Urinary Bladder Detrusor Smooth Muscle Research Articles

Analysis of spontaneous depolarization-linked hyperpolarizations in mouse detrusor smooth muscle cells

Background: Urinary bladder detrusor smooth muscle cells exhibit spontaneous electrical activities comprising various signal types. Aims and Objectives: This article introduces and analyzes a rare category of signals observed in such activity, named spontaneous depolarization-linked hyperpolarization (sDLH). Materials and Methods: A mouse model was used in the study, where all the occurrences of sDLHs were pooled together from multiple intracellular recording sessions. Four features – (i) resting membrane potential (RMP) (R, in mV), (ii) depolarization amplitude (D, in mV), (iii) hyperpolarization amplitude (H, in mV), and (iv) time course of the hyperpolarization (T, in ms) – were evaluated from all sDLHs. Results: The analysis of results indicated that (a) the signals appear more frequently in cells with higher RMP, (b) the depolarization amplitudes seem to be distributed randomly and have no correlation with other features, (c) hyperpolarization amplitudes show two distinct clusters and exhibit strong correlation with the RMP, and (d) time course of hyperpolarization phase shows no distinct groups and is distributed in a window larger than that of any other signals seen in the intracellular recordings. With the help of the results obtained from the analysis, a hypothesis for the biophysical origin of these signals is proposed. Conclusions: This needs to be tested experimentally, and if proved right, would help extend the boundaries of our current understanding of the detrusor smooth muscle system.

Biophysical Processes in a Urinary Bladder Detrusor Smooth Muscle Cell during Rehabilitation Stimulation of Parasympathetic Efferents: a Simulation Study

In a computer model of an urinary bladder detrusor (UBD) smooth muscle cell (SMC), we investigated coupled changes in the membrane potential (MP), partial transmembrane currents, and intracellular calcium concentration ([Ca2 +]i), caused by parasympathetic stimulation. The respective data should help to determine the biophysical basis for the choice of parameters of medical rehabilitation electrical stimulation of the UBD. Ionotropic and metabotropic effects of such stimulation having different latency periods (LPs) were simulated by, respectively, an increase in the conductivity of purinoreceptor channels of the sarcolemma and a delayed (by 70 msec) increase in the permeability of calcium channels of the sarcoplasmic reticulum (store) sensitive to inositol triphosphate (IP3), the end-product of a chain of reactions triggered by activation of muscarinic cholinergic receptors. The modeled SMC responded to single stimulation by generation of action potential (AP) close in its parameters to those in the prototype. Application of long series of identical stimuli caused, after a transient interval, forced stationary oscillations of the MP and [Ca2 +]i. The magnitudes of the latter decreased, and the mean level increased with shortening of the interstimulus intervals (ISIs). It was found that the stimulation outcome needing prior experimental refining was determined by such parameters of the parasympathetic action as the durations of the purine and muscarinic components and the delay between these components. It has been shown that, during stimulation with ISIs shorter than the above-mentioned delay, the subsiquent stimulus activates P2X channels before expiration of the delay of the IP3-channel activation evoked by the preceding stimulus. This is actually equivalent to shortening of the interval between successive activations of P2X- and IP3-channels. Simple quantitative expressions were obtained for the parameters of stimulation; a given difference in the LPs of purine and muscarinic effects of parasympathetic stimulation allowed us to calculate the ISIs providing more efficient rehabilitation stimulation of the UBD parasympathetic nerves.

Biophysical Processes in a Urinary Bladder Detrusor Smooth Muscle Cell during Rehabilitation Electrostimulation: a Simulation Study

