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A Close Look at the Contraction and Relaxation of the Myometrium; the Role of Calcium Myometriyumun Kasılma Ve Gevşeme Mekanizmalarında Kalsiyumun Rolü

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A Close Look at the Contraction and Relaxation of the Myometrium; the Role of Calcium Myometriyumun Kasılma Ve Gevşeme Mekanizmalarında Kalsiyumun Rolü 230 This article will be credited by TMA within the scope of CME/CPD. Review A close look at the contraction and relaxation of the myometrium; the role of calcium Myometriyumun kasılma ve gevşeme mekanizmalarında kalsiyumun rolü Bilge Pehlivanoğlu, Sibel Bayrak, Murat Doğan Department of Physiology, Hacettepe University Faculty of Medicine, Ankara, Turkey Abstract Özet The function and regulation of the myometrium, especially during Üreme fizyolojisi açısından myometriyumun işlevleri ve bu işlevle- pregnancy, labour and birth are important in reproductive physiology. rin özellikle gebelik süreci ve doğum sırasında düzenlenmeleri çok It is crucial to understand the mechanisms that generate and modu- önemlidir. Uterus kasılmalarını başlatan ve etkileyen faktörlerin anla- late uterine contractility in order to be able to prevent and/or treat the şılabilmesi myometriyumu ilgilendiren patolojik durumların engellen- problems related with the myometrium. A limited understanding of mesi ve/veya tedavi edilebilmesi için çok önemlidir. Ancak bu fizyo- the cellular and molecular events underlying these phenomena com- lojik düzenlemeyi kontrol eden hücresel ve moleküler olayların tam plicates the situation. Various agonists, hormones, transmitters and/or olarak açıklanamamış olması nedeni ile tablo hala karmaşıktır. Çok chemicals are related to the regulation of the functions of the myome- sayıda agonistin, hormonların, transmiterlerin ve kimyasal maddele- trium. Although notable advances regarding the key steps in receptor rin myometriyum işlevlerinin düzenlenmesinde rolü olduğu gösteril- signalling explaining the actions of these factors have been achieved, miş ve bunların etki mekanizmalarındaki bazı anahtar basamaklar ile a good deal of information is still necessary to understand this vital ilgili gelişmeler kaydedilmiş olmasına karşın, bu yaşamsal işlevi daha process. A better comprehension of myometrium physiology and the translation of research findings to clinical settings will help progress iyi açıklayabilmek için daha fazla bilgiye ihtiyaç vardır. Myometriyu- in women’s health. In this review, we attempt to present a critical mun fizyolojik özelliklerinin anlaşılması, araştırma sonuçlarının klinik overview of myometrial functions and focus specifically on the role of uygulamalara yansıtılması kadın sağlığı açısından önemli katkı sağla- calcium. (J Turkish-German Gynecol Assoc 2013; 14: 230-4) yacaktır. Bu derlemede myometriyumun işlevlerini ve özellikle kalsi- Key words: Myocytes, contraction, relaxation, calcium, sensitisation yumun rolünü özetlemeyi amaçladık. (J Turkish-German Gynecol Assoc 2013; 14: 230-4) Received: 20 June, 2013 Accepted: 12 August, 2013 Anahtar kelimeler: Miyositler, kasılma, gevşeme, kalsiyum, sensi- tizasyon Geliş Tarihi: 20 Haziran 2013 Kabul Tarihi: 12 Ağustos 2013 Introduction are not strong enough and/or contractions are irregular (1, 2). Since the mechanisms that generate and modulate uterine The uterus, more specifically the smooth muscle layer of the contractility are not fully known, the aberrant patterns of uterus, the myometrium, is an often overlooked tissue. Even contractile activity remain unsolved (3). Due to these limited though the myometrium is active throughout a woman’s life, answers, the therapeutic or preventive approaches to clinical not just during labour and delivery, it is rarely considered until conditions are not as successful as desired. In this review, we aimed to summarise the current knowledge about the mech- things go wrong which may have devastating consequences. anisms of contraction and relaxation of the myometrium and However, the uterine myometrium serves life-long by con- specifically the role of calcium (Ca+2). tracting at the right time with exactly the desired amount of force during labour and the postpartum period and remain- Contraction of the Myometrium ing relaxed even though distended enormously during the period of pregnancy, as well as maintaining its tonus in non- Uterine contractility, which occurs throughout the menstrual pregnant, non-menstruating periods. Some of the unwanted cycle in non-pregnant and pregnant states, is a complex but frequently seen results of myometrial dysfunction are and dynamic physiological phenomenon (2). Non-pregnant untimed contractions leading to abortions or preterm deliv- myometrium exhibits different contractions at different ery, or stronger than necessary contractions causing foetal phases of the menstrual cycle; the first one is rhythmic, distress, hypoxia and even death of the foetus. In addition, ‘wave-like’ contractions, which