Characterization of the alpha-adrenoceptors in the female rabbit urethra

Ca2+ signalling in mouse urethral smooth muscle in situ: role of Ca2+ stores and Ca2+ influx mechanisms

KEY POINTS

Contraction of urethral smooth muscle cells (USMCs) contributes to urinary continence. Ca2+ signalling in USMCs was investigated in intact urethral muscles using a genetically encoded Ca2+ sensor, GCaMP3, expressed selectively in USMCs. USMCs were spontaneously active in situ, firing intracellular Ca2+ waves that were asynchronous at different sites within cells and between adjacent cells. Spontaneous Ca2+ waves in USMCs were myogenic but enhanced by adrenergic or purinergic agonists and decreased by nitric oxide. Ca2+ waves arose from inositol trisphosphate type 1 receptors and ryanodine receptors, and Ca2+ influx by store-operated calcium entry was required to maintain Ca2+ release events. Ca2+ release and development of Ca2+ waves appear to be the primary source of Ca2+ for excitation-contraction coupling in the mouse urethra, and no evidence was found that voltage-dependent Ca2+ entry via L-type or T-type channels was required for responses to α adrenergic responses.

ABSTRACT

Urethral smooth muscle cells (USMCs) generate myogenic tone and contribute to urinary continence. Currently, little is known about Ca2+ signalling in USMCs in situ, and therefore little is known about the source(s) of Ca2+ required for excitation-contraction coupling. We characterized Ca2+ signalling in USMCs within intact urethral muscles using a genetically encoded Ca2+ sensor, GCaMP3, expressed selectively in USMCs. USMCs fired spontaneous intracellular Ca2+ waves that did not propagate cell-to-cell across muscle bundles. Ca2+ waves increased dramatically in response to the α1 adrenoceptor agonist phenylephrine (10 μm) and to ATP (10 μm). Ca2+ waves were inhibited by the nitric oxide donor DEA NONOate (10 μm). Ca2+ influx and release from sarcoplasmic reticulum stores contributed to Ca2+ waves, as Ca2+ free bathing solution and blocking the sarcoplasmic Ca2+ -ATPase abolished activity. Intracellular Ca2+ release involved cooperation between ryanadine receptors and inositol trisphosphate receptors, as tetracaine and ryanodine (100 μm) and xestospongin C (1 μm) reduced Ca2+ waves. Ca2+ waves were insensitive to L-type Ca2+ channel modulators nifedipine (1 μm), nicardipine (1 μm), isradipine (1 μm) and FPL 64176 (1 μm), and were unaffected by the T-type Ca2+ channel antagonists NNC-550396 (1 μm) and TTA-A2 (1 μm). Ca2+ waves were reduced by the store operated Ca2+ entry blocker SKF 96365 (10 μm) and by an Orai antagonist, GSK-7975A (1 μm). The latter also reduced urethral contractions induced by phenylephrine, suggesting that Orai can function effectively as a receptor-operated channel. In conclusion, Ca2+ waves in mouse USMCs are a source of Ca2+ for excitation-contraction coupling in urethral muscles.

Alpha adrenergic receptors in renal pelvis and calyces: can rat models be used?

We aimed, in this study, to determine the distribution of α-1 AR subtypes in rat and human pelvis and calyces, and to evaluate, by comparing these two species, the possibility of rats to be used as models for humans. Twenty patients with renal carcinoma were included into the study. The patients underwent radical nephrectomy for renal cell carcinoma (RCC). After nephrectomy, specimens were evaluated and excisional biopsies from healthy pelvis and calyces tissues were performed. When pathology confirmed the non-invasion of RCC, specimen was included into the study. A total of 7 adult Wistar Albino (250-300 g) female rats were used in this study. Specimens included renal pelvis and calyces. All specimens were evaluated under light microscope histopathologically. The concentrations of the receptor densities did not differ between the two groups. With the demonstration of the α receptors in rat kidneys and calyces, many receptor-based studies concerning both humans and rats can take place. Novel medication targeting these subtypes -in this matter α1A and α1D for renal pelvis and calyces- may be helpful for expulsive therapy and/or pain relief. With the demonstration of similar receptor densities between human and rat tissues, rat model may be useful for α-receptor trials for renal pelvis and calyces.

α(1)-Adrenoceptors in the urinary tract

α(1)-Adrenoceptors have been identified and characterized extensively by functional, radioligand-binding, and molecular biological techniques. Molecular clones have been isolated for three α(1)-subtypes (α(1a), α(1b), and α(1d)), and these subtypes are also functionally characterized. α(1)-Adrenoceptors are present in the prostate, urethra, bladder (urothelium, smooth muscle, and afferent nerves), ureter, vas deferens, peripheral ganglia, nerve terminals, vascular tissues, and central nervous system (CNS), and they could all potentially influence overall urinary function and contribute to both the therapeutic and adverse effects of α(1)-adrenoceptor antagonists in patients with benign prostatic hyperplasia (BPH) and lower urinary tract symptoms (LUTS). This review aimed to discuss the relevant physiological and pharmacological roles and molecular biology of α(1)-adrenoceptor subtypes in the prostate, urethra, bladder, ureter, and CNS.

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.

Effects of different alpha-1 adrenoceptor blockers on proximal urethral function using in vivo isovolumetric pressure changes

The effects of different alpha-1 adrenoceptor blockers on the urethra and the cardiovascular system were evaluated using an in vivo isovolumetric intra-urethral pressure model in New Zealand white rabbits. The urethra of anesthetized male rabbits was cannulated through the bladder and secured at the vesico-urethral junction. The distal side of urethra under the pubic bone was also closed to allow measurement of the intra-urethral pressure. Both the intra-urethral pressure and the femoral arterial pressure were monitored. The effects of five different alpha-1 adrenoceptor blockers on the increases in both the intra-urethral pressure and blood pressure induced by phenylephrine were then examined. The inhibition rate of the alpha-1 adrenoceptor blockers prazosin, bunazosin, terazosin, alfuzosin and tamsulosin on the increase in intra-urethral pressure caused as a result of contraction by phenylephrine was 87.5 +/- 4.5% (mean +/- S.E.), 88.0 +/- 7.2%, 86.2 +/- 6.2%, 81.4 +/- 4.8% and 92.5 +/- 5.0% respectively. The potency ranking of these alpha-1 adrenoceptor blockers was tamsulosin > bunazosin > prazosin > terazosin > alfuzosin. Their inhibition rate of the arterial pressure increase induced by phenylephrine was 81.9 +/- 5.0%, 86.2 +/- 5.9%, 76.0 +/- 6.0%, 63.6 +/- 5.7% and 58.0 +/- 5.2% respectively, with a potency ranking of bunazosin > prazosin > terazosin > alfuzosin > tamsulosin. We therefore conclude that the alpha-1 adrenoceptor blockers bunazosin and prazosin have a more potent action on both the urethra and the vascular system. However, tamsulosin and alfuzosin displayed a marked blockade of the increased urethral pressure induced by phenylephrine, with much less of a blockade of arterial pressure. In the present study, tamsulosin has been shown to be the most sensitive and powerful of the alpha-1 adrenoceptor blockers on urethral smooth muscle.

Effects of castration on contraction and alpha(1)-adrenoceptor expression in rat prostate

1. The prostate function is regulated by androgens and alpha-adrenergic activity. Clinically, antiandrogens and/or alpha(1)-adrenergic antagonists are commonly used to treat symptomatic prostatic hypertrophy. To elucidate the effects of androgen deprivation on prostate contractility via alpha(1)-adrenoceptor, the characteristics and expression of alpha(1)-adrenoceptors were examined in castrated rats. 2. Isolated prostate strips from intact and castrated rats were subjected to a phenylephrine stimulated contraction. Prazosin (10 nM), [(3)H]-prazosin and phenoxybenzamine (3 - 300 nM) were used for inhibition assay, receptor characterization and partial alkylation of alpha-adrenoceptor, respectively. The mRNA content of three subtypes of alpha-adrenoceptors was determined by reverse transcription combined with polymerase chain reaction (RT - PCR). 3. Contractile response to phenylephrine increased in castrated rats, which could be explained by a relative increase of the stromal component. A lowered contraction potency was also noted in castrated rats. Receptor binding assay indicated minimal changes in the affinity or density of alpha(1)-adrenoceptor. Escalating alkylation of the alpha(1)-adrenoceptor population resulted in a rightward shift in the contraction-response curves before depressing maximal contractile force, and the suppression was detected at lower doses in castrated rats. RT - PCR study confirmed the expression of three types of alpha(1)-adrenoceptor, alpha(1a), alpha(1b) and alpha(1d)-adrenoceptors, in intact rat prostate, and revealed that alpha(1a)-adrenoceptor, but not alpha(1b) or alpha(1d)-adrenoceptors, was down-regulated in castrates. 4. The results show that androgen deprivation suppressed alpha(1)-adrenergic contractility of rat prostate strips, and the suppression was associated with down-regulation of receptor reserve for the alpha(1a)-adreneroceptor population expressed in intact rat prostate.

