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Am J Physiol Regul Integr Comp Physiol 278: R964-R972, 2000;
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Vol. 278, Issue 4, R964-R972, April 2000

Functional and morphological modifications of the urinary bladder in aging female rats

P. Lluel1, S. Palea1, M. Barras1, F. Grandadam1, D. Heudes2, P. Bruneval2, B. Corman3, and D. J. Martin1

1 Synthélabo Recherche, Internal Medicine Research, 92504 Rueil-Malmaison; 2 INSERM Unité 430, Hopital Broussais, 75014 Paris; and 3 Service de Biologie Cellulaire, CEA/Saclay, 91191 Gif-sur-Yvette, France


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
MATERIAL AND METHODS
RESULTS
DISCUSSION
REFERENCES

In female Wistar/Rij rats, 10 and 30 mo old, the micturition profiles in conscious animals, the contractile responses of the isolated urinary bladder, and the histology of the vesical tissue have been investigated. During cystomanometry, 60% of conscious senescent rats, but only 25% of young adult rats, showed spontaneous contractions during the bladder-filling phase. In aging rats, micturition pressure and duration of micturition were significantly higher by ~40-50%. In contrast, bladder capacity, bladder compliance, micturition volume, and residual volume were not modified with age. In vitro, the contractile responses of the bladder body to KCl, carbachol, arecoline, and alpha ,beta -MeATP were similar in tissues from young adult and senescent rats. In contrast, maximum responses to noradrenaline, but not phenylephrine, were two times greater in the older rats. Isoprenaline exhibited the same potency in relaxing KCl-precontracted bladder body of 10- and 30-mo-old animals. Morphometric analysis showed a significant increase in the mean thickness of the muscularis layer with age, whereas the collagen density significantly decreased in the muscularis and in the lamina propria layers. The fact that the majority of senescent rats displayed bladder instability and increased response to alpha -adrenergic agonists suggests that this strain of rat seems a good model for the aged human. However, other characteristics of the aging human urinary tract (urinary frequency, decreased cystometric capacity, and decreased detrusor contractility associated with fibrosis) are not present.

aging; bladder base; cystometry; urodynamics; Wistar/Rij rats


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIAL AND METHODS
RESULTS
DISCUSSION
REFERENCES

IT IS WELL KNOWN THAT DISTURBANCES of the urinary bladder function are common in elderly people who often complain of urgency, frequency, nocturia, and incontinence (30, 33). Age-related changes in the lower urinary tract functions include decreased bladder capacity, increased detrusor instability, and, particularly in women, decreased urethral resistance (4, 9, 33, 36).

To date, the effect of age-related changes in urinary bladder function in humans is not well characterized because it involves invasive procedures, such as catheterization for urodynamic studies and cystoscopy. Moreover, in practice, it is difficult to precisely discriminate between dysfunctions belonging to pathology from those pertaining to aging. The complexity of studies in human aging is complicated by associated diseases and drugs that may affect the lower urinary tract.

The normal functions of the urinary bladder are storage and expulsion of urine (micturition). It is thought that the sympathetic nervous system exerts a tonic influence on the bladder and urethra that facilitates the storage by predominantly stimulating alpha -adrenergic receptors in the bladder base and urethra. At the same time, sympathetic stimulation of predominantly beta -adrenergic receptors in the bladder body causes detrusor muscle relaxation. Micturition reflex manifests principally through activation of a cholinergic pathway on the bladder, but a purinergic system has also been described (3).

Several factors have been put forward to explain the voiding dysfunctions observed in the elderly, including detrusor fibrosis and consequent impairment of contractility (21), deposition and cross-linking of collagen and elastin (12), and loss of acetylcholinesterase-positive nerves (10). Despite the extensive use of the rat in aging research, the effect of senescence on rat urinary bladder function is not well characterized. Studies on the rat isolated detrusor muscle are numerous (18, 20, 22, 24, 25, 27, 28, 34, 41), but in vivo studies are scarce (5, 6, 16), and to our knowledge, there is no report on the effects of aging on bladder function in conscious animals.

