Afferent bladder nerve activity in the rat: a mechanism for starting and stopping voiding contractions

Selective recording of physiologically evoked neural activity in a mixed autonomic nerve using a minimally invasive array

Real-time closed-loop control of neuromodulation devices requires long-term monitoring of neural activity in the peripheral nervous system. Although many signal extraction methods exist, few are both clinically viable and designed for extracting small signals from fragile peripheral visceral nerves. Here, we report that our minimally invasive recording and analysis technology extracts low to negative signal to noise ratio (SNR) neural activity from a visceral nerve with a high degree of specificity for fiber type and class. Complex activity was recorded from the rat pelvic nerve that was physiologically evoked during controlled bladder filling and voiding, in an extensively characterized in vivo model that provided an excellent test bed to validate our technology. Urethane-anesthetized male rats (n = 12) were implanted with a four-electrode planar array and the bladder instrumented for continuous-flow cystometry, which measures urodynamic function by recording bladder pressure changes during constant infusion of saline. We demonstrated that differential bipolar recordings and cross-correlation analyses extracts afferent and efferent activity, and discriminated between subpopulations of fibers based on conduction velocity. Integrated Aδ afferent fiber activity correlated with bladder pressure during voiding (r2: 0.66 ± 0.06) and was not affected by activating nociceptive afferents with intravesical capsaicin (r2: 0.59 ± 0.14, P = 0.54, and n = 3). Collectively, these results demonstrate our minimally invasive recording and analysis technology is selective in extracting mixed neural activity with low/negative SNR. Furthermore, integrated afferent activity reliably correlates with bladder pressure and is a promising first step in developing closed-loop technology for bladder control.

Reflex voiding in rat occurs at consistent bladder volume regardless of pressure or infusion rate

AIMS

The central nervous system (CNS) regulates lower urinary tract reflexes using information from sensory afferents; however, the mechanisms of this process are not well known. Pressure and volume were measured at the onset of the guarding and micturition reflexes across a range of infusion rates to provide insight into what the CNS is gauging to activate reflexes.

METHODS

Female Sprague Dawley rats were anesthetized with urethane for open outlet cystometry. A set of 10 infusion rates (ranging 0.92-65.5 mL/h) were pseudo-randomly distributed across 30 single-fill cystometrograms. Bladder pressure and external urethral sphincter electromyography were used for the determination of the onset of the micturition and guarding reflexes, respectively. The bladder volume at the onset of both reflexes was estimated from the total infusion rate during a single fill.

RESULTS

In response to many single-fill cystometrograms, there was an increased volume the bladder could store without a significant increase in pressure. Volume was adjusted for this effect for the analysis of how pressure and volume varied with infusion rate at the onset of the micturition and guarding reflexes. In 25 rats, the micturition reflex was evoked at similar volumes across all infusion rates, whereas the pressure at micturition reflex onset increased with increasing infusion rates. In 11 rats, the guarding reflex was evoked at similar pressures across infusion rates, but the volume decreased with increasing infusion rates.

CONCLUSIONS

These results suggest that the CNS is interpreting volume from the bladder to activate the micturition reflex and pressure from the bladder to activate the guarding reflex.

Loss of resilience contributes to detrusor underactivity in advanced age

Volume hyposensitivity resulting from impaired sympathetic detrusor relaxation during bladder filling contributes to detrusor underactivity (DU) associated with aging. Detrusor tension regulation provides an adaptive sensory input of bladder volume to the brainstem and is challenged by physiological stressors superimposed upon biological aging. We recently showed that HCN channels have a stabilizing role in detrusor sympathetic relaxation. While mature mice maintain homeostasis in the face of stressors, old mice are not always capable. In old mice, there is a dichotomous phenotype, in which resilient mice adapt and maintain homeostasis, while non-resilient mice fail to maintain physiologic homeostasis. In this DU model, we used cystometry as a stressor to categorize mice as old-responders (old-R, develop a filling/voiding cycle) or old-non-responders (old-NR, fail to develop a filling/voiding cycle; fluctuating high pressures and continuous leaking), while also assessing functional and molecular differences. Lamotrigine (HCN activator)-induced bladder relaxation is diminished in old-NR mice following HCN-blockade. Relaxation responses to NS 1619 were reduced in old-NR mice, with the effect lost following HCN-blockade. However, RNA-sequencing revealed no differences in HCN gene expression and electrophysiology studies showed similar percentage of detrusor myocytes expressing HCN (I_h) current between old-R and old-NR mice. Our murine model of DU further defines a role for HCN, with failure of adaptive recalibration of HCN participation and intensity of HCN-mediated stabilization, while genomic studies show upregulated myofibroblast and fibrosis pathways and downregulated neurotransmitter-degradation pathways in old-NR mice. Thus, the DU phenotype is multifactorial and represents the accumulation of age-associated loss in homeostatic mechanisms.

