Alpha 2 adrenergic receptors in hyperplastic human prostate: identification and characterization using [3H] rauwolscine

A prospective pilotstudy comparing the anesthetic effects of an alpha-2 agonist during holmium laser resection of the prostate and transurethral resection for prostate surgery for benign prostatic hyperplasia patients using selective alpha-1 blockers

BACKGROUND

To examine the response to an α2receptor agonist used as a sedative for patients using long-term selective α1 blockers.

METHODS

Sixty-nine consecutive patients undergoing transurethral prostate resection or holmium laser resection of the prostateunder spinal anesthesia were divided into two groups; group N (n = 37), which did not receive α1 blockers, and group T (n = 32), which was administered tamsulosin for at least 1 month before the study. Bispectral index scores, Modified Observer's Assessment of Alertness/Sedation scale scores, heart rate, and mean blood pressure were obtained under sedation using dexmedetomidine for 30 min during surgery.

RESULTS

The only significant difference found between the groups were mean bloodpressure 15 min after the first loading dose injection of dexmedetomidine. Differencesbetween both groupswere noted at 15 min(group T: 100.2 ± 12.9 mmHg; group N: 90.0 ± 17.5 mmHg; P = 0.08), 20 min (group T: 99.8 ± 12.3 mmHg; group N: 87.4 ± 15.0 mmHg; P < 0.00), 25 min (group T: 99.3 ± 13.4 mmHg; group N: 85.4 ± 13.8 mmHg; P < 0.00), and 30 min (group T: 98.8 ± 13.1 mmHg; group N: 84.5 ± 13.5 mmHg; P < 0.00).

CONCLUSIONS

The use of α2 agonists is appropriate during surgery for benign prostatic hyperplasia patients using tamsulosin, and there is no need to alter the dose. Alertness with anesthesia involving α2 agents was maintained for patients using long-term tamsulosin and patients who did not use tamsulosin.

TRIAL REGISTRATION

The study was retrospectively registered with the Clinical Research Informational Service ( KCT0002967 , July 2, 2018).

Lower urinary tract symptoms in men

Benign prostatic hyperplasia (BPH) is a highly prevalent and costly condition that affects older men worldwide. Many affected men develop lower urinary tract symptoms, which can have a negative impact on their quality of life. In the past, transurethral resection of the prostate (TURP) was the mainstay of treatment. However, several efficacious drug treatments have been developed, which have transformed BPH from an acute surgical entity to a chronic medical condition. Specifically, multiple clinical trials have shown that α adrenoceptor antagonists can significantly ameliorate lower urinary tract symptoms. Moreover, 5α reductase inhibitors, alone or combined with an α adrenoceptor antagonist, can reverse the natural course of BPH, reducing the risk of urinary retention and the need for surgical intervention. Newer medical regimens including the use of antimuscarinic agents or phosphodiesterase type 5 inhibitors, have shown promise in men with predominantly storage symptoms and concomitant erectile dysfunction, respectively. For men who do not adequately respond to conservative measures or pharmacotherapy, minimally invasive surgical techniques (such as transurethral needle ablation, microwave thermotherapy, and prostatic urethral lift) may be of benefit, although they lack the durability of TURP. A variety of laser procedures have also been introduced, whose improved hemostatic properties abrogate many of the complications associated with traditional surgery.

α-Blockers for benign prostatic hyperplasia: the new era

PURPOSE OF REVIEW

α1-Adrenoceptor blockers are the most frequently prescribed medical therapy in the treatment of lower urinary tract symptom suggestive of benign prostatic hyperplasia (LUTS/BPH). The purpose of this review is to highlight the evolution of adrenoceptor blockers with emphasis on newly approved drugs.

RECENT FINDINGS

Over the past years new formulations of several α1-adrenoceptor blockers were introduced to the market. Five long-acting α1-blockers are currently approved by the Food and Drug Administration for treatment of symptomatic LUTS/BPH: terazosin, doxazosin, tamsulosin, alfuzosin and silodosin. Silodosin is the only adrenoceptor blocker that exhibits true selectivity for the α1-adrenoceptor subtypes. This unique adrenoceptor selectivity profile likely accounts for the very favorable cardiovascular safety profile.

SUMMARY

Tamsulosin, alfuzosin slow release and silodosin do not require dose titration. Alfuzosin, terazosin, doxazosin and silodosin have all been shown to be effective in relieving LUTS/BPH independent of prostate size. Low incidence of orthostatic hypotension has been reported for silodosin, but abnormal ejaculation is the most commonly reported adverse effect.

Alpha1-, alpha2- and beta-adrenoceptors in the urinary bladder, urethra and prostate

1 We have systematically reviewed the presence, functional responses and regulation of alpha(1)-, alpha(2)- and beta-adrenoceptors in the bladder, urethra and prostate, with special emphasis on human tissues and receptor subtypes. 2 Alpha(1)-adrenoceptors are only poorly expressed and play a limited functional role in the detrusor. Alpha(1)-adrenoceptors, particularly their alpha(1A)-subtype, show a more pronounced expression and promote contraction of the bladder neck, urethra and prostate to enhance bladder outlet resistance, particularly in elderly men with enlarged prostates. Alpha(1)-adrenoceptor agonists are important in the treatment of symptoms of benign prostatic hyperplasia, but their beneficial effects may involve receptors within and outside the prostate. 3 Alpha(2)-adrenoceptors, mainly their alpha(2A)-subtype, are expressed in bladder, urethra and prostate. They mediate pre-junctional inhibition of neurotransmitter release and also a weak contractile effect in the urethra of some species, but not humans. Their overall post-junctional function in the lower urinary tract remains largely unclear. 4 Beta-adrenoceptors mediate relaxation of smooth muscle in the bladder, urethra and prostate. The available tools have limited the unequivocal identification of receptor subtypes at the protein and functional levels, but it appears that the beta(3)- and beta(2)-subtypes are important in the human bladder and urethra, respectively. Beta(3)-adrenoceptor agonists are promising drug candidates for the treatment of the overactive bladder. 5 We propose that the overall function of adrenoceptors in the lower urinary tract is to promote urinary continence. Further elucidation of the functional roles of their subtypes will help a better understanding of voiding dysfunction and its treatment.

A novel approach to the treatment of benign prostatic hyperplasia

OBJECTIVE

To assess the clinical efficacy and safety of the combined alpha1- and postsynaptic alpha2-blocker GYKI-16084 compared to placebo during a 28-day active treatment of patients with benign prostatic hyperplasia (BPH).

PATIENTS AND METHODS

After a 28-day placebo run-in phase, 7.5 and 15 mg GYKI-16084 or placebo were administered twice daily for 28 days to patients with BPH in a randomized single-blind Phase II study. Efficacy was primarily determined by changes in the American Urological Association (AUA) symptom scores and maximum urinary flow (Q(max)), while safety was assessed by orthostatic changes and adverse-event profile. A simplified International Index of Erectile Function questionnaire was used to assess effects on erectile function.

