Inositol 1,4,5-trisphosphate and oxytocin binding in human myometrium

The expression of genes involved in myometrial contractility changes during ex situ culture of pregnant human uterine smooth muscle tissue

Background: Ex situ analyses of human myometrial tissue has been used to investigate the regulation of uterine quiescence and transition to a contractile phenotype. Following concerns about the validity of cultured primary cells, we examined whether myometrial tissue undergoes culture-induced changes ex situ that may affect the validity of in vitro models. Objectives: To determine whether human myometrial tissue undergoes culture-induced changes ex situ in Estrogen receptor 1 (ESR1), Prostaglandin-endoperoxide synthase 2 (PTGS2) and Oxytocin receptor (OXTR) expression. Additionally, to determine whether culture conditions approaching the in vivo environment influence the expression of these key genes. Methods: Term non-laboring human myometrial tissues were cultured in the presence of specific treatments, including; serum supplementation, progesterone and estrogen, cAMP, PMA, stretch or NF-κB inhibitors. ESR1, PTGS2 and OXTR mRNA abundance after 48 h culture was determined using quantitative RT-PCR. Results: Myometrial tissue in culture exhibited culture-induced up-regulation of ESR1 and PTGS2 and down-regulation of OXTR mRNA expression. Progesterone prevented culture-induced increase in ESR1 expression. Estrogen further up-regulated PTGS2 expression. Stretch had no direct effect, but blocked the effects of progesterone and estrogen on ESR1 and PTGS2 expression. cAMP had no effect whereas PMA further up-regulated PTGS2 expression and prevented decline of OXTR expression. Conclusion: Human myometrial tissue in culture undergoes culture-induced gene expression changes consistent with transition toward a laboring phenotype. Changes in ESR1, PTGS2 and OXTR expression could not be controlled simultaneously. Until optimal culture conditions are determined, results of in vitro experiments with myometrial tissues should be interpreted with caution.

Myometrial cytokines and their role in the onset of labour

Human labour is an inflammatory event, physiologically driven by an interaction between hormonal and mechanical factors and pathologically associated with infection, bleeding and excessive uterine stretch. The initiation and communicators of inflammation is still not completely understood; however, a key role for cytokines has been implicated. We summarise the current understanding of the nature and role of cytokines, chemokines and hormones and their involvement in signalling within the myometrium particularly during labour.

Cyclic AMP Effectors Regulate Myometrial Oxytocin Receptor Expression

The factors that initiate human labor are poorly understood. We have tested the hypothesis that a decline in cAMP/protein kinase A (PKA) function leads to the onset of labor. Initially, we identified myometrial cAMP/PKA-responsive genes (six up-regulated and five down-regulated genes) and assessed their expression in myometrial samples taken from different stages of pregnancy and labor. We found that the oxytocin receptor (OTR) was one of the cAMP-repressed genes, and, given the importance of OTR in the labor process, we studied the mechanisms involved in greater detail using small interfering RNA, chemical agonists, and antagonists of the cAMP effectors. We found that cAMP-repressed genes, including OTR, increased with the onset of labor. Our in vitro studies showed that cAMP acting via PKA reduced OTR expression but that in the absence of PKA, cAMP acts via exchange protein activated by cAMP (EPAC) to increase OTR expression. In early labor myometrial samples, PKA levels and activity declined and Epac1 levels increased, perhaps accounting for the increase in myometrial OTR mRNA and protein levels at this time. In vitro exposure of myometrial cells to stretch and IL-1β increased OTR levels and reduced basal and forskolin-stimulated cAMP and PKA activity, as judged by phospho-cAMP response element-binding protein levels, but neither stretch nor IL-1β had any effect on PKA or EPAC1 levels. In summary, there is a reduction in the activity of the cAMP/PKA pathway with the onset of human labor potentially playing a critical role in regulating OTR expression and the transition from myometrial quiescence to activation.

