Comparison of the contractile properties, oxidative capacities and fibre type profiles of the voluntary sphincters of continence in the rat

What do we really know about the external urethral sphincter?

The external urethral sphincter (EUS), composed of skeletal muscle, along with a smooth muscle-lined internal urethral sphincter (IUS), have crucial roles in maintaining continence during bladder filling and facilitating urine flow during voiding. Disruption of this complex activity has profound consequences on normal lower urinary tract function during the micturition cycle. However, relatively little is known about the normal and pathological functions of these particular muscle types, how activity can be manipulated and regulated and why, for example, loss of EUS function and sarcopenia is associated with ageing. Here we discuss the unique physiological, biochemical and metabolic properties of striated and smooth muscle components of the urethral sphincter, which have distinct roles in maintaining continence during bladder filling. Relevant in vivo models for investigation of pathophysiological mechanisms, and for pre-clinical evaluation of therapeutic approaches are reviewed. Electromyography and Urethral Pressure Profile recordings are pivotal to understanding the function and dysfunction of the EUS and for clinical evaluation of e.g. urinary retention. Pre-clinical and clinical studies have revealed that age- or disease-related tissue remodelling that lead to filling/voiding disorders may be mitigated with emerging therapeutic approaches.

Birth trauma simulated by vaginal distention induced possible irreversible changes in the composition of the fiber types in the external urethral sphincter of rats

The external urethral sphincter (EUS) is crucial in urinary continence development. Understanding the morphological features of the EUS in female rats after vaginal distention (VD), using a model of birth trauma, would aid in evaluating its functional and metabolic properties. Our recent study demonstrated that the EUS in female rats expresses one slow (type 1) and two fast (types 2A and 2B) myosin heavy chain (MHC) isoforms. Our preliminary experiment revealed that type 2B isoform expression was markedly reduced in the EUS 4 weeks after VD. Here, we aimed to examine the expression patterns of these three types of MHC isoforms, and an embryonic MHC, a marker of regeneration fibers, in the EUS of rats 3 days and 1, 2, and 8 weeks after VD using immunofluorescence staining. Hence, type 2B fibers were selectively damaged early in post-VD and did not recover fully later. Muscle regeneration in the sphincter peaked 1 week after trauma using a marker of immature fibers, embryonic myosin heavy chain. Electron microscopy revealed that the EUS of female rats was composed of mitochondria-rich muscle fibers. Myoblasts or immature muscle fibers were discovered in the sphincter layer 1 week after trauma. These results suggest that myogenesis after VD may not contribute to restoring normal fiber composition in a female rat's EUS.

Optical EUS Activation to Relax Sensitized Micturition Response

This study aims to activate the external urethral sphincter (EUS), which plays a critical role in micturition control, through optogenetics and to determine its potential contribution to the stabilization of sensitized micturition activity. The viral vector (AAV2/8-CMV-hChR2(H134R)-EGFP) is utilized to introduce light-gated ion channels (hChR2/H134R) into the EUS of wild-type C57BL/6 mice. Following the induction of sensitized micturition activity using weak acetic acid (0.1%) in anesthetized mice, optical stimulation of the EUS muscle tissue expressing channel rhodopsin is performed using a 473 nm laser light delivered through optical fibers, and the resulting changes in muscle activation and micturition activity are examined. Through EMG (electromyography) measurements, it is confirmed that optical stimulation electrically activates the EUS muscle in mice. Analysis of micturition activity using cystometry reveals a 70.58% decrease in the micturition period and a 70.27% decrease in the voiding volume due to sensitized voiding. However, with optical stimulation, the micturition period recovers to 101.49%, and the voiding volume recovered to 100.22%. Stimulation of the EUS using optogenetics can alleviate sensitized micturition activity and holds potential for application in conjunction with other micturition control methods.