Contractile properties of urethral smooth muscles of young and aged female dogs: morphological and pharmacological aspects

BACKGROUND

Alpha1-adrenoceptors are highly concentrated in the urethral smooth muscles and may play an important role in the contraction of this area. However, detailed examinations of age-related changes of the properties of urethral smooth muscle have rarely been undertaken.

METHODS

The contractile properties of urethras from young non-parous and old parous female beagles were determined with a urethral function study, macroscopic autoradiography for urethras using [3H]-labeled tamsulosin and morphometry of the urethral muscles.

RESULTS

The antagonistic effect (pA2) of prazosin for norepinephrine was 7.76+/-0.13 in young dogs and 7.62+/-0.06 in aged dogs. The specific binding of [3H]-tamsulosin (a relatively selective alpha1A-adrenoceptor antagonist) was recognized diffusely in proximal urethras with in vitro autoradiography. The density of binding in smooth muscles was approximately 60 and 40% in circular longitudinal layers, respectively, for both dogs.

CONCLUSIONS

The female canine urethra had alpha1A, and alpha1L-adrenoceptors. No age-related changes were seen in the function of the proximal urethra, distribution of alpha1-adrenoceptor binding sites and smooth muscle densities.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

The adrenergic, cholinergic and NANC nerve-mediated contractions of the female rabbit bladder neck and proximal, medial and distal urethra

1. The nerve-mediated contraction of the female rabbit bladder neck and different portions of the urethra (proximal, medial and distal) was studied in vitro by electrical stimulation (50 V, 30 Hz, 0.05 ms width, trains of 5 s every 5 min) by use of a superfusion system. 2. The amplitude (Emax) and the duration (Dmax) of the stimulated contraction were studied in the four tissues. The Emax value was significantly higher in distal urethra (2.07+/-0.15 g) compared to the bladder neck (1.08+/-0.10 g), proximal urethra (0.73+/-0.07 g) and medial urethra (0.87+/-0.07 g). In contrast, the Dmax value appeared slightly but significantly lower (P<0.05) in distal urethra (68.5+/-2.3 s) than in bladder neck (76.7+/-6.0 s), proximal urethra (84.5+/-5.0 s) and medial urethra (81.3+/-3.5 s). 3. Cocaine (1 microM) significantly increased the basal Emax values in medial and distal urethra and the basal Dmax values in the four tissues. 4. Prazosin (1 microM) significantly reduced E max value in proximal, medial and distal urethra and Dmax value in bladder neck and proximal urethra. Atropine (1 microM) also significantly reduced Emax values in bladder neck and proximal urethra and reduced Dmax value in bladder neck, but not in other tissues. Yohimbine (0.1 microM) was devoid of effect in the four tissues. 5. The association of prazosin (1 microM) and atropine (1 microM) did not modify the Emax and the Dmax values of the electrically-induced contractions, except in proximal urethra and in bladder neck where an additive inhibitory effect (on Emax only) was observed compared to prazosin and atropine alone. 6. The residual contractile response after combined treatment with prazosin and atropine was significantly diminished by tetrodotoxin (TTX; 1 microM) but not completely abolished. These NANC contractions were insensitive to P2X-purinoceptor desensitization by continuous tissue perfusion with alpha,beta-methylene ATP (30 microM). 7. These results demonstrate that bladder neck and proximal urethra are mainly innervated by the parasympathetic nervous system, whereas medial and distal urethras are to a greater extent under the control of the sympathetic innervation. The residual responses, insensitive to prazosin and atropine, may indicate a NANC innervation in the four tissues. However, the nature of the NANC neurotransmitter remains to be identified.

Prejunctional facilitatory and inhibitory modulation of parasympathetic nerve transmission in the rabbit urinary bladder

Release of [3H]choline and muscle contraction in response to electrical field stimulation were measured from rabbit detrusor muscle strips previously loaded with [3H]choline. The importance of different stimulation frequencies (1 and 10 Hz) for activating either facilitatory or inhibitory prejunctional effects was examined in the presence of muscarinic and adrenergic (alpha2) receptor selective substances. At 1 Hz, neither [3H]choline overflow nor contraction was affected by the M1-selective receptor antagonist pirenzepine (10(-7) M), whereas overflow and contraction decreased at 10 Hz. The M1-selective receptor agonist McN-A-343 (10(-6) M) caused no significant changes except for reducing contractions at 10 Hz. The M2-selective receptor antagonist methoctramine (10(-6) M), on the other hand, increased overflow as well as contraction at both frequencies, most conspicuously at 1 Hz. Atropine (10(-7) M) caused a significant increase with respect to overflow only at 1 Hz, while quite the opposite effect occurred with respect to contractions (reduced only at 10 Hz). Clonidine (10(-6) M) induced inhibition of [3H]choline overflow at 10 Hz only, but without significantly changing contractile responses. The results show that in the rabbit urinary bladder a muscarinic autoreceptor mediated inhibition (M2) of the transmitter release dominates during low frequency stimulation and that a facilitation (M1) may be present at stimulations with higher frequencies. However, this amplification may also be influenced by alpha2-adrenoceptor mediated inhibition.

Innervation of the human middle meningeal artery: immunohistochemistry, ultrastructure, and role of endothelium for vasomotility

The majority of nerve fibers in the middle meningeal artery and branching arterioles are sympathetic, storing norepinephrine and neuropeptide Y (NPY). A sparse supply of fibers contain acetylcholinesterase activity and immunoreactivity toward vasoactive intestinal peptide (VIP), peptidine histidine methionine (PHM), and calcitonin gene-related peptide (CGRP). Only few substance P and neuropeptide K immunoreactive fibers are noted. Electronmicroscopy shows axons and terminals at the adventitial medial border of the human middle meningeal artery, with a fairly large distance to the smooth muscle cells (>500 nM). Several axon profiles contain vesicles of different types, including putative sensory profiles. The perivascularly stored signal substances, norepinephrine and NPY induced vasoconstrictor. Relaxations were induced by acetylcholine and substance P, and these were significantly reduced in arteries without endothelium, while the responses to norepinephrine, NPY, VIP, PHM, and CGRP were not changed by endothelium removal. Blockade experiments showed that the vasomotor responses to norepinephrine were blocked by prazosin, to NPY by BIBP 3226, acetylcholine by atropin, substance P by RP 67580, and the human alpha-CGRP response by human alpha-CGRP(8-37).

