Endothelin induces two types of contractions of rat uterus: phasic contractions by way of voltage-dependent calcium channels and developing contractions through a second type of calcium channels

Structural and Functional Diversity of Animal Toxins Interacting With GPCRs

Peptide toxins from venoms have undergone a long evolutionary process allowing host defense or prey capture and making them highly selective and potent for their target. This has resulted in the emergence of a large panel of toxins from a wide diversity of species, with varied structures and multiple associated biological functions. In this way, animal toxins constitute an inexhaustible reservoir of druggable molecules due to their interesting pharmacological properties. One of the most interesting classes of therapeutic targets is the G-protein coupled receptors (GPCRs). GPCRs represent the largest family of membrane receptors in mammals with approximately 800 different members. They are involved in almost all biological functions and are the target of almost 30% of drugs currently on the market. Given the interest of GPCRs in the therapeutic field, the study of toxins that can interact with and modulate their activity with the purpose of drug development is of particular importance. The present review focuses on toxins targeting GPCRs, including peptide-interacting receptors or aminergic receptors, with a particular focus on structural aspects and, when relevant, on potential medical applications. The toxins described here exhibit a great diversity in size, from 10 to 80 amino acids long, in disulfide bridges, from none to five, and belong to a large panel of structural scaffolds. Particular toxin structures developed here include inhibitory cystine knot (ICK), three-finger fold, and Kunitz-type toxins. We summarize current knowledge on the structural and functional diversity of toxins interacting with GPCRs, concerning first the agonist-mimicking toxins that act as endogenous agonists targeting the corresponding receptor, and second the toxins that differ structurally from natural agonists and which display agonist, antagonist, or allosteric properties.

Inflammatory Amplification: A Central Tenet of Uterine Transition for Labor

In preparation for delivery, the uterus transitions from actively maintaining quiescence during pregnancy to an active parturient state. This transition occurs as a result of the accumulation of pro-inflammatory signals which are amplified by positive feedback interactions involving paracrine and autocrine signaling at the level of each intrauterine cell and tissue. The amplification events occur in parallel until they reach a certain threshold, ‘tipping the scale’ and contributing to processes of uterine activation and functional progesterone withdrawal. The described signaling interactions all occur upstream from the presentation of clinical labor symptoms. In this review, we will: 1) describe the different physiological processes involved in uterine transition for each intrauterine tissue; 2) compare and contrast the current models of labor initiation; 3) introduce innovative models for measuring paracrine inflammatory interactions; and 4) discuss the therapeutic value in identifying and targeting key players in this crucial event for preterm birth.

Endothelin as a local regulating factor in the bovine oviduct

Endothelin (EDN) is a possible regulating factor of oviductal motility, which is important for the transport of gametes and embryo. To clarify the factors that control the secretion of EDN in the bovine oviduct, the expression of EDNs, EDN-converting enzymes (ECEs) and EDN receptors (EDNRs) were investigated. All isoforms of EDN (EDN1–3), ECE (ECE1 and ECE2) and EDNR (EDNRA and EDNRB) were immunolocalised in the epithelial cells of the ampulla and the isthmus. EDNRs were also immunolocalised in smooth-muscle cells. The mRNA expression of EDN2 and ECE2 was higher in cultured ampullary oviductal epithelial cells than in isthmic cells. The expression of EDN1, EDN2 and ECE2 in the ampullary tissue was highest on the day of ovulation. Oestradiol-17β increased EDN2 and ECE1 expression, while progesterone increased only ECE1 expression in cultured ampullary epithelial cells. These results indicate that EDNs are produced by epithelial cells and their target site is smooth-muscle and epithelial cells, and suggest that ovarian steroids are regulators of endothelin synthesis in ampullary oviductal epithelial cells.

Effects of progesterone treatment on expression of genes involved in uterine quiescence

An important action of progesterone during pregnancy is to maintain the uterus in a quiescent state and thereby prevent preterm labor. The causes of preterm labor are not well understood, so progesterone action on the myometrium can provide clues about the processes that keep the uterus from contracting prematurely. Accordingly, we have carried out Affymetrix GeneChip analysis of progesterone effects on gene expression in immortalized human myometrial cells cultured from a patient near the end of pregnancy. Progesterone appears to inhibit uterine excitability by a number of mechanisms, including increased expression of calcium and voltage-operated K(+) channels, which dampens the electrical activity of the myometrial cell, downregulation of agents, and receptors involved in myometrial contraction, reduction in cell signal components that lead to increased intracellular Ca(2+) concentrations in response to contractile stimuli, and downregulation of proteins involved in the cross-linking of actin and myosin filaments to produce uterine contractions.

Endothelins in reproduction

Endothelin-1 (ET-1) is a 21 amino acid peptide, first isolated in 1988 from porcine aortic endothelial cells in tissue culture (Figure 1). The peptide was shown to be the most potent known vasoconstrictor of porcine coronary arteries. A powerful endothelium-derived vasoconstrictor had been predicted for some time, but it was when Yanagisawa and his colleagues elucidated the structure, and provided information about the molecular biology and mode of action of the peptide that an unprecedented interest was stimulated in the endothelins.

Relationship between gene expression and function of uterotonic systems in the rat during gestation, uterine activation and both term and preterm labour

We have documented gestation- and labour- (preterm and term) dependent changes in expression of genes encoding contraction associated proteins in the rat uterus and correlated these changes with various parameters of uterine contractility. The data demonstrate increased expression of contractile agonist systems concurrent with decreased expression of relaxant systems after gestational day 20. Significant increases in expression of oxytocin (OT), its receptor (OTR), prostaglandin (PG) H synthase isoform 1 (PGHS-1) and PGF(2alpha) receptor (FP) occurred first, followed by increases in PGHS-2, connexin-43, endothelin-1 (ET-1) and the ET-1 receptor isoform ET(A). Expression of OTR and FP was significantly reduced during mid-gestation compared to non-pregnant animals. Expression of inducible nitric oxide synthase (iNOS) increased significantly during pregnancy then decreased concurrently with the increase in OTR and FP. Functional changes in uterine contractility accompany changes in gene expression. OT was the most potent contractile stimulant. Sensitivity of uterine strips to OT was reduced in early and mid-pregnancy then increased at uterine activation. Progesterone antagonist-induced preterm labour caused changes similar to those at normal term. Comparison of mRNA transcripts in separated endometrium and myometrium suggested that the endometrium is an important regulator of myometrial contractility, analogous to the relationship between endothelium and vascular smooth muscle. This novel combination of functional and genetic expression analyses provides new insight into the physiology of parturition.

