AGE-RELATED CHANGES IN CONTRACTILE RESPONSES OF RABBIT LOWER URINARY TRACT TO ENDOTHELIN

Endothelin ET(B) receptors are involved in the relaxation to the pig urinary bladder neck

AIMS

The involvement of endothelin receptors in the contraction of the lower urinary tract smooth muscle is well established. There is scarce information, however, about endothelin receptors mediating relaxation of the bladder outlet region. The current study investigates the possible existence of endothelin ET(B) receptors involved in the relaxation of pig bladder neck.

METHODS

ET(B) receptor expression was determined by immunohistochemistry and urothelium-denuded bladder neck strips were mounted in organ baths for isometric force recording.

RESULTS

ET(B) -immunoreactivity (ET(B) -IR) was observed within nerve fibers among smooth muscle bundles and urothelium. BQ3020 (0.01-300 nM), an ET(B) receptor agonist, produced concentration-dependent relaxations which were reduced by BQ788, an ET(B) receptor antagonist, and by inhibitors of protein kinase A (PKA) and large (BK(Ca) )- or small (SK(Ca) )-conductance Ca(2+) -activated K(+) channels. Pretreatment with BK(Ca) or SK(Ca) channel inhibitors plus PKA blocking did not cause further inhibition compared with that exerted by inhibiting BK(Ca) or SK(Ca) channels only. BQ3020-induced relaxation was not modified by blockade of either nitric oxide (NO) synthase, guanylyl cyclase, cyclooxygenase (COX) or of intermediate-conductance Ca(2+) -activated-(IK(Ca) ), ATP-dependent-(K(ATP) ), or voltage-gated-(K(v) ) K(+) channels. Under non-adrenergic non-cholinergic (NANC) conditions, electrical field stimulation (0.5-16 Hz) evoked frequency-dependent relaxations, which were reduced by BQ788 and potentiated by threshold concentrations of BQ3020.

CONCLUSIONS

These results suggest that BQ3020 produces relaxation of the pig bladder neck via activation of muscle endothelin ET(B) receptors, NO/cGMP- and COX-independent-, cAMP-PKA pathway-dependent-mechanisms, and involving BK(Ca) and SK(Ca) channel activation. ET(B) receptors are also involved in the NANC inhibitory neurotransmission.

Contractile effects of endothelins on isolated human ureter

The aim of our study was to investigate mechanism of action of endothelins 1, 2 and 3 on spontaneous activity, tone and intraluminal pressure of human ureter. Both longitudinal tension and intraluminal pressure were recorded from the isolated segments of proximal human ureter. Endothelins 1, 2 and 3 (5.35x10(-11) M - 5.05x10(-8) M) produced concentration-dependent tonic contraction and sustained increase in intraluminal pressure of isolated preparations of human ureter. Endothelins 1 and 3 produced also concentration-dependent inhibition of spontaneous, phasic contractions of the isolated preparations. Selective antagonist of ET(A) receptors BQ123 and selective antagonist of ET(B) receptors BQ788 produced significant inhibition of endothelin-1-induced tonic contraction (pA(2)=8.80 and 6.55, respectively) and increase in intraluminal pressure (pA(2)=8.68 and 7.02, respectively), while they did not affect endothelin-1-induced inhibition of spontaneous activity. Endothelin 1 produces increase in tone and intraluminal pressure of isolated human ureter acting on both ET(A) and ET(B) receptors, the first one being functionally more important. Only endothelins 1 and 3 inhibit spontaneous, phasic activity of human ureter, but this effect was not blocked by selective antagonists of ET(A) and ET(B) receptors.

Neuropeptides in lower urinary tract function

Numerous neuropeptide/receptor systems including vasoactive intestinal polypeptide, pituitary adenylate cyclase-activating polypeptide, calcitonin gene-related peptide, substance P, neurokinin A, bradykinin, and endothelin-1 are expressed in the lower urinary tract (LUT) in both neural and nonneural (e.g., urothelium) components. LUT neuropeptide immunoreactivity is present in afferent and autonomic efferent neurons innervating the bladder and urethra and in the urothelium of the urinary bladder. Neuropeptides have tissue-specific distributions and functions in the LUT and exhibit neuroplastic changes in expression and function with LUT dysfunction following neural injury, inflammation, and disease. LUT dysfunction with abnormal voiding, including urinary urgency, increased voiding frequency, nocturia, urinary incontinence, and pain, may reflect a change in the balance of neuropeptides in bladder reflex pathways. LUT neuropeptide/receptor systems may represent potential targets for therapeutic intervention.

