Differential vasodilation of human placental and myometrial arteries related to myofilament Ca(2+)-desensitization and the expression of Hsp20 but not MYPT1.

Endothelial-dependent regulation of vascular tone occurs in part via protein kinase G1α-mediated changes in smooth muscle myofilament sensitivity to Ca(2+). Tissue-specific differences in PKG-dependent relaxation have been attributed to altered expression of myofilament-associated proteins that are substrates for PKG binding. These include the alternative splicing of the myosin targeting subunit (MYPT1) of myosin light chain phosphatase to yield leucine zipper positive (LZ(+)) and negative (LZ(-)) isovariants, with the former being required for PKG-mediated relaxation, and/or altered expressions of telokin, vasodilator-stimulated phosphoprotein (VASP) or heat shock protein Hsp20. During human pregnancy the uterine and placental circulations remain distinct entities and, as such, their mechanisms of vascular tone regulation may differ. Indeed, the sensitivity of myometrial arteries to endothelial-dependent agonists has been suggested to be greater than that of placental arteries. We tested the hypothesis that this was related to tissue-specific changes in PKG-mediated myofilament Ca(2+)-desensitization and/or the expressions of PKG-interacting myofilament-associated proteins. Permeabilized human placental and myometrial arteries were constricted with maximal activating Ca(2+) (pCa 4.5), or sub-maximal Ca(2+) (pCa 6.7) and the thrombane mimetic U46619, and exposed to 8-Br-cGMP. In each case, relaxation was significantly greater in myometrial arteries (e.g. relaxation in pCa 4.5 to 8-Br-cGMP was 49 ± 9.7%, n = 7) than placental arteries (relaxation of 23 ± 6.6%, n = 6, P < 0.05). MYPT1 protein levels, or MYPT1 LZ(+)/LZ(-) mRNA ratios, were similar for both artery types. Of other proteins examined, only Hsp20 expression was significantly elevated in myometrial arteries than placental arteries. These results demonstrate that the reduced human placental artery relaxation to PKG stimulation lies partly at the level of myofilament (de)activation and may be related to a lower expression of Hsp20 than in myometrial arteries.

Changes in myometrial 'perfusion' during normal labor as visualized by three-dimensional power Doppler angiography.

OBJECTIVES: Myometrial contractions are one of the most important aspects of effective labor. For cells within the myometrium to work efficiently they need to be well oxygenated and this requires an adequate blood supply. This study used quantitative three-dimensional (3D) power Doppler angiography to calculate the percentage change in myometrial blood flow during a relaxation-contraction-relaxation cycle of active labor. METHODS: Transabdominal 3D power Doppler ultrasound imaging was used to acquire volumetric data during the first stage of spontaneous labor in 20 term, nulliparous women. 3D datasets were acquired during a single cycle of uterine relaxation, contraction and subsequent relaxation for each subject. The resultant datasets were analyzed independently by two investigators on two occasions using Virtual Organ Computer-aided AnaLysis to define a volume of interest within the myometrium; the power Doppler signal within this volume was quantified to provide 3D indices of vascularity: vascularization index (VI), flow index (FI) and vascularization flow index (VFI). The percentage change in these indices, during a uterine contraction, was calculated from the baseline value during the initial uterine relaxation phase (taken as a maximum of 100%). RESULTS: Myometrial blood flow fell significantly during the uterine contraction and returned during the subsequent relaxation phase of the cycle (P < 0.001 for VI and VFI, P = 0.002 for FI). From the initial baseline relaxation value, VI dropped to 43.9%, FI to 85.5% and VFI to 40.8% during uterine contraction, and returned to 86.7%, 98.1% and 89.1%, respectively, during the subsequent relaxation. The intraclass correlation coefficients in blood flow measurements of 0.982-0.999 between the two investigators were indicative of good interobserver reliability. CONCLUSIONS: This study confirms that myometrial perfusion, as measured by quantitative 3D power Doppler angiography, significantly falls during uterine contractions, returns during the subsequent relaxation phase, and can be quantified reliably from stored datasets. Further work is now required to establish clinical applicability for this non-invasive investigation.

