Pharmacology of neurotransmission to the smooth muscle of the rat and the guinea-pig prostate glands

Aqueous extracts of Urtica dioica (stinging nettle) leaf contain a P2-purinoceptor antagonist-Implications for male fertility

Stinging nettle root and leaf extracts were tested for their effect on prostatic smooth muscle contractility. Root extract did not affect electrical field stimulation induced-nerve mediated contractions of isolated rat prostates. On the other hand, leaf extract attenuated electrical field stimulation-induced contractions at all frequencies. Similarly, contractions elicited by exogenous administration of ATP and αβ-methylene ATP were inhibited by leaf extract, whereas contractions elicited by exogenous administration of noradrenaline or acetylcholine were unaffected. The active component was present within the aqueous phase of the leaf extract. In mouse mating studies, stinging nettle leaf extract (50 mg p.o. daily) reduced male fertility by 53% compared to vehicle-treated male mice. Cardiovascular parameters were unaffected by administration of stinging nettle leaf extract (p ≥ 0.057). Treated mice exhibited normal mating behaviour. Bladder and testes weighed less in stinging nettle leaf extract treated mice. All other organs and total body weight were unaffected. It is concluded that stinging nettle leaf extract reduces contractility of genitourinary smooth muscle by acting as an antagonist at postjunctional P2X1-purinoceptors. These data indicates that blocking sperm transport through pharmacological blockade of P2X1-purinoceptors via oral administration is consistent with an effective and convenient biological strategy male contraception.

A mechanism linking perinatal 2,3,7,8 tetrachlorodibenzo-p-dioxin exposure to lower urinary tract dysfunction in adulthood

Benign prostatic hyperplasia/lower urinary tract dysfunction (LUTD) affects nearly all men. Symptoms typically present in the fifth or sixth decade and progressively worsen over the remainder of life. Here, we identify a surprising origin of this disease that traces back to the intrauterine environment of the developing male, challenging paradigms about when this disease process begins. We delivered a single dose of a widespread environmental contaminant present in the serum of most Americans [2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD), 1 µg/kg], and representative of a broader class of environmental contaminants, to pregnant mice and observed an increase in the abundance of a neurotrophic factor, artemin, in the developing mouse prostate. Artemin is required for noradrenergic axon recruitment across multiple tissues, and TCDD rapidly increases prostatic noradrenergic axon density in the male fetus. The hyperinnervation persists into adulthood, when it is coupled to autonomic hyperactivity of prostatic smooth muscle and abnormal urinary function, including increased urinary frequency. We offer new evidence that prostate neuroanatomical development is malleable and that intrauterine chemical exposures can permanently reprogram prostate neuromuscular function to cause male LUTD in adulthood.

Summary: We describe a new mechanism of benign prostate disease, initiated by fetal chemical exposure, which durably increases prostatic noradrenergic axon density and causes smooth muscle hyperactivity and urinary voiding dysfunction.

Generation and Regulation of Spontaneous Contractions in the Prostate

Spontaneous myogenic contractions have been shown to be significantly upregulated in prostate tissue collected from men with Benign Prostatic Hyperplasia (BPH), an extremely common disorder of the ageing male. Although originally thought likely to be involved in 'housekeeping' functions, mixing prostatic secretions to prevent stagnation, these spontaneous myogenic contractions provide a novel opportunity to understand and treat BPH. This treatment potential differs from previous models, which focused exclusively on attenuating nerve-mediated neurogenic contractions. Previous studies in the rodent prostate have provided an insight into the mechanisms underlying the regulation of myogenic contractions. 'Prostatic Interstitial Cells' (PICs) within the prostate appear to generate pacemaker potentials, which arise from the summation of number of spontaneous transient depolarisations triggered by the spontaneous release of Ca²⁺ from internal stores and the opening of Ca²⁺-activated Cl^- channels. Pacemaker potentials then conduct into neighbouring smooth muscle cells to generate spontaneous slow waves. These slow waves trigger the firing of 'spike-like' action potentials, Ca²⁺ entry and contraction, which are not attenuated by blockers of neurotransmission. However, these spontaneous prostatic contractions can be modulated by the autonomic nervous system. Here, we discuss the mechanisms underlying rodent and human prostate myogenic contractions and the actions of existing and novel pharmacotherapies for the treatment of BPH. Understanding the generation of human prostatic smooth muscle tone will confirm the mechanism of action of existing drugs, inform the identification and effectiveness of new pharmacotherapies, as well as predict patient outcomes.

Purinergic signalling in the reproductive system in health and disease

There are multiple roles for purinergic signalling in both male and female reproductive organs. ATP, released as a cotransmitter with noradrenaline from sympathetic nerves, contracts smooth muscle via P2X1 receptors in vas deferens, seminal vesicles, prostate and uterus, as well as in blood vessels. Male infertility occurs in P2X1 receptor knockout mice. Both short- and long-term trophic purinergic signalling occurs in reproductive organs. Purinergic signalling is involved in hormone secretion, penile erection, sperm motility and capacitation, and mucous production. Changes in purinoceptor expression occur in pathophysiological conditions, including pre-eclampsia, cancer and pain.

Tamsulosin modulates, but does not abolish the spontaneous activity in the guinea pig prostate gland

AIMS

To examine the effects of the α1A -adrenoceptor antagonist, tamsulosin, on spontaneous contractile and electrical activity in the guinea-pig prostate gland.

METHODS

The effects of tamsulosin (0.1 and 0.3 nM) were investigated in adult and ageing male guinea pig prostate glands using conventional tension recording and electrophysiological intracellular microelectrode recording techniques.

RESULTS

Tamsulosin reduced spontaneous activity, and had different age-dependent effects on adult and ageing guinea pigs at different concentrations. 0.1 nM tamsulosin caused a significantly greater reduction of spontaneous contractile and electrical activity in ageing guinea pigs in comparison to adult guinea pigs. In contrast, 0.3 nM tamsulosin had a significantly greater reduction of spontaneous contractile and electrical activity in adult guinea pigs in comparison to ageing guinea pigs.

CONCLUSIONS

This study demonstrates that tamsulosin can modulate spontaneous myogenic stromal contractility and the underlying spontaneous electrical activity; tamsulosin does not block spontaneous activity. This reduction in spontaneous activity suggests that downstream cellular mechanisms underlying smooth muscle tone are being targeted, and these may represent novel therapeutic targets to better treat benign prostatic hyperplasia.

