Bradykinin receptor-like binding studied with iodinated analogues

Novel Bradykinin Analogues Modified in the N-Terminal Part of the Molecule with a Variety of Acyl Substituents

In the current work we present some pharmacological characteristics of ten new analogues of bradykinin (Arg–Pro–Pro–Gly–Phe–Ser–Pro–Phe–Arg) modified in the N-terminal part of the molecule with a variety of acyl substituents. Of the many acylating agents used previously with B₂ receptor antagonists, the following residues were chosen: 1-adamantaneacetic acid (Aaa), 1-adamantanecarboxylic acid (Aca), 4-tert-butylbenzoic acid (t-Bba), 4-aminobenzoic acid (Aba), 12-aminododecanoic acid (Adc), succinic acid (Sua), 4-hydroxybenzoic acid, 4-hydroxy-3-methoxybenzoic acid, 3-(4-hydroxyphenyl)propionic acid and 6-hydroxy-2-naphthoic acid. Biological activity of the compounds was assessed in the in vivo rat blood pressure test and the in vitro rat uterus test. Surprisingly, N-terminal substitution of the bradykinin peptide chain itself with aforementioned groups resulted in antagonists of bradykinin in the pressor test and suppressed agonistic potency in the uterotonic test. These interesting findings need further studies as they can be helpful for designing more potent B₂ receptor blockers.

Characterization of endogenous calcium responses in neuronal cell lines

An increasing number of putative therapeutic targets have been identified in recent years for the treatment of neuronal pathophysiologies including pain, epilepsy, stroke and schizophrenia. Many of these targets signal through calcium (Ca(2+)), either by directly facilitating Ca(2+) influx through an ion channel, or through activation of G proteins that couple to intracellular Ca(2+) stores or voltage-gated Ca(2+) channels. Immortalized neuronal cell lines are widely used models to study neuropharmacology. However, systematic pharmacological characterization of the receptors and ion channels expressed in these cell lines is lacking. In this study, we systematically assessed endogenous Ca(2+) signaling in response to addition of agonists at potential therapeutic targets in a range of cell lines of neuronal origin (ND7/23, SH-SY5Y, 50B11, F11 and Neuro2A cells) as well as HEK293 cells, a cell line commonly used for over-expression of receptors and ion channels. This study revealed a remarkable diversity of endogenous Ca(2+) responses in these cell lines, with one or more cell lines responding to addition of trypsin, bradykinin, ATP, nicotine, acetylcholine, histamine and neurotensin. Subtype specificity of these responses was inferred from agonist potency and the effect of receptor subtype specific antagonist. Surprisingly, HEK293 and SH-SY5Y cells responded to the largest number of agonists with potential roles in neuronal signaling. These findings have implications for the heterologous expression of neuronal receptors and ion channels in these cell lines, and highlight the potential of neuron-derived cell lines for the study of a range of endogenously expressed receptors and ion channels that signal through Ca(2+).

New bradykinin analogues substituted in positions 7 and 8 with sterically restricted 1-aminocyclopentane-1-carboxylic acid

A sterically constrained non-coded amino acid, 1-aminocyclopentane-1-carboxylic acid (Apc), was introduced in position 7 or 8 of the bradykinin (BK) B(2) receptor antagonist, [D-Arg(0), Hyp(3), Thi(5, 8), D-Phe(7)]BK, previously synthesized by Stewart's group. This modification is believed to reduce the flexibility of the peptides, thereby forcing the peptide backbone and side chains to adopt specific orientations. Apc substitution was combined with acylation of the N-terminus with 1-adamantaneacetic acid (Aaa). The activity of four new analogues was assayed in isolated rat uterus and in rat blood pressure tests. The results clearly demonstrated that the Apc residue inserted in position 7 led to a reduction of antagonistic properties in the rat uterus assay or even restored the agonism in the blood pressure test, whereas Apc at position 8 enhanced antagonistic potency in both the tests. In both cases, acylation of the N-terminus led to the enhancement of the antagonistic potency. On the basis of these findings, new potent and selective B(2) blockers might be designed.

Mechanisms regulating the expression, self-maintenance, and signaling-function of the bradykinin B2 and B1 receptors

Bradykinin (BK) is a potent short-lived effector belonging to a class of peptides known as kinins. It participates in inflammatory and vascular regulation and processes including angioedema, tissue permeability, vascular dilation, and smooth muscle contraction. BK exerts its biological effects through the activation of the bradykinin B2 receptor (BKB2R) which is G-protein-coupled and is generally constitutively expressed. Upon binding, the receptor is activated and transduces signal cascades which have become paradigms for the actions of the Galphai and Galphaq G-protein subunits. Following activation the receptor is then desensitized, endocytosed, and resensitized. The bradykinin B1 (BKB1R) is a closely related receptor. It is activated by desArg(10)-kallidin or desArg(9)-BK, metabolites of kallidin and BK, respectively. This receptor is induced following tissue injury or after treatment with bacterial endotoxins such as lipopolysacharide or cytokines such as interleukin-1 or tumor necrosis factor-alpha. In this review we will summarize the BKB2R and BKB1R mediated signal transduction pathways. We will then emphasize the relevance of key residues and domains of the intracellular regions of the BKB2R as they relate to modulating its function (signal transduction) and self-maintenance (desensitization, endocytosis, and resensitization). We will examine the features of the BKB1R gene promoter and its mRNA as these operate in the expression and self-maintenance of this inducible receptor. This communication will not cover areas discussed in earlier reviews pertaining to the actions of peptide analogs. For these we refer you to earlier reviews (Regoli and Barabé, 1980, Pharmacol Rev 32:1-46; Regoli et al., 1990, J Cardiovasc Pharmacol 15(Suppl 6):S30-S38; Regoli et al., 1993, Can J Physiol Pharmacol 71:556-557; Marceau, 1995, Immunopharmacology 30:1-26; Regoli et al., 1998, Eur J Pharmacol 348:1-10).

