Tachykinin NK2 receptors predominantly mediate tachykinin-induced contractions in ovine trachea

[Lys5,MeLeu9,Nle10]-NKA(4-10) Elicits NK2 Receptor-Mediated Micturition and Defecation, and NK1 Receptor-Mediated Emesis and Hypotension, in Conscious Dogs

Tachykinin neurokinin 2 (NK2) receptor agonists may have potential to alleviate clinical conditions associated with bladder and gastrointestinal underactivity by stimulating contraction of visceral smooth muscle. The ability of [Lys⁵,MeLeu⁹,Nle¹⁰]-neurokinin A_(4-10) (LMN-NKA) to elicit micturition and defecation was examined after repeated administration in groups of 2-10 conscious dogs. Administration of 10-100 μg/kg, i.v., four times daily for six consecutive days, reliably elicited micturition after ≥90% of doses and defecation after ≥50% of doses. Voiding occurred <4 minutes after dosing and was short lasting (<10 minutes). LMN-NKA was well tolerated, with emesis after ∼25% of doses at 100 μg/kg, i.v. Hypotension was induced by 100 μg/kg, i.v., of LMN-NKA but not by lower doses. Administration of 30-300 μg/kg, s.c., twice daily for seven consecutive days, reliably elicited both urination and defecation after 88%-100% of doses, and was accompanied by a high rate of emesis (50%-100%). The onset of voiding was rapid (<7 minutes) but was more prolonged than after intravenous administration (30-60 minutes). Emesis induced by 30 or 300 μg/kg, s.c., of LMN-NKA was significantly reduced (from 58% to 8% and from 96% to 54%, respectively) by a 30-minute pretreatment with the neurokinin 1 (NK1) receptor antagonist, (2S,3S)-N-(2-methoxybenzyl)-2-phenylpiperidin-3-amine (CP-99,994; 1 mg/kg, s.c.). The ability of selective NK2 receptor agonists to elicit on-demand voiding could potentially address a major unmet need in people lacking voluntary control of micturition and/or defecation. LMN-NKA unexpectedly activated NK1 receptors at doses that stimulated voiding, causing emesis and hypotension that may limit the clinical utility of nonselective NK2 receptor agonists.

Involvement of Ca2+ signaling in tachykinin-mediated contractile responses in swine trachea

Neuropeptide tachykinins, present within sensory nerves, have been implicated as neurotransmitters involved in nonadrenergic and noncholinergic airway muscle contraction. The signal transduction pathways of tachykinins on muscle contraction and Ca2+ mobilization were investigated in swine trachea. Tachykinins, substance P (SP) and neurokinin A (NKA), concentration (1 nM to 1 microM)-dependently induced contractile responses with removal of epithelium, whereas neurokinin B (NKB) did not alter the muscle tension. The SP- and NKA-evoked muscle contractions were inhibited by NK1-R antagonist L732138, but not by either NK2-R antagonist MDL29913 or NK3-R antagonist SB218795. Consistently, SP-elicited increase in [Ca2+]i was abolished by NK1-R antagonist, neither by NK2-R nor NK3-R antagonists. The SP-induced muscular responses were significantly inhibited by L-type Ca2+ channel blocker verapamil and withdrawal of external Ca2+. Caffeine (10 mM) or ryanodine (50 microM) also partly suppressed the SP-induced muscle responses. Inhibition of inositol 1,4,5-trisphosphate (InsP3) receptor with 2-APB (75 microM) potently attenuated SP-evoked Ca2+ mobilization and muscle contraction, which was further inhibited by 2-APB under Ca2+-free external solution, but not completely. Unexpectedly, simultaneous blockade of InsP3 receptor and ryanodine receptor (RyR) by 2-APB and ryanodine enhanced SP-evoked muscle contraction and Ca2+ mobilization. This potentiation was virtually abolished by removal of external Ca2+, suggesting native Ca2+ channels may contribute to this phenomenon. These results demonstrate that tachykinins produce a potent muscle contraction associated with Ca2+ mobilization via tachykinin NK1- R-dependent activation of multiple signal transduction pathways involving Ca2+ influx and release of Ca2+ from InsP3- and ryanodine-sensitive Ca2+ stores. Blockade of both InsP3 receptor and RyR enhances the Ca2+ influx through native Ca2+ channels in plasma membrane, which is crucial to Ca2+ signaling in response to NK1 receptor activation.

Conformational solution studies of neuropeptide gamma using CD and NMR spectroscopy

Neuropeptide gamma is one of the largest members of the tachykinin family of peptides, exhibiting strong agonistic activity towards the NK-2 tachykinin receptor. This peptide was synthesized by the solid-phase method using the Fmoc chemistry. Circular-dichroism spectroscopy (CD) investigations of this peptide were performed in phosphate buffer, in the presence of sodium dodecylsulphate (SDS) micelles and trifluoroethanol (TFE) solutions and in DMSO-d6 using the 2D NMR technique in conjunction with two different theoretical approaches. The first assumes multiconformational equilibrium of the peptide studied characterized by the values of statistical weights of low-energy conformations. These calculations were performed using three different force fields ECEPP/3, AMBER4.1 and CHARMM (implemented in the X-PLOR program). The second method incorporates interproton distance and dihedral angle constraints into the starting conformation using the Simulated Annealing algorithm (X-PLOR program). The CD experiments revealed that although the peptide studied is flexible in polar solvents, a tendency to adopt a helical structure was observed in the hydrophobic environment. The NMR data (NOE effects) indicate a helical or reverse structure in the Ile7-His12 fragment of the peptide studied in DMSO-d6 solution. The results obtained cannot be interpreted in terms of a single conformation. Most of the conformations obtained with the ECEPP/3 force field possess a high content of a helical structure. None of the conformers, obtained with the AMBER4.1 and CHARMM force fields, can be considered as the dominant one. In all conformations several beta-turns were detected and in some cases gamma-turns were also found. But in fact, it is rather difficult to select the position of the secondary element(s) present in the structure of NPgamma in solution. All conformers calculated with the X-PLOR program (with using NMR derived distance and torsion angle constraints) are stabilized by several beta-turns. Common structural motives are a type IV beta-turn in the Gln6-His12 fragment. All conformations obtained using two approaches adopt very similar turn shapes in the middle region of molecule and a random structure on the N- and C-terminal fragments.

