A high-affinity bradykinin receptor in membranes from rat myometrium is coupled to pertussis toxin-sensitive G-proteins of the Gi family

Unveiling the participation of avian kinin ornithokinin and its receptors in the chicken inflammatory response

Vasoactive peptides are key early mediators of inflammation released through activation of different enzymatic systems. The mammalian kinin-kallikrein (K-KLK) system produces bradykinin (BK) through proteolytic cleavage of a kininogen precursor by enzymes named kallikreins. BK acts through specific ubiquitous G-protein coupled receptors (B1R and B2R) to participate in physiological processes and inflammatory responses, such as activation of mononuclear phagocytes. In chickens, the BK-like nonapeptide ornithokinin (OK) has been shown to promote intracellular calcium increase in embryonic fibroblasts and to be vasodilatory in vivo. Also, one of its receptors (B2R) was already cloned. However, the participation of chicken K-KLK system components in the inflammatory response remains unknown and was therefore investigated. We first showed that B1R, B2R and kininogen 1 (KNG1) are expressed in unstimulated chicken tissues and macrophages. We next showed that chicken B1R and B2R are expressed at transcript and protein levels in chicken macrophages and are upregulated by E. coli LPS or avian pathogenic E. coli (APEC) infection. Interestingly, exogenous OK induced internalization and degradation of OK receptors protein, notably B2R. Also, OK induced intracellular calcium increase and potentiated zymosan-induced ROS production and Dextran-FITC endocytosis by chicken macrophages. Exogenous OK itself did not promote APEC killing and had no pro-inflammatory effect. However, when combined with LPS or APEC, OK upregulated cytokine/chemokine gene expression and NO production by chicken macrophages. This effect was not blocked by canonical non-peptide B1R or B2R receptor antagonists but was GPCR- and PI3K/Akt-dependent. In vivo, pulmonary colibacillosis led to upregulation of OK receptors expression in chicken lungs and liver. Also, colibacillosis led to significant upregulation of OK precursor KNG1 expression in liver and in cultured hepatocytes (LMH). We therefore provide hitherto unknown information on how OK and its receptors are involved in inflammation and infection in chickens.

Bradykinin promotes neuron-generating division of neural progenitor cells through ERK activation

During brain development, cells proliferate, migrate and differentiate in highly accurate patterns. In this context, published results indicate that bradykinin functions in neural fate determination, favoring neurogenesis and migration. However, mechanisms underlying bradykinin function are yet to be explored. Our findings indicate a previously unidentified role for bradykinin action in inducing neuron-generating division in vitro and in vivo, given that bradykinin lengthened the G1-phase of the neural progenitor cells (NPC) cycle and increased TIS21 (also known as PC3 and BTG2) expression in hippocampus from newborn mice. This role, triggered by activation of the kinin-B2 receptor, was conditioned by ERK1/2 activation. Moreover, immunohistochemistry analysis of hippocampal dentate gyrus showed that the percentage of Ki67(+) cells markedly increased in bradykinin-treated mice, and ERK1/2 inhibition affected this neurogenic response. The progress of neurogenesis depended on sustained ERK phosphorylation and resulted in ERK1/2 translocation to the nucleus in NPCs and PC12 cells, changing expression of genes such as Hes1 and Ngn2 (also known as Neurog2). In agreement with the function of ERK in integrating signaling pathways, effects of bradykinin in stimulating neurogenesis were reversed following removal of protein kinase C (PKC)-mediated sustained phosphorylation.

Mechanisms in bradykinin stimulated arachidonate release and synthesis of prostaglandin and platelet activating factor

Regulatory mechanisms in bradykinin (BK) activated release of arachidonate (ARA) and synthesis of prostaglandin (PG) and platelet activating factor (PAF) were studied in bovine pulmonary artery endothelial cells (BPAEC). A role for GTP binding protein (G-protein) in the binding of BK to the cells was determined. Guanosine 5-O- (thiotriphosphate), (GTPtauS), lowered the binding affinity for BK and increased the Kd for the binding from 0.45 to 1.99 nM. The Bmax remained unaltered at 2.25 x 10(-11) mole. Exposure of the cells to aluminium fluoride also reduced the affinity for BK. Bradykinin-induced release of ARA proved pertussis toxin (PTX) sensitive, with a maximum sensitivity at 10 ug/ml PTX. GTPtauS at 100 muM increased the release of arachidonate. The effect of GTPtauS and BK was additive at suboptimal doses of BK up to 0.5 nM but never exceeded the levels of maximal BK stimulation at 50 nM. PTX also inhibited the release of ARA induced by the calcium ionophore, A23187. Phorbol 12-myristate 13-acetate or more commonly known as tetradecanoyl phorbol acetate (TPA) itself had little effect on release by the intact cells. However, at 100 nM it augmented the BK activated release. This was downregulated by overnight exposure to TPA and correlated with down-regulation of protein kinase C (PKC) activity. The down-regulation only affected the augmentation of ARA release by TPA but not the original BK activated release. TPA displayed a similar, but more potent amplification of PAF synthesis in response to both BK or the calcium ionophore A23187. These results taken together point to the participation of G-protein in the binding of BK to BPAEC and its activation of ARA release. Possibly two types of G-protein are involved, one associated with the receptor, the other activated by Ca(2+) and perhaps associated with phospholipase A(2) (PLA(2)). Our results further suggest that a separate route of activation, probably also PLA(2) related, takes place through a PKC catalysed phosphorylation.

Cardioprotection in ischemia/reperfusion injury: spotlight on sphingosine-1-phosphate and bradykinin signalling

Complex signal-transduction cascades are known to be involved in regulating cardiomyocyte function, death and survival during acute cardiac ischemia-reperfusion process, but detailed survival signalling pathways are not clear. This review presents and discusses the recent findings bearing upon the evidence on the cardioprotective effect of sphingosine-1-phosphate (S1P) and bradykinin in acute cardiac ischemia-reperfusion and underlying signalling mechanisms, particularly, through activation of P21 activated kinase.

