Voltage‐dependent Ca2+ Currents Contribute to Spontaneous Ca2+ Waves in Rabbit Corpus Cavernosum Myocytes

Evidence of an excitatory purinergic innervation in mouse corpus cavernosum smooth muscle

BACKGROUND

Evidence suggests that the corpus cavernosum smooth muscle (CCSM) cells of several species, including humans, express purinergic P2X receptors, but it is not known if the corpus cavernosum has an excitatory purinergic innervation.

AIM

In this study we aimed to determine if the mouse CCSM has a functional purinergic innervation.

METHODS

Mouse CCSM myocytes were enzymatically isolated and studied using the perforated patch configuration of the patch clamp technique. Isometric tension was measured in whole cavernosum tissue subjected to electrical field stimulation (EFS) to evoke nerve-mediated responses.

OUTCOMES

The mouse CCSM myocytes expressed P2X1 receptors, and adenosine triphosphate (ATP) evoked inward currents in these cells. In addition, P2X1-mediated contractions were recorded in whole tissue in response to EFS.

RESULTS

In cells held under a voltage clamp at -60 mV, ATP (1 μm) evoked large inward currents (mean approximately 900 pA). This current rapidly declined but was repeatable at 8-minute intervals. α,β-methylene ATP (10 μM), an agonist of P2X1 and P2X3 receptors, caused a similar current that also rapidly declined. Desensitization to α,β-methylene ATP negated the effect of ATP, but the ATP effect was restored 8 minutes after washout of α,β-methylene ATP. The effect of ATP was reversibly blocked by NF449 (1 μm), a selective antagonist of P2X1 receptors. In isometric tension experiments electrical field stimulation (EFS) at 0.5-8 Hz evoked frequency-dependent contractions in the presence of l-nitro arginine (l-NO-Arg) (100 μm). When phentolamine (3 μm) and atropine (1 μm) were applied, there remained a nonadrenergic, noncholinergic component of the response to EFS, consisting mainly of a transient contraction. This was significantly reduced by NF449 (1 μm). Finally, in immunocytochemistry experiments, isolated CCSM myocytes stained positively when exposed to an antibody raised against P2X1 receptors.

CLINICAL IMPLICATIONS

Previous studies have shown that P2X1 receptors in CCSM are upregulated in diabetes. These findings, taken together with the functional evidence presented here, indicate that P2X1 receptors may provide an alternative therapeutic target for treatment of erectile dysfunction in patients with diabetes, which is known to be relatively resistant to treatment with phosphodiesterase 5 inhibitors.

STRENGTHS AND LIMITATIONS

Strengths of this study are the use of a combination of functional experiments (patch clamp) and immunocytochemical analyses to show expression of P2X1 receptors on CCSM myocytes while also performing functional experiments to show that stimulation these receptors results in contraction of CCSM. A limitation of this study was the use of animal rather than human tissue.

CONCLUSION

This investigation provides evidence that mouse corpus cavernosum smooth muscle cells express P2X1 receptors and that these receptors are involved in mediating part of the contractile response to nerve stimulation evoked by EFS.

Ca2+ -activated Cl- channels (TMEM16A) underlie spontaneous electrical activity in isolated mouse corpus cavernosum smooth muscle cells

