Laboratory practical to study the differential innervation pathways of urinary tract smooth muscle

Interstitial cell of Cajal-like cells (ICC-LC) exhibit dynamic spontaneous activity but are not functionally innervated in mouse urethra

Urethral smooth muscle cells (USMC) contract to occlude the internal urethral sphincter during bladder filling. Interstitial cells also exist in urethral smooth muscles and are hypothesized to influence USMC behaviours and neural responses. These cells are similar to Kit+ interstitial cells of Cajal (ICC), which are gastrointestinal pacemakers and neuroeffectors. Isolated urethral ICC-like cells (ICC-LC) exhibit spontaneous intracellular Ca2+ signalling behaviours that suggest these cells may serve as pacemakers or neuromodulators similar to ICC in the gut, although observation and direct stimulation of ICC-LC within intact urethral tissues is lacking. We used mice with cell-specific expression of the Ca2+ indicator, GCaMP6f, driven off the endogenous promoter for Kit (Kit-GCaMP6f mice) to identify ICC-LC in situ within urethra muscles and to characterize spontaneous and nerve-evoked Ca2+ signalling. ICC-LC generated Ca2+ waves spontaneously that propagated on average 40.1 ± 0.7 μm, with varying amplitudes, durations, and spatial spread. These events originated from multiple firing sites in cells and the activity between sites was not coordinated. ICC-LC in urethra formed clusters but not interconnected networks. No evidence for entrainment of Ca2+ signalling between ICC-LC was obtained. Ca2+ events in ICC-LC were unaffected by nifedipine but were abolished by cyclopiazonic acid and decreased by an antagonist of Orai Ca2+ channels (GSK-7975A). Phenylephrine increased Ca2+ event frequency but a nitric oxide donor (DEA-NONOate) had no effect. Electrical field stimulation (EFS, 10 Hz) of intrinsic nerves, which evoked contractions of urethral rings and increased Ca2+ event firing in USMC, failed to evoke responses in ICC-LC. Our data suggest that urethral ICC-LC are spontaneously active but are not regulated by autonomic neurons.

Cells and ionic conductances contributing to spontaneous activity in bladder and urethral smooth muscle

Smooth muscle organs of the lower urinary tract comprise the bladder detrusor and urethral wall, which have a reciprocal contractile relationship during urine storage and micturition. As the bladder fills with urine, detrusor smooth muscle cells (DSMCs) remain relaxed to accommodate increases in intravesical pressure while urethral smooth muscle cells (USMCs) sustain tone to occlude the urethral orifice, preventing leakage. While neither organ displays coordinated regular contractions as occurs in small intestine, lymphatics or renal pelvis, they do exhibit patterns of rhythmicity at cellular and tissue levels. In rabbit and guinea-pig urethra, electrical slow waves are recorded from USMCs. This activity is linked to cells expressing vimentin, c-kit and Ca2+-activated Cl- channels, like interstitial cells of Cajal in the gastrointestinal tract. In mouse, USMCs are rhythmically active (firing propagating Ca2+ waves linked to contraction), and this cellular rhythmicity is asynchronous across tissues and summates to form tone. Experiments in mice have failed to demonstrate a voltage-dependent mechanism for regulating this rhythmicity or contractions in vitro, suggesting that urethral tone results from an intrinsic ability of USMCs to 'pace' their own Ca2+ mobilization pathways required for contraction. DSMCs exhibit spontaneous transient contractions, increases in intracellular Ca2+ and action potentials. Consistent across numerous species, including humans, this activity relies on voltage-dependent Ca2+ influx in DSMCs. While interstitial cells are present in the bladder, they do not 'pace' the organ in an excitatory manner. Instead, specialized cells (PDGFRα+ interstitial cells) may 'negatively pace' DSMCs to prevent bladder overexcitability.

