Electrophysiological properties of the bladder

Site-Specific Neuromodulation of Detrusor and External Urethral Sphincter by Epidural Spinal Cord Stimulation

Impairments of the lower urinary tract function including urine storage and voiding are widely spread among patients with spinal cord injuries. The management of such patients includes bladder catheterization, surgical and pharmacological approaches, which reduce the morbidity from urinary tract-related complications. However, to date, there is no effective treatment of neurogenic bladder and restoration of urinary function. In the present study, we examined neuromodulation of detrusor (Detr) and external urethral sphincter by epidural electrical stimulation (EES) of lumbar and sacral regions of the spinal cord in chronic rats. To our knowledge, it is the first chronic study where detrusor and external urethral sphincter signals were recorded simultaneously to monitor their neuromodulation by site-specific spinal cord stimulation (SCS). The data obtained demonstrate that activation of detrusor muscle mainly occurs during the stimulation of the upper lumbar (L1) and lower lumbar (L5-L6) spinal segments whereas external urethral sphincter was activated predominantly by sacral stimulation. These findings can be used for the development of neurorehabilitation strategies based on spinal cord epidural stimulation for autonomic function recovery after severe spinal cord injury (SCI).

Activation of the spinal neuronal network responsible for visceral control during locomotion

It has been established that stepping of the decerebrate cat was accompanied by involvement of the urinary system: external urethral sphincter (EUS) and detrusor muscle activation, as well as the corresponding increase of the intravesical pressure. Detrusor and EUS evoked EMG activity matched the limbs locomotor movements. Immunohistochemical labeling of the immediate early gene c-fos expression was used to reveal the neural mechanisms of such somatovisceral interconnection within the sacral neural pathways. Study showed that two locomotor modes (forward and backward walking) had significantly different kinematic features. Combining the different immunohistochemical methods, we found that many c-fos-immunopositive nuclei were localized within several visceral areas of the S2 spinal segment which matched the sacral parasympathetic nucleus and dorsal gray commissure. Cats stepping backward had 4-fold more c-fos-immunopositive nuclei within the ventrolateral part of the sacral parasympathetic nucleus apparently correspondent to the "lateral band" contained cells controlling bladder function. The present work provides the direct evidences of visceral neurons activation depending on the specific of locomotor pattern and confirms the somatovisceral integration carrying out on the spinal cord level.

A computational model of large conductance voltage and calcium activated potassium channels: implications for calcium dynamics and electrophysiology in detrusor smooth muscle cells

The large conductance voltage and calcium activated potassium (BK) channels play a crucial role in regulating the excitability of detrusor smooth muscle, which lines the wall of the urinary bladder. These channels have been widely characterized in terms of their molecular structure, pharmacology and electrophysiology. They control the repolarising and hyperpolarising phases of the action potential, thereby regulating the firing frequency and contraction profiles of the smooth muscle. Several groups have reported varied profiles of BK currents and I-V curves under similar experimental conditions. However, no single computational model has been able to reconcile these apparent discrepancies. In view of the channels' physiological importance, it is imperative to understand their mechanistic underpinnings so that a realistic model can be created. This paper presents a computational model of the BK channel, based on the Hodgkin-Huxley formalism, constructed by utilising three activation processes - membrane potential, calcium inflow from voltage-gated calcium channels on the membrane and calcium released from the ryanodine receptors present on the sarcoplasmic reticulum. In our model, we attribute the discrepant profiles to the underlying cytosolic calcium received by the channel during its activation. The model enables us to make heuristic predictions regarding the nature of the sub-membrane calcium dynamics underlying the BK channel's activation. We have employed the model to reproduce various physiological characteristics of the channel and found the simulated responses to be in accordance with the experimental findings. Additionally, we have used the model to investigate the role of this channel in electrophysiological signals, such as the action potential and spontaneous transient hyperpolarisations. Furthermore, the clinical effects of BK channel openers, mallotoxin and NS19504, were simulated for the detrusor smooth muscle cells. Our findings support the proposed application of these drugs for amelioration of the condition of overactive bladder. We thus propose a physiologically realistic BK channel model which can be integrated with other biophysical mechanisms such as ion channels, pumps and exchangers to further elucidate its micro-domain interaction with the intracellular calcium environment.

