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.     

Platelet-activating factor contracts human myometrium in vitro

The myometrial contractile responses to synthetic 1-0-octadecyl-2-acetyl-sn-glyceryl-3-phosphorylcholine (platelet-activating factor, PAF) and to oxytocin were evaluated in vitro on uterine (lower segment) strips obtained from pregnant women at term (39th week), undergoing elective cesarean section. Contractility was measured isometrically in an isolated organ bath using a superfusion technique. PAF in a concentration range between 5 and 100 nM as well as oxytocin (0.1-10 mU/ml) induced a dose-dependent contraction which could be categorized in two patterns, depending on whether spontaneous activity was present. In resting strips, oxytocin induced a prompt (0.5-1 min) development of active tension, followed by a prolonged (6-18 min), slow contraction and a final relaxation. However, at variance with oxytocin, PAF-induced contractions were rhythmic (3-8/hr), and characterized by a prompt (0.5-2 min) development of tension, followed by a brief (0.5-2 min) plateau, and a final, rapid relaxation. In spontaneously active strips, both stimuli induced a marked potentiation of the contractile activity. PAF response was dependent on both cyclooxygenase- and lipoxygenase-derived products as inferred from the abrogating effects of indomethacin and FPL 55712. A receptor-mediated mechanism of action was inferred from the occurrence of specific desensitization to PAF (but not to oxytocin), and from the blocking effect of CV 3988, a specific PAF receptor antagonist. The present study indicates that PAF stimulates the contraction of human myometrium in vitro and suggests that this mediator may have a role in labor.     

Complex effects of imatinib on spontaneous and oxytocin-induced contractions in human non-pregnant myometrium

Human myometrium includes two important cell populations involved in its contractility: smooth muscle fibers and interstitial cells. The pacemaking mechanism is not yet identified, but it is possible that myometrial smooth muscle cells contract in response to a signal generated by c-kit positive interstitial cells. The aim of this study was to investigate the effects of imatinib as a c-kit receptor antagonist on the spontaneous or oxytocin (OT) induced contractions of human non-pregnant myometrium in vitro. Myometrial strips were obtained from non-pregnant women (reproductive age) undergoing hysterectomy for benign indications. The strips were suspended in organ baths for recording of isometric tension. Imatinib effects were assessed on spontaneous contraction and after preexposure to OT.Direct exposure of myometrial strips to imatinib inhibits both amplitude and frequency of contractions (80-320 μM) in a dose dependent manner. Amplitude reverted back to 90% of the baseline amplitude by consequent addition of imatinib (until 480 μM). Total inhibition of myometrial contraction was obtained after addition of OT 60 nM. If myometrium was pre-exposed to OT (320 nM), imatinib 80-160 μm increased amplitude, while decreasing frequency. These data provide evidence that telocytes may be involved as modulators of the spontaneous contractions of the non-pregnant human uterus, via a tyrosine-kinase independent signaling pathway.     

Influence of platelet-activating factor on leukotriene D4-induced contractions of the guinea pig parenchymal strip

Platelet-activating factor (PAF) and sulphidopeptide leukotrienes, such as leukotriene D4 (LTD4), are potent constrictors that are probably released simultaneously in a variety of inflammatory respiratory events. The purpose of the present study was to determine whether LTD4-induced contractions of guinea pig parenchymal lung strips (GPPS) are modified in the presence of PAF. The contractile responses of isolated GPPS to cumulative doses of LTD4, acetylcholine, histamine, and potassium chloride in the presence of PAF (0.1 nM, 0.1 microM) were compared with parallel controls. There was no significant alteration of the response to acetylcholine and potassium chloride and the PAF-induced inhibition of the response to histamine, although significant, was not concentration dependent. In contrast, PAF in a concentration range from 0.1 nM to 1.0 microM caused a marked, concentration-dependent reduction of LTD4-induced contractions. Pretreatment with the PAF receptor antagonist, BN52021, prevented the attenuation of LTD4-induced contraction by PAF. The attenuation of LTD4-induced contraction by PAF was also prevented by pretreatment with indomethacin or with the thromboxane synthase inhibitor U63,557A, but not by pretreatment with the lipoxygenase inhibitors BW755c or nordihydroguaiaretic acid. Thus inhibition of LTD4-induced GPPS contraction by PAF is receptor dependent and probably secondary to thromboxane generation. The respiratory smooth muscle response to leukotrienes may be modified significantly by concomitant PAF release.     