Penile detumescence is maintained by tonic contraction of corpus cavernosum smooth muscle cells (CCSMC), but the underlying mechanisms have not been fully elucidated. The purpose of this study was to characterize the mechanisms underlying activation of TMEM16A Ca2+ -activated Cl- channels in freshly isolated murine CCSMC. Male C57BL/6 mice aged 10-18 weeks were euthanized via intraperitoneal injection of sodium pentobarbital (100 mg.kg-1 ). Whole-cell patch clamp, pharmacological, and immunocytochemical experiments were performed on isolated CCSM. Tension measurements were performed in whole tissue. TMEM16A expression in murine corpus cavernosum was confirmed using immunocytochemistry. Isolated CCSMC developed spontaneous transient inward currents (STICs) under voltage clamp and spontaneous transient depolarizations (STDs) in current clamp mode of the whole cell, perforated patch clamp technique. STICs reversed close to the predicted Cl- equilibrium potential and both STICs and STDs were blocked by the TMEM16A channel blockers, Ani9 and CaCC(inh)-A01. These events were also blocked by GSK7975A (ORAI inhibitor), cyclopiazonic acid (CPA, sarcoplasmic reticulum [SR] Ca2+- ATPase blocker), tetracaine (RyR blocker), and 2APB (IP3 R blocker), suggesting that they were dependent on Ca2+ release from intracellular Ca2+ stores. Nifedipine (L-type Ca2+ channel blocker) did not affect STICs, but reduced the duration of STDs. Phenylephrine induced transient depolarizations and transient inward currents which were blocked by Ani9. Similarly, phenylephrine induced phasic contractions of intact corpus cavernosum muscle strips and these events were also inhibited by Ani9. This study suggests that contraction of CCSM is regulated by activation of TMEM16A channels and therefore inhibition of these channels could lead to penile erection.

Fast voltage-dependent sodium (NaV ) currents are functionally expressed in mouse corpus cavernosum smooth muscle cells

BACKGROUND AND PURPOSE

Corpus cavernosum smooth muscle (CCSM) exhibits phasic contractions that are coordinated by ion channels. Mouse models are commonly used to study erectile dysfunction, but there are few published electrophysiological studies of mouse CCSM. We describe, for the first time, voltage-dependent sodium (Na_V ) currents in mouse CCSM and investigate their function.

EXPERIMENTAL APPROACH

Electrophysiological, pharmacological, and immunocytochemical studies on isolated CCSM cells. Tension measurements in whole tissue.

KEY RESULTS

A fast, voltage-dependent sodium current was induced by depolarising steps. Steady-state activation and inactivation curves revealed a window current between -60 and -30 mV. Two populations of Na_V currents, ('TTX-sensitive') and ('TTX-insensitive'), were distinguished. TTX-sensitive current showed 48% block with the Na_V -subtype-specific blockers ICA-121431 (Na_V 1.1-1.3), PF-05089771 (Na_V 1.7), and 4,9-anhydro-TTX (Na_V 1.6). TTX-insensitive current was insensitive to A803467, a Na_V 1.8 blocker. Immunocytochemistry confirmed the expression of Na_V 1.5 and Na_V 1.4 in freshly dispersed CCSM cells. Veratridine, a Na_V activator, reduced time-dependent inactivation of the current and increased the duration of evoked action potentials. Veratridine induced phasic contractions in CCSM strips. This effect was reversible with TTX and nifedipine but not by KB-R7943.

CONCLUSION AND IMPLICATIONS

We report, for the first time, a fast voltage-dependent sodium current in mouse CCSM. Stimulation of this current increases the contractility of corpus cavernosum in vitro, suggesting that it may contribute to the mechanisms of detumescence, and potentially serve as a clinically relevant target for pharmaceutical intervention in erectile dysfunction. Further work will be necessary to define its role.

Human Tissue Kallikrein 1 Improves Erectile Dysfunction of Streptozotocin-Induced Diabetic Rats by Inhibition of Excessive Oxidative Stress and Activation of the PI3K/AKT/eNOS Pathway

Objective

To investigate the protective effects and mechanisms of human tissue kallikrein 1 (hKLK1) on type 1 diabetes mellitus- (DM-) induced erectile dysfunction in rats. Materials and Methods. The homozygous transgenic rats (TGR) harboring the hKLK1 gene and age-matched wild-type Sprague Dawley rats (WTR) were involved, and intraperitoneal injection of streptozotocin was utilized to induce diabetes in rats. Forty-eight-week-old male rats were randomly divided into a WTR group, TGR group, diabetic WTR group (WTDM), diabetic TGR group (TGDM), and TGDM with HOE140 group (TGDMH), with eight rats in each group. Twelve weeks later, the erectile response of all rats was detected by cavernous nerve electric stimulation, and corpus cavernosums were harvested to evaluate the levels of cavernous oxidative stress (OS), apoptosis, fibrosis, and involved pathways. Moreover, cavernous smooth muscle cells (CSMC) and endothelial cells (EC) were primarily isolated to build a coculture system for a series of in vitro verification.

