Potential vascular mechanisms in an ex vivo functional pig bladder model

A porcine bladder model of pre-clinical urodynamics demonstrates increased afferent nerve activity during filling

INTRODUCTION AND OBJECTIVES

Urodynamics are the accepted gold standard for the evaluation of multiple forms of voiding dysfunction. However, the tests are expensive, invasive, poorly reproducible, and often prone to artifacts. Therefore, there is a pressing need to develop next-generation urodynamics. The purpose of this study was to develop a novel ex vivo porcine bladder urodynamics model with afferent pelvic nerve signaling that can be used as a preclinical surrogate for bladder sensation.

METHODS

Porcine bladders including the ureters and vascular supply were harvested from local abattoirs using an established protocol in both male and female animals. Ex vivo bladder perfusion was performed using physiologic MOPS (3-(N-morpholino) propanesulfonic acid) buffer solution. The pelvic nerve adjacent to the bladder was grasped with micro-hook electrodes and electroneurogram (ENG) signals recorded at 20 kHz. Bladders were filled with saline at a nonphysiologic rate (100 mL/min) to a volume of 1 L using standard urodynamics equipment to simultaneously record intravesical pressure. ENG amplitude was calculated as the area under the curve for each minute, and ENG firing rate was calculated as number of spikes (above baseline threshold) per minute. At the conclusion of the experiment, representative nerve samples were removed and processed for nerve histology by a pathologist (hematoxylin and eosin and S100 stains).

RESULTS

A total of 10 pig bladders were used, and nerve histology confirmed the presence of nerve in all adequately processed samples. Vesical pressure, ENG firing rate, and ENG amplitude all increased as a function of filling. During filling tertiles (low fill: min 1-3, med fill: min 4-6, and high fill: min 7-10), normalized pressures were 0.22 ± 0.04, 0.38 ± 0.05, and 0.72 ± 0.07 (cmH2O). Similarly, normalized ENG firing rates were 0.08 ± 0.03, 0.31 ± 0.06, and 0.43 ± 0.04 spikes/minute, respectively, and normalized nerve amplitudes were 0.11 ± 0.06, 0.39 ± 0.06, and 0.56 ± 0.14) μV, respectively. Strong relationships between average normalized pressure values and averaged normalized ENG firing rate (r2  = 0.66) and average normalized ENG amplitude (r2  = 0.8) were identified.

CONCLUSIONS

The ex vivo perfused porcine bladder can be used as a preclinical model for the development of next-generation urodynamics technologies. Importantly, the model includes a reproducible method to measure afferent nerve activity that directly correlates with intravesical pressure during filling and could potentially be used as a surrogate measure of bladder sensation.

Continuous bladder urinary oxygen tension as a new tool to monitor medullary oxygenation in the critically ill

Acute kidney injury (AKI) is common in the critically ill. Inadequate renal medullary tissue oxygenation has been linked to its pathogenesis. Moreover, renal medullary tissue hypoxia can be detected before biochemical evidence of AKI in large mammalian models of critical illness. This justifies medullary hypoxia as a pathophysiological biomarker for early detection of impending AKI, thereby providing an opportunity to avert its evolution. Evidence from both animal and human studies supports the view that non-invasively measured bladder urinary oxygen tension (PuO₂) can provide a reliable estimate of renal medullary tissue oxygen tension (tPO₂), which can only be measured invasively. Furthermore, therapies that modify medullary tPO₂ produce corresponding changes in bladder PuO₂. Clinical studies have shown that bladder PuO₂ correlates with cardiac output, and that it increases in response to elevated cardiopulmonary bypass (CPB) flow and mean arterial pressure. Clinical observational studies in patients undergoing cardiac surgery involving CPB have shown that bladder PuO₂ has prognostic value for subsequent AKI. Thus, continuous bladder PuO₂ holds promise as a new clinical tool for monitoring the adequacy of renal medullary oxygenation, with its implications for the recognition and prevention of medullary hypoxia and thus AKI.

Review of Animal Models to Study Urinary Bladder Function

Simple Summary

The treatment of urinary bladder dysfunction requires the knowledge of bladder function, which involves physiology, pathology, and even psychology. Several animal models are available to study a variety of bladder disorders. These models include animals from rodents, such as mice and rats, to nonhuman primates, such as rabbits, felines, canines, pigs, and mini pigs. This review adapted animal models to study bladder function according to facility, priority, and disease.

