The effect of nifedipine on spontaneous, drug-induced and reflexly-activated contractions of the rat urinary bladder: evidence for the participation of an intracellular calcium store to micturition contraction

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.

Purinergic signalling in the urinary bladder - When function becomes dysfunction

Knowledge of the participation of ATP and related purines in urinary tract physiology has been established over the last five decades through the work of many independent groups, inspired by, and building on the pioneering studies of Professor Geoffrey Burnstock and his coworkers. As part of a series of reviews in this tribute edition, the present article summarises our current understanding of purines and purinergic signalling in modulating and regulating urinary tract function. Purinergic mechanisms underlying the origin of bladder pain; sensations of bladder filling and urinary tract motility; and regulation of detrusor smooth muscle contraction are described, encompassing the relevant history of discovery and consolidation of knowledge as methodologies and pharmacological tools have developed. We consider normal physiology, including development and ageing and then move to pathophysiology, discussing the causal and consequential contribution of purinergic signalling mechanism and their constituent components (receptors, signal transduction, effector molecules) to bladder dysfunction.

Purinergic signalling in the urinary tract in health and disease

Purinergic signalling is involved in a number of physiological and pathophysiological activities in the lower urinary tract. In the bladder of laboratory animals there is parasympathetic excitatory cotransmission with the purinergic and cholinergic components being approximately equal, acting via P2X1 and muscarinic receptors, respectively. Purinergic mechanosensory transduction occurs where ATP, released from urothelial cells during distension of bladder and ureter, acts on P2X3 and P2X2/3 receptors on suburothelial sensory nerves to initiate the voiding reflex, via low threshold fibres, and nociception, via high threshold fibres. In human bladder the purinergic component of parasympathetic cotransmission is less than 3 %, but in pathological conditions, such as interstitial cystitis, obstructed and neuropathic bladder, the purinergic component is increased to 40 %. Other pathological conditions of the bladder have been shown to involve purinoceptor-mediated activities, including multiple sclerosis, ischaemia, diabetes, cancer and bacterial infections. In the ureter, P2X7 receptors have been implicated in inflammation and fibrosis. Purinergic therapeutic strategies are being explored that hopefully will be developed and bring benefit and relief to many patients with urinary tract disorders.

Relaxant effects of the essential oil of Mentha pulegium L. in rat isolated trachea and urinary bladder

Objectives

We evaluated the relaxant activity of the essential oil of Mentha pulegium L. (EOMP) and pulegone in rat isolated tracheal and bladder smooth muscles.

Methods

Isometric contractions of isolated tracheal and bladder strips from male Wistar rats were induced by KCl (K60; 60 mm) or acetylcholine (ACh; 10 µm). EOMP and its majory compound pulegone were incubated, after contracting agent, with the tissues in cumulating concentrations.

Key findings

EOMP (3–300 µg/ml) inhibited the contractions induced by ACh and K60 in both tissues, but was more effective against the contractions induced by K60 in trachea (IC50 = 40.47 ± 3.27 µg/ml) compared with ACh. Its relaxant action rules out ganglia and NO participation. Pulegone (10−7 to 10−3  m) inhibited the contractions induced by ACh and K60 in both tissues. EOMP concentration-dependently inhibited the contractions evoked by addition of CaCl2 in depolarised trachea, suggesting inhibition of extracellular calcium entry.

Conclusions

These findings suggests that EOMP induced relaxant responses in pre-contracted smooth muscles of rat trachea and bladder, which are likely to be mediated via inhibition of calcium entry, mainly by its major compound, pulegone. These effects are coherent with the popular use of EOMP as an antispasmodic agent.

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.

Evolving mechanisms of action of alverine citrate on phasic smooth muscles

BACKGROUND AND PURPOSE

We have investigated the mechanisms underlying the paradoxical ability of the antispasmodic, alverine, to enhance spontaneous activity in smooth muscles while suppressing evoked activity.

EXPERIMENTAL APPROACH

The effects of alverine on spontaneous and induced contractile activity were examined in preliminary experiments with various smooth muscles. More detailed effects were also investigated by recording membrane potential, intracellular Ca2+ concentration ([Ca2+]i) and tension from single-bundle detrusor smooth muscle (DSM) of the guinea-pig urinary bladder.

KEY RESULTS

Alverine (10 microM) increased the frequency and amplitude of spontaneous action potentials, transient increases in [Ca2+]i and associated contractions. Alverine also decreased action potential rate of decay, suggesting inhibition of L-type Ca channel inactivation. Charybdotoxin (50 nM) but neither cyclopiazonic acid (10 microM) nor Bay K 8644 (10 microM) attenuated alverine-induced enhancement of spontaneous contractions. Alverine suppressed contractions produced by high K (40 mM) or ACh (10 microM), without affecting electrical responses and with little suppression of increases in [Ca2+]i. This feature was very similar to that of the effects of the Rho kinase inhibitor Y-27632 (10 microM).

CONCLUSIONS AND IMPLICATIONS

Alverine may increase Ca influx during action potentials due to inhibition of the inactivation of L-type Ca channels, but may also suppress evoked activity by inhibiting the sensitivity of contractile proteins to Ca2+. The proportional contribution of Ca-dependent and Ca-independent contractions in DSM may differ between spontaneous and evoked activity, necessitating further investigations into the interactions between these pathways for assessing the therapeutic potential of alverine to treat DSM dysfunction.

