Oxytocin and vasopressin stimulate inositol phosphate production in human gestational myometrium and decidua cells

The role of oxytocin and vasopressin in the pathophysiology of heart failure in pregnancy and in fetal and neonatal life

Pregnancy and early life create specific psychosomatic challenges for the mother and child, such as changes in hemodynamics, resetting of the water-electrolyte balance, hypoxia, pain, and stress, that all play an important role in the regulation of the release of oxytocin and vasopressin. Both of these hormones regulate the water-electrolyte balance and cardiovascular functions, maturation of the cardiovascular system, and cardiovascular responses to stress. These aspects may be of particular importance in a state of emergency, such as hypertension in the mother or severe heart failure in the child. In this review, we draw attention to a broad spectrum of actions exerted by oxytocin and vasopressin in the pregnant mother and the offspring during early life. To this end, we discuss the following topics: 1) regulation of the secretion of oxytocin and vasopressin and expression of their receptors in the pregnant mother and child, 2) direct and indirect effects of oxytocin and vasopressin on the cardiovascular system in the healthy mother and fetus, and 3) positive and negative consequences of altered secretion of oxytocin and vasopressin in the mother with cardiovascular pathology and in the progeny with heart failure. The present survey provides evidence that moderate stimulation of the oxytocin and vasopressin receptors plays a beneficial role in the healthy pregnant mother and fetus; however, under pathophysiological conditions the inappropriate action of these hormones exerts several negative effects on the cardiovascular system of the mother and progeny and may potentially contribute to the pathophysiology of heart failure in early life.

Utero-relaxant effect of PDE4-selective inhibitor alone and in simultaneous administration with beta2-mimetic on oxytocin-induced contractions in pregnant myometrium

BACKGROUND

The objective of the study was to observe the effect of rolipram, the prototype phosphodiesterase 4 selective inhibitor, on oxytocin-induced contractions of human term myometrial strips, and compare the effect with salbutamol, beta(2)-adrenergic agonist, in single and the simultaneous application.

METHODS

Human myometrium was obtained from pregnant women in term that had a term delivery by the caesarian section. Myometrial strips were excised from the lower uterine segment and placed into an organ-bath with Krebs-Henseleit buffer. The mean peak amplitude of contraction (mN) of the myometrial smooth muscle to the doses of oxytocin (10(-6) mmol/L(-1)) with subsequent single administration of rolipram (10(-4) mmol/L(-1)), salbutamol (10(-4) mmol/L(1)) and simultaneous administration of rolipram and salbutamol (both 10(-4) mmol/L(-1)), was used as a parameter of myometrial reactivity.

RESULTS

Rolipram alone decreased the oxytocin-induced contractile amplitude to 47.98%, single salbutamol application resulted in amplitudinal decrease to 56.07%, and the combination of both compounds in their simultaneous administration resulted in the decrease of oxytocin-induced contractile amplitude to 29.1%.

CONCLUSION

Our data are consistent with previous studies of the enhanced efficiency of the beta(2)-adrenergic agonist, when administered together with the phosphodiesterase 4-inhibitor. Moreover we have shown that rolipram alone has a more profound effect on oxytocin-induced contractions than salbutamol alone.

The influence of interleukin-1beta on oxytocin signalling in primary cells of human decidua

OBJECTIVE

Oxytocin (OT) and its corresponding receptor (OTR), synthesized within the pregnant uterus, play a key role in the process of (preterm) labor as part of a paracrine system that regulates symmetrical contractility. In the setting of intrauterine infection, a major cause of preterm labour and birth, decidua serves as a major source of cytokine production. The present study evaluates the time-dependent effect [0-24 h] of the inflammatory cytokine Interleukin-1beta (IL-1beta) treatment on OT signalling and OT stimulated prostaglandin release in primary cultures of human decidua.

STUDY DESIGN

Primary cultures of human decidua (n=6) were treated with IL-1beta [5 ng/ml] for 0-24h and or indomethacin [100 microM]--an inhibitor of the prostaglandin synthesis--for 0-24 h or for 24 h. OT peptide expression, OTR binding, Inositol triphosphate production (IP(3)), and arachidonic acid (AA) as well as prostaglandin (PGE(2)) release were measured.

RESULTS

IL-1beta transiently reduced cytoplasmic OT peptide at 4-6 h of IL-1beta incubation, while its secretion into the media was increased after 6 h of stimulation. The later was completely blocked by indomethacin. A decrease in OTR mRNA expression and a loss of OTR binding were detected after 8 h and 16 h of IL-1beta treatment, respectively. IL-1beta also decreased IP(3) production and AA release, but significantly enhanced OT mediated PGE(2) production. This effect was completely suppressed by the cyclooxygenase-2 (COX-2) inhibitor NS-398.

CONCLUSION

Our data suggest, that IL-1beta indirectly increases OT secretion in primary cultures of human decidua in a time dependent fashion through the production of prostaglandins through COX-2 and that this increase in OT peptide may secondarily down-regulate the OTR and its signalling cascade. These findings might explain the poor effectiveness of oxytocin receptor antagonists as tocolytic agents in the setting of intrauterine infection.