Results

The hKLK1 gene and age-matched wild-type Sprague Dawley rats (WTR) were involved, and intraperitoneal injection of streptozotocin was utilized to induce diabetes in rats. Forty-eight-week-old male rats were randomly divided into a WTR group, TGR group, diabetic WTR group (WTDM), diabetic TGR group (TGDM), and TGDM with HOE140 group (TGDMH), with eight rats in each group. Twelve weeks later, the erectile response of all rats was detected by cavernous nerve electric stimulation, and corpus cavernosums were harvested to evaluate the levels of cavernous oxidative stress (OS), apoptosis, fibrosis, and involved pathways. Moreover, cavernous smooth muscle cells (CSMC) and endothelial cells (EC) were primarily isolated to build a coculture system for a series of

Conclusions

hKLK1 preserves erectile function of DM rats through its antitissue excessive OS, apoptosis, and fibrosis effects, as well as activation of the PI3K/AKT/eNOS/cGMP pathway in the penis. Moreover, hKLK1 promotes relaxation and prevents high glucose-induced injuries of CSMC mediated by EC-CSMC crosstalk.

Divalent cation influx and calcium homeostasis in germinal vesicle mouse oocytes

Prior to maturation, mouse oocytes are arrested at the germinal vesicle (GV) stage during which they experience constitutive calcium (Ca²⁺) influx and spontaneous Ca²⁺ oscillations. The oscillations cease during maturation but Ca²⁺ influx continues, as the oocytes' internal stores attain maximal content at the culmination of maturation, the metaphase II stage. The identity of the channel(s) that underlie this Ca²⁺ influx has not been completely determined. GV and matured oocytes are known to express three Ca²⁺ channels, Ca_V3.2, TRPV3 and TRPM7, but females null for each of these channels are fertile and their oocytes display minor modifications in Ca²⁺ homeostasis, suggesting a complex regulation of Ca²⁺ influx. To define the contribution of these channels at the GV stage, we used different divalent cations, pharmacological inhibitors and genetic models. We found that the three channels are active at this stage. Ca_V3.2 and TRPM7 channels contributed the majority of Ca²⁺ influx, as inhibitors and oocytes from homologous knockout (KO) lines showed severely reduced Ca²⁺ entry. Sr²⁺ influx was promoted by Ca_V3.2 channels, as Sr²⁺ oscillations were negligible in Ca_V3.2-KO oocytes but robust in control and Trpv3-KO GV oocytes. Mn²⁺ entry relied on expression of Ca_V3.2 and TRPM7 channels, but Ni²⁺ entry depended on the latter. Ca_V3.2 and TRPV3 channels combined to fill the Ca²⁺ stores, although Ca_V3.2 was the most impactful. Studies with pharmacological inhibitors effectively blocked the influx of divalent cations, but displayed off-target effects, and occasionally agonist-like properties. In conclusion, GV oocytes express channels mediating Ca²⁺ and other divalent cation influx that are pivotal for fertilization and early development. These channels may serve as targets for intervention to improve the success of assisted reproductive technologies.