Contractile activity of ATP and diadenosine tetraphosphate on urinary bladder in the rat: role of A1- and P2X-purinoceptors and nitric oxide

1. Both adenosine-5'-triphosphate (ATP) and diadenosine tetraphosphate (AP4A) produced a dose-dependent contraction of the isolated rat urinary bladder rings. AP(4)A dose-response curve was to the left of that of ATP, and maximum response was greater than that produced by ATP. 2. 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), the A1-purinergic receptor blocker (0.01 mm) significantly inhibited the ATP- and AP4A-induced contractions at the whole dose range. The inhibition was between 31-41%, and 15-25% for ATP and AP4A respectively. 3. Pyridoxal phosphate 6-azophenyl-2',4'-disulphonic acid (PPADS), the P2X-purinoceptor antagonist (0.01 mm) potently inhibited the bladder contractions in response to ATP and AP4A by around 75-80%. 4. The nitric oxide (NO) precursor L-arginine reduced the bladder contractile response to ATP by about 22-41% and that of AP4A to a lesser extent by around 20-32%. 5. The nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME, 0.1 mM), did not produce any significant effect on ATP except for a weak inhibition of about 14% at the lowest dose of ATP. The contractions in response to AP4A were only slightly reduced by L-NAME by about 20%. 6. In conclusion, the contractile response of the bladder to ATP and to the dinucleotide AP4A is mediated mainly through P2X-purinoceptors and A1-purinergic receptors. In the detrusor muscle, NO donation possesses an inhibitory effect on ATP-mediated contractility more than that produced by the dinucleotide AP4A.

Contractile activity of ATP and diadenosine tetraphosphate on urinary bladder in the rats: role of superoxide anion and urothelium

Both ATP and diadenosine tetraphosphate (AP(4)A) produced a dose-dependent contraction of rat isolated urinary bladder rings. The AP(4)A dose-response curve was to the left of that of ATP, and the maximum response was greater than that produced by ATP. Mechanical removal of the urothelium increased the contractile response to ATP by between 53% and 71%, and that to AP(4)A by 42% (at highest AP(4)A concentration) to 68% at lower concentration. Inhibition of Cu/Zn superoxide dismutase with diethylthiocarbamate (DETCA, 5 mm) significantly reduced the ATP-evoked contraction by 31% (at high ATP concentration) to 40% at low ATP concentration. Similarly, the AP(4)A-induced contractions were significantly decreased by 27% at low AP(4)A level to 38% at higher concentrations. Induction of exogenous superoxide anion stress by the use of the superoxide anion generator, pyrogallol (0.5 mm), significantly decreased both ATP- and AP(4)A-induced contractions of the rat urinary bladder over the whole dose range. Contractile responses to ATP decreased by 36-40%, and those to AP(4)A by 44-49%. In conclusion, the urinary bladder urothelium exerts an inhibitory control over the purinergic contractility produced by adenine mononucleotides and dinucleotides. Superoxide anion stress, whether endogenous or exogenous, attenuates the ATP-induced as well as AP(4)A-induced contractility.

Muscarinic regulation of neonatal rat bladder spontaneous contractions

In vitro preparations of whole urinary bladders of neonatal rats exhibit prominent myogenic spontaneous contractions, the amplitude and frequency of which can be increased by muscarinic agonists. The muscarinic receptor subtype responsible for this facilitation was examined in the present experiments. Basal spontaneous contractions in bladders from 1- to 2-wk-old Sprague-Dawley rats were not affected by M2 or M3 receptor antagonists. However, administration of 0.5 microM physostigmine, an anticholinesterase agent that increases the levels of endogenous acetylcholine, or 50-100 nM carbachol, a cholinergic agonist at low concentrations, which did not cause tonic contractions, significantly augmented the frequency and amplitude of spontaneous contractions. Blockade of M2 receptors with 0.1 microM AF-DX 116 or 1 microM methoctramine or blockade of M3 receptors with 50 nM 4-diphenylacetoxy-N-methylpiperidine methiodide or 0.1 microM 4-diphenylacetoxy-N-(2-chloroethyl)piperidine hydrochloride (4-DAMP mustard) reversed the physostigmine and carbachol responses. M2 and M3 receptor blockade did not alter the facilitation of spontaneous contractions induced by 10 nM BAY K 8644, an L-type Ca2+ channel opener, or 0.1 microM iberiotoxin, a large-conductance Ca2+-activated K+ channel blocker. NS-1619 (30 microM), a large-conductance Ca2+-activated K+ channel opener, decreased carbachol-augmented spontaneous contractions. These results suggest that spontaneous contractions in the neonatal rat bladder are enhanced by activation of M2 and M3 receptors by endogenous acetylcholine released in the presence of an anticholinesterase agent or a cholinergic receptor agonist.

