[Ca2+]i signaling in pregnant human myometrium

Modeling and experimental approaches for elucidating multi-scale uterine smooth muscle electro- and mechano-physiology: A review

The uterus provides protection and nourishment (via its blood supply) to a developing fetus, and contracts to deliver the baby at an appropriate time, thereby having a critical contribution to the life of every human. However, despite this vital role, it is an under-investigated organ, and gaps remain in our understanding of how contractions are initiated or coordinated. The uterus is a smooth muscle organ that undergoes variations in its contractile function in response to hormonal fluctuations, the extreme instance of this being during pregnancy and labor. Researchers typically use various approaches to studying this organ, such as experiments on uterine muscle cells, tissue samples, or the intact organ, or the employment of mathematical models to simulate the electrical, mechanical and ionic activity. The complexity exhibited in the coordinated contractions of the uterus remains a challenge to understand, requiring coordinated solutions from different research fields. This review investigates differences in the underlying physiology between human and common animal models utilized in experiments, and the experimental interventions and computational models used to assess uterine function. We look to a future of hybrid experimental interventions and modeling techniques that could be employed to improve the understanding of the mechanisms enabling the healthy function of the uterus.

Contractions in the Isolated Uterus of a Rat Model of Polycystic Ovary Syndrome Compared to Controls in Adulthood

BACKGROUND

Polycystic ovary syndrome (PCOS) is one of the most common endocrine disorders in women during the reproductive age. Considering disorders reported in women with PCOS, e.g. infertility, pregnancy complications (premature delivery, caesarean section and spontaneous miscarriages), and hormonal disorders, we hypothesized that uterine contractions in PCOS rats may be different from controls. We aimed to compare uterine contractions in PCOS rats with controls.

METHODS

Rats in the experimental group were subcutaneously injected with 5 mg of free testosterone on gestational day 20, while controls received solvent. The contractions of isolated uterus in offspring of both groups were recorded by the power lab system, after exposure to carbachol and oxytocin. Two-way analysis of variance was used to compare results between 2 groups.

RESULTS

PCOS rats showed more irregular uterine contractions compared to controls. After exposure to carbachol, frequency, and resting tone in the PCOS rats were significantly increased compared to controls (P = 0.004, P = 0.02, respectively). After exposure to oxytocin, the frequency, resting tone and amplitude of rhythmic contractions did not differ between the 2 groups.

CONCLUSIONS

Our study indicates irregular uterine contractions and different mechanical responses of isolated uterus in PCOS rats compared to controls.

BKCa channel regulates calcium oscillations induced by alpha-2-macroglobulin in human myometrial smooth muscle cells

The large-conductance, voltage-gated, calcium (Ca(2+))-activated potassium channel (BKCa) plays an important role in regulating Ca(2+)signaling and is implicated in the maintenance of uterine quiescence during pregnancy. We used immunopurification and mass spectrometry to identify proteins that interact with BKCain myometrium samples from term pregnant (≥37 wk gestation) women. From this screen, we identified alpha-2-macroglobulin (α2M). We then used immunoprecipitation followed by immunoblot and the proximity ligation assay to confirm the interaction between BKCaand both α2M and its receptor, low-density lipoprotein receptor-related protein 1 (LRP1), in cultured primary human myometrial smooth muscle cells (hMSMCs). Single-channel electrophysiological recordings in the cell-attached configuration demonstrated that activated α2M (α2M*) increased the open probability of BKCain an oscillatory pattern in hMSMCs. Furthermore, α2M* caused intracellular levels of Ca(2+)to oscillate in oxytocin-primed hMSMCs. The initiation of oscillations required an interaction between α2M* and LRP1. By using Ca(2+)-free medium and inhibitors of various Ca(2+)signaling pathways, we demonstrated that the oscillations required entry of extracellular Ca(2+)through store-operated Ca(2+)channels. Finally, we found that the specific BKCablocker paxilline inhibited the oscillations, whereas the channel opener NS11021 increased the rate of these oscillations. These data demonstrate that α2M* and LRP1 modulate the BKCachannel in human myometrium and that BKCaand its immunomodulatory interacting partners regulate Ca(2+)dynamics in hMSMCs during pregnancy.

