Involvement of lipoxygenase products in myometrial contractions

Studies with an in situ preparation of guinea pig uterus suggest the possible involvement of the lipoxygenase pathway of arachidonic acid (AA) metabolism in myometrial contractions. Female guinea pigs were sensitized to ovalbumin (OA) on day one of their estrous cycle. On day 14, these pigs were anesthetized and the uterus was cannulated for measuring contractions. OA challenge, with histamine antagonism (methapyrilene) resulted in uterine contractions which significantly raised myometrial tonus, presumably due to AA metabolites. Pretreatment with high doses of indomethacin resulted in only 60% inhibition of the OA induced contraction, suggesting the remaining contraction was due to something other than cyclooxygenase products. In the presence of indomethacin and methapyrilene, the addition of AA to increase available substrate caused increased myometrial tone following antigen challenge. This increase in uterine tone was inhibited in a dose dependent fashion by FPL-55712 demonstrating that leukotrienes can contract the uterus and that antigen challenge may provide a means for studying leukotriene involvement in uterine pathophysiology.     

Endogenous prostaglandins regulate spontaneous contractile activity of uterine strips isolated from non-pregnant pigs

Myometrial strips isolated from non-pregnant pigs show spontaneous contractile activity. In the present study, the involvement of endogenous prostaglandins in regulation of uterine spontaneous contraction was investigated using mechanical, immunohistochemical and biochemical approaches. Immunohistochemical study and Western blot analysis for immunoreactive cyclooxygenase (COX) indicated that COX-1 but not COX-2 was expressed predominantly in the myometrium of non-pregnant pigs in a muscle layer-dependent manner (longitudinal muscle>circular muscle). Pretreatment of uterine strips with indomethacin and selective COX-1 inhibitors (SC-560 and FR122047) significantly reduced both the amplitude and frequency of spontaneous contraction in the longitudinal muscle, but inhibition by COX inhibitors was negligible in the circular muscle. On the other hand, CAY10404, a COX-2 inhibitor, did not change the spontaneous contraction in either of the muscle layers. Pretreatment with SC-560 reduced myometrial PGF(2alpha) and PGE(2) levels. Contractile FP and EP(3) receptors were expressed in a muscle layer-dependent manner (longitudinal muscle>circular muscle), similar to the expression pattern of COX-1. In conclusion, endogenous prostaglandins produced by COX-1 regulate spontaneous contractile activity of non-pregnant porcine uterine longitudinal muscle selectively due to the heterogeneous expression of contractile prostanoid receptors and COX-1.     

Smooth muscle contraction as a model to study the mediator role of endogenous lipoxygenase products of arachidonic acid

In the lung, the contraction of smooth muscle, or bronchospasm, is generally caused by an immunologic insult resulting in mast cell degranulation and the release of histamine, slow reacting substances, and other mediators of inflammation (1). Although the immediate response is bronchospasm, continued activation of this sequence of events results in a chronic inflammatory disease. In the uterus, numerous conditions can result in smooth muscle contraction. One major pathophysiological syndrome associated with increased uterine tone and severe rhythmic contraction is primary dysmenorrhea (2). In this disease state, prostaglandins have been shown to play a major role in these contractions (3,4), and inhibitors of cyclooxygenase have proven beneficial in clinical practice (5). Both dysmenorrhea and cervical ripening have been likened to inflammatory reactions due to varying degrees of vasodilation, invasion by inflammatory cells, proliferation of fibroblasts and smooth muscle contraction (6,7). Metabolism of arachidonic acid (AA) via cyclooxygenase to prostaglandins and thromboxanes and via lipoxygenase to hydroxyeicosatetraenoic acids (HETEs) and leukotrienes is an integral part of both the acute and chronic inflammatory reaction in the lung or uterus. The material reviewed here examines the effect of endogenous leukotrienes on both the lung and uterus and suggests that other smooth muscles and pathophysiological states may be more involved with the lipoxygenase pathway of AA metabolism than previously believed.     

