THE RESPONSE OF FETAL SHEEP BLADDER TISSUE TO PARTIAL OUTLET OBSTRUCTION

Neurophysiological control of urinary bladder storage and voiding-functional changes through development and pathology

The effective storage of urine and its expulsion relies upon the coordinated activity of parasympathetic, sympathetic, and somatic innervations to the lower urinary tract (LUT). At birth, all mammalian neonates lack the ability to voluntary regulate bladder storage or voiding. The ability to control urinary bladder activity is established as connections to the central nervous system (CNS) form through development. The neural regulation of the LUT has been predominantly investigated in adult animal models where comparatively less is known about the neonatal and postnatal neurophysiological development that facilitate urinary continence. Furthermore, congenital neurological or anatomical defects can adversely affect both storage and voiding functions through postnatal development and into adulthood, leading to secondary conditions including vesicoureteral reflux, chronic urinary tract infections, and end-stage renal disease. Therefore, the aim of the review is to provide the current knowledge available on neurophysiological regulation of the LUT through pre- to postnatal development of human and animal models and the consequences of congenital anomalies that can affect LUT neural function.

Development of contractile properties in the fetal porcine urinary bladder

BackgroundIn early fetal life, the bladder is merely a conduit allowing urine to pass through freely into the amniotic cavity. As the striated external urethral sphincter evolves, the bladder acquires its reservoir and voiding functions. We characterized the myogenic and neurogenic contractions of the normal fetal porcine bladder from midterm until close to full-term gestation.MethodsContractile responses were measured in vitro using bladder strips from fetuses at 60 (N=23) and 100 days (N=21) of gestation. Spontaneous activity, and the responses to potassium chloride (KCl) solution, electrical field stimulation (EFS), and receptor activation were recorded. The smooth muscle content was evaluated histologically.ResultsHistological studies revealed that the fractional content of smooth muscle doubled between the two time points, and passive tension was adjusted to take that into account. Spontaneous activity was regular at 60 days, changing toward an irregular pattern at 100 days. Contractile force elicited by KCl and carbachol increased significantly with gestational age, while contractions to the purinergic agonist, α-β-methylene adenosine 5'-triphosphate did not. The responses to EFS were almost completely blocked by atropine.ConclusionSpontaneous myogenic contractions become irregular and contractile responses to muscarinic receptor stimulation increase during gestation, as the bladder reservoir and voiding functions develop.

Ca 2+ Regulation in Detrusor Smooth Muscle From Ovine Fetal Bladder After In Utero Bladder Outflow Obstruction

PURPOSE

We characterized intracellular Ca(2+) regulation in fetal bladders following outflow obstruction by examining the Ca(2+) response to agonists in smooth muscle cells.

MATERIALS AND METHODS

Severe bladder outflow obstruction was induced in male fetal sheep by placing a urethral ring and urachal ligation midway through gestation at 75 days. Fetuses were examined 30 days after surgery. Intracellular Ca(2+) in single smooth muscle cells isolated from the bladder wall was measured with epifluorescence microscopy using fura-2(AM) during exposure to agonists, such as carbachol and adenosine triphosphate, and to other activators, such as caffeine and KCl.

RESULTS

Detrusor smooth muscle cells from obstructed bladders had resting intracellular Ca(2+) similar to that in sham operated controls. The maximal response to carbachol was decreased following obstruction (p <0.05). Construction of dose-response curves also demonstrated higher EC(50) (p <0.05). However, these changes were not mirrored by caffeine evoked Ca(2+) release, which was not significantly different between the obstruction group and sham operated controls. Kinetic analysis of carbachol transients further revealed an attenuated maximal rate of increase in obstructed bladders (p <0.01). The magnitude of intracellular Ca(2+) to purinergic neurotransmitter adenosine triphosphate was also found to be smaller in cells from obstructed bladders (p <0.05), although transmembrane influx by high K depolarization was not significantly affected.

CONCLUSIONS

Muscarinic and purinergic pathways were down-regulated in fetal detrusor muscle following outflow obstruction. These major functional receptors appeared to be more susceptible to obstruction than other Ca(2+) regulators. Their impairment may contribute to the compromised contractile function seen in in utero bladder outflow obstruction.