Dexmedetomidine modifies uterine contractions in pregnancy terms of rats

OBJECTIVES

The present study was aimed at determining the effective doses of Dexmedetomidine (Dex) involved in amplitude of contraction-force and frequency of uterine rings in pregnancy terms of rats. All experiments involving animal subjects were carried out with the approval of animal care and use Ethical Committee of Cukurova University. Experiments were performed on female Albino-Wistar rats (200-260 g; n = 40).

MATERIALS AND METHODS

Uterine rings from pregnant rats were placed in organ bath with Krebs and calcium ion (Ca(2+))-free solutions to record and exposed to serially increasing log10 concentrations of Dex.

RESULTS

In Krebs solution, while Dex caused an increase in the spontaneous contraction-forces in all pregnancy terms of rats in a significant dose-dependent manner, it led to a decrease in contraction-frequency in late-pregnancy term of rats. In Ca(2+)-free, the spontaneous contraction-force decreased in late-pregnancy term and increased in early and middle-pregnancy terms. In addition, while Dex increased the contraction-frequency in early and middle-pregnancy terms, it decreased in late-pregnancy term in a dose-dependent manner.

STATISTICAL ANALYSIS USED

The data were subjected to one-way analysis of variance. Repeated measures were employed for comparison of several group means through the Tukey post-hoc test (SPSS 10.00 Inc., Chicago, Ill, USA). P < 0.05 was considered statistically significant.

CONCLUSIONS

This study suggested that Dex might differently alter the spontaneous contraction-forces and contraction-frequencies of uterine rings in all pregnancy terms of rats in Krebs and Ca(2+)-free solutions.

A new slow releasing, H₂S generating compound, GYY4137 relaxes spontaneous and oxytocin-stimulated contractions of human and rat pregnant myometrium

Better tocolytics are required to help prevent preterm labour. The gaseotransmitter Hydrogen sulphide (H₂S) has been shown to reduce myometrial contractility and thus is of potential interest. However previous studies used NaHS, which is toxic and releases H₂S as a non-physiological bolus and thus alternative H₂S donors are sought. GYY4137 has been developed to slowly release H₂S and hence better reflect endogenous physiological release. We have examined its effects on spontaneous and oxytocin-stimulated contractility and compared them to NaHS, in human and rat myometrium, throughout gestation. The effects on contractility in response to GYY4137 (1 nM–1 mM) and NaHS (1 mM) were examined on myometrial strips from, biopsies of women undergoing elective caesarean section or hysterectomy, and from non-pregnant, 14, 18, 22 day (term) gestation or labouring rats. In pregnant rat and human myometrium dose-dependent and significant decreases in spontaneous contractions were seen with increasing concentrations of GYY4137, which also reduced underlying Ca transients. GYY4137 and NaHS significantly reduced oxytocin-stimulated and high-K depolarised contractions as well as spontaneous activity. Their inhibitory effects increased as gestation advanced, but were abruptly reversed in labour. Glibenclamide, an inhibitor of ATP-sensitive potassium (K_ATP) channels, abolished the inhibitory effect of GYY4137. These data suggest (i) H₂S contributes to uterine quiescence from mid-gestation until labor, (ii) that H₂S affects L-type calcium channels and K_ATP channels reducing Ca entry and thereby myometrial contractions, (iii) add to the evidence that H₂S plays a physiological role in relaxing myometrium, and thus (iv) H₂S is an attractive target for therapeutic manipulation of human myometrial contractility.

What do we know about what happens to myometrial function as women age?