RESULTS

Data from 63 patients were evaluated; the decrease in the AUA score during the active phase was greater in the 15 mg group (-6.05, -32.7%) than in the placebo (-4.3, 22.7%) or 7.5 mg (-3.55, -19.5%) groups. Q(max) improved in both active treatment groups (+3.3 and +2.16 mL for the 7.5 and 15 mg groups, respectively) compared to placebo (+1.29 mL). None of the drug-related adverse events associated with selective alpha1-blockers were reported.

CONCLUSION

The combined alpha1- and postsynaptically selective alpha2-blocker GYKI-16084 significantly improved the AUA symptom scores and increased Q(max) in patients with BPH, without inducing any adverse reaction, orthostatic changes or erectile dysfunction.

Enhancement of alpha-adrenoceptor-mediated responses in prostate of testosterone-treated rat

The present study was undertaken to investigate the effect of testosterone on the alpha-adrenoceptor-mediated contractile responses in ventral lobe of rat prostate. Contractile responses to various alpha-adrenoceptor agonists (phenylephrine, A61603 (N-[5-(4,5-dihydro-1H-imidazol-2-yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl] methanesulphonamide), clonidine, guanfacine, ST587 ((2,3-dihydro-benzo[1,4]dioxin-2-ylmethyl)-[2-(2,6-dimethoxy-phenoxy)-ethyl]-amine) and xylazine) were tested in prostate strips obtained from control and testosterone (3 mg/kg, s.c. 5 days a week for 15 days-10 doses total)-treated rats. Dose-response curves for alpha-adrenoceptor agonists in testosterone-treated animals showed a leftward shift, indicating increased sensitivity of tissue to alpha-adrenoceptor agonists. To find the mechanism of increased sensitivity, K(A) value and receptor reserve of phenylephrine were estimated. Neither the K(A) value nor the receptor reserve of phenylephrine was altered in testosterone-treated rats. The concentration-occupancy curve for A61603 was shifted leftward and the K(A) value for A61603 decreased about four-fold. The K(B) value of 2-(2,6-dimethoxyphenoxyethyl) aminomethyl-1,4-benzodioxane (WB4101) was not altered, however, the K(B) value for prazosin was decreased approximately 5.5-fold. These findings indicate that the testosterone-mediated increase in sensitivity of prostate to alpha-adrenoceptor agonists is due to alterations in the alpha(1)-adrenoceptor pool.

In vitro alpha-adrenoceptor autoradiography of the urethra and urinary bladder of the female pig, cat, guinea-pig and rat

OBJECTIVE

The aim of this study was to investigate the distribution of alpha1- and alpha2-adrenoceptors in the urethra and urinary bladder of the female pig, cat, guinea-pig and rat.

MATERIALS AND METHODS

The binding distributions of an alpha1-adrenoceptor ligand (3H-prazosin) and an alpha2-adrenoceptor ligand (3H-rauwolscine) were determined using in vitro autoradiography. Autoradiograms were analysed by combining computer-based image analysis and light microscopy.

RESULTS

In the pig, guinea-pig and rat urethra 3H-prazosin binding was highest in the muscle layer. In the cat urethra 3H-prazosin binding could not be analysed due to a negative chemography artefact. In the pig, cat and guinea-pig urethra 3H-rauwolscine binding was highest in the urothelium, followed by the sub-mucosa, with low levels in muscle. Little 3H-rauwolscine binding was observed in the rat urethra. In the urinary bladder of all species 3H-prazosin binding was low. In the rat bladder, binding was higher in the trigone than in the dome. In the pig, cat and guinea-pig bladder 3H-rauwolscine binding was highest in the mucosa. with little binding in muscle or lamina propria. In the rat bladder, there was little binding and no regional differences.

CONCLUSIONS

Alpha1-adrenoceptors were predominantly located in urethral smooth muscle, indicating their contractile importance in maintaining continence. Alpha2-Adrenoceptors were present in the urethral submucosa and bladder mucosa, but not in muscle, suggesting a role in regulation of blood flow, urethral lubrication and tumescence, but not in contraction.

Effect of JTH-601, a novel alpha1-adrenoceptor antagonist, on the function of lower urinary tract and blood pressure

In the present study, we investigated the effect of JTH-601 (3-{N-[2-(4-hydroxy-2-isopropyl-5-methylphenoxy)ethyl]-N-methylaminomethyl}-4-methoxy-2,5,6-trimethylphenol hemifumarate), a novel alpha1-adrenoceptor antagonist, in vitro and in vivo. JTH-601 (10(-9)-3 x 10(-8) M) competitively antagonized phenylephrine-induced contraction in lower urinary tract tissues (prostate, urethra and bladder trigon) in a concentration-dependent manner. The mean pA2 values for JTH-601 were 8.59+/-0.14, 8.74+/-0.09 and 8.77+/-0.11 for prostate, urethra and bladder trigon, respectively. In anesthetized rabbits, intraduodenal administration of JTH-601 (0.3-3 mg/kg), prazosin (0.03-0.3 mg/kg) and tamsulosin (0.03-0.3 mg/kg) dose dependently inhibited the phenylephrine-induced increase in urethral pressure for 3 h. Although these drugs also decreased mean blood pressure, JTH-601 was less potent than prazosin or tamsulosin. In conscious rabbits, administered JTH-601 (0.01-1 mg/kg, i.v.) had a tendency to augment orthostatic hypotension, but dose dependency was not evident. Prazosin (0.01-1 mg/kg) and tamsulosin (0.001-1 mg/kg) dose dependently augmented orthostatic hypotension. These results indicate that JTH-601 antagonized alpha1-adrenoceptor-mediated contractile responses more potently than prazosin or tamsulosin in rabbit lower urinary tract both in vitro and in vivo. JTH-601 is therefore expected to be effective in the treatment of urinary outlet obstruction in benign prostatic hypertrophy.

Naftopidil, a novel alpha1-adrenoceptor antagonist, displays selective inhibition of canine prostatic pressure and high affinity binding to cloned human alpha1-adrenoceptors

The pharmacological profiles of the alpha1-adrenoceptor antagonists naftopidil, tamsulosin and prazosin were studied in an anesthetized dog model that allowed the simultaneous assessment of their antagonist potency against phenylephrine-mediated increases in prostatic pressure and mean blood pressure. The intravenous administration of each of these compounds dose-dependently inhibited phenylephrine-induced increases in prostatic pressure and mean blood pressure. To further assess the ability of the three compounds to inhibit phenylephrine-induced responses, the doses required to produce a 50% inhibition of the phenylephrine-induced increases in prostatic and mean blood pressure and the selectivity index obtained from the ratio of those two doses were determined for each test compound. Forty minutes after the intravenous administration of naftopidil, the selectivity index was 3.76, and those of tamsulosin and prazosin were 1.23 and 0.61, respectively. These findings demonstrated that naftopidil selectively inhibited the phenylephrine-induced increase in prostatic pressure compared with mean blood pressure in the anesthetized dog model. The selectivity of naftopidil for prostatic pressure was the most potent among the test compounds. In addition, using cloned human alpha1-adrenoceptor subtypes, naftopidil was selective for the alpha1d-adrenoceptor with approximately 3- and 17-fold higher affinity than for the alpha1a- and alpha1b-adrenoceptor subtypes, respectively. The selectivity of naftopidil for prostatic pressure may be attributable to its high binding affinity for alpha1a- and alpha1d-adrenoceptor subtypes.