Ion Channels and Intracellular Calcium Signalling in Corpus Cavernosum

The corpus cavernosum smooth muscle is important for both erection of the penis and for maintaining penile flaccidity. Most of the time, the smooth muscle cells are in a contracted state, which limits filling of the corpus sinuses with blood. Occasionally, however, they relax in a co-ordinated manner, allowing filling to occur. This results in an erection. When contractions of the corpus cavernosum are measured, it can be deduced that the muscle cells work together in a syncytium, for not only do they spontaneously contract in a co-ordinated manner, but they also synchronously relax. It is challenging to understand how they achieve this.In this review we will attempt to explain the activity of the corpus cavernosum, firstly by summarising current knowledge regarding the role of ion channels and how they influence tone, and secondly by presenting data on the intracellular Ca²⁺ signals that interact with the ion channels. We propose that spontaneous Ca²⁺ waves act as a primary event, driving transient depolarisation by activating Ca²⁺-activated Cl^- channels. Depolarisation then facilitates Ca²⁺ influx via L-type voltage-dependent Ca²⁺ channels. We propose that the spontaneous Ca²⁺ oscillations depend on Ca²⁺ release from both ryanodine- and inositol trisphosphate (IP₃)-sensitive stores and that modulation by signalling molecules is achieved mainly by interactions with the IP₃-sensitive mechanism. This pacemaker mechanism is inhibited by nitric oxide (acting through cyclic GMP) and enhanced by noradrenaline. By understanding these mechanisms better, it might be possible to design new treatments for erectile dysfunction.

The role of Ca2+-activated Cl- current in tone generation in the rabbit corpus cavernosum

Rabbit corpus cavernosum smooth muscle (RCCSM) cells express ion channels that produce Ca²⁺-activated Cl^- (I_ClCa) current, but low sensitivity to conventional antagonists has made its role in tone generation difficult to evaluate. We have reexamined this question using two new generation I_ClCa blockers, T16A_inh-A01 and CaCC_inh-A01. Isolated RCCSM cells were studied using the perforated patch method. Current-voltage protocols revealed that both L-type Ca²⁺ current and I_ClCa T16A_inh-A01 and CaCC_inh-A01 (10 μM) reduced I_ClCa by ~85%, while 30 μM abolished it. L-type Ca²⁺ current was unaffected by 10 μM CaCC_inh-A01 but was reduced by 50% at 30 μM CaCC_inh-A01, 46% at 10 μM T16A_inh-A01, and 78% at 30 μM T16A_inh-A01. Both drugs reduced spontaneous isometric tension in RCCSM strips, by 60-70% at 10 μM and >90% at 30 μM. Phenylephrine (PE)-enhanced tension was also reduced (ED₅₀ = 3 μM, CaCC_inh-A01; 14 μM, T16A_inh-A01). CaCC_inh-A01 at 10 μM had little effect on 60 mM KCl contractures, though they were reduced by 30 μM CaCC_inh-A01 and T16A_inh-A01 (10 μM and 30 μM) consistent with their effects on L-type Ca²⁺ current. Both drugs also reversed the stimulatory effect of PE on intracellular Ca²⁺ waves, studied with laser scanning confocal microscopy in isolated RCCSM cells. In conclusion, although both drugs were effective blockers of I_ClCa, the effect of T16A_inh-A01 on L-type Ca²⁺ current precludes its use for evaluating the role of I_ClCa in tone generation. However, 10 μM CaCC_inh-A01 selectively blocked I_ClCa versus L-type Ca²⁺ current and reduced spontaneous and PE-induced tone, suggesting that I_ClCa is important in maintaining penile detumescence.

Effect of a novel BKCa opener on BKCa currents and contractility of the rabbit corpus cavernosum