The Interleukin 1beta-Induced Expression of Human Prostaglandin F2alpha Receptor Messenger RNA in Human Myometrial-Derived ULTR Cells Requires the Transcription Factor, NFkappaB1

The molecular mechanisms that regulate the expression of genes involved in parturition are poorly understood. The mRNA expression of the prostaglandin F(2alpha) receptor (PTGFR), a uterine activating gene, is increased at labor and is required for uterine contractile activity in numerous animal models, although the signaling pathways responsible for this increased expression have not been identified. Proinflammatory cytokines have been proposed to regulate the expression of the uterine activating genes via activation of the nuclear transcription factor, NFkappaB, and initiate labor. However, it is uncertain whether uterine PTGFR is regulated this way. In this report, we demonstrate for the first time that treatment of immortalized human myometrial-derived ULTR cells with the proinflammatory cytokine IL1beta causes an increase in PTGFR mRNA levels. Furthermore, IL1beta treatment increased the nuclear levels of the RELA subunit of NFkappaB and increased binding of RELA to the NFkappaB DNA-binding site. Inhibition of NFkappaB activation with either the proteasome inhibitor MG132 or phenethyl caffeiate reduced PTGFR mRNA levels, which indicates that this transcription factor is important for basal transcription. Furthermore, this inhibition prevented IL1beta induction ofPTGFRmRNA, which confirms that NFkappaB is required for the IL1beta-induced increase inPTGFR. These results are consistent with the proposal that proinflammatory cytokines directly regulate uterine activation genes and that the transcription factor NFkappaB is involved in both basal and IL1beta-stimulated transcription of the PTGFR gene.

Molecular signaling through G-protein-coupled receptors and the control of intracellular calcium in myometrium

Cellular mechanisms regulating myometrial intracellular free calcium (Ca2+(i)) are addressed in this review, with emphasis on G-protein-coupled receptor pathways. An increase in myometrial Ca2+(i) results in phosphorylation of myosin light chain, an increase in myosin adenosine monophosphatase (ATPase) activity and contraction. Dephosphorylation of myosin light chain and a decline in Ca2+(i) are associated with relaxation. Increases in Ca2+(i) are controlled by multiple signaling pathways, including receptor-mediated activation of phospholipase Cbeta (PLCbeta), leading to release of Ca2+ from intracellular stores. Ca2+ also enters myometrial cells through plasma membrane Ca2+ channels. Conversely, adenosine triphosphate (ATP)-dependent Ca2+ pumps lower Ca2+(i) concentrations and potassium channels promote hyperpolarization that can decrease Ca2+ entry. Receptor-coupled pathways that promote uterine relaxation primarily involve activation of cyclic adenosine monophosphate (cAMP)- or cyclic guanosine monophosphate (cGMP)-stimulated protein kinases that phosphorylate proteins regulating Ca2+ homeostasis. cAMP has inhibitory effects on myometrial contractile activity, agonist-stimulated phosphatidylinositide turnover and increases in Ca2+(i). Some of these effects require association of protein kinase A (PKA) with a plasma membrane-associated A-kinase-anchoring-protein (AKAP). Near term in the rat, there is a decline in the plasma membrane localization of PKA associated with this anchoring protein. This correlates with changes in the regulation of signaling pathways controlling Ca2+(i). L-type voltage-operated Ca2+ entry is an important regulator of myometrial contraction. In addition, putative signal-regulated or capacitative Ca2+ channel proteins, TrpCs, are expressed in myometrium, and signal-regulated Ca2+ entry is observed in human myometrial cells. This Ca2+ entry mechanism may play a significant role in the control of myometrial Ca2+(i) dynamics and myometrial contraction. The regulation of myometrial Ca2+(i) is complex. Understanding the mechanisms involved may lead to design of tocolytics that target multiple pathways and achieve improved suppression of premature labor.

Administration of oxytocin affects vasopressin V2 receptor and aquaporin-2 gene expression in the rat

Oxytocin (OT) binds to the vasopressin V2 receptor (V2R) because of its structural similarity to arginine vasopressin (AVP). Though the affinity of OT for V2R is low, it is known that OT causes antidiuresis. To clarify the effect of OT as an agonist of V2R, we investigated the influence of acute elevation of plasma OT levels on the rat mRNA expression of V2R and aquaporin-2 (AQP2), the water channel regulated by V2R. The plasma OT level increased from 11.1+/-1.6 pg/ml to 331.0+/-67.9 pg/ml by 1 h after subcutaneousinjection of 20 microg OT. V2R mRNA expression decreased to 68.3+/-4.1% of the control at 3 h, and AQP2 mRNA expression increased to 239.3+/-26.8% of the control at 6 h. The plasma AVP level did not change significantly during the experiment. The influence of a subcutaneous injection of 20 microg OT on V2R and AQP2 mRNA expression is comparable to that of 10 microg AVP that we documented in the previous study. In conclusion, OT can downregulate V2R mRNA expression and upregulate AQP2 mRNA expression in the collecting duct as an agonist of the V2R like AVP.

