Comparison of the effects of nicardipine and nifedipine on isolated human myometrium

The effects of nicardipine, a calcium-entry blocker which also has a potent phosphodiesterase inhibitory action, were investigated on isolated human term-pregnant and nonpregnant myometrium, and compared with those of nifedipine. Both drugs relaxed pregnant and nonpregnant myometrial preparations contracted by potassium (127 mM), and also reduced or abolished contractions occurring spontaneously, or induced by prostaglandin F2 alpha, oxytocin and vasopressin. However, the effect of nicardipine had a slower onset of action than that of nifedipine, and the drug was significantly more potent than nifedipine is at least as effective as nifedipine. If the differences between the drugs can be reproduced also in vivo, nicardipine offers an interesting alternative to nifedipine for inhibition of undesired uterine activity.     

Effects of histamine and serotonin on the contractility of isolated pregnant and nonpregnant human myometrium

The effects of histamine and serotonin and their antagonists on contractile activity of pregnant and nonpregnant human uterine strips were studied. Both histamine and serotonin (5-HT) increased contractions, pregnant preparations were more responsive than nonpregnant ones. The effects of histamine and 5-HT were blocked by pyrilamine and methysergide, respectively. Pyrilamine (10(-7)-10(-6) M) acted as a competitive antagonist of the effect of histamine, whereas methysergide (10(-7)-10(-6) M) inhibited the response to 5-HT in a noncompetitive manner. Cimetidine and ketanserine were completely ineffective at the doses tested. It is suggested that the increase of contractile activity observed at the end of pregnancy could be partly mediated by the effect of histamine and 5-HT on smooth muscle of the human myometrium.     

The effects of metabolic inhibition on intracellular calcium and contractility of human myometrium

Objective Hypoxia occurs in the uterus during labour and may contribute to dysfunctional labours. We wanted to establish its effects on pregnant human myometrium and elucidate the mechanisms involved.
Design Scientific study.
Setting University Hospital and laboratories.
Population or Sample Term pregnant women.
Methods We measured contractions and intracellular [Ca²⁺] ([Ca²⁺]_i), in biopsies from term pregnant women undergoing elective caesarean section, and used cyanide to block oxidative phosphorylation.
Main outcome measures Changes in contractility and calcium.
Results Although basal levels of [Ca²⁺]_i and tone rose, spontaneous and agonist-induced Ca²⁺ transients and phasic contractions were rapidly reduced and abolished by cyanide. Neither stimulation of the uterus with oxytocin nor the Ca channel agonist, Bay K8644, prevented the changes produced by cyanide. The tonic force produced by depolarising the myometrium was also decreased by cyanide, but slowly recovered towards control levels, whereas [Ca²⁺]_i was maintained throughout. Similar data were obtained when nitrogen, rather than cyanide, was applied to the depolarised uterus.
Conclusions Impairment of oxidative phosphorylation is a potent depressor of phasic activity in human myometrium, irrespective of how it is produced, and our data suggest its effects lie at and beyond the surface membrane. Stimulation of the hypoxic uterus was not effective, which may explain the unpredictability of oxytocin application in some dysfunctional labours.     

Degranulation of uterine mast cell modifies contractility of isolated myometrium from pregnant women

OBJECTIVE: This study was undertaken to test that uterine mast cell degranulation alters human myometrial contractility in vitro and to define what mediators are involved in this process. STUDY DESIGN: Longitudinal myometrial strips prepared from biopsy specimen obtained from the lower uterine segment of women at preterm and term gestation (with and without labor) were studied. Contractile responses to compound 48/80, a mast cell degranulator, were compared in the absence or presence of a mast cell stabilizer, H 1 and H 2 receptor antagonists, cyclooxygenase, and lipoxygenase inhibitors. RESULTS: Compound 48/80 increased myometrial contractility in all groups. The mast cell stabilizer cromolyn inhibited contractility, whereas the cyclooxygenase inhibitor ibuprofen, the H 1 -receptor antagonist S(+)-chlorpheniramine maleate, but not the H 2 antagonist cimetidine, only slightly attenuated this effect. The lipoxygenase inhibitor linoleyl hydroxamic acid augmented the responses to compound 48/80 in the preterm but not in the term group. CONCLUSION: Uterine mast cell degranulation, or the effects of their mediators, can modulate uterine contractility during pregnancy.     