We have therefore studied changes in micturition profiles in conscious young adult and senescent animals and the pharmacological responses to various agonists both in vitro and in vivo. An additive histological and morphometrical study of the bladder, using the same strain of rats, has been included.


    MATERIAL AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIAL AND METHODS
RESULTS
DISCUSSION
REFERENCES

Animals

Young adult (10 mo old) and senescent (30 mo old) virgin female Wistar WAG/Rij rats were obtained from the specific pathogen-free husbandry of the Centre d'Etudes de Saclay (Gif sur Yvette, France). The mean survival time of individuals in this colony was ~30 mo. They were fed ad libitum and had free access to water. All experiments were performed in accordance with the guidelines for animal experiments and principles for the care and use of laboratory animals established by Synthélabo Research Ethical Committee following the national and international directives.

Cystomanometry in Conscious Animals

Surgical procedure. Rats were anesthetized with ketamine (Imalgene, Rhône Mérieux, France), 100 mg/kg body wt ip. The abdomen was opened through a midline incision, and the bladder was exposed. A polyethylene catheter (Merk Biotrol, E03403) was implanted in the bladder through the dome and exteriorized at the scapular level. Each rat was housed individually after surgery, and food and water were given ad libitum. Animals were allowed to recover for at least 48 h.

Cystomanometric recordings. Cystomanometric investigations were performed in conscious animals 2 days after the bladder catheter implantation in young adult and senescent rats, as previously described (23). The bladder catheter was connected via a T tube to a strain gauge and an injection pump (Harvard apparatus) (22). The rats were held under partial restraint, and a vial was placed under the animals to measure volume of urine expelled. Warmed saline (37°C) was infused into the bladder at a rate of 6 ml/h.

Intravesical pressure was continuously recorded using a Maclab/8e interface (AD Instruments) and Chart software (version 3.4.2). Data were analyzed with Microsoft Excel software on Power Macintosh. The following cystomanometric parameters were measured or calculated (Fig. 1): basal pressure (BaP, cmH2O), peak micturition pressure (MP, cmH2O), threshold pressure (ThP, pressure at which micturition occurs, cmH2O), micturition duration (D, s), interval (time between 2 subsequent micturitions, min), micturition volume (MV, ml), residual volume (volume infused minus MV, ml). Five reproducible micturition cycles were analyzed, and means of the different cystometric parameters were calculated.


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  		Fig. 1.   Cystomanometric parameters investigated in conscious female rats. Basal pressure (BaP, cmH2O), peak micturition pressure (MP, cmH2O), threshold pressure (pressure at which micturition occurs; ThP, cmH2O), micturition duration (D, s), interval (time between 2 subsequent micturitions, min). Five reproducible micturition cycles were analyzed, and means of different cystomanometric parameters were calculated.

Bladder Contractility in Anesthetized Animals

Experiments were performed in young adult and senescent female Wistar/Rij rats. Under anesthesia (pentobarbital 30 mg/kg ip and 9 mg/kg sc), the urethra and the two ureters were ligated and a polyethylene catheter was positioned into the bladder to continuously record intravesical pressure. Saline was infused into the bladder to obtain a basal vesical pressure of ~20 cmH2O. Cumulative doses of drugs (arecoline, phenylephrine, and alpha ,beta -MeATP, 3, 10, 30, 100, and 300 µg/kg iv) were injected in a volume of 1 ml over 1 min every 10 min for arecoline and phenylephrine and every 15 min for alpha ,beta -MeATP. Results were expressed as bladder pressure increases (Delta BP, cmH2O). The dose required to elicit a 10 cmH2O increase in intravesical pressure (ED10, µg/kg, intravenous) was calculated by linear regression as it corresponds to an ~50% increase of the basal value.

