Ryanodine receptor type 3 does not contribute to contractions in the mouse myometrium regardless of pregnancy

Steady-state approximations for Hodgkin-Huxley cell models: Reduction of order for uterine smooth muscle cell model

Multi-scale mathematical bioelectrical models of organs such as the uterus, stomach or heart present challenges both for accuracy and computational tractability. These multi-scale models are typically founded on models of biological cells derived from the classic Hodkgin-Huxley (HH) formalism. Ion channel behaviour is tracked with dynamical variables representing activation or inactivation of currents that relax to steady-state dependencies on cellular membrane voltage. Timescales for relaxation may be orders of magnitude faster than companion ion channel variables or phenomena of physiological interest for the entire cell (such as bursting sequences of action potentials) or the entire organ (such as electromechanical coordination). Exploiting these time scales with steady-state approximations for relatively fast-acting systems is a well-known but often overlooked approach as evidenced by recent published models. We thus investigate feasibility of an extensive reduction of order for an HH-type cell model with steady-state approximations to the full dynamical activation and inactivation ion channel variables. Our effort utilises a published comprehensive uterine smooth muscle cell model that encompasses 19 ordinary differential equations and 105 formulations overall. The numerous ion channel submodels in the published model exhibit relaxation times ranging from order 10-1 to 105 milliseconds. Substitution of the faster dynamic variables with steady-state formulations demonstrates both an accurate reproduction of the full model and substantial improvements in time-to-solve, for test cases performed. Our demonstration here of an effective and relatively straightforward reduction method underlines the particular importance of considering time scales for model simplification before embarking on large-scale computations or parameter sweeps. As a preliminary complement to more intensive reduction of order methods such as parameter sensitivity and bifurcation analysis, this approach can rapidly and accurately improve computational tractability for challenging multi-scale organ modelling efforts.

Modeling and experimental approaches for elucidating multi-scale uterine smooth muscle electro- and mechano-physiology: A review

The uterus provides protection and nourishment (via its blood supply) to a developing fetus, and contracts to deliver the baby at an appropriate time, thereby having a critical contribution to the life of every human. However, despite this vital role, it is an under-investigated organ, and gaps remain in our understanding of how contractions are initiated or coordinated. The uterus is a smooth muscle organ that undergoes variations in its contractile function in response to hormonal fluctuations, the extreme instance of this being during pregnancy and labor. Researchers typically use various approaches to studying this organ, such as experiments on uterine muscle cells, tissue samples, or the intact organ, or the employment of mathematical models to simulate the electrical, mechanical and ionic activity. The complexity exhibited in the coordinated contractions of the uterus remains a challenge to understand, requiring coordinated solutions from different research fields. This review investigates differences in the underlying physiology between human and common animal models utilized in experiments, and the experimental interventions and computational models used to assess uterine function. We look to a future of hybrid experimental interventions and modeling techniques that could be employed to improve the understanding of the mechanisms enabling the healthy function of the uterus.

Expression and functional maintenance of volume-regulated anion channels in myometrial smooth muscles of pregnant mice

Pregnancy causes changes in the uterus, such as increased cell volume and altered water content. However, the mechanisms that protect the structure and maintain the function of uterine smooth muscle cells against these changes during pregnancy have not been clarified. This study focused on the volume-regulated anion channel (VRAC), which opens with cell swelling under low osmotic pressure and releases Cl- ions and various organic osmolytes to resist cell swelling and regulates a wide range of biological processes such as cell death. In this study, myometrial smooth muscle (MSM) tissues and cells (MSMCs) were collected from non-pregnant and pregnant mice. Using western blotting and immunocytochemistry, leucine-rich repeat containing protein 8A (LRRC8A), an essential membrane protein that constitutes part of the VRAC, was determined to be diffused throughout MSMCs including in the cell membrane. Patch-clamp experiments were performed to investigate the electrophysiology of swelling-induced Cl- currents (ICl, swell) mediated by the VRAC. No significant changes between non-pregnancy and pregnancy groups were observed in either the expression density of LRRC8A or the current density of ICl, swell, however the presence of LRRC8A on the cell membrane was significantly increased in the third trimester of pregnancy compared to the non-pregnancy. This study suggests that the VRAC may play a role, such as maintaining cellular homeostasis in the pregnant MSM.

