The effects of clonidine on electrically-induced contractions of rat detrusor strips in vitro

Pre-junctional alpha2-adrenoceptors modulation of the nitrergic transmission in the pig urinary bladder neck

AIMS

To investigate the nitric oxide (NO)-mediated nerve relaxation and its possible modulation by pre-junctional alpha2-adrenoceptors in the pig urinary bladder neck.

METHODS

Urothelium-denuded bladder neck strips were dissected, and mounted in isolated organ baths containing a physiological saline solution (PSS) at 37 degrees C and continuously gassed with 5% CO2 and 95% O2, for isometric force recording. The relaxations to transmural nerve stimulation (electrical field stimulation [EFS]) or exogenously applied NO were carried out on strips pre-contracted with 1 microM phenylephrine (PhE) and treated with guanethidine (10 microM) and atropine (0.1 microM), to block noradrenergic neurotransmission and muscarinic receptors, respectively.

RESULTS

EFS (0.2-1 Hz, 1 msec duration, 20 sec trains, current output adjusted to 75 mA) evoked frequency-dependent relaxations which were abolished by the neuronal voltage-activated Na+ channel blocker tetrodotoxin (TTX, 1 microM). These responses were potently reduced by the nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine (L-NOARG, 30 microM) and further reversed by the NO synthesis substrate L-arginine (L-ARG, 3 mM). The alpha2-adrenoceptor agonist BHT-920 (2 microM) reduced the electrically evoked relaxations, its effectiveness being higher on the responses induced by low frequency stimulation. BHT-920-elicited reductions were fully reversed by the alpha2-adrenoceptor antagonist rauwolscine (RAW, 1 microM). Exogenous NO (1 microM-1 mM) induced concentration-dependent relaxations which were not modified by BHT-920, thus eliminating a possible post-junctional modulation.

CONCLUSIONS

These results indicate that NO is involved in the non-adrenergic non-cholinergic (NANC) inhibitory neurotransmission in the pig urinary bladder neck, the release of NO from intramural nerves being modulated by pre-junctional alpha2-adrenoceptor stimulation.

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.

Nerve induced responses and force-velocity relations of regenerated detrusor muscle after subtotal cystectomy in the rat

AIMS

To study the pharmacological and mechanical properties of newly developed detrusor muscle after subtotal cystectomy, to explore if the regenerated detrusor has characteristics similar to the normal bladder base, from which it regenerated, or to the normal bladder body which it replaces.

METHODS

Partial cystectomy was performed in female rats. Fifteen weeks later, detrusor strips were cut from supratrigonal and equatorial segments. Unoperated rats served as controls. Responses to field stimulation were obtained in the absence and presence of scopolamine, prazosin, and P2X1 blockade. Dose-response curves were obtained for carbachol, alpha,beta-methylene-ATP, and phenylephrine. Force-velocity data were obtained on maximally activated chemically skinned preparations.

RESULTS

Maximal contractile response to field stimulation was 60% of that to high-K+ with no difference between strips from control and cystectomy bladders. Prazosin had no effect. Scopolamine decreased maximal response of supratrigonal strips to 62 +/- 6 (controls) and 61 +/- 4% (operated) of that without blocker. For equatorial strips the decrease was to 81 +/- 5 (controls) and 58 +/- 8% (operated). Frequency-response relations were obtained during blockade with scopolamine, alpha,beta-methylene-ATP, and prazosin. Supratrigonal strips showed a pronounced additional inhibition up to 40 Hz. Equatorial strips from controls were completely inhibited at all frequencies. Equatorial strips from operated bladders were inhibited up to 20 Hz but not at 40 and 60 Hz. Carbachol EC(50) values were similar in all groups. Maximum response to phenylephrine was 10-20% of high-K+ response. Maximal shortening velocity (Vmax) was similar in control supratrigonal and equatorial strips, but was significantly lower in the operated bladders.

CONCLUSIONS

(1): A regional difference exists in pharmacological properties of control detrusor, with a considerable contractile response to stimulation remaining in the supratrigonal muscle after simultaneous cholinergic, adrenergic, and purinergic blockade. (2): The new detrusor was functionally well innervated with no supersensitivity to muscarinic stimulation. (3): The newly formed bladder body had pharmacological properties specific for the supratrigonal segment from which it had developed. (4): There was no regional difference in force-velocity characteristics of the control detrusor. (5): The lowered Vmax in the newly formed bladder might thus be related to growth and regeneration of muscle cells.

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.