The oxytocin-induced inward current in vagal neurons of the rat is mediated by G protein activation but not by an increase in the intracellular calcium concentration

The neuropeptide oxytocin can depolarize parasympathetic preganglionic neurons in the dorsal motor nucleus of the vagus nerve of the rat by generating a sustained inward current, which is sodium-dependent and tetrodotoxin-insensitive. The second messenger activated by oxytocin receptor binding is, however, not yet known. In the present study, we attempted to characterize it by using the whole-cell recording technique and brainstem slices. When loaded with GTP-gamma-S, a non-hydrolysable analogue of GTP, vagal neurons generated a persistent inward current in the absence of agonist and the oxytocin effect was suppressed, suggesting that the peptide-evoked current was mediated by G-protein activation. Loading vagal neurons with the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N',-tetraacetic acid (BAPTA) suppressed a calcium-dependent, slowly decaying potassium aftercurrent but did not affect the oxytocin response, suggesting that the latter was not mediated by an agonist-induced increase in the intracellular calcium concentration. Protein kinase C (PKC) activation was probably not involved, since the peptide-evoked current was not modified by loading neurons with the PKC inhibitor H7. Thus, the oxytocin-evoked current in vagal neurons was probably not mediated by phospholipase C-beta (PLC-beta) activation. Loading neurons with 8-Br-cAMP or with an adenylyl cyclase activator (forskolin) reduced the oxytocin-evoked current by about half. SQ 22536, an adenylyl cyclase inhibitor, reduced this current by a similar amount. However, the peptide-evoked current was unaffected by Rp-cAMPS and Sp-cAMPS, an inhibitor and an activator, respectively, of cAMP-dependent protein kinase (PKA). We suggest that oxytocin activates two distinct signalling pathways in vagal neurons: one which is cAMP-dependent, but PKA-independent, and one, unidentified, which is PLC-beta-and cAMP-independent. Each pathway accounts for about half of the peptide effect and both appear to involve G-protein activation.

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.

Molecular cloning and functional characterization of the oxytocin receptor from a rat pancreatic cell line (RINm5F)

Oxytocin (OT) and vasopressin (AVP) stimulate insulin and glucagon release from the pancreas, and evoke insulin secretion from the rat insulinoma cell line, RINm5F. To determine which AVP/OT receptor subtype is expressed in RINm5F cells, we used PCR with degenerate primers to two transmembrane domains of the AVP (V1a, V1b (or V3), V2) and OT receptors (OTRs). The single PCR fragment identified was used to obtain a full length cDNA from a RINm5F cDNA library. Comparison of the deduced amino acid sequence of this clone with uterine OTR sequences from several species (human, sheep, bovine) and to the pig kidney epithelial cell (LLC-PK1) OTR reveals a very high degree of homology. After the RIN cell OTR cDNA was stably transfected into CHO cells (CHO-OTR), the cell membranes bound iodinated oxytocin antagonist with an apparent Kd comparable to that of RIN cell membranes and those from other OT target cells. Comparison of the ligand specificities of CHO-OTR and RIN cells membranes showed that the relative Ki values of a series of OT analogues were approximately equivalent in both preparations. The rank order of apparent Ki values also corresponded to published values for the rat myometrium, where OT elicits intracellular calcium transients, and increases inositol phosphate production. In uterin endometrium and amnion cells, OT stimulates prostaglandin release. Stimulation of CHO-OTR cells with OT caused an increase in cytosolic calcium concentration originating from both intracellular and extracellular sources, and a dose-dependent increase in inositol phosphate levels. Arachidonic acid release and PGE2 synthesis were also stimulated by OT. These findings (amino acid sequence homology, binding specificity, and signal transduction/second messenger production) suggest that OTRs from RINm5F cells are indistinguishable from OTRs that have been described in other tissues. The expression of OTR in pancreatic cells implies that OT plays a role in pancreatic function.

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.

Effects of progesterone and oxytocin on intracellular elemental composition of term human myometrium in vitro

OBJECTIVE

To study the effects of progesterone and oxytocin on contractile activity related to the intracellular elemental composition of human pregnant myometrium.

STUDY DESIGN

Myometrial strips were mounted in tissue baths and superfused with plain buffer, oxytocin or progesterone. Progesterone was initially added to the tissue bath or at the onset of spontaneous contraction of the strips. The myometrial contractile activities were recorded isometrically and the results were analyzed by a specific computer program. The effects of oxytocin and progesterone on the intracellular elemental composition were studied by X-ray microanalysis in an electron microscope.

RESULTS

The contractile activity in terms of frequency and tonus of the muscle strips was increased by oxytocin and progesterone. This increase was more pronounced if progesterone was added to the bath at the start of the experiment. After 120 min of incubation with oxytocin and progesterone the total intracellular concentration of calcium was significantly higher (P < 0.0001 and P < 0.002, respectively) compared to buffer.

CONCLUSIONS

(i) Oxytocin increases total intracellular calcium concentration concomitant with an increase in uterine contractile activity. (ii) Progesterone increases the myometrial tone and frequency of contractions, simultaneously with an increase in total intracellular calcium.

Regulation of endometrial prostaglandin synthesis during early pregnancy in cattle: effects of phospholipases and calcium in vitro

In experiment 1, endometrial explants from 3 cyclic (Day 17) cows were incubated with arachidonic acid (AA), phospholipase A-2 (PLA-2) and calcium ionophore A23187 (CaI) or control. AA (0.2 mg), PLA-2 (1 U/ml) and CaI (4 micrograms/ml) increased PGF and PGE secretion. In experiment 2, endometrial explants from cyclic (n = 4) and pregnant (n = 3) cows were incubated +/- Ca++ and with either: control, AA, PLA-2, CaI, PLA-2 + CaI, or AA + CaI. PG secretion was higher in cultures with Ca++. In presence of Ca++, PGF secretion was lower for pregnant than cyclic endometrium. AA with Ca++ stimulated PGF and PGE secretion, indicating that AA availability may limit PG secretion. The stimulatory effect of PLA-2 on PGF and PGE secretion was greater in pregnant than cyclic endometrium. However, CaI inhibited the PLA-2 response of pregnant, but not cyclic endometrium. In experiment 3, endometrium (4 cyclic cows) failed to convert 3H-PGF2 alpha to PGE2 or 3H-PGE2 to PGF2 alpha. Responsiveness of PG secretion to PLA-2, and CaI is altered by reproductive status suggesting that these factors may be involved in the differential regulation of PG production during early pregnancy in cattle.

