Increased sensitivity of rat myometrium to the contractile effect of platelet activating factor before delivery

Platelet-activating factor (PAF) enhances guinea pig detrusor smooth muscle contractile activities by stimulating voltage-dependent Ca2+ channels and store-operated Ca2+ channels

We investigated the extracellular Ca²⁺ influx pathways involved in platelet-activating factor (PAF)-enhanced guinea pig detrusor smooth muscle (DSM) contractile activities. One micromolar PAF-enhanced DSM contractile activities were completely inhibited by extracellular Ca²⁺ removal and strongly suppressed by voltage-dependent Ca²⁺ channel (VDCC) inhibitors. PAF-enhanced DSM contractile activities remaining in the presence of verapamil (10 μM) were not inhibited by LOE-908 (30 μM, an inhibitor of receptor-operated Ca²⁺ channels (ROCCs)), but were almost completely inhibited by SKF-96365 (30 μM, an inhibitor of store-operated Ca²⁺ channels (SOCCs) and ROCCs). These results suggest that VDCCs and SOCCs are responsible for PAF-enhanced DSM contractile activities.

Pharmacological Studies on the Ca2+ Influx Pathways in Platelet-Activating Factor (PAF)-Induced Mouse Urinary Bladder Smooth Muscle Contraction

Platelet-activating factor (PAF) not only acts as a mediator of platelet aggregation, inflammation, and allergy responses but also as a constrictor of various smooth muscle (SM) tissues, including gastrointestinal, tracheal/bronchial, and pregnancy uterine SMs. Previously, we reported that PAF induces basal tension increase (BTI) and oscillatory contraction (OC) in mouse urinary bladder SM (UBSM). In this study, we examined the Ca²⁺ influx pathways involved in PAF-induced BTI and OC in the mouse UBSM. PAF (10^-6 M) induced BTI and OC in mouse UBSM. However, the PAF-induced BTI and OC were completely suppressed by extracellular Ca²⁺ removal. PAF-induced BTI and OC frequencies were markedly suppressed by voltage-dependent Ca²⁺ channel (VDCC) inhibitors (verapamil (10^-5 M), diltiazem (10^-5 M), and nifedipine (10^-7 M)). However, these VDCC inhibitors had a minor effect on the PAF-induced OC amplitude. The PAF-induced OC amplitude in the presence of verapamil (10^-5 M) was strongly suppressed by SKF-96365 (3 × 10^-5 M), an inhibitor of receptor-operated Ca²⁺ channel (ROCC) and store-operated Ca²⁺ channel (SOCC), but not by LOE-908 (3 × 10^-5 M) (an inhibitor of ROCC). Overall, PAF-induced BTI and OC in mouse UBSM depend on Ca²⁺ influx and the main Ca²⁺ influx pathways in PAF-induced BTI and OC may be VDCC and SOCC. Of note, VDCC may be involved in PAF-induced BTI and OC frequency, and SOCC might be involved in PAF-induced OC amplitude.

Platelet-activating factor (PAF) strongly enhances contractile mechanical activities in guinea pig and mouse urinary bladder

In this study, we investigated the effects of platelet-activating factor (PAF) on the basal tone and spontaneous contractile activities of guinea pig (GP) and mouse urinary bladder (UB) smooth muscle (UBSM) tissues to determine whether PAF could induce UBSM tissue contraction. In addition, we examined the mRNA expression of the PAF receptor, PAF-synthesizing enzyme (lysophosphatidylcholine acyltransferase, LPCAT), and PAF-degrading enzyme (PAF acetylhydrolase, PAF-AH) in GP and mouse UB tissues using RT-qPCR. PAF (10^-9-10^-6 M) strongly enhanced the basal tone and spontaneous contractile activities (amplitude and frequency) of GP and mouse UBSM tissues in a concentration-dependent manner. The enhancing effects of PAF (10^-6 M) on both GP and mouse UBSM contractile activities were strongly suppressed by pretreatment with apafant (a PAF receptor antagonist, GP: 10^-5 M; mouse: 3 × 10^-5 M). The PAF receptor (Ptafr), LPCAT (Lpcat1, Lpcat2), and PAF-AH (Pafah1b3, Pafah2) mRNAs were detected in GP and mouse UB tissues. These findings reveal that PAF strongly enhances the contractile mechanical activities of UBSM tissues through its receptor and suggest that the PAF-synthesizing and -degrading system exists in UBSM tissues. PAF may serve as both an endogenous UBSM constrictor and an endogenous mediator leading to detrusor overactivity.

