Historically the hypermobility theory has dominated the field of research that aimed at identifying SUI’s causes. The acceptance of this theory and tissue availability issues may explain why most histological studies in SUI research have focused on periurethral rather than urethral tissues. However, the study by DeLancey et al 7
, which concludes that changes in the urethra play the most significant role in SUI development, point to the need to refocus our attention on the urethra. For the undertaking of this challenge, urethral tissues from SUI animal models are practical alternatives to human tissues.
The rat’s urethra is anatomically similar to humans’ and therefore suitable for studying SUI-related changes 19
. Since the majority of SUI patients are parous and menopausal, we established a rat SUI model by treating postpartum rats with VD and ovariectomy. While this rat model has been useful for assessing SUI-related functional and histological changes, interpretation of the experimental data has been complicated by the multifactorial nature of the intervention procedure. Thus, in the present study we focused on testing VD, which is singularly the most critical factor for inducing SUI symptoms 11
, in combination with BAPN, which is well known for its effects on the extracellular matrix but has not been investigated in a urological setting. The results show that, at 3 weeks post-treatment, the majority of VD rats exhibited normal urinary patterns while the majority of VD+BAPN rats abnormal. This was paralleled by changes in the urethral collagen, elastic fiber, and striated muscle as VD alone had milder effects when compared to VD+BAPN.
By examining urethral needle biopsy specimens of continent and SUI patients under electron microscopy, FitzGerald et al 20
observed SUI-related alterations in collagen fibril morphologic characteristics. While it is not known whether urethral collagen is quantitatively affected in SUI patients, we have previously shown that urethral collagen content was decreased in rats with SUI symptoms 12
, and others have reported decreased periurethral collagen content in SUI patients 21-23
. In addition, increased collagenolysis in the vaginal mucosa of patients with prolapse disorder has also been observed 24
. Thus, it appears that that a decrease in collagen content is associated with SUI-related urethral, periurethral, and vaginal abnormalities.
In the periurethral tissue of SUI women with a hypotonic urethra, irregular fragmented elastic fibers have been observed 25
. In lysyl oxidase like-1 knockout (LOXL1-KO) mice, which are unable to assemble elastic fibers properly and exhibit SUI symptoms, a disorganized elastic fiber system in the urethra has also been demonstrated 26
. In our SUI rat model we also found that the urethral elastic fibers were disorganized and fragmented 12,13
. In the present study we found that urethral elastic fibers were slightly affected by VD but became fragmented in VD+BAPN rats. Thus, it appears that birth trauma, as represented by VD, disturb the elastic fiber system, which further becomes fragmented due to BAPN’s blockade of elastic fiber biosynthesis.
Magnetic resonance imaging has found that the urethral striated muscle layer was significantly thinner in women with SUI than in continent women 27
. In the present study we found that the urethral striated muscle fibers became sparse and shorter in VD and VD+BAPN rats. In particular, those in the VD+BAPN group also exhibited a crooked “worm-like” shape suggestive of lacking architectural support. Since it has been shown that normal muscular architecture requires an intact elastic fiber system 28
, it is possible that the fragmentation of elastic fibers and their loss of inter-muscle fiber connections in VD+BAPN rats are responsible for the abnormal striated muscle morphology.
Most SUI patients incurred parturition-induced urethral injuries years or decades earlier. However, current SUI animal models simulate the acute but not the clinically relevant chronic phase of disease progression. Specifically, we and others have observed that VD-induced incontinent rats spontaneously regained normal urinary function 11,29
. Thus, in order to better simulate clinical SUI, it is necessary to inhibit this spontaneous recovery of urinary function; and, to this end, our present study points to the feasibility of adding BAPN to the SUI-induction regimen. By chronically suppressing collagen and elastic fiber biosynthesis, BAPN is expected to delay or prevent the spontaneous recovery of urinary function in VD-treated rats and thus make them more clinically relevant.
As mentioned in Introduction, BAPN has been used to create animal models of aortic aneurysm 16
. In regard to treatments for aortic aneurysm, one of the most promising strategies is the administration of doxycycline (DOX), an inhibitor of matrix metalloproteinases 30
. In our ongoing research, we are also testing the effects of DOX on SUI. Thus, while the present study reports the utilization of BAPN for the creation of a long-term SUI animal model, we have also obtained valuable data on the effects of DOX on SUI (manuscript in preparation). We believe that this combination of exploring BAPN as a negative effector and DOX as a positive effector of urinary continence is the best approach in the search of effective treatments for SUI.