Vaso-active factors in pregnancy

Pregnancy results in profound physiological changes in the cardiovascular system, yet these changes are completely reversible. It is apparent that vaso-active factors, some as yet probably unidentified, which act as humoral or local autocrine or paracrine regulators of vasular resistance, play a major role in these cardio-vascular changes. This role may be heightened in pregnancy when there has to be a large increase in blood flow to the uterus and placenta while maintaining adequate flow to other vascular beds. Our knowledge of the mechanisms of action of these vaso-active factors and their interactions with each other still remains incomplete. Alterations in synthesis and action of these vaso-active factors may occur in pregnancies associated with pregnancy-induced hypertension, pre-eclampsia or intra-uterine growth retardation. Investigation of such alterations may help to elucidate the roles of vaso-active factors in both normal and pathological situations. The gestational hormones oestrogen and progesterone, are obviously prime candidates as overall regulators of the cardiovascular changes of pregnancy and as agents which alter the synthesis or action of other vaso-active factors. Currently, much attention is being focused on the role of local autocrine or paracrine vaso-active factors which may be produced by the endothelium or by the underlying vascular smooth muscle cells and alterations in their production or action in the hyptertensive disorders of pregnancy. The endothelium forms the largest endocrine organ within the body and so its importance in the mediation of vascular events should not be under-estimated. The principal objective of this review is to examine the roles of these many autocrine and paracrine vaso-active factors during pregnancy and their relation with the overall regulation of the vascular system. Changes which may occur and be involved in the aetiology of pre-eclampsia and growth retardation will also be examined.

Response of substances co-expressed in hypothalamic magnocellular neurons to osmotic challenges in normal and Brattleboro rats

The intention of this review is to emphasize the current knowledge about the extent and importance of the substances co-localized with magnocellular arginine vasopressin (AVP) and oxytocin (OXY) as potential candidates for the gradual clarification of their actual role in the regulation of hydromineral homeostasis. Maintenance of the body hydromineral balance depends on the coordinated action of principal biologically active compounds, AVP and OXY, synthesized in the hypothalamic supraoptic and paraventricular nuclei. However, on the regulation of water-salt balance, other substances, co-localized with the principal neuropetides, participate. These can be classified as (1) peptides co-localized with AVP or OXY with unambiguous osmotic function, including angiotensin II, apelin, corticotropin releasing hormone, and galanin and (2) peptides co-localized with AVP or OXY with an unknown role in osmotic regulation, including cholecystokinin, chromogranin/secretogranin, dynorphin, endothelin-1, enkephalin, ferritin protein, interleukin 6, kininogen, neurokinin B, neuropeptide Y, vasoactive intestinal peptide, pituitary adenylate cyclase-activating polypeptide, TAFA5 protein, thyrotropin releasing hormone, tyrosine hydroxylase, and urocortin. In this brief review, also the responses of these substances to different hyperosmotic and hypoosmotic challenges are pointed out. Based on the literature data published recently, the functional implication of the majority of co-localized substances is still better understood in non-osmotic than osmotic functional circuits. Brattleboro strain of rats that does not express functional vasopressin was also included in this review. These animals suffer from chronic hypernatremia and hyperosmolality, accompanied by sustained increase in OXY mRNA in PVN and SON and OXY levels in plasma. They represent an important model of animals with constantly sustained osmolality, which in the future, will be utilizable for revealing the physiological importance of biologically active substances co-expressed with AVP and OXY, involved in the regulation of plasma osmolality.

The effect of phosphoramidon on inflammation-mediated preterm delivery in a mouse model

OBJECTIVE

Several metallopeptidases have been implicated in both term and preterm parturition. We hypothesize that endotoxin-induced preterm delivery can be prevented by the administration of a metallopeptidase inhibitor.

STUDY DESIGN

We used an animal model of endotoxin-induced preterm delivery in timed pregnancy C57Bl/6 mice. Test animals received lipopolysaccharide followed by phosphoramidon, either every 1.5 or every 3 hours. Control mice received lipopolysaccharide followed by buffer injections at the same intervals. The primary outcome was a preterm delivery rate.

RESULTS

The rate of preterm delivery for the control animals was 88.0% compared with the treatment groups of 45.5% for the mice that received phosphoramidon every 3 hours and 30.8% for the group that received it every 1.5 hours (P<.01).

CONCLUSION

The administration of a metallopeptidase inhibitor resulted in a decreased rate of preterm delivery in this animal model.

Sustained arterial narrowing after prolonged exposure to perivascular endothelin

OBJECTIVE

The vasoconstrictor peptide endothelin-1 (ET-1) produces narrowing of cerebral arteries and has been implicated in the pathogenesis of cerebral vasospasm after subarachnoid hemorrhage. Little is known, however, regarding the physiological consequences of prolonged exposure of arterial wall to ET-1.

METHODS

In 30 rats, normal saline or 10(-8) mol/h of ET-1 was continuously applied for 3 or 5 days to the adventitial surface of the femoral artery in a Silastic cuff via an osmotic infusion pump. Vessels were examined for histopathological changes and luminal narrowing during ET-1 infusion (3 or 5 d) or at intervals from 2 to 9 days after infusion was stopped.

RESULTS

Marked arterial constriction (30-40% arterial diameter reduction) was present during continuous ET-1 infusion for 3 or 5 days. For both 3- and 5-day ET-1 infusions, significant reduction in arterial cross sectional area persisted up to 4 days after cessation of infusion, after which normal caliber returned. In arteries with persistent luminal narrowing after cessation of ET-1 infusion, light microscopic findings revealed morphological changes in the vessel wall similar to those observed in cerebral vasospasm after subarachnoid hemorrhage, with apparent increased collagen deposition in media and adventitia.

CONCLUSION

Continuous infusion of ET-1 produces reversible arterial narrowing that persists beyond the usual interval of physiological effect for this agent. Prolonged arterial constriction may produce physiological changes in arterial wall that act to maintain a narrowed lumen.