Quantitative properties and receptor reserve of the DAG and PKC branch of G(q)-coupled receptor signaling

G_q protein–coupled receptors (G_qPCRs) of the plasma membrane activate the phospholipase C (PLC) signaling cascade. PLC cleaves the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP₂) into the second messengers diacylgycerol (DAG) and inositol 1,4,5-trisphosphate (IP₃), leading to calcium release, protein kinase C (PKC) activation, and in some cases, PIP₂ depletion. We determine the kinetics of each of these downstream endpoints and also ask which is responsible for the inhibition of KCNQ2/3 (K_V7.2/7.3) potassium channels in single living tsA-201 cells. We measure DAG production and PKC activity by Förster resonance energy transfer–based sensors, and PIP₂ by KCNQ2/3 channels. Fully activating endogenous purinergic receptors by uridine 5′triphosphate (UTP) leads to calcium release, DAG production, and PKC activation, but no net PIP₂ depletion. Fully activating high-density transfected muscarinic receptors (M₁Rs) by oxotremorine-M (Oxo-M) leads to similar calcium, DAG, and PKC signals, but PIP₂ is depleted. KCNQ2/3 channels are inhibited by the Oxo-M treatment (85%) and not by UTP (<1%), indicating that depletion of PIP₂ is required to inhibit KCNQ2/3 in response to receptor activation. Overexpression of A kinase–anchoring protein (AKAP)79 or calmodulin (CaM) does not increase KCNQ2/3 inhibition by UTP. From these results and measurements of IP₃ and calcium presented in our companion paper (Dickson et al. 2013. J. Gen. Physiol.http://dx.doi.org/10.1085/jgp.201210886), we extend our kinetic model for signaling from M₁Rs to DAG/PKC and IP₃/calcium signaling. We conclude that calcium/CaM and PKC-mediated phosphorylation do not underlie dynamic KCNQ2/3 channel inhibition during G_qPCR activation in tsA-201 cells. Finally, our experimental data provide indirect evidence for cleavage of PI(4)P by PLC in living cells, and our modeling revisits/explains the concept of receptor reserve with measurements from all steps of G_qPCR signaling.

Endocrinology of parturition

The myometrium must remain relatively quiescent during pregnancy to accommodate growth and development of the feto-placental unit, and then must transform into a highly coordinated, strongly contracting organ at the time of labour for successful expulsion of the new born. The control of timing of labour is complex involving interactions between mother, fetus and the placenta. The timely onset of labour and delivery is an important determinant of perinatal outcome. Both preterm birth (delivery before 37 week of gestation) and post term pregnancy (pregnancy continuing beyond 42 weeks) are both associated with a significant increase in perinatal morbidity and mortality. There are multiple paracrine/autocrine events, fetal hormonal changes and overlapping maternal/fetal control mechanisms for the triggering of parturition in women. Our current article reviews the mechanisms for uterine distension and reduced contractions during pregnancy and the parturition cascade responsible for the timely and spontaneous onset of labour at term. It also discusses the mechanisms of preterm labour and post term pregnancy and the clinical implications thereof.