Characterisation of tone oscillations in placental and myometrial arteries from normal pregnancies and those complicated by pre-eclampsia and growth restriction.

Agonist-induced tone oscillations (rhythmic contractions and relaxations) occur in vascular beds to allow acute regulation of volume flow and thus the delivery of oxygen and nutrients to the tissue. Mechanisms responsible for the control of human placental vasomotor tone and blood flow are poorly characterized. This study aimed to characterise thromboxane-induced tone oscillations in human placental and myometrial arteries. Chorionic plate and myometrial arteries obtained from biopsies at term were mounted for isometric tension measurement. Tone oscillations were observed in chorionic arteries only when exposed to sub-maximal (<1 microM) concentrations of U46619. Slow (mean+/-SEM) frequency (2.6+/-0.5 per hour), large amplitude (39+/-7% of peak contraction) tone oscillations were elicited by 0.03 microM U46619 (n=18). In the presence of the nitric oxide synthase (NOS) inhibitor l-NNA (100 microM) the amplitude was significantly reduced (40+/-13% to 18+/-8%, P<0.05, n=6), frequency was unaltered and the bradykinin-dependent vasodilator response was reduced (68+/-13% to 40+/-19%, P<0.05, n=6). Myometrial arteries exposed to 1 microM U46619 developed tone oscillations within 10 min, which increased in amplitude over 30min occurring at relatively constant frequency. The mean amplitude of oscillations at 30 min (31+/-7%, n=16) was similar to that in chorionic arteries but the occurrence more frequent (42.8+/-9.7 per hour, P<0.001). Inhibition of NOS did not alter tone oscillations in myometrial arteries. Tone oscillations in chorionic arteries from pre-eclamptic and growth restricted (FGR) pregnancies were reduced in amplitude whereas those in myometrial arteries had increased frequency. Inhibition of NOS further reduced oscillation amplitude in chorionic arteries from FGR pregnancies. The alterations may contribute to the vasculopathology of these conditions, or, may represent compensatory mechanisms to maintain a matching of materno-placental blood flow.

TNF-alpha alters nitric oxide- and endothelium-derived hyperpolarizing factor-mediated vasodilatation in human omental arteries.

OBJECTIVE: To investigate the effect of TNF-alpha on the endothelial function of human systemic arteries. METHODS: Omental arteries were obtained from healthy pregnant women undergoing Cesarean section and examined using isometric wire myography. RESULTS: Incubation with TNF-alpha (1nM) alone did not alter bradykinin-mediated endothelium-dependent relaxation of arteries. However, TNF-alpha did attenuate nitric oxide- (NO) and prostacyclin-independent endothelial-mediated relaxation. Similarly, in vessels constricted with a high potassium solution (60 mM), which inhibits vasodilatation via endothelial-derived hyperpolarising factor (EDHF), TNF-alpha incubation also attenuated bradykinin-induced vasodilatation. CONCLUSIONS: The vasorelaxant capacity of human systemic arteries is compromised by TNF-alpha incubation in the presence of NO/prostacyclin or EDHF-blockade.

Histamine-induced contraction and relaxation of placental chorionic plate arteries.

Studies of the human placental vasculature suggest a low resistance circulation. Using wire myography, endothelial-dependent relaxation of human chorionic plate arteries has been difficult to demonstrate with any consistency. However, histamine has been suggested to relax placental vessels in the perfused organ in vitro. Here we aimed to demonstrate endothelial-dependent relaxation to histamine under physiological conditions of stretch and oxygenation. Histamine administration to pre-contracted arteries induced a triphasic response; an initial contraction followed by a dilatation which stabilized to a significant relaxation compared to time control arteries. Relaxation was partially inhibited by blockers of endothelial-dependent relaxation pathways. The initial contraction was abolished by H(1)-receptor blockade with mepyramine. The relaxation was significantly reduced by H(2)-receptor blockade with famotidine but only abolished in the presence of both H(1)- and H(2)-receptor antagonists. In conclusion, histamine induced contraction and relaxation of human chorionic plate arteries. Our data suggest that contraction is mediated by activation of H(1)-receptors. Relaxation occurs directly, via activation of H(2)-receptors on vascular smooth muscle cells, and indirectly via H(1)-receptor stimulation of endothelial-dependent relaxation.