The work was aimed at the search for approaches to solving the problem of biophysically reasonable selection of the parameters of electrical stimulation of smooth muscle cells (SMCs) of the urinary bladder detrusor (UBD). Such stimulation is widely used in the rehabilitation of patients with surgical correction of congenital malformations accompanied by total or partial deficiency of the M2/M3 cholinergic receptors in the UBD. A computer model built on the basis of experimental data on ion channels and pumps of the sarcolemma and mechanisms of regulation of the intracellular calcium concentration ([Ca22+]i), providing both electrogenesis and the contractile function of the cell inherent to the biological prototype, was used. We studied changes in the membrane potential, partial transmembrane currents, and [Ca2+]i, caused by depolarizing current pulses applied with constant frequencies and combined in “packs” or “envelopes” typical of the protocols of rehabilitation stimulation; the stimuli had constant or trapezoid-modulated amplitudes. The examined UBD SMC responded to a single pulse by generation of the action potential (AP) close in its properties to the prototype. Stimulation by both packs and envelopes of identical pulses eventually led to the establishing of equal forced electrical and concentration oscillations with the parameters depending on the duration of interpulse intervals (IPIs). Such oscillations caused by stimulation with 5- and 50-msec-long IPIs, typical of the rehabilitation protocols and comparable with the durations of the absolute and relative refractoriness of the model SMC, significantly differed in the pattern of the regenerative responses (APs) and in the range and mean levels of depolarization shifts of the membrane potential and those of [Ca2+]i, which were greater at high-frequency stimulation. In the case of short IPIs, [Ca2+]i, having no time to return to the basal level, oscillated within a range of values which in other excitable cells are considered to exceed significantly the physiological norm. These data emphasize the necessity to estimate the exact kinetic characteristics of the mechanisms underlying the inflow and extrusion of Ca2+ in the UBD SMC necessary for a biophysically justified choice of the parameters of rehabilitation stimulation that would prevent possible cytotoxic side effects associated with excessively long-lasting high levels of [Ca2+]i. Essential for the observed processes and, therefore, requiring targeted studies, was such a parameter of UBD SMCs as the reversal potential for Ca2+-dependent chloride current (E Cl); this current is activated, in particular, by parasympathetic action on the M2/M3 receptors. When high-frequency oscillations of the membrane potential periodically exceeded the E Cl level, the mentioned current changed its main (depolarizing) direction to the opposite (hyperpolarizing) one.

Excitability Characteristics of a Urinary Bladder Detrusor Smooth Muscle Cell as a Basis for Choosing Parameters of Rehabilitation Electrostimulation: A Simulation Study

Electrical stimulation targeted at smooth muscle cells (SMCs) of the urinary bladder detrusor (UBD) is used in a complex of rehabilitation procedures performed during treatment of neurourological diseases, in particular of those associated with the deficiency of М2/М3 acetylcholine receptors modifying the efficacy of parasympathetic innervation of these cells. The choice of parameters of such stimulation meets difficulties because of limited knowledge about biophysical and physiological processes induced in the stimulated cells. Certain information about such processes can be obtained with the use of a UBD SMC computer model built according to the data of biological experiments characterizing ion channels, ion exchangers, and other mechanisms regulating the intracellular calcium concentration ([Ca2+] i ) typical of the prototype. We explored the standard characteristics of the electrical excitability of the model SMC (“strength – duration” relationship for the threshold current and dynamics of the refractoriness following generation of action potentials, APs), as well as coupled changes in the membrane potential, partial transmembrane currents, and [Ca2+] i values evoked by depolarizing current pulses. In some computational experiments, such stimulation was performed under conditions of a higher conductivity of purinergic ionotropic receptor channels, thereby mimicking the action of purinomimetics. The model UBD SMC generated APs with parameters close to those of the prototype; AP generation was accompanied by long periods of absolute and relative refractoriness (up to 30 and 600 msec, respectively). The relative refractoriness period included an early phase (AP half-recovery lasting about 220 msec) and a late phase; each of these phases included “fast” and “slow” components with the time constants differing from each other by an order of magnitude. These time characteristics of the refractoriness were determined by the kinetic characteristics of the processes of activation/inactivation of voltage- and calcium-dependent ion channels and by those of the [Ca2+] i return to the basal level under the action of a set of Са2+ extrusion mechanisms. An important UBD SMC biophysical parameter was also the reversal potential (E Cl) for calcium-dependent chloride current (which is activated, in particular, due to the parasympathetic action on М2/М3 receptors). This current changed its main “depolarizing” direction to the hyperpolarizing one when the membrane potential exceeded the E Cl level.

Changes in Urinary Bladder Smooth Muscle Contractility and the Role of Rho Kinase and Isoprostane after Partial Bladder outlet Obstruction in rats