Inositol trisphosphate receptors in smooth muscle cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of tetrameric intracellular calcium (Ca(2+)) release channels that are located on the sarcoplasmic reticulum (SR) membrane of virtually all mammalian cell types, including smooth muscle cells (SMC). Here, we have reviewed literature investigating IP(3)R expression, cellular localization, tissue distribution, activity regulation, communication with ion channels and organelles, generation of Ca(2+) signals, modulation of physiological functions, and alterations in pathologies in SMCs. Three IP(3)R isoforms have been identified, with relative expression and cellular localization of each contributing to signaling differences in diverse SMC types. Several endogenous ligands, kinases, proteins, and other modulators control SMC IP(3)R channel activity. SMC IP(3)Rs communicate with nearby ryanodine-sensitive Ca(2+) channels and mitochondria to influence SR Ca(2+) release and reactive oxygen species generation. IP(3)R-mediated Ca(2+) release can stimulate plasma membrane-localized channels, including transient receptor potential (TRP) channels and store-operated Ca(2+) channels. SMC IP(3)Rs also signal to other proteins via SR Ca(2+) release-independent mechanisms through physical coupling to TRP channels and local communication with large-conductance Ca(2+)-activated potassium channels. IP(3)R-mediated Ca(2+) release generates a wide variety of intracellular Ca(2+) signals, which vary with respect to frequency, amplitude, spatial, and temporal properties. IP(3)R signaling controls multiple SMC functions, including contraction, gene expression, migration, and proliferation. IP(3)R expression and cellular signaling are altered in several SMC diseases, notably asthma, atherosclerosis, diabetes, and hypertension. In summary, IP(3)R-mediated pathways control diverse SMC physiological functions, with pathological alterations in IP(3)R signaling contributing to disease.

Overview. Preterm labour: mechanisms and management

Preterm birth remains a major cause of perinatal mortality and long term handicap in surviving infants. This is one of the most important clinical problems in Europe and across the world. While some preterm births are iatrogenic, associated with severe complications of pregnancy (e.g. hypertensive disorders, antepartum haemorrhage, infection), or the result of multiple pregnancies following assisted reproduction, a high proportion of preterm births occur following spontaneous preterm labour of unknown cause. Early intervention in this group of women would have a significant impact on neonatal mortality and morbidity figures. However, the endocrine changes preceding parturition in women remain elusive and this makes it difficult to predict spontaneous labour at term, let alone preterm labour. Moreover our understanding of myometrial physiology remains rudimentary, limiting our options to devise improved pharmacological strategies to control uterine contractility when this is indicated. There is a need for concerted European and international research efforts to improve our knowledge of the mechanism of labour in women, to identify diagnostic markers to predict preterm labour and to develop uterine selective drugs to inhibit uterine contractions in a safe and efficient manner. This aim will be achieved by multidisciplinary research efforts from academics and industry, using traditional laboratory and clinical research methods, as well as novel technologies.

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.

Oxytocin receptors in guinea pig myometrium near term and during labor

Oxytocin receptors in myometrium of women, rats, and rabbits rise markedly before the onset of labor, suggesting a role in the initiation of labor. In guinea pigs, a previous study reported no such rise by one-point determination of oxytocin binding. The purpose of this study was to use a more rigorous method to determine whether the binding characteristics of myometrial oxytocin receptors change in relation to labor in guinea pigs. Competitive binding studies were carried out in microsomes from inner and outer myometrium between 42 days of gestation and labor. Binding to analogs was also tested. Data were analyzed with affinity spectra and LIGAND. Oxytocin bound to one site with a dissociation constant of 6.3 +/- 0.65 x 10(-9) M. Binding capacity was 1.0 +/- 0.1 x 10(-12) mol/mg protein. The Hill coefficient was near unity. No significant changes occurred with gestation or labor in dissociation constant, binding capacity, or Hill coefficient (all P >/= 0.2, nested ANOVA). Binding capacity was higher in the outer than in the inner layer (1.2 +/- 0.2 vs. 0.8 +/- 0.1 x 10(-12) mol/mg protein, P = 0.02), but the dissociation constants were similar. Differences existed in the dissociation constants of the analogs tested. The main conclusion is that oxytocin receptors are unlikely to have a regulatory role in the initiation of labor in guinea pigs.

