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

Steroid modulation of oxytocin/vasopressin receptors in the uterus

Oxytocin (OT) and V1 vasopressin (VP) receptors are present simultaneously in several tissues, including the uterus. In myometrium these receptors mediate contractility, while in endometrium they mediate the release of other uterotonic substances as endothelin (ET). In rabbit myometrium, estrogens increase, while progesterone blunts neurohypophysial hormone receptors. However, the action of sex steroids on OT and V1 VP receptors differs in terms of the ED50 and maximal effect. Therefore, at parturition, only OT receptors show a dramatic rise, while V1 VP receptors do not change, suggesting a major role for OT in labor. ET is a potent stimulator of uterine activity acting through specific receptors present on myometrial cells. These receptors as well as the endometrial localization of ET are modulated by sex steroids, indicating that ET might represent a paracrine regulator of uterine activity. In humans, OT but not V1 VP receptors increase as pregnancy progresses, confirming the primary relevance of OT in timing delivery.

Actions of endothelin-1 on prostaglandin production by gestational tissues

Endothelin-1 (10(-11)M-10(-7)M) was incubated with human umbilical vein endothelial cells and cells derived from amnion and decidua and prostaglandin production was determined. The rates of biosynthesis of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and prostaglandin E2 (PGE2) by endothelial cells were increased significantly by treatment with endothelin-1. Amnion cell PGE2 production was reduced significantly by endothelin-1 treatment whereas decidual PGE2 and prostaglandin F2 alpha production was unaffected by this treatment. Thus, it is possible that endothelins may play a part in the regulation of uteroplacental hemodynamics and the mechanisms of parturition.

Effects of endothelin-1 on the isolated guinea-pig ileum: role of Na+ ions

Endothelin-1 induced a unusual transient biphasic effect (relaxation followed by contraction) in the isolated guinea-pig ileum. The contractile component of the response was concentration-dependent in the dose range studied, but the relaxant component was not. Neuronal mechanisms and cyclic GMP do not seem to be involved in the relaxing effect since this component was not affected by either tetrodotoxin, atropine or methylene blue. Endothelin-1 induced desensitization which was well characterized for the contractile component of the response, and was only partially reversed after a 3-h resting period. The effects of endothelin-1 were particularly sensitive to decreases in the Na+ gradient induced either by lowering the Na+ concentration in the medium or by treating the tissue with ouabain or by a previous treatment with acetylcholine. Amiloride partially inhibited the response induced by endothelin-1 indicating that Na+ channels and/or Na+/H+ exchange are probably involved in its action. Our results suggest that the contractile response induced by endothelin-1 in the isolated guinea-pig ileum depends greatly on Na+ ions, in contrast to the responses observed in vascular smooth muscle.

Increased maternal plasma concentration of endothelin-1 during labor pain or on delivery and the existence of a large amount of endothelin-1 in amniotic fluid

The concentration of endothelin-1 (ET-1) in plasma and amniotic fluid from normal pregnant women was determined by a sensitive sandwich-enzyme immunoassay system, established recently. The plasma ET-1 level increased gradually during normal pregnancy as the pregnancy advanced, the levels (0.40 +/- 0.02 pmol/l, n = 45) being significantly (p less than 0.05) higher after 29 weeks of gestation than those (0.32 +/- 0.01 pmol/l, n = 30) before 28 weeks of gestation. The plasma ET-1 level during labor pain was significantly higher (0.59 +/- 0.06 pmol/l, n = 10) than that (0.40 +/- 0.02 pmol/l, n = 45) in the 3rd trimester of pregnancy without labor pain (p less than 0.02). Moreover, a high level of ET-1 (17.38 +/- 0.25 pmol/l, n = 18) was detected in amniotic fluid on term delivery. The ET-1 level in amniotic fluid was significantly higher than the levels in maternal and umbilical cord plasma (p less than 0.001 and p less than 0.001, respectively). After delivery the maternal ET-1 level decreased gradually and 2 day postpartum ET-1 levels reached the normal non-pregnant level.