Comparative studies on urethral function

Urethral pressure profiles (in vivo), opening pressures, and flow rates at opening pressure (in vitro) were determined for female, male, pregnant, ovariectomized (OVX), OVX and then estrogen-treated, and OVX and then progesterone-treated rabbits. Using the isolated whole-urethra preparation, we determined the opening pressures and flow rates as well as the effects of 250 microM phenylephrine, 250 microM bethanechol, and 120 mM KCl on the urethral opening pressure and flow rate. The results demonstrated that (1) the urethral pressure profiles were similar for male and female rabbits, (2) ovariectomy and pregnancy decreased the urethral pressure profiles, (3) estrogen therapy partially reversed the effect of ovariectomy on the urethra, and (4) progesterone therapy had little effect on the urethral pressure profile. With regard to opening pressure and flow, (1) flow at opening pressure was lower in the male than in the female; (2) the opening pressure was increased by ovariectomy; (3) phenylephrine and KCl stimulated a greater response in the male than in the female, whereas the response to bethanechol was significantly lower in the male than in the female; (4) the male had the greatest resistance to flow among all the groups; and (5) ovariectomy increased the resistance to flow and estrogen treatment reversed the effect of ovariectomy. In conclusion, ovariectomy had significant effects on urethral function that were reversed by estrogen therapy but not by progesterone therapy.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Influence of alpha-receptor blockade on the time-dependent change in the effect of furosemide

Influence of alpha-receptor blockade on the time-dependent change in the effect of furosemide (a loop diuretic agent) was examined. Furosemide (30 mg/kg) was given orally to the doxazosin (an alpha 1-blocker)- or vehicle-treated rats at 12 AM or 12 PM. Urine volume and urinary excretions of of sodium and furosemide for 8 hr were significantly greater at 12 AM than at 12 PM in the vehicle-treated animals. However, such time-dependent changes in these parameters disappeared in the doxazosin-treated rats. These results suggest that the alpha 1-receptor-mediated stimuli are involved in the mechanism of the time-dependent change in the effect of furosemide.

Regulation of tone in pig urethral smooth muscle

Cystometry and urethral pressure profilometry performed in Large White pigs reveal a fall in urethral pressure before micturition. The area of highest pressure in the female is in the mid-urethra. Circular smooth muscle dissected from this region generates a high spontaneous tone, which is increased by phenylephrine and carbachol, but is unaffected by guanethidine and atropine. Electrical field stimulation using different parameters produces frequency-dependent relaxations or a combination of relaxation and contraction, all sensitive to tetrodotoxin. Only the contraction is inhibited by atropine and guanethidine. However, phasic relaxations are abolished by N-nitro-L-arginine, an effect partially reversed by L- but not by D-arginine. Oxyhemoglobin also marginally reduces the relaxations. Sodium nitroprusside produces concentration-dependent relaxations of the urethral smooth muscle, maximal at 10(-5) M. It is suggested that the urethral muscle generates a high myogenic tone that maintains continence and can be enhanced by adrenergic and cholinergic input, or inhibited by NANC innervation, partially mediated by the transmitter nitric oxide.

Noradrenaline modulates smooth muscle activity of the isolated intravesical ureter of the pig through different types of adrenoceptors

1. We have studied the effects of alpha- and beta-adrenoceptor agonists and antagonists on both phasic peristaltic activity and basal tone of the isolated intravesical ureter of the pig by means of isometric techniques in vitro. 2. Spontaneous phasic activity was exhibited by 21% of pig intravesical ureter preparations manifested as rhythmic contractions with average frequency and amplitude of 2.54 +/- 0.18 min-1 and 1.48 +/- 0.16 g (n = 31), respectively. 3. Adrenaline, noradrenaline and phenylephrine induced concentration-dependent increases in both phasic activity and basal tone of ureteral preparations, all three agonists being more potent in modifying ureteral phasic activity than baseline tone. B-HT 920, B-HT 933 and clonidine had no significant effect. 4. Phentolamine (10(-9)-10(-7) M) and prazosin (3 x 10(-11)-3 x 10(-8) M) significantly inhibited increases in both frequency of phasic activity and baseline tone induced by a submaximal dose of noradrenaline. Rauwolscine (10(-9)-10(-7) M) affected only the tone evoked by noradrenaline and higher concentrations of this antagonist were needed to block phasic activity. 5. Pretreatment of ureteral strips with the beta-adrenoceptor antagonist, propranolol (10(-6) M), significantly increased the maximum contraction evoked by noradrenaline. After incubation with phentolamine (10(-6) M), noradrenaline (10(-7)-10(-6) M) decreased phasic activity induced by prostaglandin F2 alpha (10(-5) M). Isoprenaline and salbutamol also abolished PGF2 alpha-induced phasic activity. Pafenolol (10(-6) M) and butoxamine (10(-6) M) blocked the inhibitory effect of noradrenaline, isoprenaline, and salbutamol on PGF2 alpha-induced phasic activity. 6. These results suggest that noradrenaline may modulate both phasic peristaltic activity and basal tone of pig intravesical ureter through both alpha- and beta-adrenoceptors.

Role of adrenoceptors in the proximal urethral function of female and male rabbits using an in vivo model of isovolumetric pressure generation

The role of alpha-1 and alpha-2 adrenoceptors in the contractile response of the proximal urethra to agonists and antagonists was studies in female and male New Zealand White rabbits. The response pattern for alpha-1 agonists showed a sharp rise, while that for alpha-2 agonists showed a gradual increase to a maximum pressure. The duration of the response of alpha-2 agonists was significantly longer than that of alpha-1 agonists. A difference in the response between alpha-1 and alpha-2 would be produced mainly through different receptor functions. The magnitude of the response of alpha-1 adrenoceptors in the male rabbits was almost twice that of the female rabbits. In contrast, the response of alpha-2 in the male rabbits was less than that of the female rabbits. Our experimental findings indicate that alpha-2 stimulation mediates a slow and prolonged response that is essential for urinary continence in female rabbits.

Effect of the optical isomers of YM-12617 on increased intra-urethral pressure induced by phenylephrine in anaesthetized dogs

1. Effects of the individual optical isomers of YM-12617 (R, S 5-[2-[[2-(o-ethoxyphenoxy)ethyl]-amino]propyl]-2- methoxybenzenesulphonamide HCl, (+/-)-YM), a potent selective alpha 1-adrenoceptor antagonist, on the increase in intraurethral pressure (IUP) and diastolic blood pressure (DBP) induced by phenylephrine were evaluated in pentobarbital-anaesthetized female dogs. 2. Phenylephrine (0.1-30 micrograms kg-1 i.v.), an alpha 1-adrenoceptor agonist, dose-dependently increased IUP and DBP. 3. alpha-adrenoceptor antagonists (i.v.) dose-dependently antagonized the increase in IUP and DBP induced by phenylephrine, with a potency ranking of R(-)-YM greater than (+/-)-YM greater than prazosin greater than phentolamine greater than S(+)-YM greater than yohimbine. 4. These results suggest that the alpha 1-adrenoceptor mediates the increase in IUP as well as DBP in anaesthetized dogs, and that alpha 1-adrenoceptors in the urethra and arteries show a stereochemical preference for R(-)-YM.

The contribution of alpha-adrenoceptors to neurally-mediated contractions of the rabbit urethral smooth muscle

1. The nature of the nerve-mediated contractions in the urethral smooth muscle from the rabbit was studied in vitro. Field stimulation caused smaller contractile responses than in the detrusor of the rabbit. 2. There was no significant difference in response to field stimulation or exogenous agents acting on adrenoceptors between longitudinal and circular strips from the rabbit urethra. Histological studies showed that the urethral muscle is arranged in three layers, which run circularly and longitudinally. 3. Atropine had very little effect on the response to field stimulation, phentolamine almost abolished the contractile response to nerve stimulation and sometimes unmasked a relaxation. 4. The alpha 1-adrenoceptor blocking agent, prazosin, blocked both the contractile response to the alpha 1-receptor agonist phenylephrine and that to intrinsic nerve stimulation, with similar potencies. The alpha 2-blocking agent yohimbine shifted the dose-response curve of the contractile response to the alpha 2-agonist, clonidine, in a dose-dependent manner, 10(-7) M causing a 10 fold shift. This concentration had no effect on the response to intrinsic nerve stimulation, suggesting that alpha 2-receptors are not involved in the response. Higher concentrations of yohimbine caused a suppression of the nerve-evoked response which is assumed to be non-specific. 5. Noradrenaline, phenylephrine, and clonidine caused dose-dependent contractile responses in the rabbit urethral strips. The contractions induced by clonidine developed more slowly than those induced by noradrenaline and phenylephrine. 6. These results demonstrate that the rabbit urethral smooth muscle contains both alpha 1- and alpha 2-adrenoceptors, and the nerve-mediated contraction of the rabbit urethra is adrenergic in nature and mediated mainly via alpha 1-adrenoceptors.