Spasmogenic effect of platelet activating factor (PAF) on isolated rat stomach fundus strip

Platelet activating Factor (PAF) produced an increase in resting tension of isolated rat stomach fundus strips. The spasmogenic effect of a 90 nM dose was equivalent to the contraction to 110 nM acetylcholine (ACh). Tissues exposed once to PAF became refractory to re-challenge with a dose of PAF normally producing maximum contraction (desensitization). PAF desensitized tissues remained responsive to the contraction effects of ACh and KCl (80 mM). Lyso-PAF failed to produce any effect. PAF contraction was dose-dependently antagonized by pretreatment of tissues with the PAF receptor antagonist L-652,731. PAF contractions were not blocked by antagonists of cholinergic, adrenergic, histaminergic, and serotonergic receptors, nor by inhibition of cyclooxygenase. PAF is a potent spasmogen on the isolated rat stomach fundus strip, and this effect is PAF and PAF-receptor specific.     

Oxytocin receptors and contractile response of the myometrium after long term infusion of prostaglandin F2 alpha, indomethacin, oxytocin and an oxytocin antagonist in rats

Binding of [3H]oxytocin to isolated myometrial plasma membranes was not affected by the presence of prostaglandin (PG)F2 alpha or E2 in the incubation medium. Long-term treatment with PGF2 alpha or indomethacin had no effect on oxytocin receptor concentrations and dissociation constants of myometrial plasma membranes nor on maximal contractility or KM values of isolated uterine strips exposed to oxytocin. Infusion of oxytocin for 5 days in non-pregnant rats resulted in a decrease in oxytocin receptor concentrations in myometrial plasma membranes whereas the binding affinity to oxytocin was unaffected. Isolated uterine strips from similarly treated rats showed a reduced maximal contractile response to oxytocin and an elevated KM value, possibly indicating an influence of oxytocin on the coupling between receptor occupancy and contractility. Treatment for 5 days with desamino1-[D-Tyr(O-ethyl)2-Thr4-Orn8] oxytocin (an oxytocin antagonist) increased the concentration of myometrial oxytocin receptors. In addition KD values of these receptors were elevated. The present results indicate that prolonged exposure to oxytocin leads to a down-regulation of the myometrial receptor concentration, which is not caused by ligand-receptor interaction in itself. The concerted effect of oxytocin and prostaglandins on myometrial contraction does not appear to involve modulation of the oxytocin receptor by prostaglandins.     

Effects of calcium channel blocker, mibefradil, and potassium channel opener, pinacidil, on the contractile response of mid-pregnant goat myometrium

The present study was undertaken to investigate the in vitro influence of mibefradil, a calcium channel blocker, and pinacidil, a potassium channel opener, on pregnant goat myometrial spontaneous rhythmic contractility and contractions induced with the agonist, oxytocin. Longitudinal strips from the distal region of uterus, collected from goats at midgestation, were mounted in an organ bath for recording isometric contractions. Mibefradil (10(-8)-10(-4) M) or pinacidil (10(-10)-10(-4) M), added cumulatively to the bath at an increment of 1 log unit, caused concentration-dependent inhibition of the spontaneous rhythmic contractions of isolated uterine strips. The rhythmic contraction was, respectively, abolished at 100 and 10 microM concentrations of mibefradil and pinacidil. In a concentration-dependent manner, mibefradil (1 and 10 microM) antagonized the contractions elicited with oxytocin (10(-5)-10(-2) IU). Pretreatment of uterine strips with glibenclamide (10 microM), a selective KATP channel blocker, caused a rightward shift of the concentration-response curve of pinacidil with a concomitant decrease in its pD2 value. Pinacidil (0.3, 1 and 3 microM), in a concentration-related manner, antagonized the oxytocin (10(-5)-10(-2) IU)-induced contractile response. The inhibition of spontaneous rhythmic contractions and antagonism of oxytocin-induced contraction by mibefradil in the pregnant goat myometrium may be related to the antagonism of voltage-dependent Ca2+ channels, while by pinacidil suggests that KATP channel could be a therapeutic target for tocolysis.     

