Relaxin inhibits the plateau component of the action potential in the circular myometrium of the rat

Mapping uterine calcium dynamics during the ovulatory cycle in live mice

Uterine contraction patterns vary during the ovulatory cycle and throughout pregnancy but prior measurements have produced limited and conflicting information on these patterns. We combined a virally delivered genetically encoded calcium reporter (GCaMP8m) and ultra-widefield imaging in live nonpregnant mice to characterize uterine calcium dynamics at organ scale throughout the estrous cycle. Prior to ovulation (proestrus and estrus) uterine excitations primarily initiated in a region near the oviduct, but after ovulation (metestrus and diestrus), excitations initiated at loci homogeneously distributed throughout the organ. The frequency of excitation events was lowest in proestrus and estrus, higher in metestrus and highest in diestrus. These results establish a platform for mapping uterine activity, and show that the question of whether there is an anatomically localized trigger for uterine excitations depends on the estrous cycle phase.

Functional and morphological development of the womb throughout life

The uterus undergoes changes throughout a woman's life, beginning with her own embryonic development when she is still in the womb, commencing a monthly cycle at the onset of adulthood, and undergoing dramatic changes during pregnancy and parturition. The impact of preterm labour and other perinatal health problems is significant, both in human and financial terms; therefore the study of the physiological and regulatory changes which the uterus undergoes can be of enormous potential benefit. Here we briefly review the current state of knowledge, with an emphasis on the importance of changes in connectivity in the uterine smooth muscle cell network and on recent mathematical modelling work aimed at elucidating the role of spatial heterogeneity in this connected network.

Relaxin induces rapid, transient vasodilation in the microcirculation of hamster skeletal muscle

Relaxin produces a sustained decrease in total peripheral resistance, but the effects of relaxin on skeletal muscle arterioles, an important contributor to systemic resistance, are unknown. Using the intact, blood-perfused hamster cremaster muscle preparation in situ, we tested the effects of relaxin on skeletal muscle arteriolar microvasculature by applying 10(-10) M relaxin to second-, third- and fourth-order arterioles and capillaries. The mechanisms responsible for relaxin-induced dilations were explored by applying 10(-10) M relaxin to second-order arterioles in the presence of 10(-5) M N(G)-nitro-l-arginine methyl ester (l-NAME, nitric oxide (NO) synthase inhibitor), 10(-5) M glibenclamide (GLIB, ATP-dependent potassium (K(+)) channel inhibitor), 10(-3) M tetraethylammonium (TEA) or 10(-7) M iberiotoxin (IBTX, calcium-associated K(+) channel inhibitor). Relaxin caused second- (peak change in diameter: 8.3 ± 1.7 μm) and third (4.5 ± 1.1 μm)-order arterioles to vasodilate transiently while fourth-order arterioles did not (0.01 ± 0.04 μm). Relaxin-induced vasodilations were significantly inhibited by l-NAME, GLIB, TEA and IBTX. Relaxin stimulated capillaries to induce a vasodilation in upstream fourth-order arterioles (2.1 ± 0.3 μm), indicating that relaxin can induce conducted responses vasodilation that travels through blood vessel walls via gap junctions. We confirmed gap junction involvement by showing that gap junction uncouplers (18-β-glycyrrhetinic acid (40 × 10(-6) M) or 0.07% halothane) inhibited upstream vasodilations to localised relaxin stimulation of second-order arterioles. Therefore, relaxin produces transient NO- and K(+) channel-dependent vasodilations in skeletal muscle arterioles and stimulates capillaries to initiate conducted responses. The transient nature of the arteriolar dilation brings into question the role of skeletal muscle vascular beds in generating the sustained systemic haemodynamic effects induced by relaxin.

Physiology and electrical activity of uterine contractions

Presently, there is no effective treatment for preterm labor. The most obvious reason for this anomaly is that there is no objective manner to evaluate the progression of pregnancy through steps leading to labor, either at term or preterm. Several techniques have been adopted to monitor labor, and/or to diagnose labor, but they are either subjective or indirect, and they do not provide an accurate prediction of when labor will occur. With no method to determine preterm labor, treatment might never improve. Uterine electromyography (EMG) methods may provide such needed diagnostics.