Neurohormonal stimulation of histamine release from neuroendocrine cells of the human adenomatous prostate

BACKGROUND

Neuroendocrine cells (NE) constitute a population of highly specialized cells in prostatic glands; histamine has never been described in these cells. This article shows the presence and the regulation of release of histamine in NE.

METHODS

In 21 prostatic adenomas, NE were identified by specific antisera against neuroendocrine markers (chromogranin-A, synaptophysin), histamine, and histidine decarboxylase (HDC); a rate HDC-cDNA probe was used to detect this enzyme by in situ hybridization.

RESULTS

Immunoreactive cells for chromogranin-A, histamine, and HDC were found among luminal epithelial glandular cells. Similar cells were also labeled with the HDC-cDNA probe. Glandular cells, isolated from prostatic adenomas, were shown to contain histamine (7-40 pmol/mg cellular protein). L(-) norepinephrine causes a time-dependent (t1/2 = 22 min) histamine release; the alpha 1-receptor antagonists WB-4101 and YM-617 specifically inhibited this release, in agreement with a mediation by alpha 1-adrenoreceptor subtype.

CONCLUSIONS

There is some evidence for the presence in prostatic adenomas of histamine-forming cells of neuroendocrine type; histamine release from these cells is under the control of alpha 1-adrenoreceptor subtype.

Functional property, norepinephrine content and morphometric findings in human hyperplastic prostate

BACKGROUND

Although alpha-adrenergic blockers are widely used as a treatment of benign prostatic hyperplasia (BPH), it is not clear whether contractile property of hyperplastic prostate to alpha-adrenergic agonist depends upon an area of density of smooth muscle within the respective BPH tissue.

METHODS

Functional study and quantitative morphometric analysis were performed on human prostatic specimens obtained by transurethral resection from 22 men with symptomatic BPH. Tissue norepinephrine content was also evaluated.

RESULTS

There was a linear correlation between the area of density of smooth muscle and maximum response to phenylephrine (r = 0.457, P = 0.0362). Although the area of density of smooth muscle showed a positive correlation with norepinephrine content (r = 0.437, P = 0.0471), norepinephrine content was not correlated with maximum phenylephrine response.

CONCLUSIONS

Contractile response to alpha-adrenergic agonist was directly influenced by the area of density of the smooth muscle within an individual prostate.

Prostatic alpha 1-adrenoceptors and uroselectivity

BACKGROUND

alpha 1-adrenoceptor antagonists (blockers) are now commonly used in the treatment of the symptoms of lower urinary tract obstruction. Originally phenoxybenzamine, a non-selective antagonist at both alpha 1- and alpha 2-adrenoceptors, was used by Marco Caine. In an attempt to minimize side effects, selective alpha 1-antagonists, e.g. prazosin, were subsequently developed. More recently, agents such as alfuzosin, doxazosin, terazosin, and tamsulosin have been introduced and claims of "uroselectivity" and "prostate" selectivity have emerged.

METHODS

This review attempts to put these claims into perspective and represents a comprehensive analysis of all pre-clinical and clinical data including several papers from the Japanese literature. An attempt is made to define what is meant by selectivity at various levels including the test tube, in the laboratory animal and, most importantly, in the clinical context of the whole patient.

CONCLUSIONS

The conclusions are interpreted within the context of the subdivision of the alpha 1-adrenoceptor into alpha 1A, alpha 1B, and alpha 1D subtypes.

The alpha-adrenoceptor antagonist properties of the enantiomers of doxazosin in the human prostate

The alpha-adrenoceptor antagonist properties of doxazosin and its enantiomers were characterized using human prostate tissue and cell membranes isolated from rat-1 fibroblast expressing each of the cloned human alpha 1-adrenoceptor subtypes. In the alpha 1-adrenoceptor-binding studies on the human prostate with [3H]doxazosin and 2-{[beta-(3-[125I],4-hydroxyphenyl)ethyl]aminomethyl}-l-tetralone ([125I]HEAT), no significant differences were observed between racemic doxazosin, R-doxazosin and S-doxazosin (mean -log Ki (pKi) values were 8.60-8.63, 8.47-8.55 and 8.61-8.65, respectively), whereas the alpha 2-adrenoceptor-binding studies with [3H]rauwolscine and [3H]clonidine revealed that the alpha 2-adrenoceptor-binding affinity of S-doxazosin (pKi = 5.91-5.94) was slightly (3- or 4-fold), but significantly lower than that of R-doxazosin (pKi = 6.47-6.54). Studies in phenylephrine-contracted prostatic tissue showed no significant difference in alpha 1-adrenoceptor antagonist potency between racemic doxazosin, R-doxazosin and S-doxazosin (pA2 values were 8.43 +/- 0.28, 8.64 +/- 0.56 and 8.75 +/- 0.38, respectively). In the binding studies with cloned alpha 1-adrenoceptor subtypes using [3H]prazosin and [125I]HEAT, racemic doxazosin, R-doxazosin and S-doxazosin showed no selectivity for the alpha 1-adrenoceptor subtypes. The present study demonstrated that doxazosin and its enantiomers are highly selective alpha 1-adrenoceptor antagonists and that there is no evidence suggesting differential alpha 1-adrenoceptor antagonist effects of doxazosin and its enantiomers in the human prostate. Doxazosin, therefore, could be described as displaying balanced activity across all three alpha 1-adrenoceptor subtypes.

Analysis of the relative biomechanical effects of alpha 1 and alpha 2 antagonists in modifying the compliance of the prostate and micturition parameters of the hormonally manipulated male rat

The potential of the alpha 1 and alpha 2 antagonists to modify prostate compliance, and micturition characteristics of rats with hormonally enlarged prostates was studied. Prostate growth was induced in Sprague-Dawley rats using dihydrotestosterone (DHT) and estradiol (E) by daily subcutaneous injections of DHT 1.25 mg/kg and E 0.25 mg/kg together with 0.1ml of sesame oil, as a vehicle, for a period of 3 weeks. A control group of six rats was used wherein the vehicle alone was administered. Dose levels of 3, 10, 30, and 300 micrograms/kg of alpha 1 or alpha 2 antagonist were given at weekly intervals to each of the groups defined above. Voiding characteristics, in terms of micturition frequency and volume per micturition, were measured and correlated with the pharmacological and hormonal stimulus. Prostate compliance and weight was evaluated in each of the groups after rats were terminated and the ventral prostate was dissected and removed in in toto. Compliance measurements were made using a new biosensor system which is based on the principle of detecting the shift in the resonance frequency of the biosensor produced by the hormones on the acoustic impedance of prostate. The results show that DHT and [DHT+E] significantly increased prostate weight and decreased prostate compliance. The alpha 2 antagonist atipamezole significantly increased the compliance of all prostates, including controls, while the alpha 1 antagonist did not alter the compliance. It is concluded that this alpha 2 antagonist is more effective than the prazosin in reversing the hardening effect of hormones on the prostate.