New analogues of bradykinin containing a conformationally restricted dipeptide fragment in their molecules

The present paper describes the synthesis and some pharmacological properties of two new bradykinin analogues containing the ethylene-bridged dipeptide Phe-Phe in their molecules. In a further two peptides this modification was combined with acylation of the N-terminus with 1-adamantaneacetic acid. Finally, we synthesized four analogues by removing the Ser6 residue from the four peptides mentioned above. The activity of the new analogues was assayed on isolated rat uterus (RUT) and in rat blood pressure tests (BPT). The results clearly indicate that the proposed modification, alone or in combination with other changes, resulted in either a drop in antiuterotonic activity or even in conversion to an agonism. Although this tendency is not so distinct in blood pressure assays, the antagonistic potency of the new analogues is also diminished. Nevertheless, it was demonstrated that the D-amino acid in position 7 which, until recently, was considered necessary for antagonism, may be replaced, together with the amino acid occupying position 8, by a suitable, sterically restricted L,L-dipeptide unit.

New bradykinin analogs in contraction of rat uterus

In this study, we evaluated 20 of our previously synthesized peptides on isolated rat uterus by Holton's procedure with minor modifications, and compared their activity with that assessed previously by their ability to inhibit vasodepressor response to exogenous bradykinin (BK) in conscious rats. We used [D-Arg(0), Hyp(3), Thi(5, 8), (D-Phe)(7)]BK, the B(2) antagonist of Vavrek and Stewart as a model when designing our analogs. We observed that, in the case of the rat uterus test, the activity of peptides modified by acylation of the N-terminus with various bulky groups depends substantially on the chemical character of the substituent. We also learned that, contrary to previous examples, acylation of the N-terminus of antagonists, which contain a sterically restricted fragment in the C-terminal part, may not improve their antagonistic potencies. Besides an improved characterization of a series BK analogs, our studies have provided new information on the structure-activity relationship, which in turn may be of value in the design of more potent and selective bradykinin antagonists. The results of our studies appear to support the hypothesis of others about the presence of different subtypes of B(2) receptors in rat uterus and blood vessels.

Cyclic and linear bradykinin analogues: implications for B2 antagonist design

Bradykinin (BK, Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) antagonists are potentially useful for treating inflammation, pain and severe trauma. To identify what chemical features might promote effective antagonism, we replaced Arg1 and Pro7 with structurally constrained and proteolytic-resistant residues, such as Bip (biphenylalanine), Dip (diphenylalanine) or 2Ind (indane amino acid). To determine which BK folding might lead to favourable interactions with receptors, the effects of cyclo(3,8) vs. cyclo(5,8) analogues were compared. The resulting BK analogues were examined for their agonistic and antagonistic activities in guinea pig ileum, rat uterus and depressor assays. The results suggest that co-planarity of the residue-7 side chain with its backbone NH is important for potent agonism as well as antagonism, and a D-directed side chain is crucial for antagonism. For residue-1 an L-orientation is important, and Dip1 may mimic a folded Arg1 side chain to elicit agonistic activities, with Bip1 mimicking an extended Arg1 side chain to elicit inhibitory activities. However, ileal and uterine receptors appear to prefer differently folded BK. For ileum, a BK conformation in which residues-3 and -8 are proximal to each other, but apart from residue-5, led to improved pA2.

B2 kinin receptor upregulation by cAMP is associated with BK-induced PGE2 production in rat mesangial cells

In the rat mesangial cell (MC), activation of the bradykinin B2 receptor (B2R) by bradykinin (BK) is associated with both phospholipase C (PLC) and A2 (PLA2) activities and with inhibition of adenosine 3',5'-cyclic monophosphate (cAMP) formation leading to cell contraction. Because cAMP plays an important role in the regulation of gene expression in general, we investigated the effect of increasing the intracellular cAMP concentration ([cAMP]i) in mesangial cells on the B2 mRNA expression, on the density of B2 receptor binding sites, on the BK-induced increase in both the free cytosolic Ca2+ concentration ([Ca2+]i), and in the prostaglandin E2 (PGE2) production. Forskolin, PGE2, and cAMP analog, 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP), were used to increase [cAMP]i. Twenty-four-hour treatment with forskolin, PGE2, and 8-BrcAMP resulted in significant increases in B2 receptor binding sites, which were inhibited by cycloheximide. The maximum B2 receptor mRNA expression (160% above control) was observed in cells treated during 24 h with forskolin and was prevented by actinomycin D. In contrast, the D-myo-inositol 1,4,5-trisphosphate (IP3) formation and the BK-induced increase in [Ca2+]i, reflecting activation of PLC, were not affected by increased levels of [cAMP]i. However the BK-induced PGE2 release, reflecting PLA2 activity, was significantly enhanced. These data bring new information regarding the dual signaling pathways of B2 receptors that can be differentially regulated by cAMP.