In Vitro Experiments

Tissue preparation. Two groups of 10- and 30-mo-old female Wistar/Rij rats were specifically used for in vitro studies. The urinary bladder was excised at the level of the bladder base, immediately cleaned from surrounding tissues, and opened longitudinally. Two equal strips were cut from these bladders in a longitudinal direction. The bladder base was isolated, and one circular strip was prepared for each animal. Bladder body and bladder base smooth muscle strips were suspended in 20 ml organ bath containing a modified Krebs solution at 37°C (in mM: 114 NaCl, 4.7 KCl, 1.2 KH2PO4, 11.7 glucose, 2.5 CaCl2, 1.2 MgSO47H2O, 25 NaHCO3, and 1.1 ascorbic acid) with 1 µM propranolol and gassed with a mixture of 95% oxygen and 5% carbon dioxide. One end of each bladder strip was connected to a force displacement transducer (type 351, Hugo Sachs, Germany), and change in muscle tension was measured on a chart recorder (Gould) and on a real-time data-acquisition system (Software Jad, Notocord, France). A basal tension of 0.7 g for the bladder body and 0.5 g for the bladder base was applied to each strip that was washed every 15 min during 1 h equilibration time.

Contractile and relaxant responses in bladder body and bladder base. After the equilibration period, 80 mM KCl was added in the organ baths to measure the maximum contractile response of the strip. Then, the preparations were rinsed several times and allowed to equilibrate for 60 min before performing a cumulative concentration response curve (CRC) to noradrenaline (0.01-100 µM). Muscle strips were rinsed, after obtaining a plateau of response, to recover their basal tension. After a further 60 min of equilibration time, CRC for carbachol or arecoline, in the range 0.01-30 (or 100) µM, was obtained in the bladder body, whereas a CRC for phenylephrine (0.1-1,000 µM) was done on the bladder base.

In separate experiments on the bladder body, cumulative CRCs for phenylephrine (0.03- 1,000 µM) and for the selective alpha 2-adrenoceptor agonist UK 14,304 (0.1-100 µM) were obtained after the CRC for noradrenaline, as described above.

In another series of detrusor muscle strips, alpha ,beta -MeATP (0.1-300 µM) was tested in a noncumulative manner to avoid receptor desensitization. Each agonist concentration was added every 25 min with washouts of tissue strips every 5 min. Contractile responses to each agonist tested were expressed as a percentage of the contractile response to 80 mM KCl.

In another set of experiments, the relaxant effect induced by beta -adrenoceptor activation was tested. As previously described (29), the detrusor muscle was contracted with 50 mM KCl, rinsed, and equilibrated for 30 min, then again contracted with 50 mM KCl, and, on the plateau of contraction, isoprenaline was added in a cumulative manner in the range 0.001-10 µM. Each concentration was added when a plateau of relaxation was reached. Results are expressed as percentage of the plateau of contraction obtained by the second addition of 50 mM KCl. For this experiment, propranolol was not added in the Krebs solution.

For all these experiments, agonist potencies expressed as pD2 values (-log EC50) and maximum responses (Emax) were calculated by a regression analysis of the full curves using the ALLFIT software.

Histology and Morphometry

For histology and morphometry, the urinary bladders were removed from 12 rats in each group. They were fixed in 10% Formalin and embedded in paraffin. Transverse histological sections were performed at three levels and were stained with hematoxylin-eosin and orcein elastic stain for qualitative assessment and with Sirius red for morphometric measurement of collagen (35).

Morphometric measurement was based on computerized image analysis using a Nachet NS15,000 processor (Nachet, Evry, France) driven by a microcomputer following a program written in the INSERM Unit (35). For each urinary bladder, three microscopic fields, each in three different transversal sections, were observed through a Nachet microscope and a video camera (Sony, Tokyo, Japan) with a ×4 objective lens, yielding a final calibration of 3.5 µm/pixel. For collagen measurement, the Sirius red-stained sections were observed using polarized light (26). Collagen density in the muscularis and in lamina propria layers as well as mean thickness of the muscularis layer were measured.