Targeting Toll-like receptor-4 to tackle preterm birth and fetal inflammatory injury

Every year, 15 million pregnancies end prematurely, resulting in more than 1 million infant deaths and long-term health consequences for many children. The physiological processes of labour and birth involve essential roles for immune cells and pro-inflammatory cytokines in gestational tissues. There is compelling evidence that the mechanisms underlying spontaneous preterm birth are initiated when a premature and excessive inflammatory response is triggered by infection or other causes. Exposure to pro-inflammatory mediators is emerging as a major factor in the 'fetal inflammatory response syndrome' that often accompanies preterm birth, where unscheduled effects in fetal tissues interfere with normal development and predispose to neonatal morbidity. Toll-like receptors (TLRs) are critical upstream gatekeepers of inflammatory activation. TLR4 is prominently involved through its ability to sense and integrate signals from a range of microbial and endogenous triggers to provoke and perpetuate inflammation. Preclinical studies have identified TLR4 as an attractive pharmacological target to promote uterine quiescence and protect the fetus from inflammatory injury. Novel small-molecule inhibitors of TLR4 signalling, specifically the non-opioid receptor antagonists (+)-naloxone and (+)-naltrexone, are proving highly effective in animal models for preventing preterm birth induced by bacterial mimetic LPS, heat-killed Escherichia coli, or the TLR4-dependent pro-inflammatory lipid, platelet-activating factor (PAF). Here, we summarise the rationale for targeting TLR4 as a master regulator of inflammation in fetal and gestational tissues, and the potential utility of TLR4 antagonists as candidates for preventative and therapeutic application in preterm delivery and fetal inflammatory injury.

Toll-Like Receptor-4 Antagonist (+)-Naltrexone Protects Against Carbamyl-Platelet Activating Factor (cPAF)-Induced Preterm Labor in Mice

Spontaneous preterm labor is frequently caused by an inflammatory response in the gestational tissues elicited by either infectious or sterile agents. In sterile preterm labor, the key regulators of inflammation are not identified, but platelet-activating factor (PAF) is implicated as a potential rate-limiting effector agent. Since Toll-like receptor (TLR)-4 can amplify PAF signaling, we evaluated whether TLR4 contributes to inflammation and fetal loss in a mouse model of PAF-induced sterile preterm labor, and whether a small-molecule TLR4 inhibitor, (+)-naltrexone, can mitigate adverse PAF-induced effects. The administration of carbamyl (c)-PAF caused preterm labor and fetal loss in wild-type mice but not in TLR4-deficient mice. Treatment with (+)-naltrexone prevented preterm delivery and alleviated fetal demise in utero elicited after cPAF administered by i.p. or intrauterine routes. Pups born after cPAF and (+)-naltrexone treatment exhibited comparable rates of postnatal survival and growth to carrier-treated controls. (+)-Naltrexone suppressed the cPAF-induced expression of inflammatory cytokine genes Il1b, Il6, and Il10 in the decidua; Il6, Il12b, and Il10 in the myometrium; and Il1b and Il6 in the placenta. These data demonstrate that the TLR4 antagonist (+)-naltrexone inhibits the inflammatory cascade induced by cPAF, preventing preterm birth and perinatal death. The inhibition of TLR4 signaling warrants further investigation as a candidate strategy for fetal protection and delay of preterm birth elicited by sterile stimuli.

Multifactorial Regulation of Myometrial Contractility During Pregnancy and Parturition

The steroid hormones progesterone (P₄) and estradiol-17β (E₂), produced by the placenta in humans and the ovaries in rodents, serve crucial roles in the maintenance of pregnancy, and the initiation of parturition. Because of their critical importance for species survival, the mechanisms whereby P₄ and its nuclear receptor (PR) maintain myometrial quiescence during pregnancy, and for the decline in P₄/PR and increase in E₂/estrogen receptor (ER) function leading to parturition, are multifaceted, cooperative, and redundant. These actions of P₄/PR include: (1) PR interaction with proinflammatory transcription factors, nuclear factor κB (NF-κB), and activating protein 1 (AP-1) bound to promoters of proinflammatory and contractile/contraction-associated protein (CAP) genes and recruitment of corepressors to inhibit NF-κB and AP-1 activation of gene expression; (2) upregulation of inhibitors of proinflammatory transcription factor activation (IκBα, MKP-1); (3) induction of transcriptional repressors of CAP genes (e.g., ZEB1). In rodents and most other mammals, circulating maternal P₄ levels remain elevated throughout most of pregnancy and decline precipitously near term. By contrast, in humans, circulating P₄ levels and myometrial PR levels remain elevated throughout pregnancy and into labor. However, even in rodents, wherein P₄ levels decline near term, P₄ levels remain higher than the K_d for PR binding. Thus, parturition is initiated in all species by a series of molecular events that antagonize the P₄/PR maintenance of uterine quiescence. These events include: direct interaction of inflammatory transcription factors (e.g., NF-κB, AP-1) with PR; increased expression of P₄ metabolizing enzymes; increased expression of truncated/inhibitory PR isoforms; altered expression of PR coactivators and corepressors. This article will review various mechanisms whereby P₄ acting through PR isoforms maintains myometrial quiescence during pregnancy as well as those that underlie the decline in PR function leading to labor. The roles of P₄- and E₂-regulated miRNAs in the regulation and integration of these mechanisms will also be considered.