Actions of endothelin on isolated corpus cavernosum from rabbit and man

The effects of endothelin, a vasoconstrictor peptide produced by vascular endothelial cells, were investigated in isolated rabbit and human corpus cavernosum (CC). Preparations from both rabbit and man were potently contracted by endothelin in a concentration-dependent manner. The contractions developed slowly, could not be reversed despite frequent washings, and were only partly inhibited by the Ca2+ channel blocker nimodipine. Even in Ca2(+)-free medium containing the chelator EGTA a small contractile component persisted. In rabbit CC, the contractions in Ca2(+)-free medium were not affected by nimodipine, the Ca2(-)-channel agonist BAY K 86(44), or by depletion of intracellular Ca2(+) stores sensitive to noradrenaline (NA) and caffeine, but were almost abolished by the protein kinase C inhibitor H7. In both rabbit and man, carbachol and vasoactive intestinal polypeptide concentration-dependently relaxed preparations contracted by endothelin. The relaxations induced by carbachol were antagonized by atropine. Endothelin enhanced concentration-dependently the contractions induced by exogenously applied NA in rabbit CC. The enhancement was more pronounced at low concentrations of NA. This study shows that endothelin potently contracts isolated penile erectile tissue. The contraction seems to be mediated mainly by influx of Ca2+ through the cell membrane, which partly occurs through a pathway other than voltage-operated calcium channels. However, involvement of other mechanisms cannot be excluded. The results suggest that endothelin can play a role in penile erectile mechanisms.

Contractile activity of three endothelins (ET-1, ET-2 and ET-3) on the human isolated bronchus

1. The effects of three endothelins: (i) the classical or human/porcine endothelin (ET-1); (ii) [Trp6, Leu7] endothelin (ET-2) and (iii) [Thr2, Phe4, Thr5, Tyr6, Lys7, Tyr14] endothelin or rat endothelin (ET-3) were tested on the human isolated bronchus. 2. ET-1 produced a concentration-dependent contraction of the human isolated bronchus that proceeded in two different steps. The first step was observed at very low concentrations (pD2 = 11.01 +/- 0.17, n = 10) but corresponded to a low intrinsic activity (Emax = 15.6 +/- 1.8% of Emax = 26.1 +/- 2.9% of ACh 3 x 10(-3) M, n = 5, P less than 0.05), reduced by nicardipine 10(-6) M (Emax = 6.0 +/- 2.6% of ACh 3 x 10(-3) M, n = 5, P less than 0.05) and strongly inhibited in calcium-free medium. The second step of the action of ET-1 corresponded to a lesser potency (pD2 = 7.90 +/- 0.17, n = 9) but a higher intrinsic activity (Emax = 82.5 +/- 4.7% of ACh 3 x 10(-3) M). This effect was not significantly modified by nicardipine 10(-6) M or by Bay K 8644 10(-7) M. Neither of the two effects was modified by indomethacin 3 x 10(-6) M. 3. The effects of ET-2 and ET-3 were qualitatively similar to those of ET-1 but quantitatively different; for these two steps of contracting activity and for potency and efficacy the ranking was: ET-1 greater than ET-2 = ET-3. 4. Thus, ET-1 appears to be the most potent of these three substances in its effect on the human isolated bronchus. Its activity seems to involve the action of voltage-dependent calcium channels at low concentrations (10-12 to 10-9M), whereas other mechanisms are involved at higher concentrations (10-8 to 3 x 10-7M).