Sex differences in lower urinary tract biology and physiology

Females and males differ significantly in gross anatomy and physiology of the lower urinary tract, and these differences are commonly discussed in the medical and scientific literature. However, less attention is dedicated to investigating the varied development, function, and biology between females and males on a cellular level. Recognizing that cell biology is not uniform, especially in the lower urinary tract of females and males, is crucial for providing context and relevance for diverse fields of biomedical investigation. This review serves to characterize the current understanding of biological sex differences between female and male lower urinary tracts, while identifying areas for future research. First, the differences in overall cell populations are discussed in the detrusor smooth muscle, urothelium, and trigone. Second, the urethra is discussed, including anatomic discussions of the female and male urethra followed by discussions of cellular differences in the urothelial and muscular layers. The pelvic floor is then reviewed, followed by an examination of the sex differences in hormonal regulation, the urinary tract microbiome, and the reticuloendothelial system. Understanding the complex and dynamic development, anatomy, and physiology of the lower urinary tract should be contextualized by the sex differences described in this review.

Sexual and Regional Differences in Myosin Heavy Chain Expression in the Rat External Urethral Sphincter

The external urethral sphincter is a unique striated muscle surrounding the urethra that plays a crucial role in urinary continence, and a comprehensive understanding of its morphology is needed to determine the pathophysiology underlying urinary incontinence and find suitable therapies. Differences between the sexes and among species regarding the fiber types present remain controversial. This study used triple immunofluorescence labeling to visualize one slow (Type 1) and two fast (Types 2A and 2B) myosin isoforms in rat external urethral sphincters from both sexes. Type 2A fibers predominated throughout the sphincter and Type 2B fibers were restricted to the proximal one-third of the external urethral sphincter in the female rats. Type 1 fibers were present adluminally and were concentrated in the proximal and distal segments of the sphincter. While most of the male external urethral sphincter comprised Type 2B fibers, Type 2A fibers intermingled among these fibers in the proximal one-third of the sphincter, and a few Type 1 fibers were present that were restricted to the adluminal region of the proximal segment. The fiber-type compositions and their areal densities changed in both sexes after gonadectomy. The areal density of the Type 1 fibers increased significantly in the ovariectomized females, especially in the distal segment. In the orchidectomized males, the areal densities of the Types 1 and 2A fibers increased significantly, but that of the Type 2B fibers decreased. These results indicate that myosin heavy chain expression in the rat external urethral sphincter is sexually dimorphic and shows regional differences. Anat Rec, 2017. © 2017 Wiley Periodicals, Inc. Anat Rec, 300:2058-2069, 2017. © 2017 Wiley Periodicals, Inc.

Systematic Review of Animal Models Used in Research of Origins and Treatments of Fecal Incontinence

BACKGROUND

Fecal incontinence is a common disorder, but its pathophysiology is not completely understood.

OBJECTIVE

The aim of this review is to present animal models that have a place in the study of fecal incontinence.

DATA SOURCES

A literature review following Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines performed in August 2016 revealed 50 articles of interest. Search terms included fecal/faecal incontinence and animal model or specific species.

STUDY SELECTION

Articles not describing an animal model, in vitro studies, veterinary literature, reviews, and non-English articles were excluded.

MAIN OUTCOME MEASURES

The articles described models in rats (n = 31), dogs (n = 8), rabbits (n = 7), and pigs (n = 4).

RESULTS

Different fecal incontinence etiologies were modeled, including anal sphincter lesions (33 articles) ranging from a single anal sphincter cut to destruction of 50% of the anal sphincter by sharp dissection, electrocautery, or diathermy. Neuropathic fecal incontinence (12 articles) was achieved by complete or incomplete pudendal, pelvic, or inferior rectal nerve damage. Mixed fecal incontinence (5 articles) was modeled either by the inflation of pelvic balloons or an array of several lesions including nervous and muscular damage. Anal fistulas (2 articles), anal sphincter resection (3 articles), and diabetic neuropathy (2 articles) were studied to a lesser extent.

LIMITATIONS

Bias may have arisen from the authors' own work on fecal incontinence and the absence of blinding to the origins of articles.

CONCLUSIONS

Validated animal models representing the main etiologies of fecal incontinence exist, but no animal model to date represents the whole pathophysiology of fecal incontinence. Therefore, the individual research questions still dictate the choice of model and species.