Age-related changes in the innervation of the prostate gland: implications for prostate cancer initiation and progression

The adult prostate gland grows and develops under hormonal control while its physiological functions are controlled by the autonomic nervous system. The prostate gland receives sympathetic input via the hypogastric nerve and parasympathetic input via the pelvic nerve. In addition, the hypogastric and pelvic nerves also provide sensory inputs to the gland. This review provides a summary of the innervation of the adult prostate gland and describes the changes which occur with age and disease. Growth and development of the prostate gland is age dependent as is the occurrence of both benign prostate disease and prostate cancer. In parallel, the activity and influence of both the sympathetic and parasympathetic nervous system changes with age. The influence of the sympathetic nervous system on benign prostatic hyperplasia is well documented and this review considers the possibility of a link between changes in autonomic innervation and prostate cancer progression.

Novel drug targets for the pharmacotherapy of benign prostatic hyperplasia (BPH)

Benign prostatic hyperplasia (BPH) is the major cause of lower urinary tract symptoms in men aged 50 or older. Symptoms are not normally life threatening, but often drastically affect the quality of life. The number of men seeking treatment for BPH is expected to grow in the next few years as a result of the ageing male population. Estimates of annual pharmaceutical sales of BPH therapies range from $US 3 to 10 billion, yet this market is dominated by two drug classes. Current drugs are only effective in treating mild to moderate symptoms, yet despite this, no emerging contenders appear to be on the horizon. This is remarkable given the increasing number of patients with severe symptoms who are required to undergo invasive and unpleasant surgery. This review provides a brief background on prostate function and the pathophysiology of BPH, followed by a brief description of BPH epidemiology, the burden it places on society, and the current surgical and pharmaceutical therapies. The recent literature on emerging contenders to current therapies and novel drug targets is then reviewed, focusing on drug targets which are able to relax prostatic smooth muscle in a similar way to the α(1) -adrenoceptor antagonists, as this appears to be the most effective mechanism of action. Other mechanisms which may be of benefit are also discussed. It is concluded that recent basic research has revealed a number of novel drug targets such as muscarinic receptor or P2X-purinoceptor antagonists, which have the potential to produce more effective and safer drug treatments.

Contractility and pacemaker cells in the prostate gland

PURPOSE

We focused on the current opinion on mechanisms generating stromal tone in the prostate gland.

MATERIALS AND METHODS

We selected the guinea pig as the main species for investigation since its prostate has a high proportion of smooth muscle that undergoes age related changes similar in many respects to that in humans. The main techniques that we used were tension recording and electrophysiology.

RESULTS

We previously reported distinct electrical activity and cell types in the prostate, and speculated on their functional roles. We believe that a specialized group of c-kit immunoreactive prostatic interstitial cells that lie between glandular epithelium and smooth muscle stroma have a role similar to that of gastrointestinal interstitial cells of Cajal, generating the pacemaker signal that manifests as slow wave activity and triggers contraction in smooth muscle cells in guinea pig prostates.

CONCLUSIONS

Since changes in muscle tone are involved in the etiology of age dependent prostate specific conditions such as benign prostatic hyperplasia, knowledge of the electrical properties of the various prostatic cell types and their interactions with each other, with nerves and with the hormonal environment, and how these factors change with age is of considerable medical importance.

Ethanol extracts of saw palmetto contain the indirectly acting sympathomimetic: tyramine

BACKGROUND

To identify the bioactive components of saw palmetto ethanol extracts that affect contractility in the rat prostate gland.

METHODS

A commercially available saw palmetto ethanol extract was lyophilized then subjected to fractionation using silica gel column chromatography. Composition of fractions was assessed by proton nuclear magnetic resonance ((1)H NMR) spectroscopy and mass spectrometry (MS). Contractile activity of these fractions was evaluated pharmacologically using isolated preparations of rat prostate gland and compared to the activity of crude ethanol extract.

RESULTS

Saw palmetto ethanol extract caused contractions of the rat prostate gland which were consistent with indirectly acting sympathomimetic activity. Fractions resulting from chromatography produced contractions of isolated rat prostates that were similar in magnitude to the contractions produced by the crude extracts. Analysis of NMR and mass spectra revealed that this bioactivity was due to tyramine in the active fraction.

CONCLUSIONS

Tyramine is present in saw palmetto ethanol extracts and causes indirect α(1)-adrenoceptor mediated contractions via the release of noradrenaline from sympathetic neurons. This has clinical implications, as tyramine interacts with MAO inhibitors to cause hypertensive crisis.

The residual nonadrenergic contractile response to nerve stimulation of the mouse prostate is mediated by acetylcholine but not ATP in a comparison with the mouse vas deferens

Neuronal release of noradrenaline is primarily responsible for the contraction of prostatic smooth muscle in all species, and this forms the basis for the use of α(1)-adrenoceptor antagonists as pharmacotherapies for benign prostatic hyperplasia. Previous studies in mice have demonstrated that a residual nonadrenergic component to nerve stimulation remains after α(1)-adrenoceptor antagonism. In the guinea pig and rat prostate and the vas deferens of guinea pigs, rats, and mice, ATP is the mediator of this residual contraction. This study investigates the mediator of residual contraction in the mouse prostate. Whole prostates from wild-type, α(1A)-adrenoceptor, and P2X1-purinoceptor knockout mice were mounted in organ baths, and the isometric force that tissues developed in response to electrical field stimulation or exogenously applied agonists was recorded. Deletion of the P2X1 purinoceptor did not affect nerve-mediated contraction. Furthermore, the P2-purinoceptor antagonist suramin (30 μM) failed to attenuate nerve-mediated contractions in wild-type, α(1A)-adrenoceptor, or P2X1-purinoceptor knockout mice. Atropine (1 μM) attenuated contraction in prostates taken from wild-type mice. In the presence of prazosin (0.3 μM) or guanethidine (10 μM), or in prostates taken from α(1A)-adrenoceptor knockout mice, residual nerve-mediated contraction was abolished by atropine (1 μM), but not suramin (30 μM). Exogenously administered acetylcholine elicited reproducible concentration-dependent contractions of the mouse prostate that were atropine-sensitive (1 μM), but not prazosin-sensitive (0.3 μM). Acetylcholine, but not ATP, mediates the nonadrenergic component of contraction in the mouse prostate. This cholinergic component of prostatic contraction is mediated by activation of muscarinic receptors.

Inhibition of prostate efferent neurotransmission by amikacin

BACKGROUND

It has been suggested that manipulation of the autonomic nerve supply to the prostate leads to loss of functional and structural integrity of the gland, and that these changes may be useful in treating prostatic diseases. This study investigates the effect of amikacin on prostate efferent neurotransmission in vitro, in both rat and human prostate samples.

METHODS

Prostate samples, obtained from male Wistar rats and 8 patients undergoing endoscopic surgery for benign prostatic hyperplasia, were studied by measurement of isometric contraction induced by electrical field stimulation (EFS), noradrenalin, carbachol, serotonin and ATP, in the presence or absence of amikacin 10(-3) M in a low-Ca medium.