Long-term hypoxia alters calcium regulation in near-term ovine myometrium

Previous studies showed that long-term hypoxia (LTH) during pregnancy alters myometrial contractility. The present study was designed to test the hypothesis that LTH during pregnancy suppresses myometrial contractility in sheep by affecting the calcium signaling cascade. Pregnant sheep were maintained at high altitude (3820 m) from Day 30 to Day 139 of gestation, when the animals were killed for collection of myometrial tissue. Tissue was also collected from age-matched, normoxic controls. Circular and longitudinal layers were separated, and strips from each layer were mounted in a muscle bath. After pretreatment with 10(-8) M oxytocin, the strips were exposed to increasing half- or quarter-log doses of nifedipine (L-type calcium-channel blocker), ruthenium red, ryanodine (blockers of inositol 1,4,5-trisphosphate-insensitive calcium stores), or 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC; phospholipase C inhibitor). Area under the contraction curve was analyzed, and pD(2) (log of concentration yielding 50% of maximum response) values and maximum relaxation responses were calculated. The maximum relaxation response to nifedipine was increased in both longitudinal (P < 0.01) and circular (P < 0.05) myometrial layers from LTH compared to control tissue, whereas no difference was observed in response to ruthenium red or ryanodine. The maximum relaxation response to NCDC was lower in the LTH circular layer (P < 0.05). Together, these data are indicative of an increase in the dependence of ovine uterine smooth muscle on extracellular calcium influx through the L-type, voltage-gated calcium channels following LTH. This appears to occur not through an increase in L-type calcium channels but, rather, through a possible decline in importance of the oxytocin-induced, phospholipase C-mediated pathway, resulting in a greater proportion of extracellular calcium contributing to contraction. Layer-dependent differences also exist between the circular and longitudinal myometrium in response to phospholipase C inhibition.     

Long-term hypoxia changes myometrial responsiveness and oxytocin receptors in the pregnant ewe: differential effects on longitudinal versus circular smooth muscle

Previous studies from our laboratory demonstrated that long-term hypoxia (LTH) altered in vitro contractile responses to oxytocin in full-thickness myometrial strips from pregnant sheep. The present study was designed to determine, first, if the reduced contractile response to oxytocin following LTH is the result of combined effects on longitudinal and circular smooth muscle or if the effect is specific to a single muscle layer and, second, if the reduced contractile response to oxytocin following LTH is caused by changes in oxytocin-receptor protein. Pregnant ewes were maintained at high altitude (3820 m) from Day 30 to Days 137-142 of gestation, when the ewes were killed for collection of myometrial tissue. Tissue was also collected from age-matched, normoxic controls. Longitudinal and circular layers were separated, length-tension curves generated to determine optimal resting tension, and all strips exposed to increasing half-log doses of oxytocin ranging from 10-12 to 10-6.5 M. The expression of oxytocin-receptor protein was measured using Western blot analysis. We found that LTH did not affect KCl-induced contraction of either smooth muscle layer, whereas the sensitivity of both myometrial layers to oxytocin was altered. A decreased maximum contractile response of the circular layer to oxytocin was also observed. Additionally, LTH decreased expression of oxytocin-receptor protein in the circular layer and increased levels in the longitudinal layer. Results from the present study indicate that LTH alters contractile responses and oxytocin-receptor protein expression in a layer-specific manner in the pregnant sheep myometrium.     