Multidirectional contraction of human hypertrophied prostate

1. The purpose of the present investigation is to evaluate muscle contraction in two directions (longitudinal and circumferential to urethra) physiologically and morphologically for alpha-adrenoceptor agonists. 2. Norepinephrine (10(-7)-10(-4) M), phenylephrine (10(-7)-10(-4) M) and clonidine (10(-7)-10(-4) M) induced contractions in a dose-dependent manner on human prostate from patients with benign prostatic hypertrophy (BPH). 3. No significant differences were observed between longitudinal and circumferential directions of human prostate in 50% of the maximal muscle contraction (EC50 values) and the maximal muscle contractions caused by any agents used. 4. Morphometric analysis for muscle in prostates was performed using formalin-fixed, paraffin-embedded sections stained by the Mallory-Azan method. 5. There was no significant difference in the density of the muscle area between longitudinal and circumferential directions of prostatic strips. 6. These results suggest that there are no significant differences in responsiveness of alpha-adrenoceptor agonists and the smooth muscle contents in longitudinal and circumferential directions to urethra, for human hypertrophied prostate.

Alpha 1-adrenoceptors in human prostate: characterization and binding characteristics of alpha 1-antagonists

The role of alpha 1-adrenoceptors in the mediation of autonomic functions, particularly in the control of the cardiovascular system, is widely known. It has been shown that alpha 1-adrenoceptors localized in human prostate mediate the contraction of prostatic smooth muscles which produces an increase in the intraurethral pressure and thus, these receptors are important in the regulation of bladder outlet resistance. Alpha 1-antagonists such as prazosin relieve the symptoms of bladder outlet obstruction in men with symptomatic benign prostatic hypertrophy (BPH) by blocking alpha 1-adrenoceptors, thereby decreasing prostatic tone and urethral resistance. Thus, alpha 1-adrenergic stimulation may be one of the most important factors in the development of urinary obstruction in BPH. Alpha 1-adrenoceptors in human prostate have been identified and characterized extensively by functional, radioligand binding and molecular biological techniques. These studies provide evidence in support of the concept that the alpha 1C-subtype forms the majority of alpha 1-adrenoceptors in human prostatic smooth muscles. It has been shown that YM617 (tamsulosin) and naftopidil have higher affinities to alpha 1-adrenoceptors in the prostate than in the aorta. Some alpha 1-antagonists, such as prazosin and terazosin, are not selective with respect to alpha 1-adrenoceptor subtypes, while others, such as 5-methylurapidil and indoramin, show higher potencies for alpha 1C-adrenoceptors and much lower potencies for alpha 1A- and alpha 1B-subtypes. In conclusion, the recent findings from pharmacological and molecular biological studies indicate that selective antagonists of alpha 1C-adrenoceptors could be effective in the treatment of urinary obstruction in symptomatic BPH with fewer cardiovascular side effects.

Identification of alpha 1-adrenoceptor subtypes present in the human prostate

alpha 1-Adrenoceptors (ARs) play an important role in mediating human prostatic smooth muscle contraction. In the present study cDNA fragments covering different domains of 3 alpha 1-AR subtypes (alpha 1b, alpha 1c and alpha 1d) were generated from human prostate by reverse transcription coupled to the polymerase chain reaction (RT-PCR). The reconstituted partial sequence (349 amino acids) of the human prostatic alpha 1c-AR PCR products showed 94% identity at the amino acid level with that of the corresponding region of the cloned bovine brain alpha 1c-AR. Using human alpha 1-AR subtype selective cDNA probes in Northern blot analysis, the co-expression of mRNA transcripts corresponding to alpha 1b-, alpha 1c- and alpha 1d-AR subtypes was detected in 4 different regions (apex, base, periurethra and lateral lobe) of the human prostate. Competitive inhibition experiments of [3H]-prazosin binding to membrane preparations of human prostate revealed that the non-selective alpha 1-subtype antagonist, alfuzosin, produced a monophasic inhibition curve, whereas oxymetazoline produced a 2-component inhibition curve with pKi values of 8.54 and 5.46. The high-affinity alpha 1-AR component of the oxymetazoline inhibition curve was predominant (57%-66%) and showed an affinity for oxymetazoline comparable to that of the alpha 1c-AR subtype. As such our results illustrate the expression of different alpha 1-AR subtypes in human prostate and importantly that alpha 1c represents the predominant alpha 1-AR subtype present in this tissue.

Characterization and localization of prostatic alpha 1 adrenoceptors using radioligand receptor binding on slide-mounted tissue section

Alpha 1 adrenoceptor binding sites have been characterized in prostatic tissue homogenates using radioligand receptor binding studies. The objective of the present study was to characterize and localize prostatic alpha 1 adrenoceptor binding sites using slide-mounted tissue sections and the ligand 3H-prazosin. The present study demonstrated that preincubation is not required; the optimal incubation interval is 40 minutes; and a 1-minute wash (once or twice) maximizes the proportion of specific 3H-prazosin binding. Saturation studies were performed at 8 different concentrations of 3H-prazosin ranging between 0.0625 nM. to 8.0 nM. The binding of 3H-prazosin was consistently saturable and of high affinity. The mean Kd and Bmax determined from 6 saturation studies was 4.16 x 10(-10) M. and 1.30 fmol./mg. wet weight, respectively. The pharmacology of these 3H-prazosin binding sites was characterized using competitive displacement experiments. The mean IC50 corrected for prazosin, phentolamine and yohimbine was 7.8 x 10(-10) M., 6.0 x 10(-9) M. and 2.1 x 10(-6) M. The rank order of the IC50 corrected values indicates that alpha 1 binding sites were measured under the assay conditions. In the present study, the mean values for Kd, Bmax and IC50 corrected are similar to values previously reported using prostatic tissue homogenates. Prostatic tissue sections were apposed to x-ray film after being incubated with 3 nM. 3H-prazosin (total prazosin binding) and 3 nM. 3H-prazosin + 8 microM. prazosin (nonspecific prazosin binding). The autoradiograms were analyzed using a computerized analyzing system. The specific radioactive densities of 3H-prazosin in the stroma and glandular epithelium were 1099 +/- 48 pCi/mg. and 163 +/- 42 pCi/mg. The present study validates the technique of assaying alpha 1 adrenoceptor binding sites on slide-mounted prostatic tissue sections and provides further evidence that alpha 1 adrenoceptor binding sites are localized primarily to the stromal elements of the prostate.