Reciprocal Relationship between Ca2+ Signaling and Ca2+-Gated Ion Channels as a Potential Target for Drug Discovery

Cellular Ca²⁺ signaling functions as one of the most common second messengers of various signal transduction pathways in cells and mediates a number of physiological roles in a cell-type dependent manner. Ca²⁺ signaling also regulates more general and fundamental cellular activities, including cell proliferation and apoptosis. Among ion channels, Ca²⁺-permeable channels in the plasma membrane as well as endo- and sarcoplasmic reticulum membranes play important roles in Ca²⁺ signaling by directly contributing to the influx of Ca²⁺ from extracellular spaces or its release from storage sites, respectively. Furthermore, Ca²⁺-gated ion channels in the plasma membrane often crosstalk reciprocally with Ca²⁺ signals and are central to the regulation of cellular functions. This review focuses on the physiological and pharmacological impact of i) Ca²⁺-gated ion channels as an apparatus for the conversion of cellular Ca²⁺ signals to intercellularly propagative electrical signals and ii) the opposite feedback regulation of Ca²⁺ signaling by Ca²⁺-gated ion channel activities in excitable and non-excitable cells.

Inhibitory effect of Pyr6 (an Orai channel blocker) on agonist-induced contractions in rat uterus

AIM

Both human and rat myometrium express stromal interaction molecule (STIM) and Orai/ transient receptor potential canonical (TRPC) proteins, which are components of plasma membrane Ca²⁺ store-operated channels. There are reports that these proteins mediate agonist-induced Ca²⁺ influx in cultured myometrial cells. In this study, we aimed to determine the effects of Pyr6, an Orai channel blocker, on different agonist-induced contractions in isolated segments of rat uterus.

MAIN FINDINGS

In Ca²⁺ -free Tyrode's solution, Pyr6 (3 μM) promoted a reduction in both the magnitude and frequency of Ca²⁺ (1 mM)-induced uterine contractions after the addition of carbachol (CCh, 100 μM), but not after the addition of oxytocin (OT, 150 nM). In Ca²⁺ (0.18 mM)-Tyrode's solution, Pyr6 completely relaxed uterine contractions induced by both CCh and cloprostenol (300 nM), but not those induced by either KCI (40-80 mM) or OT. The addition of Pyr6 abolished the oscillatory uterine contractions induced by Ca²⁺ after the addition of cyclopiazonic acid (CPA, 10 μM). When pre-incubated (5 min), Pyr6 reduced the magnitude of both CCh-induced phasic and tonic contractions. The addition of Pyr2 (3 μM), an Orai and TRPC channel blocker, abolished uterine contractions induced by CCh or OT.

CONCLUSION

Considering Pyr6 as an Orai channel blocker and its inhibitory effect on uterine contractions induced by CCh, CPA, and cloprostenol, we suggest that Orai channels are required for the maintenance of contractions induced by these agonists in rat uterus.

Uterine contractions in rodent models and humans

Aberrant uterine contractions can lead to preterm birth and other labour complications and are a significant cause of maternal morbidity and mortality. To investigate the mechanisms underlying dysfunctional uterine contractions, researchers have used experimentally tractable small animal models. However, biological differences between humans and rodents change how researchers select their animal model and interpret their results. Here, we provide a general review of studies of uterine excitation and contractions in mice, rats, guinea pigs, and humans, in an effort to introduce new researchers to the field and help in the design and interpretation of experiments in rodent models.

