Effects of calcium, calcium channel blockers and Bay K 8644 on contractions induced by muscarinic receptor stimulation of isolated bladder muscle from rabbit and man

Reviving Cav1.2 as an attractive drug target to treat bladder dysfunction

Inhibition of bladder contraction with antimuscarinics is a common approach to treat bladder hyperactivity, and the L-type voltage-gated calcium channel α_1C (Cav1.2) is crucial for bladder contractility. Therefore, strategies aimed at inhibiting Cav1.2 appear warranted. However, multiple clinical trials that attempted to treat bladder overactivity with calcium channel blockers (CCBs) have been unsuccessful, creating an unsolved mystery. In contrast, cardiologists and epidemiologists have reported strong associations between CCB use and bladder hyperactivity, opposing expectations of urologists. Recent findings from our lab offer a potential explanation. We have demonstrated that ketamine which can cause cystitis, functions, like nifedipine, as a Cav1.2 antagonist. We also show that a Cav1.2 agonist which potentiates muscle contraction, rather than antagonizing it, can increase the volume of voids and reduce voiding frequency. This perspective will discuss in detail the unsuccessful urological trials of CCBs and the promise of Cav1.2 agonists as potential novel therapies for bladder dysfunctions.

Muscarinic receptor-induced contractions of the detrusor are impaired in TRPC4 deficient mice

Acetylcholine contracts the bladder by binding to muscarinic M3 receptors on the detrusor, leading to Ca²⁺ influx via voltage-gated Ca²⁺ channels. The cellular mechanisms linking these events are poorly understood, but studies have suggested that activation of TRPC4 channels could be involved. The purpose of this study was to investigate if spontaneous and cholinergic-mediated contractions of the detrusor were impaired in TRPC4 deficient (TRPC4^−/−) mice. Isometric tension recordings were made from strips of wild-type (WT) and TRPC4^−/− detrusor. Spontaneous phasic detrusor contractions were significantly smaller in TRPC4^−/− mice compared to wild-type, however no difference in response to exogenous application of 60 mM KCl was observed. Cholinergic responses, induced by electric-field stimulation (EFS), bath application of the cholinergic agonist carbachol, or the acetylcholinesterase inhibitor neostigmine were all significantly smaller in TRPC4^−/− detrusor strips than wild-type. Surprisingly, the TRPC4/5 inhibitor ML204 reduced EFS and CCh-evoked contractions in TRPC4^−/− detrusor strips. However, TRPC5 expression was up-regulated in these preparations and, in contrast to wild-type, EFS responses were reduced in amplitude by the TRPC5 channel inhibitor clemizole hydrochloride. This study demonstrates that TRPC4 channels are involved in spontaneous and cholinergic-mediated contractions of the murine detrusor. TRPC5 expression is up-regulated in TRPC4^−/− detrusor strips, and may partially compensate for loss of TRPC4 channels.

Muscarinic Receptor Induced Contractions of the Detrusor are Mediated by Activation of TRPC4 Channels

PURPOSE

Muscarinic receptor mediated contractions of the detrusor rely on Ca²⁺ influx through voltage-gated Ca²⁺ channels but to our knowledge the mechanism linking stimulation of M3Rs to the activation of voltage dependent Ca²⁺ channels has not been established. TRPC4 channels are receptor operated cation channels that couple muscarinic receptor activation to depolarization of intestinal smooth muscle cells, voltage-activated Ca²⁺ influx and contraction. We investigated whether TRPC4 channels are involved in cholinergic mediated contractions of the detrusor.

MATERIALS AND METHODS

Isometric tension recordings were made on strips of murine detrusor and intracellular Ca²⁺ measurements were made on isolated detrusor myocytes using confocal microscopy. Transcriptional expression of TRPC and IP₃R subtypes in intact detrusor strips and isolated detrusor myocytes was assessed using reverse transcriptase-polymerase chain reaction.

RESULTS

Cholinergic stimulation of the detrusor induced by electrical field stimulation or exogenous application of carbachol or neostigmine evoked contractions consisting of a transient plus a tonic response, which was blocked by ML204, an inhibitor of TRPC4 channels. A phasic oscillatory component was blocked by the IP₃R inhibitor 2-APB. Carbachol evoked reproducible Ca²⁺ responses in isolated detrusor myocytes, consisting of an initial Ca²⁺ transient followed by Ca²⁺ oscillations. ML204 inhibited the initial Ca²⁺ transient whereas 2-APB inhibited the Ca²⁺ oscillations. Reverse transcriptase-polymerase chain reaction experiments showed that TRPC4β, TRPC6 and IP₃R1 were selectively expressed in isolated detrusor myocytes. Control experiments demonstrated that ML204 did not affect L-type Ca²⁺ or BK current amplitude, caffeine induced Ca²⁺ transients or KCl induced contractions of the detrusor.

CONCLUSIONS

Muscarinic receptor mediated contractions of the detrusor involve the activation of TRPC4β channels.

Resveratrol prevents bradykinin-induced contraction of rat urinary bladders by decreasing prostaglandin production and calcium influx

Resveratrol, a polyphenol found in grapes and peanuts, exerts beneficial effects on a number of diseases of cardiovascular and central nervous system. However, effects of resveratrol on the urinary system have not been fully investigated. In the present study, we examined effects of resveratrol on bradykinin-induced contraction and release of prostaglandin E2 in isolated rat urinary bladders. The effects of resveratrol on contractions induced by several agonists (prostaglandin E2, prostaglandin F2α and carbachol) and high K+ were also examined. We found that resveratrol concentration-dependently reduced the bradykinin-induced contraction in the rat urinary bladder preparations. The higher concentration of resveratrol (100 μM) abolished the bradykinin-induced prostaglandin E2 release. Similar results were obtained when the cyclooxygenase inhibitor indomethacin (10 μM) was used instead of resveratrol. Resveratrol also attenuated the prostaglandin E2-, prostaglandin F2α-, and to a lesser extent carbachol-induced contractions. Contractile responses to bradykinin, prostaglandin E2 and carbachol were largely prevented by blockade of Ca2+ channels with diltiazem. Both resveratrol and diltiazem prevented contractions induced by an addition of Ca2+ (2.5- 10 mM) into Ca2+-free/50 mMK+ solution or by 50 mMK+ solution containing normal Ca2+ (2.5 mM). These results suggest that resveratrol prevents bradykinin-induced contractions by attenuating not only the production of prostaglandins but also actions of them. The effect of resveratrol on contractile actions seems to be in part due to inhibition of Ca2+ influx. Because bradykinin plays an important role in pathological conditions of urinary bladder function, resveratrol may exert beneficial effects on the urinary bladder diseases.