Intravenous oxytocin in patients undergoing elective cesarean section

The objective of this study was to compare four different doses of oxytocin to determine its minimal effective dose during elective cesarean section. A prospective, double-blind, randomized study was undertaken in 40 healthy term parturients presenting for elective cesarean section under regional anesthesia. Subjects were assigned to one of four groups. Group I received 5 IU, Group II 10 IU, Group III 15 IU, and Group IV 20 IU of oxytocin after clamping of the umbilical cord. Uterine tone was assessed by palpation on a linear analog scale (LAS) of 0 to 10 (0 = completely atonic, 10 = fully contracted) at 5, 10, 15 and 20 min after the start of oxytocin infusion. Estimated blood loss (EBL) and the difference in pre- and postoperative hematocrit (delta Hct) were also recorded. At alpha = 0.05, the study design had a power of 95% to detect a 25% difference in the LAS between the four groups. There were no differences in the uterine tone in the four groups at any of the four intervals. EBL and delta Hct were similar in all four groups. There appears to be no benefit in terms of degree of uterine contraction and amount of blood loss to administering more than 5 IU of intravenous oxytocin to term parturients undergoing elective cesarean section with a neuraxial block.

Correlation between the stimulatory effect of oxytocin on the formation of inositol phosphates and the oxytocin receptor level in the pregnant rabbit myometrium

OBJECTIVE

In order to elucidate the roles of inositol trisphosphate (IP3) and oxytocin (OT) receptors in rabbit parturition, the concentration of IP3 induced by OT and the OT receptor levels were determined in rabbit myometria before and after parturition.

METHODS

The effects of OT on IP3 formation and OT receptor levels were determined in the myometria of non-pregnant rabbits, Days 26, 28 and 30 of pregnancy rabbits, postpartum rabbits within 12 hours and steroid-treated ovariectomized rabbits. Prostaglandins (PGs) levels were also measured in the myometrial and decidual tissues.

RESULTS

OT receptors were not detectable in the myometria of non-pregnant rabbits, and OT had no effect on the formation of inositol phosphates (IPs). On Day 28 of pregnancy, OT receptors became detectable, and then OT could induce the formation of IPs. Thereafter, the stimulatory effects of OT on IPs formation and the OT receptor levels dramatically increased toward the end of pregnancy and reduced rapidly after parturition. When the ovariectomized pregnant rabbits were treated with estrogen, OT receptors in the myometrium were induced, and OT acquired the ability to stimulate IP3 formation. However, OT had no effect on the production of cAMP, cGMP, prostaglandin (PG) E2 and F2 alpha in the myometria, even if receptors existed, although PGE2 production in the decidual tissues was markedly stimulated. In addition, an OT receptor antagonist inhibited the stimulatory effects of OT on IP3 formation.

CONCLUSIONS

These results suggest that the formation of IPs by OT, the OT receptor levels in the myometrium, and the production of PGF2 in the decidua might play crucial roles in parturition.

Characterization of an oxytocin-induced rise in [Ca2+]i in single human myometrium smooth muscle cells

The effect of the uterotonic pituitary hormone oxytocin on the regulation of intracellular calcium concentration ([Ca2+]i) was studied in single cells of a smooth muscle cell line derived from human non-pregnant myometrium. [Ca2+]i was measured with fluorescence microscopy, and by recording the activity of Ca(2+)-activated potassium currents (IK(Ca)) on the whole cell and single channel level. Oxytocin induced a rapid and transient increase in [Ca2+]i that was paralleled by a significant increase in IK(Ca) activity. After removal of extracellular Ca2+, repetitive stimulation with oxytocin did not alter the [Ca2+]i transients initially; however, their amplitude became progressively smaller and the response was eventually abolished completely, indicating that oxytocin increased [Ca2+]i by release of Ca2+ from intracellular stores. Nifedipine did not alter the oxytocin-induced [Ca2+]i-transients suggesting that oxytocin failed to activate Ca2+ entry through voltage-operated Ca2+ channels. Thapsigargin abolished the oxytocin-induced [Ca2+]i transient. Caffeine alone had no effect on [Ca2+]i, however it diminished the oxytocin-induced [Ca2+]i transients. Ryanodine did not affect the oxytocin response indicating that these cells lack release of Ca2+ from the ryanodine receptor release channel. These results demonstrate that oxytocin elicited [Ca2+]i transients predominantly through Ca2+ release from thapsigargin-sensitive stores, presumably by activating an inositol 1,4,5-trisphosphate dependent pathway.