Ca2+ regulation in detrusor smooth muscle from developing fetal sheep bladders

Sheep fetus is a useful model to study in utero bladder outflow obstruction but little is known about cell physiology of fetal bladders. To remedy this defect we have characterised intracellular Ca(2+) regulation in fetal sheep myocytes of different developmental ages. Fetal detrusor myocytes had a similar resting [Ca(2+)](i) to adult cells and exhibited transient [Ca(2+)](i) increases in response to carbachol, ATP, high-K, caffeine and low-Na. The carbachol transients were abolished by atropine and caffeine; the ATP response was blocked by alpha,beta-methylene ATP; high-K-evoked [Ca(2+)](i) rises were antagonised by verapamil. The maximal responses to carbachol, high-K, caffeine and low-Na in fetal cells were similar to those of adult counterparts, whilst the ATP response was smaller (p < 0.05). These variables were largely similar between the three gestational groups with the exception of ATP-induced response between early fetal and adult bladders (p < 0.05). Dose-response curves to carbachol demonstrated an increase of potency between mid-gestation and early adulthood (p < 0.05). These data show that muscarinic receptors coupled to intracellular Ca(2+) release, P2X receptor-linked Ca(2+) entry, depolarisation-induced Ca(2+) rise via L-type Ca(2+) channels, Na(+)/Ca(2+) exchange and functional intracellular Ca(2+) stores are all operational in fetal bladder myocytes. Whilst most of Ca(2+) regulators are substantially developed and occur at an early fetal age, a further functional maturation for cholinergic sensitivity and purinergic efficacy continues throughout to adulthood.

Contractile properties of the developing fetal sheep bladder

AIM

To characterise the developmental changes to the normal bladder by examining the in vitro contractile properties of the fetal sheep detrusor smooth muscle bladder at different gestational ages.

METHODS

Three groups of normally developing fetal sheep bladders were investigated: (1) 65-70 days gestation (2nd trimester); (2) 105-110 days (3rd trimester); (3) 135-140 days (term). Isometric contractions in isolated detrusor strips were measured during either electrical field stimulation (EFS) or exposure to agonists: carbachol, alpha-beta methylene-ATP (ABMA) and KCl.

RESULTS

There was a significant increase of absolute force elicited by EFS with fetal age. Contractures elicited by carbachol, ABMA and KCl increased between Groups 1 and 2, but not thereafter. The proportional increase of contractions elicited by carbachol and ABMA was also greater between Groups 1 and 2, than for EFS and KCl.

CONCLUSIONS

Fetal development between 65 and 140 days in the sheep is associated with increased contractile activation. The data are consistent with an increase of muscle development in the earlier stages (65-110 days). In the later stage (110-140 days) muscle development is complete but functional innervation of the tissue continues.

Neurotransmission and viscoelasticity in the ovine fetal bladder after in utero bladder outflow obstruction

Fetal bladder outflow obstruction, predominantly caused by posterior urethral valves, results in significant urinary tract pathology; these lesions are the commonest cause of end-stage renal failure in children, and up to 50% continue to suffer from persistent postnatal bladder dysfunction. To investigate the physiological development of the fetal bladder and the response to urinary flow impairment, we performed partial urethral obstruction and complete urachal ligation in the midgestation fetal sheep for 30 days. By electrical and pharmacological stimulation of bladder strips, we found that muscarinic, purinergic, and nitrergic mechanisms exist in the developing fetal bladder at this gestation. After bladder outflow obstruction, the fetal bladder became hypocontractile, producing less force after nerve-mediated and muscarinic stimulation with suggested denervation, and also exhibited greater atropine resistance. Furthermore, fetal bladder urothelium exerted a negative inotropic effect, partly nitric oxide mediated, that was not present after obstruction. Increased compliance, reduced elasticity, and viscoelasticity were observed in the obstructed fetal bladder, but the proportion of work performed by the elastic component (a physical parameter of extracellular matrix) remained the same. In addition to denervation, hypocontractility may result from a reduction in the elastic modulus that may prevent any extramuscular components from sustaining force produced by detrusor smooth muscle.