Isolation and partial cloning of ryanodine-sensitive Ca2+ release channel protein isoforms from human myometrial smooth muscle

Partial cDNAs of the ryanodine receptor were cloned using PCR analysis from reverse transcribed total and mRNA, extracted from freshly isolated pregnant, non-pregnant, and cultured human myometrial smooth muscle. The identity of these clones was confirmed by nucleotide sequencing of the fragments and indicate the expression of both the skeletal and brain ryanodine receptor isoforms in these preparations. In freshly isolated non-pregnant myometrial tissue, membrane fractions displaying specific [3H]ryanodine binding activities were isolated using density gradient centrifugation. SDS-PAGE of the sucrose gradient fractions indicated the specific comigration of a polypeptide with a molecular mass of approximately 544 kDa with the ryanodine binding activity.

Oxytocin induced a biphasic increase in the intracellular Ca2+ concentration of porcine myometrial cells: participation of a pertussis toxin-insensitive G-protein, inositol 1,4,5-trisphosphate-sensitive Ca2+ pool, and Ca2+ channels

This study investigated the underlying mechanisms of oxytocin (OT)-induced increases in intracellular Ca2+ concentrations ([Ca2+]i) in acutely dispersed myometrial cells from prepartum sows. A dose-dependent increase in [Ca2+]i was induced by OT (0.1 nM to 1 microM) in the presence and absence of extracellular Ca2+ ([Ca2+]e). [Ca2+]i was elevated by OT in a biphasic pattern, with a spike followed by a sustained plateau in the presence of [Ca2+]e. However, in the absence of [Ca2+]e, the [Ca2+]i response to OT became monophasic with a lower amplitude and no plateau, and this monophasic increase was abolished by pretreatment with ionomycin, a Ca2+ ionophore. Administration of OT (1 microM) for 15 sec increased inositol 1,4,5-trisphosphate (IP3) formation by 61%. Pretreatment with pertussis toxin (PTX, 1 microgram/ml) for 2 hr failed to alter the OT-induced increase in [Ca2+]i and IP3 formation. U-73122 (30 nM to 3 microM), a phospholipase C (PLC) inhibitor, depressed the rise in [Ca2+]i by OT dose dependently. U-73122 (3 microM) also abolished the OT-induced IP3 formation. Thapsigargin (2 microM), an inhibitor of Ca(2+)-ATPase in the endoplasmic reticulum, did not increase [Ca2+]i. However, it did time-dependently inhibit the OT-induced increase in [Ca2+]i. Nimodipine (1 microM), a voltage-dependent Ca2+ channel (VDCC) blocker, inhibited the OT-induced plateau by 26%. La3+ (1 mM), a nonspecific Ca2+ channel blocker, abrogated the OT-induced plateau. In whole-cell patch-clamp studies used to evaluate VDCC activities, OT (0.1 microM) increased Ca2+ current (ICa) by 40% with no apparent changes in the current-voltage relationship.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-activated K+ channels as modulators of human myometrial contractile activity

The role of Ca(2+)-activated potassium (KCa) channels in the regulation of membrane potential, intracellular free calcium ([Ca2+]i) and contraction was investigated in uterine smooth muscle and myometrial cells. In an immortalized human myometrial cell line, oxytocin increased [Ca2+]i and [3H]inositol phosphate formation. Relaxin attenuated the oxytocin-induced increase in [Ca2+]i. In cell-attached patches, membrane depolarization activated a large-conductance KCa channel (179 +/- 4 pS). Iberiotoxin (IbTX), a potent blocker of "maxi" KCa channels (A. Galvez, G. Gimenez-Gallego, J. P. Reuben, L. Roy-Contanciin, P. Feigenbaum, G. J. Kaczorowski, and M. L. Garcia. J. Biol. Chem. 265: 11083-11090, 1990) produced long closed events (approximately 6 min) in these channels. In agreement with this blockage, IbTX depolarized the cells by 9.8 +/- 2.8 mV and caused a dose-dependent increase in [Ca2+]i with a half-maximal effective concentration of 0.79 nM. IbTX also caused phasic contractions in human myometrial strips and increased both the frequency and force of spontaneous contractions in estrogen-primed rat myometrial strips. Moreover, myometrial contractility was also affected by 1 mM tetraethylammonium, a concentration that blocks uterine smooth muscle KCa channels when applied to the extracellular side (G. J. Perez, L. Toro, S. D. Erulkar, and E. Stefani. Am. J. Obstet. Gynecol. 168: 652-660, 1993). These results strongly suggest that the large conductance KCa channels may actively participate in the control of human myometrial cell membrane potential and [Ca2+].

