Competitive and non competitive interactions of BQ-123 with endothelin ETA receptors

Mechanisms involved in the long-term modulation of tyrosine hydroxylase by endothelins in the olfactory bulb of normotensive rats

The olfactory bulbs play a relevant role in the interaction between the animal and its environment. The existence of endothelin-1 and -3 in the rat olfactory bulbs suggests their role in the control of diverse functions regulated at this level. Tyrosine hydroxylase, a crucial enzyme in catecholamine biosynthesis, is tightly regulated by short- and long-term mechanisms. We have previously reported that in the olfactory bulbs endothelins participate in the short-term tyrosine hydroxylase regulation involving complex mechanisms. In the present work we studied the effect of long-term stimulation by endothelins on tyrosine hydroxylase in the rat olfactory bulbs. Our findings show that endothelin-1 and -3 modulated catecholaminergic transmission by increasing enzymatic activity. However, these peptides acted through different receptors and intracellular pathways. Endothelin-1 enhanced tyrosine hydroxylase activity through a super high affinity ET(A) receptor and cAMP/PKA and CaMK-II pathways, whereas, endothelin-3 through a super high affinity atypical receptor coupled to cAMP/PKA, PLC/PKC and CaMK-II pathways. Endothelins also increased tyrosine hydroxylase mRNA and the enzyme total level as well as the phosphorylation of Ser 19, 31 and 40 sites. Furthermore, both peptides stimulated dopamine turnover and reduced its endogenous content. These findings support that endothelins are involved in the long-term regulation of tyrosine hydroxylase, leading to an increase in the catecholaminergic activity which might be implicated in the development and/or maintenance of diverse pathologies involving the olfactory bulbs.

Receptor changes in cerebral arteries after subarachnoid haemorrhage

Subarachnoid haemorrhage (SAH), occurring with a delay of 4-10 days is linked to cerebral vasospasm (CVS), a pathological constriction of the cerebral arteries. Several agents have been suggested as being responsible - amongst these perhaps 5-hydroxytryptamine (5-HT) and endothelin-1 (ET-1) are the most prominent, given their ability to elicit powerful constriction of arteries. Investigating both 5-HT and ET receptors we observed distinct changes in the receptor phenotype after experimental SAH - namely upregulation of the ETB and 5-HT1B receptors - linked to a higher sensitivity to the endogenous agonists. This multiple receptor upregulation may explain the failure in treating CVS using single receptor antagonists, and may also significantly change our understanding of the effector mechanism behind CVS. So far only the ET and 5-HT receptors have been studied in this regard, but other receptor systems may also undergo changes.

New insights in insurmountable antagonism

Antagonists that produce parallel rightward shifts of agonist dose-response curves with no alteration of the maximal response are traditionally classified as surmountable, while insurmountable antagonists also depress the maximal response. Although the longevity of the antagonist-receptor complex is quoted in many studies to explain insurmountable antagonism, slowly interconverting receptor conformations, allosteric binding sites, and receptor internalization have been evoked as alternative explanations. To complicate matters even further, insurmountable antagonism is not only drug-related; it may also depend on the tissue, species and experimental design. For the sake of drug development, it is important to elucidate the molecular mechanisms of insurmountable antagonism. New experimental approaches, such as intact cell studies and the use of computer-assisted simulations based on dynamic receptor models, herald the advent of better insight in the future.

Endothelin A receptor antagonist LU 135252 inhibits hypercholesterolemia-induced, but not deendothelialization-induced, atherosclerosis in rabbit arteries

RATIONALE AND OBJECTIVES

The purpose of the study was to test the capability of the endothelin A receptor antagonist LU 135252 to reduce neointimal formation in rabbits after balloon denudation with and without the presence of hypercholesterolemia.

METHODS

Twenty-eight male New Zealand White rabbits underwent balloon denudation of the infrarenal aorta. The animals were randomly assigned to 1 of the 4 groups. After balloon denudation, group 1 (n = 6) and 2 (n = 7) received a standard diet, and group 3 (n = 8) and 4 (n = 7) were fed a 0.5% cholesterol diet. All interventional procedures were performed while the rabbits were under general anesthesia. One week prior to intervention treatment with LU 135252 was started in group 2 and 4. After 6 weeks the animals were killed for morphometric and histological analysis.

RESULTS

Rabbits in all treatment groups developed neointimal hyperplasia. By additional systemic treatment with LU 135252, the mean neointima to media ratio was significantly reduced only in the hypercholesterolemic animals of group 4 (neointimal to media ratio area of group 3 vs group 4: 2.07 +/- 0.62 vs 1.41 +/- 0.45, P < 0.05). ET receptor blockade in group 2 and 4 did not have an effect on plasma levels of cholesterol, very low-density lipoprotein-, high-density lipoprotein-, and low-density lipoprotein-cholesterol.

CONCLUSION

LU 135252 was efficient in reducing lipid induced atherosclerotic changes but was ineffective in inhibiting restenosis induced by balloon denudation.