Prostate expression of endothelins and their receptors in rat growth

Endothelins (ETs) have been shown to have mitogenic effects on prostate cells in vitro. To study a relationship between ETs and prostate growth, we investigated expression of ET-1 and ET-3 and their receptors ETA and ETB in the dorsolateral (DLP) and ventral (VP) prostate lobes of rats aged 3 weeks, 3 months, and 22 months. The weight ratio of each lobe to the body was not different at 3 weeks but increased more in the DLP than in the VP at 3 months; the ratio did not change at 22 months. The density of the major receptor ETA, measured by radio-labelled ET-1 binding to prostate membrane particulates, was highest at 3 weeks in both prostate lobes and decreased thereafter. mRNA expression levels of ETA, ETB, ET-1, and ET-3 in the DLP and ETA in the VP, as assessed by relative multiplex reverse transcription-polymerase chain reaction, were highest at 3 weeks, and no significant change was seen in mRNA expression of ET-converting enzyme-1. The highest expression of ETA and the ligands at 3 weeks, followed by dramatic growth at 3 months in the DLP, suggests a role of ETA-mediated signal transduction in prostate growth of rats and possibly of humans.

Age-related changes in endothelin-1 receptor subtypes in rat heart

Density, affinity, and subtype distribution of endothelin-1 (ET-1) binding sites were determined in rat cardiac tissue as a function of age in order to evaluate the association of alterations in the endothelin receptor system and aging in the heart. A significant decrease in the receptor subtype ET-A, which represents 70% to 80% of the total receptor population in cardiac tissue of 3- and 12-month-old rats, was observed in 24-month-old rats with respect to the younger groups. These findings indicate an alteration in ET-1 cardiac receptors associated with aging, mainly due to a variation in the receptor subtype distribution.

Therapeutic effects of endothelin-A receptor antagonist on bladder overactivity in rats with chronic spinal cord injury

OBJECTIVES

We investigated the effects of suppression of endothelin-A (ET(A)) receptors on bladder function and ET-1 levels in the bladder in rats with chronic spinal cord injury (SCI).

METHODS

We transected the spinal cord of female Sprague-Dawley rats at the level of Th 8-9. Awake cystometrograms were performed 4 weeks after spinal cord transection. We evaluated cystometric parameters such as mean amplitudes of nonvoiding contractions (NVCs), the number of NVCs, voided volume, voiding efficiency, and micturition pressure before and after intravenous (i.v.) injection of ABT-627, an ET(A) antagonist, or A-19261, an ET(B) antagonist, in SCI animals. Four weeks after spinalization, we also measured the protein and mRNA levels of ET-1 in the bladder using enzyme-linked immunosorbent assay (ELISA) and quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS

ABT-627 (1 mg/kg, i.v.) but not A-192621 (10 mg/kg, i.v.) significantly decreased the amplitude of NVCs and the number of NVCs in SCI rats. There were no significant changes in pressure threshold, maximum voiding pressure, voided volume, or voiding efficiency. ELISA analysis for ET-1 showed significantly elevated protein concentrations in SCI rats compared with spinal cord intact rats. Significant upregulation of the ET-1 mRNA was also noted in SCI bladders.

CONCLUSIONS

These results suggest that upregulation of ET-1 is involved in the mechanism inducing bladder overactivity in chronic SCI rats, and that an ET(A) receptor antagonist can suppress SCI-induced bladder overactivity as indicated by a reduction in NVCs. Thus, ET(A) receptor inhibition could be an effective treatment for neurogenic bladder overactivity in pathological conditions such as SCI.

Age Related Changes in the Functional, Biochemical and Molecular Properties of α 1 -Adrenoceptors in the Rat Genitourinary Tract

PURPOSE

Because age related changes occur in the properties of alpha(1)-adrenoceptor in several mammalian tissues and alpha(1)-adrenoceptor antagonists are extensively used to treat lower urinary tract symptoms secondary to benign prostatic hyperplasia, we investigated age related changes in the functional, biochemical and molecular properties of alpha(1)-adrenoceptor in the rat genitourinary tract.

MATERIALS AND METHODS

The characteristics of alpha(1)-adrenoceptor in the ventral and dorsolateral prostate, and bladder base and dome of 3 and 22-month-old rats were determined using an isolated muscle bath, radioligand receptor binding and real-time reverse transcriptase-polymerase chain reaction techniques.