Novel bradykinin signaling in adult rat cardiac myocytes through activation of p21-activated kinase

Although bradykinin (BK) is known to exert effects on the myocardium, its intracellular signaling pathways remain poorly understood. Experiments in other cell types indicated that p21-activated kinase-1 (Pak1), a Ser/Thr kinase downstream of small monomeric G proteins, is activated by BK. We previously reported that the expression of active Pak1 in adult cardiac myocytes induced activation of protein phosphatase 2A and dephosphorylation of myofilament proteins (Ke et al. Circ Res 94: 194–200, 2004). In experiments reported here, we tested the hypothesis that BK signals altered protein phosphorylation in adult rat cardiac myocytes through the activation and translocation of Pak1. Treatment of myocytes with BK resulted in the activation of Pak1 as demonstrated by increased autophosphorylation at Thr423 and a diminished striated localization, which is present in the basal state. BK induced dephosphorylation of both cardiac troponin I and phospholamban. Treatment of isolated myocytes with BK also blunted the effect of isoproterenol to enhance peak Ca²⁺ and relaxation of Ca²⁺ transients. Protein phosphatase 2A was demonstrated to associate with both Pak 1 and phospholamban. Our studies indicate a novel signaling mechanism for BK in adult rat cardiac myocytes and support our hypothesis that Pak 1 is a significant regulator of phosphatase activity in the heart.

Pharmacological characterization of the bradykinin B2 receptor antagonist MEN16132 in rat in vitro bioassays

The pharmacological profile of the bradykinin B(2) receptor antagonist MEN16132 at the rat B(2) receptor has been investigated and compared with that of icatibant (formerly Hoe 140). Antagonist affinity has been measured through radioligand binding experiments with membranes prepared from uterine and airway tissue. MEN16132 inhibited [(3)H]bradykinin binding with subnanomolar affinity (pK(i) values 10.4 and 10.1 in the uterus and airways, respectively), and was about 3-fold less potent than icatibant (pK(i) values 10.9 and 10.5). Antagonist potency has been estimated towards bradykinin-induced contractility of uterine and urinary bladder smooth muscle preparations. In these assays MEN16132 (pK(B): 9.7 both in uterus and bladder) was about 10-fold more potent than icatibant [pK(B): 8.8 in uterus, and pK(B) 8.0 in urinary bladder, as from Meini, S., Patacchini, R., Giuliani, S., Lazzeri, M., Turini, D., Maggi, C.A., Lecci, A., 2000a. Characterization of bradykinin B(2) receptor antagonists in human and rat urinary bladder. Eur. J. Pharmacol. 388, 177-182]. Washout experiments conducted in the uterine preparation indicated for MEN16132 (100 nM) a slower reversibility than icatibant (300 nM).Altogether present results indicate that MEN16132 displays high affinity and potency also for the rat bradykinin B(2) receptor, and thus is suitable for further investigations in pathophysiological models in this species.

Regulation of cardiac excitation and contraction by p21 activated kinase-1

Cardiac excitation and contraction are regulated by a variety of signaling molecules. Central to the regulatory scheme are protein kinases and phosphatases that carry out reversible phosphorylation of different effectors. The process of beta-adrenergic stimulation mediated by cAMP dependent protein kinase (PKA) forms a well-known pathway considered as the most significant control mechanism in excitation and contraction as well as many other regulatory mechanisms in cardiac function. However, although dephosphorylation pathways are critical to these regulatory processes, signaling to phosphatases is relatively poorly understood. Emerging evidence indicates that regulation of phosphatases, which dampen the effect of beta-adrenergic stimulation, is also important. We review here functional studies of p21 activated kinase-1 (Pak1) and its potential role as an upstream signal for protein phosphatase PP2A in the heart. Pak1 is a serine/threonine protein kinase directly activated by the small GTPases Cdc42 and Rac1. Pak1 is highly expressed in different regions of the heart and modulates the activities of ion channels, sarcomeric proteins, and other phosphoproteins through up-regulation of PP2A activity. Coordination of Pak1 and PP2A activities is not only potentially involved in regulation of normal cardiac function, but is likely to be important in patho-physiological conditions.

Anti-inflammatory effects of kinins via microglia in the central nervous system

Kinins are important biologically active peptides that are up-regulated after lesions in both the peripheral and central (CNS) nervous systems. Microglia are immune cells in the CNS and play an important role in the defense of the neuronal parenchyma. In cultured murine microglia, bradykinin (BK) induces mobilization of intracellular Ca2+, microglial migration, and increases the release of nitric oxide and prostaglandin E2. On the other hand, BK attenuates lipopolysaccharide-activated TNF-α and IL-1β release. These results suggest that BK functions as a signal in brain trauma and may have an anti-inflammatory role in the CNS.

Bradykinin B2 and GPR100 receptors: a paradigm for receptor signal transduction pharmacology

The aim of the present report was to investigate the ligand selectivity of the human orphan G-protein-coupled receptor GPR100 (hGPR100), recently identified as a novel bradykinin (BK) receptor, as compared with that of the human B(2) receptor (hB(2)R) stably transfected in Chinese hamster ovary cells. BK was able to inhibit the cAMP production induced by forskolin with a potency 100-fold lower at the hGPR100 (pEC(50) = 6.6) than that measured at the hB(2)R (pEC(50) = 8.6). Both effects were inhibited by the B(2) receptor antagonist Icatibant (1 microM). The nonpeptide B(2) receptor agonist FR190997 (8-[2,6-dichloro-3-[N-methylcarbamoyl)cinnamidoacetyl]-N-methylamino]benzyloxy]-2-methyl-4-(2-pyridylmethoxy)quinoline) did inhibit the forskolin-induced cAMP production (pEC(50) = 7.7) at the hB(2)R, whereas it was not able to exert any effect at the hGPR100. The human insulin-like peptide relaxin 3 did inhibit the cAMP production at the hGPR100 (pEC(50) = 7.3) at a greater extent than BK, and was devoid of any effect at the hB(2)R. FR190997 and relaxin 3 responses at the hB(2)R and hGPR100, respectively, were not inhibited by Icatibant (1 microM). These data indicate FR190997 and relaxin 3 as selective agonists for hB(2)R and hGPR100, respectively, and support the concept that different agonists may specifically bias the conformational states of a receptor to result in a final common G protein coupling, which is differentially recognized by antagonists.