Comparative study of the contractile activity evoked by ATP and diadenosine tetraphosphate in isolated rat urinary bladder

The present study was designed to investigate the effect and possible mechanism(s) of action of ATP and diadenosine tetraphosphate (AP(4)A) on the isolated rat urinary bladder rings. ATP ( 0.1- 1 x 10(-3)M) or AP(4)A ( 0.01- 0.1 x 10(-3)M) produced contractions of the isolated bladder rings in a concentration-dependent manner. The contraction-induced by AP(4)A in the bladder rings was approximately ten times more potent than that produced by ATP. Addition of ATP prior to addition of AP(4)A or vice versa desensitized bladder tissue to the second agonist with great reduction in the contraction produced. Electrical field stimulation (EFS, 40 V, 0.5 ms, 2 Hz) produced contraction (79.8 +/-7.1 g tension x g(-1)tissue) in the bladder rings that can be greatly reduced by prior addition of ATP or AP(4)A. Theophylline, a P(1)-purinoceptor antagonist, significantly reduced the contraction-induced by AP(4)A and did altered that produced by ATP in bladder rings. Atropine, a non-selective muscarinic receptor antagonist, or indomethacin, a cyclo-oxygenase inhibitor, significantly suppressed the contractions of the bladder rings to ATP or AP(4)A. Similarly, nifedipine, an l -type Ca(2+)channel blocker, significantly attenuate the contractions induced by ATP and AP(4)A in the isolated rat urinary bladder rings. In conclusion, the results of the present study show that ATP, AP(4)A, and EFS evoked contractions in the rat urinary bladder rings and that the contractions induced by AP(4)A was more potent than that produced by ATP. Furthermore, the contractions evoked by ATP or AP(4)A were Ca(2+)-dependent and mediated at least in part through one of the cyclo-oxygenase products. Also, the present results suggested the involvement of the P(1)-purinoceptor in mediating the contractions evoked by AP(4)A but not ATP in the bladder rings.

The role of the L-type Ca(2+) channel in refilling functional intracellular Ca(2+) stores in guinea-pig detrusor smooth muscle

The transient rise of intracellular Ca(2+) in detrusor smooth muscle cells is due to the release of Ca(2+) from intracellular stores. However, it is not known how store refilling is maintained at a constant level to ensure constancy of the contractile response. The aim of these experiments was to characterise the role of L-type Ca(2+) channels in refilling. Experiments used isolated guinea-pig detrusor myocytes and store Ca(2+) content was estimated by measuring the magnitude of change to the intracellular [Ca(2+)] ([Ca(2+)](i)) after application of caffeine or carbachol using epifluorescence microscopy. Membrane potential was controlled when necessary by voltage clamp. After Ca(2+) stores were emptied they refilled with an exponential time course, with a time constant of 88 s. The value of the time constant was similar to that of the undershoot of [Ca(2+)](i) following store Ca(2+) release. The degree of store filling was enhanced by maintained depolarisation, or by transient depolarising pulses, and attenuated by L-type Ca(2+) channel antagonists. Inhibition of the sarcoplasmic reticular Ca(2+)-ATPase prevented refilling. Reduction of the resting [Ca(2+)](i) was accompanied by membrane depolarisation; under voltage clamp reduction of [Ca(2+)](i) decreased the number and magnitude of spontaneous transient outward currents. Ca(2+) release from intracellular stores, elicited by caffeine or carbachol, is independent of membrane potential under physiological conditions. However, store refilling occurs via Ca(2+) influx through L-type Ca(2+) channels. Ca(2+) influx is regulated by a feedback mechanism whereby a fall of [Ca(2+)](i) reduces the activity of Ca(2+)-activated K(+) channels, causing cell depolarisation and an enhancement of L-type Ca(2+) channel conductance.

Effect of diltiazem and pinacidil on the response of the rabbit urinary bladder to repetitive stimulation and in vitro ischemia

The effect of repetitive stimulation, in the presence and absence of diltiazem or pinacidil, on the contractile responses of isolated strips of rabbit bladder detrusor to field stimulation and carbachol, after 2 hr of incubation in a medium that serves as an in vitro model of ischemia (oxygen and substrate depleted Tyrode's solution), was determined. Our results are summarized as follows: a) The magnitude of the contractile dysfunctions after in vitro ischemia was enhanced by repetitive stimulation. b) Pre-incubation of isolated strips of detrusor with diltiazem (50 microM) inhibited the contractile responses to field stimulation (FS) and carbachol by 43 and 50%, respectively. Pinacidil (100 microM) inhibited the contractile responses to FS and carbachol by 37 and 32%, respectively. c) Neither diltiazem nor pinacidil protected the bladder strips against the effects of 2 hr of incubation in in vitro ischemia medium. However, d) both pinacidil and diltiazem reduced the level of contractile dysfunctions induced by repetitive stimulation. In conclusion, the contractile response to FS was significantly more sensitive to in vitro ischemia and repetitive stimulation than was the contractile response to carbachol. Both diltiazem and pinacidil protected the contractile responses to FS and carbachol from the degenerative effects of repetitive stimulation, but not from the effects of in vitro ischemia.