Statistical Analysis

The results are given as mean values ± SE in each group of rats. Unpaired Student's test was used for in vivo studies. For morphometric analysis, the comparison of the measurements (the mean of the 9 measurements/rat) between groups was performed using analysis of variance (Statview program, Abacus, Berkeley, CA) with age as a factor. Statistical significance was accepted for P < 0.05.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIAL AND METHODS
RESULTS
DISCUSSION
REFERENCES

Body Weight and Bladder Weight

The average body weight of 30-mo-old rats (248 ± 5 g; n = 14) was significantly (P < 0.05) greater than that of 10-mo-old rats (221 ± 5 g; n = 13). Similarly, a significant (P < 0.05) increase in bladder weight was observed in senescent rats (0.151 ± 0.007 g) compared with young adult rats (0.133 ± 0.008 g). However, when the bladder weight was expressed as a percentage of body mass, no difference was observed between groups (0.060 ± 0.003% and 0.061 ± 0.002%, respectively).

Cystomanometry in Conscious Animals

Five reproducible micturition cycles were analyzed for each rat of both groups, thereby determining individual micturition patterns.

In 30-mo-old rats, we were able to discriminate between three types of micturition patterns: type I (20% of the animals) was characterized by a higher MP without bladder instability; type II, (60% of the animals) by the presence of bladder instability; and type III, by unstable contractions associated with urinary leakage without micturition. In 10-mo-old rats, however, type I (but with MP values significatively smaller) was present in 75% of animals and type II in 25% of the animals only. Typical micturitions patterns are shown in Fig. 2.


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  		Fig. 2.   Different types of micturition profiles obtained in conscious 10- and 30-mo-old female rats. Type I: no instabilities; type II: instabilities mainly before micturition; type III: no micturition, dribbling. BP, bladder pressure.

Mean values of cystomanometric parameters obtained in conscious animals are shown in Table 1. It is important to note that aging rats showing the type III pattern (Fig. 2) were not included in the cystomanometric analysis. In aging rats (type I and II), MPs and D were significantly higher by ~40-50%. In contrast, bladder capacity, as reflected by interval between micturition, micturition volume, and residual volume were not modified.

                              
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  		Table 1.   Cystomanometric parameters in conscious 10- and 30-mo-old female rats

Bladder Contractility in Anesthetized Animals

All agonists tested dose-dependently induced bladder contractions (Fig. 3). The response to arecoline was significantly reduced in the senescent group (ED10 = 36 ± 10 and 129 ± 45 µg/kg in the 10- and 30-mo-old group, respectively, P < 0.05). In contrast, the bladder response to phenylephrine was significantly greater in older rats (ED10 = 31 ± 4 and 14 ± 1 µg/kg in the 10- and 30-mo-old group, respectively, P < 0.05). The response to alpha ,beta -MeATP was not modified with age (ED10 = 29 ± 18 and 13 ± 2 µg/kg in the 10- and 30-mo-old group, respectively, P > 0.05).


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  		Fig. 3.   Bladder pressure increases (Delta BP, cmH2O) induced by arecoline (A), phenylephrine (B), and alpha ,beta -Me ATP (C) in anesthetized female Wistar/Rij rats. * Statistically different from 10-mo-old rats; unpaired t-test, P < 0.05.

Responses in the Bladder Body

The intrinsic contractility of the bladder body was unaffected by age because no difference was found in the magnitude of the contraction induced by 80 mM KCl between 10- and 30-mo-old rats (1.94 ± 0.12 and 1.79 ± 0.13 g, respectively, n = 8 for each). Similarly, the contractile responses to carbachol, arecoline, and alpha ,beta -MeATP were similar in tissues from young adult and senescent rats (Fig. 4, A-C, respectively; Table 2).


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  		Fig. 4.   Concentration-response curves for carbachol (A), arecoline (B), alpha ,beta -Me ATP (C), and isoprenaline (D) from 10- and 30-mo-old rat isolated detrusor muscle. Responses are expressed as percentage of 80 mM KCl-induced contractions (A-C) or 50 mM KCl (D).


                              
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  		Table 2.   pD2 and Emax values obtained in 10- and 30-month-old rats isolated bladder body

Isoprenaline exhibited the same potencies in relaxing KCl-precontracted bladder body from 10- and 30-mo-old rats; pD2 values being equal to 6.22 ± 0.13 and 6.25 ± 0.19, respectively (Fig. 4D; n = 4 for each).