A four-component model of the action potential in mouse detrusor smooth muscle cell

Background and hypothesis

Detrusor smooth muscle cells (DSMCs) of the urinary bladder are electrically connected to one another via gap junctions and form a three dimensional syncytium. DSMCs exhibit spontaneous electrical activity, including passive depolarizations and action potentials. The shapes of spontaneous action potentials (sAPs) observed from a single DSM cell can vary widely. The biophysical origins of this variability, and the precise components which contribute to the complex shapes observed are not known. To address these questions, the basic components which constitute the sAPs were investigated. We hypothesized that linear combinations of scaled versions of these basic components can produce sAP shapes observed in the syncytium.

Methods and results

The basic components were identified as spontaneous evoked junction potentials (sEJP), native AP (nAP), slow after hyperpolarization (sAHP) and very slow after hyperpolarization (vsAHP). The experimental recordings were grouped into two sets: a training data set and a testing data set. A training set was used to estimate the components, and a test set to evaluate the efficiency of the estimated components. We found that a linear combination of the identified components when appropriately amplified and time shifted replicated various AP shapes to a high degree of similarity, as quantified by the root mean square error (RMSE) measure.

Conclusions

We conclude that the four basic components—sEJP, nAP, sAHP, and vsAHP—identified and isolated in this work are necessary and sufficient to replicate all varieties of the sAPs recorded experimentally in DSMCs. This model has the potential to generate testable hypotheses that can help identify the physiological processes underlying various features of the sAPs. Further, this model also provides a means to classify the sAPs into various shape classes.

Tyrosine kinase inhibitors and ATP modulate the conversion of smooth muscle L-type Ca2+ channels toward a second open state

Properties of smooth and cardiac L-type Ca2+ channels differ prominently in several physiological aspects, including sympathetic modulation. To assess the possible underlying mechanisms, we applied the whole cell patch-clamp technique to guinea pig detrusor smooth muscle cells, in which only L-type Ca2+ channel currents are observed in practice. During depolarization to large positive potentials, the conformation of the majority of L-type Ca2+ channels is converted from the normal (O1) to a second open state (O2), which undergoes little inactivation during depolarization. Extracellular application of genistein, a known tyrosine kinase inhibitor, significantly attenuated the voltage-dependent conversion of Ca2+ channels to O2, accompanied by reduction of availability, whereas genistin, an inactive analog, had little effect. In the absence of ATP in the patch pipette, intracellular application of either genistein or tyrphostin-47 suppressed the conversion to O2. Computer calculation revealed that the acceleration of the O1 to an inactivated state qualitatively reconstructs the unique effects of PTK inhibitors antagonized by ATP. We concluded that under normal conditions smooth muscle L-type Ca2+ channels are already modulated by tyrosine-kinase and ATP-related mechanism(s) and thereby easily achieve the second conversion, which yields voltage-dependent modulation of L-type Ca2+ current analogous to that in cardiac myocytes during beta-adrenoceptor stimulation.

Urinary bladder contraction and relaxation: physiology and pathophysiology

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

Phospholipase C inhibitors suppress spontaneous mechanical activity of guinea pig urinary bladder smooth muscle