Immunohistochemical localization of ECE-1 in the human placenta

Over the last several years, endothelin (ET-1) has emerged as an important mediator in the pathogenesis of pre-eclampsia and preterm labour, as well as in the normal function of gestational tissues. While the distribution of ET and its binding sites in the human placenta have been well studied, much less has so far been reported about the distribution of placental ET-1 processing enzymes. By immunohistochemical analysis and immunofluorescence, endothelin-converting enzyme-1 (ECE-1), the enzyme that synthesizes ET-1, is localized to five distinct cell populations in the human placenta: (1) the endothelial cells lining the maternal basal plate blood vessels, (2) the intermediate trophoblasts, (3) the endothelial cells lining the chorionic villous blood vessels, (4) the chorionic villous stromal cells and (5) the chorionic villous trophoblasts. The localization of ECE-1 corresponds with the previously reported distribution of ET-1 in the human placenta and is in accordance with the function of this enzyme in regulating vascular tone through synthesis of ET-1. The abundance of ECE-1 in the basal plate is consistent with a second possible function of this enzyme in affecting uterine contractions. ECE-1 may serve as a target for prognosis and therapy in states of pathologically altered vascular tone and/or altered myometrial smooth muscle tone in gestation.

New peptides, hormones and parturition

This brief review emphasizes the importance of three novel discovered factors produced by fetal membranes, placenta and/or by the fetus itself in regulating uterine contractility. We have shown that, as reported for other hormones and substances, nitric oxide and endothelin may influence myometrial activity in an autocrine/paracrine manner interacting with other well-known agents such as prostaglandins, oxytocin and hormones. We also demonstrated that different isoforms of nitric oxide synthase (NOS) may play different roles throughout gestation and during labor. We have suggested that another peptide produced by trophoblast cells, adrenomedullin, may affect, directly or indirectly, myometrial contractility during pregnancy, although much remains to be learned about the mechanisms controlling adrenomedullin expression by the feto-placental tissues cells during pregnancy. Continued research is necessary to better define the complex interactions that result in parturition, both at term and preterm, and to allow a more rational approach to management of the premature labor, exploring new possible pharmacological solutions.

Effects of nimodipine and isradipine on endothelin-1-induced contraction of pregnant rat myometrium

The purpose of this study was to investigate the effect of nimodipine and isradipine on endothelin-1-induced contractions of isolated pregnant rat myometrium. Endothelin-1 at 10(-10)-10(-8) M dose-dependently increased the amplitude and frequency of contractions and decreased the duration of contractions. Basal tone of myometrium was increased with the higher endothelin-1 concentrations (10(-9) and 10(-8) M). After pretreatment for 30 min with 10(-6) M nimodipine or 10(-5) M isradipine, amplitude and duration of endothelin-1-induced contractions were significantly decreased, but frequency of myometrial contractions increased markedly. However, neither agent abolished the stimulating effect of higher endothelin-1 concentrations (10(-9) and 10(-8) M) on the basal tone. Our study showed that phasic activity is inhibited by nimodipine and isradipine, whereas tonic activity is not inhibited. The contraction of myometrial strips isolated from the pregnant rat may be modulated by endothelin-1, and this effect is only partly modulated by dihydropyridine-type Ca2+ channels. The remaining resistance to nimodipine- and isradipine-induced inhibition may be explained by pregnancy-associated changes in the other electrophysiological and biochemical factors.

Endopeptidase 24.11 inhibition does not modify uterotonic effects of endothelins in rat uterus

We investigated effects of the endopeptidase 24.11 inhibitor, SCH 39370, on uterotonic effects of endothelins (ETs) and sarafotoxin S6b. Responses of uteri from non-pregnant rats were inhibited by the ETA receptor antagonist, BQ123 (1 microM) but not the ETB receptor antagonist, BQ 788 (1 microM). ET-1, sarafotoxin S6b and ET-2 were more potent than ET-3 in tissues from non-pregnant and pregnant rats. SCH 39370 (10 microM) did not affect uterotonic responses to these peptides in either group, but inhibited those of big ET-1 in non-pregnant rat tissues, indicating inhibition of conversion of big ET-1 to ET-1. These data indicate that endopeptidase 24.11 does not inactivate the endothelin peptides in the rat uterus.

Endothelin-induced contractions in human placental blood vessels are enhanced in intrauterine growth retardation, and modulated by agents that regulate levels of intracellular calcium

Endothelin-1 (ET-1) is a strongly vasoactive polypeptide that may be involved in the regulation of the uteroplacental blood flow. In the present study we have examined the contractile response to ET-1 in human placental arteries in the presence of several agents that interfere with storage of intracellular calcium, e.g. caffeine, ryanodine and thapsigargin. We have also compared the contractile response to ET-1 in normal pregnancies with that of patients with foetal intrauterine growth retardation (IUGR), a condition with reduced uteroplacental blood flow. We found that the response to ET-1 in the placental arteries from women with normal pregnancies was reduced by 20% in the absence of extracellular calcium. Caffeine relaxed the basal tone of the vessels and reduced the contractile response to ET-1 by 51%. Nifedipine in addition to caffeine resulted in a reduction of 70%. Ryanodine also reduced the tone. Thapsigargin had no effect on the placental arteries at lower concentrations, but gave a progressive and slow contraction at 10(-6) M. The ET-1 induced contraction in placental arteries from IUGR patients was 67% more potent than in placental arteries from women with normal pregnancies, 129% as compared with 77% of the maximal K(+)-induced contraction. We conclude that the ET-1-induced contractile response in the human placental artery is dependent on influx of extracellular calcium as well as mobilization of calcium from intracellular stores. An increased sensitivity to ET-1 in placental arteries may contribute to the reduced uteroplacental blood flow in intrauterine growth retardation.

Immunohistochemical localization of endothelin-1 in placenta and fetal membranes in term and preterm human pregnancy

PROBLEM

The aim of the study was to determine the ET-1 localization on human placenta and fetal membranes and to compare its distribution between term and preterm pregnancies in laboring and non-laboring tissues.