Much has been written about the effects of aging on reproductive function, especially female fertility. Much less is known about how aging may affect the contractility of the smooth muscle within the uterus, the myometrium. The myometrium is active through a woman’s entire life, not just during pregnancy. Here we will discuss briefly the contractile functions of the uterus and the changes it undergoes throughout the stages of a woman’s life from menstruation and the menopause, before evaluating the evidence for any changes in myometrial contractility and responses as women age, with a particular focus on women of advanced maternal age. We present original contractility analysis for the widest data set for human myometrium so far examined, and determine inherent spontaneous activity as well as responses to depolarisation and stimulation with oxytocin. Our data show that in the non-pregnant state there is a significant decrease in contractility for both spontaneous and depolarised-induced contractions, with age. We suggest that muscle atrophy and down regulation of Ca channels may account for this. Interestingly in pregnant myometrium we found a wide range of contractile ability between women and little evidence for decreased spontaneous activity between the ages of 25–40. Oxytocin responses appear to be more affected by aging, a finding that is consistent with previously reported clinical findings, and may partly be the result of membrane lipids such as cholesterol, increasing as women age. The marked differences between the age-related decline of force beyond age 30 in non-pregnant uterus, and the lack of difference in the pregnant state over this period, shows that the uterus retains its ability to respond to gestational hormones. The growth of the pregnant uterus and increase in content of myofibrillar proteins, may abolish any previous age-related force deficit. This finding is consistent with what is apparent for postmenopausal women in their 50s and 60s; that with the appropriate hormonal stimulation the uterus can allow an embryo to implant, and then without further intervention, carry the foetus to term. It is tempting therefore to speculate that unlike other well documented declines in female reproductive functions with age, the myometrium remains able to function into a woman’s 7th decade.

Phosphorylation of h1 calponin by PKC epsilon may contribute to facilitate the contraction of uterine myometrium in mice during pregnancy and labor

BACKGROUND

The timely onset of powerful uterine contractions during parturition occurs through thick and thin filament interactions, similar to other smooth muscle tissues. Calponin is one of the thin filament proteins. Phosphorylation of calponin induced by PKC-epsilon can promote the contraction of vascular smooth muscle. While the mechanism by which calponin regulates the contraction of pregnant myometrium has rarely been explored. Here, we explore whether PKC-epsilon/h1 calponin pathway contribute to regulation of myometrial contractility and development of parturition.

METHODS

We detected the expression of h1 calponin, phosphorylated h1 calponin, PKC-epsilon and phosphorylated PKC-epsilon in the different stages of mice during pregnancy and in labor by the method of western blot and recorded the contraction activity of myometrium strips at the 19th day during pregnancy with different treatments by the organ bath experiments.

RESULTS

The level of the four proteins including h1 calponin, phosphorylated h1 calponin, PKC-epsilon and phosphorylated PKC-epsilon was significantly increased in pregnant mice myometrium as compared with that in nonpregnant mice. The ratios of phosphorylated h1 calponin/h1 calponin and phosphorylated PKC-epsilon/PKC-epsilon were reached the peak after the onset of labor in myometrium in the mice. After the treatment of more than 10(9-) mol/L Psi-RACK (PKC-epsilon activator), the contractility of myometrium strips from mice was reinforced and the level of phosphorylated h1 calponin increased at the same time which could be interrupted by the specific inhibitor of PKC-epsilon. Meanwhile, the change of the ratio of phosphorylated h1 calponin/h1 calponin was consistent with that of contraction force of mice myometrium strips.

CONCLUSIONS

These data suggest that in mice myometrium, phosphorylation of h1 calponin induced by the PKC-epsilon might facilitate the contraction of uterine in labor and regulate pregnant myometrial contractility.