Fetal-to-maternal signaling in the timing of birth

Preterm birth remains the major cause of neonatal morbidity and mortality throughout the world. This is due, in part, to our incomplete understanding of the mechanisms that underlie the maintenance of pregnancy and the initiation of parturition at term. In this article, we review our current knowledge of the complex, interrelated and concerted mechanisms whereby progesterone maintains myometrial quiescence throughout most of pregnancy, as well as those that mediate the upregulation of the inflammatory response and decline in progesterone receptor function leading to parturition. Herein, we review findings that demonstrate a role of the fetus in the timing of birth. Specifically, we focus on our own studies indicating that maturation of the fetal lung and enhanced secretion of the surfactant components, surfactant protein A (SP-A) and the potent inflammatory glycerophospholipid, platelet-activating factor (PAF), initiate a signaling cascade culminating in parturition. Our studies suggest an essential role of steroid receptor coactivators, SRC-1 and SRC-2, which activate expression of genes encoding SP-A and LPCAT1. LPCAT1 is a key enzyme in the synthesis of PAF, as well as DPPC, a highly surface-active glycerophospholipid component of surfactant. Thus, we describe a novel pathway through which the fetus contributes to the initiation of labor by signaling the mother when its lungs have achieved sufficient maturity for survival in an aerobic environment.

Steroid receptor coactivators 1 and 2 mediate fetal-to-maternal signaling that initiates parturition

The precise mechanisms that lead to parturition are incompletely defined. Surfactant protein-A (SP-A), which is secreted by fetal lungs into amniotic fluid (AF) near term, likely provides a signal for parturition; however, SP-A-deficient mice have only a relatively modest delay (~12 hours) in parturition, suggesting additional factors. Here, we evaluated the contribution of steroid receptor coactivators 1 and 2 (SRC-1 and SRC-2), which upregulate SP-A transcription, to the parturition process. As mice lacking both SRC-1 and SRC-2 die at birth due to respiratory distress, we crossed double-heterozygous males and females. Parturition was severely delayed (~38 hours) in heterozygous dams harboring SRC-1/-2-deficient embryos. These mothers exhibited decreased myometrial NF-κB activation, PGF2α, and expression of contraction-associated genes; impaired luteolysis; and elevated circulating progesterone. These manifestations also occurred in WT females bearing SRC-1/-2 double-deficient embryos, indicating that a fetal-specific defect delayed labor. SP-A, as well as the enzyme lysophosphatidylcholine acyltransferase-1 (LPCAT1), required for synthesis of surfactant dipalmitoylphosphatidylcholine, and the proinflammatory glycerophospholipid platelet-activating factor (PAF) were markedly reduced in SRC-1/-2-deficient fetal lungs near term. Injection of PAF or SP-A into AF at 17.5 days post coitum enhanced uterine NF-κB activation and contractile gene expression, promoted luteolysis, and rescued delayed parturition in SRC-1/-2-deficient embryo-bearing dams. These findings reveal that fetal lungs produce signals to initiate labor when mature and that SRC-1/-2-dependent production of SP-A and PAF is crucial for this process.

The role of platelet-activating factor in the mammalian female reproductive tract

Platelet-activating factor (PAF) is a potent lipid mediator produced by various cell types of mammals and is involved in an inflammatory-like process with increased vascular permeability. Platelet-activating factor exerts its actions through the activation of specific PAF receptors (PAF-R) found in cells and tissues of the female reproductive tract. The aim of this article was summarized briefly in the current research on the role of PAF in female reproductive functions. Platelet-activating factor has been implicated in processes of ovulation, implantation and parturition because of its angiogenic and growth factor properties. This factor is influenced by ovarian steroid hormones in bringing about changes in the uterus and is a candidate molecule for initial embryo-maternal dialogue. Tissue concentrations of PAF are regulated by the equilibrium between biosynthesis and degradation by PAF-acetylhydrolase (PAF-AH). Antagonists of PAF interfere with ovulation and implantation. Platelet-activating factor, its receptor, and PAF-AH activity play an important role in the maintenance of pregnancy.