Inhibition by indomethacin of naloxone-precipitated opiate withdrawal contraction in isolated guinea pig myenteric plexus-longitudinal muscle preparation

The isolated myenteric plexus-longitudinal muscle preparation from guinea pigs which had previously been administered morphine chronically 100 mg/kg/day, 7 days) shows a pronounced contraction when exposed to naloxone. Exposure of the preparation to indomethacin (6.7 micrograms/ml) for 10 minutes prior to naloxone inhibits this withdrawal response. A single dose of indomethacin, 1 mg/kg, given on the 6th day on morphine, also inhibits the naloxone contraction. These results suggest the involvement of a prostaglandin in the opiate withdrawal response in the ileum.     

Effect of indomethacin on airway contraction and the release of LTC4-like material

Ovalbumin (OA) and arachidonic acid (AA) were used to induce contractions of sensitized guinea-pig tracheal and lung preparations in the presence and absence of indomethacin. Leukotriene (LT)C4-like material released from these tissues was extracted from the bathing fluid and measured by radioimmunoassay. Challenge with either OA or AA induced release of LTC4-like material from both parenchyma and trachea, AA inducing a greater release than OA although OA induced greater contractions. This suggested that OA-induced the synthesis of other bronchoconstrictor compounds than LTC4. Although indomethacin enhanced OA- and AA-induced contractions of trachea, there was no enhancement of the release of LTC4-like material, suggesting enhancement by indomethacin was a result of the inhibition of the synthesis of prostaglandin E2 and not diversion of AA into the lipoxygenase pathway. Indomethacin had no effect on OA-induced contractions of parenchyma, but attenuated those induced by AA. Indomethacin had no modulatory effect on the release of LTC4-like material in the parenchyma. The results demonstrate that indomethacin does not result in increased synthesis of LTs in the airways.     

Leukotriene D4 and platelet-activating factor-acether antagonists on allergic and arachidonic acid-induced reactions in guinea pig airways

Arachidonic acid (AA) and ovalbumin (OA) were used to induce contractions of sensitized guinea pig tracheal spiral (indomethacin-pretreated) and lung parenchymal strip preparations. This model was used to examine the properties of three leukotriene (LT) D4 antagonists and a platelet-activating factor (PAF)-acether receptor antagonist. The three LTD4 antagonists, L-649,923, FPL 57231, and LY163443, inhibited AA-induced contractions of indomethacin-pretreated tracheal spirals selectively. The PAF-acether antagonist, L-652,731, did not inhibit AA-induced contractions of either trachea or parenchyma. This confirmed that AA-induced contractions of trachea involved release and activity of LTD4. The LTD4 antagonists and L-652,731 partially inhibited OA-induced contractions of both trachea and parenchyma. When L-649,923 and L-652,731 or FPL 57231 and L-652,731 were combined, an additive inhibitory effect on OA-induced contractions was observed. When LY163443 and L-652,731 were combined, the inhibitory effect was synergistic. This may be due to the additional effect of LY163443 to inhibit phosphodiesterase. Total inhibition of OA-induced contractions was obtainable with relatively low concentrations when a LTD4 and PAF-acether antagonist were combined. These results suggested that LTD4 and PAF-acether may be the two major mediators in our model of allergic bronchospasm. The LTD4 and PAF-acether antagonists had the capacity to decrease baseline tone, even on tissues that were already relaxed with indomethacin, suggesting that LTD4 and PAF-acether may contribute to intrinsic tone in airway smooth muscle.     

Synchronization analysis of the uterine magnetic activity during contractions

Background  

Our objective was to quantify and compare the extent of synchronization of the spatial-temporal myometrial activity over the human uterus before and during a contraction using transabdominal magnetomyographic (MMG) recordings. Synchronization can be an important indicator for the quantification of uterine contractions.     

