Tachykinin receptors in smooth muscles: a study with agonists (substance P, neurokinin A) and antagonists

Four preparations, sensitive to tachykinins, the guinea-pig urinary bladder, the rat duodenum, the hamster and dog urinary bladders have been investigated and compared with four other preparations described before: the guinea-pig ileum and trachea, the dog carotid artery and the rabbit mesenteric vein. On the basis of the order of potency of agonists, evaluated with substance P, physalaemin, eledoisin, kassinin and neurokinin A, the preparations can be separated into three groups, the guinea-pig urinary bladder and the dog carotid artery, in which substance P is the most potent and neurokinin A the weakest tachykinin, the rabbit mesenteric vein, the guinea-pig trachea and the rat duodenum, in which the opposite order is observed and the hamster and dog urinary bladders, in which kassinin is the most potent agonist. The guinea-pig ileum shows similar sensitivity to the five tachykinins. C-terminal partial sequences appear to be weaker than SP-(1-11) in three of the four new preparations, SP-(6-11) being first in the rat duodenum and slightly weaker than SP-(1-11) in the hamster and dog urinary bladders. Studies performed with antagonists or inhibitors of endogenous agents suggest that substance P and neurokinin A act directly on specific receptors. The effects of the two peptides are reduced by antagonists analogues of the sequence SP-(4-11). One of the antagonists, [D-Pro4,Lys6,D-Trp7,9,10, Phe11]SP-(4-11) has been shown to be competitive against substance P and neurokinin A in the guinea-pig ileum, the guinea-pig urinary bladder and the rat duodenum. This compound, shows definitely higher activity against neurokinin A and kassinin, compared to substance P in various preparations. [D-Tyr4,D-Trp7,9,Nle11]SP-(4-11) is the most potent tachykinin antagonist in the hamster and dog urinary bladders. In these preparations, the antagonists act also against substance P, but with lower affinity. These findings with antagonists support the indication, emerged from the order of potency of agonists, that tachykinins may act on two and possibly three different receptor types.     

A pharmacological study of NK1 and NK2 tachykinin receptor characteristics in the rat isolated urinary bladder.

1. We have estimated potencies of tachykinin receptor agonist and antagonist analogues in order to determine the recognition characteristics of tachykinin receptors mediating phasic contractile responses of the rat isolated urinary bladder in vitro. 2. The NK1-selective synthetic agonists, substance P methyl ester and GR73632, the synthetic NK2-selective agonists [beta-Ala8]-NKA(4-10) and GR64349, and the mammalian tachykinins, neurokinin A and neurokinin B, were assayed relative to substance P and were found to be approximately equipotent. The NK3-selective agonist, senktide, was inactive (10 microM). 3. Potencies of all these agonists were not significantly different (P > 0.05) when experiments were carried out in the presence of the neutral endopeptidase inhibitor, phosphoramidon, and the kininase II inhibitor, enalaprilat (both 1 microM). 4. The NK1-selective antagonist, GR82334, inhibited responses to substance P methyl ester in a competitive manner in the rat urinary bladder and the rat ileum, and also in the guinea-pig ileum. Markedly different pKB estimates were obtained in the rat bladder (6.38) and rat ileum (6.56) compared to the guinea-pig ileum (7.42). GR82334 (3 microM) was inactive against responses of the rat bladder to [beta-Ala8]-NKA(4-10). 5. The NK1-selective antagonist (+/-)-CP-96,345 also inhibited responses of the rat bladder and guinea-pig ileum to substance P methyl ester; however, in the rat bladder at 1 microM, this antagonist reversibly inhibited responses both to the NK2-selective agonist [beta-Ala8]-NKA(4-10) and to the muscarinic agonist carbachol (P < or = 0.01), thus showing evidence of some non-selective depressant actions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for the involvement of endogenous substance P in the motor effects of capsaicin on the rat urinary bladder

[pro4,trp7,9,Leu11]SP-(4-11), a substance P (SP) antagonist, selectively antagonized contractions produced by either capsaicin or SP on the rat isolated urinary bladder. These experiments provide direct evidence indicating that the motor effects of capsaicin on rat urinary bladder are attributable, at least in part, to the release of endogenous SP.     