Role of protein kinase Cbeta in rhythmic contractions of human pregnant myometrium

To assess the role of protein kinase Cbeta (PKCbeta) in human myometrial contractions during pregnancy, we evaluated the effect of a PKCbeta inhibitor (LY333531) on the pregnant and nonpregnant myometrial contractions and compared the level of PKCbeta in the pregnant myometrium with that in the nonpregnant myometrium. The effects of LY333531 on the myometrial contractions were examined by measuring contractile activity (frequency and amplitude). PKCbeta in human myometrium was assessed at mRNA level using real-time PCR method. The characteristics of contractile activity were different between the pregnant and the nonpregnant myometrium. The amplitude of rhythmic contractions in the preterm and term myometrium was increased 2- to 2.5-fold when compared with that in the nonpregnant myometrium, but the frequency of rhythmic contractions was decreased by about half. LY333531 (10(-6) M) reduced the increased amplitude in the preterm and term myometrium by about 50%, and the inhibitory effects of LY333531 in the pregnant myometrium were significantly greater than that in the nonpregnant myometrium (about 50 vs 25%). However, the frequency in the pregnant and nonpregnant myometrium was not influenced by LY333531. Real-time PCR revealed a significant, five- to sevenfold increase in the expression of PKCbeta mRNA in the preterm and term myometrium when compared with the nonpregnant myometrium. These findings suggest that the increased amplitude of human myometrial contractions during pregnancy is related to the increased level of PKCbeta. A PKCbeta inhibitor may reduce preterm uterine contractions and prevent preterm delivery.

Prostaglandins and the myometrium and cervix

Prostaglandins have long been thought to play important roles in the mechanism of parturition. Here we review the involvement of prostaglandins in myometrial and cervical functions with emphasis on human labor and birth. In addition, the cellular sources of prostaglandins as well as their interactions with various other endocrine, paracrine and physical factors, such as oxytocin, corticotropin releasing hormone, nitric oxide, platelet activating factor, cytokines, endothelin and stretch are also addressed together with their potential role in the molecular reorganization of cervical structure associated with labor and delivery. Finally, the premier role of progesterone in pregnancy maintenance and parturition is juxtaposed with the proposed "fine-tuning", modulatory role of prostaglandins and the above listed factors in the regulation of parturition.

A new model for inflammation-induced preterm birth: the role of platelet-activating factor and Toll-like receptor-4

Preterm birth is a leading cause of neonatal morbidity and mortality. Despite a growing body of evidence correlating inflammation with preterm birth, the signal transduction pathways responsible for the emptying of the uterus in the setting of intrauterine inflammation has not been elucidated. We now report a unique, reproducible mouse model of localized intrauterine inflammation. This model results in 100% preterm delivery with no maternal mortality. Using our model, we also show that platelet-activating factor is a crucial mediator of both inflammation-induced preterm birth and fetal demise. Using C3H/HeJ mice, we demonstrate that toll-like receptor-4 (TLR-4) plays a role in lipopolysaccharide-induced preterm birth but not in inflammation-induced fetal death. Immunohistochemistry studies demonstrate the presence of the platelet-activating factor receptor in both endometrial glands and smooth muscle in uterine tissues. Molecular studies demonstrate the differential expression of platelet-activating factor receptor and TLR-4 in uterine and cervical tissue throughout gestation. Quantitative polymerase chain reaction revealed an up-regulation of TLR-4 in the fundal region of the uterus in response to intrauterine inflammation. The use of this model will increase our understanding of the significant clinical problem of inflammation-induced preterm birth and will elucidate signal transduction pathways involved in an inflammatory state.