METHODS

Tissues obtained from nine term elective cesarean section, eight spontaneous vaginal term delivery, and 13 preterm delivery from both cesarean section (N = 6) and vaginal delivery (N = 7) were studied by immunohistochemistry.

RESULTS

Immunoreactive ET-1 (IR-ET-1) was detected in villous and nonvillous trophoblast in all groups, although laboring tissues showed strong staining in the syncytiotrophoblast of the villi. ET-1 immunostaining of endothelial cells was observed in all placental villous vessels with a considerable variability within groups. In the fetal membranes, intensive immunopositive staining was observed in the chorionic trophoblast following vaginal deliveries in term and preterm tissues.

CONCLUSIONS

This is the first study to report the localization of IR-ET-1 in human fetal membranes and placenta, and suggests that amnion and trophoblast represents a source of ET-1 production or, alternatively, a site for ET-1 binding.

Endothelin expression in the uterus

The endothelins (ETs) comprise a family of 21 amino acid peptides, ET-1, ET-2 and ET-3, first demonstrated as products of vascular endothelium. Subsequent work showed that they are also found in non-endothelial cells from a variety of tissues such as breast, parathyroid and adrenal gland. At first, the ETs were recognized for their pressor effects. However, ET administration in vivo initially caused hypotension at low concentrations by triggering the paracrine release of endothelial-derived vasodilators. The ETs exert powerful contractile actions on myometrium and other types of smooth muscle and are mitogenic, or co-mitogenic for fibroblasts, vascular smooth muscle and other cells. Demonstration of extravascular ET in endometrium has revealed a powerful vasoconstrictor which might act on the spiral arterioles to effect a powerful and sustained contraction of vascular smooth muscle. ETs might also contribute to the process of endometrial repair. In addition, the ETs appear to play a fundamental role in the control of uterine function in pregnancy. Effects on myometrial contractility have been implicated in the mechanisms governing the onset of normal and pre-term labour, and the peptides are likely to be key determinants of placental blood flow by binding to vascular smooth muscle receptors in the placenta.

Gestational changes in oxytocin- and endothelin-1-induced contractility of pregnant rat myometrium

The mechanical effects of KCl, oxytocin and endothelin-1 on pregnant rat myometrium were examined using intact strips and beta-escin-treated skinned strips. Myometrial tissues from delivering rats were more sensitive to 10.7 mM K+ compared to mid and late gestation. Maximum contractions induced by K+ were obtained at concentrations of 118 mM at mid and late gestation and during delivery. The maximum amplitude of contractions induced by oxytocin and endothelin-1 compared to the 118 mM K(+)-induced contraction increased during the progress of gestation. Maximum contractions induced by oxytocin and endothelin-1 were greater than those induced by 118 mM K+ at delivery, and maximum contractions by oxytocin were larger than those by endothelin-1 during delivery. In 10 microM nifedipine and Ca(2+)-free (containing 2 mM EGTA) solutions, 118 mM K+ contractions were completely abolished; however, both oxytocin and endothelin-1 produced contractions. In Ca(2+)-free solutions, contractions by oxytocin were larger than those by endothelin-1. In skinned myometrial strips, guanosine 5'-O-thiotriphosphate (GTP, 1 microM-1 mM), guanosine 5'-O-(gamma-thiotriphosphate) (GTP gamma S, 0.1-100 microM) and oxytocin (1 nM-0.1 microM) with 10 microM GTP, but not endothelin-1 with 10 microM GTP increased Ca2+ sensitivity of contractile force.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of endothelin-1 on [Ca2+]i and pHi in trabecular meshwork cells

We examined the effects of endothelin-1 (ET-1) on [Ca2+]i and intracellular pH in cultured bovine trabecular meshwork cells and compared the effects of ET-1 with those of angiotensin II (another phospholipase C activating peptide). [Ca2+]i was measured with the Ca2+ fluorescent dye indo-1. Intracellular pH was measured using the pH sensitive fluorescent dye BCECF. Exposure to ET-1 (10 nM) produced a transient increase in [Ca2+]i (from 77.3 +/- 17.3 nM to 503.0 +/- 64.8 nM, p < 0.05, n = 12). Intracellular pH was also increased during exposure to 10 nM ET-1 (+0.081 unit, p < 0.05, n = 6). In the presence of 10 microM EIPA, ET-1 (10 nM) did not change intracellular pH. Angiotensin II did not significantly change [Ca2+]i or intracellular pH. These results suggest that ET-1 may be involved in the regulation of aqueous humor dynamics by changing [Ca2+]i and intracellular pH in trabecular meshwork cells.

The role of endothelium in phenylephrine- and potassium-induced contractions of the rat aorta during pregnancy

The involvement of endothelium in the contractile responses of rat aortic rings to phenylephrine and potassium chloride in pregnancy was examined. Contractions in response to both agents were significantly greater in rings from non-pregnant rats than in rings from pregnant rats, and they were unaltered by treatment of the rings with indomethacin. De-endothelialization potentiated the contractions of rings from pregnant rats in response to phenylephrine, but had no significant effect on similar rings contracted with potassium chloride. Whereas de-endothelialization had no significant effect on the contractions to phenylephrine in rings from non-pregnant rats, it decreased those of rings from the same type of rats, contracted with potassium chloride. Pregnancy significantly inhibited contractions in response to calcium chloride of rings treated with phenylephrine or potassium chloride. The effect of endothelium removal on contractions to calcium chloride in rings from pregnant and non-pregnant rats treated with phenylephrine or potassium chloride was similar to that observed for phenylephrine-induced and potassium chloride-induced contractions, respectively. Contractions of intact aortic rings from pregnant and non-pregnant rats to phenylephrine in calcium-free medium were similar. Results of the study suggest that the effect of pregnancy on the contractions of the rat aorta in response to phenylephrine and potassium chloride is at least partly mediated by the endothelium and is independent of prostaglandin synthesis. The endothelial factor involved in this effect appears to modulate contractions by interfering with calcium influx through the receptor-operated calcium channels and the voltage-operated calcium channels.