A comparison of the effects of three substance P antagonists on tachykinin-stimulated [3H]-acetylcholine release in the guinea-pig ileum

The potencies of three tachykinin antagonists [D-Pro4,D-Trp7,9,10]SP(4-11), [D-Arg1,D-Pro2,D-Trp7,9,Leu11]SP(1-11) and [D-Arg1,D-Trp7,9,Leu11]SP(1-11) (spantide) against eledoisin were examined in the guinea-pig ileum myenteric plexus, where a continuous superfusion system was employed to examine evoked release of [3H]-acetylcholine [( 3H]-ACh]); effects on mechanical activity of the preparations were also measured. Eledoisin was chosen as the standard tachykinin agonist since the rank order of potency observed in evoking release was eledoisin, kassinin, substance P, physalaemin; on this basis is may be presumed that an 'SP-E' type receptor was involved in the release process. The two undecapeptide antagonists both significantly reduced the response to eledoisin (10 nM) as assessed by both [3H]-ACh release and mechanical activity which under these conditions was largely dependent on ACh release, and the response levels could be restored by increasing the concentration of eledoisin to 100 nM. The pA2 values for the two antagonists were estimated as 5.3 for [D-Arg1,D-Pro2,D-Trp7,9,Leu11]SP(1-11) and 5.2 for [D-Arg1,D-Trp7,9,Leu11]SP(1-11). [D-Pro4,D-Trp7,9,10]SP(4-11) was markedly less potent with a pA2 value of less than 4.8. All three antagonists possessed considerable inherent stimulatory activity as measured both by [3H]-ACh release and mechanical activity, [D-Pro4,D-Trp7,9,10]SP(4-11) being the most active in this respect, a 10 microM concentration producing 50% of the response seen with 10 nM eledoisin. These findings are discussed both in relation to tachykinin receptor classifications and limitations in the use of such antagonists in the study of the role of tachykinins in neurotransmission.     

Receptors for substance P. III. Classification by competitive antagonists

A series of ten octa- and five undecapeptide antagonists of SP have been tested in four isolated smooth muscle preparations in order to characterise the receptors mediating the SP-induced contractions of the guinea pig ileum (G.P.I.), the guinea pig trachea (G.P.T.) and the rabbit mesenteric vein (R.M.V.) and the relaxation of the dog carotid artery (D.C.A.). It has been shown that: (a) Indirect effects by substance P and related peptides, mediated by acetylcholine or prostaglandins reduce the affinity of [pro4,trp7,9,10]SP (4-11) (an SP-antagonist) only in the G.P.I. (b) Octapeptide antagonists are specific for SP, have no agonistic activities and exert a competitive antagonism against SP, its homologues and fragments. (c) Undecapeptide antagonists are weaker than the octapeptides in the G.P.I. and G.P.T. and slightly stronger in the D.C.A. and R.M.V. However these compounds still have variable degrees of agonistic activity in some tissues. Affinity of octapeptide antagonists bearing the basic structure [pro4,trp7,9]SP-(4-11) is increased by the additional replacement of Leu10 with trp, Met11 with Leu, Nle or Phe, but it is slightly reduced by substituting Phe8 with Val. Antagonists containing aliphatic residues at the C-terminal end, for instance [pro4,trp7,9,Nle11]SP-(4-11) are more potent than others in the R.M.V., while those with aromatic residues, for instance [pro4,trp7,9,10]SP-(4-11) are weak on the R.M.V. but fairly active on the G.P.T. These antagonists do not show any selectivity on the G.P.I. and the D.C.A. Comparison of antagonists affinities for receptor characterisation suggest the existence of three different functional sites for SP-related peptides. The site of the G.P.I. and D.C.A., which accepts both the antagonists containing aromatic or aliphatic groups at the C-terminal end; the site of the G.P.T. which prefers the aromatic and that of the R.M.V. which shows high affinity for aliphatic residues. The receptor classification emerging from data obtained with antagonists is compared with the classifications of the literature and with those based on the order of potencies of SP homologues and fragments.     