A complex of Ca(V)1.2/PKC is involved in muscarinic signaling in smooth muscle

Here we present functional and biochemical evidence for a Ca(2+) channel (Ca(V)1.2)/protein kinase C (PKC) signaling complex being a key player in muscarinic regulation of urinary bladder smooth muscle. Muscarinic stimulation induced Ca(2+) signals and concomitant contractions in detrusor muscle from mice that were dependent on functional Ca(2+) channels. These signals were still present in muscles being depolarized by 85 mM extracellular K(+). Muscarinic-induced contractions were reduced by a PKC inhibitor [bisindolylmaleimide I (BIM-I)] and a phospholipase D (PLD) inhibitor (1-butanol). A phorbol ester (PDBu) enlarged muscarinic-induced Ca(2+) signals and contractions. The effects of BIM-I and PDBu were inhibited by isradipine and/or absent in muscles from Ca(V)1.2-deficient mice. Both carbachol and PDBu increased Ca(V)1.2 channel currents in isolated bladder myocytes. Blue native-PAGE electrophoresis revealed that Ca(V)1.2, PKC, and PLD are closely associated in muscles being previously stimulated by carbachol. Immunoprecipitation using anti-Ca(V)1.2 followed by Western blotting demonstrated that Ca(V)1.2 and PKC are coupled in stimulated muscles from wild-type mice. Autoradiography on immunoprecipitates showed that Ca(V)1.2 is a substrate for PKC-mediated phosphorylation. These findings suggest that a signaling complex consisting of Ca(V)1.2, PKC, and, probably, PLD controls muscarinic-mediated phasic contraction of urinary bladder smooth muscle.

Phospholipase C mediated Ca2+ signals in murine urinary bladder smooth muscle

Muscarinic stimulation of urinary bladder induces contraction via an increase in intracellular Ca(2+) concentration that results from Ca(2+) influx through Ca(2+) channels and/or IP(3)-mediated Ca(2+) release controlled by phospholipase C (PLC) signalling. The significance of PLC/IP(3) signalling in this cascade has recently been questioned because PLC inhibitors were without effect on carbachol-induced contractions in detrusor muscle strips. However, PLC/IP(3)-mediated Ca(2+) release was clearly observed in recordings of Ca(2+) signals in isolated myocytes. Therefore, we investigated the presence of PLC/IP(3)-dependent Ca(2+) release by directly monitoring Ca(2+) signals in intact detrusor muscle strips. Concomitant Ca(2+) signals from Ca(2+) channel activity were eliminated by the Ca(2+) channel antagonist isradipine (3 microM) or by the use of muscles from Ca(v)1.2 channel-deficient (SMACKO) mice. In absence of Ca(2+) channel activity, carbachol elicited contractions and Ca(2+) signals in muscles from wild type and SMACKO mice that were inhibited by the PLC inhibitor U73122 (10 microM). The results show that PLC/IP(3)-dependent Ca(2+) release is activated by stimulation with carbachol in urinary bladder smooth muscle but has a minor contribution to overall carbachol-induced Ca(2+) signals.

Intracellular calcium in hypertrophic smooth muscle from rat urinary bladder

OBJECTIVE

To explore whether infravesical outlet obstruction is associated with alterations in calcium activation of detrusor smooth muscle.

MATERIAL AND METHODS

Outlet obstruction was created by partial ligature of the urethra in female rats. Western blotting was performed using an antibody against the cytoplasmatic region of the alpha1c subunit of the L-type Ca2+ channel. Intracellular calcium was measured using Fura-2 in detrusors that had been obstructed for 10 days and activated by high K+ concentrations at different extracellular Ca2+ concentrations. The rate of force development after rapid opening of L-type Ca2+ channels was measured in contractions initiated by flash photolysis of nifedipine in Ca2(+)-containing depolarizing solution.

RESULTS

Bladder weight increased from 62 +/- 3 to 254 +/- 43 mg after 10 days of obstruction. Expression of the alpha1c subunit increased after 3 days and continued to increase until it was about fourfold greater after 10 days; however, it had not increased further at 6 weeks. This change was reversible after removal of obstruction. Activation with K+ produced a stable force at different extracellular Ca2+ concentrations, with no difference in response between controls and rats that had been obstructed for 10 days. Intracellular Ca2+ concentrations were lower in the obstructed group, showing that the calcium sensitivity of the contraction force had increased. The delay between the opening of L-type channels and the onset of contraction was longer in obstructed detrusors.

CONCLUSIONS

Growth of detrusor muscle following obstruction is accompanied by attenuated calcium transients following activation, despite upregulation of L-type Ca2+ channels. The Ca2+ sensitivity of contraction was increased in obstructed detrusors. We suggest that the decreased surface: volume ratio in hypertrophic smooth muscle cells is partly involved in the lowered Ca2+ transients. The increases in L-type calcium channels and in calcium sensitivity may be compensatory mechanisms.

Effect of letrozole on urinary bladder function in the female rabbit

OBJECTIVE

To investigate the effect of letrozole (a potent aromatase inhibitor that effectively inhibit the synthesis of oestrogen) on bladder contraction with changes in morphology and biochemistry.

MATERIALS AND METHODS

Sixteen female New Zealand white rabbits were separated into four equal groups; groups 1-3 were given oral letrozole for 1, 2 and 3 weeks, and group 4 was given saline and served as the control group. At the end of the medication period each rabbit was anaesthetized and the bladder muscle strips were used for contractile, histological and biochemical studies.