Possible role of the protein kinase C/CPI-17 pathway in the augmented contraction of human myometrium after gestation

1. Activation of protein kinase C (PKC) by phorbol 12,13-dibutylate (PDBu, 1 microm) induced sustained contractions with no increase in [Ca2+]i in nonpregnant and pregnant human myometria. The contractile effects of PDBu in pregnant myometrium were much greater than those in nonpregnant myometrium, and the contractions in pregnant myometrium were accompanied by an increase in myosin light chain (MLC) phosphorylation at Ser19. 2. The contraction induced by PDBu in pregnant myometrium was inhibited by the inhibitors of conventional PKC isoforms, bisindolylmaleimides and indolocarbazole, such as Go6976, Go6983, and Go6850 (1 microM). LY333531 (1 microM), a specific inhibitor of PKC beta, also inhibited the PDBu-induced contraction in the pregnant myometrium. 3. In the pregnant myometrium permeabilized with alpha-toxin, PDBu increased the contractions induced at fixed Ca2+ concentration (0.3 microM) both in nonpregnant and pregnant myometria, indicating Ca2+ sensitization of contractile elements. 4. Western immunoblot analysis indicated that pregnant myometrium contained PKC isozymes such as conventional PKC (alpha, beta, gamma), novel PKC (delta, epsilon, theta), and atypical PKC (zeta but not iota and lambda). RT-PCR and real-time RT-PCR analysis indicated that, among the conventional PKC, the levels of mRNA of beta isoform in pregnant human myometrium were greater than those in nonpregnant myometrium. 5. CPI-17 is a substrate for PKC, and the phosphorylated CPI-17 is considered to inhibit myosin phosphatase. The levels of CPI-17 mRNA and protein expression were also greater in the pregnant myometrium. 6. These results suggest that the PKC-mediated contractile mechanism is augmented in human myometrium after gestation, and that this augmentation may be attributable to the increased activity of the beta PKC isoform and CPI-17.

Effects of L-arginine on spontaneous contraction of the rat portal vein

The effects of L-arginine on spontaneous contraction of endothelium-denuded longitudinal preparations of the rat portal vein were studied. L-arginine increased the frequency of spontaneous contraction concentration-dependently between 10 microM and 1 mM. Changes in contraction amplitude and duration were not remarkable. D-arginine had a negligible effect on spontaneous contraction. N(omega)-nitro-L-arginine (1 mM) did not affect spontaneous contraction or the response to L-arginine. Addition of N(G)-monomethyl-L-arginine (1 mM), l-lysine (1 mM) or N-ethymaleimide (0.1 mM) increased the frequency of spontaneous contractions and inhibited the effect of L-arginine. Glibenclamide (10 microM) did not affect spontaneous contraction or the response to L-arginine. Spontaneous increase in concentration of intracellular Ca2+, estimated as the ratio of Fura-PE3 fluorescence occurred synchronously with spontaneous contraction. Spontaneous increase in concentration of intracellular Ca2+ occurred more frequently in the presence of L-arginine (1 mM). L-arginine (1 mM) also increased the number of action potential bursts/min in the longitudinal smooth muscle layer. L-arginine (1 mM) also depolarized cell membranes. This study indicates that L-arginine increases the frequency of spontaneous contraction of longitudinal muscle in the rat portal vein by membrane depolarization through mechanisms that do not involve nitric oxide or the inhibition of ATP-sensitive K+ channels.

Platelet-activating factor acetylhydrolase isoforms I and II in human uterus. Comparisons with pregnant uterus and myoma

The concentrations of platelet-activating factor (PAF) that possesses the ability to stimulate myometrial contraction are partially regulated by intracellular type of platelet-activating factor acetylhydrolase (PAF-AH) in many tissues. Tissue cytosol contains at least two intracellular PAF-AH, isoforms I and II. To examine the relationship between the activity and isoforms of intracellular PAF-AH in human uterine myometrium and myoma, we assayed the PAF-AH activity and identified the PAF-AH isoforms I and II by Western blot analysis. The intense bands of the alpha2 and ss subunits of PAF-AH isoform I were detected in nonpregnant uterus; however, the specific bands of the alpha1 subunit of PAF-AH isoform I and the PAF-AH isoform II were extremely weak. The levels of the alpha2 and ss subunits and PAF-AH activity in pregnant uterus (37-39 wk gestation) were significantly lower than those in nonpregnant uterus. On the other hand, the level of ss subunit and the PAF-AH activity in myoma were significantly higher than those in nonpregnant uterus. No significant difference was found in the expression of the PAF-AH isoform II among three tissues. These results indicate that the change in the PAF-AH activity observed in pregnant uterus and myoma are due to the lower or higher protein expression of the PAF-AH isoform I, especially the alpha2 and/or ss subunits. The decrease of the uterine PAF-AH activity in the late stage of pregnancy may facilitate the action of PAF to stimulate myometrial contraction.