Medical management of benign prostatic hyperplasia: a canine model comparing the in vivo efficacy of alpha-1 adrenergic antagonists in the prostate

Medical management of benign prostatic hyperplasia (BPH) is an alternative to surgical treatment of this disease. A major target for pharmacologic therapy is the alpha-1 adrenergic receptor, since activation of this receptor by endogenous catecholamines is thought to contribute to outlet obstruction. In the present study, we compared the potency of various alpha-1 adrenergic antagonists against epinephrine-induced contraction of the canine prostate. The drugs tested were dibenzyline, prazosin, terazosin and YM617. The rank order of potency, comparing inhibitory constants (Ki's), was found to be YM617 >> prazosin > terazosin > dibenzyline. This study is the first to compare all of these drugs directly in the prostate in vivo. The rank order of potency of the drugs is similar to the rank order of potency at other alpha-1 receptors. These results demonstrate that 1) our model is useful in confirming activity of drugs at the alpha-1 receptor and 2) the prostate alpha-1 receptor is similar to other alpha-1 receptors. Whether activity at the alpha-1 adrenergic receptor is a sufficient determinant of clinical efficacy of these drugs remains to be determined.

The effect of alpha 2 agonists and antagonists on the lower urinary tract of the rat

The pharmacologic potential of selective alpha 2 adrenergic agents in modulating vesicourethral function was evaluated in urethane-anesthetized and conscious rats. The bladder was exposed through a midline incision and intubated via a cystostomy to measure pressure. The bladder was filled with saline at 0.05 ml./min. In six animals intravenous dexmedetomidine 0.6 micrograms./kg. followed by 2 micrograms./kg. was given. In another six rats the infusion was made with atipamezole at the same concentration. The parameters obtained were pressure/volume during cystometry, detrusor voiding pressures, and evidence of urine leakage. Conscious rats were placed in a restrainer outfitted with a voided volume sensor and a data recorder. The animals were divided in three groups. In controls, group I (n = 8), the animals were injected with a subcutaneous (SC) load of 5 ml. saline in which furosemide 10.0 mg./kg. was added. In group II (n = 6), dexmedetomidine was added in group III (n = 6), atipamezole was added. Parameters obtained were volume per micturition, latency, frequency of micturition, and patterns of micturition (such as dribbling), and total volume produced after a three hour recording. Values are expressed as mean +/- standard deviation. The results show that in an anesthetized preparation, the alpha 2 agonist dexmedetomidine produced inhibitory effects on the volume evoked micturition reflex (VEMR) at a dose level of 2.0 micrograms./kg. There was a significant decrease in peak pressure from 26.4 +/- 3.6 to 6.8 +/- 2.1 cm. H2O (p < 0.01). Urine leakage was observed at the external meatus. Atipamezole, the alpha 2 agonist, gradually increased bladder baseline pressure and inhibited VEMR. At a critical pressure there was continuous leakage from the external meatus. There was a significant elevation of baseline bladder pressure by atipamezole from 6.3 +/- 1.7 to 29.8 +/- 4.6 cm. H2O (p < 0.005). In conscious rats, the alpha 2 agonist produced urinary dribbling at approximately 20 minutes after SC injection, while the antagonists produced delayed dribbling 70 minutes after SC injection. Leakage occurred in 3 of 6 rats at a dose of 10 micrograms./kg. of atipamezole and in 5 of 6 rats at a dose of 30 micrograms./kg. The alpha 2 agonist caused a diuretic effect and a dose-dependent increase in the frequency of voiding (p < 0.05), while the antagonist decreased the frequency of voiding. Bladder capacity was decreased with the alpha 2 agonist while the antagonist increased capacity.

Binding characteristics of naftopidil and α1-adrenoceptor antagonists to prostatic α-adrenoceptors in benign prostatic hypertrophy

Binding properties of naftopidil and alpha 1-adrenoceptor antagonists to alpha-adrenoceptors in prostates from benign prostatic hypertrophy (BPH) were characterized by radioreceptor assays using [3H]prazosin and [3H]rauwolscine. Specific binding of [3H]prazosin and [3H]rauwolscine in human prostatic membranes was saturable and of high affinity, and it showed a pharmacological specificity which characterized alpha 1 and alpha 2-adrenoceptors, respectively. Naftopidil and several alpha 1 antagonists competed for prostatic [3H]prazosin binding in order: R-(-)-YM-12617 greater than prazosin greater than bunazosin greater than terazosin greater than naftopidil greater than urapidil, and the inhibitory effect (Ki = 11.6 nM) of naftopidil was 10 to 45 times less potent than quinazoline derivatives such as prazosin, bunazosin and terazosin. The potencies of these antagonists in competing for [3H]prazosin binding sites in human prostates correlated well with their pharmacological potencies (pA2). Scatchard analysis indicated that the decrease of prostatic [3H]prazosin binding by naftopidil was due to a marked increase in the Kd value without a change in the Bmax value. The inhibition of prostatic [3H]prazosin binding by naftopidil was reversible. Naftopidil also inhibited prostatic [3H]rauwolscine binding (Ki = 70.0 nM). Thus, it is suggested that naftopidil antagonizes alpha 1-adrenoceptors in human prostates in a competitive and reversible manner.

Medical management of benign prostatic hyperplasia

Medical management of clinical BPH is a reality. The only effective nonsurgical treatment now recommended is aimed at relieving the dynamic component of clinical BPH. Pharmacologic treatment using alpha-adrenoceptor antagonists may be used appropriately to manage patients with prostatism who are poor surgical risks but who could benefit from reduced sympathetic tone. In addition, alpha blockers are used to relieve acute retention, and to prevent retention when increased sympathetic discharge is expected. Thus far, nonsurgical therapy aimed at reducing the mechanical obstruction associated with BPH by prostatic size reduction has failed to produce consistent objective improvement. However, several drugs are now being investigated and may be effective for reducing prostatic size in patients with BPH. Clinical trials are complicated by a number of factors, especially very variable symptoms. Moreover, reduction in prostatic size induced by drugs is not permanent and regrowth occurs with drug withdrawal, necessitating chronic treatment. Ideally, future research should be aimed at the prevention of BPH at an early age. However, this presupposes a better understanding of the pathogenesis of BPH. BPH may not be a single, variable disease but a family of diseases with a number of predictable clinical courses. In the future, we should pay particular attention to histologic variability, to see if in fact different pathologic forms of BPH follow different clinical patterns. If urologists are to keep their predominant position in managing the patient with BPH, they will have to keep informed of medical treatment trials and of potential alternative treatment strategies to prostatectomy.