RESULTS

The concentration of serum oestrogen was significantly lower and testosterone was significantly higher in letrozole-treated rabbits than in the control group. The rabbits treated for 1 week with letrozole showed significant decreases in the contractile responses to electrical field stimulation, ATP and carbachol, but not to KCl. Contractility returned to normal in the rabbits treated for 2 and 3 weeks. Letrozole resulted in an increased volume percentage of collagens and decreased bladder compliance. The volume percentage of the smooth muscle component also changed, with a significant decrease at 1 week and then a gradual increase at 2 and 3 weeks. Contractile dysfunction was absent at 2 and 3 weeks, which was consistent with no change in sarcoplasmic reticulum Ca(2+)-ATPase content or mitochondrial function.

CONCLUSIONS

The bladder contractility decline in the first week and was restored at 2 and 3 weeks. The present study unexpectedly showed the possibility that testosterone might be as important as oestrogen in the contractile function of the female bladder.

Urinary incontinence: classification and pharmacological therapy

Pharmacological therapy has been developed which can have significant impact in the management of many forms of urinary incontinence and voiding dysfunction. In general the clinical laboratory studies which have supported or challenged the efficacy of many of the commonly prescribed drugs for voiding dysfunction are often difficult to interpret and contradictory. The available clinical studies often do not demonstrate a lack of bias. Nor do they include an adequate number of subjects, use appropriate and sensitive methods of evaluation, employ double-blind placebo-controlled design, or appear statistically valid. Although the contribution of laboratory research has been of unquestionable value in the development of our current knowledge of lower urinary tract pharmacology it is difficult to interpret the results of in vitro pharmacological studies because of the array of experimental models used and the need to extrapolate to in vivo activity. This paper utilizes a functional scheme which classifies agents by their effects on urinary storage and emptying. The purpose of this review is to promote discussion of the application of uropharmacological investigation to the development of newer, more efficacious forms of drug therapy.

Analysis of calcium channels by conditional mutagenesis

Ca2+ influx through various ion channels is an important determinant of the cytosolic Ca2+ concentration, which plays a pivotal role in countless cellular processes. The cardiac L-type Ca2+ channel, Ca(v)1.2, represents a major pathway for Ca2+ entry and is in many cells expressed together with other high- and low-voltage-activated Ca2+ channels. This article will focus on the use of conditional transgenic mouse models to clarify the roles of Ca2+ channels in several biological systems. The phenotypes of conditional Ca2+ channel transgenic mice have provided novel, and often unexpected, insights into the in vivo function of L-type and T-type Ca2+ channels as mediators of signaling between cell membrane and intracellular processes in blood pressure regulation, smooth muscle contractility, insulin secretion, cardiac function, sleep, learning, and memory.

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

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

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

An essential role of Cav1.2 L-type calcium channel for urinary bladder function

Mice deficient in the smooth muscle Cav1.2 calcium channel (SMACKO, smooth muscle alpha1c-subunit calcium channel knockout) have a severely reduced micturition and an increased bladder mass. L-type calcium current, protein, and spontaneous contractile activity were absent in the bladder of SMACKO mice. K+ and carbachol (CCh)-induced contractions were reduced to 10-fold in detrusor muscles from SMACKO mice. The dihydropyridine isradipine inhibited K+- and CCh-induced contractions of muscles from CTR but had no effect in muscles from SMACKO mice. CCh-induced contraction was blocked by removing extracellular Ca2+ but was unaffected by the PLC inhibitor U73122 or depletion of intracellular Ca2+ stores by thapsigargin. In muscles from CTR and SMACKO mice, CCh-induced contraction was partially inhibited by the Rho-kinase inhibitor Y27632. These results show that the Cav1.2 Ca2+ channel is essential for normal bladder function. The Rho-kinase and Ca2+-release pathways cannot compensate the lack of the L-type Ca2+ channel.

Ca(2+) channel properties in smooth muscle cells of the urinary bladder from pig and human

Ca(2+) channel properties of pig and human bladder smooth muscle were investigated utilizing standard whole-cell patch clamp techniques. Both the amplitude obtained and the current density of Ca(2+) channel current evoked by step depolarization were larger in human than in pig myocytes. The inward currents were sensitive to an L-type Ca(2+) channel antagonist, nifedipine, the effects of which were not significantly different between species. In both species, prior application of ATP (0.1 mM) had no effect on activation of this voltage-sensitive channel current, while a muscarinic receptor agonist, carbachol (0.1 mM), significantly attenuated the amplitude of this current. Furthermore, inclusion of GDP-beta-S or Heparin in the pipette abolished or had no effect on the suppression of Ca(2+) current by carbachol, respectively. These results forward the pig as a good model for the human in detrusor Ca(2+) channel properties, especially with regard to neural modulation, although voltage-sensitive Ca(2+) channels seem to make greater contribution in human bladder physiology.

Dependency of detrusor contractions on calcium sensitization and calcium entry through LOE-908-sensitive channels

1. The subcellular mechanisms regulating stimulus-contraction coupling in detrusor remain to be determined. We used Ca(2+)-free solutions, Ca(2+) channel blockers, cyclopiazonic acid (CPA), and RhoA kinase (ROK) inhibitors to test the hypothesis that Ca(2+) influx and Ca(2+) sensitization play primary roles. 2. In rabbit detrusor, peak bethanechol (BE)-induced force was inhibited 90% by incubation for 3 min in a Ca(2+)-free solution. By comparison, a 20 min incubation of rabbit femoral artery in a Ca(2+)-free solution reduced receptor-induced force by only 5%. 3. In detrusor, inhibition of sarcoplasmic reticular (SR) Ca(2+) release by 2APB, or depletion of SR Ca(2+) by CPA, inhibited BE-induced force by only 27%. The CPA-insensitive force was abolished by LaCl3. By comparison, 2APB inhibited receptor-induced force in rabbit femoral artery by 71%. 4. In the presence of the non-selective cation channel (NSCC) inhibitor, LOE-908, BE did not produce an increase in [Ca(2+)]i but did produce weak increases in myosin phosphorylation and force. 5. Inhibitors of ROK-induced Ca(2+) sensitization, HA-1077 and Y-27632, inhibited BE-induced force by approximately 50%, and in combination with LOE-908, nearly abolished force. 6. These data suggest that two principal muscarinic receptor-stimulated detrusor contractile mechanisms include NSCC activation, that elevates [Ca(2+)]i and ROK activation, that sensitizes cross bridges to Ca(2+).

