Smooth muscle contribution to murine vaginal mechanical function and potential interactions with elastic fibers
The vagina is a fibromuscular organ that supports the pelvic organs. An increase in intrabdominal pressure and disruption of elastic fibers are suggested to contribute to a failure in pelvic support (prolapse). Further, vaginal smooth muscle content is significantly decreased with prolapse. In other tissues elastic fibers interact with smooth muscle cells maintaining their contractile function. Further, smooth muscle cells are sensitive to and respond to their mechanical environment. These relationships, however, are unknown in the vagina and may be critical for pelvic support and facilitating physiologic processes (e.g., pregnancy and vaginal delivery). Therefore, the objective of this study was to develop extension-inflation biomechanical testing protocols to determine the role of pressure and elastic fiber insufficiency on vaginal contractile, elastic, and viscoelastic biomechanical properties. An extension-inflation device was used to quantify circumferential and axial maximum contractility by subjecting the murine vagina to varied constant pressures then contracting with potassium chloride. Further, the elastic biomechanical behavior (material stiffness) was quantified by subjecting the vagina to increasing physiologic pressures with and without smooth muscle tone using mice with elastic fiber defects that developed prolapse. Lastly, the viscoelastic behavior was evaluated by subjecting the vagina to varied constant pressures with and without smooth muscle tone and measuring changes in vaginal geometry over time (creep). These studies demonstrated that smooth muscle tone decreased vaginal material stiffness and increased vaginal creep. Further, that an increase in pressure decreased circumferential contractility and decreased creep (with muscle tone). These results suggest that the smooth muscle cells provide mobility but stabilize the vagina under increased pressure. This work also demonstrated that both the contractile response and interactions with elastic fibers were direction dependent. Axial contractility was greater than circumferential contractility and disrupting elastic fibers significantly decreased axial contractility. Overall, this work highlights that smooth muscle cells are an important structural component for vaginal function, are mechano-sensitive, and may interact with elastic fibers in a direction dependent manner. Accounting for biaxial smooth muscle tone under physiologically-relevant pressures may be critical for understanding vaginal function with physiologic processes (e.g., pregnancy) and pathologic conditions (e.g., prolapse).