Research into the impact of menopause on bladder physiology has traditionally used ovariectomised rodent models (1, 2), which mimic an abrupt loss of ovarian hormones rather than the gradual decline and fluctuating hormonal changes of perimenopause. Yet it is during this transitional period that endocrine variability begins to influence multiple organ systems, potentially including the lower urinary tract. While genitourinary symptoms are more prevalent in women post-menopause (3), we hypothesise that physiological changes begin to emerge during the perimenopausal transition.  Therefore, this study aimed to examine the effect of abnormal oestrus cycling in perimenopause on physiological bladder function.

Female C57Bl/6J mice (12-week-old) were randomly allocated to Control or Perimenopause groups (n=8 per group). Mice in the perimenopause group were treated with daily i.p. injections of 4-vinylcyclohexene diepoxide (VCD) (160 mg/kg/day) from day 0, for 10 consecutive days. Age-matched control mice received daily vehicle injections. Vaginal cytology was performed to determine changes in oestrus cycle length, and voiding behaviour was assessed. Mice were sacrificed on day 100, whole bladders were removed, catheterised, and intravesical pressure was recorded during distension with saline and in response to pharmacological stimulation and following electrical field stimulation (EFS). Data (mean ± SEM) were compared via Student’s t-tests.

Vaginal cytology revealed that all animals had normal oestrus cycle length (4.7±0.2 days) prior to treatment, whilst those in the perimenopause group exhibited abnormal cycles in the 14 days preceding sacrifice, characterised by extended days in oestrus. Voiding behaviour was not altered in perimenopausal animals. Bladder capacity at 20 mmHg during filling was also not significantly different between groups (226 ± 33.3 µL control vs 231 ± 34.8 µL perimenopause), nor were bladder weight or the stretch-induced relaxation response to filling. The contractile response to 60mM KCl was significantly depressed in the perimenopause group (18.01±1.59mmHg vs control 23.96±1.10mmHg (p<0.01)), as was the response to ATP (10mM), but not the responses to cumulative concentrations of muscarinic receptor agonist carbachol (Figure 1A). However, the maximal contraction to carbachol was enhanced once normalised to the KCl response (maximum response 193±6.05% control v 239±9.45% perimenopause, p<0.01, Figure 1B). Bladder relaxation from baseline pressure upon addition of β-adrenoceptor agonist isoprenaline was significantly depressed in perimenopause mice, as were nerve-evoked contractile responses to EFS (p<0.01, Figure 1C), while the relative contribution of neurotransmitters (acetylcholine and ATP) to the response was unchanged. Normalisation of the EFS response revealed that the depression was a function of the overall decrease in detrusor contractility (Figure 1D).

Induction of perimenopause using VCD produced abnormal oestrous cycling, without altering bladder weight, capacity or passive mechanical properties. However, bladders from perimenopausal mice exhibited a general decrease in detrusor contractility and relaxation responses. Notably, when muscarinic contractions were normalised to the KCl response, the maximal response was enhanced, suggesting a compensatory upregulation of muscarinic signalling despite the broader decline in smooth muscle function. Altogether, these findings indicate that early hormonal disruption impairs multiple aspects of smooth muscle physiology even before overt changes in voiding behaviour emerge.

This study demonstrates that bladder physiology is altered prior to menopause, resulting in distinct functional changes independent of changes in bladder capacity or compliance. The VCD model successfully captures early transitional effects and may provide a more physiologically relevant platform for investigating how fluctuating ovarian hormones contribute to lower urinary tract dysfunction.

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