Simultaneous recording of mechanical and intracellular electrical activity in human urinary bladder smooth muscle

OBJECTIVE

To elucidate the role of the membrane potential in human detrusor smooth muscle contraction, by simultaneously recording mechanical and intracellular electrical activity in muscle strips. Materials and methods The agonists acetylcholine and carbachol were applied to induce a contraction on muscarinic receptor stimulation; to block the response, atropine was added to the bath. The Ca2+ necessary for activating the contractile machinery can be recruited via two pathways: release from intracellular stores or influx from the extracellular matrix. High potassium was applied to induce Ca2+ influx through voltage-sensitive Ca2+ channels.

RESULTS

There were significant changes in the force when agonist, antagonist and high potassium was administered. However, there were significant changes in membrane potential only when KCl was applied to the bath and not with muscarinic agonist or antagonist application. Activity in the form of spike potentials did not change significantly on applying any of the test substances.

CONCLUSION

The present results indicate that the Ca2+ mobilized on M3 receptor stimulation originates primarily from intracellular stores, with no systematic changes in membrane potential. Atropine only caused a relaxation in muscle previously contracted by M3-receptor agonist stimulation; it had no effect on relaxed muscle strips.

Smooth muscle fatigue due to repeated urinary bladder neurostimulation: an in vivo study

The presented study investigates the influence of different pause lengths between two consecutive stimulations of the S3 roots on intravesical pressure during bladder neurostimulation. In eight male foxhounds (aged 7-18 months), laminectomy and placement of a modified Brindley electrode were performed. In four series with different pause lengths between two consecutive stimulations (1, 3, 5, and 15 min), the maximum intravesical pressure was measured during stimulation. The changes in intravesical pressure were registered in these four series, each series with six stimulations. A 15-min interval elapsed before the commencement of each series. In the series with a pause length of 15 min, the consecutive stimulations did not result in significant changes in maximum intravesical pressure. In the 5-min series, a significant decrease in intravesical pressure was not observed after the third stimulation. In the 3-min series, a significant decrease was seen at almost every stimulation (average decrease of 3.8% per stimulation) and in the 1-min series, a significant decrease was also observed at almost every stimulation (average decrease of 5.9% per stimulation). The results of repeated bladder neurostimulation demonstrate that the maximum intravesical pressure is dependent on the pause length between two consecutive stimulations. The detrusor muscle showed reversible and short-lived signs of fatigue. This implies the importance of a minimum 5-min interval between two subsequent stimulations. A pause length <5 min leads to a falsification of the results and thus to lower validity of the investigation.

The contribution of intracellular Ca2+ release to contraction in human bladder smooth muscle

1. The importance of Ca2+ release from the sarcoplasmic reticulum (SR) in excitation contraction (EC) coupling in human detrusor muscle remains controversial. In this paper the contribution of Ca2+ release to agonist induced contraction is assessed. 2. Dose response curves to carbachol (0.01 - 10 microM) were constructed before and after exposure to 200 nM Thapsigargin (Tg). Tg pre-treatment reduced the force of contraction at all agonist concentrations however, the reduction was dose dependent. At 0.1 microM the contractions were reduced to 14.5 +/- 7% (mean +/- s.e.mean) of controls (n = 8) while at 10 microM the contractions were only reduced to 92 +/- 3% of controls (n = 10). 3. The role of external Ca2+ was examined by measuring the magnitude of contraction to low and high doses of agonist in the presence and absence of external Ca2+. With (0.1-0.3 microM) carbachol the contractions in nominally Ca2+ free media were 4+/-4% of controls (n = 7) whilst with (1 - 10 microM) carbachol the contractions were 36 +/- 8% of controls (n=7) suggesting that at low agonist concentrations the release of Ca2+ has a requirement for external Ca2+. 4. Pre-treatment of muscle strips with the Ca2+ channel blocking agent diltiazem reduced the contractile responses to carbachol. Contractions induced by 0.1 microM were reduced to 29+/-11% (P<0.05) of controls while those activated by 10 microM were reduced to 86+/-6% (P= 0.1) of controls (n = 4) suggesting the Ca2+ influx needed to activate internal store release at low agonist stimulation is through L-type Ca2+ channels. 5. These observations confirm the importance of thapsigargin sensitive intracellular Ca2+ store release in the activation of contraction of detrusor smooth muscle and suggest the overall contribution of this store depends upon the magnitude of the agonist stimulation.

Smooth muscle of the bladder in the normal and the diseased state: pathophysiology, diagnosis and treatment

The smooth muscle of the normal bladder wall must have some specific properties. It must be very compliant and able to reorganise itself during filling and emptying to accommodate the change in volume without generating any intravesical pressure, but whilst maintaining the normal shape of the bladder. It must be capable of synchronous activation to generate intravesical pressure at any length to allow voiding. The cells achieve this through spontaneous electrical activity combined with poor electrical coupling between cells, and a dense excitatory innervation. In the diseased state, alterations of the smooth muscle may lead to failure to store or failure to empty properly. The diseased states discussed are bladder instability and diabetic neuropathy. Bladder instability is characterised urodynamically by uninhibitable rises in pressure during filling, and is seen idiopathically and in association with bladder outflow obstruction and neuropathy. In diabetic neuropathy, many of the smooth muscle changes are a consequence of diuresis, but there is evidence for alterations in the sensory arm of the micturition reflex. In the unstable bladder, additional alterations of the smooth muscle are seen, which are probably caused by the patchy denervation that occurs. The causes of this denervation are not fully established. Nonsurgical treatment of instability is not yet satisfactory; neuromodulation has some promise, but is expensive, and the mechanisms poorly understood. Pharmacological treatment is largely through muscarinic receptor blockade. Drugs to reduce the excitability of the smooth muscle are being sought, since they may represent a better pharmacological option.