RESULTS

Amikacin significantly reduced EFS-induced contraction of isolated rat and human prostate samples by 45 +/- 6.5% (P < 0.01) and 47 +/- 6% (P < 0.01), respectively. Contraction was restored after addition of calcium chloride 2 x 10(-3) M. Amikacin-induced relaxation in rat prostate samples was greater than the effect of magnesium and weaker than those of prazosin and verapamil, but comparable in extent with the latter. No significant effect was observed on rat prostate contractile response to noradrenaline as to ATP in the presence of amikacin. Rat prostate contraction to carbachol and serotonin was inhibited by 35 +/- 11% (P < 0.05) and 59 +/- 11.7% (P < 0.01), respectively, after addition of amikacin 10(-3) M.

CONCLUSIONS

Amikacin reduces in vitro both rat and human prostate contraction elicited by pre-junctional stimulation, but does not affect prostate contraction evoked by post-junctional stimulation. Our results indicate that amikacin affects prostatic efferent neurotransmission.

Amikacin: A Novel Modulator of Vesical and Prostate Efferences. An in vitro Experimental Study

The autonomic efferent neurotransmission to the bladder and prostate smooth muscle is a potential target for drug therapy of specific low urinary tract disfunction (LUTD). Since amikacin and other amynoglicosides were reported to affect neurotransmission by a pre-junctional mechanism, we investigated the effect of amikacin on isolated rat and human detrusor smooth muscle contraction and on isolated rat and human prostate contraction, to further evaluate its potential relaxant properties.

Materials and Methods

Samples of detrusor smooth muscle and prostate tissue, obtained from 97 rats and 16 patients undergoing surgery, were studied through the measurement of isometric contraction induced by electrical field stimulation (EFS) and other pharmacological stimuli in the presence or absence of 1mM amikacin in a low-Ca medium.

Results

Amikacin 1 mM significantly reduced contraction of isolated rat and human detrusor muscle and prostate, achieved with pre-junctional stimulation, while no significant effect was observed on contraction induced by pharmacological post-junctional stimulators. EFS contraction inhibited by amikacin was restored after addition of calcium chloride. The amikacin effect was comparable to the effect of magnesium ions, which are known to exert a pre-junctional inhibition of neurotransmitter release.

Conclusions

Amikacin significantly inhibited rat and human detrusor and prostate contraction evoked by pre-junctional stimulation in vitro, suggesting a depressant effect on autonomic efferent neurotransmission. Further pharmacokinetics studies and researches on related compounds may hold potential for future development in the treatment of specific low urinary tract disfunction (LUTD).

K+ channel modulation of slow wave activity in the guinea-pig prostate

BACKGROUND AND PURPOSE

The aim of this study was to investigate the role of different K(+) channel populations and the inhibitory effect of various exogenously applied K(+) channel openers in the regulation of slow wave activity in the guinea-pig prostate.

EXPERIMENTAL APPROACH

Recordings of membrane potential were made using intracellular microelectrodes.

KEY RESULTS

Tetraethylammonium (TEA 300 micro M and 1 mM), iberiotoxin (150 nM) and 4-aminopyridine (4-AP 1 mM) increased the frequency of slow wave discharge. Apamin (1-200 nM) and glibenclamide (1 micro M) had no effect on slow wave activity. Lemakalim (1 micro M) and PCO-400 (1 micro M) abolished the slow waves, as did sodium nitroprusside (SNP 10 micro M) and calcitonin gene-related peptide (CGRP 100 nM). The inhibitory effect of these agents was independent of a significant change in membrane potential. In the presence of 4-AP (1 mM), TEA (1 mM) or glibenclamide (1 micro M) the inhibitory actions of SNP (10 micro M) were attenuated. The inhibitory actions of CGRP (100 nM) were also reversed by glibenclamide (1 micro M). In contrast, isoprenaline (1 micro M) did not alter the frequency of slow wave discharge.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that BK(Ca) and 4-AP-sensitive K(+) channels regulate the frequency of prostatic slow wave discharge. SNP and CGRP abolish slow waves in a hyperpolarisation-independent manner, partially via opening of K(ATP) channels. BK(Ca) and 4-AP-sensitive K(+) channels also play an important role in the SNP-induced inhibition of slow wave activity. The lack of membrane hyperpolarisation associated with the SNP- and CGRP-induced inhibition implies that the channels involved in this action are not predominantly located on the smooth muscle cells.

α1L-Adrenoceptors mediate contractions of the isolated mouse prostate

The subtype of alpha1-adrenoceptor mediating noradrenaline-induced contractile responses in isolated mouse prostate glands was investigated. Adrenoceptor agonists were able to produce concentration-dependent contractions with the following rank order of potency: adrenaline > or = noradrenaline > or = clonidine = phenylephrine > dopamine > or = isoprenaline. Concentration-response curves to noradrenaline of the prostatic smooth muscle were antagonised by prazosin, N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha, alpha-dimethyl-1H-indole-3-ethanamine (RS-17053), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), tamsulosin and yohimbine with mean antagonist affinity estimates (pA2 or apparent pKB) of 8.12+/-0.10, 6.56+/-0.11, 8.38+/-0.06, 10.14+/-0.19 and 7.38+/-1.36 respectively. Propranolol (1 microM) had no antagonist activity (P = 0.994, n = 6). Yohimbine (0.01, 0.1, 1 microM) had no antagonist activity in the presence of prazosin (0.1 microM) (P > or = 0.059). The results obtained indicate that alpha1-adrenoceptors mediate the contractile response in isolated preparations of the mouse prostate. Furthermore, the particular subtype of alpha1-adrenoceptor mediating the response to exogenously administered noradrenaline corresponds to the alpha1L-subtype, the same subtype as that which has been shown to mediate noradrenaline-induced contractile activity in the human prostate.

Spontaneous electrical activity in the prostate gland

The cellular mechanisms that underlie the initiation, maintenance and propagation of electrical activity in the prostate gland remain little understood. Intracellular microelectrode recordings have identified at least two distinct electrical waveforms: pacemaker potentials and slow wave activity. By analogy with the intestine, we have proposed that pacemaker activity arises from a morphologically distinct group of c-Kit positive interstitial cells that lie mainly between the glandular epithelium and smooth muscle layers. We speculate that pacemaker activity arising from the prostatic interstitial cells (PICs) is likely to propagate and initiate slow wave activity in the smooth muscle cells resulting in contraction of the stromal smooth muscle wall. While spontaneous electrical activity in the prostate gland is myogenic in origin, it is clear that nerve-mediated agents are able to modulate this activity. Excitatory agents such as histamine, phenylephrine and a raised potassium saline all increase slow wave discharge. In contrast, nitric oxide donors reduce or abolish the spontaneous electrical events. However, the cellular mechanisms underlying the action of various endogenously released agents remain to be elucidated.