Abstract

The urinary bladder (UB) serves as a storage and elimination organ for urine. UB dysfunction can cause multiple symptoms of failure to store urine or empty the bladder, e.g., incontinence, frequent urination, and urinary retention. Treatment of these symptoms requires knowledge on bladder function, which involves physiology, pathology, and even psychology. There is no ideal animal model for the study of UB function to understand and treat associated disorders, as the complexity in humans differs from that of other species. However, several animal models are available to study a variety of other bladder disorders. Such models include animals from rodents to nonhuman primates, such as mice, rats, rabbits, felines, canines, pigs, and mini pigs. For incontinence, vaginal distention might mimic birth trauma and can be measured based on leak point pressure. Using peripheral and central models, inflammation, bladder outlet obstruction, and genetic models facilitated the study of overactive bladder. However, the larger the animal model, the more difficult the study is, due to the associated animal ethics issues, laboratory facility, and budget. This review aims at facilitating adapted animal models to study bladder function according to facility, priority, and disease.

Regulation of bladder dynamic elasticity: a novel method to increase bladder capacity and reduce pressure using pulsatile external compressive exercises in a porcine model

PURPOSE

Dynamic elasticity is a biomechanical property of the bladder in which muscle compliance can be acutely adjusted through passive stretches and reversed with active contractions. The aim of this study was to determine if manipulating dynamic elasticity using external compression could be used as a novel method to acutely increase bladder capacity and reduce bladder pressure in a porcine model.

METHODS

Ex vivo experiment: bladders underwent continuous or pulsatile compression after establishing a reference pressure at bladder capacity. Bladders were then filled back to the reference pressure to determine if capacity could be acutely increased. In-vivo experiments: bladders underwent five cycles of pulsatile external compression with ultrasound confirmation. Pre and post-compression pressures were measured, and pressure was measured again 10 min post-compression.

RESULTS

Ex vivo experiment: pulsatile compression demonstrated increased bladder capacity by 16% (p = 0.01). Continuous compression demonstrated increased capacity by 9% (p < 0.03). Comparison of pulsatile to continuous compression showed that the pulsatile method was superior (p = 0.03). In-vivo experiments: pulsatile external compression reduced bladder pressure by 19% (p < 0.00001) with a return to baseline 10 min post-compression.

CONCLUSIONS

These results suggest that regulation of bladder dynamic elasticity achieved with external compression can acutely decrease bladder pressure and increase bladder capacity. Pulsatile compression was found to be more effective as compared to continuous compression. These results highlight the clinical potential for use of non-invasive pulsatile compression as a therapeutic technique to increase bladder capacity, decrease bladder pressure, and reduce the symptoms of urinary urgency.

A preliminary study of bilateral color mapping of pig bladder vasculature demonstrates potential for acute hemi-ischemic events

Background

Chronic ischemia is a known risk factor for the development of lower urinary tract symptoms (LUTS) and bladder hypocontractility. Less is known, however, about the impact of acute ischemia. Classic teaching suggests that collateral circulation is robust in the bladder and, therefore, loss of a single source of blood flow should have no deleterious effect. This study aims to provide visual evidence that segmental vascular supply is critical for maintaining adequate perfusion to the bladder.

Methods

Ex vivo pig bladders were cannulated bilaterally in the superior vesical arteries and perfused using contrasting red and green dye. Images were collected at each step of the perfusion dyeing process and these images were analyzed using a custom program to calculate the average hue of each side. Statistical analysis was performed using Student’s t-test.

Results

The two halves being perfused by separate arteries showed a statistically significant difference when compared (P<0.05) on both the outer wall (n=9) and in the mucosal layer (n=4). On the outside wall, the average normalized hue of the green halves was 27.5°±14.3°, while the average normalized hue of the red halves was −58.7°±3.1°. In the mucosa, the average normalized hue of the green halves was 34.5°±17.4°, while the average normalized hue of the red halves was −51.5°±3.5°.

Conclusions

This study identified a novel color mapping method to study pig bladder vascular supply. The results demonstrated a lack of collateral blood flow, highlighting the possibly of acute hemi-ischemic event. However, further research in the effect of acute ischemia on bladder function is necessary.

Phases of decompensation during acute ischemia demonstrated in an ex vivo porcine bladder model

Background

The aim of this project was to develop an ex-vivo porcine bladder model to test the effects of increasing durations of acute ischemia on detrusor function.

Methods

Porcine bladders were perfused through bilateral vesical arteries at physiologic flow (4 mL/min) and filled through a urethral catheter. Intravesical pressures were continuously recorded using standard urodynamics equipment. Bladder contractions, with simulated voiding, were induced by arterial infusion of KCl at 250 mL. Total, passive, and active pressures were recorded for each contraction and data were normalized to the control fill. Bladders underwent the following perfusion protocol by adjusting the arterial flow rates: Equilibration (4 mL/min), control (4 mL/min), partial ischemia (2 mL/min), global ischemia (0 mL/min) and reperfusion (4 mL/min). Perfusion periods were held for 15 min for one group and 30 min for another group of bladders.

Results

Porcine bladders (N=19) including 8 (15 min group) and 11 (30 min group) were used. With 15 min ischemia, passive pressure increased 39% (P=0.03) and the active pressure decreased 23% (P=0.002). Total pressure remained constant, identifying a compensated phase. Values returned to baseline with reperfusion. With 30 min ischemia, passive pressure remained unchanged. However, there was a decrease in total pressure 34% (P<0.001) and active pressure 61% (P<0.001), which incompletely recovered to baseline values, identifying a decompensated phase with incomplete recovery upon reperfusion.