Characterization of postsynaptic alpha-adrenoceptors in the arteries supplying the oviduct

1. In vitro experiments were designed to characterize postjunctional alpha-adrenoceptor subtypes in ring segments (1 mm length; outer diameter 300-500 microns) from arteries supplying the oviduct of the heifer. 2. Noradrenaline, adrenaline and phenylephrine evoked concentration-dependent contractile responses. The pD2 values were 5.67, 5.89 and 5.93, respectively. Medetomidine clonidine and B-HT 920 (2-amino-6-allyl-5,6,7,8-tetra-hydro-4H-(thiazo)-4,5-d-azepoine ) were ineffective. 3. The alpha-adrenoceptor selective antagonists, prazosin (1 nM-0.1 microM) and rauwolscine (0.1-10 microM) competitively antagonized the response to noradrenaline. The pA2 values were 9.38 and 6.83, respectively. 4. The dissociation constant (KD) for noradrenaline calculated by use of the irreversible antagonist, dibenamine, was 3.95 (2.09-5.81) microM. The occupancy-response relationship was non-linear. Half-maximal response to noradrenaline was obtained with 22% receptor occupancy while maximal response required 100% occupancy. 5. B-HT 920 evoked a biphasic contractile concentration-dependent response in preparations incubated in a physiological solution containing 20 mM K+, 0.1 microM prazosin and 1 microM propranolol. Rauwolscine 0.1 microM significantly (P less than 0.01) blocked the first component of the B-HT 920 concentration-response curve with an apparent pA2 value of 8.52 (7.86-9.18). 6. These results strongly suggest that alpha-adrenoceptors in oviductal arteries are mainly of the alpha 1 subtype, although a possible role for alpha 2-adrenoceptors cannot be excluded.

Neurogenic responses of urethra isolated from the dog

Electrical transmural stimulation evoked contraction and relaxation in isolated urethral circular muscle of the dog. The responses were abolished by tetrodotoxin, indicating their neurogenic origin. The contractile force in the middle urethra was greater than that in the proximal and distal urethra. The contractions were not affected by atropine and propranolol, but were completely inhibited by phenoxybenzamine, prazosin and guanethidine. In preparations contracted with prostaglandin F2 alpha, electrical stimulation induced frequency-dependent relaxation in all urethral portions. Atropine, phenoxybenzamine, prazosin and guanethidine had no effect on the relaxation, while propranolol slightly attenuated the relaxation induced at the highest frequency used (5 Hz). The non-adrenergic, non-cholinergic relaxation was also not affected by ketanserin, methysergide, diphenhydramine, alpha,beta-methylene ATP or capsaicin. Exogenously applied phenylephrine and clonidine both produced contractions but the maximal response to clonidine was much smaller than that to phenylephrine. Acetylcholine produced no or feeble contractions. In the preparations contracted with prostaglandin F2 alpha, isoproterenol and vasoactive intestinal polypeptide (VIP) produced relaxation. These results suggest that the circular muscle of dog urethra is reciprocally innervated by sympathetic adrenergic and non-adrenergic, non-cholinergic nerves, and that the neurogenic responses are markedly affected by muscle tension and the portion of the urethra examined.

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.

Characterization of alpha-adrenoceptors in the preprostatic urethra of sexually immature male lambs

Strips of preprostatic urethra from immature male lambs (3-4 months old) were used to study alpha-adrenoceptor-mediated contractions in vitro. The rank order of potency of the agonists was noradrenaline = phenylephrine greater than adrenaline. B-HT 920 was ineffective and clonidine behaved as a weak partial agonist. The rank order of efficacy was found to be noradrenaline greater than phenylephrine greater than adrenaline. Prazosin (10(-9)-10(-6) M) and rauwolscine (10(-6)-10(-4) M) antagonized noradrenaline-induced contractions. The pA2 values were 7.89 and 5.38 for prazosin and rauwolscine, respectively. Phenoxybenzamine reduced the maximal response to noradrenaline and shifted the concentration-response curve to the right. The dissociation constant (KA) of noradrenaline was calculated to be 4.05 +/- 0.21 x 10(-6) M and the occupancy-response plot for noradrenaline showed a linear relationship. It is concluded that alpha-adrenergic contraction of the preprostatic urethra from immature lambs are mediated by alpha 1-adrenoceptors. It is also suggested that the lack of a receptor reserve may affect the degree of urethral constriction and could be related to hormonal status.

Calcium dependence of contractile activation of isolated sheep urethra. I: Responses to electrical stimulation

In isolated sheep urethral smooth muscle at resting tension, responses elicited by electrical stimulation of nerves were either contraction (40%), or contraction preceded by relaxation (60%). All responses were suppressed by tetrodotoxin (TTX), but at stimulation frequencies exceeding 16 Hz, there was a small TTX resistant contraction, which at the highest frequency used (50 Hz) amounted to about 15% of the total response. alpha-Adrenoceptor blockade suppressed the contractions, as did chemical sympathectomy with 6-hydroxydopamine. In calcium-free medium all electrically induced contractions were abolished. Nifedipine, verapamil, and lanthanum had concentration-dependent depressant actions, whereas Bay K 8644 (10(-6) M) significantly increased the responses. In preparations prelabelled with 3H-noradrenaline, electrical stimulation caused a release of 3H that could be blocked by TTX, and effectively reduced by calcium-free medium and lanthanum. However, the release was unaffected by nifedipine, and moderately reduced by high concentrations of verapamil (10(-5) M). It is suggested that contractile responses to electrical stimulation in isolated sheep urethral smooth muscle are mediated by the sympathetic nervous system, mainly through release of noradrenaline stimulating postjunctional alpha 1-adrenoceptors. The release of noradrenaline, as well as the action of released noradrenaline on postjunctional membranes are calcium-dependent. However, the calcium entry pathways used are partly different from those inhibited by organic calcium antagonists.

Differential effects of calcium antagonists and Bay K 8644 on contractile responses to exogenous noradrenaline and adrenergic nerve stimulation in the rabbit ear artery

1. The effects of three calcium antagonists (nifedipine, verapamil, diltiazem) and the calcium agonist Bay K 8644 were compared on contractile responses of similar amplitude elicited by noradrenaline (NA) and electrical nerve stimulation (ENS) in the rabbit isolated ear artery. 2. Contractions induced by both NA (3 x 10(-7) M) and ENS (10 Hz, 10s) were almost exclusively mediated by alpha 1-adrenoceptors, since 10(-7) M prazosin abolished (NA) or almost abolished (ENS) the responses, and prazosin was more than three orders of magnitude more potent than rauwolscine on both types of response. 3. ENS-induced contractions were considerably less inhibited by nifedipine, verapamil and diltiazem than were those elicited by NA. Bay K 8644 enhanced responses to NA more than those to ENS. 4. The inhibitory effect of nifedipine and Ca2+ deprivation on NA-induced contractions decreased with increasing NA concentration. Reduction of the NA response by prazosin or phenoxybenzamine increased the nifedipine inhibition. 5. Reduction of the ENS-induced contractions by prazosin or phenoxybenzamine, or by use of a lower stimulation frequency did not increase the inhibitory effect of nifedipine. 6. In conclusion, the differential effects of the calcium antagonists on NA- and ENS-induced contractions were not related to differences in alpha-adrenoceptor subtype (alpha 1/alpha 2), receptor reserve or response amplitude, but may rather reflect temporal and spatial differences in alpha-adrenoceptor activation between the responses.

Influence of temperature and extracellular pH on contractile responses in isolated human hand veins

The effects of moderate cooling and extracellular pH changes on concentration-response relationships for noradrenaline (NA), 5-hydroxytryptamine (5-HT) and prostaglandin F2 alpha (PGF2 alpha) were investigated in isolated human hand veins. pH changes were achieved by altering the NaHCO3 content of the Krebs solution. Cooling to 24 degrees reduced the maximum contractile responses to K+ (124 mM), NA and PGF2 alpha by 20-30%, whereas that to 5-HT was unchanged. The NA and 5-HT potencies were increased 8-10 times, whereas the PGF2 alpha potency was unaffected. A shift from alkalotic (pH 7.6) to acidotic (pH 6.9) conditions did not influence the contractile response to 124 mM K+, whereas the responses to NA, 5-HT and PGF2 alpha were inhibited with regard to both potency and maximum contraction. When related to neutral pH, acidosis significantly reduced only the 5-HT potency (4 times), whereas alkalosis selectively increased the NA and PGF2 alpha potencies (3 times). In the presence of prazosin (10(-7) M) cooling to 24 degrees significantly increased the NA potency, whereas no such increase was seen in the presence of rauwolscine (10(-7) M). Alkalosis significantly increased the NA potency in the presence of either antagonist. In conclusion, temperature and extracellular pH influenced the contractile responses to NA, 5-HT and PGF2 alpha in a differentiated manner. Alkalosis appeared to increase the response to both alpha 1- and alpha 2-adrenoceptor stimulation, whereas cooling preferentially increased that to alpha 2-adrenoceptor stimulation.