Oxytocin and glucose oxidation in the rat uterus

The effects of oxytocin on the biochemical pathways of glucose oxidation were investigated in the rat uterus. In the presence of oxytocin, glucose oxidation in uterine segments obtained from Sprague-Dawley rats at diestrus increased 1.5–2.0-fold above the basal rate. A half-maximal response was observed at about 3 nM oxytocin; the maximum response was equal to or greater than the response to 1.7 nM insulin. In stripped myometrial segments (denuded of the endometrial component), oxytocin stimulated glucose oxidation at estrus only; whereas in intact uterine segments, the stimulation of oxidation was observed at both estrus and diestrus. In contrast, stimulation of oxidation by carbachol in stripped myometrial segments was independent of the estrous state of the tissue. The ratio of [1-¹⁴C]glucose to [6-¹⁴C]glucose oxidation was measured to estimate the relative involvement of the pentose phosphate and the tricarboxylic acid pathways of metabolism. In myometrial tissue, stimulation of glucose oxidation by oxytocin appeared to proceed through the tricarboxylic acid cycle. In intact uterine segments, at diestrus, glucose oxidation involved largely the pentose phosphate pathway (suggesting increased glucose metabolism in endometrial tissue), whereas at estrus, in the intact tissue segments, oxytocin increased glucose oxidation largely via the tricarboxylic acid cycle, and appeared to do so predominantly in the myometrial tissue. Carbachol-stimulated glucose oxidation appeared to proceed mainly via the tricarboxylic cycle in the myometrial tissue, irrespective of the stage of the estrous cycle. In the uterus of the Brattleboro rat (either intact uterine segments or stripped myometrial strips), oxytocin stimulated glucose oxidation only at estrus, predominantly through the tricarboxylic acid cycle. These findings suggest that oxytocin, in addition to its known effect on the contractility of uterine and myoepithelial smooth muscle, may regulate glucose metabolism in both the myometrial and endometrial components of uterine tissue.     

Apamin inhibits NO-induced relaxation of the spontaneous contractile activity of the myometrium from non-pregnant women

There is now considerable evidence for the involvement of K⁺ channels in nitric oxide (NO) induced relaxation of smooth muscles including the myometrium. In order to assess whether apamin-sensitive K⁺ channels play a role in NO – induced relaxation of the human uterus, we have studied the effect of specific blockers of these channels on the relaxation of myometrium from non-pregnant women. In vitro isometric contractions were recorded in uterine tissues from non-pregnant premenopausal women who had undergone hysterectomy. Apamin (10 nM) and scyllatoxin (10 nM) did not alter spontaneous myometrial contractions. However, 15-min pretreatment of the myometrium strips with apamin completely inhibited relaxation caused by diethylamine-nitric oxide (DEA/NO). The pretreatment with scyllatoxin significantly reduced (about 2.6 times) maximum relaxation of the strips induced by DEA/NO (p < 0.05). These results strongly suggest that, beside Ca²⁺ and voltage dependent charybdotoxin-sensitive (CTX-sensitive) K⁺ channels, apamin-sensitive K⁺ channels are also present in the human non-pregnant myometrium. These channels offer an additional target in the development of new tocolytic agents.     

Dose-Dependent Differential Effects of In Vivo Exposure of Cadmium on Myometrial Activity in Rats: Involvement of VDCC and Ca<Superscript>2+</Superscript>-Mimicking Pathways

Present study was undertaken to study the effect of 28-days exposure of female adult rats to cadmium (Cd) in drinking water @ 3, 10 and 30 parts per million (ppm) on myometrial responsiveness to different spasmogens and unravel the possible mechanism of alterations in myometrial activity. Cadmium and Ca²⁺ levels in blood and uterus were measured by atomic absorption spectroscopy while isometric tension in myometrial strips was measured using data acquisition system-based physiograph. Dose-dependent increase in levels of cadmium was observed in both blood and uterus while calcium was increased only in the uterus as compared to those in control. Significant increase in absolute tension and mean integral tension along with non-significant increase in frequency of myometrial contraction was observed in rats of Cd-treated groups. As compared to the control, cadmium decreased and increased the effects of calcium chloride, 80 mM KCl, histamine (0.1 μM) and oxytocin (10^?2 IU/ml) in lower-dose (3 ppm) and higher-dose groups (10 and 30 ppm), respectively. Cadmium potentiated and inhibited the relaxant response to phenylephrine in myometrium of rats at lower-dose (3 ppm) and highest-dose (30 ppm) Cd-treated groups, respectively. Results of our study revealed that Cd accumulates in the myometrium of rats and alters its responsiveness to oxytocin, histamine, 80 mM KCl, calcium chloride and phenylephrine, and these effects are differentially mediated depending on levels of exposure possibly through voltage-dependent calcium channel (VDCC) and Ca²⁺-mimicking pathways.     