Endothelin-A receptors and NO mediate decrease in arterial pressure during recovery from restraint

We investigated the role of central endothelin-A (ET(A)) receptors and nitric oxide (NO) in regulating arterial pressure during restraint stress and recovery from stress. Rats received intracerebroventricular (icv) injections of the ET(A) receptor antagonist BQ123 (24 microg/kg) and were then subjected to two restraint-rest cycles (1 h of restraint and 1 h of rest/cycle). Although mean arterial pressure (MAP) values in BQ123-treated and control rats increased at the onset of restraint and remained elevated during restraint, MAP values in BQ123-treated rats were consistently greater than in control rats. During rest periods, MAP values in control rats decreased to below baseline levels, whereas those in BQ123-treated rats remained significantly higher. NO content was decreased in the brain stems of BQ123-treated compared with control rats after the 4-h protocol. Injections (icv) of the NO synthase (NOS) inhibitor N(G)-nitro-L-arginine (L-NNA) eliminated the decreases in MAP values during rest periods in both BQ123-treated and control rats. Inhibition of neuronal NOS with icv injection of 7-nitroindazole sodium salt resulted in MAP values intermediate between control rats and rats receiving L-NNA. These results support the hypothesis that endothelin acts through ET(A) receptors in the brain, possibly via release of NO, to decrease arterial pressure during restraint and recovery from restraint.

Endothelin-1 mediates prostaglandin F(2alpha)-induced luteal regression in the ewe

A diversified series of experiments was conducted to determine the potential role of endothelin-1 (ET-1) in ovine luteal function. Endothelin-1 inhibited basal and LH-stimulated progesterone production by dispersed ovine luteal cells during a 2-h incubation. This inhibition was removed when cells were preincubated with cyclo-D-Asp-Pro-D-Val-Leu-D-Trp (BQ123), a highly specific endothelin ET(A) receptor antagonist. Administration of a luteolytic dose of prostaglandin F(2alpha) (PGF(2alpha)) rapidly stimulated gene expression for ET-1 in ovine corpora lutea (CL) collected at midcycle. Intraluteal administration of a single dose of BQ123 to ewes on Day 8 or 9 of the estrous cycle mitigated the luteolytic effect of PGF(2alpha). Intramuscular administration of 100 microg ET-1 to ewes at midcycle reduced plasma progesterone concentrations for the remainder of the estrous cycle. Following pretreatment with a subluteolytic dose of PGF(2alpha), i.m. administration of 100 microg ET-1 caused a rapid decline in plasma progesterone and shortened the length of the estrous cycle. These data complement and extend previously published reports in the bovine CL and are the strongest evidence presented to date in support of a role for ET-1 in PGF(2alpha)-mediated luteal function in domestic ruminants.

Allosteric inhibition of endothelin ETA receptors by 3, 5-dibromosalicylic acid

Derivatives of salicylic acid (SA) and benzoic acid prevent endothelin-1 (ET-1) binding to ETA receptors. This study analyzed actions of 30 derivatives of benzoic acid and salicylic acid on (125)I-ET-1 binding to recombinant rat ETA receptors. The most active compounds were 3,5-dibromosalicylic acid (Br2SA, K(i) = 0.5 mM) and 3,5-diiodosalicylic acid (K(i) = 0.3 mM). They were about 50 times more potent than SA and aspirin. Br2SA inhibited equilibrium (125)I-ET-1 binding in an apparently competitive manner. It accelerated 8-fold the dissociation of (125)I-ET-1 receptor complexes and did not modify the second order rate constant of association of (125)I-ET-1 to its receptors. Br2SA also decreased the affinity of ETA receptors for receptor antagonists BQ-123 and bosentan. Br2SA accelerated dissociation of (125)I-ET-1-solubilized ETA receptor complexes and decreased the apparent molecular size of solubilized receptors. Br2SA and 3,5-diiodosalicylic acid inhibited two cellular actions of ET-1: the mobilization of intracellular Ca(2+) stores in isolated cells and contractions of rat aortic rings. They accelerated the relaxing action of BQ-123 and bosentan in ET-1-treated aortic rings. The results suggest the existence of an allosteric modifier site on ETA receptors that recognizes selected derivatives of SA. SA derivatives might be of therapeutic interest to relieve tight ET-1 binding and to favor actions of receptor antagonists.

Radioiodinated endothelin-1: a radiotracer for imaging endothelin receptor distribution and occupancy

Endothelin (ET) is one of the most potent vasoconstrictors known. Recently, ET has been implicated in various diseases, e.g., acute renal failure and congestive heart failure, which present the possibility of treating such diseases with endothelin receptor antagonists. However, establishing the dosages for these antagonists may be difficult because no convenient physiologic indicator of action exists, and because of complexities in receptor function. Two receptor subtypes have been identified for which selective antagonists have been reported (e.g., BQ-123 for the ETA receptor and BQ-788 for the ETB receptor). Of the three natural peptide hormones (ET-1, ET-2, and ET-3), ET-1 exhibits high affinity for both subtypes of receptor. Using the selective peptide antagonists, and a nonpeptide antagonist with relatively balanced affinity for the two subtypes (L-749,329), we have characterized the interactions of [125I]ET-1 with its receptors in vivo (in rat). BQ-123, BQ-788, and L-749,329 inhibited binding consistent with binding to a single receptor site. However, the sum of inhibition by the selective antagonist was greater than 100% (as defined by inhibition with L-749,329), which suggests (a) lower in vivo selectivity than determined in vitro and/or (b) receptor subtype interactions. The latter explanation is supported, in part, by in vitro autoradiographic studies as well as studies in isolated tissues and cells. We synthesized ET-1 labeled with I-123 and obtained images of receptor distribution in both rat and rhesus monkey and have demonstrated our ability to visualize, via planar, noninvasive imaging, the occupancy of endothelin receptor by antagonists in both kidney and lung. [123I]ET-1 can therefore be used to determine clinical dosages of antagonist needed for receptor saturation.