Neutral metalloendopeptidase associated with the smooth muscle cells of pregnant rat uterus

The pregnant rat uterus contains a membrane-bound metalloendopeptidase that is biochemically and immunologically similar to kidney enkephalinase (E.C.3.4.24.11). The uterus enzyme readily cleaved specific neutral endopeptidase substrates and oxytocin as well as the synthetic elastase substrate, Suc(Ala)3-pNA, yet did not digest native elastin. Using specific inhibitors, the uterus endopeptidase was identified as a metallopeptidase and not a serine protease, having an absolute requirement for zinc and perhaps calcium for maximal activity. The uterus endopeptidase cross-reacted with polyclonal antiserum to kidney microvillar endopeptidase and a monoclonal antibody to common acute lymphocytic leukemia antigen. Immunohistochemical localization of the enzyme in a 17 day pregnant uterus indicated that the enzyme was localized on the smooth muscle bundles of the myometrium and the endometrial epithelium. Total enzyme activity was 25 times higher in the late-term pregnant uterus (17th day of pregnancy) than in the nonpregnant uterus. Enzyme levels dropped rapidly prior to parturition and within 4 days after delivery the enzyme activity had returned to control levels. Inhibition of NEP in uterine strips with phosphoramidon resulted in a marked potentiation of oxytocin-induced contractions. Our results suggest that the uterine endopeptidase may have an important role in regulating uterine smooth muscle cell contraction during the later stages of pregnancy through its action on oxytocin and perhaps other biologically active peptides.     

Potassium channels and uterine function

Ion channels are effector proteins that mediate uterine excitability throughout gestation. This review will focus primarily on the role of potassium channels in regulating myometrial tone in pregnancy and labor contractions. During gestation, potassium channels maintain the uterus in a state of quiescence by contributing to the resting membrane potential and counteracting contractile stimuli. This review summarizes the current knowledge about the significance of the potassium channels in maintaining a normal gestational period and initiating labor contractions at term.     

Prostaglandin E2 receptor subtype EP-2 is not involved in the induction of non-pregnant guinea pig uterine contractions associated with terminal pregnancy

Prostaglandin E2 (PGE2) exerts its biological effects through 4 different receptor subtypes, EP-1, EP-2, EP-3, and EP-4. Recently we have demonstrated the importance of the prostaglandin E2 receptor subtype EP-2 in the healing of bone defects and fractures. This discovery led to the identification of CP-533,536, an EP-2 selective agonist, a promising therapeutic alternative for the enhancement of bone healing and the treatment of fractures (J Bone Miner Res 18 (2003) 2033). PGE2 has a myriad of effects throughout the body including the induction of uterine contractions, which results in termination of pregnancies. Our objective in this study was to determine the role of the EP-2 receptor and specifically that of CP-533,536, an EP-2 specific agonist, to induce uterine contractions and terminate pregnancy in guinea pigs, an animal model of human pregnancy. Preliminary experiments confirmed earlier reports that the guinea pig uterus was more sensitive than that of the rat. The guinea pig uterus contains the four PGE2 receptor subtypes, and ex vivo treatment of the uterus with PGE2 as expected causes profound uterine contractions. However, using receptor selective prostaglandin agonists including CP-533,536 we showed that the EP-1 and 3 receptors not the EP-2 receptor is responsible for the induction of uterine contractions of PGE2. Further, CP-533,536 did not antagonize the ability of PGE2 to induce uterine contractions in this model.     

Arachidonate lipoxygenase inhibitors in guinea-pig isolated trachea. Effects on contractions to antigen and various agonists

We compared the effects of the leukotriene (LT) D4 receptor antagonist FPL55712 and some lipoxygenase inhibitors on contractions of isolated guinea-pig trachea induced by antigen (ovalbumin, OA) and calcium ionophore A23187 in the presence of the cyclooxygenase inhibitor indomethacin (5 microM), and by arachidonic acid (AA), melittin and LTD4. FPL55712 (0.1 and 1 microM) inhibited contractions induced by AA (100 microM) and the phospholipase A2 activator melittin (3 micrograms/ml), while the lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA, 10 microM) was a more effective inhibitor of the melittin response than the AA response. FPL55712 inhibited contractions induced by OA (100 micrograms/ml) more than by A23187 (1 microgram/ml), and these inhibitory effects of FPL55712 were much less in the presence of l-serine-borate complex (45 mM), an inhibitor of LTC4 conversion to LTD4. NDGA (10 microM) had no significant effect on the OA response, whereas the lipoxygenase inhibitors 1-phenyl-3-pyrazolidone (phenidone, 10 microM) and 5,8,11,14-eicosatetraynoic acid (ETYA, 10 microM) clearly inhibited it. In contrast, NDGA and phenidone inhibited the A23187 response, but ETYA had no effect on it. FPL55712, phenidone and ETYA, but not NDGA, had a large inhibitory effect on LTD4-induced contractions, but these inhibitors had no effect on histamine-induced contractions. These results suggest that in the guinea-pig trachea inhibitors of LTD4-induced contractions decrease antigen-induced contractions, whereas lipoxygenase inhibitors reduce the contraction to A23187.     