Biphasic effect of a prostacyclin methyl ester analog on uterine contractility in the chronically catheterized pregnant rhesus monkey in late gestation

We determined the in vivo effects of a prostacyclin methyl ester analog (PGI2) administration on myometrial activity in twelve chronically- catheterized pregnant rhesus monkeys during the last third of pregnancy under three different states of myometrial contractility: postsurgical contractions, spontaneous contractures and oxytocin induced contractions. Prostacyclin (200 micrograms) was administered to four monkeys 2-3 h after surgery, to eight monkeys having only contractures, and to four monkeys having oxytocin induced contractions. Vehicle administration was performed in six animals having contractures and to four having oxytocin induced contractions. In all three experimental paradigms PGI2 administration elicited a biphasic response in uterine contractility. An initial increase in the 5 min average intrauterine pressure (IUP) value from 2.3 +/- 1.29 to 5.8 +/- 2.29; 3.6 +/- 2.38 to 7.6 +/- 3.59; and 2.3 +/- 0.65 to 8.6 +/- 0.72 (SD); was observed in the post-surgical, spontaneous contracture and oxytocin induced contraction groups respectively (mmHg, P less than 0.05). This increase was followed by a fall in the 5 min average IUP from 2.3 +/- 1.29 to 0.6 +/- 0.49; 3.6 +/- 2.38 to 1.3 +/- 0.86; and 2.3 +/- 0.65 to 0.1 +/- 0.11 in the post-surgical, spontaneous contracture and oxytocin induced contraction groups respectively (mmHg, P less than 0.05). A fall in mean arterial blood pressure from 89 +/- 9.0 to 55 +/- 10.9 (mmHg, P less than 0.05) and a compensatory tachycardia from 108 +/- 18.2 to 164 +/- 56.2 (beats.min-1, P less than 0.05) was observed by 10 min after PGI2 administration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for an intrinsic control of myometrial contractile periodicity in sheep during pregnancy.

Reduction in concentration of prostaglandins in plasma by administration of sodium meclofenamate to pregnant sheep failed to alter the frequency or duration of electromyographic activity bursts or the response to oxytocin of myometrial tissue transplanted to the omentum. However, a significant (P < 0.05) delay (8.6 +/- 3.8 versus 1.3 +/- 0.3 min) in the myometrial response to oxytocin was observed when the hormone was administered 1 min after a spontaneous burst of electromyographic activity compared with 15 min after a burst, indicating a period of refractoriness. Similarly, the myometrial threshold for electrical stimulation was higher at 10-25% of the interval between contractions than close to the expected time of the next contraction. Stimulation of the myometrium at intervals of 30 s revealed a cycling of the electrical stimulation threshold: significantly higher voltages were required to elicit responses between spontaneous bursts of electromyographic activity (18.0 +/- 2.2 V) than during bursts (11.3 +/- 1.6 V). In contrast, there was no voltage differential in animals close to labour (< 24 h). These data provide no evidence to support a role for prostaglandins in the generation of contractions during pregnancy, but suggest that periodicity of contractions is associated with inherent changes in myometrial responsiveness to stimulation, which could occur as a result of a cycling of the resting membrane potential.     

Inhibitor effect of omeprazole in isolated human myometrial smooth muscle.

The aim of the present study was to investigate the effect of omeprazole, an H+-K+-ATPase inhibitor, in myometrial smooth muscle strips from women undergoing elective caesarean section at term. Isolated myometrial strips taken with informed consent were obtained from eight pregnant women undergoing elective caesarean section at term (not in labour) and mounted in organ baths for recording of isometric tension. We recorded the effect of increasing concentrations of omeprazole on spontaneous and Ca2+-induced contractions of myometrial smooth muscle and on contractions of myometrial smooth muscle pretreated with indomethacin (3 x 10(-6) M) and L-NAME (3 x 10(-5) M). Omeprazole (10(-4)-10(-3) M) decreased the amplitude and frequency of spontaneous contractions in a time- and concentration-dependent manner in all myometrial smooth muscle isolated from pregnant women. The decrease in amplitude of contractions in myometrial smooth muscle reached statistical significance beginning from the concentration of 3 x 10(-4) M. Addition of indomethacin (3 x 10(-6) M) and L-NAME (3 x 10(-5) M) in to the organ baths 30 min before did not change relaxation responses to omeprazole. When 8 mM Ca2+-precontracted in Ca2+-free medium myometrial smooth muscle were exposed to increasing concentrations of omeprazole (10(-5)-10(-3) M), omeprazole produced relaxation responses in a time- and concentration-dependent manner, reaching statistical significance at 10(-4) M. These results show: (1) omeprazole time- and concentration-dependently decreased spontaneous contractile activity in myometrial smooth muscle isolated from pregnant women, (2) omeprazole-induced relaxations was not influenced by indomethacin and N(G)-nitro-L-arginine methyl ester (L-NAME), suggesting that it is not mediated by cyclooxygenase products and nitric oxide, and (3) omeprazole brought about time- and concentration-dependently relaxation of myometrial smooth muscle precontracted by 8 mM Ca2+ in Ca2+-free medium. This effect of omeprazole may be due to blockade of the calcium channels.     