Involvement of the endothelin receptor subtype A in neuronal pathogenesis after traumatic brain injury

Endothelin-1 (ET-1) is a 21 amino acid peptide that has been closely linked to cerebral vasospasm and more recently to oxidative stress after traumatic brain injury. In this study, we have examined the effects of the endothelin receptor subtype A antagonist, Ro 61-1790, on acute cortical neuronal injury and delayed neuronal death in the cerebellum after mild traumatic brain injury. Rats were administered Ro 61-1790 or vehicle for 24 h after injury and euthanized at 1 day, 3 days, or 7 days. Heat shock protein70 (HSP70), a marker of neuronal stress/injury, was immunolocalized in the cortex. Induction of heme oxygenase-1 (HO-1) and enhanced immunoexpression of the complement C3bi receptor, both of which are indicators of cerebellar glial reactivity, and Purkinje cell loss were evaluated in the cerebellum. There was maximal induction of HSP70 in cortical neurons at 24 h postinjury in all animals. Drug treated animals showed significantly fewer HSP70 labeled cortical neurons at this time point. There were fewer reactive glia in the cerebellum of drug treated animals as compared to vehicle controls at 3 days postinjury. However, at 7 days postinjury glial reactivity and Purkinje cell loss were similar in both groups. These findings demonstrate that Ro 61-1790, when administered for the first 24 h postinjury, limits acute neuronal injury in the cortex, transiently influences glial reactivity in the cerebellum, and does not attenuate delayed Purkinje cell death. The latter finding may reflect the duration of infusion of the drug.

Involvement of ET(A) and ET(B) receptors in the activation of phospholipase D by endothelins in cultured rat cortical astrocytes

This study was performed to characterize the receptor subtypes involved in the endothelin stimulation of phospholipase D (PLD) in rat cortical astrocytes in primary culture. PLD activity was determined by measuring the formation of [32P]phosphatidylbutanol in [32P]orthophosphate prelabelled cells stimulated in the presence of 25 mM butanol. The agonists endothelin-1 (ET-1), endothelin-3 (ET-3), sarafotoxin 6c (S6c) and IRL 1620 elicited PLD activation in a concentration-dependent manner. The potencies of ET-1, ET-3 and S6c were similar. The maximal effects evoked by the ET(B)-preferring agonists, ET-3, S6c and IRL 1620, were significantly lower than the maximal response to the non-selective agonist ET-1. The response to 1 nM ET-1 was inhibited by increasing concentrations of the ET(A) receptor antagonist BQ-123 in a biphasic manner. A high potency component of the inhibition curve (24.2+/-3.5% of the ET-1 response) was defined at low (up to 1 microM) concentrations of BQ-123, yielding an estimated Ki value for BQ-123 of 21.3+/-2.5 nM. In addition, the presence of 1 microM BQ-123 significantly reduced the maximal response to ET-1 but did not change the pD2 value. Increasing concentrations of the ET(B) selective antagonist BQ-788 inhibited the S6c response with a Ki of 17.8+/-0.8 nM. BQ-788 also inhibited the effect of ET-1, although, in this case, two components were defined, accounting for approximately 50% of the response, and showing Ki values of 20.9+/-5.1 nM and 439+/-110 nM, respectively. The ET-1 concentration-response curve was shifted to the right by 1 microM BQ-788, also revealing two components. Only one of them, corresponding to 69.8+/-4.4% of the response, was sensitive to BQ-788 which showed a Ki value of 28.8+/-8.9 nM. Rapid desensitization was achieved by preincubation with ET-1 or S6c. In cells pretreated with S6c neither ET-3 nor S6c activated PLD, but ET-1 still induced approximately 40% of the response shown by non-desensitised cells. This remaining response was insensitive to BQ-788, but fully inhibited by BQ-123. In conclusion, endothelins activate PLD in rat cortical astrocytes acting through both ET(A) and ET(B) receptors, and this response desensitizes rapidly in an apparently homologous fashion. The percentage contribution of ET(A) and ET(B) receptors to the ET-1 response was found to be approximately 20% and 80%, respectively, when ET(B) receptors were not blocked, and 30-50% and 50-70%, respectively, when ET(B) receptors were inhibited or desensitized. These results may be relevant to the study of a possible role of PLD in the proliferative effects shown by endothelins on cultured and reactive astrocytes.