Big-endothelin-1 contracts rat isolated uterus via a phosphoramidon-sensitive endothelin ETA receptor-mediated mechanism

The presence of a phosphoramidon-sensitive endothelin-1-converting enzyme was investigated in the rat isolated uterus. Endothelin-1 and its precursor, big-endothelin-1, increased the rate of spontaneous contractions and caused tonic contractions. Responses to big-endothelin-1 had a slower start than those to endothelin-1. The tonic contraction induced by big-endothelin-1 (10 nM) was nearly abolished by phosphoramidon (100 microM), but the response to an equieffective concentration of endothelin-1 (3 nM) was not affected. Big-endothelin-1 (EC50 6.7 nM) was only 7-fold less potent than endothelin-1 (EC50 0.9 nM), whereas endothelin-3 was much less potent (EC50 > 100 nM). The endothelin ETA receptor antagonist, BQ-123 (40, 150 and 600 nM), induced graded rightward shifts of the concentration-response curve for endothelin-1. Schild analysis yielded a straight line with a slope not different from unity, and a pA2 value of 7.76. At 100 nM, BQ-123 specifically blocked responses to both endothelin-1 (3 nM) and big-endothelin-1 (10 nM), without modifying those to oxytocin (5 nM), acetylcholine (3 microM) or bradykinin (0.5 nM). Our results suggest the presence of phosphoramidon-sensitive endothelin-converting enzyme and demonstrate the occurrence of functional endothelin ETA receptors in the rat uterus.

Endothelin-1 receptors in the human myometrium: evidence for different binding properties in post-menopausal as compared to premenopausal and pregnant women

OBJECTIVE

We investigated the binding properties of the endothelin receptors in the human myometrium in clinical situations associated with different ovarian steroid levels.

SUBJECTS AND METHODS

Binding properties of the endothelin receptors were studied in myometrial membranes from post-menopausal women (n = 12), myomatous premenopausal women (n = 14) and pregnant women (n = 14), using 125I-labelled endothelin-1.

RESULTS

The mean (+/- SD) maximal receptor density (Bmax) was significantly higher in samples from premenopausal and pregnant women than from post-menopausal women (983 +/- 196, 1116 +/- 201 and 490 +/- 145 pmol/g protein, respectively). Receptor affinity (Kd) did not differ significantly between these groups. Among the pregnant women, mean Bmax and Kd values were similar in those who electively underwent Caesarean section prior to the onset of labour and those operated on during the second stage of spontaneous labour. Binding properties of myometrial membranes of either pre or post-menopausal women were unaffected by the presence of high levels of beta-oestradiol or progesterone in the medium. Among samples of premenopausal women, no significant difference was found in binding properties between those operated on either during mid-follicular phase or during mid-luteal phase.

CONCLUSIONS

In clinical situations associated with relatively high levels of ovarian steroids, the density of endothelin receptors in the myometrium is higher than in situations associated with low ovarian steroid level. Ovarian steroids may exert their influence via the production of other mediators. Changes in density of the endothelin receptor, induced by change in ovarian steroids activity, might play a role in the regulation of myometrial contractility.

Endothelin contracts human uterine myometrium by a partly dihydropyridine-sensitive mechanism

The effect of endothelin (ET), a recently discovered 21-amino-acid polypeptide with powerful vasoconstrictor properties, was examined on human uterine myometrial strips in vitro. ET dose-dependently (10(-11)-10(-7) M) increased the contractile force (monitored as contraction amplitude) of the myometrium with significant effects at 10(-8) and 10(-7) M. ET (10(-8) M and up) also increased the basal tone of the myometrium. The calcium channel blocking agents nifedipine (10(-7) M) and diltiazem (10(-6) M) both inhibited the spontaneous tonic contractions of the myometrium. When ET was given in the presence of nifedipine, the tonic contractions were further inhibited, whereas the ET-induced increase in basal tone remained. The same result was obtained with diltiazem (10(-6) M). The results indicate that the contractility of human myometrium may be modulated by ET, and that the effects of ET on the human myometrium are only partly mediated by dihydropyridine-sensitive calcium channels.

Solution conformation of endothelin and point mutants by nuclear magnetic resonance spectroscopy

Two-dimensional NMR techniques were utilized to determine the secondary structural elements of endothelin-1 (ET-1), a potent vasoconstrictor peptide, and two of its point mutants, Met-7 to Ala-7 (ETM7A), and Asp-8 to Ala-8 (ETD8A) in acetic acid-d3/water solution. Sequence specific NMR assignments were determined for all three peptides, as well as chemical shifts and NOE connectivity patterns. The chemical shifts of ET-1 and ETM7A are identical (+/- 0.05 ppm) except for the site of substitution, whereas marked shift changes were detected between ET-1 and ETD8A. These chemical shift differences imply that the Asp-8 to Ala-8 mutation has induced a conformational change relative to the parent conformation. All three molecules show the same basic nuclear Overhauser effect (NOE) pattern, which suggests that the gross conformation of all three molecules is the same. Small changes in sequential NOE intensities and changes in medium-range NOE patterns indicate that there are subtle conformational differences between ET-1 and ETD8A.

The comparative effects of big endothelin-1, endothelin-1, and endothelin-3 in the human fetal-placental circulation

OBJECTIVE

This study compared the effects of big endothelin-1, endothelin-1, and endothelin-3 and whether endothelin-converting enzyme was present in the human fetal-placental circulation.

STUDY DESIGN

Single cotyledons of term placentas were dually perfused in vitro, and increases in fetal-placental perfusion pressure to bolus injections of big endothelin-1, endothelin-1, and endothelin-3 (8 x 10(-10) to 1 x 10(-7) mol/L) were recorded. Responses to big endothelin-1 (10(-7) mol/L) were measured in the same placenta before and after perfusion of the fetal-placental circulation with the neutral metalloprotease inhibitor phosphoramidon (10(-5) mol/L), which acts as an endothelin-converting enzyme inhibitor. All experiments were performed in at least five separate placentas.

RESULTS

Significant concentration-dependent increases in fetal-placental perfusion pressure were seen with endothelin-1 (p < 0.0005), endothelin-3 (p < 0.0256), and big endothelin-1 (p < 0.0034, analysis of variance). Big endothelin-1 always elicited transient vasodilatation before constriction. Phosphoramidon significantly inhibited the vasoconstrictor effect of big endothelin-1 (p < 0.039, paired t test).

CONCLUSION

The three endothelins tested are vasoconstrictors, and endothelin-converting enzyme is present in the fetal-placental circulation.