Role of Ano1 Ca2+-activated Cl- channels in generating urethral tone

Urinary continence is maintained in the lower urinary tract by the contracture of urethral sphincters, including smooth muscle of the internal urethral sphincter. These contractions occlude the urethral lumen, preventing urine leakage from the bladder to the exterior. Over the past 20 years, research on the ionic conductances that contribute to urethral smooth muscle contractility has greatly accelerated. A debate has emerged over the role of interstitial cell of Cajal (ICC)-like cells in the urethra and their expression of Ca²⁺-activated Cl^- channels encoded by anoctamin-1 [Ano1; transmembrane member 16 A (Tmem16a) gene]. It has been proposed that Ano1 channels expressed in urethral ICC serve as a source of depolarization for smooth muscle cells, increasing their excitability and contributing to tone. Although a clear role for Ano1 channels expressed in ICC is evident in other smooth muscle organs, such as the gastrointestinal tract, the role of these channels in the urethra is unclear, owing to differences in the species (rabbit, rat, guinea pig, sheep, and mouse) examined and experimental approaches by different groups. The importance of clarifying this situation is evident as effective targeting of Ano1 channels may lead to new treatments for urinary incontinence. In this review, we summarize the key findings from different species on the role of ICC and Ano1 channels in urethral contractility. Finally, we outline proposals for clarifying this controversial and important topic by addressing how cell-specific optogenetic and inducible cell-specific genetic deletion strategies coupled with advances in Ano1 channel pharmacology may clarify this area in future studies.NEW & NOTEWORTHY Studies from the rabbit have shown that anoctamin-1 (Ano1) channels expressed in urethral interstitial cells of Cajal (ICC) serve as a source of depolarization for smooth muscle cells, increasing excitability and tone. However, the role of urethral Ano1 channels is unclear, owing to differences in the species examined and experimental approaches. We summarize findings from different species on the role of urethral ICC and Ano1 channels in urethral contractility and outline proposals for clarifying this topic using cell-specific optogenetic approaches.

ANO1 in urethral SMCs contributes to sex differences in urethral spontaneous tone

Stress urinary incontinence (SUI) is more common in women than in men, and sex differences in anatomic structure and physiology have been suggested as causes; however, the underlying cellular and molecular mechanisms remain unclear. The spontaneous tone (STT) of the urethra has been shown to have a fundamental effect on preventing the occurrence of SUI. Here, we investigated whether the urethral STT exhibited sex differences. First, we isolated urethral smooth muscle (USM) and detected STT in female mice and women. No STT was found in male mice or men. Furthermore, caffeine induced increased contractility and intracellular Ca²⁺ concentration in urethrae from female mice compared with male mice. EACT [an N-aroylaminothiazole, anoctamin-1 (ANO1) activator] elicited increased intracellular Ca²⁺ concentration and stronger currents in female mice than in male mice. Moreover, ANO1 expression in single USM cells from women and female mice was almost twofold higher than that found in cells from men and male mice. In summary, ANO1 in USM contributes to sex differences in urethral spontaneous tone. This finding may provide new guidance for the treatment of SUI in women and men.

Contribution of Cav1.2 Ca2+ channels and store-operated Ca2+ entry to pig urethral smooth muscle contraction

Urethral smooth muscle (USM) generates tone to prevent urine leakage from the bladder during filling. USM tone has been thought to be a voltage-dependent process, relying on Ca²⁺ influx via voltage-dependent Ca²⁺ channels in USM cells, modulated by the activation of Ca²⁺-activated Cl^- channels encoded by Ano1. However, recent findings in the mouse have suggested that USM tone is voltage independent, relying on Ca²⁺ influx through Orai channels via store-operated Ca²⁺ entry (SOCE). We explored if this pathway also occurred in the pig using isometric tension recordings of USM tone. Pig USM strips generated myogenic tone, which was nearly abolished by the Ca_v1.2 channel antagonist nifedipine and the ATP-dependent K⁺ channel agonist pinacidil. Pig USM tone was reduced by the Orai channel blocker GSK-7975A. Electrical field stimulation (EFS) led to phentolamine-sensitive contractions of USM strips. Contractions of pig USM were also induced by phenylephrine. Phenylephrine-evoked and EFS-evoked contractions of pig USM were reduced by ~50-75% by nifedipine and ~30% by GSK-7975A. Inhibition of Ano1 channels had no effect on tone or EFS-evoked contractions of pig USM. In conclusion, unlike the mouse, pig USM exhibited voltage-dependent tone and agonist/EFS-evoked contractions. Whereas SOCE plays a role in generating tone and agonist/neural-evoked contractions in both species, this dominates in the mouse. Tone and agonist/EFS-evoked contractions of pig USM are the result of Ca²⁺ influx primarily through Ca_v1.2 channels, and no evidence was found supporting a role of Ano1 channels in modulating these mechanisms.