Mitogenic activity of endothelin on human cultured prostatic smooth muscle cells

The effects of endothelins on human prostatic smooth-muscle cell growth were examined. Endothelin-1 and endothelin-3 induced a concentration-dependent increase in DNA synthesis and also promoted cell growth. Use of subtype selective antagonists BQ-123 ((cyclo(D-Trp-D-Asp(ONa)-Pro-D-Val-Leu); endothelin ET(A) receptor selective) and BQ-788 ((N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methyl Leu-D-Trp-(COOMe)-D-Nle-ONa); endothelin ET(B) receptor selective), indicated that mitogenic effects of endothelin were mediated through activation of both endothelin ET(A) and ET(B) receptors. The mitogenic effects of endothelin-1 and endothelin-3 were significantly inhibited by pretreatment of the cells with pertussis toxin. However, mitogenesis due to basic fibroblast growth factor was not affected. In conclusion, endothelin has mitogenic effects on human prostatic smooth muscle cells through activation of both endothelin ET(A) and ET(B) receptors via different signalling pathways from basic fibroblast growth factor. This may contribute to smooth muscle hyperplasia associated with benign prostatic hyperplasia.

Pseudo-noncompetitive antagonism of M1, M3, and M5 muscarinic receptor-mediated Ca2+ mobilization by muscarinic antagonists

Muscarinic receptors M1, M3, and M5 were expressed in Sf9 cells. Three different patterns of inhibition of Ca2+ elevations could be resolved for the subtype nonselective muscarinic receptor antagonists: (i) a right shift of the agonist dose-response curve, (ii) a right shift of the agonist dose-response curve and a depression of the maximum signal, and (iii) an intermediate pattern where the antagonist apparently behaved more competitively at higher concentrations. A simulation performed assuming that these differences are due to differences in the dissociation rates of the antagonists reproduced all three different modes of inhibition; the novel intermediate pattern (iii) is suggested to be caused by an intermediate antagonist dissociation rate. A direct correlation between the type of inhibition and the measured dissociation rate of the antagonists was also observed. Functional selectivity between receptor subtypes based on the dissociation constants is suggested based on the results.

Different apparent modes of inhibition of alpha2A-adrenoceptor by alpha2-adrenoceptor antagonists

The inhibition of alpha2A-adrenoceptor-mediated Ca2+ elevation by alpha2-adrenoceptor antagonists was measured in HEL human erythroleukemia cells. The antagonists could be divided in two classes: those that displayed surmountable inhibition (right-shift of the agonist dose-response curve), and those that displayed different degrees of insurmountable inhibition (depression of the maximum signal and a possible right-shift of the agonist dose-response curve). The degree of surmountability of the inhibition correlated well with the measured antagonist dissociation rates, suggesting that the hypothesis of the antagonist dissociation rate governing the mode of inhibition of fast responses, holds true. HEL cells thus provide a useful model system for the investigation of physiological consequences of different dissociation rates. Also, the dissociation rates of antagonists not available in radiolabelled form can be predicted from the functional data. The data stresses the importance of measurement of kinetic parameters of the drug-receptor interaction in addition to the equilibrium binding constants.

Comparison of the contractile effects and binding kinetics of endothelin-1 and sarafotoxin S6b in rat isolated renal artery

1. To date, only two mammalian endothelin (ET) receptors, termed ETA and ETB, have been cloned, sequenced and characterized. However, several functional studies of isolated blood vessels suggest that ET-1-induced contractions may be mediated by multiple ETA receptors. In this study, the ETA receptors in renal arteries isolated from Wistar rats were characterized by isometric tension recording and radioligand binding techniques. 2. ET-1, sarafotoxin S6b (StxS6b) and ET-3 produced concentration-dependent contraction with similar response maxima in endothelium-denuded arteries, whereas the ETB receptor-selective agonist StxS6c was inactive. ET-1 and StxS6b were equipotent and 30 times more potent than ET-3. This agonist profile, together with the findings that the ETA receptor-selective antagonists, BQ-123 and FR-139317 caused concentration-dependent, rightward shifts of the concentration-effect curves to each agonist indicated that ET-1-induced contractions in rat renal artery were mediated via ETA receptors. 3. BQ-123 and FR-139317 were both significantly more potent inhibitors of contractions induced by StxS6b or ET-3 than of responses to ET-1, raising the possibility that a component of ET-1-induced contraction was mediated through atypical, BQ-123 (or FR-139317)-insensitive ETA receptors. However, in competition binding studies, specific [125I]-ET-1 and [125I]-StxS6b binding to rat renal artery sections was completely abolished by BQ-123 in a manner consistent with an action at a single site. Thus, competition binding studies did not provide any supportive evidence of the existence of a BQ-123-insensitive ETA receptor. 4. Additional studies revealed marked differences in the kinetics of [125I]-ET-1 and [125I]-StxS6b binding. Following a 3 h period of association of [125I]-ET-1 with its receptors, no significant dissociation of receptor-bound [125I]-ET-1 was observed during a 4 h washout period. In stark contrast, dissociation studies revealed that specific [125I]-StxS6b binding to ETA receptors was reversible (t0.5diss, 100 min). A series of association binding studies were also consistent with the specific binding of [125I]-ET-1 and [125I]-StxS6b being irreversible and reversible processes, respectively. 5. Thus, differences in BQ-123 potency against ET-1 and StxS6b-induced contractions in rat renal arteries might be due to differences in the kinetics of agonist binding, rather than due to the existence of atypical ETA receptors.