Glibenclamide inhibits agonist-induced vasoconstriction of placental chorionic plate arteries.

BACKGROUND: Preliminary data suggest that K(ATP) channels may be expressed in placental arteries and veins [Wareing M, Turner C, Greenwood SL, Baker PN, Fyfe GK. Expression of mRNA encoding K+ channels in chorionic plate arteries and veins. J Soc Gynecol Investig 2004;11:353A]. However, no data exist on glibenclamide's effects in placental chorionic plate arteries. AIM: To assess the effect of glibenclamide on placental chorionic plate arterial vasoconstriction. METHODS: Arteries were dissected from placental chorionic plate biopsies obtained at term from uncomplicated pregnancies (N=63). Arteries were mounted onto a wire myograph in HCO3- -buffered physiological salt solution (PSS) at 37 degrees C (5% O2/5% CO2 bubbling) and normalised at 0.9 of L5.1 kPa. Constriction viability was assessed with 120 mmol l(-1) potassium solution (KPSS). Dose-response curves were produced with the thromboxane-mimetics U46619 and U44069 (10(-10)-2 x 10(-6)M), arginine vasopressin (10(-10)-5 x 10(-8)M) and endothelin-1 (10(-11)-3 x 10(-7)M) in the presence or absence of 50 micromol l(-1) glibenclamide. The effect of glibenclamide on arginine vasopressin- and U46619-induced constriction was also assessed in the presence of the cyclo-oxygenase inhibitor indomethacin (10 micromol l(-1)). RESULTS: Pre-incubation with 50 micromol l(-1) glibenclamide significantly right-shifted dose-response curves to all vasoconstrictive agonists tested (repeated measures ANOVA). Indomethacin did not modify the inhibitory effect of glibenclamide. CONCLUSION: Glibenclamide's effects on agonist-induced constrictions are unlikely to be via an inhibition of ATP-sensitive K+ channels, and with U46619- and U44069-induced constrictions, glibenclamide may be acting as a competitive antagonist of thromboxane receptors.

Ultrastructural features of smooth muscle and endothelial cells of isolated isobaric human placental and maternal arteries.

The ability of a blood vessel to develop tone is dependent upon morphological parameters of the smooth muscle cells (SMC), including density, relationship with the endothelium and subcellular distribution of myofilaments and intracellular organelles. Consequently, wall ultrastructure of isolated human placental chorionic plate arteries (n=12), fixed when pressurised to mimic their in vivo geometry, was examined qualitatively using electron microscopy, and compared with maternal arteries (omental, n=10, myometrial, n=6). Arteries from women with uncomplicated pregnancy were tested for contractile viability before fixing, with some vessels post-fixed in osmium-ferricyanide for sarcoplasmic reticulum (SR) identification. In contrast to maternal arteries, placental arteries had no internal elastic lamina but exhibited considerable extracellular matrix separating circularly orientated SMC. Human SMC contained tightly packed arrays of myofilaments running parallel to the plasma membrane, enveloping cellular organelles. Synthetic SMC, with few myofilaments and much rough SR, were observed in placental arteries only. SR in SMC from maternal arteries was located centrally, often encircling mitochondria, and also near the plasma membrane associated with caveolae. Positive SR staining was rarely observed in SMC of placental arteries. This study highlights ultrastructural differences between placental and maternal arteries that may underlie specialised mechanisms of regulating vascular tone in the placenta.

The mechanism of Ca2+ release from the SR of permeabilised guinea-pig and rat ureteric smooth muscle.