RESULTS

Old rats had significantly higher body weight, lower testosterone, a smaller ventral prostate and a larger bladder dome than young rats. Although there was no significant age dependent difference in the properties of alpha(1)-adrenoceptor in the bladder base and dome, total alpha(1)-adrenoceptor density, mRNA expression of all 3 alpha(1)-adrenoceptor subtypes (alpha(1A), alpha(1B) and alpha(1D)) and the maximum contractile responses to phenylephrine were significantly lower in the ventral and dorsolateral prostate of 22 vs 3-month-old rats.

CONCLUSIONS

Age related differences in the molecular, biochemical and functional properties of alpha(1)-adrenoceptors in the rat genitourinary tract may indicate potential differences in the response to alpha(1)-adrenoceptor antagonists with aging, ie a decrease in the therapeutic response in old vs young rats in the response to alpha(1)-adrenoceptor antagonists when used to treat lower urinary tract symptoms secondary to benign prostatic hyperplasia.

Participation of endogenous endothelin and ETA receptor in premicturition contractions in rats with bladder outlet obstruction

A relationship between endogenous endothelins and bladder overactivity has recently been suggested, but the related endothelin receptor subtype has not been identified. Here, to evaluate the involvement of endothelin-1 and its receptors in bladder overactivity, we investigated endothelin-1 levels and the expression of its receptors in the bladder of rats with bladder outlet obstruction (BOO), a model for bladder overactivity. We also investigated the effects of a selective endothelin ET(A) receptor antagonist, (E)-N-[6-methoxy-5-(2-methoxyphenoxy)[2,2'-bipyrimidin]-4-yl]-2-phenylethenesulfonamide monopotassium salt (YM598), on bladder functions in conscious BOO rats. Partial obstruction of the urethra led to a progressive increase in bladder weight from weeks 1 to 6. Binding assays performed using plasma membranes prepared from these bladders to estimate endothelin receptor density from the maximum [(125)I]endothelin-1 binding showed increased endothelin receptor density (about double) at 1, 2, and 6 weeks after the operation in the BOO bladder. The densities of endothelin ET(A) receptors in the bladder of sham-operated and BOO rats at 2 weeks after operation were about 3.5 and 5 times those of endothelin ET(B) receptors respectively. Furthermore, the endothelin-1 level was also increased in the BOO bladder. Two weeks after operation, BOO rats showed an increase in maximum bladder capacity and micturition volume and the generation of premicturition contractions. The frequency of premicturition contractions was dose-dependently reduced by YM598 (0.1-3 mg/kg, i.v.) without any effect on other voiding parameters in BOO rats. These data suggest that endothelin-1 and endothelin ET(A) receptors might be involved in the generation of premicturition contractions in BOO rats, and that endothelin ET(A) receptor antagonists such as YM598 may have ameliorating effects in patients with bladder overactivity associated with BOO.

Pharmacology of the lower urinary tract: basis for current and future treatments of urinary incontinence

The lower urinary tract constitutes a functional unit controlled by a complex interplay between the central and peripheral nervous systems and local regulatory factors. In the adult, micturition is controlled by a spinobulbospinal reflex, which is under suprapontine control. Several central nervous system transmitters can modulate voiding, as well as, potentially, drugs affecting voiding; for example, noradrenaline, GABA, or dopamine receptors and mechanisms may be therapeutically useful. Peripherally, lower urinary tract function is dependent on the concerted action of the smooth and striated muscles of the urinary bladder, urethra, and periurethral region. Various neurotransmitters, including acetylcholine, noradrenaline, adenosine triphosphate, nitric oxide, and neuropeptides, have been implicated in this neural regulation. Muscarinic receptors mediate normal bladder contraction as well as at least the main part of contraction in the overactive bladder. Disorders of micturition can roughly be classified as disturbances of storage or disturbances of emptying. Failure to store urine may lead to various forms of incontinence, the main forms of which are urge and stress incontinence. The etiology and pathophysiology of these disorders remain incompletely known, which is reflected in the fact that current drug treatment includes a relatively small number of more or less well-documented alternatives. Antimuscarinics are the main-stay of pharmacological treatment of the overactive bladder syndrome, which is characterized by urgency, frequency, and urge incontinence. Accepted drug treatments of stress incontinence are currently scarce, but new alternatives are emerging. New targets for control of micturition are being defined, but further research is needed to advance the pharmacological treatment of micturition disorders.