Omega conotoxin and prejunctional modulation of the biphasic response of the rat isolated urinary bladder to single pulse electrical field stimulation

1. Single pulse electrical field stimulation (EFS) produces a biphasic response of muscle strips of the rat isolated urinary bladder consisting of an early and a late contraction which were atropine-resistant and atropine-sensitive, respectively. Repeated application of desensitizing doses of the P2 purinoceptor agonist, alpha, beta-methylene ATP (mATP) inhibited the early response while leaving unaffected the late component. 2. Omega conotoxin (CTX, 0.1 microM) inhibited both the early and the late response either in control conditions or after enhancement by physostigmine (0.1 microM). The effect of CTX was, in both cases, more pronounced on the late than the early response to EFS. CTX (0.1 microM) failed to affect contraction produced by ATP or acetylcholine at concentrations (0.3 mM and 0.5 microM) which produced a response similar to that to EFS. 3. The effect of physostigmine was more intense for the late than the early response and was abolished by atropine. In the presence of CTX, physostigmine enhanced both the early and the late components of the mechanical response to EFS. 4. Nifedipine (0.1-1 microM) reduced to a similar extent both the early and late responses. Bay K 8644 (1 microM) produced a marked enhancement of the response to EFS, which, however, did not have a distinct late peak. In the presence of Bay K 8644, either atropine (3 microM) or tetrodotoxin (1 microM) had minor inhibitory effects indicating the myogenic origin of the response. 5. Neurokinin A (0.1-1 nM) enhanced both the early and late responses to EFS without affecting the contraction produced by exogenous acetylcholine or ATP. A consistent potentiation was evident also in the presence of CTX and for the early response, in the presence of atropine. Clonidine (3 microM) inhibited the response to EFS either in the absence or the presence of physostigmine. The inhibitory effect of clonidine, shown previously to depend upon activation of prejunctional alpha 2-adrenoceptors, was still observed in presence of CTX or atropine. 6. It is concluded that CTX-sensitive voltage dependent calcium channels play a more important role in determining the cholinergic rather than the non-cholinergic, putatively purinergic, component of the biphasic response of the rat bladder to single pulse EFS. The action of CTX is likely to be exerted on N-type rather than L-type (dihydropyridine-sensitive) calcium channels. Prejunctional modulation (enhancement by neurokinin A, inhibition by clonidine) occurs even in the presence of CTX-sensitive channels blockade.

The nonstop transvesical cystometrogram in urethane-anesthetized rats: a simple procedure for quantitative studies on the various phases of urinary bladder voiding cycle

A new cystometric procedure that involves a nonstop transvesical (through a needle inserted into the bladder dome) infusion of saline in urethane-anesthetized rats is described. This permits the obtainment of a series of repetitive voiding cycles in both male and female rats. Recording at high-chart speed permits quantitative measurements of the various phases of the voiding cycle. Micturition was almost invariably associated with appearance, during a sustained increase of intravesical pressure, of a series of high-frequency oscillations, which were paralleled by a stream-like emission of the infused fluid. Micturition was not observed after topical tetrodotoxin, after bilateral transection of pelvic nerves, or in acute spinal rats. Intravenous hexamethonium produced a dose-related impairment of the voiding cycle, and, at high doses, suppressed micturition, and overflow incontinence ensued. The experimental procedure described herein appears to be suitable for physiopharmacological studies dealing with regulation of voiding cycle.

The postganglionic excitatory innervation of the mouse urinary bladder and its modulation by prejunctional GABAB receptors