Platelet-activating factor-induced intracellular calcium waves in human uterine myometrial cells

OBJECTIVE

We visualized and investigated the intracellular calcium waves propagated by platelet-activating factor (PAF) in cultured human myometrial cells.

STUDY DESIGN

Myometrial cells were stimulated with PAF ranging between 10(-8) and 10(-15) M. For the observation of calcium waves, calcium green-1 and a confocal laser microscopy were used. Cells were also stimulated with 10(-9) M of PAF in a calcium-free solution.

RESULTS

In physiological solution, PAF at concentrations ranging between 10(-9) and 10(-15) M induced intracellular calcium waves. Mean wave speed was 16.1+/-5.6 microm/s. Wave speeds were independent of the PAF concentration. Similar results were observed in the absence of added calcium, with the exception that the wave speeds were significantly slower (7.3+/-3.3 microm/s). CONCLUSIONS This is the first study to demonstrate the calcium waves propagated by PAF stimulation in human myometrial cells. These observations further support the proposed role of PAF in parturition.

Cellular mechanisms involved in iso-osmotic high K+ solutions-induced contraction of the estrogen-primed rat myometrium

The aim of the present study was to investigate the mechanisms involved in the contraction evoked by iso-osmotic high K+ solutions in the estrogen-primed rat uterus. In Ca2+-containing solution, iso-osmotic addition of KCl (30, 60 or 90 mM K+) induced a rapid, phasic contraction followed by a prolonged sustained plateau (tonic component) of smaller amplitude. The KCl (60 mM)-induced contraction was unaffected by tetrodotoxin (3 microM), omega-conotoxin MVIIC (1 microM), GF 109203X (1 microM) or calphostin C (3 microM) but was markedly reduced by tissue treatment with neomycin (1 mM), mepacrine (10 microM) or U-73122 (10 microM). Nifedipine (0.01-0.1 microM) was significantly more effective as an inhibitor of the tonic component than of the phasic component. After 60 min incubation in Ca2+-free solution containing 3 mM EGTA, iso-osmotic KCl did not cause any increase in tension but potentiated contractions evoked by oxytocin (1 microM), sodium orthovanadate (160 micrM) or okadaic acid (20 microM) in these experimental conditions. In freshly dispersed myometrial cells maintained in Ca2+-containing solution and loaded with indo 1, iso-osmotic KCl (60 mM) caused a biphasic increase in the intracellular Ca2+ concentration ([Ca2+]i). In cells superfused for 60 min in Ca2+-free solution containing EGTA (1 mM), KCl did not increase [Ca2+]i. In Ca2+-containing solution, KCl (60 mM) produced a 76.0 +/- 16.2% increase in total [3H]inositol phosphates above basal levels and increased the intracellular levels of free arachidonic acid. These results suggest that, in the estrogen-primed rat uterus, iso-osmotic high K+ solutions, in addition to their well known effect on Ca2+ influx, activate other cellular processes leading to an increase in the Ca2+ sensitivity of the contractile machinery by a mechanism independent of extracellular Ca2+.

Platelet-activating factor (PAF) receptor and genetically engineered PAF receptor mutant mice

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was initially recognized for its potential to induce platelet aggregation and secretion, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues, many of which also produce the molecule. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor. PAF receptor is linked to intracellular signal transduction pathways, including turnover of phosphatidylinositol, elevation in intracellular calcium concentration, and activation of kinases, resulting in versatile bioactions. On the basis of numerous pharmacological reports, PAF is thought to have many pathophysiological and physiological functions. Recently advanced molecular technics enable us not only to clone PAF receptor cDNAs and genes, but also generate PAF receptor mutant animals, i.e., PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice gave us a novel and specific approach for identifying the pathophysiological and physiological functions of PAF. This review also describes the phenotypes of these mutant mice and discusses them by referring to previously reported pharmacological and genetical data.