Recent work has indicated that there is a major difference in the Ca2+ store of smooth muscle from rat and guinea-pig ureter; with the rat store being agonist-sensitive but ryanodine insensitive and the guinea-pig store being ryanodine sensitive but agonist insensitive [Th. V. Burdyga, M.J. Taggart, S. Wray, J. Physiol. 489 (1995) 327-335]. We have therefore examined directly the mechanism of Ca2+ release from the internal Ca2+ store (SR). Following permeabilisation with alpha-toxin or beta-escin the SR was Ca(2+)-loaded before application of carbachol or caffeine. Only carbachol evoked a transient contraction in rat ureter. The carbachol-induced contraction was blocked by heparin and cyclopiazonic acid (CPA) but not ryanodine. Only caffeine produced contraction in guinea-pig ureter, and this was blocked by ryanodine. Direct application of IP3 caused a small transient contraction in rat but not guinea-pig ureter. We conclude that rat ureter possesses only an IP3 sensitive store while guinea-pig ureter only has a ryanodine sensitive store.

Agonist mobilization of sarcoplasmic reticular calcium in smooth muscle: functional coupling to the plasmalemmal Na+/Ca2+ exchanger?

There is a close association of peripheral sarcoplasmic reticulum (SR), containing IP3 receptors, and regions of the plasma membrane enriched in the Na+/Ca2+ exchanger in smooth muscle. We have tested the possibility in rat uterine smooth muscle that Ca2+ released from the SR is preferentially removed from the cytosol by the Na+/Ca2+ exchanger. In Ca(2+)-free solution, carbachol stimulation of myometria of non-pregnant rats resulted in transient increases in [Ca2+]i and force due entirely to the release of SR Ca2+. Inhibition of Na+/Ca2+ exchange by removal of extracellular Na+ did not alter the agonist-induced transients suggesting that Na+/Ca2+ exchange was not involved in the removal of SR released Ca2+. However, in myometria of pregnant rats, Na+/Ca2+ exchange inhibition resulted in changes in the agonist-induced [Ca2+]i transient profiles. The peak amplitude, duration and integral of carbachol-induced [Ca+]i transients were enhanced in Ca(2+)-free/Na(+)-free solution without significantly affecting force transients. The lower rate of decay of [Ca2+]i transients in Na(+)-free solution leads us to suggest that up to 35% of the SR released Ca2+ may be extruded by the Na+/Ca2+ exchanger in myometria of pregnant rats. Thus, in uterine smooth muscle, there is a gestational-dependent coupling of SR releasable Ca2+ and plasmalemmal Na+/Ca2+ exchange activity.

The effects of vascular smooth muscle caldesmon on force production by 'desensitised' skeletal muscle fibres.

Smooth muscle caldesmon inhibits actomyosin MgATPase in the absence of Ca2+ and is the key regulatory component of Ca2+-regulated thin filaments. We now show that caldesmon can affect contractility as well. Glycerinated skeletal muscle fibres were treated so as to produce substantial contractions without Ca2+ as an activator. Addition of caldesmon caused reductions in force in a concentration- and time-dependent manner. Perfusion with caldesmon concentration for less than 5 min gave up to 48% reduction in isometric tension with a half-maximal effect at 1.5 micron caldesmon. Perfusion with 15 microM caldesmon for 30 min gave an irreversible tension drop.

Mechanisms of 17 beta-oestradiol induced vasodilatation in isolated pressurized rat small arteries.

1. The influence of 17 beta-oestradiol on pressurized isolated rat mesenteric and coronary small arteries was investigated. 2. 17 beta-oestradiol caused rapid (t1.0 < 5 mins) concentration-dependent relaxations of pre-contracted pressurized (50 mmHg) isolated rat mesenteric and coronary arteries. Similar responses were observed in both vessel types. Significant relaxations were only observed at concentrations exceeding 3 microM. 3. The vasodilatory responses in both types of artery were unaffected by 10 microM L-nitro arginine (L-NNA) alone or in the presence of 10 microM indomethacin, inhibitors of nitric oxide and prostaglandin synthesis respectively. They were also unaffected by the pre-contracting agent used i.e. high K+ or U46619 (a thromboxane analogue). 4. Neither the oestrogen receptor antagonist ICI 182,780 (10 microM) nor the protein synthesis inhibitor cycloheximide (100 microM) had any effect on the responses of mesenteric arteries to 17 beta-oestradiol. 5. 17 alpha-oestradiol had only a minor effect on mesenteric arterial diameter over a concentration range similar to the effective vasodilatory range for 17 beta-oestradiol. 6. Membrane impermeant 17 beta-oestradiol conjugated to bovine serum albumin (beta-oestradiol-17-hemisuccinate-BSA) (E-H-BSA) resulted in a vasodilatation of pressurized arteries. 7. Wortmannin, an inhibitor of myosin light chain kinase, near maximally relaxed pressurized mesenteric arteries although the time course for the response was significantly slower than that for 17 beta-oestradiol. 8. These results taken together suggest that the acute effects of 17 beta-oestradiol on isolated pressurized arterial tone may be due to effects directly on the vascular smooth muscle via non-genomic mechanisms that involve a stereospecific interaction at the plasma membrane.

