were applied to explore the effects of streptozotocin?STZ?induced diabetes on the stress softening and its recovery in rat intact esophagus,esophageal mucosa-submucosa layer and muscle layer.The results showed that:1)The intensity of the KCl stimulated contractions in the intact esophagus,esophageal mucosa-submucosa layer and muscle layer of diabetic rat were significantly weaker than that of normal rat;2)The activation of KCl could not recover the stress softening induced stored energy loss and passive stiffness loss in the intact esophagus,esophageal mucosa-submucosa layer and muscle layer of diabetic rat;3)The alteration of esophageal stiffness induced by diabetes mainly occurred in rat esophageal muscle layer;4)Comparing with normal rat,the passive stiffness loss caused by stress softening in the esophageal muscle layer of diabetic rat apparently increased and the KCl induced passive stiffness recovery in the intact esophagus and esophageal muscle layer of diabetic rat significantly weakened;5)In the esophageal tissues of diabetic rat,the expressions of AGE,RAGE,TGF-?1 and TGF-?1R were significantly up-regulated,while the expressions of BDNF and TRKB were significantly down-regulated,so that these proteins and factors might be related to the diabetes induced inhibition on the recovery of stress softening in rat esophagus.The study in this thesis is the first systematic study on the stress softening and its recovery characteristics of intact esophagus,esophageal mucosa-submucosa layer and muscle layer in normal and diabetic rats.This study reveals the adjustable passive biomechanical properties of normal rat esophageal tissue under the activation of KCl and the effects of diabetes on the stress softening and its recovery in different esophageal layers of rat.The results of this study have a certain reference value for a more comprehensive understanding of esophageal biomechanical characteristics and physiological functions,and can provide some inspirations for the clinical diagnosis and treatment on diabetic gastrointestinal dysfunction.