Tachykinin-induced bronchoconstriction in sheep is NK-1 receptor mediated and exhibits tachyphylaxis

OBJECTIVE

Tachykinins are mediators of airway hyper-reactivity and inflammation. There is in vitro evidence that ovine responses to tachykinins correlate closely to human responses. This study was designed to characterize the effect of intravenously administered tachykinins on sheep lung resistance in vivo to determine the effect of dose timing on reproducibility of responses and the induction of tachyphylaxis. We then used this information to help further characterize the response with several pharmacological agents.

METHODOLOGY

Substance P (SP) was administered by infusion to conscious merino ewes and lung resistance (RL) was measured. Infusions were given at 30, 60, 120 min and 24 h intervals. The effect of various agents on the response to SP was then assessed.

RESULTS

Substance P led to a transient increase in RL, mean (+/- SEM) 754.8 (+/- 139)% of baseline, with marked tachyphylaxis at 30, 60 and 120 min. Phosphoramidon increased the peak response to 1151.5 +/- 196%. Atropine and CP 96 345 abolished the response to SP, while indomethacin, sodium cromoglycate and pyrilamine had no significant effect. Substance P had a greater effect on RL than did neurokinin A.

CONCLUSIONS

Substance P increases RL in sheep via a cholinergic mechanism which is mediated by NK-1 receptors, and is subject to tachyphylaxis. These findings have implications for the design of studies using the ovine model in the evaluation of tachykinin antagonists as potential therapeutic agents.

Cytokines enhance airway smooth muscle contractility in response to acetylcholine and neurokinin A

OBJECTIVE

In vivo airway hyperresponsiveness has been demonstrated following inhalation of specific cytokines in normal individuals. Whether this airway hyperresponsiveness results from a direct effect of cytokines on airway smooth muscle contractility, or via changes in airway wall structure is not known. The aim of the present study was to determine the effect of the pro-inflammatory cytokines interleukin-1beta (IL-1beta) and tumour necrosis factor-alpha (TNF-alpha) on airway smooth muscle contractility in vitro.

METHODOLOGY

Ovine tracheal smooth muscle strips were incubated for 18 h at room temperature in Dulbecco's modified Eagle's medium, supplemented with antibiotics, with IL-1beta (10 ng/mL) and TNF-alpha (100 ng/mL), in an atmosphere of 5%CO2:95%O2. Following incubation cumulative concentration-response curves to acetylcholine (ACh) and neurokinin A (NKA) were obtained. Antagonist affinity studies were performed to determine whether the cytokine-induced enhanced contractility to ACh and NKA resulted from a functional alteration to specific M3 and NK2 receptors. Cumulative concentration-response curves to NKA were performed in the presence of phosphoramidon to determine if the enhanced contractility to NKA following cytokine exposure was due to a reduction in endogenous neutral endopeptidase activity. To assess the calcium dependence of the hyperresponsiveness, cumulative concentration-responses to ACh were conducted in calcium-free Krebs'-Henseleit solution.

RESULTS

Pre-incubation with TNF-alpha and IL-1beta caused a significant leftward shift, and an increase in the magnitude, of the concentration-response curves to both ACh and NKA. No difference in M3 and NK2 receptor antagonist affinity (pA2) values between the control and cytokine-treated tissue was observed. Neurokinin A contractility in the presence of phosphoramidon indicated that the enhanced contractility following cytokine exposure was not due to a reduction in endogenous neutral endopeptidase activity. Removal of extracellular calcium ions attenuated the contractile response to low concentrations of ACh in the control and cytokine-pretreated tissue. However, enhanced contractility following TNF-alpha and IL-1beta pretreatment was still present.

CONCLUSION

Pro-inflammatory cytokines induce in vitro hyperresponsiveness in normal airway smooth muscle via a mechanism involving intracellular calcium mobilization.

Inhibitory effect of capsaicin on cholinergic transmission in ovine airways: evidence for non-cholinergic contractions

Electrical stimulation of ovine trachealis smooth muscle and bronchial ring segments induced neurogenic and monophasic atropine-sensitive contractions. Pretreatment of the tissues with capsaicin (100 microM) significantly reduced these contractions indicating a possible contribution of a peptidergic neurotransmitter to the contractions. The effect of capsaicin on electrically induced contractions was significantly inhibited by capsazepin indicating an action on vanilloid receptors. In both preparations, electrically induced contractions were not modified by tachykinin NK(1)- and NK(2)-receptor antagonists singly and in combination. It was therefore concluded that a component of the atropine-sensitive electrically induced contractions of ovine airways smooth muscles involved the release of a peptide neurotransmitter which is probably not a tachykinin. However, an action of capsaicin on prejunctional vanilloid receptors located on cholinergic nerves cannot be ruled out.