On the other hand, the maximum contractile responses to noradrenaline were two times greater (P < 0.01) in senescent rats, whereas contractile responses to phenylephrine were not modified by aging (Fig. 5, Table 2). No difference was observed in contractile responses to noradrenaline and phenylephrine in the same age group. Importantly, the selective alpha 2-adrenoceptor agonist UK 14,304, in the range 0.01-100 µM, did not induce any contraction in either group of rats (Table 2; n = 4 for each).


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  		Fig. 5.   Concentration-response curves for noradrenaline (A and C) and phenylephrine (B and D) from 10- and 30-mo-old rat isolated bladder base and bladder body. Responses are expressed as percentage of 80 mM KCl-induced contractions.

Responses in the Bladder Base

No difference was noted in the contractile response to noradrenaline or phenylephrine in the bladder base between 10 and 30 mo (Fig. 5), both in terms of pD2 or Emax responses. For noradrenaline, pD2 and Emax values were 5.62 ± 0.07 and 49.7 ± 5.6% for 10-mo-old rats (n = 5), respectively, and 5.56 ± 0.08 and 49.9 ± 4.4% for 30-mo-old rats (n = 5), respectively. For phenylephrine, pD2 and Emax values were 5.32 ± 0.12 and 54.1 ± 4.9% for 10-mo-old rats (n = 5), respectively, and 5.30 ± 0.18 and 49.9 ± 4.4% for 30-mo-old rats (n = 5), respectively.

Histology and Morphometry

Qualitative analysis of the hematoxylin-eosin and elastic stained sections did not show any difference between the two groups. A tiny elastic network in the urinary bladder wall was present in both groups, and no inflammation was observed.

Morphometric analysis showed (Fig. 6) an increase in the mean thickness of the muscularis layer with age: 799.6 ± 12.9 µm in the 10-mo-old rats vs. 1,011.3 ± 18.2 µm in the 30-mo-old rats (P < 0.0001, n = 12 for each group). The collagen density decreased with age: 16 ± 1.1% in the 10-mo-old rats vs. 9.4 ± 0.7% (P < 0.0001) in the 30-mo-old rats in the muscularis layer and 27.7± 2.2% in the 10-mo-old rats vs. 12.9 ± 1.5% (P < 0.0001) in the 30-mo-old rats in the lamina propria layer.


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  		Fig. 6.   Transverse histological bladder section from 10 (A)- and 30-mo-old rat (B; Sirius red stained; bar = 200 µm). * Muscularis layer, dagger  lamina propria layer.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIAL AND METHODS
RESULTS
DISCUSSION
REFERENCES

The aim of the present study was to investigate the effect of normal aging on bladder function in female rats. For this purpose, we have used a strain of female Wistar rat (WAG/Rij) obtained from a specific pathogen-free husbandry. The low incidence of bladder cancers in this strain of rats is in sharp contrast to the high incidence of spontaneously occurring cancers in most other rat strains (2, 14). Moreover, all the female rats used were nulliparous because it is known that parity and/or parturition may induce bladder dysfunctions (11).

Modification of Cystomanometric Parameters

The most important result of our investigation is the observation that 60% of aging rats (but only 25% of 10-mo-old rats) showed a pronounced spontaneous bladder activity during the filling phase (bladder instability). Moreover, an increase of the peak micturition pressure and a longer duration of micturition were noticed in senescent rats. In these rats, we were able to discriminate between three types of micturition patterns. These altered micturition profiles appeared to have common features with those we have reported in conscious female rats following bladder outlet obstruction (23), in which ~80% of the rats presented a detrusor instability and a high MP. In contrast, no modification of the pressure at which micturition occurs (ThP), BaP, bladder capacity, and bladder compliance were observed in our study. These results vary from those previously reported in anesthetized rats, in which an increased ThP for micturition in older animals was demonstrated in the absence of modification in bladder capacity (5, 6). Another study on anesthetized male Wistar rats (16) concluded that aging was associated with a large increase in bladder capacity and a lower MP, which is the opposite of our findings. These contrasting results may be explained by the use of anesthesia that modify detrusor responses (39) or by a difference in strain, gender, and age of animals used. In our study, bladder capacity (as reflected by interval between micturitions) is essentially similar in 10- and 30-mo-old rats. These results are at variance with those of Chun and co-workers (5) but in accordance with another study in WAG/Rij rats, which reported that the frequency of micturition only slightly decreased with age (7). The differences in terms of micturition frequency is probably due to the bladder hypertrophy of the aging rats used by Chun and collaborators (male Fisher 344 rats).