Traditional Chinese medicine Bak Foong Pills alters uterine quiescence – Possible role in alleviation of dysmenorrhoeal symptoms

Since contractility of the uterus appears to be the major source of pain during dysmenorrhoea, alleviation of the contractions is believed to be a possible treatment strategy. Bak Foong Pills, a traditional Chinese formulation for use in gynaecological disorders, has long been thought as effective in the treatment of dysmenorrhoeal symptoms. The present study thus aims to investigate whether ethanol extract of Bak Foong Pills (BFP-Ex) or its constituent herbs may have direct effects on alleviating dysmenorrhoeal symptoms by altering uterine tone. This was investigated using isolated uterine preparations and intracellular messenger analysis of adenylate cyclase, via [³H]-adenine assay, and calcium, with fluorometry imaging, in myometrial cultures. BFP-Ex can stimulate uterine relaxation following oxytocin-induced contractions ex-vivo. Attempted inhibition of BFP-Ex's relaxatory response with a nitric oxide inhibitor and adenylate cyclase inhibitor, however, had no significant effect, suggesting that most of BFP-Ex's relaxatory response was not due to increases in NO or cAMP. Further studies on tetramethylpyrazine (TMP), a major active ingredient of BFP-Ex, indicated that TMP could modulate intracellular calcium levels in favour of uteri relaxation. The ability of Bak Foong Pills to alleviate menstrual pain may be due to direct regulation of uterine tone.     

Effect of indomethacin on airway contraction and the release of LTC4-like material

Ovalbumin (OA) and arachidonic acid (AA) were used to induce contractions of sensitized guinea-pig tracheal and lung preparations in the presence and absence of indomethacin. Leukotriene (LT)C₄-like material released from these tissues was extracted from the bathing fluid and measured by radioimmunoassay. Challenge with either OA or AA induced release of LTC₄-like material from both parenchyma and trachea, AA inducing a greater release than OA although OA induced greater contractions. This suggested that OA-induced the synthesis of other bronchoconstrictor compounds than LTC₄. Although indomethacin enhanced OA- and AA-induced contractions of trachea, there was no enhancement of the release of LTC₄-like material, suggesting enhancement by indomethacin was a result of the inhibition of the synthesis of prostaglandin E₂ and not diversion of AA into the lipoxygenase pathway. Indomethacin had no effect on OA-induced contractions of parenchyma, but attenuated those induced by AA. Indomethacin had no modulatory effect on the release of LTC₄-like material in the parenchyma. The results demonstrate that indomethacin does not result in increased synthesis of LTs in the airways.     

Role of cyclooxygenase activation and prostaglandins in antigen-induced excitability changes of bronchial parasympathetic ganglia neurons

In vitro antigen challenge has multiple effects on the excitability of guinea pig bronchial parasympathetic ganglion neurons, including depolarization, causing phasic neurons to fire with a repetitive action potential pattern and potentiating synaptic transmission. In the present study, guinea pigs were passively sensitized to the antigen ovalbumin. After sensitization, the bronchi were prepared for in vitro electrophysiological intracellular recording of parasympathetic ganglia neurons to investigate the contribution of cyclooxygenase activation and prostanoids on parasympathetic nerve activity. Cyclooxygenase inhibition with either indomethacin or piroxicam before in vitro antigen challenge blocked the change in accommodation. These cyclooxygenase inhibitors also blocked the release of prostaglandin D(2) (PGD(2)) from bronchial tissue during antigen challenge. We also determined that PGE(2) and PGD(2) decreased the duration of the action potential after hyperpolarization, whereas PGF(2alpha) potentiated synaptic transmission. Thus prostaglandins released during antigen challenge have multiple effects on the excitability of guinea pig bronchial parasympathetic ganglia neurons, which may consequently affect the output from these neurons and thereby alter parasympathetic tone in the lower airways.     