Effects of cyclopiazonic acid on contraction and intracellular Ca2+ in oesophageal striated muscle of normotensive and spontaneously hypertensive rats

1. The effects of cyclopiazonic acid (CPA), a selective inhibitor of sarcoplasmic reticulum (SR) Ca2+-ATPase, on twitch contraction and on the resting state of tension and intracellular Ca2+ level ([Ca2+]i) of the oesophageal striated muscle of stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar Kyoto rats (WKY) were compared. 2. CPA (10 micronM) augmented the twitch contraction of oesophageal striated muscle preparations from both SHRSP and WKY, reducing the rate of relaxation (-dT/dt), and thus resulting in the prolongation of the time to 80% relaxation. The effect was significantly smaller in the SHRSP preparations. 3. In the resting state, CPA caused a sustained elevation of [Ca2+]i. The elevation was greater in the WKY preparations. Tension development accompanied by the elevation was observed in WKY preparations, but not in SHRSP preparations. 4. The sustained elevation of [Ca2+]i induced by CPA was eliminated by the removal of extracellular Ca2+. Both the elevated [Ca2+]i and tension in the preparations from WKY were reduced by flufenamic acid (100 micronM), mefenamic acid (100 micronM), lanthanum (La3+, 100 micronM), gadolinium (Gd3+, 100 micronM) and SK&F 96365 (100 micronM) but not by verapamil (10 micronM). 5. Thapsigargin (3 micronM), another SR Ca2+-ATPase inhibitor, produced similar effects on basal tension to those of CPA, although it reduced the amplitude of twitch contraction. 6. These results suggest that in the rat oesophageal striated muscle, (1) CPA extends the sequestrating time of Ca2+ into the SR, (2) CPA induces a Ca2+ influx mediated through verapamil-insensitive pathways, possibly nonselective cation channels, and (3) the mechanism of [Ca2+](i) modulation due to CPA-sensitive SR Ca2+-ATPase is deteriorated in the oesophageal striated muscle from SHRSP as compared with WKY preparations.

Estrogen effects on platelet-activating factor and platelet-activating factor acetylhydrolase activity in rat uterus during the late stages of pregnancy

The platelet-activating factor (PAF) concentration of the uterus spontaneously increased during pregnancy. When 17alpha-ethynylestradiol (0.25 mg/kg) was administered subcutaneously to pregnant rats for 3 days starting on Day 17 of pregnancy, some rats delivered prematurely on Day 20. However, none of the vehicle-treated (80% dimethylsulfoxide and 20% ethanol) pregnant rats delivered prematurely. The PAF concentration of the uterus in pregnant rats treated with 17alpha-ethynylestradiol was significantly higher than in those treated with vehicle on Days 19 and 20. On the other hand, the specific activity of uterine PAF-acetylhydrolase (PAF-AH) in pregnant rats treated with 17alpha-ethynylestradiol was significantly lower than in those treated with vehicle on Days 19 and 20, and the plasma PAF-AH activity in pregnant rats treated with estrogen was also significantly lower than in treated with vehicle on Days 18, 19, and 20. These findings indicate that estrogen increases PAF concentrations in the rat uterus, and this was correlated with a decrease in PAF-AH in the uterus and plasma. The increase in PAF concentrations in the uterus may be related to premature delivery and labor caused by PAF's known effect on myometrial contraction.

Increased contractility of rat uterine smooth muscle at the end of pregnancy

Upon stimulation with high K+, oxytocin, prostaglandin E2, prostaglandin F2 alpha or carbachol, myometrium isolated from pregnant rats (21 days after pregnancy) developed 2-3 times greater isometric force than that from non-pregnant rats (estrus). High K+ increased the level of myosin light chain (MLC) phosphorylation to a similar extent in these tissues, and therefore pregnant myometrium developed greater contraction than non-pregnant myometrium at a given MLC phosphorylation. In the permeabilized muscle with alpha-toxin, Ca2+ (0.1-10 microM) induced greater contraction in pregnant myometrium than in non-pregnant myometrium. Ca2+ sensitivity was not altered after pregnancy. MLC kinase and phosphatase activities did not differ significantly between pregnant and non-pregnant myometria. Stimulation with 10 microM Ca2+ and 1 microM calyculin-A elicited similar magnitudes of contractions in the permeabilized muscles isolated from non-pregnant and pregnant rats. SDS-PAGE showed that the percentage of the content of MLC was not altered between these preparations, although actin content increased after pregnancy. These results suggest that the stress generating capacity of myometrium is increased after pregnancy without changing the MLC phosphorylation step. The equal capacity of force generation after the maximum phosphorylation by Ca2+ and phosphatase inhibitor suggests that a MLC phosphorylation-independent mechanism is responsible for the development of greater force in the pregnant myometrium.