New developments in the management of preterm labor

Current management of preterm labor has not changed the incidence of preterm delivery; therefore, significant research effort has been concentrated on the search for new methods of management. New tocolytics like inhibitors of cyclooxygenase 2 and nitric oxide donors have been tested in animal models and in preliminary clinical trials with promising results. Inhibition of cervical ripening may be one alternative to tocolysis. This new approach has a potential to be a valuable method of management of preterm labor if human studies confirm the promising results reported in animals. Growing evidence suggests that premature delivery may be associated with infection or fetal growth abnormalities, with dire consequences to the fetus. If these associations are to be included in risk and benefit assessment, then inhibition of preterm labor may prove to be detrimental to the fetus.

Augmentation and suppression of action potentials by estradiol in the myometrium of pregnant rat

The purpose of this study was to investigate the actions of estradiol on spontaneous and evoked action potentials in the isolated longitudinal smooth muscle cells of the pregnant rat. Single cells were obtained by enzymatic digestion from pregnant rat longitudinal myometrium. Action potentials and currents were recorded by whole-cell current-clamp and voltage-clamp methods, respectively. The acute effects of 17β-estradiol on action potentials and inward and outward currents were investigated. The following results were obtained. The average resting membrane potential of single myometrial cells was -54 mV (n = 40). In many cells, an electrical stimulation evoked a membrane depolarization, and action potentials were superimposed on the depolarization. In some cells, spontaneous action potentials were observed. Estradiol (30 µM) slightly depolarized the membrane (ca. 5 mV) and attenuated the generation of action potentials by reducing the frequency and amplitude of the spikes. Afterhyperpolarization was also attenuated by estradiol (30 µM). On the other hand, in 5 of 35 cells, estradiol increased the first spike amplitude and action potential duration, while frequency of the spike generation and afterhyperpolarization were inhibited. In voltage-clamped muscle cells, estradiol inhibited both inward and outward currents. Acute inhibition or augmentation of spike generation by estradiol is due to the balance of inhibition of inward and outward currents. Inhibition of both currents also prevented afterhyperpolarization, causing potential-dependent block of Ca spikes.Key words: estradiol, progesterone, rat myometrium, action potential, channel current.

Instrumentation for the diagnosis of term and preterm labour

The problems associated with labor during pregnancy are among the most important health issues facing physicians. Understanding the role of the uterus and cervix in labor and developing methods to control their function is essential to solving problems relating to labor. At the moment, only crude, inaccurate and subjective methods are used to assess changes in the uterus and cervix that occur in preparation for or during labor. In the past several years, we have developed noninvasive methods to quantitatively evaluate the uterus and cervix based respectively on recording of uterine electrical signals from the abdominal surface (uterine EMG) and measurement of light-induced cervical collagen fluorescence (LIF) with an optical device (Collascope). The methods are rapid and allow assessment of uterine contractility and cervical ripening. Studies in rats and humans indicate that uterine and cervical function can be successfully monitored during pregnancy using these approaches and that these techniques might be used in a variety of conditions associated with labor to better define management. The potential benefits of the proposed instrumentation and methods include a reducing the rate of preterm delivery, improving maternal and perinatal outcome, monitoring treatment, decreasing cesarean section rate and improving research methods to understand uterine and cervical function.

Enhancement of K+ currents by stimulation of protein kinase C in the uterine smooth muscle cells of the pregnant rat

1. Effects of phorbol esters on the K+ currents in isolated rat uterine smooth muscle cells during (18-day) pregnancy were examined using whole-cell voltage-clamp modes. All experiments were performed at room temperature. 2. Test pulses were applied between -20 to + 90 mV from a holding potential of -40 mV. Initially, a transient outward current (ITO) was activated, and outward K+ current (IK) was followed. Threshold potential was - 10 to 0 mV, and the activation was voltage-dependent. At - 80 mV, ITO and IK were 17.8 +/- 3.3 and 13.2 +/- 2.6 pA/pF as a current density. Membrane capacitance was 64.0 +/- 11.5 pF (n = 8). 3. At 0.1 microM, 12-O-tetradecanoylphorbol-13-acetate (TPA) and 4-beta-phorbol-12-13-dibutyrate (PDB) enhanced IK at +80mV by 14.5 +/- 2.0% (n = 8, P < 0.05) and 23.5 +/- 2.2% (n = 7, P < 0.01). Also, ITO at +80mV was increased by 22.1 +/- 2.1% (n = 8, P < 0.01) at 1 microM TPA and by 22.7 +/- 3.0 (n = 7, P < 0.05) at 0.1 microM PDB, significantly. 4. These results indicate that the IK and ITO currents are present in the uterine smooth muscle cells of pregnant rat, and PK-C stimulation modulates the K+ currents, resulting in the regulation of physiological contraction of the uterine muscle during pregnancy.