Early life exposure to chronic intermittent hypoxia causes upper airway dilator muscle weakness, which persists into young adulthood

NEW FINDINGS

What is the central question of this study? Chronic intermittent hypoxia (CIH) is a dominant feature of respiratory control disorders, which are common. We sought to examine the effects of exposure to CIH during neonatal development on respiratory muscle form and function in male and female rats. What is the main finding and its importance? Exposure to CIH during neonatal development caused sternohyoid muscle weakness in both sexes; an effect that persisted into young adult life upon return to normoxia. Upper airway dilator muscle dysfunction in vivo could predispose to airway collapse, leading to impaired respiratory homeostasis. Chronic intermittent hypoxia (CIH) is a feature of sleep-disordered breathing, which is very common. Exposure to CIH is associated with aberrant plasticity in the respiratory control system including the final effector organs, the striated muscles of breathing. We reasoned that developmental age and sex are key factors determining the functional response of respiratory muscle to CIH. We tested the hypothesis that exposure to CIH causes persistent impairment of sternohyoid muscle function due to oxidative stress and that males are more susceptible to CIH-induced muscle impairment than females. Wistar rat litters (with respective dams) were exposed to intermittent hypoxia for 12 cycles per hour, 8 h per day for 3 weeks from the first day of life [postnatal day (P) 0]. Sham experiments were run in parallel. Half of each litter was studied on P22; the other half was returned to normoxia and studied on P42. Functional properties of the sternohyoid muscle were determined ex vivo. Exposure to CIH significantly decreased sternohyoid muscle force in both sexes; an effect that persisted into young adult life. Chronic intermittent hypoxia had no effect on sternohyoid muscle endurance. Chronic intermittent hypoxia did not affect sternohyoid myosin fibre type, succinate dehydrogenase or glycerol-3-phosphate dehydrogenase activities, or protein free thiol and carbonyl content. Muscles exposed to CIH had smaller cross-sectional areas, consistent with the observation of muscle weakness. In human infants with disordered breathing, CIH-induced upper airway dilator muscle weakness could increase the propensity for airway narrowing or collapse, which could serve to perpetuate impaired respiratory homeostasis.

Fiber type characterization of striated muscles related to micturition in female rabbits

Pelvic and perineal striated muscles are relevant for reproduction and micturition in female mammals. Damage to these muscles is associated with pelvic organ prolapse and stress urinary incontinence. The fiber type composition of skeletal muscle influences the susceptibility for damage and/or regeneration. The aim of the present study was to determine the fiber type composition of a perineal muscle, the bulbospongiosus, and a pelvic muscle, the pubococcygeus. Both muscles were harvested from adult female rabbits (8-10 months old). NADH-TR (nicotinamide adenine dinucleotide tetrazolium reductase) histochemistry was undertaken to identify oxidative and glycolytic muscle fibers. Alkaline (pH 9.4) ATP-ase (actomyosin adenosine triphosphatase) histochemistry was used to classify type I, type IIb or type IIa/IId muscle fibers. Results showed that the content of glycolytic fibers in the bulbospongiosus muscle was higher than that of oxidative fibers. Meanwhile, the opposite was true for the pubococcygeus. In the bulbospongiosus muscle, the content of type IIb muscle fibers was higher than that of type I, but was similar to that of type IIa/IId. In contrast, the content of each fiber type was similar in the pubococcygeus muscle. The relative proportion of fibers in bulbospongiosus and pubococcygeus muscles is consistent with their function during voiding and storage phases of micturition.

Nitrergic relaxations and phenylephrine contractions are not compromised in isolated urethra in a rat model of diabetes