Cross talk between beta-adrenergic and bradykinin B(2) receptors results in cooperative regulation of cyclic AMP accumulation and mitogen-activated protein kinase activity

Costimulation of G protein-coupled receptors (GPCRs) may result in cross talk interactions between their downstream signaling pathways. Stimulation of GPCRs may also lead to cross talk regulation of receptor tyrosine kinase signaling and thereby to activation of mitogen-activated protein kinase (MAPK). In COS-7 cells, we investigated the interactions between two particular mitogenic receptor pathways, the endogenously expressed beta-adrenergic receptor (beta-AR) and the transiently transfected human bradykinin (BK) B(2) receptor (B(2)R). When beta-AR and B(2)R are costimulated, we found two different cross talk mechanisms. First, the predominantly G(q) protein-coupled B(2)R is enabled to activate a G(i) protein and, subsequently, type II adenylate cyclase. This results in augmentation of beta-AR-mediated cyclic AMP (cAMP) accumulation by BK, which alone is unable to increase the cAMP level. Second, independently of BK-induced superactivation of the cAMP system, costimulation of beta-AR leads to protein kinase A-mediated blockade of phospholipase C activation by BK. Thereby, the pathway from B(2)R to MAPK, which essentially involves protein kinase C activation, is selectively switched off. The MAPK activation in response to isoproterenol was not affected due to costimulation. Furthermore, in the presence of isoproterenol, BK lost its ability to stimulate DNA synthesis in COS-7 cells. Thus, our findings might establish a novel paradigm: cooperation between simultaneously activated mitogenic pathways may prevent multiple stimulation of MAPK activity and increased cell growth.

Regulation of MAP kinase activity by peptide receptor signalling pathway: paradigms of multiplicity

G protein-coupled receptors (GPCRs) can stimulate the mitogen-activated protein kinase (MAPK) cascade and thereby induce cellular proliferation like receptor tyrosine kinases (RTKs). Work over the past 5 years has established several models which reduce the links of G(i)-, G(q)-, and G(s)-coupled receptors to MAPK on few principle pathways. They include (i) Ras-dependent activation of MAPK via transactivation of RTKs such as the epidermal growth factor receptor (EGFR), (ii) Ras-independent MAPK activation via protein kinase C (PKC) that converges with the RTK signalling at the level of Raf, and (iii) activation as well as inactivation of MAPK via the cAMP/protein kinase A (PKA) pathway in dependency on the type of Raf. Most of these generalizing hypotheses are founded on experimental data obtained from expression studies and using a limited set of individual receptors. This review will compare these models with pathways to MAPK found for a great variety of peptide hormone and neuropeptide receptor subtypes in various cells. It becomes evident that under endogenous conditions, the transactivation pathway is less dominant as postulated, whereas pathways involving isoforms of PKC and, especially, phosphoinositide 3-kinase (PI-3K) appear to play a more important role as assumed so far. Highly cell-specific and unusual connections of signalling proteins towards MAPK, in particular tumour cells, might provide points of attacks for new therapeutic concepts.

Bradykinin signalling to MAP kinase: cell-specific connections versus principle mitogenic pathways

Mitogenic signalling pathways from G protein-coupled receptors (GPCRs) to the mitogen-activated protein kinase (MAPK) cascade may involve alpha- or betagamma-subunits of heterotrimeric G proteins, receptor or non-receptor tyrosine kinases, adaptor molecules, phosphoinositide 3-kinases, protein kinase C, and probably other proteins. The majority of models describing the connection of different signalling proteins within a mitogenic pathway are based on experimental data obtained by co- and overexpression of epitope-tagged MAPK together with the respective GPCR and other signalling proteins of interest in transfectable cell lines. Here the link of the bradykinin B2 receptor (B2R) to MAPK in the COS-7 cell expression system is compared with mitogenic signalling pathways of bradykinin in various tumour cell lines. It becomes evident that in natural or tumour cells expressing individual amounts and different isoforms of signalling proteins completely other relations between B2R and MAPK may exist than in COS-7 cells, suggesting a high degree of cellular specificity in mitogenic signalling.

G protein-coupled receptor-mediated mitogen-activated protein kinase activation through cooperation of Galpha(q) and Galpha(i) signals

G protein-coupled receptors (GPCRs) have been shown to stimulate extracellular regulated kinases (ERKs) through a number of linear pathways that are initiated by G(q/11) or G(i) proteins. We studied signaling to the ERK cascade by receptors that simultaneously activate both G protein subfamilies. In HEK293T cells, bradykinin B(2) receptor (B(2)R)-induced stimulation of ERK2 and transcriptional activity of Elk1 are dependent on Galpha(q)-mediated protein kinase C (PKC) and on Galpha(i)-induced Ras activation, while they are independent of Gbetagamma subunits, phosphatidylinositol 3-kinase, and tyrosine kinases. Similar results were obtained with m(1) and m(3) muscarinic receptors in HEK293T cells and with the B(2)R in human and mouse fibroblasts, indicating a general mechanism in signaling toward the ERK cascade. Furthermore, the bradykinin-induced activation of ERK is strongly reduced in Galpha(q/11)-deficient fibroblasts. In addition, we found that constitutively active mutants of Galpha(q/11) or Galpha(i) proteins alone poorly stimulate ERK2, whereas a combination of both led to synergistic effects. We conclude that dually coupled GPCRs require a cooperation of Galpha(i)- and G(q/11)-mediated pathways for efficient stimulation of the ERK cascade. Cooperative signaling by multiple G proteins thus might represent a novel concept implicated in the regulation of cellular responses by GPCRs.

Expression and functional characterization of a pHis-tagged human bradykinin B2 receptor in COS-7 cells

A polyHis-tagged bradykinin (BK) B2 receptor (pHis-BKR) cDNA was constructed and expressed in COS-7 cells. The pHis-BKR is suitable for both immunoprecipitation and immunoblotting with anti-polyHis antibodies and can be easily purified using Ni-NTA columns. Immunochemical detection revealed a molecular mass of approximately 66 kDa. The pHis-BKR is capable of mediating BK-induced stimulation of inositol phosphate formation as well as of mitogen-activated protein kinase (MAPK) activity. Compared with the wild-type receptor (WT-BKR) the tagged receptor showed a slightly enhanced affinity towards BK but a reduced expression level. Despite these modified pharmacological properties the pHis-tagged BKR may be a useful tool for studying BKR modifications and signaling.

Sympathetic potentiation of cyclic ADP-ribose formation in rat cardiac myocytes

We examined the role of cyclic ADP-ribose (cADP-ribose) as a second messenger downstream of adrenergic receptors in the heart after excitation of sympathetic neurons. To address this question, ADP-ribosyl cyclase activity was measured as the rate of [(3)H]cADP-ribose formation from [(3)H]NAD(+) in a crude membrane fraction of rat ventricular myocytes. Isoproterenol at 1 microM increased ADP-ribosyl cyclase activity by 1.7-fold in ventricular muscle; this increase was inhibited by propranolol. The stimulatory effect on the cyclase was mimicked by 10 nM GTP and 10 microM guanosine 5'-3-O-(thio)triphosphate, whereas 10 microM GTP inhibited the cyclase. Cholera toxin blocked the activation of the cyclase by isoproterenol and GTP. The above effects of isoproterenol and GTP in ventricular membranes were confirmed by cyclic GDP-ribose formation fluorometrically. These results demonstrate the existence of a signal pathway from beta-adrenergic receptors to membrane-bound ADP-ribosyl cyclase via G protein in the ventricular muscle cells and suggest that increased cADP-ribose synthesis is involved in up-regulation of cardiac function by sympathetic stimulation.