Comparative studies of phosphoinositide hydrolysis induced by endothelin-related peptides in cultured cerebellar astrocytes, C6-glioma and cerebellar granule cells

Effects of endothelin (ET) homologues (ET-1, 2, 3 and sarafotoxin S6b) and its precursor (big ET-1) on phosphoinositide (PI) turnover were compared in neurally-related cell cultures. All ET-related peptides induced a robust increase of PI turnover in cerebellar astrocytes, C6-glioma and cerebellar granule cells. The rank order of potency in stimulating PI turnover was ET-1 = ET-2 greater than or equal to S6b greater than ET-3 greater than big ET-1 for granule cell neurons, while it was ET-1 greater than or equal to ET-2 greater than or equal to S6b greater than big ET-1 greater than ET-3 for astrocytes and C6-glioma cells. Short-term pretreatment with phorbol dibutyrate (PDBu) attenuated the ET-1-induced PI response in all three types of cultures. However, long-term pretreatment with PDBu attenuated the response in granule cells and C6-gliomas, but enhanced responses to ET and ATP in astrocytes. Long-term exposure of cells to pertussis toxin (PTX) attenuated the PI response to ET in astrocytes and C6-gliomas, but not in granule cells. Thus, phospholipase C-coupled ET receptors are expressed in both neurons and glial cells, but they differ considerably in their pharmacological selectivity and signal transduction mechanisms in stimulating PI hydrolysis.

Endothelin: a review of its effects and possible mechanisms of action

The recently identified endothelium-derived peptide, endothelin, is a potent vasoconstrictor, but also binds specifically to many types of smooth muscle and to nerve tissue. Endothelin has been detected in plasma and may have physiological or pathological functions. Like other agonists, endothelin increases the turnover of phosphatidyl inositol and liberates intracellular stocks of Ca2+. It also increases plasmalemmal Ca2+ permeability, an effect that is antagonized by calcium entry blockers in some tissues. However, the characteristics of this antagonism are not always typical of that seen when other types of agonists are employed. It seems that in at least some cell types endothelin might activate specific L-type Ca2+ channels indirectly, perhaps secondarily to the activation of another type of cation channel. The endothelin originally described is one of a family of peptides that are closely related to the sarafotoxins. The comparative pharmacology of these peptides and of some analogues of the originally described endothelin have revealed some surprising differences and may indicate the existence of different endothelin receptors.

Endothelin increases cytoplasmic calcium and myosin phosphorylation in human myometrium

Endothelin, a recently discovered sarafotoxin-like peptide secreted by endothelial cells, is a potent stimulator of vascular smooth muscle contraction. We found that the action of endothelin is not restricted to the vasculature; we demonstrated that endothelin causes an increase in the concentration of intracellular Ca++ and phosphorylation of the 20 kd light chain of myosin in human uterine smooth muscle cells in culture. In the absence of Ca++ in the buffer medium of myometrial cells, the effects of endothelin on intracellular Ca++ and myosin light chain phosphorylation are attenuated but not abolished. Endothelin also increases the frequency of contraction of human uterine smooth muscle (longitudinal and circular). The contractile effects of endothelin on myometrial strips are diminished in the presence of nifedipine. We conclude that (1) human myometrium is responsive to endothelin, (2) endothelin promotes contraction in myometrium by effecting an increase in intracellular Ca++ and thus an increase in myosin light chain phosphorylation, and (3) endothelin acts in myometrium by stimulating Ca++ influx as well as Ca++ release from intracellular stores.

Biphasic effects of endothelin in the guinea-pig ileum

The intestinal effects of porcine endothelin (ET-1), a potent endothelium-derived vasoconstrictor, were studied on the guinea-pig isolated ileum. ET-1 (3 x 10(-10)-3 x 10(-7) M) caused biphasic responses on spontaneous smooth muscle tone, an initial relaxation followed by a late contraction. The contractile response was elicited in a concentration-dependent manner. Both the relaxing and contractile phases were not affected by pretreatment with tetrodotoxin, phentolamine, tolazoline, propranolol, guanethidine, 8-phenyl-theophylline, naloxone, methylsergide, [D-Pro4,D-Try7,9]SP-(4-11), diphenylhydralamine and indomethacin. Atropine (3 x 10(-7) M) also had no effect on the ET-1-induced contraction. The ET-1-induced contraction, however, was markedly inhibited by verapamil (greater than 10(-8) M) and H-7 (3 x 10(-5) M). These results suggest that, in intestinal smooth muscle, ET-1-induced contraction can be attributed to the direct effect on muscle and appears to be mediated by increased Ca2+ influx through voltage-dependent Ca2+ channels as well as protein kinase C activation. On the other hand, ET-1-induced relaxation is due neither to the indirectly evoked release of inhibitory neurotransmitters nor to the direct activation of adrenoceptors, and purine and opiate receptors. The exact mechanism responsible for ET-1-induced relaxation needs to be further studied.