Effect of bladder filling volume on detrusor contractility in men with bladder outlet obstruction

INTRODUCTION

Bladder outlet obstruction alters detrusor contractility, reducing the bladder's ability to respond to large filling with a risk of urinary retention. The objective was to assess the effect of bladder filling volume on detrusor contractility in men with bladder outlet obstruction.

METHODS

A prospective multicenter study in two pelviperineology departments. Male patients eligible for urodynamics (IPSS score > 7) were included from January to July 2022. In case of absence of bladder outlet obstruction on pressure-flow studies, they were secondarily excluded. The primary endpoint was the maximum isometric detrusor pressure during a stop-test, corresponding to detrusor contractility, measured at 3 filling volumes (50%, 75%, and 100% of cystometric capacity).

RESULTS

Fifty-two patients performed urodynamics, of whom 12 were excluded because of lack of obstruction or inability to perform the stop-test. Detrusor contractility was significantly higher for a 75% bladder filling than 50% and for a 75% filling than 100%, with a mean difference of 19.5; confidence interval (CI) 95% [14.3; 24.8] and 12.2; CI 95% [6.9; 17.5] cmH₂ O respectively (p < 0,01).

CONCLUSION

In case of bladder outlet obstruction in men, detrusor contractility depends on bladder filling volume, with reduced contractility when the bladder was underfilled or overfilled. This phenomenon could help to explain the mechanisms of urinary retention in men with bladder outlet obstruction.

Mathematical modeling of the lower urinary tract: A review

AIMS

Understand what progress has been made toward a functionally predictive lower urinary tract (LUT) model, identify knowledge gaps, and develop from them a path forward.

METHODS

We surveyed prominent mathematical models of the basic LUT components (bladder, urethra, and their neural control) and categorized the common modeling strategies and theoretical assumptions associated with each component. Given that LUT function emerges from the interaction of these components, we emphasized attempts to model their connections, and highlighted unmodeled aspects of LUT function.

RESULTS

There is currently no satisfactory model of the LUT in its entirety that can predict its function in response to disease, treatment, or other perturbations. In particular, there is a lack of physiologically based mathematical descriptions of the neural control of the LUT.

CONCLUSIONS

Based on our survey of the work to date, a potential path to a predictive LUT model is a modular effort in which models are initially built of individual tissue-level components using methods that are extensible and interoperable, allowing them to be connected and tested in a common framework. A modular approach will allow the larger goal of a comprehensive LUT model to be in sight while keeping individual efforts manageable, ensure new models can straightforwardly build on prior research, respect potential interactions between components, and incentivize efforts to model absent components. Using a modular framework and developing models based on physiological principles, to create a functionally predictive model is a challenge that the field is ready to undertake.

Age is associated with reduced urethral pressure and afferent activity in rat

Age-related changes in the lower urinary tract (LUT) can affect the coordination of reflexes and increase the incidence of bladder disorders in elderly. This study examines the age-related loss of urethral signaling capability by measuring the afferent activity directly. We find that less urethral pressure develops in response to fluid flow in old rats compared to young rats and that pressure and flow evoke less urethral afferent activation. These findings are consistent with our previous study demonstrating that the urethra-to-bladder reflex, which is required for efficient voiding, becomes weaker with age. We measured the pudendal afferent response in young (4-7 months) and old (18-24 months) rats to fluid flow in the urethra across a range of flow rates. We used paraffin embedding and hematoxylin and eosin staining to quantify age-related changes in the sensory branch of the pudendal nerve. Urethral afferent signaling in response to the same urethral flow rates was weaker in older animals. That is, the sensitivity of urethra afferents to flow decreased with age, and higher flow rates were required in older animals to recruit urethra afferents. There was also a reduction in the myelin thickness of pudendal afferents in old rats, which is a possible contributing factor to the sensory activity. Furthermore, the same flow rates evoked less pressure in the urethras of old animals, indicating there is an age-related change of the urethral tissue that reduces the pressure stimulus to which these afferents respond. These results help characterize the underlying changes in LUT system with age.