Effect of inhibition of the electrogenic Na+/K+ pump on the mechanical activity in the rat uterus

The effects of ouabain and K(+)-free solution were studied in estrogen-primed rat uterine strips under resting tone or repeatedly stimulated with KCl, acetylcholine or oxytocin applied for 20 minutes at 60 minute intervals. These effects were compared with those of the K+ channel opener cromakalim. In preparations under resting tone, ouabain (0.1 mM and 0.3 mM) induced rhythmic contractions which disappeared after 20-30 minutes whereas at a higher concentration (1 mM) it evoked a rapid, phasic response followed by a small tonic contraction. Exposure of the strip to a K(+)-free solution induced either rhythmic waves, which ceased after 8-10 minutes, or a single phasic contraction which was followed by a small and slow increase in the resting tone (54 +/- 10 mg after 180 min exposure). Nifedipine (0.3 microM) abolished the rhythmic or phasic component of these responses but failed to modify the late small tonic contraction induced by ouabain 1 mM or by K(+)-free solution. Ouabain (0.1-1 mM) or K(+)-free-evoked responses disappeared after short (4 min) or prolonged (60 min) exposure to a Ca(2+)-free, 3 mM EGTA-containing solution. Cromakalim (10 nM-0.1 mM) did not induce any variation in the resting tone either in the presence or in the absence of Ca2+ in the medium. In strips repeatedly stimulated with acetylcholine (0.1 mM) or oxytocin (1 microM), ouabain (0.3 mM), K(+)-free-solution and cromakalim (10 microM) reduced the amplitude of the initial, phasic response and progressively decreased the oscillatory component of the response to these agonists. Conversely, the successive responses evoked by KCl 60 mM in similar experimental conditions were not affected by ouabain or cromakalim. Ouabain (0.3 mM), K(+)-free solution and cromakalim (10 microM) decreased the Ca(2+)-independent, maintained contractions induced by acetylcholine or oxytocin after prolonged exposure to a Ca(2+)-free, EGTA-containing medium. These inhibitory effects were partially or completely reversed in the presence of the non-selective potassium channel blocker tetraethylammonium (10 mM) or in a Ca(2+)-free solution containing 60 mM K+. In conclusion, these results suggest that the response induced by ouabain or K(+)-free solution in estrogen-primed rat myometrium involves Ca2+ influx through potential-operated calcium channels but not Ca2+ release from intracellular stores. In addition, our results show that prolonged exposure to ouabain or K(+)-free medium decreases membrane receptor-mediated responses in rat uterus. This inhibitory effect seems to be the result, at least in part, of a decrease in the cytosolic level of K+, due to the inhibition of the electrogenic Na+ pump.

Preterm labour: a pharmacological challenge

Preterm labour is a major cause of perinatal mortality and morbidity, but its prevention is difficult because most of the available drugs lack uterine selectivity and have potentially serious side-effects for the mother or the foetus. In this article, Andrés López Bernal and colleagues discuss new evidence that shows pregnancy is associated with changes in G protein signalling and second messenger formation in human myometrium. During gestation uterine relaxation is favoured by a pronounced increase in G alpha s levels, thereby facilitating the effect of agonists that increase cAMP formation. The change in G alpha s is reversed in spontaneous labour enabling the uterus to become responsive to contractile agents. Although it is not established that these changes in G protein function are causally related to the spontaneous onset of labour, nevertheless they provide a novel viewpoint towards increased understanding of the cellular mechanisms of uterine contractility, which may result in better drugs for the management of preterm labour.