Potentiation of the actions of bradykinin by angiotensin I-converting enzyme inhibitors. The role of expressed human bradykinin B2 receptors and angiotensin I-converting enzyme in CHO cells

Part of the beneficial effects of angiotensin I-converting enzyme (ACE) inhibitors are due to augmenting the actions of bradykinin (BK). We studied this effect of enalaprilat on the binding of [3H]BK to Chinese hamster ovary (CHO) cells stably transfected to express the human BK B2 receptor alone (CHO-3B) or in combination with ACE (CHO-15AB). In CHO-15AB cells, enalaprilat (1 mumol/L) increased the total number of low-affinity [3H]BK binding sites on the cells at 37 degrees C, but not at 4 degrees C, from 18.4 +/- 4.3 to 40.3 +/- 11.9 fmol/10(6) cells (P < .05; Kd, 2.3 +/- 0.8 and 5.9 +/- 1.3 nmol/L; n = 4). Enalaprilat preserved a portion of the receptors in high-affinity conformation (Kd, 0.17 +/- 0.08 nmol/L; 8.1 +/- 0.9 fmol/10(6) cells). Enalaprilat decreased the IC50 of [Hyp3-Tyr(Me)8]BK, the BK analogue more resistant to ACE, from 3.2 +/- 0.8 to 0.41 +/- 0.16 nmol/L (P < .05, n = 3). The biphasic displacement curve of the binding of [3H]BK also suggested the presence of high-affinity BK binding sites. Enalaprilat (5 nmol to 1 mumol/L) potentiated the release of [3H]arachidonic acid and the liberation of inositol 1,4,5-trisphosphate (IP3) induced by BK and [Hyp3-Tyr(Me)8]BK. Moreover, enalaprilat (1 mumol/L) completely and immediately restored the response of the B2 receptor, desensitized by the agonist (1 mumol/L [Hyp3-Tyr(Me)8]BK); this effect was blocked by the antagonist, HOE 140. Finally, enalaprilat, but not the prodrug enalapril, decreased internalization of the receptor from 70 +/- 9% to 45 +/- 9% (P < .05, n = 7). In CHO-3B cells, enalaprilat was ineffective. ACE inhibitors in the presence of both the B2 receptor and ACE enhance BK binding, protect high-affinity receptors, block receptor desensitization, and decrease internalization, thereby potentiating BK beyond blocking its hydrolysis.

Biologic activities of iodinated analogues of Tyr0-bradykinin and bradykinin-Ile10-Tyr11 assessed in the rat uterus and the guinea pig ileum

1. The biological activity of bradykinin (BK) and analogues containing ofr in extended N- or C-terminal portions of the molecule, as well as that of their iodinated products, was compared in isolated rat uterus and guinea pig ileum preparations. 2. BK-Tyr10 and BK-Ile10-Tyr11 were obtained by solid phase synthesis employing fmoc chemistry. 3. Iodination of BK-Ile10-Tyr11 and Tyr0-BK was performed using iodobeads, and the products were purified by reverse-phase HPLC. 4. The relative potency (RP) of noniodinated analogues in the uterus was: Tyr0-BK (1.3) = BK (1.0) > BK-Ile10-Tyr11 (0.45) > > BK-Tyr10 (0.02) and BK (1.0) > BK-Ile10-Tyr11 (0.25) = Tyr0-BK (0.22) > > > BK-Tyr10 (0.002). The RP of mono-iodo (MI) and di-iodo (DI) products was: BK (1.0) > DI-BK-Ile10-Tyr11 (0.63) = DI-Tyr0-BK (0.63) > MI-Tyr0-BK (0.46) = MI-BK-Ile10-Tyr11 (0.40). 5. The RP of noniodinated analogues in the guinea pig ileum was: BK (1.0) > MI-Tyr0-BK (0.39) > MI-BK-Ile10-Tyr11 (0.17) = DI-Tyr0-BK (0.16) = DI-BK-Ile10-Tyr11 (0.13). 6. Differences in RP of 8-10 fold for Tyr0-BK or BK-Tyr10 and 2-fold for BK-Ile10-Tyr11 were observed between the two preparations used, indicating possible receptor differences. 7. Iodination caused a reduction in the RP of the analogues in both preparations. 8. In the rat uterus, the changes in the RP of the Tyr0-BK analogues were more evident than those observed with the iodinated analogues of BK-Ile10-Tyr11, indicating that iodination causes different changes in RP, according to the localization of the Tyr in the molecule. 9. The data support the idea that iodinated analogues of BK-Ile10-Tyr11, with intact N-terminal portion, may be as useful as iodinated analogues of Tyr0-BK for the study of BK receptors.