Nerve-mediated functions in the circular and longitudinal muscle layers of the proximal female rabbit urethra

In the present study, nerve mediated functions in the circular and longitudinal muscle layers of the female rabbit urethra were characterized. Based on light microscopic findings, the interest was focused on the proximal third of the organ. A microsurgical dissection technique was developed, allowing separation of the two muscle layers. The following studies were carried out: functional in vitro investigations including exogenous application of drugs and electrical field stimulation, investigations of the release of 3H-noradrenaline (NA), and autoradiography for visualization of muscarinic receptors. The results can be summarized as follows: the adrenergic nervous influence, which was mediated mainly via alpha 1-adrenoceptors, dominated the contractile response of the circular layer and contributed significantly to the contraction of the longitudinal layer. A previously described fast, non-adrenergic, non-cholinergic response was found in the circular muscle layer. This response should probably be ascribed to striated muscle with a different innervation and/or structure than skeletal muscle in general. Muscarinic cholinoceptors were abundant in the outer parts of the urethral wall and decreased in density in luminal direction. A significant cholinergic contractile component was demonstrated in the longitudinal muscle. A non-adrenergic, non-cholinergic nerve-mediated relaxant response was found in both layers, although more pronounced in the circular one. Vasoactive intestinal polypeptide completely relaxed both circular and longitudinal preparations contracted by noradrenaline. A possible basis for an interaction between adrenergic and cholinergic nerve endings was demonstrated in the circular muscle layer. Stimulation of muscarinic receptors on the adrenergic nerve endings markedly reduced the release of 3H-NA. The present results support the findings in previous studies on circularly and longitudinally oriented full-thickness preparations of the female urethra and further underline the differences in structure, innervation and receptor functions between the different muscle layers of the urethral wall.

Functional evaluation of sympathetically mediated responses in in vivo lower urinary tract of dogs

An in vivo procedure for evaluating local effects of alpha-adrenoceptor stimuli on the lower urinary tract was developed in anesthetized dogs. Electrical stimulation of hypogastric nerve at varied frequencies (1, 2, and 4 Hz) and intraarterial (i.a.) administration of an alpha 1-adrenoceptor agonist, phenylephrine (0.3, 1, and 3 micrograms) to the urethra and bladder through the cannulated right external iliac artery caused reproducible frequency- or dose-related increases in intraurethral pressure (IUP). Intrabladder pressure (IBP) was increased by the nerve stimulation but not by i.a. phenylephrine. Acetylcholine (10 micrograms) given i.a. elicited increases in both IUP and IBP. Prazosin (0.1, 1, and 10 micrograms/kg, i.v.) dose-dependently suppressed the urethral contractile responses to the nerve stimulation and i.a. phenylephrine, but it failed to affect the bladder contraction evoked by the nerve stimulation. The results suggest that the urethral contractile responses to hypogastric nerve stimulation as well as i.a. phenylephrine were mediated via alpha 1-adrenoceptors, whereas the IBP increasing effect of hypogastric nerve stimulation was not mediated via alpha 1-adrenoceptors.

Functional studies on the cholinergic and sympathetic innervation of the rat proximal urethra: effect of pelvic ganglionectomy or experimental diabetes

1. Isolated rings of rat proximal urethra contracted in response to acetylcholine, noradrenaline, phenylephrine, tachykinins or field stimulation. In the presence of field stimulation-induced contractions, isoprenaline or CGRP produced an inhibitory effect on evoked contractions. 2. Electrical field stimulation produced frequency-related contractions which, when using trains of stimuli, consisted of a phasic and a tonic response. These contractions were abolished by tetrodotoxin (1 microM) but unaffected by hexamethonium (10 microM). 3. Atropine (3 microM) produced a partial inhibition of these nerve-mediated contractions, slightly more evident on the tonic than the phasic response. In the presence of atropine (3 microM), phentolamine (3 microM) or prazosin (1-10 nM) produced an almost complete inhibition of the response to field stimulation. 4. After bilateral removal of the pelvic ganglia, electrical field stimulation was still able to induce tetrodotoxin-sensitive contractions of the rat proximal urethra. However, the inhibitory effect of atropine was much reduced as compared to controls, while the phentolamine-sensitive portion of the response was increased. 5. DMPP (0.1 mM) a nicotinic receptor agonist, produced a contraction (phasic and tonic components) which was strongly inhibited by hexamethonium (10 microM), abolished by prazosin (1 microM) and reduced by tetrodotoxin (3 microM). 6. In urethral rings from diabetic rats (i.v. streptozotocin 7-9 weeks before) the contractile response to acetylcholine or noradrenaline as well as the response to field stimulation were unchanged as compared to controls. 7. Capsaicin (1 microM) produced an inhibitory effect on the nerve-mediated contractions of the rat proximal urethra. This response was abolished by pelvic ganglionectomy but was unaffected in streptozotocin-treated rats. 8. These findings provide functional evidence that both cholinergic and noradrenergic nerves mediate the contractions of the rat proximal urethra. In addition, capsaicin-sensitive sensory nerves can produce certain local motor responses through release of endogenous neuropeptides. In vivo studies are needed to examine the interplay between these differents sets of motor nerves in the regulation of proximal urethra motility.

Infravesical outflow obstruction in rats: a comparison of two models

1. A new model of infravesical outflow obstruction was developed in male rats by the repeated s.c. administration of testosterone for 5-15 days (3 mg/kg die). The effects of this treatment which produced a 65% increase of prostate weight (10 days) on bladder voiding was evaluated in urethane anesthetized rats by the transvesical infusion of saline and compared to the cystometric alterations produced by application of a silk ligature at urethral level in female rats (4-8 weeks before) as described by Malmgren et al. (1987a, b). 2. Testosterone-pretreatment for 10 days produced little changes in bladder weight, bladder capacity or amplitude of micturition contraction but determined a marked increase in residual volume, indicating that infravesical outflow obstruction impaired significantly bladder voiding. Furthermore, detrusor instability was observed in the majority of testosterone-treated rats. 3. The participation of an active component to voiding impairment in testosterone-treated rats was suggested by the effect of intravenous prazosin which improved voiding efficiency. 4. In urethra-ligated female rats there was a marked increase in bladder weight which was paralleled by a dramatic alteration in micturition reflex that is marked increase in bladder capacity and residual volume. 5. It is concluded that these two models of infravesical outflow obstruction produce cystometric patterns simulating the urodynamic alterations observed in patients with benign prostatic hyperplasia and are potentially suitable for development of drugs in this field.

The response of autonomic receptors to castration and testosterone in the urinary bladder of the rabbit

The effects of castration and testosterone on the autonomic receptor density and contractility in the urinary bladder smooth muscle of male rabbits were compared to untreated animals. Four groups of rabbits were studied over a similar time span with Group 1 animals serving as the untreated controls. Two groups (Groups 2 and 3) were castrated 28 days prior to sacrifice, Group 2 animals received corn oil for 14 days, and Group 3 animals received testosterone, 10 mg./day, for 14 days. The Group 4 animals were non-operated and received testosterone 10 mg./day for 14 days. Ligand saturation binding studies for alpha adrenoceptors in the bladder base and proximal urethra were performed with [3H]dihydroergocryptine ([3H]DHE). Muscarinic cholinergic receptors (MChR) were assayed with [3H]quinuclidinyl benzilate ([3H]QNB) and beta adrenoceptors with [125I]iodocyanopindolol ([125I]CYP) on the detrusor smooth muscle. Castrated Group 2 animals showed no significant change in receptor density with either [3H]QNB or [125I]CYP in detrusor muscle, but did exhibit a significant reduction (59%) of alpha adrenoceptors in the bladder base-urethra. The testosterone treated castrate and testosterone treated non-operated animals had significant increases in the MChR density, but no change in the alpha adrenergic, or beta adrenergic receptor density as compared to untreated controls. Cumulative dose response contractile studies were performed with carbachol on detrusor muscle strips and with phenylephrine on bladder base strips in isolated organ baths. The contractile studies on muscles from Groups 1, 2 and 3 showed no change in the ED50 or maximal contractile strength between control, castration or testosterone treated castrated animals. The ratio of wet bladder weight as compared to total body weight between each of the treatment groups showed a slight increase in both of the testosterone treatment groups. It was concluded that castration down regulates the alpha adrenergic receptors of the bladder base, while testosterone treatment increases the density of MChRs, and increases the ratio of the bladder to total body weight. Although no contractile changes were observed in the bladder base tissue it is conceivable that longer chronic testosterone deficits might ultimately affect the bladder outlet resistance in the male because of the reduced alpha adrenergic receptor density.