Stimulatory action of endothelin-1 on rat Leydig cells: involvement of endothelin-A subtype receptor and phospholipase A2-arachidonate metabolism system

In a previous report we have observed that endothelin-1 (ET-1) is able to stimulate testosterone (T) production by rat Leydig cells revealing an interaction with human chorionic gonadotropin (hCG). The present study was designed to further characterize the stimulatory action of ET on testicular steroidogenesis, to evaluate which subtype of ET receptors is involved in this activity and to examine the role of phospholipase A2 (PLA2)-arachidonate metabolism system in ET-1 transduction mechanism. To this purpose we investigated: i) the interaction of ET-1 with another secretagogue of T, like luteinizing hormone releasing hormone (LHRH); ii) the interference of ET(A) and ET(B) receptor antagonists (BQ-123 and BQ-788, respectively) and of inhibitors of PLA2 (quinacrine) and arachidonate lipoxygenase pathway (nordihydroguaiaretic acid:NDGA) on ET-1-induced T and PGE2 secretion from purified rat Leydig cells. Data obtained indicate that ET-1 amplified T and PGE2 response to LHRH and this secretagogue in turn potentiated testicular steroidogenesis stimulated by endothelin. The ET(A) antagonist, BQ-123, inhibited in a dose-related fashion ET-1-induced T production whereas ET(B) antagonist, BQ-788, failed to affect T response to the peptide. Furthermore, ET(A) antagonist inhibited the stimulatory effect of ET-1 on hCG- or LHRH-induced T secretion and it was able to exert a dose-dependent inhibition of ET-1-stimulated PGE2 output. Moreover, a PLA2 inhibitor quinacrine inhibited the stimulatory action of ET-1 on T production and suppressed basal and ET-1-induced PGE2 release whilst a lipoxygenase blocker NDGA did not modify T response to the peptide. Taken together these findings i) indicate additivity of effects between ET-1 and LHRH in stimulating T and PGE2 production; ii) confirm that ET(A) subtype receptors mediate the stimulatory action of ET-1 on rat Leydig cells; iii) strongly suggest that PLA2-arachidonate metabolism system is involved in endothelin transduction mechanism.

Endothelin-1 inhibits L-type Ca2+ current enhanced by isoprenaline in rat atrial myocytes

Endothelin-1 (ET-1) was shown to exert direct cardiac effects by complex signaling pathways and to interact with neurotransmitter regulation of cardiac activity. The effect of ET-1 was investigated on the beta-adrenergic stimulation of cardiac L-type Ca2+ current (ICaL) on isolated rat atrial myocytes by using the patch-clamp technique. ET-1 (5 x 10(-8) M) reversed the increase in ICaL induced by isoprenaline (10(-6) M) but had no effect on basal ICaL and on (-) Bay K 8644-increased ICaL (10(-6) M); so ET-1 might exert an effect only when the Ca2+ channels are phosphorylated. The antiadrenergic action of ET-1, blocked by BQ-123 (10(-6) M) and unaffected by IRL 1038 (3.5 x 10(-8) M) should be mediated by ET-A receptors. The inhibitory action of ET-1 was still observed when ICaL was previously increased by forskolin (3 x 10(-6) M), 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP; 200 microM), or cAMP (100 microM) in presence of isobutyl methyl xanthine (IBMX; 10(-6) M), suggesting that the antiadrenergic action of ET-1 on ICaL was exerted independent of the cAMP-dependent phosphorylation pathway. ET-1 is known to be an activator of phosphoinositide hydrolysis, resulting in an increased production of IP3 and diacylglycerol (DAG). A Ca(2+)-dependent inhibition of ICaL consequently to an elevation of the intracellular Ca2+ pool via IP3 might be excluded in the action of ET-1, because of the presence of EGTA in the intrapipette medium. ET-1 reversed the isoprenaline-induced increase in ICaL in the presence of protein kinase C inhibitor [PKC(19-31); 100 microM), making unlikely the involvement of a DAG-dependent activation of PKC. Therefore the antiadrenergic action of ET-1 might also be independent on the phosphoinositide pathway.

The effect of PPADS as an antagonist of inositol (1,4,5)trisphosphate induced intracellular calcium mobilization

1. Brain capillary endothelial cells responded to uridine 5'-triphosphate (UTP) and adenosine 5'-triphosphate (ATP) by activation of phospholipase C and by large changes in [Ca2+]i. These cells expressed mRNA sequences identical to the sequence of the P2Y2-purinoceptor of rat pituitaries. 2. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) at 100 microM did not prevent UTP and ATP induced accumulations of total [3H]-inositol (poly)phosphates. It inhibited UTP and ATP induced intracellular Ca2+ mobilization (IC50 = 30 microM) by non competitive mechanism. 3. PPADS (100 microM) inhibited endothelin-1 induced accumulation of total [3H]-inositol (poly)phosphates by less than 20% and prevented most of endothelin-1 induced intracellular Ca2+ mobilization (IC50 = 30 microM). 4. PPADS (100 microM) had no action on ionomycin induced intracellular Ca2+ mobilization. 5. Microinjection of inositol (1,4,5)trisphosphate (InsP3) into Xenopus oocytes induced large Ca2+ activated Cl- currents that were prevented by heparin and by PPADS. 6. It is concluded that PPADS does not recognize rat P2Y2-purinoceptors and prevents UTP and ATP induced intracellular Ca2+ mobilization by a non-specific mechanism that could involve the inhibition of InsP3 channels.