Aging Changes in Bladder Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels Are Associated With Increasing Heterogeneity of Adrenergic/Mucosal Influence on Detrusor Control in the Mouse

A geroscience-informed approach to the increasing prevalence of bladder control problems in older adults requires understanding the impact of aging on dynamic mechanisms that ensure resilience in response to stressors challenging asymptomatic voluntary control over urine storage and voiding. Bladder control is predicated on sensory neural information about bladder volume. Modulation of volume-induced bladder wall tensions by autonomic and mucosal factors controls neural sensitivity to bladder volume. We hypothesized that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels integrate these factors and thereby mediate adrenergic detrusor tension control. Furthermore, loss of HCN expression compromises that integration and could result in loss of precision of detrusor control. Using a life-span mouse model, reverse transcription quantitative real-time PCR and pharmacologic studies in pretensioned intact and mucosa-denuded bladder strips were made. The dominant hcn1 expression declines with maturation and aging; however, aging is also associated with increased variance around mean values. In strips from Mature animals, isoproterenol had less effect in denuded muscle strips than in intact strips, and HCN blockade diminished isoproterenol responsiveness. With aging, variances about mean response values significantly increased, paralleling hcn1 expression. Our findings support a role for HCN in providing neuroendocrine/paracrine integration and suggest an association of increased heterogeneity of HCN expression in aging with reductions in response precision to neuroendocrine control. The functional implication is an increased risk of dysfunction of brainstem/bladder regulation of neuronal sensitivity to bladder volume. This supports the clinical model of the aging bladder phenotype as an expression of loss of resilience, and not as emerging bladder pathology with aging.

Should we routinely assess psychological morbidities in idiopathic lower urinary tract dysfunction: ICI-RS 2019?

AIMS

Psychological morbidities play a major role in idiopathic lower urinary tract dysfunction (iLUTD). The aim of the Think Tank (TT) was to discuss the relevance of psychological morbidities in idiopathic LUTD over the life span, including overactive bladder (OAB) or dysfunctional voiding (DV) and methods of assessment.

METHODS

The paper is based on a selective review of the literature and in-depth discussions, leading to research recommendations regarding the assessment of psychological morbidities in iLUTD on children and adults held during the TT of the International Consultation on Incontinence Research Society in 2019.

RESULTS

Psychological comorbidities affect the health behaviors and treatment outcomes in patients with iLUTD. Both clinically relevant comorbid mental disorders, as well as subclinical psychological symptoms have a major impact and negatively influence incontinence treatment. Research is needed to elucidate mechanisms underlying iLUTD and psychological comorbidities. Clinical studies are needed to determine how perception generation and cognition impacts on the relationship of urinary perceptions, symptoms, and objective urodynamic function. Due to high psychological comorbidity rates, screening with validated, generic questionnaires for emotional and behavioral disorders in children with nocturnal enuresis, daytime urinary incontinence, and fecal incontinence is recommended. Brief screening is recommended for all adults with iLUTD, especially with OAB and DV, who are refractory to treatment.

CONCLUSIONS

Due to the high rate and relevance in clinical practice, screening for psychological comorbidities is recommended for all age groups. The research recommendations of this TT may be followed to improve the assessment of psychological morbidities in iLUTD.

The aging bladder phenotype is not the direct consequence of bladder aging

AIMS

The prevalence of urinary dysfunction increases with age, yet therapies are often suboptimal. Incomplete understanding of the linkages between system, organ, and tissue domains across lifespan remains a knowledge gap. If tissue-level changes drive the aging bladder phenotype, parallel changes should be observed across these domains. In contrast, a lack of inter-domain correlation across age groups would support the hypothesis that urinary performance is a measure of the physiologic reserve, dependent on centrally-mediated adaptive mechanisms in the aging system.

METHODS

Male and female mice across four age groups underwent sequential voiding spot assays, pressure/flow cystometry, bladder strip tension studies, histology, and quantitative PCR analyses. The primary objective of this study was to test the impact of age on the cortical, autonomic, tissue functional and structural, and molecular domains, and identify inter-domain correlations among variables showing significant changes with age within these domains.

RESULTS

Behavior revealed diminished peripheral voiding and spot size in aged females. Cystometry demonstrated increased postvoid residual and loss of volume sensitivity, but the preservation of voiding contraction power, with almost half of oldest-old mice failing under cystometric stress. Strip studies revealed no significant differences in adrenergic, cholinergic, or EFS sensitivity. Histology showed increased detrusor and lamina propria thickness, without a change in collagen/muscle ratio. Adrb2 gene expression decreased with age. No consistent inter-domain correlations were found across age groups.

CONCLUSIONS

Our findings are consistent with a model in which centrally-mediated adaptive failures to aging stressors are more influential over the aging bladder phenotype than local tissue changes.