Substance P--structure-activity studies and the development of antagonists

Various attempts to developing antagonists for substance P (SP) and related peptides, based on our past experience with angiotensin, kinins, and neurotensin, were unsuccessful. The particular features of SP, namely the high activity of C-terminal partial sequences in some pharmacological tests were used to develop one hexa and several octapeptide antagonists. Weak antagonists were obtained with a single modification, namely the replacement of Leu10 with trp in the sequences SP (6-11), SP (4-11) and SP (1-11). The affinity of octa and undecapeptide antagonists could be increased by using two (in positions 7 and 9 or 7 and 10) or 3 (in position 7, 9 and 10) substitutions of the natural residues with trp. Affinity of antagonists was further increased by replacing Met11 with either Nle or Phe. These new compounds showed some selectivity, [pro4, trp7 ,9, Nle11 ]-SP (4-11) being more potent in the rabbit mesenteric vein than all other octapeptides described in the present study; on the other hand, [pro4, trp7 ,9,10,Phe11]-SP (4-11) was found to be the most potent antagonist of SP and related peptides in the guinea pig ileum and the guinea pig trachea. Both compounds were similarly active in the dog carotid artery. Undecapeptide antagonists, bearing the same structural modifications as the octapeptides, were found to be stimulant in the guinea pig trachea and relaxant in the dog carotid artery. The agonistic property was eliminated by repeated applications of the compounds in guinea pig tracheae, and therefore the compounds could be tested as antagonists. The undecapeptides were found to be much more active antagonists against kasinin and eledoisin than against SP and physalaemin. The data obtained with the octa and undecapeptide antagonists of SP have been used for identification and characterization of SP receptors in various smooth muscles. It appears that SP and related peptides may exert their numerous pharmacological effects by activating more than one receptor type.     

Tachykinin receptors of the NK2 type involved in the acetylcholine release by nicotine in guinea-pig bladder.

1. The effects of guanethidine and tachykinins on nicotine- and electrical stimulation-induced cholinoceptor responses were studied in isolated urinary bladder from the guinea-pig. 2. Acetylcholine release and the contractile response stimulated by nicotine were partially reduced by a sympathetic nerve blocker, guanethidine. Neurokinin A (but not substance P methyl ester or senktide) enhanced both acetylcholine release and contraction by nicotine in the presence of guanethidine. 3. Frequency-contraction curves (1 to 50 Hz) for electrical field stimulation (EFS) were partially reduced by atropine (1 microM), and after desensitization to alpha,beta-methylene adenosine 5'-triphosphate, the atropine-resistant contraction to EFS was completely abolished. Guanethidine, the tachykinin antagonist [D-Arg1, D-Pro2, Trp7,9, Leu11]-substance P and application of neurokinin A or substance P did not change the contractile response to EFS. Preganglionic nerve stimulation (5 Hz and 20 Hz) also evoked a similar response to EFS and was not influenced at all by guanethidine or neurokinin A. 4. We conclude that the ability of nicotine to release acetylcholine is enhanced both by endogenous tachykinins (probably released from sympathetic nerves) and by exogenously applied tachykinins as a result of interaction with NK2 receptors in the urinary bladder.     