Increased inhibitory effect of phorbol ester on cytosolic Ca2+ level and contraction in rat myometrium after gestation

Activation of voltage-dependent Ca2+ channels by high K+ (40 mM) increased the cytosolic Ca2+ level ([Ca2+]i) (estimated by fura-PE3 fluorescence ratio) and force in myometrium isolated from pregnant (21 days after gestation) and non-pregnant (estrus) rats. 12-Deoxyphorbol 13-isobutyrate (DPB, 1 mM) decreased the high (K+)-stimulated [Ca2+]i and force in a concentration-dependent manner. The inhibitory effect was stronger in the pregnant myometrium than in the non-pregnant myometrium. In the pregnant myometrium, the increase in Ca2+ permeability by ionomycin (1 microM) greatly increased [Ca2+]i and force, which were only partially inhibited by verapamil (10 microM). DPB (1 microM) inhibited the verapamil-insensitive component of the increases in [Ca2+]i and muscle tension. Oxytocin (100 nM) and thapsigargin (1 microM) also induced a verapamil-insensitive increase in [Ca2+]i and force, and DPB (1 microM) inhibited these increments. Ca2+ sensitivity of contractile elements, estimated from the relationships between Ca2+ and muscle force in intact and alpha-toxin permeabilized muscle, was not significantly changed by DPB (1 microM). In summary, DPB inhibits the increase in [Ca2+]i more strongly in myometrium isolated from pregnant rats than that from non-pregnant rats without any change in the [Ca2+]i/tension relationship. Since DPB decreased [Ca2+]i-rise induced by three different mechanisms, DPB may activate Ca2+ extrusion, rather than to inhibit a specific influx pathway, to decrease [Ca2+]i.

Effects of a myosin light chain kinase inhibitor, wortmannin, on cytoplasmic Ca2+ levels, myosin light chain phosphorylation and force in vascular smooth muscle

Biochemical studies have shown that wortmannin is an inhibitor of myosin light chain (MLC) kinase (Nakanishi et al. (1992) J. Biol. Chem. 267: 2157-2163). To investigate the role of MLC kinase in smooth muscle contractions, we examined the effects of wortmannin on isolated smooth muscles of the rat aorta. Wortmannin (1 microM) decreased MLC phosphorylation and the amplitude of contractions induced by high K+ (72.7 mM) to a level seen at rest. This occurred without a change in cytosolic Ca2+ levels ([Ca2+]i). In contrast, wortmannin only partially inhibited the sustained contractions induced by phenylephrine (1 microM) and prostaglandin F2 alpha (PGF2 alpha, 10 microM) without a change in the [Ca2+]i. On the other hand, wortmannin (1 or 10 microM) reduced the increase in MLC phosphorylation induced by phenylephrine and PGF2 alpha to a level seen at rest. In the absence of external Ca2+, caffeine (20 mM) induced a transient increase in [Ca2+]i and force with an increase in MLC phosphorylation. Wortmannin completely inhibited the increase in MLC phosphorylation and contraction induced by caffeine without affecting the increase in [Ca2+]i. In the absence of external Ca2+, phenylephrine induced a small transient increase in [Ca2+]i, MLC phosphorylation and generation of force. This was followed by a small sustained contraction without an increase in [Ca2+]i and MLC phosphorylation. Wortmannin (1 microM) inhibited the transient phase of the contraction and the increase in MLC phosphorylation without affecting the transient increase in [Ca2+]i nor the sustained contraction. Wortmannin inhibited the Ca2(+)-induced contraction in permeabilized rat mesenteric artery, although it did not inhibit the Ca(2+)-independent, ATP-induced contraction in the thiophosphorylated muscle. These results suggest that wortmannin inhibits MLC phosphorylation due to an increase in the entry of Ca2+ or through the release of Ca2+ from the sarcoplasmic reticulum. The results also suggest that the activation of receptors by norepinephrine and PGF2 alpha induces a contraction via a MLC phosphorylation-independent pathway or through a pathway which is dependent on the resting level of MLC phosphorylation. We conclude that wortmannin is a useful tool in studies of the physiological role of MLC kinase.

Nitric oxide and the myometrium

Nitric oxide (NO) is a potent smooth muscle relaxant in blood vessels, the gastrointestinal tract and the respiratory system. Recent evidence has shown that NO has a relaxant (tocolytic) effect on myometrium. NO is produced within the female genital tract during pregnancy, and a reduction in NO synthesis may be involved in the initiation of parturition. Furthermore, the administration of NO donors may be useful in inhibiting uterine contractions in situations where such activity is unwanted, e.g., in preterm labour. NO is also produced in the myometrium in the nonpregnant state, and has potential roles in the facilitation of implantation and the prevention of dysmenorrhoea. This article aims to examine the evidence suggesting that NO has a physiological role in the maintenance of pregnancy and potential pharmacological use in the treatment of preterm labour.