Calcium independent contraction of bladder smooth muscle

BACKGROUND

Recently, it has been suggested that in vascular smooth muscle a Ca(2+)-independent mechanism or Ca(2+)-sensitization of contractile elements may participate in smooth muscle contraction. In this study, we evaluate this mechanism in detrusor muscle.

METHODS

Strips of smooth muscle from rabbit aorta, rabbit bladder and human bladder were evaluated by in vitro contraction studies.

RESULTS

The results show that (1) in Ca(2+)-free solution containing ethyleneglycol bis (-aminothylether)-N,N,-tetraacetic acid (EGTA), carbachol and phorbol ester produced sustained contractions in detrusor muscle (Ca(2+)-free contraction); (2) depletion of Ca2+ stores by caffeine did not affect Ca(2+)-free contraction induced by carbachol; and (3) W-7 (calmodulin inhibitor) and ML-9 (myosin light chain kinase [MLCK] inhibitor) did not show inhibitory effects on Ca(2+)-free contraction, while H-7 (protein kinase C. [PKC] inhibitor) abolished this contraction.

CONCLUSIONS

These results suggest that neither stored Ca2+ nor the Ca(2+)-calmodulin-MLCK system is involved in the carbachol-induced Ca(2+)-free contraction of detrusor muscle. This Ca(2+)-independent contraction seems to be mediated by the activation of PKC coupled with agonist stimulation of the muscarinic receptor.

Bladder function in experimental outlet obstruction: pharmacologic responses to alterations in innervation, energetics, calcium mobilization, and genetics

The two functions of the urinary bladder is to store urine at low intravesical pressures, and to periodically expel the urine through a coordinated contraction of the bladder and relaxation of the urethra. To a large extent, urinary bladder function depends upon the underlying structure of the organ as a whole, particularly on the inter-relationships among the smooth muscle, connective tissue, and neuronal elements. An alteration in the ratio of connective tissue to smooth muscle, for example, can significantly alter compliance and functional capacity, structurally impairing the bladder's ability to empty efficiently and fully. Thus, a change in structural compartmentation can affect bladder function independent of autonomic receptor density, response to receptor stimulation, and the contractile capabilities of the smooth muscle elements. Similarly, a selective alteration in either the afferent or efferent innervation of the bladder or urethra can induce significant alterations in the structural interrelationships between smooth muscle and connective elements. In addition, the bladder responds rapidly to alterations in urine volume and urethral resistance with marked changes in bladder and urethral structure and function, and these changes are under the controls of specific genes that are known to control cellular growth, hypertrophy, and hyperplasia. A knowledge of the mechanisms that control the response to specific forms of stress may lead to novel therapies for specific disease states.

Release of intracellular calcium by prenylamine in human ovarian tumour cells

The effects of the calcium antagonist prenylamine on intracellular calcium concentration were studied in a human ovarian carcinoma cell line, OVCAR-3. Exposure of cells to 100 microM prenylamine resulted in nearly a 10-fold increase in cytosolic free calcium concentration ([Ca2+]i) as measured by Fura-2 fluorescence. In calcium-free medium, although the increase in [Ca2+]i caused by prenylamine was smaller, it was still substantial compared with the basal level. Efflux experiments with 45Ca showed that 100 microM prenylamine increased calcium efflux by 70% compared with control, indicating active extrusion of the elevated [Ca2+]i. The sluggish nature of calcium release and its independence from the pool activated by ionomycin suggest that the calcium was probably not released from endoplasmic reticulum. These results, although paradoxical, provide a new insight into the possible mechanism of action of prenylamine in causing cancer cell death.

Role of calcium in mediating the biphasic contraction of the rabbit urinary bladder

1. The response of the urinary bladder to field stimulation is biphasic in nature consisting of an initial phasic contraction followed by a prolonged tonic phase which lasts for the duration of the stimulation. 2. The phasic response is mediated by the release of neurohumoral transmitters, primarily acetylcholine (via muscarinic receptor stimulation) and ATP (via purinergic receptor stimulation). The tonic component is mediated entirely via muscarinic receptor stimulation. 3. The present study investigates the dependence on extracellular calcium of the phasic and tonic contractile responses to field stimulation, bethanechol, and ATP. The results can be summarized as follows: 4. Field stimulation (2 and 32 Hz) and bethanechol evoke a biphasic contractile response whereas ATP evokes only a phasic response. 5. There were no significant effects of either calcium channel blockers or calcium fee EGTA medium on either spontaneous contraction or basal tension of muscle strips. 6. The calcium channel antagonists diltiazem and verapamil inhibited both the phasic and tonic responses induced by field stimulation (both 2 and 32 Hz) in a dose dependent manner. 7. For both 2 and 32 Hz stimulation, the ED50 s for the inhibition of the tonic phases of the responses to field stimulation were significantly lower than the ED50s for the inhibition of the phasic responses. 8. The tonic phase of the responses to field stimulation were inhibited to a significantly greater degree than the phasic responses by incubation in calcium-free medium containing EGTA. 9. Both the phasic and tonic components of the response to bethanechol stimulation were inhibited equally, and followed a similar time course as the tonic component of field stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effect of cromakalim in human detrusor muscle is mediated by glibenclamide-sensitive potassium channels

The effects of cromakalim, a potassium channel activating drug, and glibenclamide, a relatively selective antagonist of ATP-sensitive potassium channels, have been investigated on isolated detrusor muscle from human bladder. Specimens of human bladder were cut into strips and suspended in an organ bath filled with modified Tyrode solution for measurement of isometric contractile force. Concentration-response curves to acetylcholine were constructed before and after pretreatment with cromakalim and cromakalim plus glibenclamide. The concentration-response curves to acetylcholine were displaced to the right, and the maximal response to acetylcholine was significantly inhibited by cromakalim in a concentration-dependent manner. The inhibitory effect of cromakalim on acetylcholine-induced contraction was significantly reduced by glibenclamide. Following sustained contraction induced by 20 mM. KCl, the cumulative addition of cromakalim to the organ bath produced a concentration-dependent relaxation. However, in strips precontracted with 60 mM. KCl, the addition of cromakalim in concentrations as high as 10(-5) M. did not induce relaxation. The relaxation induced by cromakalim in strips precontracted with 20 mM. KCl was significantly inhibited by glibenclamide. These results suggest that the inhibitory effect of cromakalim in human bladder involves activation of glibenclamide-sensitive potassium channels.