Endothelin ETA receptors mediate human uterine smooth muscle contraction

Receptors mediating endothelin-induced contraction of myometrium were investigated in the human uterus. Endothelin-1 and endothelin-3 (10 pM to 0.3 microM) caused concentration-dependent contraction of myometrial strips. Endothelin-1 was approximately ten times more potent than endothelin-3, with pD2 values of 8.24 and 7.20, respectively. By contrast, two endothelin ETB receptor selective agonist, BQ 3020 (N-acetyl-[Ala11,15]endothelin-1-(6-21) and sarafotoxin 6c (up to 0.3 microM), did not induce contraction of human myometrium. The endothelin ETA receptor selective antagonist, FR139317 (1-hexahydroazepino-CO-Leu-D-Trp(CH3)-D-(2-pyridyl)alanine) (0.1, 0.3 and 1 microM), competitively antagonized the endothelin-1-elicited contraction, with a pA2 value of 7.10, whereas another endothelin ETA receptor-selective blocking drug, BQ 123 [cyclo(-D-Trp-D-Asp-Pro-D-Val-Leu)], behaved as a non-competitive antagonist. Pretreatment of myometrial strips with an endothelin ETB receptor selective antagonist, IRL 1038 ([Cys11-Cys15]endothelin-1-(11-21)), had no effect on contractions induced by endothelin-1. All these data indicate that only endothelin ETA receptors mediate endothelin-1-induced contractions of human myometrium.

Cardiovascular and respiratory effects of endothelin in the ventrolateral medulla of the normotensive rat

We studied the relevance of the ventrolateral medulla for the cardiovascular and respiratory effects of endothelin-1 in urethane-anesthetized rats. Microinjection of endothelin-1 into the rostral ventrolateral medulla (RVLM) evoked pressor and bradycardic effects followed by sustained decreases in blood pressure, bradycardia, and respiratory depression. These effects were inhibited by endothelin-A receptor antagonists (BQ-123 and BQ-610) but not by endothelin-B antagonists. In the caudal ventrolateral medulla (CVLM) endothelin-1 decreased blood pressure, renal sympathetic nerve activity, respiratory frequency, and phrenic nerve activity, whereas heart rate increased. Pretreatment with BQ-123 in the CVLM increased respiratory frequency by 15 +/- 6 breaths per minute and prevented the effects of intra-CVLM administration of endothelin-1. In separate experiments, the intracisternal administration of endothelin-1 (20 pmol) to rats pretreated with saline in both RVLM and CVLM resulted in a hypotensive and bradycardic phase that was followed by hypertension (50 +/- 15 mm Hg), bradycardia, and 100% mortality. In a separate group, pretreatment with BQ-123 in the RVLM and CVLM completely inhibited the hypotensive phase and reduced by 83% the subsequent rise in blood pressure evoked by endothelin-1. Cardiorespiratory arrest was prevented in all the rats in this group. Selective endothelin receptor blockade in the RVLM attenuated the hypertensive period of intracisternal administration of endothelin-1 and prevented mortality by 33%, whereas in the CVLM the endothelin receptor antagonist inhibited the initial hypotension and reduced mortality by 25%. Our results support the concept that in the ventral medulla, endothelin-1 can modulate cardiovascular and respiratory function.

Sensitization by calyculin A of brain capillary endothelial cells to endothelin-1

1. Cultured brain capillary endothelial cells of the rat respond to endothelin-1 (ET-1) by an increased activity of the Na+,K+,2Cl-, cotransporter and a mobilization of intracellular Ca2+ stores. 2. Calyculin A (1-30 nM), but not okadaic acid, sensitizes up to 100 fold the Na+,K+,2Cl- cotransporter to the action of ET-1. 3. Calyculin A (30 nM) does not modify the binding properties of ET-1 to ETA receptors. 4. Calyculin A (30 nM) inhibits ET-1 induced intracellular Ca2+ mobilization. 5. It is concluded that inhibition of protein phosphatase 1 selectively modifies the repertoire of intracellular actions of ET-1 and favours actions that are unrelated to the phospholipase C signalling cascade.

Endothelin receptor subtypes and their role in transmembrane signaling mechanisms

The endothelins (ETs) are potent vasoactive peptides that appear to be involved in diverse biological actions, for example, contraction, neuromodulation, and neurotransmission, as well as in various pathophysiological conditions, such as renal and heart failure. The diversity of actions of ETs may be explained in terms of (1) the existence of several receptor subtypes and (2) the activation of different signal transduction pathways. This review summarizes the state of the art in this intensively studied field, with particular focus on structural aspects, receptor heterogeneity, coupling of receptors to G-proteins, and signal transduction mechanisms mediated by the activation of ET-receptors.