The lack of a role for potassium channel opening in the action of relaxin in the rat isolated uterus; a comparison with levcromakalim and salbutamol

1. The effects of relaxin in vitro in the isolated uterus from the non-pregnant rat were compared with those of levcromakalim and salbutamol in tissue bath, 42K+ -efflux and electrophysiological studies, to determine whether relaxin exhibits the characteristics of an opener of KATP-channels. 2. In uterus exposed to oxytocin (0.2 nM), tetraethylammonium (TEA, 10 mM) and glibenclamide (10 microM) produced large rightward shifts of the log10 concentration-effect curve to levcromakalim (125 fold and 118 fold, respectively). TEA (10 mM) caused only small rightward shifts of the log10 concentration-effect curves to salbutamol and relaxin (5.2 fold and 7.5 fold respectively). Glibenclamide did not antagonize salbutamol or relaxin. 3. Levromakalim (0.2 and 2 microM) suppressed the spasm evoked by low ( < or = 40 mM) but not high ( > 40 mM) concentrations of KCl. Salbutamol (1.5 nM) inhibited the spasm evoked by low concentrations of KCl ( < or = 40 mM). Salbutamol (15 nM) and relaxin (3 and 30 nM) inhibited the spasm evoked by low and high concentrations of KCl (10-80 mM). 4. Relaxin (0.12 microM) did not produce an increase in 42K+-efflux from longitudinal segments of rat myometrium. Exposure of tissues to relaxin (0.12 microM), in the presence of diltiazem (1 microM) plus KCl (20 mM), resulted in a small increase in 42K+-efflux of short duration. 5. Electrophysiological recording showed that the phasic spasms of the uterus exposed to oxytocin (0.2 nM) were accompanied by bursts of spiking activity superimposed upon a plateau potential. Inhibition of the mechanical activity of the uterus by levcromakalim (2 and 10 microM), salbutamol (30 nM) or relaxin (0.18 microM) was accompanied by a reduction in the duration of the plateau potential and the number of spikes without membrane hyperpolarization. 6. Unlike levcromakalim, relaxin did not selectively inhibit the spasm evoked by low concentrations of KCl and was not markedly antagonized by TEA or glibenclamide. Under conditions where a cromakalim-induced increase of the 42K+-efflux rate has been demonstrated, relaxin had only a very small effect. In isolated uterus from the rat, in contrast to observations in vivo, relaxin did not exhibit the characteristics of an opener of KATP-channels suggesting that another mechanism accounts for its inhibitory action.

Plasma prostaglandin metabolite concentrations in normal and dysfunctional labour

OBJECTIVE

To determine the concentrations of the metabolites of prostaglandin E2 (PGEM) and of prostaglandin F2 alpha (PGFM) prior to the onset of labour and during spontaneous labour, and to correlate the changes in concentrations of these metabolites with labour outcome.

DESIGN

Longitudinal study throughout labour.

SETTING

Labour ward of a large maternity unit.

SUBJECTS

Seven primigravid and 11 parous women in the late third trimester with no signs of labour, and 17 primigravid and 11 parous women in spontaneous labour.

INTERVENTIONS

Six of the primigravid women required augmentation with oxytocin because of dysfunctional labour.

RESULTS

Before labour, parous women had significantly higher concentrations of both PGEM (P < 0.007) and PGFM (P < 0.006) compared with primigravid women. During labour, PGFM concentrations were significantly higher in both primigravid (P < 0.0002) and parous (P < 0.0001) women compared with the concentrations of these metabolites in women not in labour; the same was true for PGEM in primigravid (P < 0.003) but not in parous (P = 0.1) women. There was a small but significant increase (P < 0.02) in PGEM as labour progressed in both the normal groups. Amniotomy was associated with a significant increase in PGFM in primigravid and parous women (P < 0.002 and P < 0.009, respectively). The concentration of PGFM one hour following amniotomy correlated inversely with the amniotomy to delivery interval in both the normal primigravid (r = -0.624; P = 0.04) and the parous (r = 0.745; P = 0.021) groups. Women with dysfunctional labour showed no significant rise in PGEM or PGFM. Their PGFM concentrations were significantly lower than those seen in normal labour (P < 0.05). The concentration of PGFM in cord blood was significantly higher (P < 0.0001) in the parous women who laboured than in women delivered by elective caesarean section. There was no difference in the corresponding concentrations of PGEM (P = 0.9).

CONCLUSIONS

These data show that spontaneous labour is associated with increased concentrations of prostaglandin metabolites in the maternal plasma, and are consistent with PGF2 alpha being an important stimulator of uterine contractility, with a relative deficiency of PGF2 alpha being associated with dysfunctional labour.