Cyclic AMP selectively enhances bradykinin receptor synthesis and expression in cultured arterial smooth muscle. Inhibition of angiotensin II and vasopressin response

Bradykinin receptors on vascular smooth muscle may play an important role in regulating the endogenous effects of the vascular kallikrein-kinin system. The present study examined the effect of cyclic nucleotides on bradykinin-stimulated responses in cultured arterial smooth muscle cells. Short term stimulation (1 min) with cyclic AMP produced a variable inhibition of bradykinin-stimulated calcium mobilization which was lost in later passaged cells. However, long-term stimulation (24 h) produced a consistent increase in bradykinin-stimulated calcium mobilization in both early and late passaged cells. Further analysis demonstrated that chronic exposure to cAMP produced a twofold increase in both the number of cell surface bradykinin receptors and in bradykinin-stimulated phosphoinositide hydrolysis. The increase in bradykinin receptors was time dependent (> 7 h) and blocked by protein synthesis inhibitors, suggesting that cAMP enhanced the synthesis of new bradykinin receptors. The increase in bradykinin receptor binding and calcium mobilization was also stimulated by cholera toxin, forskolin, and isobutylmethylxanthine, but not isoproterenol or prostaglandin E2. Of considerable interest, prolonged exposure to cAMP inhibited both angiotensin II and arginine vasopressin-stimulated phosphoinositide hydrolysis and intracellular calcium mobilization. In summary, prolonged treatment with cAMP selectively stimulates the synthesis and expression of bradykinin receptors on arterial smooth muscle while decreasing the responsiveness to vasoconstrictor agonists such as angiotensin II and vasopressin.

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.

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.

Effect of cell culture on rabbit colonic smooth muscle bradykinin receptors

The aim of this study was to assess the effect of cell culture on the bradykinin receptor of rabbit colon myocytes. In longitudinal muscle strips prepared from distal colon, bradykinin stimulated dose-dependent contraction that was 62% of the maximal response to bethanecol. At 4 degrees C, [3H]bradykinin binding to fresh muscle homogenates from the distal colon was time dependent, saturable, and linearly related to tissue concentration. Specific binding of 0.6 nmol/L [3H]bradykinin was 80% +/- 2% of total binding. In competitive binding studies, Hill coefficients approached unity, suggesting the presence of a single class of receptors. The order of potency was bradykinin greater than [D-Phe7]bradykinin much greater than des-Arg9, [Leu8]bradykinin, which is consistent with results of a B2 receptor subclass. Colon myocytes from the longitudinal muscle layer achieved confluence and were harvested for studies after 12-14 days in culture. Bradykinin receptors were of high affinity [disassociation constant (Kd) = 672 pmol/L] and numbered 10,217 +/- 2567/cell. To show that the receptors on cultured myocytes were functional, the effect of bradykinin was measured (a) on intracellular calcium concentration using Fura 2 and (b) on prostaglandin E2 concentration in the culture media using radioimmunoassay. In cells grown to confluence on cover slips and preloaded with Fura 2, bradykinin stimulated the threshold response at 1 nmol/L and maximal response (increased intracellular calcium concentration from 229 to 633 nmol/L) at 1 mumol/L. Bradykinin, 100 nmol/L, increased Prostaglandin E2 in the culture media threefold. In summary, colon myocytes express functioning bradykinin receptors, which, unlike muscarinic receptors, persist in culture. Bradykinin appears to be a suitable agonist for studies of receptor-mediated intracellular events in cultured colon myocytes.

Expression of functional bradykinin receptors in Xenopus oocytes

mRNA prepared from various tissues and cultured cells was injected into Xenopus laevis oocytes. Three to five days after injection, the response of the oocytes to the peptide bradykinin was monitored. The oocytes were voltage clamped and the membrane currents generated on application of agonist were recorded. mRNA from NG108-15, rat uterus, and human fibroblast cell line WI38 gave similar responses to bradykinin (1 microM), with an initial inward current (10-20 nA) followed by a prolonged period of membrane current oscillations. The same pattern of response was given by total RNA from rat dorsal root ganglia. No response to bradykinin (10 microM) was recorded from oocytes injected with rat brain mRNA, although these oocytes gave peak inward currents of about 75 nA in response to serotonin (10 microM). mRNA from both NG108-15 cells and rat uterus was fractionated on sucrose gradients. This resulted in an approximately five-fold increase in the size of the response compared to that given by unfractionated mRNA. The largest responses were given by mRNA fractions with a size of approximately 4.5 kb. Data were obtained consistent with the expression of both B1 and B2 receptors by WI38 human fibroblasts and with the expression of only the B2 type of receptor by NG108-15 cells.

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.

Bradykinin recognizes different molecular forms of the B2 kinin receptor in the presence and absence of guanine nucleotides

We have previously reported that [3H]bradykinin [( 3H]BK) identifies high- and low-affinity B2 kinin receptor sites in bovine myometrial membranes which are sensitive and insensitive respectively to guanine nucleotides. Here we show that these receptor-binding sites are solubilized by the detergent CHAPS. Equilibrium binding in soluble preparations revealed that [3H]BK identified a maximal number of binding sites (Bmax) of 1119 +/- 160 fmol/mg of protein, with an equilibrium dissociation constant (KD) of 314 +/- 70 pM and with a typical B2 kinin receptor specificity. Dissociation of equilibrium binding was biphasic. In the presence of the GTP analogue guanosine 5'[beta gamma-imido]triphosphate (Gpp[NH]p, [3H]BK bound to the soluble receptors with a KD of 929 +/- 129 pM and a Bmax. of 706 +/- 38 fmol/mg of protein. The Gpp(NH)p-promoted decrease in the apparent affinity and Bmax., which was half-maximal at 0.5 microM, was due at least in part to an increase in the dissociation rate of the slowly dissociating component of the equilibrium binding. Recoveries of guanine-nucleotide-sensitivity and of rapidly and slowly dissociating binding components were essentially identical, whether or not the receptor had been occupied by an agonist before solubilization. Sucrose-density-gradient sedimentation profiles revealed that [3H]BK recognized two different molecular forms of the receptor in the absence or presence of guanine nucleotides. These results provide for the first time direct evidence that guanine nucleotides promote a change in the structure of the B2 kinin-receptor complex. We propose that this structural change is due to dissociation of a guanine-nucleotide-regulatory (G-)protein.