Rabbit versus rat urinary bladder: effects of in vitro hypoxia

PURPOSE

Studies indicate that bladder hypoxia may be an etiological factor for lower urinary tract dysfunction. Rat and rabbit are two species of experimental animals used frequently to study lower urinary tract function and dysfunction. The objective of this study was to compare directly effects of in vitro hypoxia on contractile responses of rat and rabbit urinary bladder to different forms of stimulation.

METHODS

Sexually mature male New Zealand White rabbits and Sprague-Dawley rats were compared. Each bladder was excised while the animal was anesthetized, and longitudinal bladder strips were cut, then mounted in organ baths. A tension of 2 g was placed on all strips. Effects of 1, 2, 3 and 4 h hypoxia followed by 1 h of reoxygenation on contractile responses of bladder strips to field stimulation (FS), carbachol (100 micromol/l), ATP (1 mmol/l) and KCl (120 mmol/l) were determined.

RESULTS

Contractility, per unit tissue mass, of rat bladder strips was significantly greater than that of rabbit bladder strips in response to FS (all frequencies), carbachol, KCl and ATP. Hypoxia (followed by reoxygenation) resulted in time-dependent progressive reduction in contractile responses of bladder strips to all stimuli. Rat bladder was significantly more sensitive to hypoxia than rabbit bladder in response to FS and carbachol. Hypoxia induced similar effects on rat and rabbit bladder responses to ATP and KCl.

CONCLUSION

Rat bladder neurogenic and cholinergic responses are significantly more sensitive to hypoxia than are those of rabbit bladder, which may be due to the rat bladder's greater contractile force generation and previously reported higher Ca2+-ATPase activity.

Carbachol-induced sustained tonic contraction of rat detrusor muscle

OBJECTIVE

To investigate the underlying contractile mechanism of the sustained tonic contraction (SuTC) induced by repetitive carbachol application in rat detrusor muscles.

MATERIALS AND METHODS

Longitudinal muscle strips with no mucosa were obtained from the anterior wall of the urinary bladder in 12-week-old Sprague-Dawley rats. Carbachol (5 micromol/L) was applied repetitively to induce SuTC. The carbachol-induced SuTC was assessed in the presence of various Ca2+-channel blockers and drugs affecting intracellular Ca2+ concentration.

RESULTS

The first application of carbachol elicited a large phasic contraction followed by a tonic contraction (TC); the carbachol-induced contraction was completely reversed by washing out the solution. However, the initial phasic contraction was not reproduced after a second or further application of carbachol. There was consistently only a SuTC with no phasic contraction. The amplitude of the SuTC was 85% of the TC induced by the first carbachol application. The application of atropine (1 micromol/L) to the bath completely blocked SuTC. The carbachol-induced SuTC was insensitive to nicardipine (5 micromol/L) and extracellular polyvalent cations (1 mmol/L, La3+, Co2+, Cd2+, Ni2+ ). Moreover, a similar SuTC was induced even after the complete elimination of extracellular Ca2+ by adding 2 mmol/L EGTA to the Ca2+-free Tyrode solution. To exclude intracellular Ca2+ sources related to the sarcoplasmic reticulum (SR), the effects of SR Ca2+ pump inhibitors, cyclopiazonic acid (CPA, 10 micromol/L) and thapsigargin (0.5 micromol/L) were tested. The carbachol-induced SuTC was insensitive to pretreatment with CPA and/or thapsigargin. To deplete the ryanodine-sensitive Ca2+ pool, muscle strips were repetitively stimulated with caffeine (10 mmol/L) in the presence of 10 micromol/L ryanodine, which did not affect the carbachol-induced SuTC.

CONCLUSIONS

Although the characteristics of the carbachol-induced SuTC have not been defined, these results show that a significant proportion of the carbachol-induced contraction in rats is contributed by the SuTC, which is present even in the complete absence of external Ca2+. The SuTC was not affected by limiting the contributions of internal Ca2+ sources. This suggests that the SuTC in rat bladders is unrelated to known Ca2+ mobilization mechanisms.

Correlation of EGTA and calcium-blocking agents on the response of the bladder to in vitro ischemia

The effects of repetitive field stimulation (model of hyperrelexia) on the responses of isolated strips of rabbit urinary bladder to FS and carbachol were evaluated under a variety of incubation conditions. Compared to control conditions, 2 h of repetitive FS in normal, oxygenated Tyrode's solution followed by incubation for 1 h with no stimulation resulted in a 50% decrease in contractile response to FS and a 30% decrease in the response to carbachol. Incubation in the absence of O2 and glucose was used as an in vitro model for ischemia. Repetitive stimulation during in vitro ischemia resulted in a significantly greater decrease in the contractile responses to FS and carbachol than did in vitro ischemia without repetitive stimulation. The magnitude of contractile dysfunctions in response to both stimuli were significantly reduced in the presence of EGTA (calcium chelator), diltiazem (calcium channel blocker) or pincidil (potassium channel opener). Incubation with thapsigargin (SR calcium uptake inhibitor) + ryanodine (SR calcium storage inhibitor) had no effect. The results of these studies indicate that inhibition of Ca2+ entry reduces the contractile dysfunctions induced by repetitive stimulation in the presence of in vitro ischemia. Inhibition of Ca2+i storage and release had no significant effect on the magnitude of contractile dysfunctions induced by repetitive stimulation an in vitro ischemia.