Determination of norepinephrine levels in the adult human prostate

Tissue levels of norepinephrine were measured in prostate tissue from 24 men ranging in age between 41 and 83 years. Prostatic tissue was obtained from men with subtle palpable prostate nodules undergoing transperineal needle biopsy. None of the patients were shown to have histologic evidence of adenocarcinoma. The severity of the symptoms of prostatism was evaluated prospectively using a standardized micturition symptom score questionnaire. Norepinephrine levels were quantified using a sensitive radioenzymatic assay (REA). Overall, the prostates contained relatively high levels of norepinephrine (1666 +/- 124 ng./gm.). Inverse correlations were observed between tissue norepinephrine levels and severity of symptoms of prostatism (r = -0.58; p = 0.003); age (r = -0.53; p = .008); and prostate size (r = -0.48; p = .02). Norepinephrine levels were also measured in tissue specimens obtained from men undergoing enucleation prostatectomy and transurethral resection of the prostate (TURP). The level of norepinephrine in these prostatectomy specimens (115 ng./gm.) was only 14% the level of the prostate biopsy specimens. The relatively low level of norepinephrine in the specimens obtained from patients with symptoms necessitating prostatectomy provides further evidence that norepinephrine levels are inversely related to the degree of symptomatic bladder outlet obstruction.

Density and localization of alpha 1-adrenoceptors in hypertrophied prostate

Alpha-adrenoceptors have been considered to play an important role in the regulation of the voiding force. In order to determine the density and localization of the alpha-adrenoceptors in the prostate, binding assays for alpha-adrenoceptors were performed with [3H]prazosin and [3H] yohimbine in membrane preparations from enucleated hyperplastic prostate tissues. Furthermore, autoradiographic analysis of alpha-adrenoceptors in the sliced tissue specimens from benign prostatic hypertrophy was performed. Specific binding of both ligands were saturable and of high affinity in membrane preparations, and Scatchard analyses indicated Bmax = 104 fmole/mg. protein, Kd = 0.488 nM for [3H]prazosin, and Bmax = 41 fmole/mg, protein, Kd = 1.83 nM for [3H] yohimbine. Bmax and Kd for [3H]prazosin were greater in the adenoma than in the submucosal tissue of the prostatic urethra. No relationship was noted between the size of enucleated prostate and the density of alpha-adrenoceptors. Image analysis of autoradiograms using [3H]prazosin showed specific binding sites could not be clearly demonstrated, but only [125I]HEAT slightly exposed specific binding sites in the prostate.

Autonomic receptors in human prostate adenomas

Radioligand receptor binding techniques were used to characterize alpha 1 adrenergic, alpha 2 adrenergic and muscarinic cholinergic (MCh) binding sites in human prostate adenomas obtained from men with symptomatic and asymptomatic benign prostatic hyperplasia (BPH). Prostate adenoma specimens were obtained from nine men with asymptomatic BPH undergoing cystoprostatectomy, 11 men with symptomatic BPH undergoing open prostatectomy, and 11 men with symptomatic BPH undergoing transurethral resection of the prostate (TURP). A quantitative symptoms score analysis and urinary flow rate determinations documented the absence of bladder outlet obstruction in men undergoing cystoprostatectomy and confirmed the presence of bladder outlet obstruction in men undergoing prostatectomy. The mean equilibrium dissociation constants (Kd) and the mean densities of 125I-Heat (alpha 1 adrenergic) and 3H-NMS (MCh) binding sites were similar in tissue homogenates obtained from men with asymptomatic and symptomatic BPH. The mean Kd of 3H-Rauwolscine (3H-Ra) was significantly greater in the prostatectomy specimens obtained from men with symptomatic BPH compared to the specimens obtained from men with asymptomatic BPH (p less than 0.05). The density of 3H-Ra (alpha 2 adrenergic) binding sites was significantly greater in the prostate adenomas obtained from men with symptomatic BPH compared to the prostate adenomas obtained from men with asymptomatic BPH (p less than 0.05). The difference in alpha 2 adrenoceptor density was accounted for by an increased receptor density in the open prostatectomy specimens. There was no significant correlation between alpha 2 adrenergic, alpha 1 adrenergic, and MCh receptor densities and prostate weight or patient age. This study indicates that the development of infravesical obstruction in men with BPH is not related to upregulation or altered binding affinity of alpha 1 adrenergic or MCh receptor binding sites. The significance of the observed upregulation of alpha 2 adrenoreceptors in the prostate adenomas obtained from men undergoing open prostatectomy is unknown, and requires further investigation.

Role of alpha-adrenergic blockers in the treatment of benign prostatic hyperplasia

The dynamic component of infravesical obstruction in men with symptomatic benign prostatic hyperplasia (BPH) is determined by alpha 1-adrenoceptor-mediated contractions of the prostatic capsule, prostate adenoma, and bladder neck. Since alpha 1-adrenoceptors are sparse in the bladder, medical therapy aimed at blocking the alpha receptor will relieve bladder outlet obstruction without inhibiting bladder function. Numerous clinical studies have evaluated the use of various alpha blockers for the treatment of BPH. Although the majority of these trials involved limited numbers of patients treated for only short periods, their results have consistently shown that alpha blockers improve urinary flow rates. Adverse reactions appear to be more frequent and more serious with the use of nonselective alpha blockers than with selective alpha 1 blockers, such as terazosin or prazosin. Terazosin offers the additional advantage of once-daily dosing. The common association of hypertension, hyperlipidemia, and symptomatic BPH in the aging male population may provide further impetus for initiating treatment with alpha blockers because alpha blockers are effective antihypertensive agents and may favorably alter lipid profiles.

Autoradiographic analysis of alpha-adrenoceptors and muscarinic cholinergic receptors in the hyperplastic human prostate

Radioligand receptor binding and autoradiography were used to characterize, localize and compare alpha-1 and alpha-2 adrenoceptors and muscarinic cholinergic receptor populations in human benign prostatic hyperplastic tissue. The binding of selective alpha-1 and alpha-2 ligands, [3H]-prazosin and [3H]-UK 14,304, to homogenates of human central and peripheral prostate was saturable and of high affinity. Scatchard analysis produced an equilibrium dissociation constant (KD) of 0.51 +/- 0.10 nM for alpha-1 adrenoceptors, and 2.34 +/- 0.40 nM for alpha-2 adrenoceptors. The mean densities, Bmax, of alpha-1 and alpha-2 adrenoceptors identified in the human adenomatous prostate were 65.9 +/- 12.9 and 36.1 +/- 7.0 fmoles/mg. protein respectively. Receptor autoradiography was used to examine the distribution of muscarinic cholinergic receptors [( 3H]-QNB), alpha-1 adrenoceptors [( 3H]-prazosin]), and alpha-2 adrenoceptors [( 3H]-rauwolscine) on consecutive sections of benign hyperplastic prostatic tissue. Although both subtypes of adrenoceptor were seen in the stromal component of the hyperplastic prostate, there was a substantial predominance of alpha-1 adrenoceptors. A densitometric computer-assisted analysis was performed on the autoradiographic slides to determine the mean ratio of specific alpha-1: alpha-2 adrenoceptors in the stromal compartment of the hyperplastic tissue. The ratio, expressed as % grain occupancy/unit area, was 3.9 +/- 0.75, which is in agreement with a functional alpha-1 adrenoceptor predominance shown in previous studies. Although sparsely distributed in the stroma, a dense alpha-2 adrenoceptor population was seen in association with blood vessels, and in close proximity to the base of some of the [3H]-QNB-labelled prostatic glandular epithelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of muscarinic cholinergic and alpha adrenergic receptors in canine ileum, colon, intestinal urinary reservoirs and bladder