The effect of oxytocin receptor blockade on parturition in guinea pigs

The mechanism of the onset of labor is unknown in humans and guinea pigs. Contrary to most other species, progesterone withdrawal appears not to precede the onset of labor. To elucidate the role of oxytocin in the onset and maintenance of labor, guinea pigs were fitted with vascular catheters, an intraabdominal pressure catheter and an array of uterine electromyogram electrodes. An oxytocin antagonist (des-Gly9-[D-Trp2,Thr4,Orn8]dC6-oxytocin, 20 micrograms/kg per h, n = 11) or saline solution (n = 12) was infused starting on day 66 of gestation (term is 69 d). Oxytocin receptor blockade resulted in decreased uterine activity and a prolonged expulsive phase (second stage) of labor. Fetal delivery was delayed and fetal mortality was increased. The onset of the expulsive phase of labor was delayed but maximum uterine activity occurred in time together with a timely change in uterine electromyogram activity from a prepartum to a postpartum pattern following an unaltered progressive increase in baseline uterine activity. This indicates that oxytocin is requisite for the normal progress of the first and second stage of labor, but has no involvement in the mechanism of the onset and the timing of labor.

Effect of oxytocin antagonists on the activation of human myometrium in vitro: atosiban prevents oxytocin-induced desensitization

OBJECTIVE

Our purpose was to investigate whether the sensitivity of myometrial cells to oxytocin is affected by prolonged exposure to oxytocin antagonists.

STUDY DESIGN

Tissue slices or cultured myometrial cells were exposed to peptides in vitro. Myometrial activation was studied by measuring the formation of inositol phosphates and the changes in intracellular calcium. Oxytocin binding was measured by saturation analysis.

RESULTS

Atosiban and related peptides inhibited oxytocin-induced myometrial activation as pure antagonists (inhibition constant 10 nmol/L) but had no effect on prostaglandin E2-induced activation. Long-term (> or = 24 hours) exposure to atosiban had no residual effect on oxytocin sensitivity. However, long-term exposure to oxytocin resulted in homologous desensitization and loss of oxytocin receptors. Oxytocin-induced desensitization was prevented by coincubation with atosiban.

CONCLUSIONS

Atosiban is a pure oxytocin antagonist and has a specific, reversible effect on myometrial cells in vitro. Its potential use for the management or even prevention of idiopathic preterm labor or to reverse uterine hypertony during oxytocin-induced labor should be tested in controlled clinical trials.

Biochemistry and physiology of preterm labour and delivery

Human parturition is associated with profound changes in uterine connective tissue affecting mainly the cervix, but the endocrine control of cervical ripening remains obscure. Connective tissue changes are also implicated in premature rupture of the membranes, a problem often associated with preterm delivery, and it is believed that local inflammatory infiltration may play a role in both this condition and cervical ripening, but it is difficult to define which changes precede parturition and which are a consequence of the trauma of labour. Chorioamnionitis can cause preterm labour by provoking the release of inflammatory mediators in the decidua/fetal membranes area and it is likely that activation of prostaglandin release by decidual macrophages is involved in triggering labour. However, the role of macrophages and other bone marrow derived cells in normal labour and in labour associated with chorioamnionitis needs to be defined. It is likely that treatment with a combination of antibiotics and prostaglandin synthase inhibitors and/or other anti-inflammatory drugs is the most appropriate therapeutic approach. Idiopathic preterm labour and spontaneous labour at term are probably due to changes in the sensitivity of the myometrium to endogenous agonists. Recent progress in cell signalling pathways, such as the characterization of regulatory G proteins and the cloning of hormone receptors, should clarify the mechanism of action of relaxing and contracting agents on myometrial cells and should provide the means for the development of new therapeutic agents of high effectiveness and selectivity. This approach should result in better management of both term and preterm labour.