Dynamic erectile responses of a novel penile organ model utilizing TPEM†

Male penis is required to become erect during copulation. In the upper (dorsal) part of penis, the erectile tissue termed corpus cavernosum (CC) plays fundamental roles for erection by regulating the inner blood flow. When blood flows into the CC, the microvascular complex termed sinusoidal space is reported to expand during erection. A novel in vitro explant system to analyze the dynamic erectile responses during contraction/relaxation is established. The current data show regulatory contraction/relaxation processes induced by phenylephrine (PE) and nitric oxide (NO) donor mimicking dynamic erectile responses by in vitro CC explants. Two-photon excitation microscopy (TPEM) observation shows the synchronous movement of sinusoidal space and the entire CC. By taking advantages of the CC explant system, tadalafil (Cialis) was shown to increase sinusoidal relaxation. Histopathological changes have been generally reported associating with erection in several pathological conditions. Various stressed statuses have been suggested to occur in the erectile responses by previous studies. The current CC explant model enables to analyze such conditions through directly manipulating CC in the repeated contraction/relaxation processes. Expression of oxidative stress marker and contraction-related genes, Hypoxia-inducible factor 1-alpha (Hif1a), glutathione peroxidase 1 (Gpx1), Ras homolog family member A (RhoA), and Rho-associated protein kinase (Rock), was significantly increased in such repeated contraction/relaxation. Altogether, it is suggested that the system is valuable for analyzing structural changes and physiological responses to several regulators in the field of penile medicine.

Pimpinella anisum extract attenuates spontaneous and agonist-induced uterine contraction in term-pregnant rats

ETHNOPHARMACOLOGY RELEVANCE

Pimpinella anisum is a well-known traditional medicinal herb which has been used in folk medicine as an antiulcer, anticancer, antibacterial and as a muscle relaxant.

AIM OF THE STUDY

This study was performed to explore the modulatory effects of Pimpinella anisum on term-pregnant rat uterine contractility and to investigate its possible underlying mechanisms.

MATERIAL AND METHODS

Intact uterine strips without endometrial layer were isolated from female term-pregnant Wistar rats (22 days of gestation) and mounted in a tissue bath apparatus for in vitro isometric force recording. The effects of different concentrations of Pimpinella anisum extract (PAE) (1, 3, 5, and 7 mg/mL) were examined on uterine contractions generated spontaneously or induced with oxytocin (5 nmol/L), Bay K8644 (1 μmol/L), and carbachol (10 μmol/L). In some experiments, PAE was applied on depolarized myometrium in the presence of high-KCl solution (60 mmol/L). The effect on Ca²⁺ release was also examined.

RESULTS

Application of PAE significantly reduced uterine contractions generated spontaneously or induced with oxytocin, Bay K8644, and carbachol in a concentration-dependent manner (n = 7; P < 0.01). In depolarized myometrium, PAE significantly reduced the tonic force induced by high-KCl solution (n = 7; P < 0.01). PAE prevented oxytocin-induced transient contraction in the entire absence of external calcium (n = 7; P < 0.01).

CONCLUSION

The present findings demonstrate the potentials of PAE to relax pregnant uterine contractions possibly by blocking Ca²⁺ entry via L-type calcium channels and inhibiting Ca²⁺ release from the internal store. The tocolytic effects of PAE may be a potential adjuvant against strong premature uterine contractions which threaten early pregnancy although clinical studies are required.

The Myometrium: From Excitation to Contractions and Labour

We start by describing the functions of the uterus, its structure, both gross and fine, innervation and blood supply. It is interesting to note the diversity of the female's reproductive tract between species and to remember it when working with different animal models. Myocytes are the overwhelming cell type of the uterus (>95%) and our focus. Their function is to contract, and they have an intrinsic pacemaker and rhythmicity, which is modified by hormones, stretch, paracrine factors and the extracellular environment. We discuss evidence or not for pacemaker cells in the uterus. We also describe the sarcoplasmic reticulum (SR) in some detail, as it is relevant to calcium signalling and excitability. Ion channels, including store-operated ones, their contributions to excitability and action potentials, are covered. The main pathway to excitation is from depolarisation opening voltage-gated Ca²⁺ channels. Much of what happens downstream of excitability is common to other smooth muscles, with force depending upon the balance of myosin light kinase and phosphatase. Mechanisms of maintaining Ca²⁺ balance within the myocytes are discussed. Metabolism, and how it is intertwined with activity, blood flow and pH, is covered. Growth of the myometrium and changes in contractile proteins with pregnancy and parturition are also detailed. We finish with a description of uterine activity and why it is important, covering progression to labour as well as preterm and dysfunctional labours. We conclude by highlighting progress made and where further efforts are required.