Bradykinin B(2) receptor-mediated mitogen-activated protein kinase activation in COS-7 cells requires dual signaling via both protein kinase C pathway and epidermal growth factor receptor transactivation

The signaling routes linking G-protein-coupled receptors to mitogen-activated protein kinase (MAPK) may involve tyrosine kinases, phosphoinositide 3-kinase gamma (PI3Kgamma), and protein kinase C (PKC). To characterize the mitogenic pathway of bradykinin (BK), COS-7 cells were transiently cotransfected with the human bradykinin B(2) receptor and hemagglutinin-tagged MAPK. We demonstrate that BK-induced activation of MAPK is mediated via the alpha subunits of a G(q/11) protein. Both activation of Raf-1 and activation of MAPK in response to BK were blocked by inhibitors of PKC as well as of the epidermal growth factor (EGF) receptor. Furthermore, in PKC-depleted COS-7 cells, the effect of BK on MAPK was clearly reduced. Inhibition of PI3-Kgamma or Src kinase failed to diminish MAPK activation by BK. BK-induced translocation and overexpression of PKC isoforms as well as coexpression of inactive or constitutively active mutants of different PKC isozymes provided evidence for a role of the diacylglycerol-sensitive PKCs alpha and epsilon in BK signaling toward MAPK. In addition to PKC activation, BK also induced tyrosine phosphorylation of EGF receptor (transactivation) in COS-7 cells. Inhibition of PKC did not alter BK-induced transactivation, and blockade of EGF receptor did not affect BK-stimulated phosphatidylinositol turnover or BK-induced PKC translocation, suggesting that PKC acts neither upstream nor downstream of the EGF receptor. Comparison of the kinetics of PKC activation and EGF receptor transactivation in response to BK also suggests simultaneous rather than consecutive signaling. We conclude that in COS-7 cells, BK activates MAPK via a permanent dual signaling pathway involving the independent activation of the PKC isoforms alpha and epsilon and transactivation of the EGF receptor. The two branches of this pathway may converge at the level of the Ras-Raf complex.

Pharmacology and cardiovascular implications of the kinin-kallikrein system

Kinins are peptide hormones that can exert a significant influence on the regulation of blood pressure and vascular tone due to their vasodilatatory, natriuretic and growth modulating activity. Their cardiovascular involvement in physiological and pathophysiological situations has been studied intensively since inhibitors for angiotensin I-converting enzyme and selective receptor antagonists have become available for pharmacologically potentiating or inhibiting kinin-mediated reactions. Molecular biological analysis and the establishment of genetically modified animal models have also allowed newer information to be acquired on this subject. In this review, the components and cardiovascularly relevant mechanisms of the kinin-kallikrein system shall be described. Organ-specific effects concerning the kidneys, the vascular system, the heart and nervous tissue shall also be illustrated. On this issue, the physiological functions and pathophysiological implications of the kinin-kallikrein system should be clearly distinguished from the many, mostly endothelium-mediated protective effects which occur during ACE inhibition due to the potentiation of kinin effects. Finally, a view shall also be cast upon newly discovered targets of action, which could be exploited for therapeutically altering the kinin-kallikrein system.

Bradykinin antagonists with dehydrophenylalanine analogues at position 5

Continuing the studies on structural requirements of bradykinin antagonists, it has been found that analogues with dehydrophenylalanine (deltaPhe) or its ring-substituted analogues (deltaPhe(X)) at position 5 act as antagonists on guinea pig pulmonary artery, and on guinea pig ileum. Because both organs are considered to be bradykinin B2 receptor tissues, the analogues with deltaPhe or deltaPhe(X) at position 5, but without any replacement at position 7, seem to represent a new structural type of B2 receptor antagonist. All the analogues investigated act as partial antagonists; they inhibit the bradykinin-induced contraction at low concentrations and act as agonists at higher concentrations. Ring substitutions by methyl groups or iodine reduce both the agonistic and antagonistic activity. Only substitution by fluorine gives a high potency. Incorporation of deltaPhe into different representative antagonists with key modifications at position 7 does not enhance the antagonist activity of the basic structures, with one exception. Only the combination of deltaPhe at position 5 with DPhe at position 7 increases the antagonistic potency on guinea pig ileum by about one order of magnitude. Radioligand binding studies indicate the importance of position 5 for the discrimination of B2 receptor subtypes. The binding affinity to the low-affinity binding site (KL) was not significantly changed by replacement of Phe by deltaPhe. In contrast, ring-methylation of deltaPhe results in clearly reduced binding to KL. The affinity to the high-affinity binding site (KH) was almost unchanged by the replacement of Phe in position 5 by deltaPhe, whereas the analogue with 2-methyl-dehydrophenylalanine completely failed to detect the KH-site. The peptides were synthesized on the Wang-resin according to the Fmoc/Bu(t) strategy using Mtr protection for the side chain of Arg. The dehydrophenylalanine analogues were prepared by a strategy involving PyBop couplings of the dipeptide unit Fmoc-Gly-deltaPhe(X)-OH to resin-bound fragments.