Large-conductance Ca2+-activated K+ (BKCa) channels are thought to play a key role in the regulation of corpus cavernosum smooth muscle (CCSM) excitability. Few BKCa channel openers have been accepted for clinical development. The effect of the novel BKCa channel opener GoSlo-SR5-130 on electrical activity in isolated rabbit CCSM cells and mechanical activity in strips of rabbit CCSM was examined. Single-channel currents were observed in inside-out patches. These channels were sensitive to Ca2+, blocked by penitrem A, and had a conductance of 291 ± 20 pS ( n = 7). In the presence of GoSlo-SR5-130, the number of open BKCa channels increased. Using voltage-ramp protocols, GoSlo-SR5-130 caused currents to activate at more negative potentials in a concentration-dependent manner, shifting the half-maximal activation voltage potential to the left on the voltage axis. Therefore, BKCa channels were open within the physiological range of membrane potentials in the presence of GoSlo-SR5-130. GoSlo-SR5-130 also resulted in an increase in the activity of spontaneous transient outward currents in myocytes isolated from CCSM, and this effect was reversed by iberiotoxin. In current-clamp mode, GoSlo-SR5-130 hyperpolarized the cell membrane. Isometric tension recording of strips of rabbit corpus cavernosum showed that GoSlo-SR5-130 inhibited spontaneous contractions in a concentration-dependent manner. This effect was reversed in the presence of iberiotoxin, suggesting that GoSlo-SR5-130 exerts its effect through BKCa channels. These findings suggest that GoSlo-SR5-130 is an effective tool for the study of BKCa channels and that these channels can modulate CCSM activity and are possible targets for the treatment of erectile dysfunction.

The role of Ca(2+) influx in spontaneous Ca(2+) wave propagation in interstitial cells of Cajal from the rabbit urethra

KEY POINTS

Tonic contractions of rabbit urethra are associated with spontaneous electrical slow waves that are thought to originate in pacemaker cells termed interstitial cells of Cajal (ICC). ICC pacemaker activity results from their ability to generate propagating Ca(2+) waves, although the exact mechanisms of propagation are not understood. In this study, we have identified spontaneous localised Ca(2+) events for the first time in urethral ICC; these were due to Ca(2+) release from the endoplasmic reticulum (ER) via ryanodine receptors (RyRs) and, while they often remained localised, they sometimes initiated propagating Ca(2+) waves. We show that propagation of Ca(2+) waves in urethral ICC is critically dependent upon Ca(2+) influx via reverse mode NCX. Our data provide a clearer understanding of the intracellular mechanisms involved in the generation of ICC pacemaker activity. Interstitial cells of Cajal (ICC) are putative pacemaker cells in the rabbit urethra. Pacemaker activity in ICC results from spontaneous propagating Ca(2+) waves that are modulated by [Ca(2+)]o and whose propagation is inhibited by inositol tri-phosphate receptor (IP3 R) blockers. The purpose of this study was to further examine the role of Ca(2+) influx and Ca(2+) release in the propagation of Ca(2+) waves. Intracellular Ca(2+) was measured in Fluo-4-loaded ICC using a Nipkow spinning disc confocal microscope at fast acquisition rates (50 fps). We identified previously undetected localised Ca(2+) events originating from ryanodine receptors (RyRs). Inhibiting Ca(2+) influx by removing [Ca(2+)]o or blocking reverse mode sodium-calcium exchange (NCX) with KB-R 7943 or SEA-0400 abolished Ca(2+) waves, while localised Ca(2+) events persisted. Stimulating RyRs with 1 mm caffeine restored propagation. Propagation was also inhibited when Ca(2+) release sites were uncoupled by buffering intracellular Ca(2+) with EGTA-AM. This was reversed when Ca(2+) influx via NCX was increased by reducing [Na(+)]o to 13 mm. Low [Na(+)]o also increased the frequency of Ca(2+) waves and this effect was blocked by tetracaine and ryanodine but not 2-aminoethoxydiphenyl borate (2-APB). RT-PCR revealed that isolated ICC expressed both RyR2 and RyR3 subtypes. We conclude: (i) RyRs are required for the initiation of Ca(2+) waves, but wave propagation normally depends on activation of IP3 Rs; (ii) under resting conditions, propagation by IP3 Rs requires sensitisation by influx of Ca(2+) via reverse mode NCX; (iii) propagation can be maintained by RyRs if they have been sensitised to Ca(2+).