Vasoactive responses of veins isolated from the human placental chorionic plate.

The control of the blood flow within the fetoplacental circulation is poorly understood despite the essential role of the placenta in pregnancy. Our aim was to assess the vasoactive responses of veins from the placental chorionic plate. Biopsies were obtained from term placentae from uncomplicated pregnancies. Small veins from the chorionic plate were dissected free from surrounding tissue and studied using parallel wire myography. Human placental chorionic plate veins developed maintained constrictions to the thromboxane-mimetic U46619. Endothelium-dependent agonists did not promote venous relaxation. However, NO donation with the endothelial-independent agent, sodium nitroprusside, elicited significant relaxation. Venous constriction to U46619 and relaxation to sodium nitroprusside were modified by adjustment of media oxygen tension and normalization parameters. Human placental chorionic plate veins respond to vasoactive agents and may play a role in the control of the blood flow in the fetoplacental circulation.

Characterization of small arteries isolated from the human placental chorionic plate.

Despite the essential role of the placenta in pregnancy, the control of the blood flow within the fetoplacental circulation is poorly understood. A handful of myography studies have directly assessed the role of vasoactive agonists in fetoplacental vasculature contractility but have used a range of steady-state conditions. Our aim, therefore, was to determine the optimal vessel diameter and oxygen tension to assess vascular function in small arteries isolated from the chorionic plate of normal term placentae. Biopsies were obtained from term placentae from uncomplicated pregnancies. Small arteries were dissected from the chorionic plate, mounted onto a wire myograph in HCO3(-) -buffered physiological salt solution at 37 degrees C and equilibrated for 20 min. Two methods for normalization of the optimal length/diameter for contractility of chorionic plate small arteries were assessed. Both classical normalization (CN) and length-tension curve (LTC) methods produced similar data. These data were agonist-independent. Data for CN and LTC were unaffected but maximal force generation (for U46619) was decreased in reduced oxygen tensions. Using conditions for optimal tension production in chorionic plate small arteries the thromboxane-mimetic U46619 produced the greatest and most reproducible constrictive effect. Relaxations were only achieved with endothelial-independent agonists (sodium nitroprusside and papaverine).

Towards a computational reconstruction of the electrodynamics of premature and full term human labour.

We apply virtual tissue engineering to the full term human uterus with a view to reconstruction of the spatiotemporal patterns of electrical activity of the myometrium that control mechanical activity via intracellular calcium. The three-dimensional geometry of the gravid uterus has been reconstructed from segmented in vivo magnetic resonance imaging as well as ex vivo diffusion tensor magnetic resonance imaging to resolve fine scale tissue architecture. A late-pregnancy uterine smooth muscle cell model is constructed and bursting analysed using continuation algorithms. These cell models are incorporated into partial differential equation models for tissue synchronisation and propagation. The ultimate objective is to develop a quantitative and predictive understanding of the mechanisms that initiate and regulate labour.

Responses of isolated pressurised rat uterine arteries to changes in pressure: effects of pre-constriction, endothelium and pregnancy.