Hypothyroid state reduces calcium channel function in 18-day pregnant rat uterus

Hypothyroidism significantly reduced the mean amplitude and increased the mean frequency of spontaneous rhythmic contractions in 18 day pregnant rat uterus. Nifedipine (10(-12)-10(-9) M) and diltiazem (10(-10)-10(-6) M) caused concentration related inhibition of the myogenic responses of the uterine strips obtained from both pregnant and hypothyroid state. However, nifedipine was less potent (IC50:2.11 x 10(-11) M) in pregnant hypothyroid state as compared to pregnant control (IC50: 3.1 x 10(-12) M). Similarly, diltiazem was less potent (IC50: 3.72 x 10(-9) M) in inhibiting the uterine spontaneous contractions in hypothyroid than in pregnant rat uterus (IC50:5.37 x 10(-10) M). A similar decrease in the sensitivity to nifedipine and diltiazem for reversal of K+ (100 mM)-induced tonic contraction and K(+)-stimulated 45Ca2+ influx was observed with these calcium channel antagonists in uterus obtained from hypothyroid pregnant rats compared to the controls. Nifedipine-sensitive influx of 45Ca(2+)-stimulated either by K+ (100 mM) or by Bay K8644 (1,4-dihydro-2,6-methyl-5-nitro-4-[2'-(trifluromethyl)phenyl]-3-pyridine carboxylic acid methyl ester) (10(-9) M) was significantly less in uterine strips from hypothyroid rats compared to controls. The results suggest that the inhibition of uterine rhythmic contractions may be attributable to a reduction in rat myometrial Ca2+ channel function in the hypothyroid state.     

The cyclic-ADP-ribose signaling pathway in human myometrium

Human myometrial contraction plays a fundamental role in labor. Dysfunction of uterine contraction is an important cause of failure in progression of labor. The mechanisms of control of uterine contractions are not completely understood. It appears that intracellular Ca(2+) mobilization may play an important role during uterine contraction. Several mechanisms of intracellular Ca(2+) mobilization have been described. However, in human uterus only the inositol 1,4,5-trisphosphate-induced Ca(2+) release has been extensively studied to date. In view of the identification of the presence of functional ryanodine channels in myometrium, we explored the role of the endogenous regulator of the ryanodine channel cyclic-ADP-ribose in human myometrial Ca(2+) regulation. Cyclic-ADP-ribose (cADPR) is a naturally occurring nucleotide implicated in the regulation of the gating properties of the ryanodine channel, in fact cADPR may be a second messenger that activates the ryanodine receptor. Here we explore the components of the cADPR system in human myometrium. We found that human myometrium contains all the components of the cADPR pathway including (1) cADPR-activated microsomal Ca(2+) release and (2) enzymes responsible for synthesis and degradation of cADPR and, furthermore, that intracellular levels of cADPR were detected in human myometrial tissue. These data indicate that the cADPR system is present and operational in human myometrial tissue. Further research is warranted to determine the role of this new signaling molecule in uterine contraction.     