Hydrogen sulfide inhibits the spontaneous and oxytocin-induced contractility of human pregnant myometrium

Hydrogen sulfide (H₂S) is a novel endogenous gaseous signaling transmitter in mammalian tissues including smooth muscle tissues. We investigated the effect of sodium hydrosulfide (NaHS), a H₂S donor, on the contractility of isolated human myometrium strips from term pregnant women who were undergoing labor. Cumulative effects of NaHS on spontaneous and oxytocin-induced contractility were evaluated by using isometric tension recordings. NaHS (0.1?μM–1 mM) concentration dependently inhibited spontaneous contractility of laboring myometrium, with a decrease in amplitude and frequency. NaHS (0.1?μM–1 mM) decreased the frequency but not the amplitude of oxytocin (1?μM)-induced contractions. NaHS-induced relaxation could be prevented by pretreatment with glibenclamide, an inhibitor of K⁺_ATP channels. Thus, NaHS evokes relaxation of human pregnant myometrium, suggesting a possible role of H₂S during human pregnancy.     

Interactive effects of volatile anesthetics, verapamil, and ryanodine on contractility and calcium homeostasis of isolated pregnant rat myometrium.

OBJECTIVE: Both volatile anesthetics and Ca2+ antagonists decrease uterine contractility. The interactive effects of anesthetics, verapamil, and ryanodine on myometrial muscular activity and on intracellular Ca2+ availability were examined. STUDY DESIGN: The effects of minimum alveolar concentrations of 0.5, 1.0, and 2.0 of halothane, enflurane, and isoflurane on electrically stimulated isometric mechanical activity of isolated longitudinal myometrium strips from 45 pregnant rats (15 to 21 days), superfused with Krebs' solution with or without 5.0 mmol/L Ca2+, 10(-7) and 10(-6) mol/L verapamil, and 10(-6) mol/L ryanodine, were compared. Analysis was performed by analysis of variance and Duncan test. RESULTS: All three anesthetics and verapamil produced dose-dependent depression of contractility. Increasing the Ca2+ concentration from 2.5 to 5.0 mmol/L in the superfusate partly reversed the depression. An additive effect of verapamil and anesthetics, especially enflurane, on contractile force was observed. Ryanodine had no effect on contractility, but it could partly counteract the depressant effect of anesthetics. CONCLUSION: Clinically used concentrations of volatile anesthetics modify Ca2+ availability and depress uterine contractility. General anesthesia, especially by enflurane, in patients being treated with calcium antagonists may represent a higher risk.     

The effects of Escherichia coli STa (heat stable) toxin on the contractility of isolated human myometrium in vitro.

OBJECTIVE: The purpose of the study was to assess the effects of Escherichia coli STa (heat stable) toxin on isolated human myometrial response to oxytocin. METHODS: One hundred and sixteen muscle strips were obtained from the lower uterine segment of 42 women undergoing cesarean section at term. Amniotic membranes and decidua were excluded. Uterine contractility in response to cumulative doses of E. coli STa toxin was recorded, as well as uterine response to cumulative doses of oxytocin before and after incubation with STa toxin or vehicle. The 50th percentile effective oxytocin concentration (EC50) of muscle strips with and without spontaneous activity before and after the incubation with STa toxin or vehicle was calculated. A paired t test was used for comparison. RESULTS: Muscle strips with and without spontaneous activity responded to cumulative doses of oxytocin before and after the incubation with STa toxin or vehicle. No differences in contraction force, duration, or frequency were noted between the groups (P > 0.05). Furthermore, this toxin was not able to induce uterine contractility when tested alone. CONCLUSIONS: The inability of this toxin to affect myometrial response to oxytocin in this study may be due to the absence of amnion cells, chorion, or decidua. Other possible explanations for the lack of response are discussed.     

The effects of pH change on Ca(++) signaling and force in pregnant human myometrium

OBJECTIVE: This study was designed to determine the effects of both intracellular and extracellular pH change on contractile activity and intracellular Ca(++) during spontaneous contractions, oxytocin, and depolarization-induced stimulation of human myometrium. STUDY DESIGN: Human myometrial tissue was obtained at elective caesarean delivery at term (37-41 completed weeks of gestation). Longitudinal strips were dissected and loaded with the calcium sensitive indicator Indo-1. Statistical significance was tested with the Student t test. RESULTS: Both intracellular and extracellular acidification significantly reduces or even abolishes phasic activity, whether it arises spontaneously or in the presence of oxytocin. These contractile changes can be accounted for by the changes in intracellular Ca(++). Alkalinization produced the opposite effects. However, baseline or maintained tension changes could not be accounted for by changes in intracellular Ca(++). CONCLUSION: We suggest that the effects on phasic activity are due to the inhibition of L-type calcium entry and that, during maintained or baseline activity, pH-sensitive Ca(++) release, possibly from the sarcoplasmic reticulum occurs; but it is insufficient to overcome the inhibitory effects at the myofilaments. We conclude that alterations of pH significantly affect calcium signaling and force production in the human myometrium and may contribute to dysfunction in labor.     