Field stimulation produced reproducible contractions of the mouse isolated urinary bladder whose amplitude was frequency-related. These contractions were partially sensitive to atropine (3 microM), unaffected by hexamethonium (10 microM) and almost abolished by tetrodotoxin (0.5 microM). Atropine (3 microM) suppressed contractions produced by exogenous acetylcholine thereby indicating atropine-resistance of the nerve-mediated contractions. Nerve-mediated contractions of the mouse urinary bladder were enhanced by physostigmine (0.1-0.5 microM) and inhibited by hemicholinium-3 (0.5 mM) thus confirming the presence of a cholinergic component in the excitatory postganglionic innervation. Atropine (3 microM) inhibition of the nerve-mediated contractions increased with increasing duration and strength of the train of stimulation. The nerve-mediated contractions of the mouse bladder were unaffected by phentolamine (0.2 microM), propranolol (0.3 microM) or indomethacin (5 microM). ATP (1mM) the major candidate for the role of nonadrenergic-noncholinegic (NANC) excitatory neurotransmitter in the mammalian urinary bladder produced a contraction of the mouse isolated bladder. Exposure to the stable ATP analogue alpha, beta-methylene ATP (APCPP) or beta, gamma-methylene ATP (APPCP) produced a partial desensitization of the nerve-mediated response which, for APCPP, was greater in the presence than in the absence of atropine (3 microM). In the presence of atropine (3 microM) and after APCPP desensitization the amplitude of the response to field stimulation amounted to about 20% of the original response and was sensitive to tetrodotoxin, indicating that it is nerve-mediated. GABA (0.001-0.3 mM) inhibited the amplitude of field stimulation induced contractions of mouse urinary bladder. This effect was mimicked by the selective GABAB receptor agonist, (+/-)-baclofen, but not by the selective GABAA receptor agonist, homotaurine. GABA and (+/-)-baclofen exhibited cross-desensitization. The GABA-or (+/-)-baclofen-induced inhibition of the nerve-mediated contractions were reduced by previous exposure to homotaurine (1 mM) or to 5-aminovaleric acid (2 mM), two GABAB receptor antagonists. On the other hand the inhibitory effects of GABA or (+/-)-baclofen were unaffected by picrotoxin (0.1 mM), a selective GABAA receptor antagonist. The inhibitory effect of GABA on nerve-mediated contractions was reduced in the presence of atropine or hemicholinium-3 as well as following desensitization of P2-purinoreceptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacological evidence for the existence of two components in the twitch response to field stimulation of detrusor strips from the rat urinary bladder

Isolated strips from the anterior dome of the rat urinary bladder respond to single pulse field stimulation with a contraction. Two distinct components of this contraction ("early' and "late') could be observed, both of which were unaffected by hexamethonium (10 microM) and almost abolished by tetrodotoxin (1 microM) indicating their dependence on neurotransmitter release from postganglionic nerve endings. Atropine (3 microM) inhibited the "late' component (over 60%) to a significantly greater extent than the "early' component (less than 10%) Amplitude of the "early' component was usually greater than that of the "late' component. There was almost no difference between the "early' and "late' component in respect of their relationship to stimulus strength and pulse duration. Physostigmine (0.03 microM) enhanced both components of the nerve-mediated contraction, although enhancement of the "late' component was much greater than that of the "early' one. Tetraethylammonium (TEA, 0.5-5 mM) enhanced, in a concentration-related manner, both "early' and "late' components of the nerve-mediated contraction. Following exposure to physostigmine or TEA (5 mM) both "early' and "late' components of contraction were almost completely inhibited by tetrodotoxin. Atropine inhibition was more evident on the "late' as compared to "early' component of contraction. These findings demonstrate the presence, in the twitch response of rat isolated urinary bladder to field stimulation, of two nerve-mediated components which exhibit a different susceptibility to atropine and physostigmine.

GABAB receptor mediated inhibition of field stimulation induced contractions of detrusor muscle strips from newborn rats

The effect of GABA on field stimulation (0.1 Hz) induced contractions have been examined on detrusor strips from adult and newborn rats. Field stimulation induced contractions were unaffected by hexamethonium (10(-5) M) but almost suppressed by tetrodotoxin (5 X 10(-7) M) in both age groups. Atropine (3 X 10(-6) M) produced a marked inhibition of twitches in detrusor strips from newborn rats but only a slight (10% or less) inhibition in strips from adult animals. Neither GABA (10(-4), homotaurine (10(-3) M) or (+/-)-baclofen (10(-4] affected twitches in detrusor strips from adults rats. On the other hand both GABA and (+/-)-baclofen inhibited, to about the same extent (55-60% inhibition) twitches in detrusor strips from newborn animals. Homotaurine had no significant effect. The effects of both GABA (10(-4) M) and (+/-)-baclofen (10(-4) M) on twitches in detrusor strips from newborn rats were reduced if the preparations were previously exposed to a low concentration of GABA (5 X 10(-6) M) thus indicating desensitization. The inhibitory effect of GABA on twitches in newborn rats were concentration dependent in the range of 10(-7) to 10(-4) M. The GABAA receptor antagonist picrotoxin (10(-4) M) had no effect on the GABA concentration response curve whereas the GABAB receptor antagonist, homotaurine, produced an antagonism of the competitive type. The calculated pA2 value for the GABA-homotaurine interaction was 4.4 +/- 0.07. Neither GABA (10(-4) M) nor (+/-)-baclofen (10(-4) M) produced any significant inhibition of acetylcholine (2 X 10(-7) M) induced contractions of detrusor of strips from newborn rats, while atropine (3 X 10(-6) M) completely suppressed them. These results indicate that GABA inhibits field stimulation induced contractions of detrusor strips from newborn rats by activating prejunctional receptors (presumably of the GABAB subtype) which inhibit the release of acetylcholine from the postganglionic nerve endings.