Several recent publications have identified adverse effects of HRT in a diverse group of disease processes, including stroke,12
and urinary incontinence.7
Further, the Women's Health Initiative (WHI) trial, evaluating the effects of postmenopausal estrogen plus progesterone therapy was prematurely halted secondary to multiple reported adverse trends in the treatment arm.12–14
The results of our experiment align with these aforementioned studies, indicating that extended term estrogen therapy (> 5 wk) worsened voiding dysfunction in our postpartum intravaginal balloon-dilated, ovariectomized rat model.
Using this model, we previously reported that birth trauma simulated by weighted ballooning and ovariectomy appears to contribute to SUI.9,11
We have also demonstrated that these rats rarely have UI during pregnancy or after ovariectomy alone without vaginal distension. Although the scenario does not create a true replica of prolonged labour as seen in humans, we believe this model is suitable for studying the urinary continence mechanism in the rat.
In our study, cystometric curve and the observation of urine (saline) leakage pattern clearly identified rats with normal voiding patterns and those with abnormal leakage. Since the cystometry was performed under ketamine sedation, we could not determine whether the AV pattern was due to pure sphincter insufficiency or mixed with overactive bladder, we included all rats with abnormal voiding in the AV group. Future studies with conscious cystometry will be required to further study the AV patterns.
Using immunohistochemistry, we observed a significant downregulation of α1A- and αD-adrenoceptor expression in the endothelium and subendothelial regions of the urethral submucosa in rats treated with estrogen. Conversely, there was an increase in α1A- and αD-receptor immunostaining in vaginal tissues. This differential regulation of adrenoceptor in the urethra and vagina is intriguing and warrants further investigation ().
Urinary continence is believed to be maintained by sympathetic impulses through α1
-adrenoceptors in the bladder neck and urethral smooth muscle.15,16
Binding and molecular biological studies established the α1A
-adrenoceptor to be the predominant subtype in the urethra, responsible for the contractile responses of the urethra via elevated intracellular calcium levels to adrenergic agonists.17–19
Taki and colleagues20
demonstrated that α1
-adrenoceptors play an important role in maintaining the resting urethral tonus of the female urethra. They divided the urethra into segments and administered noradrenaline (NA), an α1
agonist, demonstrating a concentration-dependent contraction with a peak amplitude in the middle to proximal urethra, while acetylcholine produced contractions only in the proximal urethra and bladder neck. Applying these results to our experiment, a downregulation of α1
-adrenoceptors would contribute to a significant decrease in contraction and tone of urethral smooth muscle.
The 4 reported α1
-adrenoceptor subtypes include α1A
In the proximal female urethra, the expression of α1
-adrenoceptor subtype mRNA has a reported ratio of 90010 for α1A
However, Nishimatsu and colleagues23
reported that phenylephrine, a nonselective α1
-adrenoceptor agonist, induced contractions through the α1L
receptor, and not through the α1A
subtype. The role of each of these α1
-adrenoceptor subtypes in relation to urethral contractile function has yet to be delineated. Regardless, our results suggest that downregulation of both α1A
adrenoceptors may represent one mechanism by which impaired contraction of the urethra can lead to incontinence.
NO, a neurotransmitter that causes relaxation of urethral smooth muscle, is produced by NOS, an enzyme located at several sites, including neurons.24,25
Recent studies suggest that adrenergic and NO-mediated nerves may function in a complex interactive manner and not as independent agents.17
In the rabbit urethra, released NO suppresses NA release from adrenergic nerve terminals; the reverse has also been reported.26
Perhaps our finding of upregulated urethral nNOS in the estrogen group results in smooth muscle relaxation by both local effect and α1
-adrenoceptor downregulation with a net decrease in NA release.
Further investigation is needed to determine the intricate mechanisms causing SUI. Both molecular study and physiological animal experiments are needed to advance our knowledge and improve clinical management. Identifying the mechanism by which estrogen modulates the α1 adrenoceptor will be valuable not only to incontinence research but also to a broad spectrum of diseases.