Expression and regulation of endothelin precursor mRNA in avascular human amnion

Posttranslational processing of preproendothelin in endothelial cells gives rise to endothelin, a 21 amino acid polypeptide that is a potent vasoconstrictor. Endothelin production is believed to be mediated principally by transcriptional mechanisms. Previously, preproendothelin mRNA expression has been detected only in vascular endothelial tissue and cells. In this study, we found that preproendothelin mRNA is expressed in an avascular human tissue, namely, amnion, an extraembryonic fetal membrane. Preproendothelin mRNA was not detected in avascular chorion laeve tissue (also an extraembryonic fetal membrane), in the highly vascularized fetal trophoblast, or in maternal uterine tissues. Furthermore, we found that preproendothelin gene expression is retained in human amnion cells maintained in primary monolayer culture. Using the amnion cells in primary monolayer culture to investigate the regulation of preproendothelin mRNA expression, we found that epidermal growth factor (EGF) and interleukin-1 (IL-1) act to stimulate preproendothelin mRNA levels; in addition, the induction of preproendothelin mRNA by either of these agents is enhanced upon simultaneous treatment with cycloheximide. These findings are indicative that preproendothelin gene expression in amnion is regulated positively by EGF and IL-1 and that inhibition of protein synthesis leads to superinduction of preproendothelin mRNA. In human umbilical cord endothelial cells, neither IL-1 nor EGF stimulate preproendothelin mRNA expression but inhibition of protein synthesis does lead to increased levels of preproendothelin mRNA. The amnion, therefore, provides a useful system for expansion of our understanding of the tissue specific expression and regulation of preproendothelin mRNA.

The activity of peptides of the endothelin family in various mammalian smooth muscle preparations

The activity of natural endothelins such as ET-1, ET-2, ET-3 and of sarafotoxin S6b (SRFTX) as compared to that of the C-terminal hexapeptide ET-(16-21) was investigated in several smooth muscle preparations in the presence of indomethacin, captopril, bestatin and thiorphan. In some tissues (rat thoracic aorta, guinea-pig ileum, human urinary bladder, renal pelvis or renal artery), ET-(16-21) had little if any agonist activity. In other preparations (guinea-pig bronchus, rat vas deferens, rabbit pulmonary artery) ET-(16-21) was a full agonist. The amidated form of ET-(16-21) was either inactive or less potent than ET-(16-21). These findings provide evidence that at least two receptors exist for peptides of the ET family; these receptors were termed ETA and ETB. ET-(16-21) is a full agonist at ETB receptors while being inactive or weakly active at ETA receptors. The free acid of tryptophan in position 21 plays a key role in the activity of these peptides at ETB receptors.

Endothelin depolarizes membrane voltage and increases intracellular calcium concentration in human ciliary muscle cells