Effects of porcine relaxin on contraction, membrane response and cyclic AMP content in rat myometrium in comparison with the effects of isoprenaline and forskolin

1. The longitudinal muscle from the uterus of oestrogen-treated rats was quiescent in Mg-free Krebs solution. Electrical stimulation generated phasic contraction, which was depressed to 35% and 18% by 50 mu and 150 mu porcine relaxin, respectively. 2. The phasic contractions were more strongly depressed to 26% by 50 mu relaxin in solution containing 0.6 mM Mg, and the depression lasted for more than 4 h after the removal of relaxin. During the persisting depression, raising the external Ca to 7.5 mM did not restore the contraction, but the contraction was restored by removal of Mg. 3. The depression of the phasic contraction by relaxin, examined in Mg-free solution, was enhanced and reduced by pretreatment of the tissue with 0.6 mM Mg and 0.6 mM Mn, respectively, for about 15 min. In contrast, the depression of contraction by isoprenaline or forskolin was enhanced by pretreatment with either Mg or Mn. 4. The cellular content of cyclic AMP was measured in Krebs solution containing 0.6 mM Mg. The values were 1.24 (pmol mg-1 protein) in control solution, and 2.31 and 1.56 when the tissues were treated with 150 mu relaxin and 10(-9) M isoprenaline, respectively. 5. The cyclic AMP production in response to 10(-7) M forskolin measured in Mg-free solution was enhanced when the tissue was pretreated with either 0.6 mM Mg or Mn for 15 min. The cyclic AMP production in response to 100 mu relaxin was increased when the tissue was pretreated with 0.6 mM Mg, and was unchanged by pretreatment with Mn. The cyclic AMP production in response to 10(-9) M isoprenaline was unchanged by pretreatment with the divalent cations. 6. The membrane potential of the muscle was -60.8 mV in Krebs solution containing 0.3 mM Mg, and electrical stimulation induced an action potential which consisted of spike and plateau components. Application of 150 mu relaxin reduced the duration of the plateau; the contractions were progressively depressed. The resting membrane potential and membrane resistance were unchanged by application of 150 mu relaxin. The membrane was hyperpolarized by 2.8 mV, accompanied by a decrease in membrane resistance, when 10(-9) M isoprenaline was applied. 7. Although there were several differences between the effects of relaxin and isoprenaline, it is probable that some process, which is cyclic AMP-dependent, accelerated by Mg and depressed by Mn, is involved in the depressant action of relaxin on contraction.

Gestational changes in the utilization of intracellularly stored calcium in the myometrium of guinea-pigs

1. The ability of oxytocin and prostaglandin F2 alpha (PGF) to induce contraction in guinea-pig myometrium in calcium-free solution was studied in an attempt to assess the extent to which intracellular calcium stores could be released by these two agonists. Both longitudinal and circular muscle layers were studied separately and the effects of gestational age were also examined. 2. In longitudinal strips, the responses to oxytocin and PGF in the absence of external calcium decreased progressively throughout gestation. Responses of circular strips to both agonists were unchanged throughout pregnancy, until day 64, when no response to PGF could be elicited. 3. Pre-treatment with high potassium (and normal calcium) increased the responses to the agonists in calcium-free medium while pre-treatment with beta-adrenoceptor agonists had no effect on responses to oxytocin or PGF. 4. Responses to both agonists decreased with time in calcium-free solution suggesting a loss of calcium from stores with a half-time of 3 min. The rate of the decline in the responses was the same in both muscle layers and did not change with gestational age. 5. In the presence of lanthanum contractions evoked by oxytocin, but not PGF, were augmented 2-3-fold. This potentiation of the response to oxytocin occurred in both muscle layers and throughout gestation. 6. Each agonist evoked only one response in calcium-free solution containing EGTA. The response to PGF in longitudinal strips following a challenge with oxytocin was reduced, compared with the response to PGF when applied first while the response to oxytocin in these strips was unchanged following exposure to PGF. In circular strips neither oxytocin- nor PGF-induced contractions were altered following prior exposure to the other agonist. 7. It is concluded that oxytocin and PGF operate via two distinct mechanisms to release intracellularly stored calcium in both longitudinal and circular components of the guinea-pig myometrium and a hypothesis to explain the results is presented.