Phenylephrine, a common cold remedy active ingredient, suppresses uterine contractions through cAMP signalling

Regulation of uterine contractility is an important aspect of women’s health. Phenylephrine, a selective agonist of the α₁-adrenoceptor and a potent smooth muscle constrictor, is widely used in women even during pregnancy to relieve cold-related symptoms, to treat postpartum haemorrhoid, and during routine eye exams. We performed isometric tension recordings to investigate the effect of phenylephrine on mouse uterine contractility. Phenylephrine decreased spontaneous and oxytocin-induced contractions in non-pregnant mouse uterine rings and strips with an IC₅₀ of ~1 μM. Prazosin, an inhibitor of α₁-adrenoceptor, did not prevent phenylephrine-mediated relaxations. Conversely, ICI118551, an antagonist of β2-adrenoceptors, inhibited phenylephrine relaxation. In the presence of ICI118551, high concentrations (>30 μM) of phenylephrine caused mouse uterine contractions, suggesting that β-adrenoceptor-mediated inhibition interferes with the phenylephrine contractile potential. Phenylephrine-dependent relaxation was reduced in the uterus of pregnant mice. We used primary mouse and human uterine smooth muscle cells (M/HUSMC) to establish the underlying mechanisms. Phenylephrine stimulated large increases in intracellular cAMP in M/HUSMCs. These cAMP transients were decreased when HUSMCs were cultured in the presence of oestrogen and progesterone to mimic the pregnancy milieu. Thus, phenylephrine is a strong relaxant in the non-pregnant mouse uterus, but exhibits diminished effect in the pregnant uterus.

Negative regulation of cellular Ca2+ mobilization by ryanodine receptor type 3 in mouse mesenteric artery smooth muscle

Physiological functions of type 3 ryanodine receptors (RyR3) in smooth muscle (SM) tissues are not well understood, in spite of their wide expression. However, the short isoform of RyR3 is known to be a dominant-negative variant (DN-RyR3), which may negatively regulate functions of both RyR2 and full-length (FL) RyR3 by forming hetero-tetramers. Here, functional roles of RyR3 in the regulation of Ca²⁺ signaling in mesenteric artery SM cells (MASMCs) were examined using RyR3 homozygous knockout mice (RyR3^-/-). Quantitative PCR analyses suggested that the predominant RyR3 subtype in MASMs from wild-type mice (RyR3^+/+) was DN-RyR3. In single MASMCs freshly isolated from RyR3^-/-, the EC₅₀ of caffeine to induce Ca²⁺ release was lower than that in RyR3^+/+ myocytes. The amplitude and frequency of Ca²⁺ sparks and spontaneous transient outward currents in MASMCs from RyR3^-/- were all larger than those from RyR3^+/+. Importantly, mRNA and functional expressions of voltage-dependent Ca²⁺ channel and large-conductance Ca²⁺-activated K⁺ (BK) channel in MASMCs from RyR3^-/- were identical to those from RyR3^+/+. However, in the presence of BK channel inhibitor, paxilline, the pressure rises induced by BayK8644 in MA vascular beds of RyR3^-/- were significantly larger than in those of RyR3^+/+. This indicates that the negative feedback effects of BK channel activity on intracellular Ca²⁺ signaling was enhanced in RyR3^-/-. Thus, RyR3, and, in fact, mainly DN-RyR3, via a complex with RyR2 suppresses Ca²⁺ release and indirectly regulated membrane potential by reducing BK channel activity in MASMCs and presumably can affect the regulation of intrinsic vascular tone.