Effect of calcium and calcium chelator on the response of the bladder to in vitro ischaemia

OBJECTIVE

To examine the effect of different concentrations of calcium on the contractile responses of isolated strips of rabbit bladder detrusor to various forms of stimulation after 2 h incubation in the presence of substrate and oxygen depletion (in vitro ischaemia), followed by 1 h of recovery. The resultant contractile responses were correlated with the level of lipid peroxidation as determined by malonedialdehyde (MDA) concentration.

MATERIALS AND METHODS

Isolated strips of rabbit bladder detrusor smooth muscle were incubated in Tyrode's solution containing different concentrations of calcium (0-5.4 mmol/L). The effect of 2 h of incubation in oxygen- and substrate-free medium (in vitro ischaemia), followed by a 1-h incubation in the presence of oxygen and substrate, on the contractile responses to field stimulation, carbachol and KCl were determined. The effects of repetitive stimulation (15 s of stimulation at 32 Hz applied every 5 min during the 2-h experimental period) were also assessed.

RESULTS

The contractile responses to all stimuli increased as the extracellular calcium concentration was increased from 0.6 to 5.4 mmol/L. A 2 h exposure to in vitro ischaemia, followed by a return to normal solution, resulted in a diminished response to all stimuli. This contractile dysfunction was least in the presence of calcium chelator (EGTA) and greatest in the presence of 5.4 mmol/L calcium. Repetitive stimulation during in vitro ischaemia also exacerbated the contractile dysfunction. Lipid peroxidation increased during in vitro ischaemia in proportion to the calcium concentration and was enhanced by repetitive stimulation during this period. Regardless of the incubation conditions, the reduction in the contractile response was significantly greater for field-stimulated tissues than for those stimulated with carbachol or KCl.

CONCLUSIONS

These results show that the magnitude of contractile dysfunction induced by incubation in the presence of substrate and oxygen depletion is reduced in the presence of low calcium concentrations, increased in the presence of high calcium levels and increased in the presence of repetitive stimulation. In addition, the level of lipid peroxidation after the recovery period was proportional to the magnitude of contractile dysfunction present.

A novel in vitro bladder pelvic nerve afferent model in the rat

OBJECTIVE

To develop an in vitro model to allow electrophysiological recordings from pelvic nerve afferents of the urinary bladder in the rat and to ascertain the stability and reproducibility of the model with time.

MATERIALS AND METHODS

Six male Wistar rats (body weight approximately 100 g) were used in the study. The bladder (complete with accessory organs of prostate and seminal vesicles), urethra and penis, together with the attached pelvic nerve and L6/S1 nerve trunk, were removed intact and placed in a specially designed recording chamber containing oxygenated Krebs solution maintained at 30 degrees C. The bladder was catheterized urethrally and attached to a continuous-infusion pump and a pressure transducer. The L6/S1 nerve trunk was placed across a silicone-gel wall into a separate chamber containing liquid paraffin, in which multiunit recordings from pelvic nerve afferents originating from the bladder were made. The afferent nerve activities in response to repeated bladder distension with saline, at 0.04 mL/min for 8 min over 3 h, were compared using the paired t-test to assess the reproducibility of the model. Conduction velocity studies were also carried out to ascertain the proportion of C- and A delta-fibres in the multiunit recordings.

RESULTS

Repeated bladder distension with saline over 3 h produced consistent and reproducible afferent nerve responses, signifying that the afferent nerves recorded in this study neither sensitize nor desensitize over time. This is an essential prerequisite when using this model to study the effects of pharmacological manipulation of the bladder on its afferent nerve response. Conduction velocity studies showed that approximately 30% of the afferent fibres recorded from were C-fibres with the remaining being A delta-fibres.

CONCLUSIONS

An in vitro bladder pelvic nerve afferent model for the rat was developed successfully; it is stable and produces reproducible results with repeated bladder distension over at least 3 h.

Role of L- and N-type Ca2+ channels in muscarinic receptor-mediated facilitation of ACh and noradrenaline release in the rat urinary bladder

1. 3H-Noradrenaline (NA) and 14C-acetylcholine (ACh) released by electrical field stimulation were measured simultaneously in strips from the body of rat urinary bladder. 2. omega-Conotoxin GVIA (omega-CgTX; 20-100 nM) suppressed the non-facilitated transmitter release evoked by intermittent stimulation (IS), whereas nifedipine (1 microM) did not affect release. 3. Continuous electrical stimulation (CS) facilitated NA and ACh release via an atropine-sensitive mechanism. omega-CgTX reduced the facilitated release of NA (44% depression) but did not affect ACh release. Nifedipine depressed ACh release (43%) but not NA release. Combined administration of nifedipine and omega-CgTX (20 nM) produced a greater suppression of NA and ACh release (86 and 91%, respectively). 4. Maximal muscarinic facilitation of NA (5-fold) and ACh (17-fold) release occurred following administration of eserine, an anticholinesterase agent. Release of both NA and ACh was depressed by nifedipine (70 and 83%, respectively) but not by omega-CgTX. Combined application of omega-CgTX and nifedipine elicited a further depression of NA (95%) but not ACh release. 5. When NA and ACh release was facilitated with phorbol dibutyrate (0.5 microM), nifedipine inhibited ACh (67%) but not NA release, whereas omega-CgTX inhibited NA (73%) but not ACh release. Combined administration of both Ca2+ channel blockers did not elicit greater inhibition. 6. Bay K 8644, the L-type Ca2+ channel activator, increased ACh release in a dose-dependent manner (up to 5-fold) but did not significantly change NA release. 7. Both omega-CgTX (20-100 nM) and nifedipine (100 nM-1 microM) significantly decreased (50-80%) the neurally evoked contractions of the bladder strips. 8. It is concluded that L-type Ca2+ channels play a major role in muscarinic facilitation of NA and ACh release in the urinary bladder but are not essential for non-facilitated release. Other types of Ca2+ channels, including N-type, are involved to varying degrees in non-facilitated and facilitated release under different experimental conditions.