The effect of histamine on field-stimulated contractions of the guinea-pig prostate

The aim of this study was to investigate the effect of histamine on field-stimulated contractions of the guinea-pig prostate using isolated preparations in organ bath experiments. The histamine receptor subtype involved in potentiating the twitch responses was characterised, and the presence of any post-synaptic effects was determined. In addition, the effects of histamine on nerve-stimulated contractions of the ventral and dorsal prostate as well as the coagulating gland were compared in preliminary experiments. Histamine (300 microM) approximately doubled the magnitude of the twitch contractions in the ventral and dorsal lobes of the prostate (nerve stimulation parameters were 10 Hz for 2 s every 50 s, 0.5 ms duration at supramaximal voltage). In the coagulating glands, histamine increased the contractions more than fourfold. Further experiments were performed only on the ventral glands. The H(1) receptor antagonist, mepyramine, exhibited an apparent competitive antagonism against the histamine-induced potentiations of the twitch responses (apparent pK(B) value = 9.21+/-0.17 (n=5). The H(2) receptor antagonist, ranitidine, produced a small, significant shift to the right, as did the time control. The H(3) receptor antagonist, thioperamide, had no significant effect on the concentration-response curve. The effects of histamine (10 microM) on exogenously applied acetylcholine (Ach), noradrenaline (NA) and adenosine 5'-triphosphate (ATP) were determined. In each case, the contractile responses were significantly potentiated: ATP (5.8-fold P<0.0001, n=8), NA (1.6-fold P<0.05, n=10) and Ach (2.1-fold P<0.0007, n=10). This is the first study that has shown the effects of histamine on field-stimulated contractions in the prostate of any species. It is concluded that histamine, acting on H(1) receptors, elicits a considerable potentiation of the nerve-stimulated twitch contractions in the guinea-pig prostate. One mechanism whereby histamine exerts its potentiating effect is a post-synaptic enhancement of the response to ATP in particular, but also to NA and Ach. It is suggested that H(1)-receptor and purinoceptor antagonists may have a role to play in the treatment of benign prostatic hyperplasia (BPH).

Muscarinic acetylcholine receptor subtypes in the rat seminal vesicle

The aim of the present study was to identify the muscarinic acetylcholine receptor (mAChR) mRNA subtypes in the rat seminal vesicle. Furthermore, the mAChR subtypes involved in the contraction of the seminal vesicle were also explored. Reverse transcriptase-polymerase chain reaction (PCR) was performed and five PCR products corresponding to M1-M5 mAChR mRNA subtypes were detected in this tissue. Functional pharmacological studies indicated that the rank order of mAChR antagonists in blocking the contractile effects of carbachol was p-fluoro-hexahydro-sila-difenidol (pF-HHSiD) >> tropicamide > methoctramine = pirenzepine. This antagonist profile indicates that M3 mAChR subtype is predominantly involved in the seminal vesicle contraction. Furthermore, immunohistochemical studies confirmed the presence of the M3 mAChR subtype in the smooth muscle layers. M2 mAChR subtype was also immunolocalized in smooth muscle cells and may be involved in the contraction of this tissue. The presence of M2 and M3 mAChR subtypes in the epithelial cells suggests that these receptors could be involved in the protein secretion. Taken together, the cholinergic neurotransmitter may be a factor controlling contractility and protein secretion in this tissue.

Saw palmetto is an indirectly acting sympathomimetic in the rat-isolated prostate gland

BACKGROUND

To investigate whether saw palmetto that inhibits alpha1-adrenoceptor binding in vitro affects contractility of the rat prostate gland.

METHODS

The effects of a commercially available saw palmetto extract were examined on the contractility of rat-isolated prostate glands. The extract was tested in the presence and absence of phentolamine, prazosin, yohimbine, propranolol, hexamethonium, cocaine, desipramine, nifedipine, guanethidine, atropine, and alpha,beta-methylene ATP to evaluate the mechanism of action. Isolated preparations of rat vas deferens and bladder were used for comparison.

RESULTS

Unexpectedly, saw palmetto extract caused contractions of the rat prostate gland that could be attenuated by prazosin, phentolamine, nifedipine, guanethidine, cocaine, and desipramine but not by any of the other pharmacological tools. Similar contractile effects were observed in rat-isolated vas deferens preparations but not in rat-isolated bladder preparations.

CONCLUSIONS

In the rat prostate gland, saw palmetto extract causes indirect alpha1-adrenoceptor-mediated contractions via the release of noradrenaline from sympathetic neurons.

Current models of human prostate contractility

1. The human prostate is a compact gland contributing to seminal fluid. With increasing age, most humans will develop benign prostatic hyperplasia, a condition of prostatic enlargement and contractility that leads to occlusion of the urethra. Over many years, investigators have used a variety of animal and cell culture models to elucidate some of the contractile and proliferative mechanisms that may be associated with the development of this condition. 2. This review briefly assesses the current state of knowledge of the mechanisms underlying human prostatic contractility and compares it with that of animal and cell culture models. It is not intended as a comprehensive methodological review, nor is it intended to indicate our preferences for either model. Our aim is to correlate findings from animal and cell culture models with the current understanding of human prostate contractility. 3. We hope that the present review will increase awareness of the suitability of the current models in developing our understanding of benign prostatic hyperplasia.

Adenosine 5'-triphosphate and noradrenaline are excitatory cotransmitters to the fibromuscular stroma of the guinea pig prostate gland

Immunohistochemical studies demonstrated abundant P2X(1)-receptor immunoreactivity colocalized with alpha-actin within the fibromuscular stroma of the guinea pig prostate. P2X(2)-, P2X(3)- and P2X(4)-receptor immunoreactivity was absent. alphabetamethylene Adenosine 5'-triphosphate (ATP) attenuated contractile responses to electrical field stimulation (50 V, 0.5 ms, 5-20 Hz) in the absence and presence of prazosin (0.3 microM). Responses to 1-2 Hz were unaffected. ARL 67156 (6-N, N-diethyl-beta-gamma-dibromomethylene-D-adenosine-5-triphosphate; 100 microM) enhanced contractile responses to electrical field stimulation (50 V, 0.5 ms, 10-20 Hz). Concentration-response curves to exogenously applied ATP analogues on unstimulated preparations elicited concentration-dependent suramin (100 microM)-sensitive contractions. The rank order of potency was: alphabetamethylene ATP>2methylthio ATP=betagammamethylene ATP>adenosine 5'-diphosphate (ADP)=ATP. Adenosine and adenosine 5'-monophosphate (AMP) did not produce contractile responses. These results demonstrate the presence of functional P2X(1)-receptors within the fibromuscular stroma of the guinea pig prostate and suggest a cotransmitter role for ATP with noradrenaline during high-frequency stimulation.