Single channel Cl- and K+ currents from cells of uterus not treated with enzymes

Single channel currents were recorded from uterine smooth muscle cells of pregnant guinea-pigs. The cells were within small bundles of smooth muscle of low input resistance. Giga-ohm seals were formed between the patch clamp electrodes and the membrane of smooth muscle cells that had not been treated with enzymes. In cell-attached mode, outward current steps were observed that had a conductance of 130-170 pS, a reversal potential close to the potassium reversal potential, and the probability of a channel being in the open state increased e-fold per 8-11 mV depolarization. In the cell-free, inside-out patch mode, currents were recorded that had a large conductance, 420 pS, and smaller conductance levels. When the chloride concentration was changed the reversal potential shifted in a manner consistent with the Nernst equation indicating that the channels were permeable to chloride ions.

Contractions of a human skeletal muscle at different temperatures

1. Influence of temperature on electrically evoked twitch contractions and maximal voluntary contractions was studied in human first dorsal interosseus muscle. The range of the muscle temperature was 35-12 degrees C. 2. The maximal twitch tension decreased by about 50% in cooling from 35-12 degrees C; the tension decrease was more pronounced below 25 degrees C. The temperature coefficients (Q10 values) estimated for muscle temperatures of 35-25 degrees C were 1.43 for time-to-peak and 1.7 for half-time of relaxation. 3. The maximum voluntary tension remained relatively constant on cooling to 25 degrees C but decreased by about 30% on cooling to 12-15 degrees C. The normalized rate of tension rise in voluntary contractions was largely independent of temperature. 4. Results are discussed in relation to previous work on temperature and muscle contraction in humans and in animals.

Evidence for physiological regulation of myometrial gap junction permeability

We have investigated whether direct intercellular communication between uterine smooth muscle cells of delivering rats is influenced by intracellular adenosine 3',5'-cyclic monophosphate concentration ([cAMPi]) and agents relevant to the hormonal control of pregnancy and parturition. The rate of diffusion of phosphorylated 2-[3H]deoxy-D-glucose (2-DG) in strips of longitudinal myometrium from rats in labor, indicated by the apparent diffusion coefficient for this molecule, was observed to be significantly reduced in tissues with elevated [cAMPi] (after treatment with dibutyryl cAMP, 8-bromo cAMP, forskolin, and theophylline) in the absence of any change in the area of gap junctions (GJs). Similarly, several agonists that elevate [cAMPi] in this tissue (e.g., isoproterenol, relaxin, carbacyclin, prostaglandin E2) also reduced 2-DG diffusion. These data suggest that the permeability of GJs in uterine smooth muscle may be regulated by [cAMPi] and physiologically relevant agonists. Control of GJ permeability may be important for the physiological regulation of intercellular communication and the extent of synchronous contractile activity in the uterine wall during pregnancy and parturition.

Some properties of the circular myometrium of the sheep throughout pregnancy and during labour

Spontaneous electrical and mechanical activity were measured in strips of circular myometrium, stretched to in vivo length, obtained from ewes during pregnancy and parturition. The resting tension was high (280 mN/mm2) early in pregnancy and decreased gradually during the course of gestation to reach low levels (20 mN/mm2) during labour. Strips showed continuous electrical and mechanical activity up to around day 50 of pregnancy. The variability in the amplitude of the action potentials, the occurrence of many small fluctuations in membrane potential and the short space constant (0.9 mm) could be explained by impaired conduction in these tissues. During the second half of pregnancy, muscle strips showed regular bursts of spike activity and contractions with a frequency of about 4/min. The space constant (lambda) increased gradually to around 2 mm at term. The membrane time constant (tau m) remained constant at approximately 130 ms throughout gestation. Tissues from ewes in labour were not spontaneously active but action potentials could be evoked by depolarizing current pulses. The space constant increased to 4.6 +/- 0.4 mm and tau m increased to 510 +/- 50 ms at this time. Verapamil (10(-6) M) rapidly abolished electrically evoked activity in strips obtained from ewes in labour. Verapamil abolished spontaneous activity in strips from animals up to day 145 of pregnancy, though a higher concentration (2 X 10(-5) M) was required. However, action potentials could be evoked by depolarizing current pulses and these were accompanied by contractions that were 50-80% of those observed in control solution. Cobalt (2mM) had an effect on spontaneous and electrically induced activity that was similar to that of verapamil. The threshold for the initiation of an action potential was increased in all tissues in the presence of the blockers. Verapamil decreased lambda by about 50% in the later stages of pregnancy and during labour. During labour alone verapamil reduced tau m by about 50%. These results show that the onset of labour in the ewe is associated with rapid and dramatic changes in both the passive and active properties of the circular muscle of the uterus.