Report and Research Agenda of the American Geriatrics Society and National Institute on Aging Bedside-to-Bench Conference on Urinary Incontinence in Older Adults: A Translational Research Agenda for a Complex Geriatric Syndrome

The American Geriatrics Society, with support from the National Institute on Aging and other funders, held its ninth Bedside-to-Bench research conference, entitled "Urinary Incontinence in the Older Adult: A Translational Research Agenda for a Complex Geriatric Syndrome," October 16 to 18, 2016, in Bethesda, Maryland. As part of a conference series addressing three common geriatric syndromes-delirium, sleep and circadian rhythm disturbance, and urinary incontinence-the series highlighted relationships and pertinent clinical and pathophysiological commonalities between these conditions. The conference provided a forum for discussing current epidemiology, basic science, and clinical and translational research on urinary incontinence in older adults; for identifying gaps in knowledge; and for developing a research agenda to inform future investigative efforts. The conference also promoted networking involving emerging researchers and thought leaders in the field of incontinence, aging, and other fields of research, as well as National Institutes of Health program personnel.

Quantification of bladder wall biomechanics during urodynamics: A methodologic investigation using ultrasound

Overactive bladder is often characterized by biomechanical changes in the bladder wall, but there is no established method to measure these changes in vivo. The goal of this study was to develop a novel method to determine detrusor wall biomechanical parameters during urodynamics through the incorporation of transabdominal ultrasound imaging. Individuals with overactive bladder (OAB) underwent ultrasound imaging during filling. The fill rate was 10% of the cystometric capacity per minute as determined by an initial fill. Transabdominal ultrasound images were captured in the midsagittal and transverse planes at 1min intervals. Using image data and Pves, detrusor wall tension, stress, and compliance were calculated. From each cross-sectional image, luminal and wall areas along with inner perimeters were measured. In the sagittal and transverse planes, wall tension was calculated as Pves∗luminal area, wall stress as tension/wall area, and strain as the change in perimeter normalized to the perimeter at 10% capacity. Elastic modulus was calculated as stress/strain in the medial-lateral and cranial-caudal directions. Patient-reported fullness sensation was continuously recorded. Data from five individuals with OAB showed that detrusor wall tension, volume, and strain had the highest correlations to continuous bladder sensation of all quantities measured. This study demonstrates how detrusor wall tension, stress, strain, and elastic modulus can be quantified by adding ultrasound imaging to standard urodynamics. This technique may be useful in diagnosing and better understanding the biomechanics involved in OAB and other bladder disorders.

Pathophysiology of the Underactive Bladder: Evolving New Concepts

PURPOSE OF REVIEW

Underactive Bladder (UAB) is an emerging concept of voiding dysfunction, building on a tradition of often overlapping and confusing terminologies describing the patient experience, urodynamic function, and pathophysiology. This confusion of symptoms with function with pathology has hampered development of an effective disease model.

RECENT FINDINGS

As with other urinary symptoms, the relationship of voiding symptoms to urodynamic dysfunction is variable. Efforts are underway to formally define UAB as a collection of symptoms related to voiding. Defined as voiding symptoms, UAB cannot be reliably linked to detrusor underactivity (DU) nor to detrusor weakness.

SUMMARY

Progress will be made by characterizing the functional disorders underlying UAB symptoms, examining the biology of these linkages, re-conceptualizing urinary control as one part of a more global biologic adaptive physiology, and determining the relationships of central and peripheral pathologies leading to disrupted control mechanisms.

Sensory feedback from the urethra evokes state-dependent lower urinary tract reflexes in rat

KEY POINTS

The lower urinary tract is regulated by reflexes responsible for maintaining continence and producing efficient voiding. It is unclear how sensory information from the bladder and urethra engages differential, state-dependent reflexes to either maintain continence or promote voiding. Using a new in vivo experimental approach, we quantified how sensory information from the bladder and urethra are integrated to switch reflex responses to urethral sensory feedback from maintaining continence to producing voiding. The results demonstrate how sensory information regulates state-dependent reflexes in the lower urinary tract and contribute to our understanding of the pathophysiology of urinary retention and incontinence where sensory feedback may engage these reflexes inappropriately.