Objective: Bladder outlet obstruction is a common urological problem that associatedwith many cellular changes and remodeling of urinary bladder detrusor smoothmuscle. This study was designed to determine the changes in Rho kinase (ROCK) andisoprostane levels in urinary bladder homogenate after partial bladder outletobstruction (PBOO); and to evaluate the contractile response of isolated rat detrusormuscle after PBOO. Methods: The present work was carried out on 30 male albinorats that were divided into 3 groups (10 rats/group): group I: consisted of normalrats, they were sham operated and considered as control group, group II: consisted ofrats that were undergone PBOO for 14days, group III: consisted of rats that wereundergone PBOO for 28 days. The following parameters were assessed in rats of allgroups: urinary bladder weight, levels of both Rho kinase and isoprostane in thetissues of urinary bladder as well as the contractile response of detrusor muscle bothspontaneously and in response to acetyl choline. Results: The present study revealeda significant increase in the weight of urinary bladder in rats of PBOO of group IIand III as compared to sham operated control of group I. There was a significantincrease in the mean value of Rho kinase and isoprostane in the tissue of the urinarybladder after obstruction. 14 days after obstruction was associated with a significantincrease in the mean value of Rho kinase which was significantly high compared togroup I (sham operated rats) and group III (28days of obstruction), 28 days ofobstruction was associated with decrease in the mean value of Rho kinase comparedto 14 days of obstruction, however, its value was above the control. Regarding thecontractile response, PBOO was associated with increased spontaneous activity ofdetrusor muscle and increased tonic response to acetyl choline which wassignificantly high after 14 and 28 days of obstruction compared to the control.Conclusion: The results of the present study demonstrated that Rho kinase andisoprostane levels were elevated after bladder outlet obstruction and that Rho kinasecould play a crucial role in the regulation of rat urinary bladder smooth musclecontraction and tone. Therefore, Rho kinase inhibitors may prove to betherapeutically useful in the treatment of lower urinary tract disorders associatedwith changes in the physiology of bladder smooth muscle contractility.

Effect of the SK/IK channel modulator 4,5‐dichloro‐1,3‐diethyl‐1,3‐dihydro‐benzoimidazol‐2‐one (NS4591) on contractile force in rat, pig and human detrusor smooth muscle

• To investigate the importance of small (SK)- and intermediate (IK)-conductance Ca2(+) -activated K(+) channels on bladder function, by studying the effects of 4,5-dichloro-1,3-diethyl-1,3-dihydro-benzoimidazol-2-one (NS4591), a new modulator of SK/IK channels, on contractions induced by electrical field stimulation (EFS) and carbachol in rat, pig and human detrusor. • Detrusor biopsies were obtained from rats, pigs and male patients undergoing cystectomy because of bladder cancer. • Force was recorded using myographs. • Intracellular free Ca(2+) was measured in myocytes using microfluorimetry. • In rat bladder rings subjected to EFS, cumulative addition of NS4591 (0.1-30 µM) decreased force by 82 ± 2.9% (n = 6).This effect was reduced by 64 ± 5.2% in the presence of 0.3 µM apamin, a specific inhibitor of SK channels. Apamin increased the force evoked by EFS significantly: force was increased by 14.2 ± 3.4% (n = 5) and 10.1 ± 2.6% (n = 7) in pig and human detrusor strips, respectively (P = 0.04 and P = 0.02). • The cumulative addition of NS4591 (0.3-30 µM) significantly reduced the amplitude of carbachol-induced rhythmic oscillations by 62.0 ± 12.0% (n = 12) and the minimum force between oscillations by 30 ± 5% (n = 9) in pig detrusor strips (P < 0.005). In the presence of 10 µM NS4591, carbachol (1 µM) induced rhythmic contractions with an amplitude and normalized mean power frequency (nmeanPF) of 8.4 ± 5.1% and 0.11 ± 0.06 mN root mean square (rms) Hz (n = 12), respectively, vs. 21 ± 3.4% and 0.17 ± 0.04 mN rms Hz in control strips (n = 13). Apamin induced 6- and 11-fold increases in amplitude and nmeanPF vs. 1.3- and 2-fold increases in control strips. • In human detrusor strips (n = 15), the cumulative addition of NS4591 (1-30 µM) significantly reduced the amplitude by 69 ± 11%, the nmeanPF by 78 ± 6% and the minimum force between carbachol-induced oscillations by 59 ± 5% (P < 0.008). The addition of apamin (0.3 µM) before application of 1 µM carbachol abolished the effects of NS4591 on amplitude and partially abolished its effect on nmeanPF by 41 ± 7%, vs. a 78 ± 6% reduction in the absence of apamin (n = 8). • In spontaneously active detrusor preparations, NS4591 reduced or abolished contractions. • Furthermore, NS4591 (10 µM) decreased the carbachol-induced increase in the fura-2 ratio by 43 ± 3% compared with control (n = 12) (P < 0.03). • The SK/IK channel modulator NS4591 inhibits EFS- and carbachol-induced contractions in rat, pig and human detrusor muscle. • NS4591 may have therapeutic potential for treatment of detrusor overactivity.