Voltage dependence of slow wave frequency in the guinea pig prostate

PURPOSE

Spontaneous phasic contractions of the guinea pig prostate stroma result from the generation of slow waves that appear to primarily rely on spontaneous Ca(2+) release from the endoplasmic/sarcoplasmic reticulum and subsequent opening of Ca(2+) activated chloride channels. We investigated voltage dependent mechanisms in the regulation of slow wave frequency.

MATERIALS AND METHODS

Changes in membrane potential were recorded using conventional intracellular recording techniques while simultaneously measuring the isometric tension of guinea pig prostate lobes. Fluorescence immunohistochemistry was done to determine the cellular composition of the prostate stroma.

RESULTS

Depolarization induced by high K(+) solution, K(+) free solution or outward current injection was associated with increased slow wave frequency. In contrast, hyperpolarization induced by the re-addition of K(+), adenosine triphosphate sensitive K(+) channel openers or inward current injection prevented slow wave generation. K(+) channel openers induced hyperpolarization and the cessation of slow waves was reversed by glibenclamide (10 μM). Nifedipine (1 to 10 μM) shortened the duration of slow waves and pacemaker potentials but often failed to prevent their generation and associated contractions. Subsequently Ni(2+) (100 μM) or mibefradil (1 μM) largely suppressed slow waves and abolished residual contractions. Immunohistochemistry revealed small interconnected smooth muscle bundles as well as vimentin positive interstitial cells but failed to show a network of Kit positive interstitial cells.

CONCLUSIONS

Prostate slow wave frequency is voltage dependent due to the significant contribution of L-type and T-type Ca(2+) channels. Prostate slow waves may arise from cooperation between spontaneous Ca(2+) release from internal stores and plasmalemmal voltage dependent Ca(2+) channels.

T-type Ca2+ channels and the urinary and male genital tracts

T-type Ca(2+) channels are widely expressed throughout the urinary and male genital tracts, generally alongside L-type Ca(2+) channels. The use of pharmacological blockers of these channels has suggested functional roles in all regions, with the possible exception of the ureter. Their functional expression is apparent not just in smooth muscle cells but also in interstitial cells that lie in close proximity to muscle, nerve and epithelial components of these tissues. Thus, T-type Ca(2+) channels can contribute directly to modulation of muscle function and indirectly to changes of epithelial and nerve function. T-type Ca(2+) channel activity modulates phasic contractile activity, especially in conjunction with Ca(2+)-activated K(+) channels, and also to agonist-dependent responses in different tissues. Upregulation of channel density occurs in pathological conditions associated with enhanced contractile responses, e.g. overactive bladder, but it is unclear if this is causal or a response to the pathological state. Moreover, T-type Ca(2+) channels may have a role in the development of prostate tumours regulating the secretion of mitogens from neuroendocrine cells. Although a number of selective channel blockers exist, their relative selectivity over L-type Ca(2+) channels is often low and makes evaluation of T-type Ca(2+) channel function in the whole organism difficult.

ATP evokes inward currents in corpus cavernosum myocytes

INTRODUCTION

Although adenosine triphosphate (ATP) has often been reported to relax the corpus cavernosum, this may be mediated by indirect effects, such as release of nitric oxide from the endothelium. Recent data suggest that P2X(1) receptors may be up-regulated in diabetes, and these exert an anti-erectile effect by causing the corpus cavernosum smooth muscle cells (CCSMCs) to contract. However, to date, there is no functional evidence that ATP can directly stimulate CCSMC.

AIMS

This study aims to (i) to directly examine the effect of ATP on membrane currents in freshly isolated CCSMC, where influences of endothelium and other cells are absent; and (ii) to determine the receptor subtypes, ionic currents, and Ca(2+) signals stimulated by ATP.

METHODS

CCSMCs were enzymatically dispersed from male New Zealand White rabbits for patch clamp recording and measurement of intracellular Ca(2+) in fluo-4-loaded cells using spinning disk confocal microscopy.