Urodynamic effects of intravesical instillation of terodiline in healthy volunteers and in patients with detrusor hyperactivity

The effects of intravesical instillation of terodiline on urodynamic parameters were investigated in 8 healthy volunteers (10(-4) M.) and in 34 patients with detrusor hyperactivity (10(-5) M.) of neurogenic (22) or nonneurogenic (12) origin. The volunteers were investigated with conventional medium-fill cystometry, while in the patients a modified cystometric technique with slow intermittent filling was used. The reproducibility of the procedure was verified in 17 patients. Instillation of terodiline had no effect on the normal bladders nor were any improvements found in the nonneurogenic patients. In 12 patients in the neurogenic group treated with terodiline instillation the bladder capacity increased significantly (p < 0.05) from 289 +/- 32 to 413 +/- 55 ml. Within this group 5 patients were responders. It is suggested that pathophysiological changes may explain the difference between the neurogenic and nonneurogenic groups, and that the number of responders within the neurogenic group may be increased by an optimal drug preparation and increased dosage. Intravesical administration of terodiline may offer an alternative treatment in selected patients with detrusor hyperreflexia.

Contractions induced by angiotensin I, angiotensin II and bradykinin in isolated smooth muscle from the human detrusor

In isolated human detrusor preparations angiotensin (At)II 10(-9)-10(-5) M caused concentration-dependent contractions. The contractile effect was immediate, and had an amplitude which at the highest concentration used, 10(-5) M, reached 103 +/- 16% of the mean contraction produced by K+ 124 mM (27.6 +/- 1.4 mN). The AtII effect was completely blocked by saralasin 10(-6) M, but was not affected by pre-treatment of the preparations with captopril or enalaprilate. There was a marked tachyphylaxis to the actions of the peptide. AtI (10(-8)-10(-5) M) also caused contractions which were rapidly developing, and subject to a marked tachyphylaxis. At a concentration of 10(-5) M, the mean amplitude was 66 +/- 9% of the K(+)-induced contraction. The contractions were blocked by saralasin 10(-6) M, but not by captopril or enalaprilate 10(-5) M. In contrast, contractions produced by AtI in rabbit mesenteric arteries were practically abolished by the angiotensin converting enzyme (ACE) inhibitors. The contractions induced by both AtI and AtII were practically abolished after pre-treatment in a nominally calcium-free Krebs solution. However, blockade of L-type calcium channels by nifedipine 10(-6) M reduced the responses to both AtI 10(-6) M (by 38 +/- 4%) and AtII 10(-6) M (by 39 +/- 7%), but never abolished the contractions. Bradykinin (Bk; 3 x 10(-8)-10(-5) M) had a contractile effect in detrusor preparations which varied widely between strips. At a concentration of 3 x 10(-6) M, a maximum was reached amounting to 30 +/- 10% of the K(+)-induced contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for the presence of regional differences in the calcium antagonist receptors in lower urinary tract smooth muscle

(+)-[3H]PN 200-100 (a dihydropyridine calcium channel antagonist) was utilized to characterize calcium channel binding sites in rabbit bladder dome, bladder base, and urethra. Specific binding of (+)-[3H]PN 200-110 to membrane particulates was saturable, reversible, linear to protein concentration, and of high affinity. The density of (+)-[3H]PN 200-110 binding sites (Bmax values in fmol/mg of protein) and the affinity constants for (+)-[3H]PN 200-110 (KD value in pM) in urethra, bladder dome and bladder base were 64.1 +/- 7.8 and 179 +/- 31; 21.9 +/- 3.0 and 213 +/- 36; and 18.8 +/- 4.2 and 140 +/- 28, respectively. Agonists and antagonists inhibited (+)-[3H]PN 200-110 binding with Ki values in the following rank order: nitrendipine less than nifedipine less than niguldipine much less than Bay K 8644 much less than verapamil. Although carbachol-induced contractile responses were 20-30 times smaller in muscle strips from urethra than from bladder base or bladder dome, KCl-induced contractions were only 3-4 times smaller in urethra than in bladder tissues. Nifedipine inhibited carbachol-induced contractions in urethra, bladder dome, and bladder base by 76%, 64%, and 60%, respectively, and completely inhibited KCl-induced contractions in all three tissues. IC50 values for nifedipine inhibition of both carbachol- and KCl-induced contractions were significantly smaller in urethra than in bladder base or bladder dome. Nitrendipine, niguldipine and verapamil inhibited urethral contractions induced by carbachol and KCl to the same degree as did nifedipine. The IC50 values, obtained from functional studies, for calcium channel antagonists were in good agreement with Ki values obtained from binding studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipase C activation by endothelin-1 and noradrenaline in isolated penile erectile tissue from rabbit

The effects of endothelin-1 and noradrenaline on phospholipase C activity in the rabbit isolated corpus cavernosum were investigated by measuring the accumulation of inositol phosphates. Both endothelin-1 and noradrenaline caused a time- and concentration-dependent increase in the accumulation of 3H-inositol phosphates in preparations prelabelled with 3H-myo-inositol. The reaction was slow in onset with no significant accumulation of 3H-inositol phosphates, including inositol trisphosphate, demonstrable during the first 15 minutes. At 60 minutes, the mean increases in 3H-inositol inositol phosphates induced by 3 x 10(-7) M endothelin-1 and 10(-3) M noradrenaline amounted to 341 and 530% of time-matched controls, respectively. However, when given at concentrations having the same contractile amplitude on rabbit corpus cavernosum, there was no difference in the amounts of 3H-inositol phosphates generated by endothelin-1 and noradrenaline. Prazosin (10(-6) M) significantly inhibited the stimulatory effect of noradrenaline on phosphoinositide hydrolysis. Pretreatment with 10(-6) M nimodipine did not reduce the increases in 3H-inositol phosphates induced by 3 x 10(-7) M endothelin-1 and 10(-3)M noradrenaline. Also in Ca(2+)-free medium, both agonists had significant stimulatory effects on phosphoinositide turnover, although under this condition, the responses were greatly reduced. The results suggest that exogenous endothelin-1 and noradrenaline activate phospholipase C in corpus cavernosum, and that this mechanism is partly independent of extracellular Ca2+. Considering the slow onset of action, phospholipase C activation is probably not directly involved in rapid contractile events, but may be of importance in the long-term regulation of penile smooth muscle tone.