Partial outlet obstruction of the rabbit urinary bladder induces selective protein oxidation

Oxidation of proteins is presumed to contribute to contractile dysfunction associated with partial outlet obstruction (PBOO) of the urinary bladder. The objective of this study was to determine the acute and chronic effects of PBOO on protein oxidation in urinary bladder detrusor smooth muscle (DSM) and mucosa, and to correlate these findings with in vitro contractile function. Urinary bladders of New Zealand White (NZW) rabbits were obstructed for 1, 3, 7 and 14 days. Proteins were extracted from the bladder tissues and protein carbonyl formation was assessed using immunoblot assays. In vitro contractile response to field stimulation (32 Hz) and carbachol was evaluated in whole bladder strips at the same time points. Significant elevations in oxidation of DSM proteins were observed in the MW range of 29-65 kDa after 3 days and 14 days of obstruction. No changes in the oxidative status of mucosal proteins were detected as a result of short or long term obstruction. The intermediate filament protein, desmin (53-55 kDa) was detected in obstructed DSM samples in the same MW range as oxidized proteins. A significant decrease in contractile response to field stimulation and carbachol was observed after 1 day and 3-days respectively, and continued to deteriorate through 14 days. The increase in protein oxidation at 14-days of obstruction correlates with impaired bladder contractility, suggesting that oxidatively modified proteins may contribute to the contractile and biochemical dysfunction associated with PBOO.

β-NAD and CD38 provide a novel mechanism of autonomic nervous system control of vascular and visceral smooth muscle

We have recently reported that β-nicotinamide adenine dinucleotide (β-NAD) is released during electrical field stimulation (EFS) of canine isolated mesenteric arteries and veins and urinary bladder detrusor smooth muscle of mouse, rat, guinea-pig, rabbit, monkey, and human (Smyth et al., J Biol Chem 279:48893, 2004; Breen et al., Am J Physiol 290:F 486, 2006). Exogenous β-NAD reduces the release of norepinephrine, causes vasorelaxation in canine mesenteric blood vessels, and inhibits the spontaneous contractions in murine and human bladders.

Effects of potassium channel modulators on human detrusor smooth muscle myogenic phasic contractile activity: Potential therapeutic targets for overactive bladder

Objectives Increased urinary bladder detrusor smooth muscle phasic contractility has been suggested to be associated with idiopathic bladder overactivity (OAB). We examined the role of voltage-dependent l-type calcium channels, adenosine triphosphate-sensitive potassium (K ATP) channels, and calcium-activated potassium (BK Ca and SK Ca) channels in the regulation of human detrusor phasic contractile activity. Methods Isolated human bladder strip phasic contractions were measured and quantified as the mean area under the force-time curve, amplitude, and frequency of phasic contractions in 22 bladder samples. Results Human detrusor strips displayed myogenic phasic contractions in the presence of atropine (10 −6 M), phentolamine (10 −6 M), propranolol (10 −6 M), suramin (10 −5 M), and tetrodotoxin (10 −6 M). The l-type calcium channel inhibitor nifedipine (300 nM) abolished the contractile activity. Blockade of K ATP channels by glibenclamide (1 and 10 μM) did not alter myogenic contractions. In contrast, the K ATP channel opener pinacidil (10 μM) markedly inhibited phasic contractility. Iberiotoxin (100 nM) and apamin (100 nM), potent and selective inhibitors of BK Ca and SK Ca channels, respectively, significantly increased the area under the force-time curve and the amplitude of contractions. Conclusions Phasic contractions of human detrusor are dependent on calcium entry through l-type calcium channels. BK Ca and SK Ca channels play a key role in the modulation of human detrusor smooth muscle phasic contractility. Furthermore, these observations support the concept that increasing conductance through K ATP, BK Ca, and SK Ca channels may represent attractive pharmacologic targets for decreasing phasic contractions of detrusor smooth muscle in OAB.