Influence of Mg2+ and pH on n.m.r. spectra and radioligand binding of inositol 1,4,5-trisphosphate

We and others have shown that the binding of Ins(1,4,5)P3 to its receptor is pH-sensitive and can be inhibited by Mg2+. In the present study we have used 1H- and 31P-n.m.r. spectroscopy to study whether these effects results from increased ionization of Ins(1,4,5)P3 and a direct interaction with Mg2+ respectively. Under near-physiological conditions of ionic strength (100 mM-KCl), three ionizable groups were observed. The pH titration curve of the 1-phosphate was monophasic, with a pKa of 6.3. The titration curves of the 4- and 5-phosphates were biphasic, suggesting that these groups interact; the pKa values for the 4-phosphate determined by 31P-n.m.r. were 5.7 and 7.8, and for the 5-phosphate they were 5.3 and 7.9. 1H- and 31P-n.m.r. measurements suggest that Mg2+ binds weakly to Ins(1,4,5)P3 at physiological pH. Mg2+ non-competitively inhibited binding of Ins(1,4,5)P3 to its receptor in rat cerebellum and bovine adrenal cortex. Inhibition curves for rat cerebellum at pH 7.1 and 8.5, and also for bovine adrenal cortex at pH 8.5, appeared to be monophasic, with IC50 values (concn. of displacer giving 50% inhibition of specific binding) of 214 microM, 572 microM and 9.1 mM respectively. Scatchard analysis revealed that Mg2+ inhibited binding of Ins(1,4,5)P3 to bovine adrenal cortex at pH 8.5 in a non-competitive manner. Our results suggest that the previously reported pH-sensitivity of the binding of Ins(1,4,5)P3 may be caused by ionization of the phosphate groups in positions 4 and 5, and that the ability of Mg2+ to inhibit the binding of Ins(1,4,5)P3 is not mediated by direct chelation but through a site located on, or close to, the Ins(1,4,5)P3 receptor. Inhibition by Mg2+ is pH-sensitive and can vary at least 10-fold between tissues, suggesting possible receptor heterogeneity. Mg2+ may exert an important regulatory control on the release of Ca2+ by Ins(1,4,5)P3.

Structure and function of inositol trisphosphate receptors

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a soluble intracellular messenger formed rapidly after activation of a variety of cell-surface receptors that stimulate phosphoinositidase C activity. The initial response to Ins(1,4,5)P3 is a rapid Ca2+ efflux from nonmitochondrial intracellular stores which are probably specialized subcompartments of the endoplasmic reticulum, although their exact identities remain unknown. This initial response is followed by more complex Ca2+ signals: regenerative Ca2+ waves propagate across the cell, repetitive Ca2+ spikes occur, and stimulated Ca2+ entry across the plasma membrane contributes to the sustained Ca2+ signal. The mechanisms underlying these complex Ca2+ signals are unknown, although Ins(1,4,5)P3 is clearly involved. The intracellular receptor that mediates Ins(1,4,5)P3-stimulated Ca2+ mobilization has been purified and functionally reconstituted, and its amino acid sequence deduced from its cDNA sequence. These studies demonstrate that the Ins(1,4,5)P3 receptor has an integral Ca2+ channel separated from the Ins(1,4,5)P3 binding site by a long stretch of residues some of which form binding sites for allosteric regulators, and some of which are substrates for phosphorylation. In this review, we discuss the ligand recognition characteristics of Ins(1,4,5)P3 receptors, and their functional properties in their native environment and after purification, and we relate these properties to what is known of the structure of the receptor. In addition to regulation by Ins(1,4,5)P3, the Ins(1,4,5)P3 receptor is subject to many additional regulatory influences which include Ca2+, adenine nucleotides, pH and phosphorylation by protein kinases. Many of the functional and structural characteristics of the Ins(1,4,5)P3 receptor show striking similarities to another intracellular Ca2+ channel, the ryanodine receptor. These properties of the Ins(1,4,5)P3 are discussed, and their possible roles in contributing to the complex Ca2+ signals evoked by extracellular stimuli are considered.