MAIN OUTCOME MEASURES

Patch clamp recordings were made of ATP-evoked membrane currents and spontaneous membrane currents. Spinning disk confocal imaging of intracellular Ca(2+) was performed, and the response to ATP was recorded.

RESULTS

ATP evoked repeatable inward currents in CCSMC (1st application: -675 ± 101 pA; 2nd application: -694 ± 120 pA, N = 9, P = 0.77). ATP-induced currents were reduced by suramin from -380 ± 121 to -124 ± 37 pA (N = 8, P < 0.05), by α,β-methylene ATP from -755 ± 235 to 139 ± 49 pA (N = 5, P < 0.05), and by NF449 from -419 ± to -51 ± 13 pA (N = 6, P < 0.05). In contrast, MRS2500, a P2Y1(1,12,13) antagonist, had no effect on ATP responses (control: -838 ± 139 pA; in MRS2500: -822 ± 184 pA, N = 13, P = 0.84) but blocked inward currents evoked by 2-MeSATP, a P2Y1,12,13 agonist (control: -623 ± 166 pA; in MRS2500: -56 ± 25 pA, N = 6, P < 0.05). The ATP-evoked inward current was unaffected by changing the transmembrane Cl(-) gradient but reversed in direction when extracellular Na(+) was reduced, indicating that it was a cation current.

CONCLUSIONS

ATP directly stimulates CCSMC by evoking a P2X-mediated cation current.

Calcium-activated chloride channels in the corpus cavernosum: recent developments and future of a key cellular component of the erectile process

Calcium-activated chloride channels (CaCCs) are one of five families of chloride channels, ubiquitously expressed, and essential for a host of biological actions. CaCCs have key roles in processes as diverse as olfactory transduction and epithelial secretion, and also CaCCs are essential in smooth muscle contraction. The corpus cavernosum is a vascular smooth muscle that must relax to facilitate erections. Parasympathetic activation produces relaxation of the corpus cavernosum through a nitric oxide-dependent pathway, and sympathetic stimulation in both preventing and terminating erections by contracting the corpus cavernosum. Both these pathways affect activity of CaCCs. The past 5 years produced many successes in CaCC research. One key area of success was the identification of the elusive 'molecular candidate' of CaCCs, as the TMEM16A protein (dubbed anoctamin-1) and potentially other members of the anoctamin family of transmembrane proteins. However, enthusiasm has been somewhat tempered because of evidence that this family of proteins may not be responsible for calcium-activated chloride currents in certain epithelial tissues. Several studies identified specific inhibitors of CaCCs as well as specific inhibitors for anoctamin-1. Despite the number of recent achievements in this field there are many details that still need to be elucidated. Of particular value would be more details on the identity of the CaCCs in corpus cavernosum smooth muscle, using new inhibitors to gain insight into the signalling pathway, and the evaluation of whether inhibition of CaCCs provides any specific benefit in different models of ED.

The promise of inhibition of smooth muscle tone as a treatment for erectile dysfunction: where are we now?

Ten years ago, the inhibition of Rho kinase by intracavernosal injection of Y-27632 was found to induce an erectile response. This effect did not require activation of nitric oxide-mediated signaling, introducing a novel target pathway for the treatment of erectile dysfunction (ED), with potential added benefit in cases where nitric oxide bioavailability is attenuated (and thus phosphodiesterase type 5 (PDE5) inhibitors are less efficacious). Rho-kinase antagonists are currently being developed and tested for a wide range of potential uses. The inhibition of this calcium-sensitizing pathway results in blood vessel relaxation. It is also possible that blockade of additional smooth muscle contractile signaling mechanisms may have the same effect. In this review, we conducted an extensive search of pertinent literature using PUBMED. We have outlined the various pathways involved in the maintenance of penile smooth muscle tone and discussed the current potential benefit for the pharmacological inhibition of these targets for the treatment of ED.