Evaluation of the mouse prostate as a suitable model for the study of human prostate function

INTRODUCTION

Research into prostate development and function is mainly carried out in rats and guinea pigs. While these animals have proven to be good models of human prostate function, their use is limited when compared with what could be achieved using the various currently available gene knockout mice. This study aimed to ascertain whether the mouse prostate was a viable model for studying human prostate function.

METHODS

Sections from mouse prostate glands were histochemically processed to visualise the neurotransmitters and receptors present. Isolated organ bath studies were conducted in Krebs-Henseleit solution at 37 degrees C to delineate the physiological mechanisms involved in contractility.

RESULTS

Positive histochemical staining for noradrenaline and acetylcholinesterase (AChE) was observed in the fibromuscular stroma. AChE-positive staining was also observed in epithelial cells lining prostatic acini. Immunoreactivity to P2X(1) and P2X(7) purinoceptors was observed in the fibromuscular stroma and immunoreactivity to P2X(4) purinoceptors in the glandular epithelium. Positive immunostaining for neuropeptide Y, big endothelin, calcitonin gene-related peptide (CGRP), substance P, and neurokinin A was observed in the fibromuscular stroma. Frequency-response curves (1.0 ms pulse duration, 60 V, 0.1-20 Hz) to electrical field stimulation yielded frequency-dependent contractions that were attenuated by tetrodotoxin (P < .001), guanethidine (P < .001), and prazosin (P = .01). Suramin (P = .21), alpha,beta-methylene ATP (P = .84), and atropine (P = .76) caused no significant effects. Concentration-response curves to endogenously administered phenylephrine yielded concentration-dependent contractions (pEC(50) = 6.1 +/- 0.3, maximum response 0.20 +/- 0.03 g), which were attenuated by prazosin (P < .001).

DISCUSSION

Histochemistry suggests that mouse prostates have a similar innervation to that of humans and other laboratory animals. Furthermore, responses to nerve stimulation are noradrenergic and mediated by alpha(1)-adrenoceptors. Therefore, the mouse prostate is a suitable model for human prostate function and a viable isolated preparation for contractility studies.

Anatomy of the pelvic plexus and innervation of the prostate gland

We have examined the anatomy of the pelvic (inferior hypogastric) plexus in six male cadavers, paying particular attention to gross anatomical landmarks that might aid in locating it and have used immunohistochemistry to study the small branches of the plexus that supply the prostate gland. The pelvic plexus was found two finger breadths lateral to the third anterior sacral foramina, lying deep to a line drawn from third sacral vertebra, the conventional level of the recto-sigmoid junction, and the palpable posterior superior surface of the pubic symphysis. Immunohistochemical staining showed small nerve branches from the pelvic plexus entering the prostate gland and the presence of ganglia within the prostate gland that contained both tyrosine hydroxylase positive and negative neuronal cell bodies. This information may be useful in nerve-sparing surgical procedures and in discussions of the functional implications of perturbations of prostate innervation.

Adenosine 5'-triphosphate (ATP) is an excitatory cotransmitter with noradrenaline to the smooth muscle of the rat prostate gland

1. This study investigated whether adenosine 5'-triphosphate (ATP) is involved in neurotransmission to the rat prostate gland. 2. Fluorescence immunohistochemistry carried out on formaldehyde-fixed and frozen sections of rat prostate showed immunoreactivity for the P2X(1)-receptor in the fibromuscular stroma surrounding the secretory acini but not in the glandular epithelium. P2X(2)-, P2X(3)-, P2X(4)- and P2X(7)-receptors were immunonegative in the rat prostate stroma. Double-staining procedures showed P2X(1)-receptor immunoreactivity to be colocalized with alpha-actin immunoreactivity. 3. Isolated organ bath studies investigated whether drugs, which modify purinergic mechanisms, are able to affect contractility of the rat prostate gland. Suramin (100 micro M) and alphabetamethylene ATP (10 micro M) inhibited contractile responses to trains of electrical-field stimulation (70 V, 0.5 ms, 0.1-2 Hz) in the absence and presence of prazosin (300 nM). Responses to 5-20 Hz were unaffected by suramin or alphabetamethylene ATP. 4. Exogenous application of ATP analogues to unstimulated isolated preparations of rat prostate produced concentration-dependent suramin (100 micro M) sensitive transient contractions with a relative order of potency: alphabetamethylene ATP>betagammamethylene ATP>ATP. 5. Adenosine and adenosine 5'-monophosphate (AMP) did not produce contractile responses. 6. These results suggest that P2X(1)-receptors for ATP, which mediate contractions are present in the fibromuscular stroma of the rat prostate. The relative order of potency of ATP analogues in producing contractions of the rat prostate is consistent with the activation of P2X(1)-receptors. Inhibition by suramin and alphabetamethylene ATP of electrically evoked nerve-mediated contractions of the rat prostate implies that ATP contributes to this contractile response and is therefore a cotransmitter with noradrenaline during low-frequency stimulation.

Method for simultaneous recording of the prostatic contractile and urethral pressure responses in anesthetized rats and the effects of tamsulosin

We simultaneously recorded the prostatic contractile and urethral pressure responses to electrical stimulation (ES) of the hypogastric nerves (HGNs) or phenylephrine in anesthetized rats and studied the effects of tamsulosin on these responses. At 0.01 and 0.1 mg/kg, i.v., tamsulosin inhibited the prostatic responses to ES of the HGNs in a dose-dependent manner, while at 1 microg/kg, i.v., it reduced the response to phenylephrine (0.01 mg/kg, i.v.) to about 26% of the nonantagonized level. These inhibitory effects on prostatic responses were maintained for 60 min. Tamsulosin exerted an inhibitory effect on the urethral response to ES of the HGNs at 0.01 mg/kg, i.v. but not at 0.1 mg/kg, i.v. At 1 microg/kg, i.v., tamsulosin also reduced the urethral response to phenylephrine to about 46% of the nonantagonized level; this effect was maintained for 60 min. Furthermore, tamsulosin was found to exert a stronger inhibitory effect on the prostatic response than on the urethral response induced by sympathetic nerve activation. Our findings suggest that rat urethral sympathetic nerve terminals may contain prejunctional alpha1 adrenoceptors that modulate the release of norepinephrine.