Progesterone enhances the tocolytic effect of ritodrine in isolated pregnant human myometrium

OBJECTIVE: To evaluate the effect of natural progesterone on the relaxant effect of ritodrine on pregnant human oxytocin-induced myometrial contractility. STUDY DESIGN: Isometric tension recordings were performed under physiologic conditions on isolated myometrial strips taken from low-risk term pregnant women undergoing elective cesarean section. Cumulative effects of natural progesterone (10 (-11) to 10 (-5) mol/L) on oxytocin-induced myometrial contractility were evaluated. Contractile activity following ritodrine exposure was also investigated in myometrium pretreated with natural progesterone. RESULTS: Natural progesterone alone exerted a concentration-dependent relaxant effect on myometrial contractions. The concentration-response curve for ritodrine from natural progesterone pretreated myometrium was shifted to the left with a significant reduction ( P < .01) of 50% of the maximal response, contraction amplitude ( P < .05), and frequency ( P < .05). However, there was no significant difference in the mean maximal inhibition achieved ( P = .95). CONCLUSION: Natural progesterone increased the relaxant effect of ritodrine by reducing 50% of the maximal response, amplitude, and frequency of myometrial contraction, most likely through nongenomic actions. These results suggest that natural progesterone may be beneficial for preventing preterm birth in a low-risk population.     

The effect of magnesium on calcium uptake and contractility in the human myometrium

High concentrations of magnesium (12 and 24 mM) in the extracellular medium markedly inhibited both spontaneous activity and K+-induced contracture in strips of nonpregnant human myometrium. Net calcium influx measured by the uptake of 45Ca2+ in the myometrium was considerably decreased by high concentrations of magnesium. This was true for both resting (unstimulated) and K+-stimulated uptake of 45Ca2. While calcium uptake in K+-stimulated tissues exposed to 24 mM magnesium was significantly lower than in those exposed to 12 mM, no difference was found in the unstimulated tissues. These data indicate that the tocolytic action of magnesium most probably results from the inhibition of calcium entry into myometrial cells.     

The pattern of spontaneous contraction and the effects of Mg++ and terbutaline on pregnant human myometrium

The isometric spontaneous contraction and K-contracture of pregnant human myometrium were recorded in order to study the effects of external Mg++ and the correlation between Mg++ and terbutaline. Electrical activities were also recorded with a single sucrose-gap method. Maximum tension, frequency and half duration of spontaneous contractions were not significantly different among all specimens. Maximum tension of the spontaneous contraction was lower than the longitudinal muscle of pregnant rat and higher than the circular one. The frequency was lower and the duration was longer than rat myometrium. Mg++ exhibited the inhibition of the spontaneous contraction and decreased the gradient of pacemaker potential and duration of action potentials. Phasic contraction of K-contracture was also inhibited by the pre-incubation with Mg++. Mg++ rich medium shifted the dose-response curve of terbutaline to the left and the pre-incubation with Mg++ increased the inhibitory effect of terbutaline on the tonic part of K-contracture. The results indicated that human myometrium might be constructed of plural muscle bundles with different characteristics, and Mg++ inhibited the spontaneous contraction by affecting the membrane activities and enhanced the terbutaline effect by penetrating into the membrane.     

Shortening increases spontaneous contractility in myometrium from pregnant women at term

OBJECTIVE: The purpose of this study was to determine whether shortening alters spontaneous contractility in myometrial strips that are obtained from pregnant women. STUDY DESIGN: Isometric contractions were measured in myometrial strips that were obtained at cesarean delivery from 14 pregnant women at term. After 2 hours of stretching, the strip lengths were decreased by 4%, 6%, or 8%. Spontaneous contractility was measured for 120 minutes with or without prostaglandin synthase inhibitor indomethacin (10 -5 mol/L), and the cumulative concentration response to oxytocin was determined. RESULTS: Contractility was increased by 29% and 34% in strips that shortened by 4% and 6%, respectively. Preincubation with indomethacin increased contractility by 15% in stretched strips and decreased contractility by 30% and 19% in 4% and 6% strips, respectively. Contraction frequency was increased by 26% and 53% for the strips that were shortened to 6% and 8%, respectively. These increases were prevented by indomethacin. The oxytocin responses were similar at all lengths. CONCLUSION: Shortening of myometrial strips from pregnant women at term increases spontaneous contractility by a mechanism that apparently involves prostaglandins.     