Molecular cloning and functional characterization of V2 [8-lysine] vasopressin and oxytocin receptors from a pig kidney cell line

[Arg8]vasopressin and oxytocin are the two main members of the neurohypophysial hormone family found to be present in nearly all mammals. [Lys8]vasopressin ([Lys8]VP) has been identified as the antidiuretic hormone in pig and some marsupial families. The porcine-derived kidney epithelial cell line, LLC-PK1, expresses both [Lys8]VP receptors coupled to the activation of adenylate cyclase (V2 receptors) and oxytocin receptors. Here we report the molecular cloning of the V2 [Lys8]VP receptor and the oxytocin receptor from LLC-PK1 cells. The cloned V2 [Lys8]VP receptor differs from human and rat V2 [Arg8] receptors mainly in its N-terminal region, in residues located in the extracellular loops and in intracellular phosphorylation sites. When expressed in COS7 cells, the V2 [Lys8]VP receptor exhibits the relative order of ligand affinity [Lys8]VP = [Arg8]VP >> 1-deamino[D-Arg8]VP > or = oxytocin and adenylate-cyclase stimulation, expected for the porcine V2 [Lys8]VP receptor but different from V2 [Arg8]VP receptors. Adenylate-cyclase activation by [Lys8]VP was inhibited in COS7 cells by a V2 antagonist. The cloned oxytocin receptor exhibits in COS7 cells a ligand specificity typical of mammalian oxytocin receptors. mRNA-distribution analysis revealed a single 5.5-kb transcript in the uterus from pregnant guinea pig.

The effect of the oxytocin antagonists, CAP 476 and F327, on calcium mobilisation in single cultured human myometrial cells

OBJECTIVE

To investigate the mechanism of action of the oxytocin (OT) antagonists, CAP 476 and F327.

DESIGN

A prospective descriptional study.

SUBJECTS

Women undergoing caesarean section at term or hysterectomy.

INTERVENTIONS

Myometrial cells were cultured from uterine biopsies.

MAIN OUTCOME MEASURES

Intracellular calcium ([Ca2+]i), determined in single cells.

RESULTS

Application of OT caused a transient increase in [Ca2+]i. CAP 476 abolished and F327 reduced the response to OT but neither reduced the [Ca2+]i transient induced by cell depolarisation with 120 mmol K+. CAP 476 did not reduce transients caused by prostaglandin E2. F327 reduced the frequency of repetitive [Ca2+]i transients occurring during continuous application of OT.

CONCLUSIONS

The results demonstrate that the antagonists reduce the effect of OT and that their action is relatively specific. Their mechanism of action as clinical tocolytic agents is discussed.

Molecular cloning and expression of rat V1a and V2 arginine vasopressin receptors

Vasopressin, one of the first characterized neuropeptides, has a wide spectrum of biological action, acting on distinct tissues. Indeed, it is involved in water retention, glucose metabolism, blood pressure and its implication in the CNS has also been described. This diversity of effects on mammalian tissues is mediated by distinct G protein-coupled receptors, acting via distinct second messenger pathways. This receptor family has been subtyped by pharmacological studies, as V1a receptor whose action is mediated by intracellular calcium mobilization, and V2 receptor which is linked to adenylyl cyclase. Since so many essential functions were ensured by vasopressin, molecular characterization of its receptors became soon a great challenge. This prompted us to isolate the cDNA of AVP V1a receptor as the first member of this family, by expression cloning. Intracellular calcium mobilization was therefore assayed after rat liver mRNA injection into Xenopus oocytes. A single clone, encoding a functional AVP receptor corresponding to the V1a subtype was finally characterized as a G protein-coupled receptor. Furthermore, we used homology cloning strategy in order to clone the AVP V2 subtype from a rat kidney cDNA library. A putative receptor clone was finally characterized as the rat V2 receptor cDNA by binding and cAMP increase experiments, on transfected cells.

Oxytocin, PGE2 and PGF2 alpha stimulate the production of inositol phosphates in the rabbit myometrium

Phosphoinositide breakdown is thought to be important in regulating a variety of transmembrane signal transduction in the action of oxytocic agent during uterine smooth muscle contraction. We investigated the effects of oxytocin and prostaglandins (PGE2 and PGF2 alpha) on phosphoinositide hydrolysis in the myometrium taken from non-pregnant and pregnant rabbits by measuring the accumulation of total inositol phosphates (IP). Oxytocin strongly, and PGE2 and PGF2 alpha slightly but significantly, stimulated IP production in both the non-pregnant and pregnant myometria. Oxytocin more markedly accelerated the IP production in pregnant myometrium than in non-pregnant myometrium. However, IP production stimulated by PGE2 and PGF2 alpha was much the same in non-pregnant and pregnant myometria. The amount and time course of the increase in the production of the total IPs by oxytocin are quite different from those by PGs. It seems that the mechanism by which oxytocin stimulates phospholipase C is different from that of the PGs. It is suggested that transmembrane signalling pathways of phosphoinositide hydrolysis play an important role in the each mechanism.

Oxytocin induced cAMP-dependent protein kinase activation and urokinase-type plasminogen activator production in LLC-PK1 renal epithelial cells is mediated by the vasopressin V2-receptor

Using a variety of peptide analogues of oxytocin (OT) and Arg8-vasopressin (AVP), OT-mediated induction of urokinase-type plasminogen activator (uPA) was examined in LLC-PK1 renal epithelial cells, which possess distinct high-affinity receptors of both the OT- and vasopressin renal (V2-) types. OT or OT-receptor specific agonists induced concentration-dependent cAMP synthesis, activation of the cAMP-dependent protein kinase (cAMP-PK) and uPA production consistent with their respective binding affinities for the V2- and not the OT-receptor. OT-mediated uPA induction could be inhibited in a concentration-dependent fashion by coincubation with a V2/V1-receptor specific antagonist, but not by an OT-receptor specific antagonist. Results implied that stimulation of cAMP- and uPA responses in LLC-PK1 cells by OT was V2-receptor-mediated.