Urinary bladder smooth muscle (UBSM) exhibits spontaneous rhythmic contraction. This spontaneous mechanical activity is generated in the presence of neuronal blockade and thus is myogenic in origin. The spontaneous myogenic contraction of UBSM may be the fundamental determinant of the physiological functions of the urinary bladder to store and excrete urine. Although the mechanisms by which UBSM generates spontaneous contraction have not been completely ascertained, its induction has been suggested to be intimately associated with smooth muscle cell action potentials to enhance extracellular Ca(2+) influx through voltage-gated L-type Ca(2+) channels. However, the alteration of membrane electrical activity does not seem to be the exclusive trigger mechanism for the generation of the spontaneous contraction. In the present study, we show that spontaneous mechanical activity of guinea pig UBSM is substantially diminished by an inhibitor of phospholipase C (PLC), U-73122, but is not affected by its inactive form, U-73343. Significant attenuation of the mechanical activity can be also obtained with another PLC inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate. Our present findings suggest a significant role for the activation of PLC and subsequent inositol 1,4,5-trisphosphate-induced Ca(2+) release mechanism as an alternative triggering system for inducing spontaneous mechanical activity of UBSM. The present results support the idea that the action potential is not the sole pacemaker mechanism by which spontaneous contraction is induced in UBSM.

Increased connexin43-mediated intercellular communication in a rat model of bladder overactivity in vivo

Bladder overactivity associated with outflow obstruction is a common human condition recapitulated in the female rat by narrowing the diameter of the urethra. The goal of these studies was to evaluate the role of intercellular communication through connexin43 (Cx43)-derived gap junction channels to bladder overactivity following partial urethral outflow obstruction of 3-day to 6-wk duration. Cx43 mRNA and protein expression were barely detectable by Northern or Western blots, respectively, in the detrusor layer of normal bladders, but bands were found with both techniques after 6 wk of obstruction. Linear regression analysis of the RT-PCR data revealed a statistically significant positive correlation between the duration of obstruction (again, ranging from 3-day to 6-wk duration) and Cx43 mRNA transcript levels, such that after 6 wk of obstruction, Cx43 transcript levels were approximately 15-fold greater than initial control values. When taking into account the approximately fivefold increase in bladder weight over this same time frame, the absolute amount of Cx43 mRNA in the bladder apparently increased by approximately 75-fold. In that regard, as anticipated, and consistent with previous observations, 6 wk of obstruction was also associated with a significant increase in spontaneous bladder contractions between micturitions. The amplitude of these contractions was significantly reduced by heptanol given intravesically. Furthermore, carbachol-precontracted bladder strips from obstructed animals were more sensitive to heptanol-induced relaxation (100 microM) than their unobstructed counterparts (n = 6; P < 0.01). When bladder strips were equivalently precontracted via electrical field stimulation (EFS; 20 Hz), similar heptanol-induced relaxation responses were observed. However, the tetrodotoxin-resistant portion of the EFS-induced contraction was greater in the obstructed than in the unobstructed animals, and this portion of the contractile response was more sensitive to heptanol-induced relaxation in obstructed than unobstructed bladders (n = 7; P < 0.01). Taken together, these observations indicate that partial outlet obstruction produces an overactive bladder that may be more dependent on intercellular communication through gap junctions for modulation of contractile responses than its normal counterpart.

Electromyographic detection of purinergic activity in Guinea pig detrusor smooth muscle

PURPOSE

We recorded nerve mediated extracellular electrical activity from guinea pig detrusor smooth muscle strips using suction electrodes and determined the electrophysiological origins of this signal and its relationship to contractile activity.

MATERIALS AND METHODS

Mucosa-free detrusor strips were prepared from male guinea pigs sacrificed under Home Office license, physiologically superfused, attached to a pressure transducer and electrically stimulated (0.1 millisecond pulses). Electrical signals recorded using a bipolar reversible suction electrode were processed and recorded simultaneously with changes in strip tension. The effect of superfusion with alpha, beta-methylene adenosine triphosphate (ATP), atropine, extracellular [CaCl(2)] depletion and pharmacological Ca2+ channel blockade on the electrical and mechanical signals was determined.

RESULTS

A biphasic electrical signal was consistently recorded from 37 detrusor strips. The signal was sensitive to graded reduction in [CaCl(2)] of the superfusate and abolished by tetrodotoxin in 7 preparations. The signal was also abolished in 12 preparations by alpha, beta-methylene ATP in association with an attenuated contraction but not significantly reduced in amplitude (p = 0.77) despite a significant reduction in tension with atropine (mean plus or minus SD 74% +/- 14% of control, p <0.001). The signal was attenuated to a mean maximum of 9% +/- 3% of control by pharmacological Ca2+ channel blockade and the remaining signal was abolished by alpha, beta-methylene ATP.