is sometimes known as dysfunctional labour can be faced and caesarean section uterine peristalsis, while the second type of contraction is may become inevitable when the force of the contractions defined as the ‘focal and sporadic bulging of the myome- Address for Correspondence: Bilge Pehlivanoğlu, Department of Physiology, Hacettepe University Faculty of Medicine, Ankara, Turkey. Phone: + 90 312 305 15 67-305 15 59 e.mail: [email protected] ©Copyright 2013 by the Turkish-German Gynecological Education and Research Foundation - Available online at www.jtgga.org doi:10.5152/jtgga.2013.67763 Pehlivanoğlu et al. J Turkish-German Gynecol Assoc 2013; 14: 230-4 Myometrial function and calcium 231 +2 trium’ (4, 5), leading to sustained contractions. These contrac- Agonist or IP3-induced Ca release: Binding of uterine agonists tions serve in the sloughing of the endometrium (6) and help to a specific G-protein coupled receptor (GPCR) in the plasma the passage of sperm. The myometrium in the pregnant uterus membrane of uterine myocytes activates trimeric G-protein (11) transforms from a silent, non-contracting state to an actively and turns on a cascade of events starting with membrane phos- and forcefully contracting organ for a successful delivery. pholipase C (PLC) stimulation (12). Stimulated PLC cleaves This is achieved by morphological changes and adaptations phosphatidylinositol bisphosphate (PIP2) to diacylglycerol under the effect of elevated oestrogen and progesterone and +2 (DAG) and IP3, the latter of which causes the release of Ca the balance between these two hormones; these interactions from the SR into the cytoplasm, as indicated above (13). The are reviewed elsewhere as they are beyond the purpose of +2 IP3-mediated Ca release from SR is the major factor adjust- this review (7, 8). Besides the above-mentioned functional ing resting membrane voltage (Vrest) to the value at which differences, regardless of the presence or absence of preg- voltage-operated Ca+2 (VOC) channels open to trigger an action nancy, uterine contractions are dependent on the contractile potential (AP) (14). There are two types of APs recorded in myo- activity of the cellular elements, the uterine myocytes. The metrial smooth muscle of various species; simple APs which uterine myocytes are smooth muscle cells exhibiting a phasic have depolarisation followed by rapid repolarisation, and com- contractile pattern which can be summarised as the mainte- plex APs which have an initial depolarisation with a sustained nance of a resting tone superimposed by separate intermittent plateau (15, 16). It has been suggested that the shape of [Ca+2]i contractions of varying frequency, amplitude and duration. It transients and contractions triggered by these action potentials is predominantly regulated by intracellular calcium concentra- may be different (17). tion ([Ca+2]i) (1, 2, 8). Ca+2-induced Ca+2 release in the myometrium: Another mecha- Cellular Organisation nism is known as Ca+2-induced Ca+2 release, whereby the The contractile machinery of the uterine smooth muscles increasing [Ca+2] sensitises other Ca+2 channels to open, thus involves actin and myosin myofilaments with six times prepon- i creating a feed-forward loop, although the mechanism of this derance of actin to myosin. The thin myofilaments composed phenomenon is not yet clear (14). of polymers of globular actin monomers and thick filaments are made up of myosin helices lying parallel to the longitudinal axis of the cell. In addition, there are intermediate filaments Extracellular calcium entry composed of predominantly desmin and vimentin. The myometrium contains predominantly voltage-operated, large conductance L-type Ca+2 channels (18, 19). The presence +2 Excitation contraction coupling of T-type Ca channels has been recently shown, although their The excitation contraction coupling is regulated predominantly role has not yet been fully elucidated (20). Massive movement +2 by [Ca+2]i. Physiologically, the changes in [Ca+2]i can be con- of Ca from the extracellular space into the cytosol through +2 +2 sidered in three phases leading to differences in contractility: these Ca channels determines [Ca ]i and force generation to basal concentrations, which are sufficient to maintain the rest- a much greater extent than the agonist-stimulated IP3-mediated ing tone of the myometrium, increased [Ca+2]i that occurs with release of Ca+2 (13, 21). contractile agonist stimulation and the restoration of [Ca+2]i to the resting state following stimulation. The contractile machinery of the myocytes involves the inter- Agonist action of myofilaments, actin and myosin. As in any other DAG AA PG Gs SP PLC ++ Ca++ Ca Ca++ muscle, the cross bridge formation and contraction is medi- IP3 Ca++-ATPase +2 PIP2 ated by elevated [Ca ]i and myosin light chain phosphoryla- Ca++ Ca++ ++ ++ +2 Calm 4 Ca -Calm Ca Ca++ tion. Irrespective of the triggering stimulus, [Ca ]i elevation
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