Characterization of bradykinin receptors solubilized from rat uterus and NG108-15 cells

In this paper we report the solubilization of bradykinin B2 binding sites from membranes prepared from NG 108-15 tumours and rat uterus with retention of binding activity. Digitonin was found to solubilize the receptors from both tissues, and the addition of CHAPS increased the yield of soluble receptor from rat uterus only. The affinity of a range of bradykinin analogues has been shown to have the same rank order for both the soluble and membrane receptors from both tissues, and in corresponding functional assays. In addition, the binding of bradykinin ligands to the soluble and membrane receptors is similarly modulated by the presence of sodium ions. We conclude that the soluble binding sites correspond to the physiological bradykinin B2 receptor present in these tissues.

Construction of a physiologically active photoaffinity probe based on the structure of bradykinin: labelling of angiotensin converting enzyme but not candidate bradykinin receptors on NG108-15 cells

The peptides bradykinin and kallidin are released in response to noxious stimuli and mediate various physiological effects, including a direct stimulation of nociceptive afferent neurones. The nature of the receptor molecules through which these ligands act is presently unknown. We synthesised an iodinatable photoaffinity probe, N epsilon-4-azidosalicylylkallidin, and used it in an attempt to identify candidate bradykinin receptors on the NG108-15 neuroblastoma X glioma hybrid cell line. The ligand bound in subdued light to a particulate fraction of NG108-15 tumours and could be displaced by bradykinin with an IC50 of 0.33 nM. In a physiological assay, it behaved as an agonist equipotent with bradykinin. Gel analysis of the labelled products after photolysis of the iodinated ligand in the presence of NG108-15 cells or tumour membranes revealed bradykinin-blockable labelling of a glycoprotein with an Mr of 166,000. The probe was also able to label purified commercial angiotensin converting enzyme. The band labelled in NG108-15 cells was immunoprecipitable with a polyclonal antiserum to angiotensin converting enzyme, an enzyme shown to be present in low amounts in these preparations by direct binding using the iodinatable specific ligand MK351A.

The competitive antagonistic effect of compounds from Mandevilla velutina on kinin-induced contractions of rat uterus and guinea-pig ileum in vitro

1. Pure non-peptide compounds obtained in crystal form from the crude extract of the plant Mandevilla velutina (Apocynaceae) were analysed for their antagonistic effects on rat uterine and guinea-pig smooth muscle contractions induced by bradykinin (Bk), lisyl-bradykinin (L-Bk), acetylcholine (ACh), oxytocin and histamine, in vitro. 2. Pre-incubation of rat uterine muscle with compounds MV 8608, MV 8609, MV 8610, MV 8611 and MV 8612 (5 to 40 micrograms ml-1) caused parallel and concentration-dependent rightward displacements of the Bk concentration-response curves (1 to 1000 nM). Schild plots of these data were linear (correlation close to 1) and yielded nominal pA2 values (g ml-1) of 5.7, 5.6, 5.4, 5.7 and 5.3, respectively. Compounds MV 8608, MV 8611 and MV 8612 (5 to 20 micrograms ml-1) also caused concentration-dependent and parallel displacements to the right of the concentration-response curve to L-Bk (1 to 10,000 nM). The Schild plots were linear and furnished nominal pA2 values (g ml-1) of 5.4, 5.8 and 5.1, respectively. With the exception of the antagonist effect of compound MV 8606 against Bk-induced contraction, all compounds behaved as simple competitive kinin antagonists since the calculated slopes were not different from unity. 3. In the guinea-pig ileum, both MV 8608 and MV 8612 (5 to 20 micrograms ml-1), produced concentration-dependent rightward displacements of the concentration-response curve to Bk (0.1 to 1000 nM) when the experiments were performed in the presence but not in the absence of atropine (2.5 microM). However, in contrast to the result obtained in the rat uterus, compound MV 8608 also caused a significant reduction of the maximal response to Bk. The Schild plot for compound MV 8612 was linear (correlation close to unity) and furnished a nominal pA2 value (g ml-1) of 5.3 and a slope not different from unity. 4. In rat uterine muscle, compounds MV 8608 and MV 8612 at concentrations producing marked rightward displacements of the kinin concentration-response curves (10 and 20 micrograms ml-1), did not influence the uterine contractile response to oxytocin or ACh, indicating some selectivity towards kinin receptors. Similarly, compound MV 8612 (10 and 20 ygml 1) did not interfere with the sensitivities or the maximal responses to ACh and histamine in the guinea-pig ileum, whereas compound MV 8608 (10 and 20ug ml-1) caused a slight reduction of ACh- and histamine-induced maximal contractions, allied to decrease of the sensitivity to histamine at concentrations of 20pgml-1 or more. 5. These results extend our previous data, indicating the existence of several non-peptide compounds in the crude extract of Mandevilla velutina that act as simple, competitive, selective and reversible kinin receptor antagonists in the rat isolated uterus and guinea-pig ileum smooth muscle.