Effects of nimodipine, Bay K 8644 and pinacidil on alpha 1- and alpha 2-adrenoceptor-mediated vasoconstriction in human hand veins

The effects of nimodipine, Bay K 8644 and pinacidil, three drugs interfering with transmembrane Ca2+ fluxes in different ways, were investigated in isolated human hand veins. Their ability to influence the concentration-response relationship for noradrenaline (NA) was assessed in the absence and presence of prazosin or rauwolscine. The contractile response to NA was almost abolished in Ca2+ -free medium. Nimodipine and pinacidil depressed the NA concentration-response curve both in the absence and presence of alpha-adrenoceptor blockers. The NA response was only partially inhibited by nimodipine, indicating that NA may activate nimodipine-insensitive influx pathways, presumably receptor-operated calcium channels. Pinacidil inhibited the contractile response to 124 mM K+ and reduced the NA-induced contraction in the presence of nimodipine, suggesting that pinacidil has actions other than the opening of potassium channels and subsequent membrane hyperpolarization. Bay K 8644 increased the NA potency fourfold in the presence of rauwolscine, whereas it had no effect on the NA response in the presence of prazosin and in the absence of alpha-adrenoceptor blockade. Such an action of Bay K 8644 can be reconciled with alpha 1-adrenoceptor activation causing membrane depolarization and opening of potential-operated calcium channels. It may be concluded that both alpha 1- and alpha 2-adrenoceptor-mediated contractions in human hand veins are highly dependent on Ca2+ influx, although the mechanisms utilized to bring about this influx partly differ between the two receptor subtypes.

Effects of some calcium antagonists on aggregation by adrenalin and serotonin and on alpha-adrenoceptor radioligand binding in human platelets

The inhibitory effects of verapamil, nifedipine and diltiazem, representatives of different classes of calcium antagonists, were studied on aggregation of human platelets induced by adrenalin and serotonin (5-HT). For references, the alpha-adrenoceptor-antagonists phentolamine (alpha 1 and alpha 2) and rauwolscine (alpha 2), and the 5-HT 2-receptor-antagonist ketanserin were included. Verapamil in the concentration range 10(-6) 10(-4) M inhibited both adrenalin- and serotonin-induced aggregation in a concentration-dependent manner, whereas nifedipine and diltiazem had little or no effect. Phentolamine and rauwolscine were clearly weaker than verapamil as antagonists of serotonin, and ketanserin lacked effect on adrenalin-induced aggregation. Binding studies with [3H]dihydro-alpha-ergocryptine and [3H]rauwolscine on human platelet membranes showed equal numbers of binding sites, suggesting that only alpha 2-adrenoceptors were present. In the same concentration range as inhibition of aggregation was obtained, verapamil inhibited binding of either radioligand. Nifedipine, diltiazem and 5-HT were all poor inhibitors of radioligand binding. The results suggest that verapamil at high concentrations not only has alpha-adrenoceptor antagonistic properties but also exerts 5-HT-receptor blocking effects. This was not found with the other calcium channel blockers examined (nifedipine, diltiazem).

Identification of receptor subtypes in the rabbit and human urinary bladder by selective radio-ligand binding

Recent advances in receptor technology have demonstrated that subtypes of each autonomic receptor exist. Using both direct radio-ligand studies and the inhibition of receptor binding by subtype-selective pharmacological antagonists, we have studied the distribution of subtypes of alpha and beta adrenergic receptors and muscarinic cholinergic receptors in the urinary bladder of the rabbit and man. Alpha adrenergic receptors were quantified by direct binding of tritiated prazosin (alpha-1), yohimbine (alpha-2), and the non-selective alpha adrenergic ligand dihydroergocriptine (DHE). These studies demonstrated that the distribution of alpha receptor subtypes in the bladder base (for both rabbit and human) is approximately 80% alpha-1 and 20% alpha-2. Beta receptor subtypes were identified by the inhibition of the non-selective ligand 3H-dihydroalprenalol (DHA) by the beta-1 selective inhibitor ICI-89 and the beta-2 selective inhibitor ICI-118. Initial studies demonstrated that the beta adrenergic density of the bladder body was 92 fmol per mg. protein for the rabbit and 32 fmol per mg. protein for human bladder body. Inhibition of DHA binding by ICI-118 demonstrated a single class of receptor with an IC50 of approximately 0.013 microM for both rabbit and human. Inhibition of DHA binding by ICI-89 also demonstrated one class of receptors with an IC50 of approximately 9.0 microM for both species. These results indicate that there are primarily beta-2 receptors in the rabbit and human bladder body. Although the number of muscarinic subtypes in existence is currently being re-evaluated, there are at least two which can be identified by the selective muscarinic agent pirenzepine (PZP). The brain has been shown to contain both high and low affinity PZP sites. Using both direct PZP binding to the bladder body, and the inhibition by PZP of the non-selective radio-ligand quinuclidinyl benzylate (QNB), we have demonstrated that both the rabbit and human bladder body have no observable high affinity PZP-selective binding and the inhibition of 3H-QNB by PZP demonstrated that there was only the low-affinity PZP binding site. Although receptor subtypes in the bladder have been the subject of numerous investigations, this is the first study describing the distribution of both adrenergic and cholinergic receptor subtypes in both the rabbit and human.

Theoretical and functional studies on alpha 1-and alpha 2-adrenoreceptors: an examination using the Schild plot

1 The influence on the shape of the Schild plot by a two-receptor system was studied in both functional and theoretical studies. In the functional studies, the alpha-adrenoreceptors in cat lingual arteries were studied since both alpha 1- and alpha 2-adrenoreceptors have been suggested to contribute to the noradrenaline-induced contractile response in this tissue. 2 The Schild plots constructed using noradrenaline (NA) as agonist and prazosin as antagonist gave a straight line with a slope close to unity. In contrast, the corresponding Schild plot constructed for rauwolscine appeared to be biphasic. 3 The results obtained in the functional study were discussed in view of Schild plots obtained from a theoretical model which was designed to take into consideration the presence of two distinct types of receptor, the percentile proportions of which could be altered. 4 The theoretical model indicates that the shape of the Schild plot can vary considerably depending on the relative contribution of each receptor subtype to the contractile response, the selectivity of the antagonist, and the range and number of antagonist concentrations used. 5 It is suggested that a response is predominantly mediated by alpha 1-adrenoreceptors and less by alpha 2-adrenoreceptors when the Schild plot for prazosin gives a slope close to unity and has a pA2-value representative for alpha 1-adrenoreceptors, and rauwolscine gives a slope less than unity and a significant shift of the NA concentration-response curve in concentrations around 10(-8) M. When the reverse is true (but with a significant shift caused by prazosin already at a concentration around 10(-9) M) the contraction elicited by NA is proposed to be predominantly mediated by alpha 2-adrenoreceptors and less by alpha 1-adrenoreceptors.