Differential stimulation of cytosolic phospholipase A2 by bradykinin in human cystic fibrosis cell lines

Tracheal epithelial cells and skin fibroblasts from different cystic fibrosis (CF) patients bearing the deltaF508 mutation of cystic fibrosis transmembrane conductance regulator (CFTR) released more arachidonic acid in response to bradykinin than do other CF and normal cells. Immortalized tracheal epithelial cell lines were used as models to study the mechanisms of this dysregulation. An 85 kD cytosolic phospholipase A2 (cPLA2) was found in these cells and bradykinin increased its binding to membranes of deltaF508 cells (CFT-2) but not to those of a double heterozygous CF cells (CFT-1), or of control cells (NT-1). The expression of G alpha(q)/11 protein was also increased in deltaF508 cells, with increased stimulation of phosphatidylinositol diphosphate-specific phospholipase C (PLC) by bradykinin, and an early, transient activation of mitogen-activated protein (MAP) kinase. As the binding of cPLA2 to membranes is Ca2+-dependent, the increased coupling to PLC could cause the hypersensitivity to bradykinin. Comparison of the effects of bradykinin to those observed with thapsigargin, an inhibitor of calcium reuptake, suggests that the increase of intracellular calcium is not the only mechanism involved in arachidonic acid release by bradykinin in deltaF508 cells. The lack of effect of calcium ionophore A23187 or TPA on arachidonic acid release from any of the cell lines suggested that activation needs a PKC-independent cPLA2 phosphorylation step, perhaps via MAP kinase activation. The binding of cPLA2 to membranes after bradykinin stimulation still occurred in CFT2 cells (deltaF508) homogenized in EDTA, suggesting that a membrane component plus increased intracellular calcium influenced cPLA2 anchoring to membranes. The defective processing of deltaF508 CFTR seems to increase cPLA2 stimulation by bradykinin, since the bradykinin-stimulated release of arachidonic acid is reversed by growing cells at 28 degrees C for 48 h. The deltaF508 mutation of CFTR appears to increase the stimulation of cPLA2 by Gq-mediated receptors in a PKC-independent and MAP kinase-dependent manner. Hence normal CFTR, or normally processed deltaF508 CFTR, inhibit cPLA2 stimulation. The greater reactivity of deltaF508 CFTR cells to inflammatory mediators might be part of the increased sensitivity of CF patients to lung inflammation.

G-protein-mediated signaling in cholesterol-enriched arterial smooth muscle cells. 1. Reduced membrane-associated G-protein content due to diminished isoprenylation of G-gamma subunits and p21ras

Mechanisms contributing to altered heterotrimeric G-protein expression and subsequent signaling events during cholesterol accretion have been unexplored. The influence of cholesterol enrichment on G-protein expression was examined in cultured smooth muscle cells that resemble human atherosclerotic cells by exposure to cationized LDL (cLDL). cLDL, which increases cellular free and esterified cholesterol 2-fold and 10-fold, respectively, reduced the cell membrane content of Galphai-1, Galphai-2, Galphai-3, Gq/11, and Galphas. The following evidence supports the premise that the mechanism by which this occurs is due to reduced isoprenylation of the Ggamma-subunit. First, the inhibitory effect of cholesterol enrichment on the membrane content of Galphai subunits was found to be post-transcriptional, since the mRNA steady-state levels of Galphai(1-3) were unchanged following cholesterol enrichment. Second, the membrane expression of alpha and beta subunits was mimicked by cholesterol and 17-ketocholesterol, both of which inhibit HMG-CoA reductase. Third, inhibition of Galphai and Gbeta expression in cholesterol-enriched cells was overcome by mevalonate, the immediate product of HMG-CoA reductase. Fourth, pulse-chase experiments revealed that cholesterol enrichment did not reduce the degradation rate of membrane-associated Galphai subunits. Fifth, cholesterol enrichment also reduced membrane expression of Ggamma-5, Ggamma-7upper; these gamma subunits are responsible for trafficking of the heterotrimeric G-protein complex to the cell membrane as a result of HMG-CoA reductase-dependent post-translational lipid modification (geranylgeranylation) and subsequent membrane association. Cholesterol enrichment did not alter expression of G-gamma-5 mRNA, as assessed by reverse transcriptase polymerase chain reaction, supporting a post-transcriptional defect in Ggamma subunit expression. Fifth, cholesterol enrichment also reduced the membrane content of p21ras (a low molecular weight G-protein requiring farnesylation for membrane targeting) but did not alter the membrane content of the two proteins that do not require isoprenylation for membrane association&sbd;PDGF-receptor or p60-src. Reduced G-protein content in cholesterol-laden cells was reflected by reduced G-protein-mediated signaling events, including ATP-induced GTPase activity, thrombin-induced inhibition of cyclic AMP accumulation, and MAP kinase activity. Collectively, these results demonstrate that cholesterol enrichment reduces G-protein expression and signaling by inhibiting isoprenylation and subsequent membrane targeting. These results provide a molecular basis for altered G-protein-mediated cell signaling processes in cholesterol-enriched cells.

Cloning and functional characterization of the ornithokinin receptor. Recognition of the major kinin receptor antagonist, HOE140, as a full agonist

Kinins are proinflammatory peptides that dilate vessels, increase vascular permeability, contract smooth muscles, and provoke pain. The known mammalian kinin receptors are classified as two subtypes, i.e. the B1 receptor triggered by [des-Arg9]bradykinin and inhibited by [des-Arg9,Leu8]bradykinin, and the B2 receptor stimulated by bradykinin and antagonized by HOE140. Here we report the cloning of a non-mammalian kinin receptor gene amplified from genomic chicken DNA. The protein predicted from the open reading frame shows 31 and 49% sequence identity to the human B1 and B2 receptors, respectively, suggesting that it represents a G protein-coupled receptor of the kinin receptor family. The recombinantly expressed chicken receptor had IC50 values of 4.7 nM for the authentic ligand, ornithokinin ([Thr6,Leu8]bradykinin), 3.8 nM for HOE140, and >/=10 microM for bradykinin, [des-Arg9]bradykinin, and [des-Arg9,Leu8]bradykinin. Ornithokinin and HOE140 at nanomolar concentrations stimulated intracellular inositol phosphate accumulation and induced a significant transient rise in intracelluar free Ca2+, whereas bradykinin was ineffective even at 100 nM. Hence the principal B2 receptor antagonist HOE140 is a potent agonist of the chicken kinin receptor. This unique pharmacological profile classifies the ornithokinin receptor as a novel subtype among kinin receptors and will facilitate further molecular studies on ligand binding and receptor activation.