Structural Changes of the Detrusor Muscle

In contrast to rats used by other authors (5, 17), in the present experiments, the increase in bladder weight was associated with a parallel increase in body weight, indicating the absence of vesical hypertrophy in this strain of Wistar rats (Wag/Rij). However, we observed structural changes in the aging bladder. An increase in the mean thickness of the muscularis layer and a decrease in collagen density in the muscularis and in the lamina propria layer was evident in aging bladders. The increase in the quantity of smooth muscle could explain the higher peak micturition pressure observed during cystomanometry in aging rats and may be due to muscle cell hypertrophy, as previously described in the bladder after outlet obstruction (8). The decrease in collagen density could be related to the increase of the mean thickness of the muscularis layer without modification of the total amount of collagen. Aging of the rat urinary bladder seems therefore not associated with bladder fibrosis, thus explaining the absence of age-related modification of bladder compliance observed in this study. To date, the influence of aging on collagen composition in the rat bladder was not reported, but no difference in collagen content has been shown in the aging mouse bladder (32). It should be noted that in the aging human detrusor muscle, a significative fibrosis does occur (15).

Bladder Contractility in Vitro

We also investigated the contractile response of urinary bladder strips to several agonists. No difference was found in the intrinsic contractility of detrusor muscle between young adult and senescent rats, as indicated by similar responses to 80 mM KCl. Moreover, no age-related differences in the contractile response to the selective P2X-receptor agonist alpha ,beta -MeATP were disclosed both in vitro and in vivo. Furthermore, the contractile responses of the isolated detrusor muscle to carbachol and arecoline were similar in the two groups of rats, in accordance with a previous report (27), and it is interesting to note that acetylcholine contractile potency on human detrusor muscle is similar in tissues taken from normal subjects or from patients with detrusor instability (19). However, we found an age-related decrease in the response to intravenous arecoline in anesthetized rats. One possible explanation for the discrepancy between in vitro and in vivo results could be a differential functional role of M2 muscarinic receptors that, being activated by arecoline together with M3 receptors, contract the bladder indirectly by reversing beta -adrenoceptor-mediated relaxation (13). Our in vitro, but not in vivo experiments, were performed in the presence of propranolol (a potent beta 2-adrenoceptor antagonist), so a possible explanation for such a decreased contractile response to arecoline in vivo in aging rats could be the lack of interaction between M2 and beta 2-adrenoceptors, which is functional in 10-mo-old rats. Alternatively, age-related changes in the pre- or postjunctional modulation of arecoline-induced contractility by other neurotransmitters or neuromodulators could be evoked.