Age-related changes in the properties of the endothelin receptor system at protein and mRNA levels in the rat vas deferens

As age-related changes occur in the properties of the endothelin (ET) receptor system in several mammalian tissues, and as there are significant amounts of functional ET receptors in the vas deferens, we investigated the age-related changes in the ET receptor system at the protein and mRNA levels in the rat vas deferens. The ET system was investigated in the vasa deferentia of 3 weeks, 3 months and 22 months old rats. ET receptors were characterized and quantified at the protein level by radioligand receptor binding, and gene transcript levels of ET-1, ET-3, ET converting enzyme-1 (ECE-1), and ET(A) and ET(B) receptor subtypes were quantified by real-time reverse transcription polymerase chain reaction (RT-PCR). The results of radioligand receptor binding assays demonstrate that there is a higher density of total ET receptors in the vas deferens of 3 weeks old rats than in 3 months and 22 months old rats, and that the predominant ET receptor is of the ET(A) subtype in all three ages. Real-time RT-PCR data show that the predominant mRNA expression of ETs and their receptors in all age groups studied are ET-1 and the ET(A) receptor subtype, respectively. Furthermore, ET-1, ET-3, ECE-1, and ET(A) and ET(B) receptor subtype mRNAs are expressed at higher levels in the 3 weeks old rats as compared with the other two age groups. These results demonstrate the presence of age-related changes in the properties of the ET receptor system at both protein and mRNA levels in the rat vas deferens.

Inward current oscillation underlying tonic contraction caused via ETA receptors in pig detrusor smooth muscle

Endothelin-1 (ET-1) is a powerful vasoconstricting peptide. Recent studies showed synthesis of ET-1 and the presence of ET receptors in urinary bladder smooth muscle cells. In the present study, we investigated the possible role of ET-1 in detrusor contraction and its underlying mechanisms in terms of electrical activity. ET-1 caused dose-dependent tonic contraction of bladder smooth muscle strips. Whole cell patch-clamp experiments revealed that ET-1 induced a single transient inward current in the majority of detrusor cells and that additional inward current oscillations were induced in one-third of the cells. The inward current oscillation and tonic contraction shared several characteristic features: 1) both activities lasted for a considerable time after ET-1 washout and 2) only prior application of ETA receptor antagonists, not ETB receptor antagonists, significantly suppressed ET-1-induced contractions and the oscillating inward currents. It was concluded that the inward current oscillation underlies ET-1-induced tonic contraction. Experiments with ion substitution and channel blockers suggested that periodic activation of Ca2+-activated Cl- channels caused the oscillating inward currents.

Role of endothelin ETA and ETB receptors in the guinea-pig urinary bladder contraction

The distribution and function of endothelin receptors in the guinea-pig urinary bladder were examined. Specific [125I]endothelin-1 binding sites with both the endothelin ET(A) and ET(B) receptor subtypes were distributed in the muscle layer. Endothelin-1 elicited a tonic contraction which was inhibited by cyclo(D-Asp-Pro-D-Val-Leu-D-Trp) (BQ123) but not by N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D-1-methoxycarbonyltryptophanyl-D-norleucine (BQ788) and which was inhibited more strongly by a combination of BQ123 and BQ788. Sarafotoxin S6c elicited a contraction which was abolished by BQ788. The concentration of endothelin-1 in the muscle layer was 707.0+/-67.5 pg/g wet weight. Thus, endothelin-1 may regulate muscle tone via both subtypes of endothelin receptors in an autocrine manner in the guinea-pig urinary bladder.

Developmental changes in the functional, biochemical and molecular properties of rat bladder endothelin receptors