Urinary outflow obstruction increases apoptosis and deregulates Bcl-2 and Bax expression in the fetal ovine bladder

During organogenesis, net growth of tissues is determined by a balance between proliferation, hypertrophy, and apoptotic death. Human fetal bladder outflow obstruction is a major cause of end-stage renal failure in children and is associated with complex pathology in the kidney and lower urinary tract. Experimental manipulation of the fetal sheep urinary tract has proved informative in understanding the pathobiology of congenital obstructive uropathy. In this study we used an ovine model of fetal bladder outflow obstruction to examine effects on apoptotic cell death in the developing urinary bladder. While 30 days of obstruction in utero between 75 and 105 days gestation resulted in overall growth of the fetal bladder as assessed by weight, protein, and DNA measurements, we found that apoptosis, as assessed by in situ end-labeling, was up-regulated in fetal bladder detrusor muscle and lamina propria cells and that this was accompanied by a down-regulation of the anti-death protein Bcl-2 and an up-regulation of the pro-death protein Bax. Moreover, activated caspase-3, an effector of apoptotic death, was increased in obstructed bladders. This is the first study to define altered death in an experimental fetal model of bladder dysmorphogenesis. We speculate that enhanced apoptosis in detrusor smooth muscle cells is part of a remodeling response during compensatory hyperplasia and hypertrophy. Conversely, in the lamina propria, an imbalance between death and proliferation leads to a relative depletion of cells.

Effects of in utero bladder outflow obstruction on fetal sheep detrusor contractility, compliance and innervation

PURPOSE

Congenital bladder outflow obstruction caused by posterior urethral valves is a common cause of end stage renal failure in boys. We hypothesized that fetal bladder outflow obstruction perturbs detrusor contractility and innervation and bladder storage volume-pressure relationships.

MATERIALS AND METHODS

Severe bladder outflow obstruction was induced in male fetal sheep by placing a urethral ring and urachal ligation midway through gestation at 75 days. Fetuses were examined 30 days after surgery, when urinary tract dilatation, enlarged bladders and histologically abnormal kidneys were documented. Isolated strips of bladder detrusor from sham operated and obstructed fetuses were subjected to electrical field stimulation, carbachol, KCl and alpha-beta methylene-adenosine triphosphate. Whole bladder storage characteristics were determined by filling cystometry and bladder innervation was investigated by immunohistochemistry and Western blot.

RESULTS

Tension-frequency contractility studies showed that obstructed fetal bladder strips were significantly hypocontractile versus sham operated controls in response to electrical field stimulation and the specific agonists carbachol, KCl and alpha-beta methylene-adenosine triphosphate. Hypocontractility was greater with nerve mediated stimulation than with carbachol, suggesting relative denervation. Reduced innervation was confirmed by S100 and protein gene product 9.5 immunohistochemistry and by measuring a significant reduction in protein gene product 9.5 protein expression using Western blot. Filling cystometry showed that obstructed fetal bladders appeared more compliant (Delta V/Delta P, where Delta V is the change in volume and Delta P is the change in pressure) with larger capacity, more flaccidity and yet retained stress relaxation.

CONCLUSIONS

In response to severe experimental fetal bladder outflow obstruction the bladder becomes large and hypocontractile, and has aberrant innervation.

Regional differences in bladder enlargement and in vitro contractility after outlet obstruction in the rabbit

PURPOSE

Bladder outlet obstruction leads to bladder enlargement and subsequent decreases in contractile function in vivo and in vitro. We determined whether there were regional differences in bladder wall properties and in vitro contractile responses after 2 weeks of bladder outlet obstruction.

MATERIALS AND METHODS

Male rabbits underwent cystometry. The bladder was then filled to 40 ml. and the surface was marked with 2-zero silk knots placed approximately 1 cm. apart. The distance between the knots was measured at 20, 40 and 80 ml. The animals then underwent the creation of surgical obstruction. After 2 weeks the obstruction was removed. Cystometry and measurements were repeated and strips were obtained from defined dorsal and ventral areas. Contractile responses to electrical field stimulation, adenosine triphosphate, carbachol and KCl were determined and compared with strips from unobstructed controls.

RESULTS

In vivo expansion during bladder filling occurred evenly throughout the bladder wall in controls and the contractile response to all stimuli was similar in ventral and dorsal strips. After 2 weeks of bladder outlet obstruction the upper dome expanded to a significantly higher degree than the lower bladder body. The response to all stimuli was significantly reduced after bladder outlet obstruction and there was a significantly decreased response to all stimuli in dorsal compared with ventral strips. Strips from the dorsal midline showed a relaxation response to electrical field stimulation at low frequencies, whereas all ventral strips contracted.

CONCLUSIONS

Functional remodeling after bladder outlet obstruction is a process that does not occur to the same extent throughout the bladder. The obstructed bladder is an inhomogeneous organ with significant regional differences in mechanical and pharmacological properties.