Cholinergic innervation and function in the prostate gland

The mammalian prostate is densely innervated by hypogastric and pelvic nerves that play an important role in regulating the growth and function of the gland. While there has been much interest in the role of the noradrenergic innervation and adrenoceptors in prostate function, the role of cholinergic neurones in prostate physiology and pathophysiology is not well understood. This review focuses on the role of acetylcholine and cholinoceptors in prostate function. Nitric oxide, vasoactive intestinal polypeptide, and/or neuropeptide Y are co-localised with cholinesterase and/or acetylcholine transporter in some of the nerve fibres supplying the prostate. Their roles are also briefly discussed in this review. A dense network of cholinesterase-staining fibres supplies both prostate epithelium and stroma, suggesting a role of acetylcholine and/or co-localised neuropeptides in the modulation of prostatic secretions, as well as smooth muscle tone. A predominantly epithelial location for prostate muscarinic receptors indicated a major secretomotor role for acetylcholine. The muscarinic receptor subtype mediating muscarinic agonist-induced smooth muscle contraction or enhancement of contractions evoked by nerve stimulation differs in different species. In the human, there is evidence for M(1) receptors on the epithelium, M(2) receptors on the stroma, and both M(1) and M(3) receptors in some prostate cancer cell lines. Several recent investigations indicate that muscarinic receptors may also mediate or modulate normal, benign, and malignant prostate growth. The role of muscarinic agonists and their receptors and the influences of age, testicular, and other steroids in regulating the effects are reviewed.

Big endothelin-1 but not endothelin-1 is present in the smooth muscle stroma of the prostate gland of the rat

Immunohistochemical techniques were employed to localize the presence of endothelins in the mature rat prostate gland. Immunoreactivity for big endothelin-1 but not endothelin-1 was observed in the fibromuscular stroma of the rat prostate gland. No immunoreactivity was seen in the glandular epithelium. Double staining procedures showed big endothelin-1 immunoreactivity to be co-localized with alpha-actin immunoreactivity. The stroma of the prostate gland also contained nerve fibres coursing through it which are immunopositive for tyrosine hydroxylase. These results suggest that big endothelin-1 but not endothelin-1 is co-localized with alpha-actin in the smooth muscle cells of the rat prostate gland. This implies that endothelin-1 is synthesized on demand from big endothelin-1 in the fibromuscular stroma of the rat prostate.

Protein kinase C and the sub-sensitivity and sub-reactivity of the diabetic rat prostate gland to noradrenaline

Concentration-response curves to noradrenaline (1 nM-100 microM) were obtained in prostates from 6-week streptozotocin diabetic, insulin-treated diabetic or control rats. Compared to the curve obtained in controls, those obtained in prostates from diabetic and insulin-treated diabetic rats were shifted rightward. The alpha1-adrenoceptor antagonist, prazosin (100 nM), caused a rightward shift of the curves in prostates from all groups. In contrast, the uptake 1 inhibitor, nisoxetine (300 nM), only produced a leftward shift of the curves in prostates from control and insulin-treated diabetic rats. However, frequency-response curves obtained in prostates from both control and diabetic rats were shifted leftward by nisoxetine (300 nM). The concentration-response curve to the alpha1-adrenoceptor agonist, methoxamine (10 nM-100 microM), obtained in prostates from diabetic rats was shifted rightward compared with controls. Calphostin C (500 nM), a protein kinase C inhibitor, caused a leftward shift of the curve in prostates from diabetic, but not control, rats. The protein kinase C inhibitor, bisindolylmaleimide I (500 nM), beta-adrenoceptor antagonist, propranolol (500 nM) and muscarinic cholinoceptor antagonist, atropine (300 nM), had no effect on the noradrenaline concentration-response curves of prostates from control or diabetic rats. Our results suggest that diabetes reduces the sensitivity and reactivity of the prostate to noradrenaline-induced stimulation, and this reduction may be due to changes in protein kinase C activity.

Beta2-adrenoceptor-mediated inhibition of field stimulation induced contractile responses of the smooth muscle of the rat prostate gland

Isolated preparations of rat prostate responded to electrical field stimulation (2 strains every 60 s, 0.5 ms, 10 Hz, 80 V) with contractions. The adrenoceptor agonists adrenaline, isoprenaline and noradrenaline (0.1 nM(-10) x microM) elicited concentration-dependent inhibition of electrical field stimulation-induced contractions of the rat prostate. Phenylephrine had no effect on the amplitude of electrical field stimulation-induced contractions. The rank order of potency was isoprenaline> or =adrenaline=noradrenaline>phenylephrine. Inhibition of electrical field stimulation-induced contractions by isoprenaline was attenuated by propranolol (1 microM). The selective beta1-adrenoceptor agonist (-)-1-(3,4-dimethoxy-phenethylamino)-3-(3,4-dihydroxyphenoxy)-2-propanol)oxalate (RO363) and the selective beta2-adrenoceptor agonist salbutamol (1 nM(-100) x microM) were approximately equipotent in inhibiting electrical field stimulation-induced contractions but the selective beta3-adrenoceptor agonist sodium 4-(2-[2-hydroxy-[3-chlorophenyl]ethylamino]propyl)phenoxyacetate (BRL 37344, 1 nM(-100) x microM) did not inhibit electrical field stimulation-induced contractions. The selective beta2-adrenoceptor antagonist, (+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol (ICI 118 551, 0.1 microM) attenuated inhibitory responses to isoprenaline and salbutamol, while the selective beta1-adrenoceptor antagonist atenolol (3 microM) did not. Contractions induced by electrical field stimulation were also inhibited by forskolin (10 nM(-3) x mM) but unaffected by sodium nitroprusside (10 nM(-1) x mM) indicating the presence of an inhibitory cAMP mechanism. These data suggest that stimulation of beta2-adrenoceptors can inhibit contractions of the rat prostate induced by electrical field stimulation.

Effects of adenine nucleosides and nucleotides on neuromuscular transmission to the prostatic stroma of the rat

1. The aim of this study was to investigate the effects of adenine nucleosides and nucleotides on contractility of the smooth muscle of rat prostate gland. 2. Nerve terminals within rat isolated prostatic tissues were electrically field stimulated (60 V, 0.5 ms, 10 Hz, 20 pulses every 60 s). Adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP) and adenosine had no effect on baseline smooth muscle tone but concentration-dependently inhibited electrically-evoked contractile responses. The relative order of potency was ATP congruent with AMP congruent with adenosine>ADP. 3. The inhibition by ATP and adenosine of field stimulation-induced contractions in the rat prostate was antagonized by 8-phenyltheophylline (10 microM), but not by suramin (100 microM) and only slightly by reactive blue 2 (5 microM). 4. The adenosine metabolizing enzyme adenosine deaminase (0.1 unit ml(-1)) inhibited the inhibitory effects of ATP and adenosine. The P2 purinoceptor agonist 2-methylthio ATP (10 nM - 0.1 mM), had no effect on field stimulation-induced contractions of the rat prostate. 5. ATP and adenosine did not modify the contractile responses of the rat prostate to exogenously added noradrenaline (10 microM). 6. Inhibitory concentration-response curves to a number of adenosine analogues with differing stabilities and selectivities for the different adenosine receptors yielded a relative rank order of agonist potency of: N(6)-cyclopentyladenosine (CPA)>N(6)-cyclohexyladenosine (CHA) congruent with (-)-N(6)-(2-phenylisopropyl)-adenosine (R-PIA) congruent with 5'-(N-ethylcarboxamido)-adenosine (NECA)>(+)-N(6)-(2-phenylisopropyl)-adenosine (S-PIA)>2-p-[2-carboxyethyl]phenethyl-amino-5'-N-ethylcarboxamido-ade nosine (CGS 21680). 7. These results indicate that adenine nucleoside and nucleotide induced inhibition of electrically-evoked contractions in the rat prostate occurs through activation of adenosine but not ATP receptors. The relative order of potency of adenosine analogues is consistent with activation of receptors of the A(1)-adenosine receptor subtype. These receptors appear to be prejunctional.