In vitro contractile effect of platelet-activating factor on guinea-pig myometrium

Platelet-activating factor (PAF) evoked myometrial contractions in two different patterns, depending on whether spontaneous activity was present. In spontaneously active myometrial strips (58%), both PAF and oxytocin enhanced the amplitude of myometrial contractions. In quiescent myometrial strips, PAF induced contractions characterized by a prompt development of tension, a plateau, and a final, rapid relaxation. In 54% of these strips, PAF-induced contraction was followed by rhythmic activity. PAF contractile response was dependent upon the concentration (0.1-100 nM); the minimal effective concentration of PAF was 0.1 nM and the EC50 was 1 nM. The response to oxytocin (0.01-10 mU/ml), assumed as reference stimulus, was characterized by a prompt development of tension, which was followed by a sustained, slow contraction and relaxation. PAF response was almost completely dependent on cyclooxygenase and partially on lipoxygenase pathways, as inferred from studies with indomethacin and FPL 55712, respectively. A receptor mediated mechanism of PAF action was suggested by specific desensitization of the myometrium to a second challenge with an equimolar concentration of PAF (but not with oxytocin) and the blocking effect of CV 3988, a specific PAF receptor antagonist.     

Repeated antigen challenge induces airway hyperresponsiveness with tissue eosinophilia in guinea pigs

To test the hypothesis that the development of airway hyperresponsiveness (AHR) lasting greater than or equal to 3 days after the last antigenic exposure required repeated mediator release, we compared dose-response changes in lung resistance (RL) to acetylcholine (ACh) in animals sensitized with 1% ovalbumin (OA), 4% Bordatella pertussis aerosol and subsequently challenged with 0.5% OA aerosol twice weekly for 4-6 wk vs. animals receiving saline aerosol instead of OA. Despite antihistamine pretreatment, each OA challenge produced cyanosis and inspiratory indrawing. Blood gas analysis in six guinea pigs revealed an immediate fall in arterial PO2 (PaO2) from 104.3 +/- 4.9 to 35.4 +/- 2.2 Torr after a 1-min exposure to aerosolized OA. ACh dose-response measurements of RL 3 days after the last OA challenge demonstrated a leftward shift and an increased magnitude of response. These differences were less marked at 7 days, and by 14 days after the last OA challenge, ACh dose-response curves were not different from those of control guinea pigs. Sensitization without repeated antigen challenge did not cause hyperresponsiveness. Morphometric analysis showed significantly increased numbers of eosinophils in the epithelium of airways in hyperresponsive guinea pigs, without neutrophil infiltration or alterations in epithelium and airway wall areas. We conclude that repeated antigenic challenge, but not sensitization alone, causes prolonged AHR in guinea pigs, which is associated with tissue eosinophilia.     

Adrenergic influence of uterine muscle contractions stimulated by a glycoside from the root of Dalbergia saxatilis

The mechanism of uterine muscle contraction stimulated by a triterpenoid glycoside (dalsaxin) isolated from the root of D. saxatilis was investigated by in vitro methods in the rat. Dalsaxin caused a dose-related increase in uterine muscle contraction. The contraction was single and transient and was abolished by moderate doses of isoprenaline (1.80 nmol-0.40 mumol) and salbutamol (0.13-25 mumol). Adrenaline (9.10 nmol) also caused a reversible decrease (92.6%; P < 0.01) in myometrial contraction stimulated by this glycoside (0.24 mg/ml). Uterine muscle responses to dalsaxin (0.24 mg/ml) were enhanced by the beta-adrenergic receptor antagonist, propranolol, in a dose related manner. Atipamezole (1.50 ng/ml) but not prazosin (7.72 nmol-15.60 nmol) substantially reduced (80%; P < 0.01) myometrial contractions induced by this uterine spasmogen. The results suggest that dalsaxin enhances uterine muscle contraction by stimulating post junctional alpha 2-adrenergic receptors, presumably by inhibiting plasma membrane adenylate cyclase system and its associated increase in intracellular cAMP content.     

Sustained inhibition of rat myometrial gap junctions and contractions by lindane

Background  

Gap junctions increase in size and abundance coincident with parturition, forming an intercellular communication network that permits the uterus to develop the forceful, coordinated contractions necessary for delivery of the fetus. Lindane, a pesticide used in the human and veterinary treatment of scabies and lice as well as in agricultural applications, inhibits uterine contractions in vitro, inhibits myometrial gap junctions, and has been associated with prolonged gestation length in rats. The aim of the present study was to investigate whether brief exposures to lindane would elicit sustained inhibition of rat uterine contractile activity and myometrial gap junction intercellular communication.