The muscarinic cholinergic (MCh) and alpha 2 adrenergic receptor densities in canine ileum, colon, ileal and colonic urinary reservoirs and bladder were determined using radioligand receptor binding methods in order to provide a rational basis for pharmacologic management of urinary incontinence following bladder replacement with intestinal segments. Muscarinic cholinergic and alpha 2 adrenergic receptor binding sites were studied in these tissues using saturation experiments with 3H-NMS and 3H-rauwolscine, respectively. The mean equilibrium dissociation constants for 3H-NMS binding (0.13 to 0.17 nM) in these tissues were similar (p greater than 0.05) indicating homogeneity of muscarinic cholinergic binding sites. The mean equilibrium dissociation constants for 3H-rauwolscine binding (1.27 to 1.98 nM) in these tissues were also similar (p greater than 0.05). A substantial density of MCh (1.06 to 1.22 fmol/mg. wet wt.) and alpha 2 adrenergic (0.47 to 1.11 fmol/mg. wet wt.) binding sites was identified in the intestinal tissues assayed. The density of ileal and colonic MCh and alpha 2 adrenergic binding sites was not altered following construction of urinary intestinal reservoirs. The presence of a substantial density of MCh and alpha 2 adrenergic binding sites in the intestinal tissues suggests that MCh and alpha 2 adrenergic analogs may be utilized for the management of urinary incontinence following bladder replacement with intestinal urinary reservoirs.

Characterisation of human prostatic adrenoceptors using pharmacology receptor binding and localisation

Benign prostatic enlargement is a common cause of bladder outlet obstruction. Recent work has demonstrated the important role played by the sympathetic nervous system in the control of prostatic muscle tone. Although isometric muscle strip studies and clinical trials have highlighted the influence of alpha-1 adrenoceptors, radioisotope ligand binding studies have demonstrated a relatively increased density of alpha-2 adrenoceptors in the muscle within prostatic tissue, the significance of which is as yet unexplained. Forty patients entered a study using pharmacological muscle strip experiments, radioligand binding assays and receptor autoradiography. Pharmacological data from these studies confirmed that contraction of prostatic muscle is mediated predominantly by alpha-1 adrenoceptor stimulation, with no evidence of significant alpha-2 adrenoceptor or cholinergic mediated effects. Radioligand binding studies confirmed that there is a higher concentration of alpha-1 binding sites as contrasted to alpha-2 within normal prostate, but that this relationship approaches equity in adenomatous prostate. Autoradiographic localisation demonstrated that alpha-1 adrenoceptor binding is predominant within prostatic stroma with only a small component of alpha-2 adrenoceptors in this compartment. This comprehensive study supports the suggestion that prostatic muscular contraction is controlled by the influence of the sympathetic nervous system acting via alpha-1 adrenoceptors. These findings support the therapeutic use of specific alpha-1 adrenoceptor blockade in the management of benign prostatic hyperplasia.

An in vivo evaluation of alpha adrenergic receptors in canine prostate

The role of alpha 2 adrenergic receptors in the prostate in vivo is unknown. A model was developed to measure canine prostatic urethral pressure in vivo, and to assess the ability of various alpha adrenergic blocking agents to affect prostatic pressure. In this model, an esophogeal pressure catheter is inserted into the prostatic urethra to record prostatic urethral pressure. We investigated the effects of alpha adrenergic agonists and antagonists on prostatic pressure using this model. Dose-response curves were generated for epinephrine, and were then repeated in the presence of either prazosin (alpha 1 antagonist), yohimbine (alpha 2 antagonist) or SK&F-86466 (alpha 2 antagonist). Prazosin was the most potent of the three drugs in competitively blocking epinephrine-induced contraction of the prostate, with an inhibition constant of 0.24 micrograms./kg. calculated from the double reciprocal plot. Clonidine, an alpha 2 adrenergic agonist, caused contraction of the prostate, which was also blocked by prazosin. Furthermore, the specific alpha 2 agonist BHT-920 was totally inactive in our system. These results demonstrate that the increase in urethral pressure caused by alpha-adrenergic agonists can be blocked by alpha adrenergic antagonists. However, the specific alpha 1 antagonist, prazosin, is more potent than alpha 2 antagonists, and is also effective against an alpha 2 agonist, clonidine. This suggests that blockade of alpha 1 receptors may be a more useful strategy for causing relaxation of the prostate than blockade of alpha 2 receptors.

Stimulation of postjunctional alpha-1 and alpha-2 adrenoreceptors increases proximal urethral perfusion pressure in the pithed rat

1. In pithed rats, the effects of postjunctional alpha-1 and alpha-2 adrenoreceptor stimulation on prostatomembranous urethral perfusion pressure (UPP) were characterized by using selective adrenoreceptor agonists and antagonists. 2. Dose-dependent increases in UPP were elicited by the intravenous administration of selective alpha-1 and alpha-2 adrenoreceptor agonists, phenylephrine (1.0-30.0 micrograms kg-1) and BHT-933 (10-3000 micrograms kg-1), respectively. 3. The effects of phenylephrine on UPP were antagonized, in a dose-related manner, by pretreatment with prazosin (0.1 and 0.5 mg kg-1, iv), a selective alpha-1 adrenoreceptor antagonist. In contrast, pretreatment with rauwolscine 0.1 and 0.5 mg kg-1, iv), a selective alpha-2 adrenoreceptor antagonist, did not alter phenylephrine-induced increases in UPP. 4. BHT-933 elicited increases in UPP that were antagonized, in a dose-related manner, by pretreatment with rauwolscine (0.1 and 0.5 mg kg-1, iv). Neither dose of prazosin altered the effects of BHT-933 on UPP. 5. These results demonstrate increases in the resistance to flow (constriction) through the proximal urethra following selective stimulation of postjunctional alpha-1 and alpha-2 urethral adrenoreceptors. 6. The role of urethral alpha-adrenoreceptor subtypes in the neurohumoral control of the urethra remains to be determined.

Contractile properties of human prostate adenomas and the development of infravesical obstruction

The contractile response of human prostate adenomas to KCl, phenylephrine (alpha 1 adrenergic agonist), UK 14304 (alpha 2 adrenergic agonist), and carbachol (muscarinic cholinergic agonist) was evaluated in tissue specimens obtained from men with symptomatic and asymptomatic BPH. Prostate specimens were obtained from 5 men with asymptomatic BPH undergoing cystoprostatectomy, 11 men with symptomatic BPH undergoing open prostatectomy, and 11 men with symptomatic BPH undergoing transurethral resection of the prostate (TURP). Quantitative symptom score analysis and urinary flow rate determination documented the absence of bladder outlet obstruction in men undergoing cystoprostatectomy and confirmed the presence of bladder outlet obstruction in men undergoing prostatectomy. The magnitude of the contractile response (Emax) and the potency of phenylephrine-induced contractions (EC50) in prostatic preparations obtained from men with symptomatic and asymptomatic BPH were similar. The IC50 for the inhibition of phenylephrine-induced contractions by prazosin was 3.2 nM, confirming that phenylephrine-induced contraction in the human prostate is mediated by the alpha 1 adrenoceptor. The contractile responses of prostate adenomas to muscarinic cholinergic and alpha 2 agonists were negligible. This study demonstrates that the development of bladder outlet obstruction in men with BPH is not related to alterations in the functional response of the smooth muscle component of the prostate adenoma.