Maternal obesity and its relationship with spontaneous and oxytocin-induced contractility of human myometrium in vitro

Maternal obesity is associated with increased rates of labor induction, dysfunctional labor requiring intrapartum cesarean delivery, and postpartum hemorrhage, implying that maternal obesity has an inhibitory effect on myometrial function and its ability to respond to oxytocin. This study aimed to use an in vitro model to investigate the relationship between maternal body mass index (BMI) and the ability of myometrium to contract spontaneously and in response to oxytocin. Linear mixed effects regression modeling was applied to contractile data from 609 strips from 85 women. No correlation was found between maternal BMI and any indices of spontaneous myometrial activity. A single addition of oxytocin increased contractility, however, this was not related to maternal BMI. Similarly, oxytocin concentration-response curves were unrelated to BMI. Overall, the results from this in vitro study suggest that the observed implications of obesity on parturition in vivo cannot be explained by a direct effect on myometrial contractile mechanisms per se.

Mathematical model of excitation-contraction in a uterine smooth muscle cell

Uterine contractility is generated by contractions of myometrial smooth muscle cells (SMCs) that compose most of the myometrial layer of the uterine wall. Calcium ion (Ca2+) entry into the cell can be initiated by depolarization of the cell membrane. The increase in the free Ca2+concentration within the cell initiates a chain of reactions, which lead to formation of cross bridges between actin and myosin filaments, and thereby the cell contracts. During contraction the SMC shortens while it exerts forces on neighboring cells. A mathematical model of myometrial SMC contraction has been developed to study this process of excitation and contraction. The model can be used to describe the intracellular Ca2+concentration and stress produced by the cell in response to depolarization of the cell membrane. The model accounts for the operation of three Ca2+control mechanisms: voltage-operated Ca2+channels, Ca2+pumps, and Na+/Ca2+exchangers. The processes of myosin light chain (MLC) phosphorylation and stress production are accounted for using the cross-bridge model of Hai and Murphy ( Am J Physiol Cell Physiol 254: C99–C106, 1988) and are coupled to the Ca2+concentration through the rate constant of myosin phosphorylation. Measurements of Ca2+, MLC phosphorylation, and force in contracting cells were used to set the model parameters and test its ability to predict the cell response to stimulation. The model has been used to reproduce results of voltage-clamp experiments performed in myometrial cells of pregnant rats as well as the results of simultaneous measurements of MLC phosphorylation and force production in human nonpregnant myometrial cells.

Role of ERK1/2 in uterine contractility and preterm labor in rats

The present study tested the hypothesis that ERK activation is an essential step in the onset of labor in a rat model of preterm labor. The administration of RU-486, an antiprogesterone agent, to rats induced preterm delivery 22.2 +/- 0.24 h after treatment. Changes in basal signaling events were studied in myometrial tissue from CO(2)-euthanized rats. Rats treated with RU-486 displayed a dramatically increased in vitro uterine contractility compared with gestational stage-matched, sham-treated rats. In vitro contractility was not significantly different from that during spontaneous labor. During RU-486-induced preterm labor, as previously described for spontaneous labor, ERK phosphorylation levels increased, as did phosphorylation of caldesmon at Ser(789), an ERK phosphorylation site. Also, a small but significant increase in 20-kDa myosin light chain phosphorylation was seen at a constant intracellular pCa of 7. When rats were chronically treated with an agent that prevents ERK activation, U-0126, the onset of RU-486-induced preterm labor was delayed in a statistically significant manner. Chronic in vivo treatment with U-0126 also significantly inhibited the RU-486-induced increase in in vitro contractility and ERK and caldesmon phosphorylation but did not alter the RU-486-induced increase in 20-kDa myosin light chain phosphorylation. These data indicate that ERK activation is a component of the multiple events leading to the development of labor in this rat model. We suggest that the ERK pathway could possibly be used to identify targets for the development of a novel class of tocolytic agents.

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.