Involvement of Prostaglandin E2 in the P2Y1 Receptor‐mediated Tonic Contraction of the Rabbit Urinary Bladder

Previously we have provided evidence that excitatory ADP activated P2Y1 receptors contribute for the rabbit urinary bladder contractility. To determine the mechanisms involved in P2Y1-mediated signaling, we have carried out experiments with the isolated rabbit urinary bladder detrusor smooth muscle strips. The contractile responses evoked by ADP or the specific P2Y1 receptor agonist ADPβS were antagonized by the non-selective cyclooxygenase inhibitors indomethacin and meclofenamic acid, as well as by AACOCF3 (10μM), a specific inhibitor of the receptor activated cytosolic Phospholipase A2 (cPLA2). The contractile responses of the detrusor smooth muscle evoked by prostaglandin E2 (PgE2) (EC50, 0.3 μM) but not by PgF2α resembled that evoked by ADPβS (EC50, 0.6 μM). Pretreatment with SC19220 (30 μM), a selective antagonist of the EP1 subtype of the PgE2 receptors abolished contractile responses evoked by ADPβS, while AL-8810, a selective FP (PgF2α) receptor antagonist had no effect. RT-PCR experiments demonstrated that the rabbit detrusor smooth muscle expresses mRNA for the microsomal prostaglandin E2 synthase-2 (mPGES-2). This evidence suggests that activation of P2Y1 receptors may lead to the synthesis of PgE2, which acts in a paracrine manner to evoke and possible amplify the contractile response of the detrusor smooth muscle of the urinary bladder. Supported by NIH HL38126 and the Foundation for Research.

Molecular biological researches of the lower urinary tract function

Adrenergic alpha1 and beta receptors are present in the target organs of sympathetic nerve and they participate in the signal transduction mechanism of the lower urinary tract. Adrenergic alpha1 receptors are present in urethral and prostatic smooth muscles, and contract these muscles. Among these receptor subtypes, the alpha1-A receptor has the most important role, and mRNA expression of the corresponding alpha1-a subtype is predominant. In the human urinary bladder detrusor smooth muscle, the expression of adrenergic beta3 receptor subtype mRNA is predominant, and relaxation of detrusor smooth muscle is mediated mainly via beta3 receptor. Afferent nerve with lower threshold can easily transmit bladder sensation and takes an important role in the pathophysiology of urge urinary incontinence. Successful molecular cloning of vanilloid receptors, which are present in these afferent nerves, revealed that vanilloid receptors are ion-channels, sensitive for heat and pH, and termed VR1 and VRL1. Among purinergic receptors, ion channel type P2X3 receptor is found in afferent nerve fibers and plays some roles in the signal transduction of bladder sensation. In the near future, agonist for the adrenergic beta3 receptor and selective antagonists for VR1, VRL1, or P2X3 will possibly become drugs for pollakisuria and urge urinary incontinence.

Pharmacological and molecular biological evidence for ETA endothelin receptor subtype mediating mechanical responses in the detrusor smooth muscle of the human urinary bladder

The aim of this study was to characterize endothelin receptor subtypes of the detrusor muscle of the human urinary bladder. The receptor subtypes mediating endothelin (ET)-1-induced activity in the human detrusor smooth muscles have been characterized using isometric contraction and reverse transcription-polymerase chain reaction (RT-PCR). ET-1 (a non-selective ET receptor agonist; 10(-10) M to 10(-6) M) exhibited concentration-dependent contractions in human urinary bladder with a plateau at concentrations above 3x10(-7) M. Neither IRL1620 nor sarafotoxin S6c (both ETB-selective agonists; 10(-10) M to 10(-6) M) elicited contractile activity in the human urinary bladder detrusor smooth muscle. FR139317 (an ETA-selective antagonist; 10(-7) M to 10(-5) M) produced a marked shift to the right of the ET-1 concentration-response curve in human urinary bladder detrusor smooth muscle (from the Schild plot TpA2=7.96; slope=0.95). In contrast, RES701-1 (an ETB-selective antagonist; 10(-7) M to 10(-5) M) had no effect on the ET-1 concentration-response curve. RT-PCR revealed positive amplification of ETA receptor mRNA fragment, but not ETB. These results indicate that the ET-1-induced contractile effects of urinary bladder detrusor smooth muscle seem to be mediated mainly by the ETA receptor, not by the ETB receptor.

Purinergic Modulation of Rat Urinary Bladder Detrusor Smooth Muscle

1. Rat detrusor muscle was responsive to both ATP and adenosine; ATP elicited an excitatory response, whereas adenosine had an inhibitory effect. 2. ATP and adenosine had an inhibitory modulatory action on responses to acetylcholine, potassium depolarization and field stimulation. 3. Quinidine inhibited the ATP response and blocked the inhibitory effect of ATP on acetylcholine, potassium-depolarization and field-stimulation responses. The effect of adenosine remained unaltered in the presence of quinidine. 4. Caffeine and theophylline blocked the adenosine inhibition of responses to field stimulation. 5. It is concluded that excitatory P 2-type purinoreceptors mediated by ATP and inhibitory P 1-type purinoreceptors mediated by adenosine exist in rat urinary bladder detrusor smooth muscle and that both ATP and adenosine exhibit a modulatory action on detrusor muscle agonist-induced responses.