Impact of tamsulosin and nifedipine on contractility of pregnant rat ureters in vitro*

Objectives: To evaluate the in vitro effect of tamsulosin and nifedipine on the contractility of pregnant rat ureters and to perform quantitative analysis of the pharmacological effects. Medical expulsive therapy (MET) is commonly used to treat urolithiasis. However, this treatment is seldom used in pregnant women since no studies support this practice.

Methods: This was an in vitro study on animal tissue derived from pregnant Sprague-Dawley rats. A total of 124 ureteral segments were mounted in an organ bath system and contractile response to methacholine (MCh) was assessed. Tamsulosin or nifedipine were added at cumulative concentrations (0.001–1?μM). The area under the curve (AUC) from isometric tension measurements was calculated. The effect of pharmacological agents and the respective controls were assessed by calculating the AUC for each 5-min interval. Statistical analyses were performed using the Mann–Whitney–Wilcoxon nonparametric test.

Results: Both drugs displayed statistically significant inhibitory activity at concentrations of 0.1 and 1?μM for tamsulosin and 1?μM for nifedipine when calculated as the AUC as compared to DMSO controls.

Conclusion: Tamsulosin and nifedipine directly inhibit MCh-induced contractility of pregnant rat ureters. Further work is needed to determine the clinical efficacy of these medications for MET in pregnancy.     

Some electrical properties of human pregnant myometrium

Membrane properties of the human pregnant myometrium were investigated with the conventional microelectrode and patch clamp methods. The majority of preparations produced spontaneous action potentials at a very low frequency, and action potentials were inhibited in sodium-deficient or calcium-free solutions. With the patch clamp technique with 120 mmol/L cesium-20 mmol/L tetraethylammonium in the pipette, the inward current was evoked by a depolarizing pulse above -40 mV from a holding potential of -60 mV and maximum amplitude was obtained at 0 mV. At a holding potential of -100 mV, the inward current could be evoked with less positive depolarizing pulses, and the membrane potential that evoked the maximum amplitude of inward current was shifted to a more negative potential. Single-channel current recording revealed that two types of calcium channels existed in human pregnant myometrium, with single-channel conductances of 12 and 29 pS. One of the calcium channels (12 pS) was inactivated at a holding potential of -60 mV.     

The effect of a nicorandil congener on isolated human myometrium

OBJECTIVE: To determine the effect of a nicorandil congener, SG-209 on the myometrium. STUDY DESIGN: Isolated strips of myometrium, from nine women who underwent hysterectomy, were made to contract with 55 mM KCl before and after incubation with three concentrations of SG-209 (1.5, 3.0 and 10 microM). The mean height of contraction and the area under the curve were analysed using a non-parametric test. RESULTS: At a 10 microM concentration, SG-209 significantly decreased the mean area under the curve from 10.8 to 2.5 cm2 (p<0.05). CONCLUSION: SG-209 significantly relaxes the human non-pregnant myometrium. Along with its congener, nicorandil, it may be useful in clinical conditions that require uterine relaxation.     

The effect of aminophylline on uterine smooth muscle contractility and prostaglandin production in the pregnant rat uterus in vitro

The effect of aminophylline on contractility and prostaglandin production was investigated in a 20-day pregnant rat uterus preparation in vitro. A significant decrease was observed in frequency (24.4% of control, p less than 0.05) and contractile force (13.8% of control, p less than 0.001) in rat uterine segments treated with aminophylline. Treated segments produced more prostaglandin E2 (139.6% of control, p less than 0.001) and less prostaglandin F2 alpha (73.8% of control, p less than 0.001) during the 45-minute observation period. No significant difference in thromboxane B2 or 6-ketoprostaglandin F1 alpha production was noted. This is the first observation of prostaglandin levels in rat uterine segments treated with aminophylline. It could not be concluded from this study whether the observed changes in prostaglandin production are directly related to the relaxant effect of aminophylline.