CONCLUSIONS

The extracellular electrical signal recorded from guinea pig detrusor strips using suction electrodes originates from a purinergic mechanism. Although an atropine sensitive component may be present, the signal does not depend on cholinergic neuromuscular transmission and would not be expected to be generated by normal human detrusor. Provided that the electrophysiological basis of purinergic neurotransmission in guinea pig and human bladders is similar suction electrodes may be a valuable tool with which to evaluate in vitro and clinically by electromyography the pathological purinergic neuromuscular transmission that can be expressed in addition to normal cholinergic mechanisms in detrusor from dysfunctional human bladders.

Intercellular communication and bladder function

There is now considerable experimental and clinical evidence supporting the supposition that overactivity of the bladder is associated with detectable alterations in the electrical properties of the detrusor smooth muscle cells. The preliminary data described in this report indicates that intercellular communication through gap junctions might play an important role in this process. Moreover, alterations in Cx43 mRNA expression may represent a tissue response to a physiologic insult (i.e., increased after load) in an attempt to further increase the syncytial nature and force of detrusor contractility to compensate for an increased pressure load. Finally, this report elucidates the rationale for suspecting that intercellular communication through gap junctions may play a role in normal bladder physiology and the pathophysiology of urinary incontinence caused by partial outlet obstruction.

Bladder injection of "naked" hSlo/pcDNA3 ameliorates detrusor hyperactivity in obstructed rats in vivo

The goal of these studies was to examine the potential utility of bladder instilled K+ channel gene therapy with hSlo cDNA (i.e., the maxi-K channel) to ameliorate bladder overactivity in a rat model of partial urinary outlet obstruction. Twenty-two female Sprague-Dawley rats were subjected to partial urethral (i.e., outlet) obstruction, with 17 sham-operated control rats run in parallel. After 6 wk of obstruction, suprapubic catheters were surgically placed in the dome of the bladder in all rats. Twelve obstructed rats received bladder instillation of 100 microg of hSlo/pcDNA in 1 ml PBS during catheterization, and another 10 obstructed rats received 1 ml PBS (7 rats) or 1 ml PBS containing pcDNA only (3 rats). Two days after surgery cystometry was performed on all animals to examine the characteristics of the micturition reflex in conscious and unrestrained rats. Obstruction was associated with a three- to fourfold increase in bladder weight and alterations in virtually every micturition parameter estimate. PBS-injected obstructed rats routinely displayed spontaneous bladder contractions between micturitions. In contrast, hSlo injection eliminated the obstruction-associated bladder hyperactivity, without detectably affecting any other cystometric parameter. Presumably, expression of hSlo in rat bladder functionally antagonizes the increased contractility normally observed in obstructed animals and thereby ameliorates bladder overactivity. These initial observations indicate a potential utility of gene therapy for urinary incontinence.

Pharmacology of the lower urinary tract

The functions of the lower urinary tract, to store and periodically release urine, are dependent on the activity of smooth and striated muscles in the urinary bladder, urethra, and external urethral sphincter. This activity is in turn controlled by neural circuits in the brain, spinal cord, and peripheral ganglia. Various neurotransmitters, including acetylcholine, norepinephrine, dopamine, serotonin, excitatory and inhibitory amino acids, adenosine triphosphate, nitric oxide, and neuropeptides, have been implicated in the neural regulation of the lower urinary tract. Injuries or diseases of the nervous system, as well as drugs and disorders of the peripheral organs, can produce voiding dysfunctions such as urinary frequency, urgency, and incontinence or inefficient voiding and urinary retention. This chapter will review recent advances in our understanding of the pathophysiology of voiding disorders and the targets for drug therapy.