The ciliary muscle which is involved in accommodation and regulation of aqueous humour outflow resistance resembles smooth muscle in other parts of the body. In the present investigation we used an established primary cell line (H7CM) to study the effects of endothelin, a novel vasoconstrictor peptide, on membrane voltage (V) and intracellular calcium in cultured human ciliary muscle cells. Membrane voltage was measured in confluent monolayers of H7CM cells using conventional microelectrodes. Intracellular calcium concentration [( Ca]i) was measured in single H7CM cells using the fluorescent calcium indicator fura-2. Under resting conditions V averaged -66.9 +/- 0.7 mV (mean +/- SEM, n = 125). Endothelin (10(-10)-10(-6)M) induced a dose-dependent reversible membrane voltage depolarization and a dose-dependent rise in [Ca]i. The initial calcium peak was followed by a recovery phase during which oscillations of [Ca]i occurred. The initial calcium peak was not dependent on the presence of extracellular calcium and was not abolished in the presence of the calcium antagonist verapamil (10(-4)M). Thus it is probably mediated by a release of calcium from intracellular reservoirs. We conclude that cultured human ciliary muscle cells express a functional endothelin receptor.

Endothelin-1 induces vasoconstriction through two functionally distinct pathways in porcine coronary artery: contribution of phosphoinositide turnover

Endothelin-1 (ET1)-induced contraction of isolated porcine coronary artery strips was previously reported to be mainly dependent on extracellular Ca2+. However, even in a Ca2+-free, EGTA-containing solution relatively high concentrations of ET1 induced a weak vasoconstriction, which was markedly but not completely inhibited by pretreatment with caffeine. Over similar dose ranges, ET1 stimulated the production of inositol phosphates in a dose-dependent manner in intact arterial tissues, which was independent of extracellular Ca2+ and was not affected by receptor blockers such as atropine, methysergide and diphenhydramine. Moreover, ET1 was shown to induce an increase in 1,2-diacylglycerol. These results indicate that the activation of ET1 receptors on porcine coronary artery smooth muscle causes phosphoinositide breakdown, leading to intracellular Ca2+ mobilization and protein kinase C activation. It is suggested that phospholipase C-mediated phosphoinositide breakdown as well as previously reported activation of voltage-dependent Ca2+ channels are involved in the mechanism of ET1-induced vasoconstriction.

Endothelin, its diverse biological activities and mechanisms of action

Endothelin, originally identified as a vasoconstrictive peptide derived from vascular endothelial cells, is now known to exert diverse biological effects on a wide variety of tissues and cell types through its own receptor(s). One of the outstanding actions of endothelin is a cell growth promoting activity which is demonstrated in several cell types including cultured vascular smooth muscle cells, fibroblasts, glomerular mesangial cells and osteoblasts. The mitogenic effect is likely mediated by stimulation of phospholipase C via receptor-G-protein coupling, and subsequent activation of protein kinase C. The effect of endothelin may contribute to the cell-proliferation response under various physiological and pathological conditions, such as wound healing and development of atherosclerosis and glomerulonephritis. Recently, three distinct endothelin-related genes have been cloned, suggesting that mammals, including humans, produce three members of this peptide family, endothelin (ET)-1 (the 'classical' endothelin), ET-2 and ET-3, which may act on distinct subtypes of endothelin receptor to induce different cellular responses.

Endothelin--a new family of endothelium-derived peptides with widespread biological properties

Endothelin (ET) is a novel family of three isopeptides (ET-1, ET-2, ET-3) each containing twenty-one amino acids and two disulfide bonds. Initially isolated from the supernatant of cultured porcine aortic endothelial cells, ET is stored as a preproform and released through an unusual proteolytic cleavage. In general, ET-1, ET-2, ET-3 differ quantitatively but not qualitatively in their biologic activity. ET have potent contractile activity in a variety of isolated tissues including arteries veins, trachea, duodenum urinary bladder and uterus. In vivo, ET possesses potent vasodilator and vasoconstrictor properties. Although the mechanisms mediating the hemodynamic effects of ET are not entirely clarified, recent evidence indicates a role for endothelium-derived relaxant factor (EDRF), protein kinase C and extracellular calcium. Moreover, ET appears to produce inflammation and bronchoconstriction through the formation of arachidonic acid metabolites via the cyclooxygenase pathway. The presence of ET binding sites in blood vessels and in several organ systems suggests ET may have important regulatory functions, which remain to be determined.