The effect of aging on functional, biochemical, anatomical and molecular properties of endothelin (ET) receptors in bladder smooth muscle of the 3-week-, 3-month- and 22-month-old rats was examined using isolated muscle bath techniques, radioligand binding on membrane particulates and slide mounted tissue sections, and real-time reverse transcription polymerase chain reaction (RT-PCR). ET-1 induced significantly larger contractile responses in bladder dome muscle strips from 3-week- than from 3-month- and 22-month-old rats. The expression level of total ET receptors, determined by saturation binding experiments with [(125)I]ET-1, was higher in detrusor from 3-week- than 22-month-old rats. Inhibition studies with BQ123, a selective ET(A) receptor antagonist, indicated the predominance of the ET(A) receptor subtype and a similar proportion of ET(A) to ET(B) receptor subtypes in the rat detrusor at all ages studied. Autoradiographic data support the age-dependent decrease in the density of ET receptors and also indicate that the ET(A) receptor subtype is primarily located in the smooth muscle layer, whereas the ET(B) receptor subtype is located in both the urothelial and smooth muscle layers. Determined by real-time RT-PCR, ET 1, ET-3, ECE-1 and ET receptor subtype (ET(A) and ET(B)) mRNAs were shown to be higher in bladders of 3-week- compared to 3-month- or 22-month-old rats. This study indicates age-dependent alterations in the ET receptor system at both gene transcript and protein levels in the Fischer rat detrusor.

[Regulation by autonomic nerves of bladder neck sphincter function--mainly on inhibitory NANC nerves]

This article describes current information concerning analyses of contraction and relaxation associated with electrical stimulation of efferent nerves in isolated mammalian sphincter muscles. Contractile responses of sphincters are mediated by alpha 1-adrenoceptors and muscarinic receptors stimulated by transmitters from adrenergic and cholinergic nerves, respectively, whereas those of the bladder body are almost exclusively mediated by transmitters from parasympathetic nerves. Relaxant responses to nerve stimulation are ascribed mainly to mechanisms that are sensitive and resistant to nitric oxide (NO) synthase inhibitors. Neurogenic calcitonin gene-related peptide and beta-adrenoceptor activation by neurogenic norepinephrine may also be involved in some mammals. Stimulus frequency is an important determinant to distinguish NO synthase-sensitive and -resistant components; responses at low frequencies are abolished by the enzyme inhibitors, whereas those at high frequencies are inhibited only partially. High and low frequency stimulation increases the cyclic GMP content in muscles, suggesting the involvement of neurogenic NO, although relaxation at high frequencies is only partially due to such a mechanism. From pharmacological studies so far analyzed, including ours performed with porcine urinary tract sphincters, it is concluded that NO synthase resistant-relaxation is not mediated by peptides nor compounds that open K+ channels in muscle cell membrane and stimulate beta-adrenoceptors. Contribution of NO and non-NO relaxing factor (s) in relaxant responses varies with animal species. Identification of this factor, determination of intracellular signaling processes and interaction with the NO/cyclic GMP system may give us a clue in developing new therapeutics to treat dysfunctions of the lower urinary tract sphincters.

Developmental changes in the biochemical and functional properties of endothelin receptors in rabbit renal pelvis

OBJECTIVE

To examine the effect of age on the biochemical and functional properties, and regional distribution of endothelin (ET) receptors in the rabbit renal pelvis.

MATERIALS AND METHODS

The properties of ET receptors in 6-week-old and 6-month-old male rabbit renal pelves were examined using isolated muscle-bath and radioligand receptor-binding techniques.

RESULTS

ET-1 caused a significant increase in the contractile force in muscle strips from all regions of the renal pelvis from both age groups, with the following rank order: upper=middle>lower. The magnitude of the ET-1-induced contractile responses were similar in the lower pelvic regions in both ages, but the responses in the upper and middle regions were significantly greater in younger rabbits. ET-1 increased the frequency of spontaneous activity in a concentration-dependent manner in the upper and middle pelvic regions in both age groups, with significantly smaller ED50 values in the younger than in the older rabbits. In both age groups the lower pelvic region lacked spontaneous activity. The density of total ET receptors was higher in the upper and middle regions of the renal pelvis than in the lower renal pelvis of both ages, with the density in the upper and middle regions being greater in older than in younger rabbits. In all regions, ET subtype selective compounds inhibited [125I]ET-1, binding consistent with the predominance of the ETA receptor subtype, except in the lower region of the older rabbits, in which the densities of ETA and ETB subtypes were similar. In all regions, the younger renal pelvis contained a higher proportion of ETA receptors than in older tissues. Light microscopic autoradiographic data indicated the presence of ETA and ETB receptors in smooth muscle and epithelial cells, respectively.

CONCLUSION

These data indicate the presence of regional differences in the density of ET receptors and in the contractile responses to ET-1 in rabbit renal pelvis, and that although older rabbit renal pelvis contains more total ET receptors than younger renal pelvis, the latter had a higher portion of the ETA receptor subtype and the younger tissues were more responsive to ET-1.