Effects of endothelin on cytosolic Ca2+ level and mechanical activity in rat uterine smooth muscle

The effects of endothelin (ET) on cytosolic Ca2+ level ([Ca2+]i) and mechanical activity were examined in isolated rat uterine smooth muscle. ET-1, ET-2, ET-3 and sarafotoxin S6b (STX) induced rhythmic contractions superimposed on an increased muscle tone. The concentration needed to induce a half-maximum contraction (EC50) was 1.6-3.3 nM for ET-1, ET-2 and STX and higher than 200 nM for ET-3, suggesting that the ET(A) receptor is responsible for these contractions. The sensitivity to ET-1 of uterus at day 20 of gestation was higher than that of non-pregnant rat uterus. Contraction induced by ET-1 followed an increase in [Ca2+]i. The relation between [Ca2+]i and muscle tension, an an indicator of Ca2+ sensitivity of contractile elements, in the presence of ET-1 was identical to that in the presence of high K+ in non-pregnant and pregnant rat uteri. The ET-1-induced increases in [Ca2+]i and muscle tension were strongly inhibited by verapamil in non-pregnant rat uterus. In pregnant rat uterus, however, verapamil only partially inhibited the increases. The verapamil-insensitive portions of [Ca2+]i and contraction were inhibited by EGTA. In the absence of external Ca2+, ET changed neither [Ca2+]i nor muscle tension. These results suggest that ET-1 acts on ET(A) receptors, increase [Ca2+]i and induces contraction without changing Ca2+ sensitivity of contractile elements. The increase in [Ca2+]i seemed to be mediated by opening of L-type Ca2+ channels in non-pregnant rat uterus and also of non-L-type Ca2+ channels in pregnant rat uterus, but not by Ca2+ release from intracellular storage sites.

Contractile effect of endothelin in the human uterine artery and autoradiographic localization of its binding sites

OBJECTIVE

The aim was to study the effects, mode of action, and binding sites of endothelin-1 in the human uterine artery.

STUDY DESIGN

The contractile effect of endothelin-1 on the human uterine artery with and without endothelium and the effect of verapamil and nicardipine on the contraction was investigated in vitro. The Student t test was used. Iodine 125-endothelin-1 binding sites were localized with autoradiography.

RESULTS

Endothelin-1 induced a contraction that was unaffected by removal of the endothelium. Verapamil antagonized the contraction, whereas nicardipine showed no effect. Iodine 125-endothelin-1 binding sites were demonstrated in the smooth muscle layer.

CONCLUSIONS

These results show that endothelin-1 is a vasoconstrictor in the human uterine artery and suggest that the effect is mediated by receptors on the smooth muscle cells. The mode of action seems to involve Ca++ influx by other than dihydropyridine-sensitive Ca++ channels. Endothelin-1 does not seem to stimulate release of other endothelium-derived vasoactive agents.

Endothelins and sarafotoxins: effects on motility, binding properties and phosphoinositide hydrolysis during the estrous cycle of the rat uterus

The effects of four peptides of the endothelin/sarafotoxin (ET/SRTX) family on the motility of the rat uterus were examined during the different stages of the estrous cycle. ET-1, ET-3, SRTX-b and SRTX-c showed similar effects on the contraction of the uterus: a slight increase in the maximum tension of the spontaneous rhythmic contractions, a suppression of the relaxation phase of these contractions and an increase in their rate. All three effects were concentration dependent. Of the four peptides, ET-1 and SRTX-b showed the highest potency and efficacy, suggesting that among the various peptides of this family so far studied, ET-1 and SRTX-b are the two full agonists. The rank order of susceptibility of the different stages was, in most cases: proestrus greater than estrus greater than metestrus. Freshly excised diestrus uteri showed no spontaneous contractions and did not respond to any of the peptides. The binding potency of ET-1 and SRTX-b to uterine membranes was similar at the various estrous stages, but their maximal binding decreased gradually from proestrus to diestrus. All four peptides induced phosphoinositide (PI) hydrolysis in uterine slices at all four different stages, with ET-1 and SRTX-b again being more potent than ET-3 or SRTX-c. The maximal PI hydrolysis correlated with the increased rate of the rhythmic contractions. It is suggested that the reaction of the rat uterus to the ET/SRTX peptides depends on its hormonal status and that ET may act in concert with steroid hormones in the modulation of the estrous cycle.

Endothelin-1,2 levels are increased in the amniotic fluid of women with preterm labor and microbial invasion of the amniotic cavity

The purpose of this study was to determine the effect of gestational age, labor, and microbial invasion of the amniotic cavity on amniotic fluid concentrations of endothelin-1,2. Amniotic fluid was retrieved by amniocentesis from 148 women: patients at term with and without labor, patients with preterm labor with and without intraamniotic infection, and women in the second trimester of pregnancy. Endothelin-1,2 was measured by a sensitive and specific radioimmunoassay. Immunoreactive endothelin-1,2 was detectable in all samples of human amniotic fluid. Advancing gestational age and spontaneous term labor did not result in changes in amniotic fluid concentrations of endothelin-1,2. Women with preterm labor and positive amniotic fluid cultures for microorganisms had higher amniotic fluid concentrations of endothelin-1,2 than did those without microbial invasion of the amniotic cavity (p less than 0.05). These results support a role for endothelins in the mechanisms responsible for preterm delivery associated with intraamniotic infection.

Endothelin-1-induced vasoconstriction is not mediated by thromboxane release and action in the human fetal-placental circulation

The vasoconstrictor peptide endothelin-1 (8 x 10(-10) to 1 x 10(-8) mol/L) significantly increased fetal-placental perfusion pressure in vitro in a cumulative manner from 30 +/- 2 to 123 +/- 25 mm Hg (mean +/- SEM, n = 5, p less than 0.0005, analysis of variance). Accompanying this vasoconstriction was a corresponding reduction in fetal-placental perfusate flow rate. Measurement of thromboxane B2 and 6-keto-prostaglandin F1 alpha in the fetal-placental perfusate revealed a significant reduction in their release (p less than 0.0096 and p less than 0.0004, analysis of variance, respectively) when corrected for flow rate. Neither the thromboxane synthesis inhibitor dazoxiben (10(-6) mol/L) nor the thromboxane receptor antagonist SQ29548 (10(-6) mol/L) was able to block the vasoconstrictor actions of endothelin-1. Therefore endothelin-1-induced vasoconstriction in the human fetal-placental circulation does not appear to be mediated by thromboxane release or action. The stimulus to eicosanoid release in the fetal-placental circulation may be hydrodynamic, i.e., flow or shear stress.