Ability of obstructed bladders to empty is dependent on method of stimulation

PURPOSE

To correlate pharmacologic changes that occur in the bladder after a partial outlet obstruction with the bladder's ability to perform work and empty.

METHODS

After 2 weeks of partial outlet obstruction, rabbit bladders were stimulated in vitro both isovolumetrically [field stimulation (FS)] and to empty (FS, bethanechol, and KCl).

RESULTS

The obstructed bladders were separated into two groups according to their ability to empty when stimulated with FS. Compensated bladders were those that could empty as much as controls. Decompensated bladders emptied significantly less than controls. With FS and bethanechol, the compensated obstructed bladders showed no difference from the control bladders in their ability to empty. In contrast, with KCl, the compensated bladders generated significantly less pressure, performed less work, and emptied less than controls. When the decompensated bladders were stimulated with all three types of stimulation, all parameters, including emptying ability, were significantly decreased.

CONCLUSIONS

The reduction in the response of compensated bladders to KCl stimulation suggested that the initial defects to the bladder after an outlet obstruction involved the interaction of smooth muscle proteins with calcium and ATP. In contrast, the response of decompensated bladders to all three forms of stimulation was equally reduced, suggesting that the degenerative processes were directly related to significant cellular damage to metabolic processes involved in energy synthesis, storage, and utilization.

Drugs acting on calcium channels modulate the diuretic and micturition effects of dexmedetomidine in rats

The purpose of this study was to assess the effects of calcium channel antagonist, verapamil, and agonist, Bay K 8644, on the alpha 2-adrenoceptor agonist, dexmedetomidine-induced (300 micrograms kg-1 subcutaneously) diuresis and overflow incontinence, in rats. Ultrasonography study revealed that verapamil (2.5 mg kg-1 subcutaneously) or Bay K 8644 (0.5 mg kg-1 intraperitoneally) coadministrations delayed dexmedetomidine-induced bladder filling and significantly prolonged the latency of urination (P < 0.05). Bay K 8644 decreased relative bladder volume and stopped continuous urination from dexmedetomidine, whereas verapamil had neither effect. However, none of the drugs eliminated the overflow incontinence. Dexmedetomidine alone increased the hourly and total (for 4 hours) urine volume. Bay K 8644 (0.5 or 1 mg kg-1) dose-dependently decreased the diuretic effect of dexmedetomidine (P < 0.01). Verapamil (0.5, 1 or 2.5 mg kg-1) dose-dependently decreased urine volume in the first hour (P < 0.01), and thereafter potentiated the diuretic effect of dexmedetomidine. Simultaneous determinations of mean arterial blood pressure (MAP) and urine output after dexmedetomidine and the highest dose of verapamil coadministration demonstrated a significant correlation between these variables (r = 0.537; P < 0.001). MAP of 100 mmHg or less was associated with a urine output significantly lower (P < 0.001) than that at higher pressures. Thus, hypotension during the first hour after dexmedetomidine-verapamil may explain the transient reduction in urination during this period. We conclude that modulation of calcium channel affects dexmedetomidine actions on both urine formation and micturition. Since both alpha 2-adrenoceptor agonists and calcium channel blockers have frequently been used for antihypertensive therapy and as adjuvant drugs during anesthesia, these interactions may have some practical importance.

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.

Effect of partial outlet obstruction of the rabbit urinary bladder on ryanodine binding to microsomal membranes

1. Partial outlet of obstruction of the rabbit urinary bladder results in a significant increase in the ability of ryanodine to inhibit field stimulated contraction. The current study determined the effect of outlet obstruction on ryanodine binding to microsomal membranes. 2. The results demonstrate: (i) Ryanodine binding is linear showing one set of ryanodine binding sites (Bmax = 28 +/- 3.5 fmol/mg protein; Kd = 4.2 nM). (ii) Total ryanodine binding was slightly increased at 1 and 3 day following obstruction. (iii) Binding increased approx 4-fold at 5 and 7 days post obstruction. (iv) No change in the Kds were noted at any time period. 3. The results of these studies indicate that smooth muscle hypertrophy secondary to partial outlet obstruction induces a marked increase in the role of intracellular calcium in the mediation of the contractile response to field stimulation.