Tachykinin antagonists have potent local anaesthetic actions

Contrary to what would have been expected, an antagonist of substance P (SP) [Arg5,D-Trp7,9]SP-(5-11) inhibited the neurogenic contraction of isolated guinea-pig hilus bronchi more readily than a contraction produced by exogenous SP. Furthermore, it has previously been shown that a tachykinin antagonist given intrathecally produced motor blockade as do local anaesthetic drugs. We therefore examined whether tachykinin antagonists had a depressant action on axonal neurotransmission. The compound action potential (APc) of the frog isolated sciatic nerve was suppressed in a concentration-dependent manner by the tachykinin antagonists [D-Pro2,D-Trp7,9]SP and [Arg5,D-Trp7,9]Sp-(5-11), both being about 4 times more potent than lidocaine. SP itself was without effect. Similarly in the rat isolated sciatic nerve [D-Pro2,D-Trp7,9]SP suppressed the APc. It was more potent in the A alpha- than in the C-fibres. SP did not affect conduction in either fibre type. In conscious guinea-pigs [D-Pro2,D-Trp7,9]SP injected adjacent to the sciatic nerve was found to block motor but not sensory functions of the limb. Thus, commonly used tachykinin antagonists, but not SP itself, have potent local anaesthetic properties. This should be considered when these agents are employed as pharmacological tools.     

Pharmacological characterization of tachykinin-stimulated inositol phospholipid hydrolysis in peripheral tissues.

1. Tachykinin-stimulated inositol phospholipid hydrolysis was examined in slices of rat parotid gland, hamster urinary bladder and guinea-pig ileum longitudinal muscle. 2. In the presence of lithium, substance P and other naturally-occurring and synthetic tachykinins induced large, dose-dependent increases in [3H]-inositol monophosphate accumulation. 3. In slices of rat parotid gland, [pGlu6,L-Pro9]SP(6-11) was considerably more potent in stimulating inositol phospholipid hydrolysis than [pGlu6,D-Pro9]SP(6-11). 4. In contrast, in slices of hamster urinary bladder, [pGlu6,D-Pro9]SP(6-11) exhibited greater potency in evoking inositol phospholipid breakdown than [pGlu6,L-Pro9]SP(6-11). 5. The differential selectivity of these C-terminal fragments of substance P suggests that they may be useful tools for distinguishing between NK1 and NK2 receptors. 6. L-659,837 and L-659,874 antagonized eledoisin-stimulated inositol phospholipid hydrolysis in slices of hamster urinary bladder. Neither compound significantly reduced substance-P evoked inositol phospholipid breakdown in slices of rat parotid gland, or senktide-induced inositol phospholipid hydrolysis in slices of guinea-pig ileum. 7. L-659,837 and L-659,874 had no effect on the atropine-sensitive, carbachol-stimulated inositol phospholipid hydrolysis in slices of rat parotid gland. 8. These data further support the notion that L-659,837 and L-659,874 are potent and selective NK2 receptor antagonists.     

Effect of nepadutant at tachykinin NK(2) receptors in human intestine and urinary bladder

We have characterized the action of the tachykinin NK(2) receptor antagonist nepadutant (c?[(beta-D-GlcNAc)Asn-Asp-Trp-Phe-Dpr-Leu]c(2 beta-5 beta)?) in the human isolated ileum, colon and urinary bladder. Nepadutant (30-1000 nM) competitively antagonized neurokinin A- or [beta Ala(8)]neurokinin A-(4-10)-induced contractions in all tissues, with pK(B)=8.3 (ileum and colon) and pK(B)=8.5 (bladder). In contrast, the nonpeptide tachykinin NK(2) receptor antagonist SR 48968 (or (S)-N-methyl-N [4-acetylamino-4-phenylpiperidino)-2-(3, 4-dichlorophenyl) butyl] benzamide) (30-1000 nM) produced insurmountable antagonism in all preparations. The tachykinin NK(2) receptor blockade produced by nepadutant in the colon was fully reversed by washout, whereas that produced by SR 48968 was not. Nepadutant (1 microM) greatly reduced (by 70-80%) the nonadrenergic noncholinergic (NANC) contractile off-response evoked by electrical field stimulation in the human ileum, and almost abolished it in the presence of the tachykinin NK(1) receptor antagonist GR 82334 (or: [[(S,S) Pro-Leu (spiro-gamma-lactam)](9,10),Trp(11)]Physalaemin (1-11)) (1 microM). The present results show that nepadutant is a potent, competitive and reversible antagonist at human tachykinin NK(2) receptors and provide further evidence that tachykinins act as excitatory NANC neurotransmitters in the human small intestine.     