Conclusion

In the porcine bladder, 15 min ischemia resulted in a compensated phase and 30 min ischemia resulted in a decompensated phase of detrusor function. This study provides mechanistic insight into the natural history of ischemia-mediated voiding dysfunction.

Are oxidative stress and ischemia significant causes of bladder damage leading to lower urinary tract dysfunction? Report from the ICI-RS 2019

Several studies indicate that pelvic ischemia and oxidative stress may play a significant role in lower urinary tract dysfunction (LUTD), including detrusor overactivity (DO)/overactive bladder (OAB) and detrusor underactivity (DU)/underactive bladder (UAB). The present article addresses proposal 1: "Are oxidative stress and ischemia significant causes of bladder damage leading to LUTD?" from the 2019 International Consultation on Incontinence-Research Society (ICI-RS) meeting. Bladder ischemia in animals and humans is briefly described, along with the proposed progression from ischemia to LUTD. Bladder ischemia is compared with ischemia of other organs, and the ongoing development of pelvic ischemia animal models is discussed. In addition, the distribution of blood within the bladder during filling and voiding and the challenges of quantification of blood flow in vivo are described. Furthermore, oxidative stress, including potential biomarkers and treatments, and challenges regarding antioxidant therapy for the treatment of LUTD are discussed. Finally, seven critical research questions and proposed studies to answer those questions were identified as priorities that would lead to major advances in the understanding and treatment of lower urinary tract symptoms (LUTS)/LUTD associated with pelvic ischemia and oxidative stress.

Bladder attack: transient bladder ischemia leads to a reversible decrease in detrusor compliance

Background

The deleterious effects of chronic ischemia on bladder function have been extensively studied; however, evaluation and characterization of the effects of acute ischemia and hypoxia are lacking. The present study examined pig and human detrusor smooth muscle (DSM) strips, in combination with an isolated perfused working pig bladder model to evaluate the relationship between transient ischemia and bladder function.

Methods

Organ bath and myographic studies were performed using pig and human DSM strips exposed to starvation/hypoxia conditions. Analogous conditions were then recreated in the ex vivo bladder preparation. Filled bladders were then treated with intravascular carbachol to induce contraction and subsequent void. An intravesical transducer continuously monitored changes in bladder pressure, while a tissue pO₂ monitor analyzed changes in oxygenation.

Results

After 120 min in starved/hypoxic conditions, both pig and human DSM strips demonstrated significantly increased resting tone, with a greater than two-fold increase in force over control. This was effectively blocked with atropine. DSM strips also demonstrated significantly weaker contractions; however, contractile force was nearly recovered following 15-min exposure to replete/oxygenated buffer. In the ex vivo bladder preparation, filling under ischemic conditions yielded a 225% increase in end-fill vesical pressures (P_ves) compared to controls. End-fill P_ves returned to baseline with reperfusion during a subsequent filling cycle.

Conclusions

Transient ischemia/hypoxia leads to an acute increase in tone in both DSM strips and ex vivo pig bladder. Remarkably, the effect is reversible with re-perfusion and may be blocked with anticholinergics, suggesting a relationship between acute ischemia and increased local acetylcholine release.

An external compress-release protocol induces dynamic elasticity in the porcine bladder: A novel technique for the treatment of overactive bladder?

INTRODUCTION

Dynamic elasticity is an acutely regulated bladder material property through which filling and passive emptying produce strain softening, and active voiding restores baseline pressure. The aim of this study was to test the hypothesis that strain softening produced by filling-passive emptying is equivalent to that produced by compression-release in a porcine bladder model.

METHODS/MATERIALS

Latex balloons and ex vivo perfused pig bladders were used for a series of alternating fill-passive emptying ("Fill") and external compress-release ("Press") protocols. For the Fill protocol balloons/bladders were (1) filled to defined volumes (prestrain softening), (2) filled to capacity to strain soften (reference), and (3) passively emptied to the original volume (poststrain softening). For the Press protocol, balloons/bladders were (1) filled to defined volumes (prestrain softening), (2) externally compressed to reference pressure and then released for five cycles (poststrain softening). After each protocol, bladders were voided with high-KCl buffer to induce "active" voiding.

RESULTS

In both balloons and porcine bladder, both the Fill and Press protocols produced significant strain softening (P < 0.05) and poststrain softening pressures were not different for Fill and Press protocols (P > 0.05), indicating a similar degree of strain softening with both methods.

CONCLUSIONS

Repeated external compression can induce bladder strain softening similar to filling and passive emptying. This technique may represent a means to acutely regulate bladder compliance and potentially be used as a mechanical treatment for urinary urgency.