Effects of Ca2+ channel antagonists and benzodiazepine receptor ligands in normal and skinned rat urinary bladder

The effects of Ca2+ channel antagonists and benzodiazepine receptor ligands against concentration-dependent contractions of rat urinary bladder induced by CaCl2 (0.1-50 mM, in K(+)-depolarized tissues), KCl (1-100 mM) and acetylcholine (0.1 microM to 1 mM) were studied. Nifedipine (0.001-0.1 microM), verapamil (0.01-1 microM), diltiazem (0.01-1 microM), cinnarizine (1-100 microM), and trifluoperazine (1-100 microM) each produced a concentration-related inhibition of the log concentration-effect curve for CaCl2. The rank order of potencies of these antagonists, measured as the IC50 against Ca2+ (25 mM)-induced contraction of depolarized bladder, was nifedipine (0.01 microM) > diltiazem (0.36 microM) approximately verapamil (0.41 microM) > or = cinnarizine (2.57 microM) > trifluoperazine (17.4 microM). These antagonists depressed KCl-induced contractions with an effectiveness and potency similar to that displayed against CaCl2-induced contractions. Nifedipine, verapamil, and diltiazem but not cinnarizine and trifluoperazine had a preferential inhibitory effect on the contractions elicited by KCl when compared to those elicited by acetylcholine. Ro 5-4864, diazepam, midazolam and the non-benzodiazepine PK 11195, each at 1-100 microM, depressed CaCl2- and KCl-induced contractions (IC50 values in the micromolar range). Benzodiazepines and PK 11195, all at 100 microM, markedly depressed acetylcholine-induced contractions. Flumazenil was scarcely effective. Cinnarizine (100 microM) and trifluoperazine (100 microM), but not the other Ca2+ channel antagonists and benzodiazepine receptor ligands tested, depressed Ca2+ (20 microM)-evoked contractions of skinned bladder. It is concluded that the action of nifedipine, verapamil, and diltiazem is restricted to the plasmalemma whereas cinnarizine and trifluoperazine also act on the intracellular contractile apparatus.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

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.

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.

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.

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.

The effects of Bay K 8644 and nifedipine on the responses of rat urinary bladder to electrical field stimulation, beta,gamma-methylene ATP and acetylcholine

1. Bay K 8644 (0.33 nM to 1 microM) greatly increased the contractions of rat urinary bladder detrusor muscle induced by beta, gamma-methylene ATP (beta, gamma-MeATP, 10 microM) and by electrical field stimulation of the purinergic component (the cholinergic response was blocked by atropine). 2. The contractions induced by acetylcholine (ACh, 10 microM) and by electrical field stimulation of the cholinergic component (the purinergic response was blocked following desensitization by alpha, beta-MeATP) were also potentiated by Bay K 8644, although to a lesser extent than the purinergic responses. 3. Nifedipine (1 nM to 3.3 microM) inhibited all the contractions induced by beta, gamma-MeATP, ACh and electrical field stimulation. However, while the responses to beta, gamma-MeATP and electrical field stimulation of the purinergic component were almost abolished, a substantial proportion of the responses to ACh and electrical field stimulation of the cholinergic component were nifedipine resistant. 4. The concentration-effect curves for the potentiation by Bay K 8644 of the responses to beta, gamma-MeATP, ACh and electrical field stimulation were shifted to the right by nifedipine (10 nM). At concentrations greater than 1 microM, Bay K 8644 inhibited contraction. 5. It is concluded that voltage-sensitive calcium channels play an important role in the excitatory mechanical action of P2X-purinoceptor-mediated purinergic responses in the rat urinary bladder, while cholinergic-mediated responses are less dependent on such channels.

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.

Multiple sources of calcium for contraction of the human urinary bladder muscle

1. KCl, carbachol, neurokinin A and endothelin produced concentration-dependent contractions of mucosa-free muscle strips from the dome of the human urinary bladder. The maximal response to carbachol or neurokinin A exceeded that to KCl, while the maximal response to endothelin approached that to KCl. 2. Nifedipine (1 microM) abolished the response to KCl, reduced the response to carbachol or neurokinin A but had no effect on the response to endothelin. Bay K 8644 (1 microM) markedly potentiated the response to KCl but had little or no effect on the response produced by the other stimulants. 3. Superfusion of the strips with a nominally calcium (Ca)-free medium containing EDTA (1 mM) for 30 min markedly reduced the response to carbachol, neurokinin A and endothelin, although a small response was still evident at high concentrations. Likewise, after a prolonged (60 min) superfusion of the strips with a high K (80 mM) Ca-free medium plus EDTA (1 mM) these three agonists still produced a small contractile response. 4. The nifedipine (1 microM) resistant response to carbachol, neurokinin A or endothelin was markedly depressed by LaCl3 (1 mM). In contrast, the nifedipine-(1 microM) resistant response to carbachol was not modified by NiCl2 (0.1 mM) or omega-conotoxin (0.1 microM). 5. Caffeine produced divergent effects depending upon the temperature of incubation: a relaxation at 37 degrees C and a concentration-dependent (2.5-20 mM) contraction at 25 degrees C. The latter was markedly inhibited by procaine (3 mM) but unaffected by nifedipine (1 microM). 6. After a prolonged (60 min) superfusion with a high K, Ca-free medium containing EDTA the response to carbachol (100 microM) was abolished by previous exposure to procaine (3 mM). Conversely, the response to endothelin (1 microM) was unaffected by procaine. The response to endothelin in these experimental conditions was also resistant to LaCl3 (1 mM). 7. These findings indicate that multiple sources of Ca are mobilized for contraction of the human bladder muscle by different stimulants. Dihydropyridine- and voltage-sensitive Ca channels provide the major if not the sole source of Ca for the response to KCl, play some role in the response to muscarinic (carbachol) or NK-2 tachykinin receptor stimulation but are not involved in the response to endothelin. Carbachol, neurokinin A and endothelin all mobilize a Ca pool (either extracellular or located at membrane level) which is LaCl3-sensitive but nifedipine-resistant. Neither T- nor N-type channels appear to be involved in the response to carbachol. In addition, these agents mobilize a tightly bound Ca pool independently from membrane depolarization. This latter pool is probably a procaine-sensitive intracellular source of activator Ca mobilized by caffeine and carbachol. The failure of procaine to prevent the response to endothelin in high K, Ca-free medium raises the possibility that this peptide mobilizes an intracellular source of activator Ca, distinct from the caffeine- and carbachol-sensitive pool.