The influence of streptozotocin-induced diabetes mellitus on the sensitivity of rat urinary bladder body and base strips to changes in extracellular calcium

1. The influence of calcium on contractile responses of bladders from control and 2 month streptozotocin-diabetic rats was investigated. 2. Removal of calcium from the bathing medium caused rapid decreases in the contractile responses of bladder body and base strips to carbachol. The responses of strips from control rats were reduced more by calcium removal than were strips from diabetics. 3. Replacement of calcium caused dose-dependent increases in contraction to carbachol. The responses of bladder body strips from diabetic rats to carbachol were significantly greater at all calcium concentrations than were those of controls. There were no differences in the responsiveness of bladder base strips to carbachol. 4. In contrast, bladder body strips from diabetic rats were more sensitive to calcium than were strips from controls, with an IC50 value for calcium of 0.38 mM vs 0.72 mM for controls. 5. At the calcium concentration of Krebs buffer (2.5 mM), contractile responses were near maximal, and there were no differences in sensitivity. 6. The calcium antagonist nifedipine caused dose-dependent decreases in the contractile responses of bladder base and body strips to nerve stimulation. The responses to nerve stimulation were more sensitive to nifedipine than were those to carbachol. There were no differences between controls and diabetics in the sensitivity of bladder strips to nifedipine. 7. The findings suggest that although increases in sensitivity to calcium are observed in bladder body strips from streptozotocin-diabetic rats, they are unlikely to be responsible for the increases in maximal contractile response to nerve stimulation and contractile agents.

Comparison of Bay K 8644, nitrendipine and atropine on spontaneous and pelvic-nerve-induced bladder contractions on rat bladder in vivo

The effects of the dihydropyridine-type calcium antagonist (nitrendipine) and agonist (Bay K 8644) in comparison to atropine have been studied after intravenous administration on spontaneous and pelvic-nerve-induced contraction of rat urinary bladder. Bay K 8644 increased the basal internal bladder pressure as well as the amplitude of the spontaneous bladder contractions in a dose-dependent manner. In addition, an increase in systemic arterial blood pressure was noted for a period of about 20 min. In the presence of atropine the effects of Bay K 8644 on the urinary bladder were almost completely antagonized. Both nitrendipine and atropine reduced in a dose-dependent manner the amplitude of spontaneous and nerve-induced bladder contraction. The spontaneous and nerve-induced bladder contractions were significantly reduced by atropine or nitrendipine. Only nitrendipine caused a reduction of the spontaneous bladder contraction frequency. The systemic blood pressure was decreased significantly by nitrendipine but not after atropine administration. We suggest that both calcium antagonist and agonist can change the tension of the urinary bladder in vivo. As a side-effect the systemic blood pressure is altered. Atropine can antagonize the effect of BayK 8644 on the urinary bladder and reduces spontaneous and nerve-induced bladder contractions more specifically than nitrendipine.

Calcium dependence of contractile activation of isolated sheep urethra. II: Responses to exogenous noradrenaline

Isolated smooth muscle of sheep urethra responded to exogenous noradrenaline (NA) with a concentration-dependent contraction. After exposing the preparations for 30 min. to calcium-free medium, NA in a submaximal concentration was still able to produce a contractile response amounting to 36% of control value in calcium-containing solution. Readmission of calcium (administered cumulatively) restored the response in a concentration-related fashion to 85% (at 5 mM calcium) of control level. Nifedipine and verapamil failed to inhibit these graded calcium contractions in the presence of NA. Verapamil and diltiazem also failed to significantly prevent contraction induced by NA in a submaximum concentration, but nifedipine showed a concentration-dependent inhibitory effect, with a potency 1000 times lower than that observed on K+ (124 mM) induced contractions. In calcium-free medium, repeated applications of NA at 30 min. intervals, induced a progressive reduction in contractile amplitude. Nifedipine or verapamil did not affect the time course for recovery of NA-induced contraction when the preparations were returned to calcium containing medium, but the recovery was blocked almost completely by lanthanum. Furthermore, lanthanum abolished the remaining NA contraction in calcium-free medium. The results suggest that NA-induced contraction in isolated sheep urethra is dependent on both influx of extracellular calcium and on release of intracellular calcium. Calcium influx for contractile activation and refilling of intracellular stores seem to occur through membrane channels that can only partly be blocked with calcium antagonists.

Muscarinic receptor stimulation of phosphoinositide hydrolysis in the human isolated urinary bladder

The stimulatory action of carbachol and acetylcholine (ACh) on phosphoinositide turnover, as well as their contractile effects, were investigated in human isolated detrusor muscle. Carbachol, and ACh in combination with 10(-7) M physostigmine, induced increases in phosphoinositide turnover. However, at all the concentrations tested, carbachol was more effective than ACh (plus physostigmine), and at the highest concentration used (10(-4) M), the difference was significant (p less than 0.05). Also in a Ca(2+)-free medium containing the chelator EGTA (10(-4) M), both agonists (10(-4) M) induced small but distinct increases in phosphoinositide breakdown. Carbachol and ACh contracted the detrusor preparations concentration-dependently, and the responses were almost identical when ACh was combined with 10(-7) M physostigmine. In Ca(2+)-free medium the agonists elicited a moderate but concentration-dependent contractile response at high concentrations. The results show that muscarinic receptor agonists stimulate phosphoinositide turnover in the human bladder. Possibly, this effect is coupled to multiple muscarinic receptor subtypes. More studies are required to elucidate to what extent phosphoinositide breakdown participates in the contractile activation of this tissue.