Mechanisms of penile erection and basis for pharmacological treatment of erectile dysfunction

Erection is basically a spinal reflex that can be initiated by recruitment of penile afferents, both autonomic and somatic, and supraspinal influences from visual, olfactory, and imaginary stimuli. Several central transmitters are involved in the erectile control. Dopamine, acetylcholine, nitric oxide (NO), and peptides, such as oxytocin and adrenocorticotropin/α-melanocyte-stimulating hormone, have a facilitatory role, whereas serotonin may be either facilitatory or inhibitory, and enkephalins are inhibitory. The balance between contractant and relaxant factors controls the degree of contraction of the smooth muscle of the corpora cavernosa (CC) and determines the functional state of the penis. Noradrenaline contracts both CC and penile vessels via stimulation of α₁-adrenoceptors. Neurogenic NO is considered the most important factor for relaxation of penile vessels and CC. The role of other mediators, released from nerves or endothelium, has not been definitely established. Erectile dysfunction (ED), defined as the "inability to achieve or maintain an erection adequate for sexual satisfaction," may have multiple causes and can be classified as psychogenic, vasculogenic or organic, neurologic, and endocrinologic. Many patients with ED respond well to the pharmacological treatments that are currently available, but there are still groups of patients in whom the response is unsatisfactory. The drugs used are able to substitute, partially or completely, the malfunctioning endogenous mechanisms that control penile erection. Most drugs have a direct action on penile tissue facilitating penile smooth muscle relaxation, including oral phosphodiesterase inhibitors and intracavernosal injections of prostaglandin E₁. Irrespective of the underlying cause, these drugs are effective in the majority of cases. Drugs with a central site of action have so far not been very successful. There is a need for therapeutic alternatives. This requires identification of new therapeutic targets and design of new approaches. Research in the field is expanding, and several promising new targets for future drugs have been identified.

Three phases of corporal tracing elicited by electrical field stimulation on rabbit corpus cavernosum smooth muscle in penile perfusion model

INTRODUCTION

Nitric oxide (NO) has been shown to mediate electrical field stimulation (EFS)-caused smooth muscle relaxation. It is known that the neural control of penile erection involves adrenergic, cholinergic, and non-adrenergic-non-cholinergic (NANC) neuro-effector systems; however, the effects of EFS on adrenergic and cholinergic nerves are not clear.

AIMS

To elucidate EFS-induced signal transductions involved in adrenergic, cholinergic, and NANC neuro-effector systems by using an in vitro penile perfusion model.

METHODS

EFS was performed on penile corpus cavernosum smooth muscle from male New Zealand White rabbits, which was pre-contracted with L-phenylephrine (10 µM). We investigated the penile tracing elicited by EFS on tissues pre-incubated with guanethidine (Guan, 50 µM), tetrodotoxin (TTX, 10 µM), Nω nitro-L-arginine-methyl ester (L-NAME, 1 mM), atropine (50 µM), or eserine (10 µM).

MAIN OUTCOME MEASURES

The time-to-peak of each phase, the percentage of relaxation, and the area under the curve (AUC).

RESULTS

We discovered an extraordinary phenomenon: three distinct phases elicited by EFS. Phase I was abolished by L-NAME. Phase II was decreased by eserine and Guan, but increased by L-NAME. Phase III was abolished by atropine, but enhanced by eserine and Guan. TTX diminished all three phases. The time to reach the top of phase I was delayed by TTX. The time to attain the peak of phase II was shortened by L-NAME, but delayed by TTX and atropine. The time to reach the top of phase III was shortened by L-NAME, eserine, and Guan. AUC was significantly decreased by L-NAME and TTX.

CONCLUSIONS

EFS stimulated adrenergic, cholinergic, and NANC neuro-effector systems simultaneously. Phase I was related to the NO pathway. Phase II was multiply affected by self-recovery properties, and adrenergic and cholinergic nerves. Phase III was related to cholinergic nerves. The corporal tracing elicited by EFS was the balanced result of multiple factors.