Characterization of tachykinin receptors in isolated basilar arteries of guinea-pig.

In circular lengths cut from the basilar artery of guinea-pig (0.2-0.3 mm o.d.) relaxations induced by substance P and neurokinin A were highly susceptible to mechanical damage of the endothelium by rubbing. The precontraction induced by prostaglandin F2 alpha but not that of 124 mM potassium was reduced considerably by the rubbing procedure. Concentration-dependent relaxations were evoked by tachykinin agonists in the following order of potency: substance P = physalaemin greater than neurokinin A greater than eledoisin. Physalaemin was, however, a partial agonist, giving only half the maximum relaxation as compared to the other tachykinins. The two putative tachykinin receptor antagonists, spantide ([D-Arg1, D-Trp7,9, D-Leu11] substance P) and [D-Pro2, D-Trp7,9] substance P, shifted the concentration-dependent relaxations of substance P to the right in a parallel manner. Calculation of pA2 values and Schild plot analysis revealed pA2 values of 7.4-7.6 for spantide and 6.9-7.0 for [D-Pro2, D-Trp7,9] substance P, irrespective of whether substance P or neurokinin A was used as agonist. The pA2 values and the Schild plot analysis suggest a specific interaction between tachykinin agonists and antagonists that follow a simple bimolecular process. The results suggest the presence of tachykinin receptors of the 'SP-P' type in guinea-pig basilar arteries which, for induction of relaxation, involves the release of an endothelium-derived relaxing factor.     

Undeca- and octa-peptide antagonists for substance P, a study on the guinea pig trachea

Undeca - and octa-peptide analogues of substance P (SP), acting as antagonists in the guinea pig ileum, have been tested on the guinea pig trachea. The antagonistic properties of several octapeptides, particularly of [pro4, trp7 ,9,10,Phe11]SP-(4-11) have been confirmed, while the undecapeptides have been found to be potent stimulants of the trachea. The contractions of the guinea pig trachea in response several undecapeptides , particularly [pro2, trp7 ,9, Leu11 ]SP, undergo rapid tachyphylaxis, are significantly reduced in the presence of diphenhydramine and are not influenced by octapeptide antagonists of SP. These contractions appear to be due to the activation of tissue sites mediating the release of intramural histamine and different from SP receptors. On repeated applications, the stimulant effects of undecapeptides are eliminated and the compounds can be tested as antagonists. Undecapeptide antagonists have been found to be more potent against eledoisin and kassinin than against SP or physalaemin, while the octapeptides are equally active against the four homologues. Both undeca - and octa-peptides seem however to exert a competitive type of antagonism against all the SP-related peptides tested in the present study. Differences of antagonistic affinities have been interpreted as indicative of the existence of two different receptor types for SP and related peptides in the guinea pig trachea. The two receptors are blocked by the octapeptide antagonists, which are not discriminatory, while undecapeptides are particularly active on the receptor subtype which shows high sensitivity for eledoisin and kassinin .     

Interaction of tachykinins with an adrenergic receptor in the rat urinary bladder