Endothelin receptor antagonism

Following the original report by Yanagisawa et al. (1988) more than 7 years ago, compelling evidence that ET plays an important role in the local regulation of smooth muscle tone and cell growth has been reported. In addition, many studies point to a significant role for endothelin in nonvascular function. The investigation of the endothelin system has been greatly advanced in the last 2 to 3 years through significant advances in the development of potent and selective ET receptor antagonists. These agents have proven to be essential tools for elucidating the biological significance of the ET system, leading to the realization that antagonism of the ET system may have significant therapeutic potential. As emphasized in this review, the importance of chronic blockade of the ET system may be a critical aspect of future research in this exciting area. Confounding issues remain the lack of information about the role of the ETB receptor, the apparent pharmacological evidence for additional ET receptor subtypes, and species variation in the tissue distribution of ET isoforms and receptor subtypes. Along with the greater ability to understand the endothelin system provided by potent and selective pharmacological agents, is the important contribution of modern molecular biology techniques, highlighted by the insights gained from recent reports of results from ET gene disruption studies. Kurihara et al. (1994) found that ET-1-deficient homozygous mice die at birth of apparent respiratory failure secondary to severe craniofacial abnormalities. Subsequently, Yanagisawa's laboratory has presented and published a series of complementary gene disruption studies. First, Hosoda et al. (1994) demonstrated remarkably, that ETA receptor knockout mice bear morphological abnormalities nearly identical to ET-1 knockout mice. Second, they found that disruption of the ET-3 peptide and ETB receptor genes result in homozygous mice that share identical phenotypic traits (i.e., coloration changes and aganglionic megacolon) which are similar to a previously known natural mutation, the Piebald-Lethal mouse (Hosoda et al., 1994; Baynash et al., 1994). This phenotype has a human corollary known as Hirschsprung's Disease and it is now known that the disease, though multigenic, results from a missense mutation of the ETB receptor gene in some individuals (Puffenberger et al., 1994). Taken together these data indicate that the endothelin system is essential to correct embryonic neural crest development, a completely novel finding within the superfamily of guanine-protein-linked receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Different endothelin receptors involved in endothelin-1- and sarafotoxin S6B-induced contractions of the human isolated coronary artery

1. Endothelin receptors, that mediate contraction of the human isolated coronary artery, were characterized by use of a number of agonists and antagonists. Contraction induced by the non-selective agonists, endothelin (ET)-1 and sarafotoxin S6b, was compared in endothelium-intact and endothelium-denuded ring segments. The effects of ET-1 and BQ-123 (an ETA receptor antagonist) were investigated both in ring segments and in spirally cut strips. Lastly, the effect of phosphoramidon was studied on contraction induced by big-ET-1. 2. The order of agonist potency (pD2) in endothelium-intact coronary artery ring segments was: ET-1 (8.27) approximately sarafotoxin S6b (8.16) > big-ET-1 (< 7.1) approximately ET-3 (< 6.9). [Ala1,3,11,15]ET-1 (ETB receptor agonist) caused significant contraction only at 1 microM, whereas 0.3 microM big-ET-3 had no effect. Removal of the endothelium in ring segments did not affect the contractile response to ET-1 or to sarafotoxin S6b. 3. After a full concentration-response curve had been obtained to ET-1 or sarafotoxin S6b, further contractions of the endothelium-intact coronary artery segments could only be achieved by applying ET-1 in segments exposed to sarafotoxin S6b, and not the reverse. 4. BQ-123 (0.1 microM) antagonized contractions of endothelium-intact ring segments induced by sarafotoxin S6b (pKB 7.86). Only 10 microM BQ-123 antagonized contractions induced by ET-1 (pKB 5.75). FR139317 was also more potent against sarafotoxin S6b (pKB 8.24-8.47) than against ET-1 (pKB 6.11). [Ala1,3,11,15]ET-1 (1 microM) had no effect on the contractile response to ET-1 or to sarafotoxin S6b. 5. In strip preparations with intact endothelium, the pD2 of ET-l increased to 9.04 =/- 0.16 (vs.8.50 +/- 0.07 in rings), and BQ-123 (1 microM) caused a rightward shift of the ET-l induced concentration response curve (pKB 6.62 vs. 5.75 in rings).6. Contractile responses to big-ET-1 of endothelium-intact coronary artery segments were attenuated in the presence of phosphoramidon (100 microM), indicating conversion of big-ET-1 to ET-1 within the coronary artery segment.7. The present study indicates that ET-1 and sarafotoxin S6b contract the human isolated coronary artery via different receptors, which can probably be best characterized as subtypes of the ETA receptor.Furthermore, it is demonstrated that the type of preparation (ring or strip) may affect the potency of ET-1 as an agonist and of BQ-123 as an antagonist.

The current endothelin receptor classification: time for reconsideration?

The possible involvement of endothelins in a variety of diseases has attracted the attention of many pharmacologists in search of a novel therapeutic approach. The rapid development of endothelin research has resulted in the molecular characterization and pharmacological recognition of ETA and ETB receptors, and in the development of compounds selective for these receptors. However, the characterization of receptors in various assays has shown that a number of effects are mediated by receptors that do not fit the present criteria for ETA or ETB receptors. In this article, Willem Bax and Pramod Saxena address endothelin receptors in general, and atypical receptors in particular.