Coupling of oxytocin receptor to G proteins in rat myometrium during labor: Gi receptor interaction

Occupancy of oxytocin receptor (OTR) binding sites in pregnant rat myometrial membranes with iodinated oxytocin antagonist (OTA), followed by detergent solubilization and size selection, showed that radioactivity eluted in two distinct peaks: one corresponding in size to the isolated receptor (approximately 60 kDa) and the other ranging from 240 to 320 kDa. The unliganded 240- to 320-kDa fraction contained OTRs coupled to G proteins, as the addition of oxytocin (OT) increased guanosine 35S-labeled 5'-O-(3-thiotriphosphate) binding up to twofold in a dose-dependent manner. The effects of OT were blocked by coincubation with OTA. G protein alpha-subunits associated with OTRs in the 240- to 320-kDa peak were identified by immunoadsorption. Significant amounts of both G alpha q/11 and G alpha i3 were associated with the OTR; a lesser amount of G alpha s was complexed. Using the same approach but with antibodies to effector enzymes, we observed that phospholipase C beta 1 (PLC beta 1) and PLA2 were also associated with the OTR. The results corroborate the well-established interaction of OTR with Gq and further show that Gi coupling might be an important component of OTR signal transduction. To further investigate the interaction of Gi with the OTR, we showed that OT stimulation of guanosine 5'-triphosphatase activity in intact myometrial membranes was inhibited by pertussis toxin. Pertussis toxin-stimulated ADP ribosylation of G alpha i in myometrial membranes was also decreased by OT treatment. These findings with pertussis toxin strongly indicate that OTR is coupled to Gi in rat myometrial membranes. The 60-kDa OTR peak (noncoupled receptor) was demonstrable in the myometrium only before the end of gestation and after parturition and accounted for about one-half the 125I-OTA binding activity. At term, there was about a fivefold increase in binding and almost a complete shift to the 240- to 320-kDa-size complex. Thus the established increased sensitivity of the myometrium to OT at term could be the result of both upregulation of OTRs and an increase in the fraction of receptors coupled to signal transduction components, one of which is Gi.

Localization of bradykinin B2 receptors in sheep uterus

The localization and characterization of bradykinin B2 receptors in sheep uterus was carried out using a radiolabelled B2 receptor ligand, 3,4-hydroxyphenypropionyl-D-Arg0-[Hyp3,Thi5,D-++ +Tic7,D-Oic8] bradykinin (HPP-HOE140). Competition of the radioligand, [125I]HPP-HOE140, from membrane preparations of anoestrus sheep uterus by bradykinin agonists and antagonists revealed the presence of high- and low-affinity binding sites with ligand specificity typical of the bradykinin B2 receptor. Using in vitro autoradiography on tissue sections, intense binding was visible over the superficial epithelial layer of the endometrium and inner third of the myometrium of anoestrus sheep uterus. Bradykinin B2 receptors in the myometrium were down regulated in pregnant sheep uterus. We demonstrate that binding studies using [125I]HPP-HOE140 offer high sensitivity and specificity for characterization, quantitation and localization of the bradykinin B2 receptors in tissues and offers new information on uterine bradykinin B2 receptors.

Highly selective bradykinin agonists and antagonists with replacement of proline residues by N-methyl-D- and L-phenylalanine

For further studies on the structural and conformational requirements of positions 2,3, and 7 in the bradykinin sequence, we replaced the proline residues by the more hydrophobic and conformationally restricted N-methyl-L- and D-phenylalanine (NMF). The biological activities of the new analogs were evaluated on rat uterus, guinea pig ileum, and guinea pig lung strip. Receptor binding of the analogs was studied in membranes from rat uterus and guinea pig ileum. Influence of bradykinin analogs on the release of cytokines from mouse spleen cell cultures was also measured. Bradykinin analogs were synthesized by the solid phase method, using Boc strategy on PAM or Merrifield resins. The best results in the formation of the N-methylamide bond were obtained with the coupling reagent PyBrop. In position 7 the substitution of D-Phe by D-NMF, retaining the configuration of the amino acid, converts bradykinin antagonists into agonists. The bradykinin analogs with D-NMF at position 7 gave the highest known tissue selectivity for rat uterus among agonists. [L-NMF(2)]bradykinin has moderate agonist activity on rat uterus but antagonist activity on guinea pig lung strip. It represents a new antagonist for B(2) receptors without any replacement at position 7. The same analog completely inhibits bradykinin-evoked cytokine expression by mononuclear cells.

Multiple mechanisms of bradykinin-induced contraction in rat and guinea pig smooth muscles in vitro

Bradykinin caused graded contraction in the guinea pig ileum, trachea and urinary bladder and rat uterus and vas deferens in vitro. The order of potency (EC50, nM) was: ileum (3) > uterus (5) > trachea (15) > vas deferens (41) > urinary bladder (52) and the maximal responses (percentage to 80 mM KCl) were: 152 +/- 8 (ileum), 122 +/- 6 (uterus), 97 +/- 3 (urinary bladder), 75 +/- 5 (trachea) and 33 +/- 3 (vas deferens). Responses to bradykinin in guinea pig ileum and urinary bladder and rat vas deferens and uterus were markedly attenuated in Ca(2+)-free medium with or without EGTA or by nicardipine, whereas those in guinea pig trachea depended almost exclusively on intracellular Ca2+ sources which were sensitive to ryanodine. Treatment of the animals with pertussis toxin only inhibited bradykinin-induced contraction of the rat uterus. Furthermore, the protein kinase C inhibitors, H7 (5-isoquinolinysulfonyl-2-methyl-piperazine) and staurosporine, antagonized in a graded manner bradykinin responses in guinea pig ileum and trachea and rat vas deferens, indicating the possible dependence on activation of protein kinase C mechanisms, while responses of the rat uterus rely on coupling by a pertussis toxin-sensitive G protein. Thus, bradykinin acting at B2 receptors may induce contractions in several smooth muscles from rat and guinea pig through activation of multiple second messenger pathways.

Bradykinin inhibits adenylate cyclase activity in guinea pig ileum membranes via a separate high-affinity bradykinin B2 receptor

In guinea pig ileum membranes, the pre-stimulated adenylate cyclase activity was dose-dependently inhibited by picomolar concentrations of bradykinin exhibiting an apparent IC50 value of approximately 30 pM. At nanomolar bradykinin concentrations (> 1 nM) this effect was attenuated. The inhibition of ileal adenylate cyclase was completely prevented by both the bradykinin B2 receptor antagonist Hoe 140 (D-Arg[Hyp3,Thi5,D-Tic7,Oic8]bradykinin) and pertussis toxin. The potency of bradykinin to inhibit ileal adenylate cyclase considerably correlates with a binding site with picomolar affinity for bradykinin. In addition, a second site was constantly found displaying nanomolar binding affinity for bradykinin. The occurrence of two independent bradykinin B2 receptors in guinea pig ileum membranes is further supported by three other lines of evidence: bradykinin stimulates [35S]GTP[S] (guanosine 5'-O-[3-thiotriphosphate]) binding to guinea pig ileum membranes in a biphasic manner with EC50 values which correspond to the affinities of both sites. In binding studies, the high-affinity site cannot be transformed into the low-affinity site in the presence of Gpp[NH]p (5'-guanylylimidodiphosphate). The specific binding of [3H]bradykinin to guinea pig ileum membranes was also biphasically inhibited by increasing concentrations of Gpp[NH]p. Thus, our results favour the existence of two separate bradykinin B2 receptors with different signal transduction pathways in guinea pig ileum membranes: one receptor with picomolar affinity for bradykinin which inhibits adenylate cyclase via a pertussis toxin-sensitive G protein of probably the Gi2 type and the other receptor with nanomolar affinity for bradykinin which might be responsible for bradykinin-induced stimulation of phosphoinositide hydrolysis.