ABSTRACT

Lower urinary tract reflexes are mediated by peripheral afferents from the bladder (primarily in the pelvic nerve) and the urethra (in the pudendal and pelvic nerves) to maintain continence or initiate micturition. If fluid enters the urethra at low bladder volumes, reflexes relax the bladder and evoke external urethral sphincter (EUS) contraction (guarding reflex) to maintain continence. Conversely, urethral flow at high bladder volumes, excites the bladder (micturition reflex) and relaxes the EUS (augmenting reflex). We conducted measurements in a urethane-anaesthetized in vivo rat preparation to characterize systematically the reflexes evoked by fluid flow through the urethra. We used a novel preparation to manipulate sensory feedback from the bladder and urethra independently by controlling bladder volume and urethral flow. We found a distinct bladder volume threshold (74% of bladder capacity) above which flow-evoked bladder contractions were 252% larger and evoked phasic EUS activation 2.6 times as often as responses below threshold, clearly demonstrating a discrete transition between continence (guarding) and micturition (augmenting) reflexes. Below this threshold urethral flow evoked tonic EUS activity, indicative of the guarding reflex, that was proportional to the urethral flow rate. These results demonstrate the complementary roles of sensory feedback from the bladder and urethra in regulating reflexes in the lower urinary tract that depend on the state of the bladder. Understanding the neural control of functional reflexes and how they are mediated by sensory information in the bladder and urethra will open new opportunities, especially in neuromodulation, to treat pathologies of the lower urinary tract.

Urethane anesthesia in acute lower urinary tract studies in the male rat

Urethane is a widely used anesthetic in animal lower urinary tract research. Our objective was to investigate the quality of anesthesia and the correlation between bladder (voiding) contractions, micturition pressure, bladder capacity and urethane dosage and body weight. Urethane was given subcutaneously and/or intraperitoneally (1.0-1.2 g/kg). The bladder was filled with saline and the bladder pressure was recorded continuously. Animals in which the subcutaneous/intraperitoneal ratio was higher needed less urethane. Heavier animals needed less extra urethane. In animals, in which no bladder contractions could be evoked, the total amount of urethane given was similar to that in those that did show contractions. In the animals that did void, the bladder never emptied completely and residual volumes remained. There was no relationship between animal weight or total amount of urethane and mean capacity. Anesthesia lasted up till 14 h, during which bladder (voiding) contractions could be recorded. Considering all results, we conclude that urethane is a well suited anesthetic for acute lower urinary tract physiological research in the intact rat.

Effect of tibial nerve stimulation on bladder afferent nerve activity in a rat detrusor overactivity model

OBJECTIVES

To study the post-stimulation effect of tibial nerve stimulation on rat bladder afferent activity, and urodynamic parameters in normal and acetic acid-induced detrusor overactivity conditions.

METHODS

In urethane anesthetized male Wistar rats, the tibial nerve was stimulated for 30 min at 5 Hz, pulse width 200 μs and amplitude approximately threefold the threshold to induce a slight toe movement. The post-stimulation effect was studied by measuring afferent nerve activity of postganglionic pelvic nerve branches and various urodynamic parameters under two different conditions: (i) in physiological saline filling experiments (simulating normal bladder condition); and (ii) in acetic acid irritated bladders (simulating detrusor overactivity).

RESULTS

After 30 min of tibial nerve stimulation in saline filling experiments, the bladder capacity, threshold pressure and afferent nerve activity were not significantly different from the prestimulation measurements. The instillation of 0.5% acetic acid significantly reduced the bladder capacity and increased the afferent nerve activity. Tibial nerve stimulation significantly improved the bladder capacity and suppressed the afferent nerve activity compared with prestimulation acetic acid measurements.

CONCLUSIONS

Tibial nerve stimulation is able to significantly restore the bladder capacity by inhibiting afferent nerve activity in chemically irritated rat bladders. The present study provides important basic electrophysiological evidence to substantiate the clinical use of tibial nerve stimulation for treatment of symptoms related to detrusor overactivity.

Sensory and circuit mechanisms mediating lower urinary tract reflexes

Neural control of continence and micturition is distributed over a network of interconnected reflexes. These reflexes integrate sensory information from the bladder and urethra and are modulated by descending influences to produce different physiological outcomes based on the information arriving from peripheral afferents. Therefore, the mode of activation of primary afferents is essential in understanding the action of spinal reflex pathways in the lower urinary tract. We present an overview of sensory mechanisms in the bladder and urethra focusing on their spinal integration, identify the cardinal spinal reflexes responsible for continence and micturition, and describe how their functional role is controlled via peripheral afferent activity.

Neurophysiological modeling of bladder afferent activity in the rat overactive bladder model

The overactive bladder (OAB) is a syndrome-based urinary dysfunction characterized by “urgency, with or without urge incontinence, usually with frequency and nocturia”. Earlier we developed a mathematical model of bladder nerve activity during voiding in anesthetized rats and found that the nerve activity in the relaxation phase of voiding contractions was all afferent. In the present study, we applied this mathematical model to an acetic acid (AA) rat model of bladder overactivity to study the sensitivity of afferent fibers in intact nerves to bladder pressure and volume changes. The afferent activity in the filling phase and the slope, i.e., the sensitivity of the afferent fibers to pressure changes in the post-void relaxation phase, were found to be significantly higher in AA than in saline measurements, while the offset (nerve activity at pressure ~0) and maximum pressure were comparable. We have thus shown, for the first time, that the sensitivity of afferent fibers in the OAB can be studied without cutting nerves or preparation of single fibers. We conclude that bladder overactivity induced by AA in rats is neurogenic in origin and is caused by increased sensitivity of afferent sensors in the bladder wall.