Conventional-type protein kinase C contributes to phorbol ester-induced inhibition of rat myometrial tension

1 Phorbol ester decreases muscle tension in the rat myometrium, and the effect is more potent in late-pregnant myometrium than in nonpregnant myometrium. In the present study, we have examined the contribution of protein kinase C (PKC) isoforms to the phorbol ester-induced inhibition of tension in rat uterine smooth muscle. 2 Thymeleatoxin (THX), a selective activator of conventional-type PKC (cPKC), and 12-deoxyphorbol 13-isobutyrate (DPB), an activator of pan PKC, inhibited the tension induced by high K(+), and inhibitions were significantly increased in pregnant myometrium compared to nonpregnant myometrium. The inhibition by DPB and THX of high K(+)-induced tension was significantly attenuated when PKC was downregulated by long-term pretreatment with THX and inhibited by Go6976, a cPKC inhibitor. 3 Of the cPKCs, PKC alpha is predominantly expressed in the rat myometrium, as detected by Western blot analysis. The expression of PKC alpha gradually increases from the beginning of gestation, reaching a maximum at day 21 of pregnancy. Treatment with DPB induced PKC alpha to translocate from the cytosol to the membrane in the pregnant myometrium. PKC epsilon and PKC zeta, other dominant PKC isoforms in the rat myometrium, decrease during gestation, reaching a minimum in late pregnancy. 4 These results suggest that cPKC may be at least partly involved in the PKC-mediated inhibition of muscle tension in the rat myometrium.

ERK1/2-mediated phosphorylation of myometrial caldesmon during pregnancy and labor

We used a timed-pregnant rat model to track changes in myometrial contractility during pregnancy and labor and to correlate these changes with upstream signaling events. Myometrium was harvested from CO(2)-euthanized rats. Although contraction amplitudes increased at 16 and 20 days of pregnancy, contraction incidence and area under the force curve were inhibited, consistent with the myometrial quiescence of pregnancy. The Ca(2+) sensitivity of contraction was decreased at 20 days of pregnancy and this was partially reversed in labor. The protein content of h-caldesmon (h-CaD) was increased in pregnancy. A 40-fold increase in the signal from a phospho-CaD antibody specific for phosphorylation at an ERK1/2 site occurred during labor. ERK1/2 activation increased significantly at the onset of labor. Myosin light chain phosphorylation (LC20-P) increased significantly in labor compared with the nonpregnant state. Thus we conclude that the increase in CaD protein content during pregnancy may contribute to a suppression of the contractility of pregnant myometrium. Conversely, CaD phosphorylation, through an ERK1/2-mediated signaling pathway, as well as an increase in basal LC20-P, is suggested to contribute to the reversal of inhibition and promote contraction of the uterus during labor.

Simultaneous measurements of changes in sarcoplasmic reticulum and cytosolic

1. The role of the sarcoplasmic reticulum (SR) was investigated in spontaneous and agonist-induced uterine Ca2+ transients, by combining low- (mag-fluo-4) and high-affinity (fura-2) indicators to measure intraluminal SR ([Ca2+]L) and cytosolic ([Ca2+]i) calcium concentration, simultaneously, in single smooth muscle cells from pregnant rat uterus. 2. Carbachol or ATP, in the absence of extracellular Ca2+, decreased [Ca2+]L and increased [Ca2+]i. Although some replenishment (around 50 %) occurred in its absence, extracellular Ca2+ was required for full replenishment of the SR Ca2+. 3. In 4/15 cells, ATP evoked oscillations of [Ca2+]i. These were accompanied by successive release and re-uptake of SR Ca2+. Inhibition of the SR Ca2+-ATPase with thapsigargin abolished the oscillations and luminal changes. 4. Spontaneous [Ca2+]i transients produced no detectable changes in [Ca2+]L. The larger [Ca2+]i transients evoked by high-K+ depolarisation increased [Ca2+]L. Spontaneous activity was inhibited when [Ca2+]L was increased. 5. These data show that it is possible to simultaneously measure SR and cytosolic [Ca2+], and to investigate their response to agonist application and spontaneous activity.