Oxytocin receptors on cultured astroglial cells. Regulation by a guanine-nucleotide-binding protein and effect of Mg2+

Specific binding sites for the radio-iodinated oxytocin (OT) antagonist d(CH2)5-[Tyr(Me)2,Thr4, Tyr-NH2(9)]OVT ([125I]OTA) have been characterized on cultured hypothalamic astroglial cell membranes. The rate of association of the ligand to OT-binding sites was identical in the presence and the absence of the non-hydrolysable GTP analogue guanosine 5'-[beta gamma-imido]triphosphate (Gpp[NH]p, 0.1 mM), whereas the monophasic dissociation reaction became biphasic in the presence of Gpp[NH]p. Scatchard analysis of equilibrium binding of [125I]OTA resulted in a linear plot with a single class of binding sites (Kd 0.06 nM) which were insensitive to the addition of Gpp[NH]p. Unlabelled OT and [Arg8]vasopressin (AVP) bound to high- (H) and low- (L) affinity states with a dissociation constant ratio (KL/KH) of 100 for both hormones. Binding with both high and low affinity required the presence of Mg2+ in the incubation buffer, and the addition of Gpp[NH]p decreased the KL/KH ratio to 10 and increased the percentage of low-affinity binding sites. On the other hand, neither omission of Mg2+ from the buffer nor the addition of Gpp[NH]p altered the binding of either OT or V1 AVP antagonists to OT receptors. In the presence of a G-protein inactivator (N-ethylmaleimide; 3 mM) during OT competition studies the affinities of the two OT-binding sites were unchanged, but 90% of the high-affinity binding sites were converted into the low-affinity state. These results obtained with cultured hypothalamic astroglial cells provide further evidence for a coupling of OT receptors with a guanine-nucleotide-binding protein, with a requirement for Mg2+.

Oxytocin receptors on cultured astroglial cells. Kinetic and pharmacological characterization of oxytocin-binding sites on intact hypothalamic and hippocampic cells from foetal rat brain

The ability of astroglial cells to exhibit oxytocin (OT)-binding sites has been investigated in embryonic hypothalamic and hippocampic astroglial cell cultures. The differential characteristics of binding of OT and [Arg8]vasopressin (AVP) agonists and antagonists to the OT-binding sites using the highly selective iodinated OT antagonist d(CH2)5-[Tyr(Me)2,Thr4,Tyr-NH2(9)]OVT ([125I]OTA) have been evaluated using intact cells maintained for 12 days in culture. The specific binding displayed features of reversibility. Computer analysis of the saturation studies using the LIGAND program indicated that, at 4 degrees C, the antagonist binds to a homogeneous population of sites with a Kd value of 0.02 nM and a low binding-site density of around 2 fmol/dish for hypothalamic cells and 6 fmol/dish for hippocampic cells. For hypothalamic cells, competition curves using unlabelled OT, AVP or V2 AVP agonist were characterized by a pseudo-Hill coefficient below unity (0.7), indicating possible heterogeneity among the binding sites. On the other hand, the dose-inhibition curves resulting from competition studies with hippocampic cells had a pseudo-Hill coefficient close to unity, except for OT. Computer analysis (LIGAND) indicated that the OT dose-inhibition curve was significantly better fitted to a two-site model, and this can be explained by two apparent forms of the receptor having high and low affinities for the displacing drug. The relative potencies of the peptides tested for binding to the high-affinity site were: AVP greater than OT greater than V1 AVP antagonist ([d(CH2)5-Tyr(Me)2]AVP) = V2 AVP agonist greater than AVP-Sar ([d(CH2)5-Sar7,Arg8]VP) in hypothalamic cultures, and OT = AVP greater than V1 AVP antagonist greater than V2 AVP agonist in hippocampic cultures. In addition, autoradiography allowed visualization of OT-binding sites, which are located on both soma and processes of astrocyte-like type of cells. In conclusion, these data provide evidence that glial cell cultures contain specific OT-binding sites which display pharmacological characteristics different from those already reported in neuronal cultures and in the adult rat brain.

Estimation by an electrophysiological method of the expression of oxytocin receptor mRNA in human myometrium during pregnancy

In order to evaluate the changes in uterine oxytocin receptor-specific mRNA during pregnancy, receptor expression in Xenopus oocytes are examined electrophysiologically following microinjection of mRNA from human uterus. In voltage-clamped oocytes injected with term myometrial mRNA, oxytocin elicited an inward current response. The amplitude of the oxytocin-induced current increased with increasing dose of oxytocin, but no current was elicited following stimulation with vasopressin. The oxytocin-induced current was completely eliminated as a result of pretreatment with a specific oxytocin antagonist. 21 of 27 oocytes injected with term myometrial mRNA showed a large amplitude (77.0 +/- 16.1 nA) reaction to oxytocin. In comparison, only 3 of 13 oocytes injected with early gestational myometrial mRNA exhibited a small amplitude (4.6 +/- 1.4 nA) reaction to oxytocin. No oxytocin response was observed in oocytes injected with non-pregnant myometrial mRNA. These results indicate that the striking increment in oxytocin sensitivity in term uterus depends on the increase in mRNA encoding oxytocin receptors.