Bradykinin as a pain mediator: receptors are localized to sensory neurons, and antagonists have analgesic actions

Autoradiographic studies localize [3H]bradykinin receptor binding sites to the substantia gelatinosa, dorsal root, and a subset of small cells in both the dorsal root and trigeminal ganglia of the guinea pig. [3H]Bradykinin labeling is also observed over myocardial/coronary visceral afferent fibers. The localization of [3H]bradykinin receptors to nociceptive pathways supports a role for bradykinin in pain mediation. Several bradykinin antagonists block bradykinin-induced acute vascular pain in the rat. The bradykinin antagonists also relieve bradykinin- and urate-induced hyperalgesia in the rat paw. These results indicate that bradykinin is a physiologic mediator of pain and that bradykinin antagonists have analgesic activity in both acute and chronic pain models.

Bradykinin receptor antagonists used to characterize the heterogeneity of bradykinin-induced responses in rat vas deferens

The application of bradykinin to the isolated, transmurally stimulated rat vas deferens caused two effects: increase of the basal tension of the tissue and potentiation of the magnitude of electrically driven twitches. These bradykinin responses were not evenly distributed along the ductus. The direct contractile action of bradykinin was found to be stronger in the epididymal half of the tissue while the potentiation of the muscle twitches was more pronounced in the prostatic half of the rat ductus. Bradykinin is more potent to potentiate the electrically driven twitches than to act as a postjunctional agonist. Tyr-bradykinin, [Tyr5]bradykinin and [Tyr8]bradykinin exhibited significant differences in the potency ratio to produce each of these responses. [Thi5,8,D-Phe7]bradykinin is a weak postjunctional agonist but was a full agonist to potentiate the electrically induced twitches. Furthermore, this compound antagonized the bradykinin-induced contractions. [Hyp3,Thi5,8,D-Phe7]bradykinin was devoid of agonist activity at either pre- or postjunctional sites; it behaved as a pure antagonist and was more than potent its non-hydroxylated analog. The addition of a D-Arg residue at the amino terminal increased the antagonist potency significantly. The pA2 of D-Arg-[Hyp3,Thi5,8,D-Phe7]bradykinin to antagonize the postjunctional effect of bradykinin was 6.35, a value that differed significantly from the value of 6.93 required to block the prejunctional effect of the peptide. The bradykinin receptor antagonists did not modify significantly the magnitude of the contractile responses caused by angiotensin II, norepinephrine or 5-hydroxy-tryptamine.

Characterization of soluble bradykinin receptor-like binding sites

Bradykinin (BK) receptor-like binding sites were solubilized from a particulate fraction of bovine uterine myometrium (BUM) using the zwitterionic detergent 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate (CHAPS). Scatchard analysis of [125I-Tyr1]kallidin (T1K) binding revealed a single class of soluble binding sites with KD = 0.35 nM and Bmax = 0.13 pmol/mg protein. The soluble binding sites exhibited a kinin-binding specificity comparable to that of the particulate BUM receptor-like sites. Soluble T1K binding association kinetics were first-order at the four T1K concentrations examined. A plot of the pseudo-first order rate constant (Kobs) versus T1K concentration was linear, and values for the association (k1) and dissociation (k-1) rate constants were obtained. These rate constants yielded a kinetically derived equilibrium dissociation constant (KD = 0.64 nM) which was comparable to that obtained by Scatchard analysis. Biphasic dissociation of bound T1K was resolved into rapid and slow dissociation phases. The rate constant (k-1) of the rapid dissociation phase was comparable to the dissociation rate constant determined in association experiments. A biphasic loss of soluble T1K binding activity was observed with storage at 10 degrees C.

Specific binding and proteolytic inactivation of bradykinin by membrane vesicles from pig intestinal smooth muscle

A preparation of closed membrane vesicles derived from the longitudinal and circular smooth muscle of pig small intestine was enriched eight-fold in the activity of 5'-nucleotidase and six-fold in the activity of peptidyl dipeptidase A relative to the tissue homogenate. The membrane vesicles specifically bound [3H]bradykinin and the concentration of bradykinin required to inhibit 50% binding was 0.76 +/- 0.05 nM. This concentration was not significantly different from the corresponding concentration of lysyl-bradykinin (0.45 +/- 0.13 nM) but was less (P less than 0.05) than the concentration of methionyl-lysyl-bradykinin (1.25 +/- 0.10 nM). The concentration of des-Arg9 bradykinin (7.5 microM) required for 50% inhibition was greater than 10(3) times less than bradykinin indicating the presence of a B2-type receptor. The membrane vesicles also degraded bradykinin and the principal metabolite was identified as bradykinin. Des-Arg1 bradykinin, des-Arg9 bradykinin and bradykinin were also formed in low yield. Cleavage of the Pro7-Phe8 bond was inhibited by phosphoramidon but not by enalapril or captopril indicating that proteolytic inactivation of bradykinin in the muscle layer of the intestine is mediated through endopeptidase 24.11 ("enkephalinase") but not through peptidyl dipeptidase A ("angiotensin-converting enzyme").