Co-solubilization of bradykinin B2 receptors and angiotensin-converting enzyme from guinea pig lung membranes

Bradykinin B2 receptor-like binding activity was solubilized from guinea pig lung using the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulphonate (Chaps). The binding of [3H]bradykinin to the soluble fraction was time-dependent and saturable. Scatchard analysis of equilibrium binding data indicated that the soluble extract contained a single class of binding sites with a Kd of 696 pM and a Bmax of 57 fmol/mg protein. Unlabelled bradykinin and B2 antagonists inhibited the binding of [3H]bradykinin to Chaps-solubilized extracts with relative potencies similar to those observed with the low-affinity membrane-bound binding sites. Following partial purification of the soluble preparation, using anion exchange (DEAE-Sephacel) and gel filtration (Aca 34) column chromatography steps, two peaks eluted off the column were able to bind [3H]bradykinin and have molecular masses of 168 and 98.5 kDa. The former seems to represent binding of bradykinin to angiotensin converting enzyme (ACE, EC 3.4.15.1) and the latter binding to bradykinin receptor. Using purified commercial ACE, we show that the binding of [3H]bradykinin to ACE can easily be distinguished from that of the bradykinin receptor, since both B1 and B2 ligands were able to inhibit bradykinin binding with affinities clearly different from that expected for a bradykinin receptor.

Characterization of receptor-mediated actions of T-kinin

T-Kinin (Ile-Ser-bradykinin) is unique to the rat. This study characterizes the receptors involved in T-kinin activity on both the intact isolated rat uterus and membrane receptor preparations of the rat uterus. The results show that T-kinin acts through kinin B2 receptors in the rat uterus as demonstrated by B2 receptor-antagonist inhibition. While the potency of T-kinin on rat uterus contraction was similar to that of bradykinin, binding studies showed that the affinity of T-kinin to the receptor was 10-fold lower than that of bradykinin. On the other hand, the D isomer of T-kinin, D-Ile-Ser-bradykinin, had an affinity for the receptor greater than that of T-kinin and was more potent in causing contraction. Comparing this finding with our previously published report that D-Ile-Ser-bradykinin is not active on the kinin receptor for vascular permeability indicates that the kinin receptors in the rat uterus are not the same as those previously reported in the smooth muscle of the vasculature, i.e. there exists subclasses of kinin B2 receptors. The data from binding studies on a variety of T-kinin analogues show that the substitution of hydroxyproline (Hyp) for Pro5, together with the D-configuration at Ile1 and/or Ser2 may be useful for the development of selective T-kinin antagonists. Studies involving pretreatment of the tissue with indomethacin demonstrated that prostaglandin release was more of a component of T-kinin's activity on the rat uterus than that of bradykinin.

Potentiation by cholera toxin of bradykinin-induced inositol phosphate production in the osteoblast-like cell line MC3T3-E1

Cells of the osteoblastic cell line MC3T3-E1 were shown to contain at least three phosphatidylinositol-specific phospholipase C (PI-PLC) isoenzymes (PLC-beta, PLC-gamma and PLC-delta) by Western blotting analysis with various anti-PLC antibodies. Stimulation of inositol phosphate production in MC3T3-E1 cells by bradykinin (BK) occurred via a GTP-binding protein. Inositol phosphate formation on stimulation by BK was not affected by pretreatment with pertussis toxin, whereas it was potentiated by cholera toxin pretreatment. Elevation of cellular cyclic AMP levels by brief pretreatment with dibutyryl cyclic AMP or forskolin failed to enhance the BK-mediated generation of inositol phosphates, but long-term preincubation with these agents partially mimicked the action of the cholera toxin. Cholera toxin also caused an increase in BK receptor number. Cycloheximide, a protein biosynthesis inhibitor, prevented the potentiating actions of the cholera toxin and the cyclic AMP-elevating agents on BK-induced inositol phosphate production, and also inhibited the increase in BK receptor number. The specific binding of [3H]BK to the whole MC3T3-E1 cells in the presence or absence of cholera toxin was completely inhibited by the B2 BK receptor antagonist D-Arg[Hyp3,Thi5,8,D-Phe7]BK, but not by the B1 BK receptor agonist des-Arg9-BK. These data suggest that the activation of PI-PLC induced by cholera toxin in BK-stimulated MC3T3-E1 cells was caused by an enhancement of the synthesis of BK receptor protein(s), at least part of which was mediated by a sustained increase in the intracellular level of cyclic AMP.

Pharmacological and molecular actions of the bradykinin B2 receptor antagonist, Hoe 140, in the rat uterus

The type of antagonism exhibited by the potent bradykinin B2 receptor antagonist, Hoe 140, on the rat uterus was investigated using various approaches. In the isolated rat uterus the concentration-response curve of bradykinin was shifted to the right and the maximum effect was reduced after pretreatment with Hoe 140 for 5 min. Shorter times of antagonist pretreatment failed to decrease the maximum effect of bradykinin. In the [3H]BK binding assay Hoe 140 bound with pM affinity to a single site and did not discriminate multiple bradykinin receptors. Studying the bradykinin-induced G protein activation we could verify that Hoe 140 at subnanomolar concentrations selectively antagonized the low-affinity BK receptor in the rat myometrium. At nM concentrations Hoe 140 itself was able to stimulate G proteins. The results suggest that in the rat uterus, differently from guinea pig ileum, Hoe 140 possibly acts as a mixed competitive as well as functional antagonist.

Bradykinin receptors: pharmacological properties and biological roles

Kinins contribute to the acute inflammatory response and are implicated in the pathophysiology of inflammatory disease. The development of therapeutically viable agents that counteract the effects of kinins is, therefore, potentially very rewarding. Since kinin actions are generally mediated via an interaction with cell-surface receptors, one approach is the development of site-specific receptor antagonists. The emphasis in this review is to outline our current understanding of the properties of bradykinin receptors and the potential therapeutic applications for drugs acting at these sites. As a result of the recent introduction of potent bradykinin receptor antagonists and the cloning of bradykinin receptor genes, considerable advances in kinin research can now be confidently anticipated.