Pregnancy-induced changes in uterine artery function play a critical role in ensuring adequate placental perfusion. Responses of these vessels to pressure (myogenic responsiveness) may contribute to this. The overall myogenic properties of uterine arteries may depend upon the integration of a number of different factors, including effects of pre-constrictor stimuli, and should be considered in terms of both initial and stable diameters both of which may be modulated by pregnancy. This study thus investigated the effects of pre-constriction, the endothelium and pregnancy on responses of isolated rat uterine arteries to changes in intravascular pressure (IvP). The effects on both the immediate transient diameter changes and stable diameters (myogenic tone) were studied. Isolated 3rd order uterine arteries from non-pregnant and days 19-21 pregnant Sprague-Dawley rats were mounted on a pressure myograph and responses to changes in IvP (20-120 mm Hg) examined. Arteries did not exhibit active responses to pressure in the absence of stimulation, however, all showed active myogenic constriction when pre-constricted by depolarization (30 or 60mM KCl) or arginine vasopressin (AVP). Pregnancy enhanced stable levels of myogenic tone with AVP, but not depolarization. This difference was not dependent upon the endothelium. Initial peak diameters were enhanced in arteries from pregnant rats due to endothelium-dependent mechanisms. Thus, both the peak and stable response of isolated rat uterine arteries to pressure can be differentially regulated and thus must both be considered when considering the influence of pressure on uterine artery reactivity during pregnancy.

Myometrial mechanoadaptation during pregnancy: implications for smooth muscle plasticity and remodelling.

The smooth muscle of the uterus during pregnancy presents a unique circumstance of physiological mechanotransduction as the tissue remodels in response to stretches imposed by the growing foetus(es), yet the nature of the molecular and functional adaptations remain unresolved. We studied, in myometrium isolated from non-pregnant (NP) and pregnant mice, the active and passive length-tension curves by myography and the expression and activation by immunoblotting of focal adhesion-related proteins known in other systems to participate in mechanosensing and mechanotransduction. In situ uterine mass correlated with pup number and weight throughout pregnancy. In vitro myometrial active, and passive, length-tension curves shifted significantly to the right during pregnancy indicative of altered mechanosensitivity; at term, maximum active tension was generated following 3.94+/-0.33-fold stretch beyond slack length compared to 1.91+/-0.12-fold for NP mice. Moreover, mechanotransduction was altered during pregnancy as evidenced by the progressive increase in absolute force production at each optimal stretch. Pregnancy was concomitantly associated with an increased expression of the dense plaque-associated proteins FAK and paxillin, and elevated activation of FAK, paxillin, c-Src and extracellular signal-regulated kinase (ERK1/2) which reversed 1 day post-partum. Electron microscopy revealed close appositioning of neighbouring myometrial cells across a narrow extracellular cleft adjoining plasmalemmal dense plaques. Collectively, these results suggest a physiological basis of myometrial length adaptation, long known to be a property of many smooth muscles, whereupon plasmalemmal dense plaque proteins serve as molecular signalling and structural platforms contributing to functional (contractile) remodelling in response to chronic stretch.

Regulation of endothelial-dependent relaxation in human systemic arteries by SKCa and IKCa channels.

Blockade of small-conductance Ca (2)(+)-activated K(+) channels (SK(Ca)) and intermediate conductance Ca(2)(+)-activated K(+) channels (IK(Ca)) can cause inhibition of endothelium-dependent hyperpolarizing factor (EDHF) in many vascular beds from animals, but there is a relative paucity of data in human vessels. Systemic arteries, isolated from women with healthy pregnancies, relax to the endothelial-dependent agonist bradykinin via a nonprostacyclin and non-nitric oxide pathway attributable to EDHF. Therefore, in this study, the authors investigated the effect of pharmacological blockade of SK(Ca) and IK(Ca) on EDHF-mediated relaxation of human omental and myometrial arteries preconstricted with either arginine vasopressin or U46619. Human arteries were isolated from omental and myometrial biopsies taken from healthy women undergoing planned cesarean section at term. Endothelial function was assessed using wire myography. In all vessels examined, nonspecific blockade of IK(Ca) with charybdotoxin attenuated EDHF-attributed relaxation. However, when Tram 34 was used to block IK(Ca), the attenuation of relaxation was evident only with U46619 preconstriction. In arteries from both vascular beds, and with either preconstrictor, a combination of either apamin and charybdotoxin or apamin plus Tram 34 almost ablated EDHF-attributable relaxation. These data support the notion that in human systemic arteries, activation of, primarily, SK(Ca) and IK(Ca)K(+) channel subtypes underlies EDHF-mediated relaxation. These results have important implications for future studies ascertaining the molecular mechanisms of hypertensive disorders (eg, preeclampsia, in which EDHF is thought to be aberrant).