Further studies on mechanisms regulating the voiding cycle of the rat urinary bladder

1. The effect of transection of pudendal or hypogastric nerves and of various pharmacological pretreatments on the bladder voiding cycle elicited by saline filling was investigated in urethane-anesthetized rats. 2. Sectioning of pudendal nerves reduced efficiency of the expulsive phase of the voiding cycle while sectioning of hypogastric nerves enhanced voiding efficiency and reduced residual volume. 3. An increased voiding efficiency was also observed in 6-hydroxydopamine-treated rats. 4. Atropine and physostigmine decreased and increased voiding efficiency, respectively. Indomethacin pretreatment produced a marked increase in residual volume. 5. These findings indicate that the autonomic innervation of the bladder and urethra as well as production of local factors such as prostanoids regulates not only the collecting phase of the cystometragram but also influence markedly voiding efficiency.

The effect of calcium free medium and nifedipine on the release of substance P-like immunoreactivity and contractions induced by capsaicin in the isolated guinea-pig and rat bladder

1. Capsaicin produced a prompt release of substance P-like immunoreactivity (SP-LI) from superfused mucosa-free muscle strips excised from the guinea-pig urinary bladder. A second application of capsaicin had no further effect, indicating desensitization. 2. Neither tetrodotoxin (1 microM) or nifedipine (10 microM) had any inhibitory effect on SP-LI release by capsaicin nor influenced the establishment of the desensitized state. Nifedipine produced per se some SP-LI release. 3. SP-LI release by capsaicin was abolished by incubation in a Calcium(Ca)-free medium containing EDTA (1.0 mM) which also afforded a partial protection toward desensitization. A lower EDTA concentration (0.1 mM) did not suppress SP-LI release by capsaicin but still inhibited desensitization. 4. When the concentration of CaCl2 in the medium was lowered to 1/10-1/100 of that present in normal Krebs solution, capsaicin still evoked a marked SP-LI release and desensitization occurred. In a nominally Ca free medium (maximal Ca concentration due to impurities was 6.7 microM) SP-LI release was still observed and desensitization was incomplete. 5. In a nominally Ca free medium, removal of Mg ions enhanced the SP-LI release induced by capsaicin and enhanced desensitization. 6. In functional studies, nifedipine greatly reduced or abolished the capsaicin- or SP-induced contraction of the rat or guinea-pig isolated bladder but did not prevent desensitization. Likewise, SP-LI depletion in the rat bladder following systemic capsaicin desensitization was not prevented by nifedipine pretreatment. On the other hand, the protective action of Ca free media (containing EDTA) was confirmed in organ bath studies (guinea-pig bladder). 7. These findings indicate that: (a) the requirements of extracellular calcium for activation of neuropeptide release from sensory nerves by capsaicin are very low; (b) both excitation of sensory fibers (SP-LI release) and desensitization are dependent upon the presence of extracellular calcium and (c) L-type voltage-sensitive Ca channels are not likely to be involved in the actions of capsaicin on sensory nerve terminals.

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 effect of SC-19220, a prostaglandin antagonist, on the micturition reflex in rats

SC-19220 (5-20 mg/kg i.v.), a competitive receptor antagonist of PGE, increased the bladder capacity and reduced the voiding efficiency of micturition (elicited by slow transvesical filling) of urethane-anesthetized rats. The effect of SC-19220 was prevented by indomethacin pretreatment, whereas indomethacin per se mimicked the effects of SC-19220. SC-19220 produced a competitive rightward shift of the dose-response curve for the contractile effect induced by PGE2 on strips of rat detrusor muscle in vitro, whereas the amplitude of nerve-mediated twitches was unaffected. These findings support the hypothesis that endogenous PGE2 is physiologically involved in the regulation of vesicourethral motility in this species by facilitating attainment of the micturition threshold during the collection phase of the cystometrogram.