Ketanserin interaction with urethral alpha-adrenoceptors

The selective 5-HT2 receptor blocker ketanserin was found to reduce maximal urethral pressures in healthy females by about 40% without reducing blood pressure. In vitro, ketanserin completely or almost completely reduced contractions of the isolated female rabbit urethra induced by phenylephrine, noradrenaline (NA) and electrical field stimulation. The drug was less effective against responses evoked by clonidine and 5-hydroxytryptamine (5-HT). 5-HT-induced contractions were effectively reduced by methysergide, but little affected by prazosin and rauwolscine. In concentrations exceeding 10(-7) M ketanserin significantly increased efflux of 3H in 3H-NA preloaded preparations of rabbit urethral muscle. Low concentrations of 5-HT, less than 10(-6) M, had slight inhibitory effects of 3H release, whereas 5-HT 10(-5) M caused a significant increase. It is concluded that ketanserin counteracts the effects of postjunctional alpha 1-adrenoceptor stimulation in isolated rabbit urethra. Such an effect might also explain its urethral pressure lowering action in man.

Postjunctional alpha-adrenoceptors in human superficial epigastric arteries and veins

A pharmacological characterization of the postjunctional alpha-adrenoceptors in human superficial epigastric artery and vein was performed, using several alpha-adrenoceptor subtype selective agonists, and the antagonists prazosin (alpha 1) and rauwolscine (alpha 2). In the arteries prazosin fulfilled the criteria for a competitive antagonism in concentrations 10(-9)-10(-7) M, giving a pA2-value of 9.17 in the Schild plot. Rauwolscine in concentrations 10(-8)-10(-6) M caused less pronounced but significant dextral shifts of the noradrenaline (NA) concentration-response curves. In the veins rauwolscine behaved like a competitive antagonist (10(-8)-10(-7) M). The pA2-value was 9.16. Prazosin 10(-9) M displaced the NA concentration-response curve, but higher concentrations (10(-8) and 10(-7) M) caused no further displacement. Prazosin reduced the Emax-values in the veins. In the arteries the rank order of potency for the agonists was: cirazoline (alpha 1) greater than NA greater than naphazoline (alpha 2) greater than guanfacine (alpha 2) greater than phenylephrine (alpha 1). The intrinsic activities of clonidine (alpha 2), ST 587 (alpha 1), B-HT 920 (alpha 2) and B-HT 933 (alpha 2) were too low to allow meaningful comparisons to be made. The rank order of potency in the veins was: NA greater than clonidine (alpha 2) greater than naphazoline (alpha 2) greater than guanfacine (alpha 2) greater than phenylephrine (alpha 1) greater than B-HT 920 (alpha 2) greater than cirazoline (alpha 1) greater than B-HT 933 (alpha 2). The intrinsic activity of ST 587 was low.(ABSTRACT TRUNCATED AT 250 WORDS)

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

[3H]Rauwolscine ([3H]Ra), a selective ligand for the alpha 2 adrenergic receptor, was used to identify and characterize alpha 2 adrenergic receptors in prostate glands of men with benign prostatic hyperplasia. Specific binding of [3H]Ra to prostatic tissue homogenates was rapid and readily reversible by addition of excess unlabelled phentolamine. Scatchard analysis of saturation experiments demonstrates a single, saturable class of high affinity binding sites (Bmax = 0.31 +/- 0.04 fmol./microgram. DNA, Kd = 0.9 +/- 0.11 nM.). The relative potency of alpha adrenergic drugs (clonidine, alpha-methylnorepinephrine and prazosin) in competing for [3H]Ra binding sites was consistent with the order predicted for an alpha 2 subtype. The role of alpha 2 adrenergic receptors in normal prostatic function and in men with bladder outlet obstruction secondary to BPH requires further investigation.

Alpha 1- and alpha 2-adrenoceptors in the smooth muscle of male and female rabbit urethra

[3H]Prazosin and [3H]rauwolscine, specific alpha 1- and alpha 2-antagonists respectively, were used to label alpha-adrenoceptors in membranes from male and female rabbit urethra. In the male rabbit, [3H]prazosin bound with high affinity (Kd = 0.56 nM) to a single population of sites with a capacity of 73 fmol/mg protein. [3H]Rauwolscine bound with a lower affinity (Kd = 2.24 nM) to another single class of sites with a capacity of 41 fmol/mg protein. The order of potencies of various adrenergic compounds in inhibiting radioligand binding suggested that [3H]prazosin and [3H]rauwolscine interacted in the urethra with sites having the characteristics of alpha 1- and alpha 2-adrenoceptors, respectively. In addition, studies on the female rabbit urethra showed that alpha 2-adrenoceptor density and affinity were respectively 6 times higher and 2 times lower than in the male. No significant sex difference was observed for urethral alpha 1-adrenoceptors.

Effects of alpha-adrenoceptor subtype-selective antagonists on the human saphenous vein in vivo

The effects of the alpha-adrenoceptor subtype-selective antagonists prazosin (alpha 1) and yohimbine (alpha 2) on the saphenous vein of six healthy male subjects were investigated in vivo. The drugs were infused locally into the congested (40 mmHg), long saphenous vein constricted by simultaneous local infusion of noradrenaline (NA). Prazosin 10(-9) M (concentration in the infusion solution, infusion rate 0.3 ml min-1) did not reduce the NA-induced venoconstriction, but at a concentration of 10(-8) M there was a significant reduction; in two subjects no response to NA could be elicited in the presence of 10(-8) M prazosin. Prazosin 10(-7) M caused no further reduction of the NA effect compared to that produced by 10(-8) M in three of the subjects, whereas in one, prazosin 10(-9), 10(-8) and 10(-7) M caused a dose-dependent blockade. Yohimbine, 10(-9), 10(-8) and 10(-7) M caused a dose-dependent reduction of the NA-induced venoconstriction in all subjects. The results suggest that the human saphenous vein is endowed with functionally important populations of both alpha 2- and alpha 1-adrenoceptors.

Regional distribution of alpha-adrenoceptor subtypes in the female rabbit urethra

The aim of the present investigation was to study the regional distribution of alpha-adrenoceptor subtypes in the female rabbit bladder base and urethra, using radioligand binding and mechanical activity studies. The binding studies performed on membranes prepared from the bladder base, and the proximal and distal part of the urethra, revealed that the number of alpha 1-adrenoceptors did not significantly differ between the three regions, whereas the number of alpha 2-adrenoceptors increased distally. The mechanical activity studies showed that noradrenaline and clonidine, but not phenylephrine, were more potent in the distal than the proximal part of the urethra. It is suggested that in the female rabbit urethra the lower EC50-value found for noradrenaline and clonidine in the distal as compared to the proximal part of the urethra, at least in part, is attributable to a higher number of postjunctional alpha 2-adrenoceptors in the distal than in the proximal urethra.

Alpha-adrenoceptors and muscarinic receptors in the isolated human prostate

Prostatic specimens of adenomatous tissue (PA) were obtained from the lateral lobe of the prostate in patients with benign prostatic hyperplasia (BPH). Non-hyperplastic specimens of the outer prostatic tissue (PC) were taken from the dorsal part of the prostate in patients undergoing cystourethrectomy. Effects of alpha-adrenoceptor and muscarinic receptor active drugs were studied. Noradrenaline (NA) and the alpha 1-adrenoceptor agonist phenylephrine induced concentration-dependent contractions in PC and PA preparations. The alpha 2-adrenoceptor agonist clonidine was without effect on PC but contracted PA preparations; it was less potent and had less intrinsic activity than phenylephrine. In PC and PA strips, the alpha 1-adrenoceptor antagonist prazosin was more effective than the alpha 2-adrenoceptor antagonist rauwolscine to inhibit NA-induced contractions. Prazosin, but not rauwolscine, inhibited electrically induced contractions in PC strips. The muscarinic receptor agonists acetylcholine and carbachol were without effect in PC and PA preparations. In both PC and PA preparations, clonidine decreased and rauwolscine increased the electrically elicited 3H-efflux after pre-loading of the tissues with 3H-noradrenaline. Carbachol and scopolamine were without consistent effects. In radioligand receptor binding experiments, the occurrence of alpha 1- and alpha 2-adrenoceptor binding sites was demonstrated; the ratio between alpha 1- and alpha 2-adrenoceptor binding sites was 3/2 in PC, but 2/3 in PA tissue. By autoradiography, muscarinic receptors were found to be localized exclusively to the glandular epithelium, consistent with the lack of contractile effects of muscarinic receptor active drugs on PC and PA preparations. Our results thus suggest that the main alpha-adrenoceptor function in human prostatic smooth muscle is of the alpha 1-type and that muscarinic receptors in the prostate are involved in processes other than control of smooth muscle contraction.