Endothelin: a potent stimulator of intestinal ion secretion in vitro

Effects of endothelin (ET) on electrical properties and Na+ and Cl- fluxes in stripped rabbit ileal mucosa were investigated in vitro in Ussing chambers. Results demonstrate that serosal addition of ET-1, ET-2, ET-3 or the precursor 38 amino acid 'big endothelin' produce dose-dependent increases in short-circuit current (Isc) with maximal effects at approx. 100 nM, 100 nM, 10 nM and 100 nM, respectively and half-maximal effects at 1.4 nM, 5 nM, 1.4 nM and 20 nM, respectively. Mucosal addition of ET-3 failed to elicit a response. Changes in Isc elicited by ET-3 are accompanied by decreases in net fluxes of both Na+ and Cl-. The cyclooxygenase inhibitors, indomethacin and piroxicam, inhibited the increase in Isc produced by ET-3 and indomethacin also abolished the changes in Na+ and Cl- fluxes produced by ET-3. However, no changes in the release of PGE2, thromboxane B2 or 6-keto-prostaglandin F1 alpha could be detected up to 20 min after the addition of ET-3. Preincubation of tissues with neuronal agonists or antagonists, antihistamines or an LTD4/LTE4 receptor antagonist, SKF 104353, failed to alter the response to ET-3. Furthermore, removal of serosal Ca2+ also failed to inhibit the change in Isc produced by ET-3. These results indicate that endothelin is a potent intestinal secretagogue which does not appear to elicit its response through stimulation of PGE2, thromboxane A2 or prostacyclin.

Endothelin-evoked contractions in bovine ciliary muscle and trabecular meshwork: interaction with calcium, nifedipine and nickel

In the present study, we compared contractile responses of isolated bovine ciliary muscle and trabecular meshwork strips to endothelin-1 and carbachol. 1. Endothelin-1 is a potent contracting agent for ciliary muscle and trabecular meshwork. The EC50 was 5 x 10(-9) mol/l for both tissues. The maximal force evoked by endothelin was 73% of the maximal carbachol response in trabecular meshwork and 52% in ciliary muscle. 2. Carbachol contracted both tissues with an EC50 of 2 x 10(-7) mol/l. 3. In ciliary muscle, the tension was completely dependent on extracellular calcium. 4. In trabecular meshwork, 23 +/- 4% of the endothelin- and 42 +/- 10% of the carbachol-induced force response remained after removal of extracellular calcium. 5. Nifedipine (10(-5) mol/l) had only a slight relaxing effect in both tissues. 6. Nickel (10(3) mol/l) inhibited the development of force in both tissues. The relaxation induced by nickel was more pronounced in endothelin- than in carbachol-induced contractions. Different intracellular mechanisms mediating the action of endothelin and carbachol can be described: i) a calcium- and a nickel-sensitive pathway in both tissues and ii) an additional mechanism independent of external calcium in trabecular meshwork. These results indicate functional differences between the contractile region of trabecular meshwork and ciliary muscle. Endothelin may participate in accommodation and regulation of the intraocular pressure.

Increased plasma levels of the novel vasoconstrictor peptide endothelin in severe pre-eclampsia

Plasma endothelin levels were studied in 29 pregnancies. Levels were found to be increased in 9 pregnancies complicated by pregnancy-induced hypertensive disease and/or pre-eclampsia as compared to levels in 14 normotensive pregnancies with gestational age varying between 24-40 weeks with normal Doppler flow velocity waveforms of the uterine arcuate arteries, and 6 normotensive pregnancies with abnormal Doppler flow velocity waveforms at 24 weeks' gestation. Two patients with severe pre-eclampsia showed a rise in plasma endothelin levels, suggesting a correlation between plasma endothelin levels and the disease process. Endothelin is produced by endothelial cells of blood vessels; it is the most potent vasoconstrictor known to date. For this reason it has been suggested that endothelin might be important in the control of systemic blood pressure and local blood flow, both disturbed in pre-eclampsia. The conclusion of this study is that the venous plasma level of endothelin would seem to be a marker for severe disease, however, without any predictive value.

An endothelin receptor (ETA) antagonist isolated from Streptomyces misakiensis

A competitive endothelin (ET) antagonist, BE-18257B, was isolated from the fermentation products of Streptomyces misakiensis. It is a novel cyclic pentapeptide, cyclo(-D-Glu-L-Ala-allo-D-Ile-L-Leu-D-Trp-), and binds to ETA receptors (ET-1 selective) in cardiovascular tissues, but not to ETB receptors (equally sensitive to isopeptides of ET family) in kidney, adrenal gland and cerebellum tissues. BE-18257B also antagonizes ET-1-induced vasoconstriction in rabbit iliac artery and pressor action in rats. Thus it is a selective ETA antagonist and should provide a valuable tool for elucidation of the pharmacological and pathophysiological roles of ET-1.

Endothelin production by amnion and its regulation by cytokines

Cells derived from human amnion, an avascular tissue, produced significant amounts of endothelin 1 and 2, although the rate of biosynthesis was approximately one tenth of that of human umbilical vein endothelial cells. Endothelin 1 and 2 was not produced in measurable amounts by cells derived from chorion laeve or decidua. Production of endothelin 1 and 2 by amnion cells was increased by treatment with the cytokines interleukin-1 beta, tumor necrosis factor, and interleukin-6. The biosynthesis of endothelin 1 and 2 by an avascular tissue is suggestive of a wider role for endothelins than has been recognized.