The contractile effects of endothelins on the smooth muscle of the rat prostate gland

Endothelin-1 elicited tonic contractions of rat prostatic smooth muscle that were unaffected by prazosin (0.5 microM), tetrodotoxin (1 microM) or guanethidine (10 microM). The rank order of potency of the endothelin isopeptides was endothelin-1>endothelin-2> or =endothelin-3. Sarafotoxin S6B was approximately equipotent with endothelin-1 in eliciting tonic contractions, but neither of the selective endothelin ET(B) receptor-agonists, sarafotoxin S6C (0.1 nM-0.3 microM) and BQ3020 (Ac-[Ala (11,15)]endothelin-1(6-21); 0.1 nM-0.3 microM), affected prostatic smooth muscle tone. The selective endothelin ET(A) receptor antagonist, BQ123 (cyclo(D-Asp-L-Pro-D-Val-L-Leu-D-Trp; 1 microM), attenuated responses to endothelin-1, -2 and -3, while the non-selective endothelin receptor antagonist bosentan (1 microM) and the selective endothelin ET(B) receptor antagonist BQ788, (Dmpc-gamma-MeLeu(9)-D-Trp(l-CO(2)CH(3))-D-Nle-OH; 1 microM) attenuated responses to endothelin-3 only. Contractions induced by exogenous administration of noradrenaline were unaffected by preincubation of tissues in BQ123 (1 microM) indicating the selectivity of this antagonist. These data suggest that endothelins mediate contractions of the rat prostate by action at endothelin ET(A) receptors.

The autonomic and sensory innervation of the smooth muscle of the prostate gland: a review of pharmacological and histological studies

1. We review literature demonstrating (a) the presence and (b) the actions of substances that mediate or modify neuroeffector transmission to the smooth muscle of the prostrate stroma of a number of species including man. 2. In all species studied prostatic stroma, but not secretory acini, receives rich noradrenergic innervation. Stimulation of these nerves causes contractions of prostate smooth muscle that are inhibited by guanethidine and by alpha1-adrenoceptor antagonists that probably act at the alpha1L-adrenoceptor. Such actions underlie the clinical use of alpha1-adrenoceptor antagonists in benign prostatic hyperplasia (BPH). 3. Acetylcholinesterase-positive nerves innervate prostatic stroma as well as epithelium. Atropine reduces nerve-mediated contractions of stromal muscle in the rat, guinea-pig and rabbit. M1, M2 and M3 muscarinic receptors have been implicated in eliciting or facilitating contraction in the prostate from guinea-pig, dog and rat, respectively. 4. Adenine nucleotides and nucleosides, nitric oxide (NO), opioids, neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) may act as co-transmitters or modulators in autonomic effector nerves supplying prostate stroma. Adenosine inhibits neurotransmission to the rat prostate, and NO is inhibitory in prostate from human, rat, rabbit, pig and dog. The activity of peptides present in the relatively sparse sensory innervation of the prostate exhibits species variation, but, when effective, calcitonin gene-related peptide is inhibitory while tachykinins are stimulant. The roles of NPY and VIP in modulating stromal contractility remain unclear. 5. Taken together the current literature indicates that, in addition to noradrenaline, other neurotransmitters and neuromodulators may regulate the tone of prostatic smooth muscle. Whether drugs that mimic or modify their actions might be useful in providing symptomatic relief of the urinary symptoms associated with BPH remains to be established.

Calcitonin gene-related peptide (CGRP) inhibits contractions of the prostatic stroma of the rat but not the guinea-pig

This study investigated the presence and effects of calcitonin gene-related peptide (CGRP) within the rat and guinea-pig prostate glands. Immunohistochemical studies demonstrated that CGRP immunoreactive nerve fibres are sparsely distributed throughout the prostatic fibromuscular stroma in both species. These CGRP immunopositive nerve fibres shared a similar distribution profile but were not colocalized with tyrosine hydroxylase immunopositive nerve fibres which also innervate the prostatic stroma of these species. Nerve terminals within rat and guinea-pig prostatic tissues were electrically field stimulated (60 V, 0.5 ms, 10 Hz, 20 pulses every 60 s). In guinea-pig preparations, application of human alpha-CGRP, rat adrenomedullin or rat amylin (0.1 nM-1 microM) had no effect on responses to field stimulation. In contrast, both rat and human alpha-CGRP (10 pM-300 nM), rat adrenomedullin (0.3 nM-1 microM) and rat amylin (3 nM-1 microM) concentration-dependently inhibited electrically evoked contractile responses in the rat prostate. The relative order of potency was rat alpha-CGRP=human alpha-CGRP>rat adrenomedullin>rat amylin. The inhibition by rat alpha-CGRP of field stimulation-induced contractions in the rat prostate was competitively antagonized by human CGRP((8-37)) (1, 3 and 10 microM) with a pA(2) of 6.20+/-0.13. Rat alpha-CGRP (10 nM) attenuated contractile responses of the rat prostate to exogenously added noradrenaline (1-100 microM). Inhibitory concentration-response curves to rat alpha-CGRP in rat prostates were unaffected by preincubation in either glibenclamide (10-100 microM), N-nitro-L-arginine methyl ester (L-NAME) (10 microM), bestatin (10 microM), captopril (10 microM) or phosphoramidon (3 microM). Our results indicate that CGRP-induced inhibition of electrically evoked contractions in the rat prostate occurs through activation of postjunctional CGRP(2) receptors which act independently of a K(ATP) channel or nitrergic mechanisms. Degradation of rat alpha-CGRP via peptidases does not appear to occur in the rat prostate.