In vivo experiments in a diabetic rat model revealed compromised nitrergic urethral relaxations and increased sensitivity to adrenergic agonists. This study evaluated contractile and relaxation properties of urethral smooth muscle after streptozotocin (STZ)-induced diabetes, in vitro, with the aim of determining whether in vivo deficiencies are related to smooth muscle dysfunction. Urethral tissue was collected from adult female Sprague-Dawley rats naive, STZ-treated, vehicle-treated and sucrose-fed at 9-12 week post treatment. Strips from proximal, mid, and distal urethra were placed in tissue baths and stimulated using electric field stimulation (EFS) and pharmacological agents. nNOS staining was evaluated using immunohistochemistry. Phenylephrine (PE, 10μM) contracted all urethral strips with the highest amplitude in mid urethra, in all treatment groups. Likewise, EFS-induced relaxation amplitudes were larger and were observed more frequently in mid urethra. Relaxations were inhibited by the NOS inhibitor, L-NAME (1-100μM). Sodium nitroprusside (0.01-1μM), an NO donor, reversed PE-induced contractions. No statistical differences were observed between treatment groups with respect to any parameters. Qualitative immunohistochemistry showed no differences in the urethral nNOS innervation patterns across the treatment groups. In summary, nitrergic relaxations and adrenergic-induced contractions in the isolated diabetic rat urethra display similar properties to controls, suggesting no dysfunction on the nitrergic or alpha1 adrenergic receptor function in the smooth muscle. This further implies that compromised urethral relaxation and increased adrenergic agonist sensitivity observed in vivo in this model may be due to the disruption of neural signaling between the urethra and the spinal cord, or within the CNS.

The distribution of muscles fibers and their types in the female rat urethra: cytoarchitecture and three-dimensional reconstruction

An attempt to explore urethral cytoarchitecture including the distribution of smooth muscles and fast and slow striated muscles of adult female Sprague Dawley rat--a popular model in studying lower urinary tract function. Histological and immunohistochemical stainings were carried out to investigate the distribution of urethral muscle fibers and motor end plates. The urethral sphincter was furthermore three-dimensionally reconstructed from serial histological sections. The mucosa at the distal urethra was significantly thicker than that of other segments. A prominent inner longitudinal and outer circular layer of smooth muscles covered the proximal end of urethra. Thick circular smooth muscles of the bladder neck region (urethral portion) decreased significantly distalward and longitudinal smooth muscles became 2- to 3-fold thicker in the rest of the urethra. An additional layer of striated muscles appeared externally after neck region (urethra) and in association with motor end plates ran throughout the remaining urethra as the striated sphincter layer. Most striated muscles were fast fibers while relatively fewer slow fibers often concentrated at the periphery. A pair of extraneous striated muscles, resembling the human urethrovaginal sphincter muscles, connected both sides of mainly the distal vagina to the dorsal striated muscles in the wall of the middle urethra. The tension provided by this pair of muscles, and in conjunction with the striated sphincter of the urethral wall, was likely to function to suspend the middle urethra and facilitates its closure. Comprehensive morphological data of urethral sphincter offers solid basis for researchers conducting studies on dysfunction of bladder outlet.

Sternohyoid and diaphragm muscle form and function during postnatal development in the rat

NEW FINDINGS

What is the central question of this study? Co-ordinated activity of the thoracic pump and pharyngeal dilator muscles is critical for maintaining airway calibre and respiratory homeostasis. Whilst postnatal maturation of the diaphragm has been well characterized, surprisingly little is known about the developmental programme in the airway dilator muscles. What is the main finding and its importance? Developmental increases in force-generating capacity and fatigue in the sternohyoid and diaphragm muscles are attributed to a maturational shift in muscle myosin heavy chain phenotype. This maturation is accelerated in the sternohyoid muscle relative to the diaphragm and may have implications for the control of airway calibre in vivo. The striated muscles of breathing, including the thoracic pump and pharyngeal dilator muscles, play a critical role in maintaining respiratory homeostasis. Whilst postnatal maturation of the diaphragm has been well characterized, surprisingly little is known about the developmental programme in airway dilator muscles given that co-ordinated activity of both sets of muscles is needed for the maintenance of airway calibre and effective pulmonary ventilation. The form and function of sternohyoid and diaphragm muscles from Wistar rat pups [postnatal day (PD) 10, 20 and 30] was determined. Isometric contractile and endurance properties were examined in tissue baths containing Krebs solution at 35°C. Myosin heavy chain (MHC) isoform composition was determined using immunofluorescence. Muscle oxidative and glycolytic capacity was assessed by measuring the activities of succinate dehydrogenase and glycerol-3-phosphate dehydrogenase using semi-quantitative histochemistry. Sternohyoid and diaphragm peak isometric force and fatigue increased significantly with postnatal maturation. Developmental myosin disappeared by PD20, whereas MHC2B areal density increased significantly from PD10 to PD30, emerging earlier and to a much greater extent in the sternohyoid muscle. The numerical density of fibres expressing MHC2X and MHC2B increased significantly during development in the sternohyoid. Diaphragm succinate dehydrogenase activity and sternohyoid glycerol-3-phosphate dehydrogenase activity increased significantly with age. Developmental increases in force-generating capacity and fatigue in the sternohyoid and diaphragm muscles are attributed to a postnatal shift in muscle MHC phenotype. The accelerated maturation of the sternohyoid muscle relative to the diaphragm may have implications for the control of airway calibre in vivo.