Ca2+ release and InsP3 production in avian uterine cells: effects of PGF2 alpha and AVT

Primary cultures of smooth muscle cells isolated from the shell gland ("uterus") of the domestic hen were permeabilized with digitonin and loaded with 45Ca2+ in the presence of ATP. When these cells were stimulated with prostaglandin F2 alpha (PGF2 alpha), arginine vasotocin (AVT), or D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], there was a rapid, biphasic, and dose-related release of 45Ca2+ from nonmitochondrial pools. 2-Nitro-4-carboxyphenyl-N,N-diphenylcarbamate, an inhibitor of phospholipase C, had no effect on PGF2 alpha - and Ins(1,4,5)P3-promoted 45Ca2+ efflux, whereas it significantly inhibited AVT-stimulated and a stable analogue of GTP-stimulated 45Ca2+ release. In fura-2-loaded intact cells, both PGF2 alpha and AVT increased intracellular Ca2+ levels [( Ca2+]i) in a dose-related manner in the presence of extracellular Ca2+. However, omission of extracellular Ca2+ prevented a PGF2 alpha, but not AVT-induced, rise in [Ca2+]i In D-myo-[3H]inositol-labeled cells, 10 nM AVT caused a rapid, two- to threefold increase in [3H]-Insp3, whereas PGF2 alpha up to 1 microM was infective. Raising PGF2 alpha to 10 microM increased total inositol phosphates by 22% over controls (P less than 0.05). These results point to marked differences in the mechanisms by which AVT and PGF2 alpha regulate [Ca2+]i in uterine smooth muscle cells. It is suggested that the two agonists act in concert to initiate oviposition.

The endocrinology of parturition in the human

Current evidence suggests that oestrogens, progesterone, relaxin, the prostaglandins, and oxytocin are all hormones concerned to a major degree with the onset and maintenance of parturition. Oestrogens, relaxin, and the prostaglandins are particularly involved with cervical ripening, while prostaglandins, progesterone and oxytocin are more involved in regulating myometrial contractility. Catecholamines may also have some regulatory function in relation to uterine contractions. Progesterone dominance during pregnancy is associated with a firm closed cervix, few myometrial gap junctions, low calcium levels in the cells, and a quiescent myometrium. At term, a change in the oestrogen/progesterone balance favours cervical ripening and increased uterine activity. Of particular importance at the level of the muscle cell are changes in the number of oxytocin receptors; a complex interaction between cAMP and phosphoinositide metabolism governs the intracellular level of calcium, thus regulating contractile activity.

Vasopressin receptors in human pregnant myometrium and decidua: interactions with oxytocin and vasopressin agonists and antagonists

Saturation analysis and competition experiments were performed to identify and characterize [3H]arginine vasopressin binding to human myometrium and decidua in late pregnancy. [3H]Arginine vasopressin bound with affinity similar to that of [3H]oxytocin to both tissues (dissociation constant 1 to 2 nmol/L). The concentration of [3H]arginine vasopressin binding sites was high, particularly in decidua, but in all instances was about 50% to 60% of [3H]oxytocin binding. Analogs with selective oxytocic potency (4-threonine oxytocin, isotocin) had high affinity to both [3H]arginine vasopressin and [3H]oxytocin binding sites, as did analogs with both oxytocic and vasopressor activity (vasotocin). Analogs with selective antidiuretic activity (1-deamino-8-D-arginine vasopressin) showed drastically reduced affinity to [3H]oxytocin binding sites and relatively low but significantly higher affinity to [3H]arginine vasopressin binding sites. A new oxytocin antagonist (RW22164 or [1-deamino-2D-tyrosine-(O-ethyl)-4-threonine-8-ornithine]oxytocin) competitively bound to both binding sites. Its affinity to [3H]oxytocin binding sites was greater than to [3H]arginine vasopressin binding sites whereas the relative affinities of a predominantly vasopressor antagonist [Manning compound) were reversed, suggesting the presence of distinct receptors for oxytocin and arginine vasopressin in pregnant human myometrium and decidua.

A role for phosphoinositide hydrolysis in human uterine smooth muscle during parturition

Phosphoinositide hydrolysis is thought to be important in regulating a variety of intracellular signals, including Ca++ and prostaglandins, both of which have been implicated in the action of oxytocin during uterine smooth muscle contraction. We investigated the in vitro effect of oxytocin and various other uterotonic agents on phosphoinositide hydrolysis in gestational myometrium by measuring the production of inositol phosphates in tissue explants prelabeled with 3H-inositol. Oxytocin caused significant increases in all three inositol phosphates in myometrium at 3 minutes. Stimulation of inositol monophosphate production was sustained for 30 minutes and was dose dependent, with a half-maximal effect around 2 X 10(-8) mol/L. Platelet activating factor and alpha-adrenergic agonists also stimulated myometrial phosphoinositide hydrolysis, but carbachol prostaglandins E2 and F2 alpha had no effect. Vasopressin had greater efficacy than oxytocin for stimulating hydrolysis in gestational myometrium. Furthermore, in contrast to vasopressin, oxytocin had no effect on inositol phosphate production in nongestational myometrium. Oxytocin also stimulated arachidonic acid release and prostaglandin E2 and F2 alpha production in gestational myometrium. The hydrolysis of phosphatidylinositol by myometrium homogenates showed a precursor-product relationship for the production of diacylglycerol, monoacylglycerol, and arachidonic acid, indicative of a sequential action of phospholipase C and diacylglycerol lipase. These data demonstrate the potential for certain uterotonic agonists to use inositol lipid signaling to mobilize free arachidonic acid for prostaglandin production and to release intracellular Ca++ during excitation-contraction coupling.