The rat urinary bladder was examined as a model for studying tachykinin receptors. The order of potency, the maximal effect and the slope of the dose-response curve were examined with six tachykinins - substance P (SP), physalaemin, phyllomedusin, uperolein, eledoisin, kassinin - and several substance P fragments - SP-(2-11), SP-(3-11), SP-(4-11) and SP-(6-11). The tachykinin receptor on the rat urinary bladder was shown to bind preferentially tachykinins having a hydrophilic amino acid residue in position 5-6, as occurs with physalaemin, phyllomedusin, eledoisin and kassinin. The N-terminal of the tachykinins and in particular substance P is suggested to play a major role in regulating affinity, intrinsic activity and the slope of the dose-response curve. The tachykinins are thought to exert a direct action on smooth muscle. An accessory binding site associated with the tachykinin receptor on rat urinary bladder was also identified. This accessory site binds the N-terminal amino acids of the tachykinins as well as some alpha-adrenergic compounds (phentolamine, prazosin, noradrenaline or adrenaline in the presence of propranolol). When the accessory binding site is occupied by adrenergic compounds, the affinity of the tachykinins is markedly reduced. This observation is interpreted to mean that catecholamines may have a modulatory influence on tachykinin activity on the rat urinary bladder.     

A pharmacological study with substance K: evidence for multiple types of tachykinin receptors

The substance P antagonist [D-Arg,D-Pro,D-Trp,Leu]SP (RPTTL-SP) produced a simple competitive antagonism of the responses to substance K (SK) and substance P (SP) in a variety of tissues. In the guinea-pig ileum and field stimulated rat vas deferens the pKB values against SK were similar to those against SP indicating a single population of receptors. In the guinea-pig urinary bladder the pKB value against SK was significantly higher suggesting the existence of at least two subtypes of tachykinin receptor in this tissue. In the hamster urinary bladder no antagonism was observed with up to 30 microM RPTTL-SP indicating the possibility of a third type of receptor.     

Pharmacological specificity of novel, synthetic, cyclic peptides as antagonists at tachykinin receptors.

1. The interaction at tachykinin receptors of a series of novel cyclic hexapeptides has been examined by use of radioligand binding assays (NK1 and NK3 sites in rat cortex, NK2 sites in hamster urinary bladder) and functional pharmacological assays (guinea-pig ileum, rat vas deferens and rat portal vein for NK1, NK2 and NK3 receptors, respectively). 2. The compounds cyclo(GlnTrpPhe(R)Gly[ANC-2]LeuMet) (L-659,837) and cyclo(GlnTrpPheGly-LeuMet) (L-659,877) were powerful and selective displacers of NK2 binding (pIC50 6.9 and 8.0, respectively), and were competitive antagonists of responses to stimulation of NK2 receptors in rat vas deferens (pKB for antagonism of responses to eledoisin 6.7 and 8.1, respectively). Responses in the NK1 and NK3 pharmacological assays were blocked only weakly, if at all. 3. In the longitudinal muscle of the small intestine of the rat, responses to stimulation of the putative NK2 receptor by eledoisin, neurokinin A or neurokinin B were antagonized by both cyclo(GlnTrpPhe(R)-Gly[ANC-2]LeuMet) and cyclo (GlnTrpPheGlyLeuMet) in a manner consistent with the presence in this tissue of a uniform population of receptors, indistinguishable from the NK2 receptor of the rat vas deferens. 4. The compounds cyclo(GlnTrpPheGlyLeuMet) and the lactam-containing analogue are among the most selective antagonists for the NK2 receptor that have been described; their availability should be of value in the characterization of the receptors mediating responses to tachykinins, and in elucidating the physiological functions of the tachykinin receptors.     

Tachykinin antagonist I: Specific, competitive and tissue-selective neurokinin B antagonists on contractile activity in smooth muscles

Two neurokinin B (NKB) analogs, [Gly6]-NKB [3-10] and [Arg3, D-Ala6]-NKB [3-10], were tested for agonistic activity as well as for their ability to antagonize the myotropic actions of NKB, neurokinin A, substance P, physalaemin and eledoisin in isolated guinea-pig ileum, guinea-pig urinary bladder, rat duodenum, rat vas deferens and rat portal vein. [Gly6]-NKB [3-10] in the guinea-pig ileum and rat portal vein and [Arg3, D-Ala6]-NKB [3-10] in the guinea-pig ileum were found to be the first specific and competitive antagonists against NKB.     