Cerebral microvascular responses to endothelins: the role of ETA receptors

The regulatory role of endothelins in cerebral microvessels was investigated in a recently developed model system which allows the study of small cerebral vessels in their normal microenvironment. Using brain slices of the rat neocortex, it was shown that the isopeptide endothelin-3 (ET-3) had no effect on cerebral microvessels, while the isopeptide endothelin-1 (ET-1) produced a potent, dose-dependent vasoconstriction. When a recently developed antagonist of ETA receptors (cyclo-[D-Asp-L-Pro-D-Val-L-Leu-D-Trp]; ETant) was administered prior to treatment with ET-1, the vasoconstrictor response to ET-1 was inhibited in a dose-dependent manner. When ETant was administered after the establishment of a constriction by ET-1, the constrictor response to ET-1 was partially reversed, and this effect was weaker than that seen in the pre-treatment paradigm. These findings indicate that constrictor responses to ET-1 in cerebral microvessels are mediated by ETA receptors. Inasmuch as endothelins have been implicated in pathological forms of vasoconstriction in the CNS, the present findings also suggest that endothelin antagonists may be useful in the treatment of cerebral ischemia.

Characterisation of the endothelin receptor mediating contraction of human pulmonary artery using BQ123 and Ro 46-2005

We have characterised the endothelin receptor mediating contraction of human isolated pulmonary artery. Endothelin-1 induced a concentration-dependent contraction of human endothelium-denuded pulmonary artery (EC50 5.6 nM). In contrast, endothelin-3 produced only a small contraction (approximately 12% of maximum endothelin-1 response) at the highest concentration tested (1 microM). The ETB receptor-selective agonist, sarafotoxin S6c (0.1 nM to 1 microM) did not cause contraction of human pulmonary artery. Pretreatment of human pulmonary artery with BQ123 (1-10 microM), an ETA receptor-selective blocking drug, resulted in a concentration-dependent, surmountable antagonism of endothelin-1-induced contractions (apparent pKB 6.6-7.0). Schild analyses yielded a shallow slope (0.58), which was significantly less than unity and, consequently, the calculated pA2 (8.1) was greater than the individual pKB values. Pretreatment of human pulmonary artery with Ro 46-2005 (30 microM), a non-peptide. non-selective endothelin receptor-blocking drug, resulted in a surmountable antagonism of endothelin-1-induced contractions (apparent pKB 5.5). In conclusion, endothelin-1-induced contraction of human pulmonary artery appears to be mediated predominantly via ETA receptors, although the shallow Schild slope observed with BQ123 indicates possible receptor heterogeneity.

Endothelins activate phospholipase A2 in brain capillary endothelial cells

Addition of endothelin-1 to cultured rat brain capillary endothelial cells induced a 2.7-fold activation of phospholipase A2, as evidenced from the release of [3H]arachidonic acid from prelabelled cells. Half maximum activation by endothelin-1 was observed at 1 nM. The action of endothelin-1 was not mimicked by low concentrations of endothelin-3 and it was largely suppressed by BQ-123, suggesting the involvement of an ETA receptor subtype. It is suggested that the activation of phospholipase A2 by endothelins plays a role in the development of delayed cerebral vasospasm following subarachnoid hemorrhage.

Cross talk among cyclic AMP, cyclic GMP, and Ca(2+)-dependent intracellular signalling mechanisms in brain capillary endothelial cells

C-type natriuretic peptide and sodium nitroprusside, a nitric oxide donor molecule, induced large increases in cyclic GMP formation in cultured rat brain capillary endothelial cells. Isoproterenol, a potent agonist of adenylate cyclase, potentiated the actions of C-type natriuretic peptide and of sodium nitroprusside. These actions were not observed in the presence of isobutylmethylxanthine and were mimicked by forskolin. Endothelin-1 had no action on basal cyclic GMP levels. It reduced cyclic GMP formation induced by C-type natriuretic peptide and sodium nitroprusside by about 50%. These actions involved an ETA receptor subtype and a Ca(2+)-dependent and protein kinase C-independent mechanism. Finally, increasing cyclic GMP slightly prolonged intracellular Ca2+ transients induced by endothelin-1. The results suggest the presence of extensive cross talk among cyclic AMP, cyclic GMP, and Ca(2+)-dependent mechanisms in endothelial cells of brain microvessels. The relevance of the results to the regulation of the blood-brain barrier permeability is discussed.

Endothelins and sarafotoxins: receptor heterogeneity

1. Endothelins (ETs) and sarafotoxins (SRTXs) belong to a family of 21-amino-acid peptides comprising at least eight isoforms. 2. ET exerts multiple pharmacological effects through its receptors. 3. This review summarizes the observations and findings pointing to the existence of receptor subtypes and leading to their identification. 4. Two receptor subtypes have been cloned and stably expressed. 5. The existence of at least two more is predicted by dissimilar ligand potencies in different tissues, kinetics of receptor-ligand interactions, and cross-linking of receptors and radiolabeled ligands.