Effects of papaverine on tension and 45Ca-uptake in isolated urinary bladder

Papaverine is believed to relax smooth muscle by reducing transmembrane calcium transport and cyclic nucleotide phosphodiesterase activity. The present study characterizes the different relaxing effects of papaverine on isolated muscle strips of rat bladder dome. Compared to histamine, norepinephrine and serotonin, carbachol and high potassium induced the most prominent contractions in rat bladder strips. For this reason both agents were used as stimulants. High-potassium-induced muscle contractions were reduced by a lower concentration of papaverine than carbachol-induced muscle concentrations. Compared to verapamil, papaverine, especially in low concentrations, was less potent on both kinds of induced muscle contractions. These tension responses correspond to a difference in 45Ca uptake, suggesting a nonspecific blocking property of papaverine on transmembrane calcium channels. The beta-sympathomimetic effects of isoprenaline on carbachol-induced contractions were not enhanced by verapamil. In contrast, papaverine increased this tension response of isoprenaline on carbachol-induced contraction. From these results it is possible that part of the papaverine action seems to be related to an intracellular mechanism probably to cAMP.

Effect of extracellular Ca2+ on cholinergic, KCl and phorbol ester-mediated phosphoinositide turnover and guinea pig urinary bladder contraction

The effect of extracellular Ca2+ ([Ca2+]o) on cholinergic, KCl and phorbol ester-mediated detrusor contractions was related to phosphoinositide (PI) breakdown in guinea pig urinary bladder. Carbachol (1.0 mM) elicited a 20-fold increase in inositol phosphate (IP) accumulation both in presence and absence of [Ca2+]o yielding the same EC50 value (approximately 12 microM). In contrast, carbachol-induced detrusor contractions were reduced by 35% without [Ca2+]o, but maximal efficacy was restored with Ca2+ replenishment. In absence of [Ca2+]o, repeated cholinergic stimulation yielded contractions only if tissues were intermittently equilibrated in [Ca2+]o. High K+ and PDBu evoked [Ca2+]o-dependent contractions. Ca2+ channel antagonists and divalent metal cations inhibited high K+ more potently than carbachol-mediated contractions. Together, these findings suggest multiple sources of Ca2+ for urinary bladder contraction, where voltage-sensitive responses depend primarily on [Ca2+]o and PI-linked muscarinic responses involved Ca2+ mobilization from intracellular stores as well. Clinical agents used for the treatment of urinary incontinence inhibited both carbachol-induced PI turnover and muscle contraction with the same rank order of potency both in presence and absence of [Ca2+]o. These findings suggest that the cholinergic mechanism of action of these agents involves the PI-Ca2+ effector system.

Effects of papaverine on human isolated bladder muscle

Papaverine is a non-specific smooth muscle relaxant and is thought to act at a site beyond the receptor sites on the cell membrane. In this study the relaxing properties of papaverine were tested in isolated muscle strips from the human bladder dome. In carbachol-induced contractions papaverine, even in high concentrations of 10(-4) mol/l had virtually no effects on peak tension generation, whereas the fading was accelerated and the steady state tension at 30 min. was reduced by about 54%. In contrast, high potassium-induced contractions were relaxed by papaverine in a concentration-dependent way; a concentration of papaverine of 10(-4) mol/l produced full relaxation. These findings might possibly be explained if it is assumed that papaverine blocks calcium ion channels in the cell membrane. However, the observation that rather high concentrations of papaverine were necessary to fully relax high potassium contractions and the fact, that papaverine affects cellular cAMP levels separate this drug from more selective calcium channel blockers. The calcium movements responsible for the peak tension generation in carbachol-induced contractions are obviously not affected by papaverine. Although papaverine had little effect on carbachol-induced contractions in vitro it cannot be excluded that the drug is effective in diseases were non-cholinergic mechanisms are involved.

Mini-pig urinary bladder function: comparisons of in vitro anticholinergic responses and in vivo cystometry with drugs indicated for urinary incontinence

1. Studies of carbachol-induced contractions on mini-pig bladder tissue strips in vitro demonstrated that antagonist drugs produced a rank order of potency similar to that observed in guinea-pig tissues: propantheline approximately atropine greater than oxybutynin greater than dicyclomine greater than HHSiD greater than imipramine greater than terodiline approximately AF-DX 116. The drugs appeared to show competitive antagonism and the tissues exhibited resistance to complete cholinergic blockade. 2. Cytometry performed in vivo on awake mini-pigs also showed that i.v. cholinergic antagonists produced a dose-dependent depression of peak intravesical bladder pressure (PvesP) during slow filling of the bladder using urethral catheters, with a rank order of potency: atropine greater than oxybutynin approximately propantheline greater than HHSiD approximately dicyclomine greater than terodiline. Other parameters of the cystometrogram were unaffected by the antagonists, except for residual volume, which generally increased after drug treatment. 3. Hexahydrosiladifenidol (HHSiD), an ileal-selective competitive muscarinic antagonist, was about as effective an antagonist as the clinically useful drugs oxybutynin or dicyclomine, both in vitro and in vivo, suggesting that HHSiD may have useful therapeutic effects for the treatment of urinary incontinence. 4. Correlation of the rank order of potency for muscarinic antagonism between mini-pigs and guinea-pigs was very high in vitro (r = 0.97, P less than 0.05), as was the correlation among the drugs for their ability to depress PvesP of the cystometrogram in vivo (r = 0.89, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of nifedipine on the contractile responses of the isolated rat bladder

The effect of the calcium channel blocker nifedipine on the motor transmission in isolated preparations of rat detrusor smooth muscle has been studied. Nifedipine blocked the major part (75 to 80%) of the contractile response to electrical field stimulation, while atropine only blocked 20 to 25%. In preparations pretreated with atropine, the response to electrical field stimulation was completely abolished by nifedipine. The converse was also true; in preparations pretreated with nifedipine the response was fully blocked by atropine. The nifedipine-resistant response was greatly potentiated by the anticholinesterase eserine. The blocking action of nifedipine on motor transmission was partially antagonised by raising Ca2(+)-concentration. Acetylcholine concentration-response curve was shifted to the right by nifedipine. It is concluded that the non-cholinergic motor neurotransmitter evokes contraction of the rat detrusor smooth muscle by activating external Ca2(+)-transport channels whereas the cholinergic contraction is mediated partly or wholly by alternative mechanisms.