External urethral sphincter motor unit recruitment patterns during micturition in the spinally intact and transected adult rat

In the rat, external urethral sphincter (EUS) activation during micturition consists of three sequential phases: 1) an increase in tonic EUS activity during passive filling and active contraction of the bladder (guarding reflex), 2) synchronized phasic activity (EUS bursting) associated with voiding, and 3) sustained tonic EUS activity that persists after bladder contraction. These phases are perturbed following spinal cord injury. The purpose of the present study was to characterize individual EUS motor unit (MU) patterns during micturition in the spinally intact and transected adult rat. EUS MU activity was recorded from either the L5 or L6 ventral root (intact) or EUS muscle (transected) during continuous flow cystometry in urethane-anesthetized adult female Sprague-Dawley rats. With the use of bladder pressure threshold and timing of activation, four distinct patterns of EUS MU activity were identified in the intact rat: low threshold sustained, medium/high threshold sustained, medium/high threshold not sustained, and burst only. In general, these MUs displayed little frequency modulation during active contraction, generated high-frequency bursts of action potentials during EUS bursting, and varied in terms of the duration of sustained tonic activity. In contrast, three general patterns of EUS MU activity were identified in the transected rat: low threshold, medium threshold, and high threshold. These MUs exhibited considerable frequency modulation during active contraction of the bladder, no bursting behavior and little to no sustained firing. The prominent frequency modulation of EUS MUs is likely due to the enhanced guarding reflex seen in EUS whole muscle electromyogram recordings in transected rats (D'Amico SC, Schuster IP, Collins WF 3rd. Exp Neurol 228: 59–68, 2011). In addition, EUS MU recruitment in transected rats more closely followed predictions by the size principle than in intact rats. This may reflect the influence of local synaptic circuits or intrinsic properties of EUS motoneurons that are active in intact rats but attenuated or absent in transected rats.

Comparison of the motor discharge to the voluntary sphincters of continence in the rat

BACKGROUND

  The rat external anal sphincter (EAS) and external urethral sphincter (EUS) are voluntary muscles of continence that can display similar synchronous electromyographic (EMG) activity patterns. However, the two sphincters are quite different in structure and function. The EUS is a fast twitch muscle and contains fibers expressing type 2B myosin. In contrast, the EAS exhibits slower kinetics and lacks type 2B fibers. This striking contrast in kinetics and fiber type profiles may be shaped by differences in the basal motor drive that each sphincter receives.

METHODS

  A double EMG approach was used to obtain spontaneously active single motor unit action potentials from the EUS and EAS simultaneously and compare their basal discharge frequencies in urethane anaesthetized rats.

KEY RESULTS

  The basal firing rates of motor units of the EUS and EAS were not significantly different (3.9 ± 0.9 Hz vs. 3.1 ± 1.6 Hz, respectively, n = 7 animals, P = 0.32, paired Student's t-test). However, auto-correlogram analysis showed that EUS is driven by neurons with faster instantaneous firing frequencies: 30.5 ± 2.4 Hz vs 14.3 ± 0.9 Hz in the EAS (P = 0.03, paired Student's t-test).

CONCLUSIONS & INFERENCES

  The oscillator(s) driving the EUS operate(s) at a frequency twice that of the EAS. This may explain the presence of type 2B fibers in the EUS. In the inter-micturition periods no cross correlation was found in motor discharge to the sphincters suggesting that the two muscles do not share a common central drive to sustain the continent tonus of the two outlet tracts.