Recent developments in the understanding of bradykinin receptors

The dramatic activities of bradykinin and related peptides as mediators of pain, inflammation and hypotension have been intensely studied for several decades. More recently, the involvement of bradykinin in regulation of ion transport by epithelia, hormone release from endocrine organs, energy metabolism, tissue growth, and leukocyte activation have become topics of study. Kininogen precursors, synthetic kallikreins, and degradative kininases have been characterized in detail with regard to catalytic mechanisms, physical structure and gene regulation; however, the actual receptors for bradykinin are still only poorly understood. This situation is caused by the lack of availability of potent, specific receptor antagonists. However, specific bradykinin receptor antagonists became available in 1985, and several very potent classes of agents are now available; also, the first bradykinin receptor has been cloned.

Analysis of the antagonistic actions of HOE 140 and other novel bradykinin analogues on the guinea-pig ileum

The type of antagonism exhibited by three novel bradykinin (BK) antagonists, D-Arg-[Hyp3,Thi5,D-Tic7,Oic8]BK (HOE 140, compound I), D-Arg-[Hyp3,D-Tic7,Oic8]BK (compound II) and [Arg(Tos)1,Hyp3,Thi5,D-Tic7,Oic8]BK (compound III), was compared with that of a conventional antagonist, D-Arg-[Hyp2,Thi5,8,D-Phe7]BK (compound IV), on the guinea-pig ileum. The novel compounds induced rightward displacements of cumulative concentration-response curves to BK, accompanied by a progressive reduction of the maximum effect (Emax) without a significant decrease in the slope, whereas no reduction of Emax was observed with compound IV. Actions of substance P on the guinea-pig ileum and of vasopressin on the rat uterus remained completely unaffected. It is concluded that as the novel BK analogues show competitive as well as non-competitive inhibition in the guinea-pig ileum, but the inhibition is reversible and specific, they are dual antagonists.

Antagonist binding reveals two heterogenous B2 bradykinin receptors in rat myometrial membranes

In rat myometrial membranes, two bradykinin binding sites with K1 values of 18 pM and 5.6 nM were identified. Three potent bradykinin antagonists were tested for their ability to compete for [3H]bradykinin binding. Two of them, D-Arg[Hyp3,Thi5.8,D-Phe7]bradykinin and [Hyp3,Thi5.8,D-Phe7]bradykinin, also bound to both the high- (KH) and the low-affinity (KL) site whereas [Thi5.8,D-Phe7]bradykinin identified only the low-affinity bradykinin receptor. There is a close correlation between the antagonistic potencies and the KL site affinities.

Bradykinin receptors of cerebral microvessels stimulate phosphoinositide turnover

We examined by ligand binding methods whether bradykinin (BK) receptors exist in rat and pig cerebral microvessels, and in the cerebral cortex from which the microvessels were isolated. We found a high-affinity and saturable BK receptor site in both rat and pig cerebral microvessels, but not in their cerebral cortex. The maximal density of binding and the dissociation constant were 8.0 +/- 4.1 and 6.8 +/- 1.5 fmol/mg of protein and 47 +/- 24 and 150 +/- 8 pM (mean +/- SD) in cerebral microvessels of the pig and rat, respectively. The high-affinity specific binding of BK was effectively displaced by des-Arg0[Hyp3-Thi5-8,D-Phe7]BK, a specific B2 receptor antagonist, but not by des-Arg9[Leu8]BK, a specific B1 antagonist. We also demonstrated that BK increases phosphatidylinositol hydrolysis in cerebral microvessels of the rat and pig. This effect was also blocked by the B2, but not by the B1, antagonist. Increased phosphatidylinositol hydrolysis was manifested by a rapid transient increase in inositol trisphosphate and the later slow accumulation of inositol bisphosphate and inositol monophosphate. Preincubation of microvessels with phorbol ester, stable GTP analogs, pertussis toxin, or in Ca(2+)-free buffer did not influence BK activation of phosphatidylinositol hydrolysis. These results demonstrate the existence of BK receptors of the B2 subtype in brain microvessels, which may play an important role in modulation of the brain microcirculation, probably via increased phosphoinositide turnover.

Evidence for two high-affinity bradykinin binding sites in the guinea-pig lung

We examined the binding of [3H]bradykinin ([3H]BK) to the guinea-pig lung which was saturable. Scatchard analysis indicated the existence of two binding sites, one with a high affinity (KD = 15 pM) and one with a low one (KD = 570 pM) with maximum number of binding sites of 12 and 45 fmol/mg protein, respectively. Kinetic studies confirmed the presence of these two types of binding sites and their affinity ranges. Neither the B1 agonist des-Arg9-BK, nor the B1 antagonist des-Arg9-[Leu8]BK displaced [3H]BK, demonstrating the absence of B1 receptors in the guinea-pig lung. Current B2 antagonists fully displaced the [3H]BK binding. Their potencies differed slightly according to the concentration of [3H]BK, suggesting a specificity of current B2 antagonists for the lower affinity site as opposed to the higher one. Altogether, these results do not allow us to confirm the occurrence of putative B3 receptors in guinea-pig lung.

High-affinity bradykinin receptor-catalyzed G protein activation in rat myometrium

Binding of the labelled GTP analog, guanosine-5'-O-(3-thiotriphosphate ([35S]GTP[S] to G proteins was studied in rat myometrial membranes in the presence of GDP (1 microM). Binding was stimulated by bradykinin at subnanomolar concentrations. while oxotremorine increased binding of [35S]GTP[S] to myometrial membranes at micromolar concentrations. The bradykinin-induced stimulation was antagonized by the receptor antagonist, [D-Arg-(Hyp3, Thi5,8, D-Phe7)]bradykinin. Addition of NaCl (150 nM) decreased control binding and abolished the stimulatory effect of bradykinin. On the other hand, addition of CaCl2 (5 mM) had no effect on control binding but also prevented the bradykinin-induced increased in [35S]GTP[S] binding. Saturation experiments revealed that activation of the bradykinin receptor leads to about a three-fold increase in the apparent GTP[S] binding affinity of about 30% of the total GTP[S] binding sites measured in these membranes. The results provide evidence for a high-affinity bradykinin receptor in rat myometrial membranes which interacts with and activates G proteins. This receptor action, which appears to be under the control of both sodium and calcium ions, is catalytic and leads to a large signal amplification, in that one agonist-liganded bradykinin receptor can apparently activate up to 100 G proteins.