[125I]Tyr-bradykinin binding in primary rat brain cultures

Kinins bind to specific, high affinity recognition sites in rat brain cell culture. Studies in these cultures minimize non-specific binding and degradation of the ligand. Binding of [125I]Tyr-bradykinin to intact cultured brain cells from neonatal rats was time- and pH-dependent. Scatchard analysis of saturation experiments yielded two affinity components with dissociation constant and maximum binding site concentration averaging 1 nM and 100 fmol/mg protein, and 16 nM and 1000 fmol/mg protein, respectively. The binding sites were specific for kinins and kinin analogues, and the order of potency in competing for [125I]Tyr-bradykinin binding was Lys-bradykinin greater than bradykinin greater than Tyr-bradykinin greater than Tyr8-bradykinin much much greater than Des-Arg9-bradykinin. Monovalent and divalent cations inhibited kinin binding. Comparison of competition curves performed in glial-enriched vs neuron-enriched cultures suggested that the kinin binding sites resided primarily on neurons. These data enhance the existing evidence suggesting kinins as neurotransmitters or neuromodulators.

B2 bradykinin receptor-like binding in rat renomedullary interstitial cells

A particulate fraction from cultured rat renomedullary interstitial cells (RRIC) was prepared for bradykinin (BK) binding studies. Incubation of three radiolabeled BK analogs, [125I-Tyr1]kallidin, [125I-Tyr5]-BK, and [125I-Tyr8]-BK, with the particulate fraction resulted in degradation of these peptides. Assay conditions which prevented hydrolysis of these radiolabeled kinins were determined. Under these conditions, direct binding studies were performed with [125I-Tyr1]kallidin (TlK) as the radioligand. BK binding affinity, apparent Kassoc. = 1.3 X 10(9) M-1, and specificity, determined with 51 BK analogs, were consistent with those expected of a B2 BK receptor.

Competitive antagonists of bradykinin

The first sequence-related competitive inhibitors of the classic kinin in vitro (rat uterus guinea pig ileum) and in vivo (rat blood pressure) assays have been developed. Replacement of the proline residue at position 7 of bradykinin (BK) with a D-phenylalanine residue is the key modification which converts BK agonists into antagonists. [D-Phe7]-BK exhibits moderate (pA2 = 5.0) inhibition of BK activity on the guinea pig ileum but possesses weak BK-like myotropic activity on the isolated rat uterus and 2-4% of BK depressor potency in the rat blood pressure assay. The additional replacement of the phenylalanine residues at positions 5 and 8 of [D-Phe7]-BK with the isosteric beta-(2-thienyl)-alanine residue produces a potent antagonist of BK activity on the uterus (pA2 = 6.4), ileum (pA2 = 6.3), and in the rat blood pressure assay. The antagonism of BK action on smooth muscle is specific for kinins (BK, kallidin, Met-Lys-BK), but neither inhibitor antagonizes the smooth muscle activity of angiotensin or substance P. Inhibition is competitive and fully reversible.

Further studies of myometrial bradykinin receptor-like binding

[125I-Tyr1]Kallidin (T1K), a bradykinin (BK) analog with biological potency comparable to BK, was used as a probe for BK receptor-like binding from bovine uterine myometrium. BK binding exhibited a high affinity, Kdissoc = 1.65 X 10(-10) M. The specificity of T1K binding was examined with forty-four BK analogs. Comparison of the binding inhibitory potencies with the relative biological potencies of these analogs on isolated rat uterus resulted in a good correlation, r = 0.87. BK binding activity was solubilized with CHAPS, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, a zwitterionic detergent. The solubilized binding activity exhibited a BK binding affinity, Kdissoc = 2.25 X 10(-10) M, and a specificity for three 125I-labeled kinins similar to those of the particulate BK receptor-like binding activity.

Iodination of [Tyr8]-bradykinin-comparison of chloramine-T and lactoperoxidase techniques

Antigen-antibody kinetics were studied using a hapten which was iodinated by two unique procedures. Using bradykinin, a vasopressor hormone as a model peptide, radioactive iodination (125I) of its 8-tyrosyl analogue was carried out both enzymatically and chemically using modified procedures. Two distinct chemical species were obtained which were characterized on a chromatographic, chemical as well as charge basis as a mono-iodinated form of [Tyr8]-bradykinin using the lactoperoxidase procedure and a di-iodinated entity using chloramine-T technique. The addition of a second iodine atom to the antigen lowers its immunoreactivity for its antibody and thus alters the kinetics of this reaction. Further experiments on the stability (temperature, time of storage, and chemical environment) of these iodinated peptides are described.

Interactions of kinins with angiotensin I converting enzyme (kininase II)

Angiotensin I converting enzyme (ACE) was purified to homogeneity from porcine kidney in order to determine whether iodobradykinins bind to the enzyme and, if so, whether pGlu-Trp-Pro-Arg-Pro-Gin-Ile-Pro-Pro, SQ20881, a competitive ACE inhibitor, changes the conformation of the enzyme in such a way that it binds kinins with an affinity and specificity expected of a bradykinin (BK) receptor, i.e. where the BK potentiating action of SQ20881 involves an increase in the number of BK receptors due to a conformational change in ACE. 125I-Labeled derivatives of [Tyr1]-kallidin and [Tyr-8]-bradykinin bound to the EDTA-inhibited enzyme, and binding was inhibited by nonradioactive BK. [125I-Tyr5]-BK was not bound by the enzyme. Specificity of [125I-Tyr5]-kallidin (T1K) binding was tested with forty-eight BK analogs, and the concentrations of analogs that inhibited 50% of T1K binding were determined. BK at 1.6 +/- 0.3 X 10(-8) M inhibited 505 of T1K binding. In addition, the concentrations of analogs that decreased by 50% the rate of [3H]-Hip-Gly-Gly ([3H]-HGG) hydrolysis by ACE were assessed. BK at 1.2 +/- 0.2 X 10(-6) M decreased the rate of [3H]-HGG hydrolysis by 50%. A comparison between these concentrations of analogs for inhibition of T1K binding and [3H]-HGG hydrolysis yielded a high correlation coefficient (r = 0.85). The specificity of ACE binding was clearly different from that expected of a BK receptor. Compounds structurally unrelated to BK, such as 5Q20881, pGlu-Lys-Trp-Ala-Pro-OH (BPP5a) and angiotensin I, inhibited T1K binding and [3H]-HGG hydrolysis by ACE.