Species differences between [3H] substance P binding in rat and guinea-pig shown by the use of peptide agonists and antagonists.

The affinities of various substance P agonists and antagonists for NK1 receptors in rat and guinea-pig tissues were compared. Striking species differences were observed. Both septide and [D-Pro4,D-Trp7,9]SP-(4-11) possessed much higher affinity for sites in the guinea-pig (brain and ileum) than for sites in the rat brain. These results could be explained by differences in the structure of the NK1 receptor according to the species, although the existence of various subtypes of NK1 binding sites in the two species cannot be excluded.     

Synthesis and biological activities of substance P antagonists

Several substance P analogues containing various D-amino acid modifications have been synthesized by the solid-phase procedure, detached from the solid support by ammonolysis, and purified by gel filtration combined with reversed-phase chromatography. Three compounds were fair to very potent competitive antagonists of substance P on three bioassays, i.e., guinea pig ileum, rabbit mesenteric vein, and guinea pig trachea. [Arg6,D-Trp10]SP(6-11) is a reasonable antagonist in all three bioassays and [D-Pro4,D-Trp7,9]SP(4-11) is a very potent competitive antagonist with pA2 values ranging around 6.0.     

Use and limitations of three TRPV-1 receptor antagonists on smooth muscles of animals and man: a vote for BCTC

Specificity of the effect is a crucial factor in using antagonists for detecting the physiological/pathophysiological roles of receptors. Here we examined the capsaicin receptor antagonist effects of three commercially-available substances, capsazepine, iodo-resiniferatoxin (I-RTX) and BCTC, on isolated smooth muscle preparations, including the human intestine. Care was taken to observe possible non-specific effects, to find out safe and effective concentrations. Capsazepine appeared to have a low margin of safety. I-RTX (up to 1μM) specifically inhibited capsaicin-induced contractions in the guinea-pig ileum and urinary bladder. I-RTX showed agonist activity on the rat urinary bladder. BCTC (1μM) abolished the contractile effects of capsaicin (1 or 2μM) on all preparations tested (guinea-pig ileum, bladder, trachea, as well as rat and mouse bladder), and on the guinea-pig renal pelvis, where it failed to influence capsaicin-sensitive, sensory neuron-mediated positive inotropy in response to field stimulation. On human intestinal preparations BCTC prevented the relaxant effect of capsaicin. It is concluded that of the three antagonists tested BCTC seems the safest one for inhibiting TRPV-1 receptors. The effect of capsazepine may be complicated by non-specific inhibition of smooth muscle contractility and that of I-RTX by agonist activity. The "local efferent" function of capsaicin-sensitive sensory neurons is not influenced by BCTC, as shown by the results obtained in the renal pelvis. In conclusion, of the TRPV-1 receptor antagonists studied, BCTC (1μM) seems the most reliable in isolated organ experiments. This substance is also effective in the human intestine.     

Tachykinin Receptor Antagonists and Cough

Several potent and selective antagonists for tachykinin receptors are now available and appear as powerful tools to investigate the physiological and pathological roles of tachykinins and to identify the type of receptor involved in their effect. Indeed, a lot of studies have shown that tachykinin NK₂receptor antagonists (SR 48968, MEN 10627) are able to inhibit cough induced by citric acid, capsaicin or allergen challenge in the unanesthetized guinea-pig or mechanical stimulation of the trachea in the cat. The effects of tachykinin NK₁receptor antagonists are still debated, whereas an inhibitory effect of SR 142801, a tachykinin NK₃receptor antagonist, has been reported against citric acid-induced cough in the guinea-pig. Experiments with tachykinin receptor antagonists which do not cross the blood brain barrier suggest that the site of action of tachykinin receptor antagonists is most probably peripheral, but a central action, at least in an area not protected by the blood brain barrier, cannot be excluded. Finally, tachykinin NK₂receptor stimulation seems to be involved in sensitisation of cough reflex.