Hypoxic inhibition of human myometrial contractions in vitro: implications for the regulation of parturition.

BACKGROUND: Insufficient tissue oxygenation is a likely contribution to weak, inco-ordinate human uterine contractile activity characteristic of prolonged, dysfunctional labour. However, the direct effects of hypoxia on human myometrial contractility has, surprisingly, not yet been detailed. Therefore, we report the influence of hypoxia on spontaneous and agonist-induced carbachol, prostaglandin (PGF2alpha), and oxytocin contractions of myometria from nonpregnant and pregnant women. MATERIALS AND METHODS: Uterine biopsies were obtained from pregnant women at term undergoing elective Caesarean section and nonpregnant women undergoing hysterectomy. Myometrial strips were equilibrated at 37 degrees C in normoxic physiological salt solution (95% air/5% CO(2)) and the influence of hypoxia (95% N(2)/5% CO(2)) on contractility was investigated. RESULTS: Hypoxia resulted in a significant reduction in spontaneous contractile function; nonpregnant tissue was less resistant to the deleterious effects of hypoxia. Agonist-induced contractions, while being more resistant to hypoxia than spontaneous contractions, were also significantly inhibited. In myometria of pregnant women the PGF2alpha- or oxytocin-induced contractility was more resistant to hypoxia than carbachol. Finally, the inhibitory actions of hypoxia were exacerbated with repeated oxytocin administration with a more severe effect on contractile integral than on initial phasic contraction amplitude. CONCLUSIONS: We detail, for the first time, the effects of hypoxia on contractility of human myometria from nonpregnant and pregnant women. Physiologically important uterotonic agents are more resistant to the effects of hypoxia than spontaneous contractions although repeated stimulation with oxytocin during hypoxia results in progressively less force. The results indicate that if significant hypoxia occurs in vivo then it is a likely contributory factor to the pathways underlying prolonged dysfunctional labour.

Expression of scaffolding, signalling and contractile-filament proteins in human myometria: effects of pregnancy and labour.

Successful parturition requires the co-ordination of numerous myometrial signalling events to allow for timely and efficient uterine contractions. Late pregnancy and labour onset in humans may be associated with changes in the expression of myometrial proteins implicated in such uterine contractile signal integration. Accordingly, in myometria from non-pregnant women and pregnant women, not in labour or in labour, we examined the content of putative plasmalemmal scaffolding proteins (caveolin-1 and -2) and compared these to the proportions of signal transducing rho-associated kinases (ROKalpha and beta) and contractile filament-associated proteins alpha-actin, myosin regulatory light chain (MLC(20)) and h-caldesmon. There was no effect of pregnancy or labour on the proportion of caveolin, ROK betaor alpha-actin. However, pregnancy was associated with a decrease in ROKalpha and MLC(20) such that ROK alpha: alpha-actin and MLC(20): alpha-actin ratios were reduced compared to myometria of non-pregnant women. In contrast, h-caldesmon was up-regulated in pregnancy resulting in an elevated h-caldesmon: alpha-actin ratio. There were, however, no further significant changes in ROK alpha, MLC(20) or h-caldesmon expression with spontaneous or oxytocin-induced labour. These data suggest that the mechanism(s) integrating myometrial signalling events with the onset of human labour does not involve differential alterations of the cellular expressions of caveolins, ROK, alpha-actin, MLC(20) or h-caldesmon.

Smooth muscle excitation-contraction coupling: a role for caveolae and caveolins?

Agonist stimulation of smooth muscle contractility involves integration of many signal-transducing events from the plasma membrane to myofilaments in the cytoplasm. Recent evidence suggests an important role for membranous invaginations termed caveolae, and their integral protein components caveolins, in the coordination of extracellular contractile stimuli and intracellular effectors in smooth muscle.