Preterm labour

Spontaneous preterm labour remains a major obstetric problem because of the high incidence of neonatal mortality or long-term handicap associated with it. The drugs available for the management of preterm labour are poorly effective and have potentially serious side-effects for the mother or fetus. In recent years, there has been a remarkable increase in the knowledge of the biochemical mechanism underlying uterine quiescence and contractility. Many of the G protein-coupled receptors that participate in the regulation of myometrial activity have been cloned and characterized, and their intracellular signalling pathways have been elucidated. The role of G protein receptor kinases in uterine tachyphylaxis is better understood. New developments in our understanding of the cellular mechanisms involved in uterine contractions in idiopathic and infection-associated preterm labour are expected, which will lead to better, more selective therapy for this problem. However, much research remains to be done before the mechanism of human parturition is fully understood.

Pharmacologic characterization of the oxytocin receptor in human uterine smooth muscle cells

[(3)H]-oxytocin was used to characterize the oxytocin receptor found in human uterine smooth muscle cells (USMC). Specific binding of [(3)H]-oxytocin to USMC plasma membranes was dependent upon time, temperature and membrane protein concentration. Scatchard plot analysis of equilibrium binding data revealed the existence of a single class of high-affinity binding sites with an apparent equilibrium dissociation constant (K(d)) of 0.76 nM and a maximum receptor density (B(max)) of 153 fmol mg(-1) protein. The Hill coefficient (n(H)) did not differ significantly from unity, suggesting binding to homogenous, non-interacting receptor populations. Competitive inhibition of [(3)H]-oxytocin binding showed that oxytocin and vasopressin (AVP) receptor agonists and antagonists displaced [(3)H]-oxytocin in a concentration-dependent manner. The order of potencies for peptide agonists and antagonists was: oxytocin>[Asu(1,6)]-oxytocin>AVP= atosiban>d(CH(2))(5)Tyr(Me)AVP>[Thr(4),Gly(7)]-oxytocin>dDAVP, and for nonpeptide antagonists was: L-371257>YM087>SR 49059>OPC-21268>SR 121463A>OPC-31260. Oxytocin significantly induced concentration-dependent increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) and hyperplasia in USMC. The oxytocin receptor antagonists, atosiban and L-371257, potently and concentration-dependently inhibited oxytocin-induced [Ca(2+)](i) increase and hyperplasia. In contrast, the V(1A) receptor selective antagonist, SR 49059, and the V(2) receptor selective antagonist, SR 121463A, did not potently inhibit oxytocin-induced [Ca(2+)](i) increase and hyperplasia. The potency order of antagonists in inhibiting oxytocin-induced [Ca(2+)](i) increase and hyperplasia was similar to that observed in radioligand binding assays. In conclusion, these data provide evidence that the high-affinity [(3)H]-oxytocin binding site found in human USMC is a functional oxytocin receptor coupled to [Ca(2+)](i) increase and cell growth. Thus human USMC may prove to be a valuable tool in further investigation of the physiologic and pathophysiologic roles of oxytocin in the uterus. British Journal of Pharmacology (2000) 129, 131 - 139

Properties of voltage-activated [Ca2+]i transients in single smooth muscle cells isolated from pregnant rat uterus

1. The intracellular calcium concentration ([Ca2+]i) was measured at 35 degrees C using the fluorescent indicator indo-1 in patch-clamped, single uterine myocytes from pregnant rats to investigate the relationship between depolarization, Ca2+ current (ICa) and [Ca2+]i. 2. Membrane depolarization activated ICa and produced a [Ca2+]i transient. The rapid increase in [Ca2+]i occurred at the same time as the inward ICa. Both ICa and the increase in [Ca2+]i were abolished by nifedipine (10 microM). 3. When the membrane potential was held at -80 mV the threshold depolarization for an increase in [Ca2+]i was about -55 to -50 mV. As the magnitude of the depolarization was increased to about 0 mV there was an increase in the size of both ICa and the increase in [Ca2+]i. As the magnitude of the depolarization was further increased both ICa and the [Ca2+]i increase declined. 4. When the depolarizing pulses were applied at 3 Hz to mimic normal action potentials then the individual [Ca2+]i transients did not fully relax and a tetanic rise of [Ca2+]i was observed. Under these conditions, there was not a simple relationship between the magnitude of the Ca2+ response and Ca2+ entry. When pairs of depolarizing pulses were applied, the increase in [Ca2+]i produced by the second pulse was larger (in relation to the magnitude of the L-type Ca2+ current) than that produced by the first pulse. This facilitation was abolished by both ryanodine and cyclopiazonic acid suggesting a role for release from intracellular stores. 5. We conclude that the L-type Ca2+ current is the major source of Ca2+ ions entering the cell to produce the [Ca2+]i transient on depolarization. The magnitude of the increase in [Ca2+]i may, however, be amplified by Ca2+-induced Ca2+ release.