cAMP dependence of Ca2+ entry during agonist-induced contractions of the uterine smooth muscles

Possible involvement of cAMP-dependent mechanisms in the development of both phasic and tonic contractions induced by oxytocin — OT (25 nM and 25 µM, respectively), as well as of KCl-induced contracture, was studied on the myometrium of estradiol-dominated rats using the myometrial strips with suppressed spontaneous mechanical activity. The intracellular cAMP level was modulated by furosemide that had been previously shown to decrease cAMP content in the rat myometrium tissue. When added to the medium in the pulse mode together with 25 nM OT, furosemide (0.02 mM) increased contraction amplitude by 224%, whereas higher, 0.2 and 20 mM, furosemide concentrations suppressed the response by 286% or totally removed it, respectively. Being present in the bath permanently, 0.2 mM furosemide progressively decreased the amplitude of OT-induced phasic contractions. Under such conditions, 0.02 mM furosemide exerted biphasic effect on the responses, so that the initial enhancement was replaced by the progressive inhibition. Dibutyryl-cAMP (dbcAMP) at a proper concentration restored the responsiveness of the tissue to OT in the presence of furosemide in the saturating concentration. Contractile responses induced by 25 µM OT comprised both phasic and tonic components. In a Ca²⁺-free medium, the OT-induced contractions seemed to be associated with Ca²⁺ release from intracellular stores. Permanent presence of furosemide in the CaCl₂-containing medium inhibited OT-induced responses in the same manner as omission of Ca²⁺ from the medium, i.e., furosemide did not affect the responses caused by Ca²⁺ release but inhibited those mediated via acceleration of the Ca²⁺ influx. The furosemide-sensitive component of responses to OT was combined with a persistent contraction caused by KCl depolarization; there was a moderate decrease in amplitude of the KCl-induced contracture due to furosemide action. The decrease could be prevented by dbcAMP addition. It is suggested that both voltage-gated and receptor-operated Ca²⁺ entries induced by OT are regulated by cAMP-dependent protein kinases, while Ca²⁺ extrusion into the extracellular space does not depend on the intracellular cAMP.Neirofiziologiya/Neurophysiology, Vol. 26, No. 1, pp. 54–60, January–February, 1994.     

Altered motility of myometrium from estrous ewes after the regulation of estrus with progestagen or prostaglandin

The $\text{in}$$\text{vitro}$ motility of strips of myometrium was studied in estrous ewes after administration of melengestrol acetate (MGA), medroxyprogesterone acetate (MAP) or prostaglandin F_2α (PG) to regulate estrus. Ewes were killed during estrus, 2 to 4 days after the end of progestagen or PG treatment. The contractions of strips of myometrium were recorded by kymograph and compared to contractions of control ewes.Muscle relaxation was indicated by an interval of tracing that was nearly horizontal and close to the baseline. A major contraction was indicated by a sharp upward movement of the tracing pen and an interval of tracing above the baseline.Treating ewes with MAP by intravaginal sponge lengthened contraction time. Feeding MGA or MAP tended to shorten relaxation time, and injecting PG tended to both lengthen contraction time and shorten relaxation time. As a result of these effects, the proportion of time that myometrium spent in contraction averaged 74% for 42 ewes treated with progestagen or prostaglandin and 56% for 24 control ewes. These results indicate that treatments that inhibit sperm transport when used to regulate estrus also affect the nature of myometrial contractions $\text{in}$$\text{vitro}$.     