Endothelin and neuropeptide Y are vasoconstrictors in human uterine blood vessels

The control of uterine blood flow is regulated by a number of factors including vasoactive present in the endothelium and the perivascular nerves. We examined the effects of two vasoactive peptides, endothelin and neuropeptide Y, on segments of human uterine arteries and veins in vitro. We found endothelin, a potent vasoconstrictor in the uterine artery and vein, giving a slow and long-lasting contraction starting at 10(-9) mol/L. At this concentration, the established vasoconstrictor noradrenaline had almost no effect. Neuropeptide Y had no effect of its own in the human uterine artery, but increased basal tone in uterine veins, and at higher concentrations triggered spontaneous contractions. Neuropeptide Y (10(-7) mol/L) often, but not always, enhanced the contraction induced by noradrenaline (10(-6) mol/L). Neuropeptide Y (10(-7) mol/L) also enhanced the contraction induced by transmural nerve stimulation (20 Hz, 0.4 msec). We conclude that endothelin and neuropeptide Y may modulate the control of human uterine circulation.

Endothelin- and oxytocin-induced calcium signaling in cultured human myometrial cells

The demonstration that endothelin (ET) induces rat uterine contraction, coupled with the observation that ET is present in human amniotic fluid, suggests that the myometrium may be an important target organ for this hormone. We show that in quiescent human myometrial cells ET produced a dose-dependent increase in cytosolic free Ca2+ (Cai2+), which was markedly attenuated when the cells were studied in Ca2(+)-free media. Preincubation with nicardipine, diltiazem, or verapamil reduced the ET-evoked Cai2+ transient by 30, 40, and 65%, respectively. The presence of voltage sensitive Ca2+ channels was demonstrated by Mn2+ quenching of fura-2. Activation of the Na+/H+ antiport could not be demonstrated with ET stimulation. In nonquiescent cells, the ET-evoked Cai2+ transient was significantly reduced, while the response to oxytocin was retained. This is at least partially explained by a reduction in Bmax (maximal binding capacity) for ET (mean +/- SEM) from 3,506 +/- 268 binding sites/cell in quiescent cells to 2,411 +/- 300 binding sites/cell, as well as 72% increase in Kd (equilibrium dissociation constant), in the nonquiescent cells. We conclude that, in human myometrial cells, ET and oxytocin modulate Cai2+ through independent receptors and propose that ET, like oxytocin, is an important endogenous modulator of uterine contractility.

Effects of endothelins, Bay K 8644 and other oxytocics in non-pregnant and late pregnant rat isolated uterus

Endothelins 1, 2 and 3 (ET-1, ET-2 and ET-3; 1-30 nM) caused long-lasting concentration-dependent tonic contractions of uterine strips from non-pregnant rats. The potency of ET-1 (EC50 7 nM) was similar to that of angiotensin II (AII) and greater than that of ET-2 or ET-3 (EC50S greater than or equal to 10 nM), bradykinin, Bay K 8644, oxytocin (OT), 5-hydroxytryptamine, prostaglandin F2 alpha (PGF2 alpha) or acetylcholine. Strips from 21-day pregnant rats were 2- to 3-fold more sensitive to ET-1, AII, OT and PGF2 alpha and 200-fold more sensitive to Bay K 8644 than non-pregnant preparations. The development of tonic responses to ET-1 (30 nM) and of phasic-rhythmic ones to Bay K 8644 (300 nM) was fully prevented in strips from non-pregnant rats bathed in Ca2(+)-free medium, but stepwise reintroduction of Ca2+ (0.03-3 mM) to the solution allowed the manifestation of contractions in response to both agonists. Responses to ET-1 required less Ca2+ than those to Bay K 8644. Strips challenged with ET-1 while in Ca2(+)-free medium developed greater contractions upon reintroduction of Ca2+ than preparations stimulated with the peptide in normal medium. The reverse occurred with Bay K 8644-induced contractions. Nicardipine (10 nM) abolished the responses of strips from non-pregnant rats to Bay K 8644 (300 nM), but only attenuated ET-1-induced (30 nM) contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple subtypes of endothelin receptors in porcine tissues: characterization by ligand binding, affinity labeling and regional distribution

To clarify the existence and the distribution of endothelin (ET) receptor subtypes, we have examined the pharmacological properties and the molecular weight (Mr) of 125I-ET-1 and 125I-ET-3 binding sites in various tissues of pigs. ET-1 and ET-2 showed almost identical potencies in displacing the bound 125I-ET-1 in all the tissues examined. ET-3, sarafotoxin S6b (SRT-b) and sarafotoxin S6c (SRT-c) displaced the 125I-ET-1 with the same sensitivity as ET-1 (IC50 = 0.1-1.4 nM) in brain, kidney, liver and adrenal, whereas the three peptides showed very weak competition (IC50 = 40-500 nM) against 125I-ET-1 binding in cardiac atria, aorta, lung, stomach and uterus. The computer analyses of the binding data suggested the presence of high (Kd1 = 0.04-0.29 nM) and low (Kd2 = 60-190 nM) affinity binding sites for ET-3 and SRT-b in lung and stomach. 125I-ET-3 bound to the high affinity sites in lung and stomach was displaced by ET/SRT isopeptides almost equipotently. Two proteins with Mr of 47,000 and 35,000 were affinity-labeled with 125I-ET-1 in cerebellum, while a protein with Mr of 123,000, in addition to the two proteins, was predominantly labeled in lung. The above findings indicated that two distinct subclasses of ET receptors, namely, ET-1-specific and ET/SRT family-common receptors were distributed in various proportions in mammalian tissues, and suggested that their molecular forms are also different.

Detection of endothelin-like immunoreactivity in epithelium and fibroblasts of the human umbilical cord

We studied tissue sections of freshly obtained full-term and premature human umbilical cords using polyclonal antibody to endothelin and immunocytochemistry. Endothelin immunoreactivity was detected in the cytoplasm of epithelial cells and primitive fibroblasts, but not in the endothelial cells of both full-term and premature umbilical cords. Immunoelectron microscopy using indirect immunogold staining technique localized endothelin immunoreactivity to the cytoplasm of the epithelial cells and fibroblasts but not confined to any particular structures. No endothelin immunoreactivity was detected in the nucleus or on the cell membrane. Pre-absorption tests with synthetic endothelin-1, -2, and -3 independently established that the immunoreactivity represented endothelin-1 and -2, but not -3. The presence of endothelin-1 and -2-like immunoreactive materials in epithelial cells and fibroblasts of human umbilical cord suggests a role of endothelin in parturition.