In humans, a clear correlation between alpha -adrenergic contractile response in the isolated detrusor muscle and bladder instability was evident in patients suffering from prostatic obstruction (31), and such a correlation to detrusor instability seems also to exist in rats. Interestingly, in the bladder body, we found a clear increase in noradrenaline maximal response in 30-mo-old rats. However, the sensitivity to noradrenaline was unchanged in the bladder base, a region rich in alpha 1-adrenoceptors. The lack of effect of the selective alpha 2-adrenoceptor agonist UK 14,304 suggests that noradrenaline acts through contractile alpha 1-adrenoceptors only. Damage of sympathetic nerves was previously reported in the urinary tract of aging rats (37). However, denervation supersensitivity is not a likely explanation of our results, because in this case, a leftward shift in the CRC for noradrenaline would be apparent (38). Moreover, an increase in alpha 1-adrenoceptor density seems not probable, because in this case, the Emax for phenylephrine would be higher also. A possible way to explain our results is to hypothesize a shift in beta -adrenoceptor density with age. Several recent papers have demonstrated that noradrenaline-induced relaxation in male rat detrusor is mediated by both beta 2- and beta 3-adrenoceptors (29, 40). In our hands, the relaxant potency of isoprenaline was similar in 10- and 30-mo-old rats; however, this compound does not discriminate between beta -adrenoceptors subtypes, whereas propranolol is 100 times more potent on beta 2- than beta 3-adrenoceptors (29). Therefore, in our in vitro conditions (1 µM propranolol), beta 3-adrenoceptors are only partially blocked on the detrusor muscle. So, if we hypothesize that in senescent rats, noradrenaline mainly acts on beta 2-adrenoceptors, whereas in young adult rats, it activates beta 3-adrenoceptors, the net result would be an increase in the response to noradrenaline in older animals, as we observed. Because phenylephrine does not activate beta -adrenoceptors, this hypothesis explains why phenylephrine responses were similar in the two groups of rats. Interestingly, a previous investigation on Fisher male rats showed no age-related changes in phenylephrine-induced contraction and isoprenaline-induced relaxation on isolated detrusor muscle (20).

In anesthetized rats, phenylephrine induced large bladder contractions, in sharp contrast to the relatively weak contractile effect displayed on the isolated detrusor muscle (~40% of KCl-induced response). It is, therefore, likely that the in vivo response to this adrenergic agonist is not only due to activation of alpha 1-adrenoceptors on detrusor muscle itself, but probably involves receptors located on cell bodies of sympathetic ganglia that, in rats, are found in plexuses near the bladder base (1). Therefore, the increase in the response to phenylephrine in senescent rats could be due to a modification in sympathetic tonus at the level of pelvic ganglia.

In conclusion, in aging rats, we observed bladder instabilities during the filling phase and higher MP associated with an increase in the urinary contractility to phenylephrine in vivo and to noradrenaline on the isolated detrusor muscle. Moreover, aged rats demonstrated a decrease in the response to arecoline in vivo but no modification in the response to a P2x-receptor agonist. Histological studies showed an increase in the mean thickness of the muscularis layer with age and a decrease in the collagen density in the muscularis and in the lamina propria layers. In contrast to other tissues, age-related changes in the bladder are not associated with fibrosis and modification of the compliance. The fact that the majority of senescent rats displayed bladder instability suggests that this strain of rat could be a good model for the aged human. However, other characteristics of the aging human urinary tract (urinary frequency, decreased cystometric capacity, and decreased detrusor contractility associated with fibrosis) are not present in this strain of rats.

Perspectives

These experiments show greater changes in the voiding pattern in female senescent rats associated with changes in the contractile response of the detrusor muscle. We propose that aging modifies urinary bladder function, leading to a dysfunction similar to that induced by obstruction. The possibility that aging induces physiological obstruction is further supported by structural changes of the bladder. External factors to the bladder, e.g., urethra, C fiber afferents, and spinal and supraspinal micturition centers, could be considered as possible underlying factors. Further investigations on age-related changes of the urethral function and age-related modifications of beta -adrenoceptor subtypes mediating bladder relaxation and beta -adrenoceptor/muscarinic receptor interaction will be particularly interesting.


    ACKNOWLEDGEMENTS

We thank Dr. Anne-Marie Galzin for the critical reading of the manuscript. The skillful technical assistance of Chantal Duquenne is gratefully acknowledged.


    FOOTNOTES

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Address for reprint requests and other correspondence: P. Lluel, Synthélabo Recherche, Dept. of Internal Medicine, 10 rue des Carrières, 92504 Rueil-Malmaison, France (E-mail: Philippe.LLUEL{at}sanofi-synthelabo.com).

Received 2 June 1999; accepted in final form 27 October 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
MATERIAL AND METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Alm, P, and Elmer P. Adrenergic and cholinergic innervation of the rat urinary bladder. Acta Physiol Scand 94: 36, 1975[ISI][Medline].

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Am J Physiol Regul Integr Comp Physiol 278(4):R964-R972
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