Myometrial expression of small conductance Ca2+-activated K+ channels depresses phasic uterine contraction

Mechanisms regulating uterine contractility are poorly understood. We hypothesized that a specific isoform of small conductance Ca²⁺-activated K⁺ (SK) channel, SK3, promotes feedback regulation of myometrial Ca²⁺ and hence relaxation of the uterus. To determine the specific functional impact of SK3 channels, we assessed isometric contractions of uterine strips from genetically altered mice (SK3^T/T), in which SK3 is overexpressed and can be suppressed by oral administration of doxycycline (SK3^T/T+Dox). We found SK3 protein in mouse myometrium, and this expression was substantially higher in SK3^T/T mice and lower in SK3^T/T+Dox mice compared with wild-type (WT) controls. Sustained contractions elicited by 60 mM KCl were not different among SK3^T/T, SK3^T/T+Dox, and WT mice. However, the rate of onset and magnitude of spontaneously occurring phasic contractions was muted significantly in isolated uterine strips from SK3^T/T mice compared with those from WT mice. These spontaneous contractions were augmented greatly by blockade of SK channels with apamin or by suppression of SK3 expression. Phasic but not tonic contraction in response to oxytocin was depressed in uterine strips from SK3^T/T mice, whereas suppression of SK3 channel expression or treatment with apamin promoted the predominance of large coordinated phasic events over tone. Spontaneous contractions and the phasic component of oxytocin contractions were blocked by nifedipine but not by cyclopiazonic acid. Our findings suggest that SK3 channels play an important role in regulating uterine function by limiting influx through L-type Ca²⁺ channels and disrupting the development of concerted phasic contractile events. uterus; Ca²⁺-activated K⁺ channel; doxycycline; mouse     

The effects of methyl palmitate,a putative regulator from perivascular fat,on the contractility of pregnant human myometrium

Aims

Methyl palmitate is thought to cause relaxation in vascular smooth muscle by opening voltage-activated potassium channels. We have tested the hypothesis that methyl palmitate, a putative regulator from perivascular fat, is an inhibitor of the contractility of human pregnant myometrium and that its effects might partially explain the higher incidence of dysfunctional labor in obese women compared to those with normal body mass indices.

Main methods

Strips of myometrium obtained with informed consent from women undergoing elective cesarean section at term were mounted in organ baths. Strips stimulated with oxytocin (1 nM) or KCl (30 mM) were exposed to cumulatively increasing concentrations of methyl palmitate up to 10 μM. Similar strips were exposed to cumulative addition of the potassium channel blockers 4-aminopyridine and tetraethylammonium. The contractility of the strips was monitored and analyzed using conventional methods.

Key findings

Methyl palmitate failed to inhibit oxytocin- or KCl-induced contractions over the concentration range tested. In fact, it exerted a slight excitatory effect in the presence of KCl, though not in the presence of oxytocin. The contractility of naïve strips was unaltered by exposure to 1 μM methyl palmitate. Both 4-aminopyridine and tetraethylammonium produced concentration-dependent contractions of human pregnant myometrium providing pharmacological evidence for the presence of voltage-activated potassium channels in this preparation.

Significance

Our findings do not support the hypothesis that methyl palmitate is an inhibitor of human pregnant myometrial contractility. Alternate hypotheses must be pursued to explain the higher incidence of dysfunctional labor in obese women.     

The "prostaglandin step", a bottleneck in the activation of the uterus

Uterine strips were excised from post partum rabbits, mounted $\text{in vitro}$ and stimulated electrically to sustain cyclic tension at a maximal value. Within 10–15 minutes after exposure to 1.5–2.0 mg/ml Naproxen (a derivative of propionic acid and an inhibitor of PG-synthesis) uterine tension decreased to less than 25% of the original value. The effect of Naproxen (N), also observed in spontaneously active (unstimulated) uteri, was suspended by removing N through washing the strips with mammalian Krebs' solution.When suppressed by N, uterine tension could be restored by exogenous PG F2α but not by oxytocin, in spite of a 1000 fold increase in that oxytocin concentration which effectively stimulated the normal uterus (unexposed to N). This failure of oxytocin in promoting activation, when the PG-synthesis of the uterus was blocked, suggest that endogenous PGs participate in a critical step of the sequence of events which provoke myometrial activity.