Does defective volume sensation contribute to detrusor underactivity?

AIMS

The urodynamic finding of detrusor underactivity (DU) in neurologically intact unobstructed patients may relate to impaired volume sensations rather than detrusor contractile defects. We hypothesized that DU patients would demonstrate higher volumes but similar wall stress at sensation thresholds, and similar voiding contractility, compared to other groups.

METHODS

Chart review of urodynamic studies in neurologically normal, nonobstructed symptomatic patients. Urodynamic studies having the primary findings of DU, stress urinary incontinence (SUI), detrusor overactivity (DO), and increased sensations without detrusor overactivity (IS) were abstracted. Age, gender, and pressure/volume data associated with sensations and voiding parameters were collected. Wall stress at sensations was calculated. Urodynamic variables at standard sensations and progression across standard sensations were compared among the four groups.

RESULTS

Fifty-one urodynamic studies were analyzed for comparison. Mean age did not differ between groups. The DU group was predominantly male versus the other groups. DU, SUI, and DO had higher volume thresholds for strong desire than did IS. DU and DO demonstrated higher wall stress at strong desire than did IS and SUI. Watts factor was not significantly different between groups, however, DU had a smaller voided volumes and a higher post-void residuals.

CONCLUSION

Increased volume and wall stress at strong desire, and similar contractility but the smaller voided volumes and elevated PVRs in DU suggest that diminished central sensitivity to volume afferent activity contributes to DU in nonobstructed, non-neurogenic symptomatic patients.

Detrusor Underactivity: The Current Concept of the Pathophysiology

Based on evidence from available literature, we review the pathophysiology of detrusor underactivity (DU). DU is likely to be multifactorial. Aging reduces detrusor activity, but other concomitant causes may aggravate this condition, resulting in decrease of detrusor contractility. Impaired detrusor contractility has been regarded as a major etiologic factor of DU. However, a more complex pathology has been proposed. As contributing factors to DU, we discuss disturbances of the sensory afferent side of the micturition reflex, the central nervous system (CNS) and the efferent side of the reflex, including nerves and the detrusor muscle. Particularly, dysfunction of afferent nerves in the bladder and urethra may play a crucial role in the pathogenesis of DU. In addition, recent studies suggest that chronic bladder ischemia and resultant oxidative stress cause detrusor overactivity progressing to DU and inability to empty the bladder.

The purinergic component of human bladder smooth muscle cells' proliferation and contraction under physiological stretch

OBJECTIVE

To investigate whether cyclic stretch induces proliferation and contraction of human smooth muscle cells (HBSMCs), mediated by P2X purinoceptor 1 and 2 and the signal transduction mechanisms of this process.

METHODS

HBSMCs were seeded on silicone membrane and stretched under varying parameters; (equibiaxial elongation: 2.5%, 5%, 10%, 15%, 20%, 25%), (Frequency: 0.05Hz, 0.1Hz, 0.2Hz, 0.5Hz, 1Hz). 5-Bromo-2-deoxyuridine assay was employed for proliferative studies. Contractility of the cells was determined using collagen gel contraction assay. After optimal physiological stretch was established; P2X1 and P2X2 were analyzed by real time polymerase chain reaction and Western Blot. Specificity of purinoceptors was maintained by employing specific inhibitors; (NF023 for P2X1, and A317491for P2X2), in some experiments.

RESULTS

Optimum proliferation and contractility were observed at 5% and 10% equibiaxial stretching respectively, applied at a frequency of 0.1Hz; At 5% stretch, proliferation increased from 0.837±0.026 (control) to 1.462±0.023%, p<0.05. Mean contraction at 10% stretching increased from 31.7±2.3%, (control) to 78.28 ±1.45%, p< 0.05. Expression of P2X1 and P2X2 was upregulated after application of stretch. Inhibition had effects on proliferation (1.232±0.051, p<0.05 NF023) and (1.302±0.021, p<0.05 A314791) while contractility was markedly reduced (68.24±2.31, p<0.05 NF023) and (73.2±2.87, p<0.05 A314791). These findings shows that mechanical stretch can promote magnitude-dependent proliferative and contractile modulation of HBSMCs in vitro, and P2X1 and 2 are at least partially responsible in this process.