Age-related differential susceptibility to calcium channel blocker and low calcium medium in rat detrusor muscle: response to field stimulation.

The influences of aging on the response of urinary bladder detrusor smooth muscle in response to neurogenic stimulation (electric field stimulation) were investigated in rats aged 3-4 months (young) and 18-20 months (old). In addition, age-associated changes in calcium homeostasis were evaluated by comparing the basal intracellular free calcium concentration ([Ca2+]i), the change of [Ca2+]i in response to field stimulation, the contractile response at different extracellular calcium concentrations (Ca2+ = 1.8, 0.6, and 0 mM), and the sensitivity to the L-type calcium channel blocker. In both groups, the frequency-response curve in response to field stimulation (1-32 Hz) was similar. The contractile response to field stimulation was biphasic (phasic and tonic). The phasic/tonic ratio at different frequencies was similar between the two groups. The contractile responses were equally inhibited by atropine (1 microM) and preincubation with alpha, beta-methylene ATP (50 microM). Tetrodotoxin (1 microM) virtually abolished the responses to field stimulation in both groups. The biphasic components showed different sensitivities to diltiazem and low calcium. The tonic component was more readily inhibited by diltiazem, whereas the phasic component was more sensitive to low calcium medium. In general, the response to lower frequency stimulations was more sensitive to both diltiazem and low calcium. The aged group was more sensitive to the inhibitory effects of both low calcium and diltiazem. In Ca2(+)-free medium, the contractile response to higher frequency (e.g., 32 Hz) was temporarily preserved in most of the preparations. This response disappeared after repeated stimulation. The magnitude of this Ca2(+)-free response to 32 Hz stimulation was significantly lower in the aged group than in the young group (18.1 +/- 1.4 vs.29.1 +/- 3.2% of the control response in normal Tyrode medium, P < 0.05). The basal [Ca2+]i levels were similar in the two groups (young: 131 +/- 12 nM, n = 5; old: 118 +/- 10 nM, n = 4, P > 0.05). The magnitude and time course of the [Ca2+]i response to field stimulation were also similar in the two groups. It is concluded that aging seems to have little effect on the neuromuscular activities of rat urinary bladder. However, the calcium homeostasis may be altered as evidenced by the increased sensitivity to diltiazem and low extracellular calcium.

Field stimulation responses of rat urinary bladder detrusor smooth-muscle. Dependence upon slow calcium channel activity determined by K + depolarization and calcium antagonists

1. 1. Field stimulation responses of rat urinary bladder detrusor smooth muscle were unaffected by atropine, phentolamine, propranolol, caffeine and theophylline. After caffeine or theophylline treatment, previously quiescent detrusor preparations developed pronounced spontaneous contractile activity. 2. 2. Both natural spontaneous contractile activity and field stimulation responses were eliminated in Ca-free EGTA media. After washout, both kinds of activity returned but over quite different time scales. 3. 3. At moderate levels of K +-induced depolarization which inactivate fast Ca-channels, field stimulation responses persisted and additively superimposed upon the K +-induced tension, suggesting that field stimulation-induced depolarization had activated slow voltage-dependent Ca-channels. 4. 4. Amost 61% of detrusor muscle preparations examined showed some spontaneous contractile activity. Such preparations were more sensitive to K +-induced depolarization than quiescent preparations and K +-depolarization greatly accelerated the spontaneous activity of these muscle strips. 5. 5. The slow Ca-channel blockers nifedipine and verapamil eliminated field stimulation responses in the 10 −6–10 −5M range, but octylonium bromide was far less effective even at 10 −4 M. 6. 6. The inorganic Ca-channel blocker Mn 2+ inhibited field stimulation responses at 0.5–1.0 mM but La 3+ was surprisingly far less effective than Mn 2+ in this concentration range, requiring a 1 mM level to achieve a 50% inhibition of response. 7. 7. It is concluded that field stimulation induced a depolarization of rat urinary bladder detrusor muscle which activates slow voltage-dependent Ca-channels. Field stimulation responses and natural spontaneous activity appear to strongly depend upon a sustained Ca influx through these channels for the sustained release of intracellular calcium and the maintenance of contractile force.