Effect of drugs interacting with adrenoreceptors and muscarinic receptors in the epididymal and prostatic parts of the human isolated vas deferens

Electrical field stimulation of ring preparations of the epididymal (Ve) and prostatic (Vp) parts of the human isolated vas deferens produced contractions with similar frequency-dependence and appearance. The contractions of Ve, but not of Vp preparations were abolished by tetrodotoxin (10(-6)M). Noradrenaline (NA), phenylephrine, and methoxamine, but not clonidine induced repetitive, phasic contractions in both Ve and Vp preparations, and increased the amplitude of electrically induced responses. Clonidine concentration-dependently decreased electrically induced contractions in Ve preparations, but had no significant effects in Vp preparations. Phentolamine and prazosin abolished electrically induced contractions in Ve but not in Vp preparations. In Ve rings the contractions were increased by rauwolscine; no such effect was observed in Vp preparations. Isoprenaline, propranolol, acetylcholine and carbachol had no effects in the Ve or Vp preparations. Scopolamine and atropine reduced electrically induced responses. Clonidine decreased and rauwolscine increased the electrically induced release of 3H in both Ve and Vp preparations pre-loaded with 3H-NA. Phenylephrine, prazosin, isoprenaline, propranolol, carbachol and scopolamine had minor or no effects on the 3H release. Radioligand receptor binding experiments using 3H-prazosin and 3H-rauwolscine as ligands revealed similar densities of alpha 1- and alpha 2-adrenoreceptors in the human vas deferens. There seemed to be no differences in their distribution between the epididymal, middle and prostatic part of the organ. It is concluded that the neurotransmission in the human vas deferens is noradrenergic and mediated via alpha 1-adrenoreceptors. The prazosin and tetrodotoxin resistant part of the electrically induced contraction in Vp preparations may be caused by direct smooth muscle stimulation.

alpha 1-Adrenoceptor subtype mediating contraction of the smooth muscle in the lower urinary tract and prostate of rabbits

The alpha-adrenoceptor subtype mediating contraction of the smooth muscle in the urinary bladder base (trigone), proximal urethra and prostate isolated from male rabbits was investigated by comparing the responsiveness to alpha-adrenoceptor agonists and antagonists under condition where beta-adrenoceptors and neuronal and extraneuronal uptakes were inhibited. Noradrenaline (non-selective), phenylephrine (alpha 1-selective) and clonidine (alpha 2-selective) caused a dose-dependent contraction in the trigone, urethra and prostate. Phenylephrine acted as a full agonist whereas clonidine was a partial agonist. YM-12617 and prazosin (alpha 1-selective), phentolamine (non-selective) and yohimbine (alpha 2-selective) produced dose-dependent shifts to the right of the dose-response curves for noradrenaline, phenylephrine and clonidine in the all three tissues. YM-12617 (pA2 = 9.77, 9.67 and 9.73 for trigone, urethra and prostate, respectively), prazosin (pA2 = 8.26, 8.20 and 8.08), phentolamine (pA2 = 7.67, 7.62 and 7.45) and yohimbine (pA2 = 6.30, 6.30 and 5.94) showed constant pA2 values irrespective of the agonists and tissues used, suggesting that only a single subclass of alpha-adrenoceptors is present. The actual pA2 values for these antagonists are comparable to those reported previously in tissues said to contain mainly alpha 1-adrenoceptors. Thus, we concluded that the postsynaptic alpha-adrenoceptors of the rabbit trigone, urethra and prostate mediating contraction belong to the alpha 1-subtype.

Adrenergic and non-adrenergic contraction of isolated urethral muscle from rabbit and man

Isolated urethral muscle from rabbit and man was subjected to electrical field stimulation and the components of the resulting contraction were analyzed. Contraction usually consisted of 2 main components. One was rapidly developing, non-adrenergic and atropine-resistant. The other developed more slowly and was sensitive to alpha-adrenoceptor blockade and to chemical sympathectomy with 6-hydroxydopamine. By lowering the temperature from 37C to 20C, these contraction components could be separated from each other. Both could be blocked by tetrodotoxin but the effects of this drug were not consistent in human tissue. Characteristic for the initial contraction component was its dependence on the tension of the preparation; it increased with increasing tension. The adrenergic part of the contraction could be effectively blocked by phentolamine and prazosin, whereas rauwolscine was less effective. Atropine and scopolamine also inhibited the adrenergic part of the contraction, whereas acetylcholine caused a transient increase. The non-adrenergic contraction component was less sensitive to deprivation of extracellular Ca2+ than the adrenergic; almost 40 per cent remained after exposure to Ca2+ free solution for 40 min., whereas the adrenergic component disappeared after 20 min. exposure. Light and electron microscopic investigation revealed 3 distinct layers of the rabbit urethral wall, 1 outer consisting of smooth and striated muscle, 1 middle consisting of smooth muscle only, and a submucosal layer where vessels and smooth muscle cells were found. Removal of the longitudinal muscle layer did not change the responses to electrical stimulation. The results suggest that the electrically induced adrenergic activation of urethral muscle of both rabbit and man was mediated mainly via alpha-adrenoceptors and that this muscle is innervated not only by sympathetic, adrenergic nerves but also by a type of nerve able to release a non-cholinergic, contraction-mediating transmitter.

Contractant and relaxant properties of the female rabbit urethral submucosa

Isolated submucosal (lamina propria) preparations from the female rabbit urethra exhibited both contractant and relaxant properties. The nerve-mediated contraction to electrical field stimulation was adrenergic in nature, and both this response and the contraction induced by exogenous application of noradrenaline were blocked to a greater extent by alpha 2 than by alpha 1-adrenoceptor blocking agents. Vasoactive intestinal polypeptide was found to be a potent inhibitor of the noradrenaline-mediated contraction. Neuropeptide Y induced contraction of the preparation, but also inhibited the nerve-mediated contractant response. In noradrenaline-contracted preparations, electrical field stimulation induced a non-adrenergic, non-cholinergic relaxation. The maximum relaxant response was significantly greater when the preparations were contracted by clonidine than by noradrenaline. Abundant smooth muscle cells with no obvious connection to vessel walls were found in the submucosa, but to what extent the contractant and relaxant responses can be ascribed to vascular or non-vascular smooth muscle is not settled. The results indicate a non-uniform distribution of the peripheral nervous control within the wall of the female rabbit urethra. The demonstrated contractant and relaxant properties of the submucosal tissue might be of importance for urethral function.

Differential effects of nifedipine, verapamil, and diltiazem on noradrenaline-induced contractions, adrenergic transmitter release, and alpha-adrenoceptor binding in the female rabbit urethra

In order to study differences in action between "Ca2+-entry blockers" on smooth muscle and peripheral nerves, the effects of nifedipine, verapamil, and diltiazem on noradrenaline (NA)-induced contractions and electrically evoked release of 3H-NA were investigated in the female rabbit urethra. In addition, possible influences of Ca2+-entry blockers on alpha-adrenoceptors were studied with radioligand binding technique. Exposure to Ca2+-free medium completely abolished the contractile response to 1 microM NA in the rabbit urethra, indicating that the contraction was entirely dependent on influx of extracellular Ca2+. The Ca2+-entry blockers inhibited the NA-induced contractions in the following order of potency: nifedipine greater than verapamil approximately equal to diltiazem. In contrast to nifedipine and diltiazem, which produced a maximum inhibition of between 50 and 60%, verapamil was able to abolish the contractile responses to NA. The electrically evoked efflux of 3H-NA was decreased by diltiazem and increased by verapamil, whereas nifedipine failed to alter the 3H-NA efflux. Only verapamil was effective in inhibiting specific 3H-DHE binding to a crude membrane preparation of the rabbit bladder base and urethra, and the inhibition appeared to be of the competitive type. It is suggested that the effects of verapamil on electrically evoked efflux of 3H-NA and on NA-induced contractions can be partly explained by blockade of pre- and post-junctional alpha-adrenoceptors. The failure of nifedipine and diltiazem to abolish the NA-induced contraction might indicate the existence of different Ca2+-entry pathways in urethral smooth muscle.