Detrusor expulsive strength is preserved, but responsiveness to bladder filling and urinary sensitivity is diminished in the aging mouse

The prevalence of urinary symptoms increases with age and is a significant source of distress, morbidity, and expense in the elderly. Recent evidence suggests that symptoms in the aged may result from sensory dysfunction, rather than abnormalities of detrusor performance. Therefore, we employed a pressure/flow multichannel urethane-anesthetized mouse cystometry model to test the hypothesis that in vivo detrusor performance does not degrade with aging. Secondarily, we sought to evaluate sensory responsiveness to volume using pressure-volume data generated during bladder filling. Cystometric data from 2-, 12-, 22-, and 26-mo-old female C57BL6 mice were compared. All 2- and 12-mo-old mice, 66% of 22-mo-old mice, and 50% of 26-mo-old mice responded to continuous bladder filling with periodic reflex voiding. Abdominal wall contraction with voiding had a minimal contribution to expulsive pressure, whereas compliance pressure was a significant contributor. Maximum bladder pressure, estimated detrusor pressure, detrusor impulse (pressure-time integral), as well as indices of detrusor power and work, did not decrease with aging. Bladder precontraction pressures decreased, compliance increased, and nonvoiding contraction counts did not change with increasing age. Intervoid intervals, per-void volumes, and voiding flow rates increased with age. Calculations approximating wall stress during filling suggested loss of bladder volume sensitivity with increasing age. We conclude that aging is associated with an impaired ability to respond to the challenge of continuous bladder filling with cyclic voiding, yet among responsive animals, voiding detrusor contraction strength does not degrade with aging in this murine model. Furthermore, indirect measures suggest that bladder volume sensitivity is diminished. Thus, changes in homeostatic reserve and peripheral and/or central sensory mechanisms may be important contributors to aging-associated changes in bladder function.

Evidence of central modulation of bladder compliance during filling phase

AIMS

Bladder compliance is one expression of the pressure and volume relationship as the bladder fills. In addition to passive elements, autonomous micromotional detrusor activity contributes to this relationship. In the mouse cystometric model, compliance pressure contributes to voiding expulsive pressure. During attempts to isolate the detrusor contractile component of this filling pressurization, we found that compliance reversibly diminishes under conditions which remove central control from the micturition cycle.

METHODS

Ten mature female mice underwent constant infusion pressure/flow cystometry under urethane anesthesia, and five awake mature female mice underwent constant infusion pressure cystometry. Following baseline cystometry, all mice were anesthetized with isoflurane to abolish the micturition reflex, and cystometry conducted with manual emptying of the bladders. Animals were then allowed to recover from isoflurane to re-establish the micturition reflex, and cystometry again conducted. The urethane group was also studied immediately post-mortem. Repeated measures comparisons of cystometric parameters were made across conditions.

RESULTS

Compliance reversibly decreased in all mice with the abolishment of micturition responses by isoflurane anesthesia. A similar decrease was observed immediately post-mortem in the urethaned mice. Bladder filling and voiding were not different between the intact micturition segments of the testing.

CONCLUSIONS

Enhanced compliance in mice with intact micturition responses suggests that autonomous micromotional activity is suppressed by central processes during normal filling. Since afferent activity during filling is also determined by the relationship between bladder pressure and volume, a feed-forward afferent signal conditioning mechanism may exist, creating novel therapeutic targets for urinary dysfunctions.

Afferent mechanism in the urinary tract

Much of the current research on lower urinary tract dysfunction is focused on afferent mechanisms. The main goals are to define and modulate the signaling pathways by which afferent information is generated and conveyed to the central nervous system. Alterations in bladder afferent mechanisms are a potential source of voiding dysfunction and an emerging source of drug targets. Even some established drug therapies such as muscarinic receptor antagonists, as well as emerging therapies such as botulinum toxin type-A, may act partly through afferent mechanisms. This review presents up-to-date findings on the localization of afferent fiber types within the bladder wall, afferent receptors and transmitters, and how these may communicate with the urothelium, interstitial cells, and detrusor smooth muscle to regulate micturition in normal and pathological bladders. Peripheral and central mechanisms of afferent sensitization and myogenic mechanisms that lead to detrusor overactivity, overactive bladder symptoms, and urgency sensations are also covered as well as new therapeutic approaches and new and established methods of measuring afferent activity.

Aging and the underactive detrusor: a failure of activity or activation?