Properties of intracellular calcium stores in pregnant rat myometrium

1. The properties of the Ca2+ stores in myometrium of 21-day pregnant rats were studied by recording the contractile responses of saponin-treated skinned muscles. 2. After accumulation of Ca2+ into the stores in the presence of 5 mM NaN3, inositol 1,4,5-trisphosphate (InsP3) at concentrations exceeding 3 microM produced a contraction. The amplitude of this contraction was maximal at about 20 microM. A second application of 20 microM InsP3 produced a smaller contraction than the first one. However after reloading the stores with Ca2+, 20 microM InsP3 produced a contraction of the same amplitude as the initial one. 3. After application of 20 microM InsP3, 1 microM A23187 still evoked a large contraction. If A23187 was applied first, the subsequent application of InsP3 or A23187 no longer induced a contraction, even after Ca2+ loading. 4. Guanosine triphosphate (GTP) or arachidonic acid, both 100 microM neither evoked a contraction nor enhanced the subsequent contraction elicited by 20 microM InsP3. 5. Caffeine 25 mM did not induce a contraction nor did it affect the contraction elicited by 20 microM InsP3. 6. The results indicate that in pregnant rat myometrium InsP3 releases Ca2+ from intracellular stores as has been proposed in vascular smooth muscles.

Identification of a myometrial oxytocin-receptor protein

The specific binding of [3H]oxytoxin to uterine membrane preparations derived from different species at late pregnancy was examined. The highest receptor density (bmax value) was found in membranes derived from the myometria of guinea pigs between day 60 post-conception (bmax = 3.6 +/- 0.1 pmol/mg) and day 65 (bmax = 4.4 +/- 0.1 pmol/mg). The similarity of Kd values for oxytocin binding (Kd = 2.6 +/- 0.2 nM) and for vasopressin binding (Kd = 2.1 +/- 0.4 nM) to the same membranes derived from a guinea pig myometrium indicate a homogeneous population of high-affinity binding sites which do not discriminate between these two hormones. Competitive binding experiments with specific oxytocin agonists containing either sarcosine or N-methylalanine in the place of Pro7 demonstrated that these myometrial receptors have the pharmacological properties of oxytocin receptors. The analogue of 1-deamino-[8-lysine]vasopressin containing a photoreactive azidophenylamidino group at the sidechain of Lys8 retained roughly the same receptor affinity as oxytocin. In photoaffinity labelling experiments with the tritium-labelled analogue a membrane protein from guinea pig myometrium with an apparent relative molecular mass Mr of 78,000 +/- 5000 (n = 13) was preferentially labelled. The labelling of this protein was completely suppressed by a 100-fold molar excess of either oxytocin, or [Sar7]oxytocin or [Thr4, Sar7]oxytocin, but not by other peptide hormones. These results provide evidence that the labelled 78,000-Mr protein is a myometrial oxytocin-receptor protein.

Stimulation of phospholipid hydrolysis and arachidonic acid mobilization in human uterine decidua cells by phorbol ester

Vasopressin and oxytocin both stimulated inositol phosphate accumulation in isolated uterine decidua cells. Pretreatment of cells with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) prevented this agonist-induced phosphoinositide hydrolysis. TPA (0.1 microM) alone had no effect on basal inositol phosphate accumulation, but stimulated phosphoinositide deacylation, as indicated by a 2-fold increase in lysophosphatidylinositol and glycerophosphoinositol. TPA also stimulated a dose-related release of arachidonic acid from decidua-cell phospholipid [phosphatidylcholine (PC) much greater than phosphatidylinositol (PI) greater than phosphatidylethanolamine]. The phorbol ester 4 beta-phorbol 12,13-diacetate (PDA) at 0.1 microM had no effect on arachidonic acid mobilization. The TPA-stimulated increase in arachidonic acid release was apparent by 2 1/2 min (116% of control), maximal after 20 min (283% of control), and remained around this value (306% of control) after 120 min incubation. TPA also stimulated significant increases in 1,2-diacylglycerol and monoacylglycerol production at 20 and 120 min. Although the temporal increases in arachidonic acid and monoacylglycerol accumulation in the presence of TPA continued up to 120 min, that of 1,2-diacylglycerol declined after 20 min. In decidua cells prelabelled with [3H]choline, TPA also stimulated a significant decrease in radiolabelled PC after 20 min, which was accompanied by an increased release of water-soluble metabolites into the medium. Most of the radioactivity in the extracellular pool was associated with choline, whereas the main cellular water-soluble metabolite was phosphorylcholine. TPA stimulated extracellular choline accumulation to 183% and 351% of basal release after 5 and 20 min respectively and cellular phosphorylcholine production to 136% of basal values after 20 min. These results are consistent with a model in which protein kinase C activation by TPA leads to arachidonic acid mobilization from decidua-cell phospholipid by a mechanism involving phospholipase A-mediated PI hydrolysis and phospholipase C-mediated PC hydrolysis, coupled with further hydrolysis of the 1,2-diacylglycerol product.