Receptors for bradykinin in intact cultured human fibroblasts. Identification and characterization by direct binding study

Bradykinin receptors on cultured human fibroblasts were characterized using [2,3-prolyl-3,4-3H(N)]bradykinin as radioligand. During incubation with intact fibroblasts, intact [3H]bradykinin was lost much more rapidly at 37 degrees than at 4 degrees C as determined by bioassay, high-performance liquid chromatography, and ion-exchange chromatography, and is likely to be degraded. At 4 degrees, but not at 37 degrees C, bradykinin remained intact in the presence of 2 mM bacitracin, but not in the presence of soybean trypsin inhibitor or SQ-20881, an inhibitor of kininase II. Specific binding at 4 degrees C was saturable with a maximum number of binding sites of 230 +/- 18 fmol/mg protein (mean +/- SE, n = 4) and a dissociation constant of 4.6 +/- 0.5 nM (mean +/- SE, n = 4). Linear Scatchard plots, Hill coefficients close to unity (0.95-1.06), and the failure of excess bradykinin to influence dissociation kinetics are consistent with a single component binding system with no significant cooperativity. Na+ at physiological concentrations and Ca++ or Mg++ at 3-10 mM reduced binding by 25%. The relative potencies of bradykinin analogues and unrelated peptides in competing for [3H]bradykinin binding indicated a specificity of the binding sites consistent with that of a B2 type receptor. Potencies of the peptides in displacing [3H]bradykinin correlated with their abilities to release prostacyclin, determined as its metabolite 6-keto-PGF1 alpha. This system, the first in which bradykinin receptors on human cells have been characterized, should prove useful for investigation of the regulation of bradykinin-influenced biological processes.

Effect of angiotensin-converting enzyme inhibition on circulating and local kinin levels

Angiotensin-converting enzyme kininase II reduces bradykinin metabolism in vitro and in vivo. However, consistent changes in circulating bradykinin levels after the angiotensin-converting enzyme inhibitor captopril have not been reported. The kallikrein-kinin system has been suggested to be a local hormonal system concerned with regional blood flow, and hence circulating levels may not reflect local tissue levels of kinins. Anesthetized dogs given captopril had a significant increase in urinary kinin excretion without a change in circulating bradykinin levels or in urinary kallikrein. These changes in renal kinins were accompanied by a decrease in blood pressure and renal vasodilation. The hypotension and renal vasodilation produced by captopril were not attenuated either by pretreatment with the angiotension receptor antagonist Sar1-Ileu8-angiotensin II or by reduction of endogenous prostaglandin production with indomethacin. Postischemic renal vasodilation after temporary renal artery occlusion was also associated with increased urinary kinin levels. These results demonstrate that captopril effectively inhibits renal angiotensin-converting enzyme and that the renal kallikrein-kinin system may play an important role in regulating the renal vasculature and may contribute to the renal hemodynamic effects of captopril. Many polypeptide hormone membrane receptors are self-regulated by endogenous tissue concentrations of the peptide hormone. Infusions of bradykinin into rats reduced specific bradykinin receptors. A similar decrease in bradykinin receptor numbers without change in receptor affinity was demonstrated after captopril administration. These results provide indirect evidence that angiotensin-converting enzyme/kininase inhibition by captopril increases local tissue concentration of kinins, which may contribute to the hypotensive effect.

Decreased uterine bradykinin receptors during captopril infusion in rats

To assess whether local tissue changes in bradykinin metabolism occur with captopril, the effect of intravenous infusion of bradykinin and captopril on bradykinin receptors in rat uterine smooth muscle was compared. Bradykinin and captopril decreased both the blood pressure and the number of bradykinin uterine receptors, estimated by direct radiolabeled binding techniques, without changing receptor affinity. These results demonstrate that bradykinin "down-regulates" its membrane receptor in rat myometrium. The similar change in receptor numbers induced by captopril could be explained by a local increase in bradykinin at receptor sites. The increase in local tissue level of bradykinin may contribute to the hypotensive action of captopril.

[3H]Bradykinin receptor binding in mammalian tissue membranes

[3H]Bradykinin binds to membranes from a variety of mammalian tissues in a saturable fashion with a dissociation constant of about 5 nM. Highest levels of binding are detected in guinea pig ileum, colon, and duodenum and in the oestrus rat uterus. The relative potencies of bradykinin derivatives in competing for these binding sites in guinea pig ileum membranes correlate with their contractile potencies in the ileum better than with contractile effects in the uterus. Thus, receptor recognition sites may differ in ileum and uterus. Monovalent and divalent cations at physiological concentrations reduce [3H]bradykinin binding. Of the monovalent cations, medium lowers binding 50% at about 80 mM. Among the divalent cations, calcium lowers binding about 50% at 5 mM, suggesting a link to the calcium conductance channel.