AIMS

To examine the known and potential contributions of motor, sensory, and biomechanical dysfunctions to the clinical problems of detrusor underactivity and detrusor hyperactivity/impaired contractility.

METHODS

A review of the current literature on non-obstructive voiding dysfunction associated with aging was conducted. The functional impact of age-induced biomechanical tissue change via alterations in sensory transduction is considered.

RESULTS

Impaired contractility has been regarded as etiologic of detrusor underactivity. However, an age-related degradation in detrusor contractility as the primary contributor to impaired bladder emptying has not been conclusively demonstrated. To the extent that detrusor contraction force and duration are dependent upon efferent nerve activity and thus reflex responses to sensory activity, there is a potential impact of impaired sensory function on voiding efficiency. Structural and functional tissue changes accompanying aging may result in altered bladder afferent function, with subsequent reflex impairment of detrusor voiding function.

CONCLUSIONS

The relative contributions of motor, sensory, and biomechanical dysfunctions to impaired voiding performance independent of outlet obstruction associated with aging remain to be elucidated.

Is abdominal wall contraction important for normal voiding in the female rat?

Background

Normal voiding behavior in urethane-anesthetized rats includes contraction of the abdominal wall striated muscle, similar to the visceromotor response (VMR) to noxious bladder distension. Normal rat voiding requires pulsatile release of urine from a pressurized bladder. The abdominal wall contraction accompanying urine flow may provide a necessary pressure increment for normal efficient pulsatile voiding. This study aimed to evaluate the occurrence and necessity of the voiding-associated abdominal wall activity in urethane-anesthetized female rats

Methods

A free-voiding model was designed to allow assessment of abdominal wall activity during voiding resulting from physiologic bladder filling, in the absence of bladder or urethral instrumentation. Physiologic diuresis was promoted by rapid intravascular hydration. Intercontraction interval (ICI), voided volumes and EMG activity of the rectus abdominis were quantified. The contribution of abdominal wall contraction to voiding was eliminated in a second group of rats by injecting botulinum-A (BTX, 5 U) into each rectus abdominis to induce local paralysis. Uroflow parameters were compared between intact free-voiding and BTX-prepared animals.

Results

Abdominal wall response is present in free voiding. BTX preparation eliminated the voiding-associated EMG activity. Average per-void volume decreased from 1.8 ml to 1.1 ml (p < 0.05), and reduced average flow from 0.17 ml/sec to 0.11 ml/sec (p < 0.05). Intercontraction interval (ICI) was not changed by BTX pretreatment.

Conclusion

The voiding-associated abdominal wall response is a necessary component of normal voiding in urethane anesthetized female rats. As the proximal urethra may be the origin of the afferent signaling which results in the abdominal wall response, the importance of the bladder pressure increment due to this response may be in maintaining a normal duration intermittent pulsatile high frequency oscillatory (IPHFO)/flow phase and thus efficient voiding. We propose the term Voiding-associated Abdominal Response (VAR) for the physiologic voiding-associated EMG/abdominal wall response, to distinguish it from the visceromotor response (VMR) to noxious bladder distension.

Bladder volume sensitivity of isovolumetric intravesical pressure

AIMS

Recently, methods have been introduced for non-invasively measuring the isovolumetric bladder pressure. This pressure can also be measured invasively using a stop test. In itself, the isovolumetric pressure is a measure of urinary bladder contractility, by combining it with maximum urinary flow rate it can be used to diagnose infravesical obstruction. We have studied the (possible) volume dependence of this pressure in order to enhance the accuracy of this type of measurements and to explore its physiological background and possible diagnostic relevance.

MATERIALS AND METHODS

In 1,020 healthy subjects, recruited for an ongoing longitudinal study of changes in bladder contractility secondary to prostatic enlargement, we measured the isovolumetric bladder pressure using the condom catheter method. In each subject, voiding was repeatedly interrupted. The resulting pressure-volume relations were normalized and averaged.

RESULTS AND CONCLUSIONS

There is an optimum bladder volume for isovolumetric pressure measurements, averaging 264 +/- 122 ml (mean +/- SD). Measurements should be taken at or above the optimum volume. At volumes below the optimum volume, the pressure decreases by approximately 5% for each 10% of volume decrease. At bladder volumes smaller than 247 ml pressure readings in 50% of subjects are suboptimal. The optimum volume for isovolumetric pressure generation is only marginally related to voiding diary parameters. Probably it represents mechanical properties of the bladder, whereas voiding diary parameters more likely represent neurophysiological properties. However, the optimum volume does not reflect the optimum (smooth) muscle length for force generation of the bladder wall: during normal voiding bladder smooth muscle always operates at a suboptimal length for force generation.