The effect of electrocautery on neurotransmitter receptor binding assays in the canine prostate

The purpose of this study was to determine whether resection of the prostate with electrocautery alters the binding properties of various neurotransmitter ligands. Prostate glands were removed from four adult male dogs. The prostates were divided in the midsaggital plane and one half of the prostate was resected using a resectoscope. Saturation experiments were performed on the resected and control prostatic tissue using 3H-NMS, 125I-Heat, and 3H-rauwolscine. The mean equilibrium dissociation constants (Kd) and the mean densities of 3H-NMS, 125I-Heat, and 3H-Rauwolscine binding sites were similar in tissue homogenates obtained from control and resected portions of the prostate (p greater than 0.05). Resection of the prostate using electrocautery did not alter the binding properties of various neurotransmitter ligands for characterizing and quantifying muscarinic cholinergic, alpha 1 adrenergic, and alpha 2 adrenergic binding sites in the canine prostate. Approximately 90% of prostatectomies for symptomatic BPH (benign prostatic hyperplasia) are performed transurethrally. The ability to accurately measure neurotransmitter receptor densities in prostate tissues obtained following transurethral resection is imperative for our future studies designed to elucidate the role of alpha adrenergic receptors in the development of bladder outlet obstruction in men with BPH.

Identification and characterization of alpha 1 adrenergic receptors in the canine prostate using [125I]-Heat

We have recently utilized radioligand receptor binding methods to characterize muscarinic cholinergic and alpha adrenergic receptors in human prostate adenomas. The primary advantages of radioligand receptor binding methods are that neurotransmitter receptor density is quantitated, the affinity of unlabelled drugs for receptor sites is determined, and receptors can be localized using autoradiography on slide-mounted tissue sections. Recently, [125I]-Heat, a selective and high affinity ligand with high specific activity (2200 Ci/mmole) has been used to characterize alpha 1 adrenergic receptors in the brain. In this study alpha 1 adrenergic receptors in the dog prostate were characterized using [125I]-Heat. The Scatchard plots were linear indicating homogeneity of [125I]-Heat binding sites. The mean alpha 1 adrenergic receptor density determined from these Scatchard plots was 0.61 +/- 0.07 fmol/mg. wet wt. +/- S.E.M. The binding of [125I]-Heat to canine prostate alpha 1 adrenergic binding sites was of high affinity (Kd = 86 +/- 19 pM). Steady state conditions were reached following an incubation interval of 30 minutes and specific binding and tissue concentration were linear within the range of tissue concentrations assayed. The specificity of [125I]-Heat for alpha 1 adrenergic binding sites was confirmed by competitive displacement assays using unlabelled clonidine and prazosin. Retrospective analysis of the saturation experiments demonstrated that Bmax can be accurately calculated by determining specific [125I]-Heat binding at a single ligand concentration. [125I]-Heat is an ideal ligand for studying alpha 1 adrenergic receptors in the prostate and its favorable properties should facilitate the autoradiographic localization of alpha 1 adrenergic receptors in the prostate.

Alpha 1 adrenergic receptors in canine lower genitourinary tissues: insight into development and function

Radioligand receptor binding methods were used to characterize the alpha 1-adrenergic receptor in the bladder body, bladder base, prostate and urethra of the male dog. Saturation experiments were performed in tissue homogenates using [125iodine]-Heat, an alpha 1-adrenergic antagonist of high specific activity (2,200 Ci. per mmol.). The equilibrium dissociation constant Kd for [125iodine]-Heat binding in the bladder body (0.56 pM.), bladder base (0.81 +/- 0.11 pM.), prostate (0.86 +/- 0.19 pM.) and urethra (0.55 pM.) was similar, suggesting homogeneity of alpha 1-adrenergic binding sites in lower genitourinary tissues. The receptor density in the bladder body, bladder base, prostate and urethra, expressed as fmol. per mg. wet weight, was 0.22 +/- 0.02, 0.82 +/- 0.09, 0.55 +/- 0.06 and 0.27 +/- 0.06, respectively (mean +/- standard error of mean). Competitive binding experiments with [125iodine]-Heat and unlabeled prazosin and clonidine confirmed the selectivity of Heat for alpha 1-adrenergic binding sites. Anatomical dissections have revealed that a major component of the smooth muscle of the bladder base and prostate originates from the ureter, whereas a major component of the smooth muscle of the urethra originates from the bladder. The measured alpha 1-adrenergic receptor densities support these developmental theories.

Alpha 2 adrenergic receptors in canine prostate: biochemical and functional correlations

The sympathetic innervation of human prostate adenomas has been previously demonstrated using fluorescence microscopy and in vitro isometric studies. A clinical implication of these observations is that bladder outlet obstruction in men with benign prostatic hypertrophy may be subject to pharmacologic manipulation using adrenergic drugs. Randomized clinical trials have demonstrated the efficacy of alpha adrenergic antagonists for symptomatic BPH. We have previously characterized the alpha1 and alpha2 adrenergic receptors in the human prostate using [3H]prazosin and [3H]rauwolscine, respectively. The mean alpha1 and alpha2 receptor densities in the adenomas studied were equivalent. The effect of alpha2 adrenergic drugs on prostatic urethral pressure has not been examined in the human or in an animal model. In this study a canine model was used to define the effect of alpha2 drugs on prostatic urethral pressure. Intravenous administration of clonidine, a selective alpha2 agonist, resulted in a dose dependent increase in prostatic urethral pressure. The maximal increase in urethral pressure ranged between 18 to 30 cm. H2O. The maximal response to clonidine was approximately 50% less than the response to epinephrine, indicating that clonidine acts as a partial agonist. Pretreatment with yohimbine, a selective alpha2 adrenergic antagonist, abolished the effects of clonidine and epinephrine. The alpha2 adrenergic receptors were then studied in the canine prostates using [3H]rauwolscine. The equilibrium dissociation constant, Kd, ranged between 0.68 to 1.80 nM and the receptor density ranged between 14.8 to 69.3 fmol./mg. protein. The receptor density was homogeneous in specimens obtained from the proximal, midportion, and distal canine prostate suggesting that the effect of alpha2 drugs is not sphincter mediated. These in vitro and in vivo studies provide the basis for investigating the effects of alpha2 antagonists in men with symptomatic BPH.