Cholinergic facilitation of neurotransmission to the smooth muscle of the guinea-pig prostate gland

1. Functional experiments have been conducted to assess the effects of acetylcholine and carbachol, and the receptors on which they act to facilitate neurotransmission to the stromal smooth muscle of the prostate gland of the guinea-pig. 2. Acetylcholine and carbachol (0.1 microM - 0.1 mM) enhanced contractions evoked by trains of electrical field stimulation (20 pulses of 0.5 ms at 10 Hz every 50 s with a dial setting of 60 V) of nerve terminals within the guinea-pig isolated prostate. In these concentrations they had negligible effects on prostatic smooth muscle tone. 3. The facilitatory effects of acetylcholine, but not those of carbachol, were further enhanced in the presence of physostigmine (10 microM). 3. The facilitatory effects of carbachol were unaffected by the neuropeptide Y Y(1) receptor antagonist BIBP 3226 ((R)-N(2)-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]-arginina mide) (0.3 microM, n=3) or suramin (100 microM, n=5). Prazosin (0.1 microM, n=5) and guanethidine (10 microM, n=5) alone and in combination (n=4), reduced responses to field stimulation and produced rightward shifts of the log concentration-response curves to carbachol. 4. The rank orders of potency of subtype-preferring muscarinic receptor antagonists in inhibiting the facilitatory actions of acetylcholine and carbachol were: pirenzepine > HHSiD (hexahydrosiladifenidol) > pF-HHSiD (para-fluoro-hexahydrosiladifenidol)>/= 5 himbacine, and pirenzepine > HHSiD > himbacine>/= 5 pF-HHSiD, respectively. These profiles suggest that muscarinic receptors of the M(1)-subtype mediate the facilitatory effects of acetylcholine and carbachol on neurotransmission to the smooth muscle of the guinea-pig prostate.

alpha(1L)-adrenoceptors mediate noradrenaline-induced contractions of the guinea-pig prostate stroma

The alpha(1)-adrenoceptor subtype mediating noradrenaline-induced contractions of the guinea-pig isolated prostatic smooth muscle was investigated. Noradrenaline produced concentration-dependent contractions of the tissue with a mean pD(2) value of 5.26+/-0.03 (n=20). These contractions were antagonised by prazosin, 2-(2, 6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride (WB-4101), N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha, alpha-dimethyl-1H-indole-3-ethanamine hydrochloride (RS-17053) and (R)-5-[2-[[2-(2-ethoxyphenoxy)ethyl]amino]-2-methylethyl]-2- methoxybenzensulfonamide methanesulphonate hydrate (tamsulosin). Mean pA(2) or apparent pK(B) estimates for the antagonism of noradrenaline were 8.15+/-0.05 for prazosin; 8.83+/-0.11 for WB-4101, 7.18+/-0.14 for RS-17053 and 10.11+/-0.12 for tamsulosin. The relatively low estimates of the apparent dissociation constant for all antagonists except tamsulosin indicate that an alpha(1L)-adrenoceptor mediates noradrenaline-induced prostatic smooth muscle contraction in the guinea-pig prostate gland.

An immunohistochemical and pharmacological study of tachykinins in the rat and guinea-pig prostate glands

This study investigated the presence and effects of tachykinin peptides within the rat and guinea-pig prostate glands. Immunohistochemical studies demonstrated the presence of substance P and neurokinin A immunoreactive nerve fibres, sparsely distributed throughout the prostatic fibromuscular stroma in both species. In organ bath experiments, nerve terminals within rat and guinea-pig prostatic tissues were electrically field stimulated (60 V, 0.5 ms, 10 Hz, 20 pulses every 50 s). In rat preparations, the exogenous application of substance P, neurokinin A and the tachykinin NK3 receptor agonist senktide (1 nM-1 microM) had no effect on contractile responses. In contrast, substance P and neurokinin A (1 nM-3 microM) concentration-dependently enhanced electrically-evoked contractile responses in the guinea-pig prostate. Senktide was without effect. The potentiation of electrical field stimulation-induced contractions by substance P and neurokinin A in the guinea-pig prostate was competitively antagonized by ((S)1-[2-[3-(3,4-dichlorophenyl)-1-(3-isopropoxyphenylacetyl) piperidin-3-yl]ethyl]-4-phenyl-1-azonia-bicyclo[2.2.2]octane , chloride) (SR 140333) at 10 nM, a tachykinin NK1 receptor antagonist. The tachykinin NK2 receptor antagonist (S)-N-methyl-N[4-(4-acetylamino-4-phenylpiperidino)-2-(3,4-dichloropheny l)butyl]benzamide (SR 48968) was without effect at 10 nM, suggesting that neuromodulation of electrically-evoked contractions in the guinea-pig prostate occurs through activation of a tachykinin NK1 receptor subtype.

Pharmacological characterization of endothelin receptor subtypes in the guinea-pig prostate gland

Experiments have been conducted to investigate the actions of endothelins on the guinea-pig prostate gland. Saturation experiments with [125I]-endothelin-1 (2-800 pM) in guinea-pig prostatic homogenates indicated the presence of high affinity binding sites with an equilibrium dissociation constant (KD) of 230+/-50 pM, a maximum number of binding sites (Bmax) of 52+/-16 fmol mg(-1) protein or 269+/-61 fmol g(-1) tissue and a Hill coefficient (nH) of 1.01+/-0.03 (n = 3). Competition experiments revealed that binding of [125I]-endothelin-1 (20 pM) was inhibited with the following order of potency: endothelin-1 >>BQ-788 (N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methyl-Leu-D-Trp[1-+ ++CO2CH3-D-Nle-ONa])> BQ-123 (cyclo-D-Asp-L-Pro-D-Val-Leu-D-Trp) > or = sarafotoxin S6c. At concentrations with negligible influence on smooth muscle tone, endothelin-1, endothelin-2 and sarafotoxin S6b (1 nM-0.1 microM) produced concentration-dependent potentiation of the contractions evoked by electrical field stimulation with trains of 20 pulses at 10 Hz every 50 s, 0.5 ms pulse width and a dial setting of 60 V. In contrast, the endothelin ET(B) receptor-preferring agonist endothelin-3 (1 nM- 1 microM) was much less potent, and the endothelin ET(B) receptor-selective agonists sarafotoxin S6c and BQ-3020 (Ac-[Ala11,15]-endothelin-1 (6-21)), up to 1 microM, were without effect. The endothelin ET(A) receptor antagonist BQ-123 (1 microM) markedly inhibited the potentiation induced by endothelin-1, endothelin-2 and sarafotoxin S6b while the endothelin ET(B) receptor antagonist BQ-788 (1 microM) was less effective. While our binding data indicates the presence of ET(A) and ET(B) binding sites in the guinea-pig prostate, the endothelin-induced facilitation of neurotransmission to the prostatic smooth muscle is mediated largely via activation of endothelin receptors of the ET(A) subtype.