Changes in responsiveness of freshly isolated longitudinal muscle cells from rat uterus towards oxytocin during gestation: contractility and calcium signaling

Changes in responsiveness of freshly isolated longitudinal muscle cells from rat uterus to oxytocin during gestation were investigated through measuring contractility as well as intracellular free calcium concentration. We have demonstrated the pregnant stage-dependent contraction of freshly isolated myometrial cells in response to an extracellular hormone, oxytocin, in Ca2+-containing medium. The oxytocin effect appeared to be through oxytocin receptor since the effect could be blocked by a specific oxytocin antagonist. The magnitude of the contraction of the isolated cells in response to extracellular oxytocin was in the order of 21 day >> 18 day > 15 day pregnant rat longitudinal muscle cells. In a concentration dependent manner, oxytocin elicited a rapid increase in [Ca2+]i of longitudinal muscle cells isolated from different stages of the pregnant rat uterus, especially at the term of pregnancy. The time (4-5 s) required to reach a maximum increase in [Ca2+]i of the isolated longitudinal muscle cells in response to oxytocin was the shortest among all previously reported studies. The results also indicated that the freshly prepared longitudinal muscle cells maintained their functional calcium signaling system. The order of the responsiveness of the isolated longitudinal muscle cells to oxytocin was 21 day >> 18 day > 15 day pregnant rats in terms of rate, affinity and magnitude. Oxytocin appears to transmit its signal mainly through stimulating a voltage-dependent and/or receptor operated nonselective calcium channel. However, the possibility that a part of the oxytocin action occurs through stimulating the release of calcium from intracellular store sites of longitudinal muscle still remains.

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

1. The contractile effects of platelet activating factor (PAF) were compared in the myometrium isolated from non-pregnant and pregnant rats. 2. In the non-pregnant myometrium, PAF, at a concentration of 0.1 microM, did not change muscle tension and induced only a small transient contraction at 10 microM. 3. The contractile responses to PAF increased with the progress of gestation. In the late pregnant myometrium (21 day after gestation), PAF (0.1 nM-10 microM) induced large and relatively sustained contractions. The threshold concentration of PAF was decreased by approximately 10,000 times and the maximum contraction was increased 5 times by day 21 of gestation. 4. PAF (10 microM) increased the cytosolic Ca2+ concentration ([Ca2+]i) and muscle contraction to levels higher than those induced by high K+ in the pregnant rat myometrium (day 21). Verapamil (10 microM), a voltage-dependent Ca2+ channel blocker, decreased the stimulated [Ca2+]i and muscle tension to 49.6% and 22.7%, respectively, while the same concentration of verapamil completely inhibited the high K(+)-induced responses. 5. PAF (10 microM) induced a transient increase in [Ca2+]i with no contraction in the absence of external Ca2+ in the pregnant myometrium (day 21). 6. These results suggest that PAF induces contraction in rat myometrium by increasing Ca2+ influx. Although PAF released Ca2+ from stored sites, this Ca2+ does not seem to contribute to the PAF-induced contraction. Our major finding is that the sensitivity of the myometrium to PAF increased after gestation and that this may play a role in delivery.