Excitatory Neuro-effector Transmission Research Articles

Excitatory effect of acotiamide on rat and human bladder: Implications for underactive bladder treatment

ObjectiveTo evaluate whether approved gastroprokinetic agent, acotiamide exerts a direct excitatory effect on bladder to help explain the reported meaningful reduction of post-void residual urine volume (PVR) in detrusor underactivity (DU) patients after thrice daily oral intake of acotiamide 100 mg for 2 weeks. MethodsEffect of acotiamide [1–16 μM] was assessed on nerve-mediated contractions evoked by electrical field stimulation (EFS) for 5 s with 5 ms pulse trains of 10 V in longitudinal, mucosa intact rat and human bladder strips to construct frequency response curve (1–32 Hz) and repeat 10 Hz stimulation at 60s interval. Effect of acotiamide 2 μM on spontaneous and carbachol evoked contractions was also assessed. ResultsAcotiamide 2 μM significantly enhanced the Atropine and Tetrodotoxin (TTX)-sensitive EFS evoked contractions of rat and human bladder at 8–32 Hz (Two-way ANOVA followed Sidak's multiple comparison; *p < 0.01) and on repeat 10 Hz stimulation (Paired Student's t-test; *p < 0.05), while producing a modest effect on the spontaneous contractions and a negligible effect on the carbachol evoked contractions. ConclusionsEnhancement of TTX-sensitive evoked contractions of rat and human bladder by acotiamide is consistent with the enhancement of excitatory neuro-effector transmission mainly through prejunctional mechanisms. Findings highlight immense therapeutic potential of antimuscarinics with low M3 receptor affinity like acotiamide in Underactive bladder (UAB)/DU treatment.

Origin and modulation of circular smooth muscle layer contractions in the porcine esophagus

The origin and modulation mechanisms controlling timing and amplitude of esophageal body peristalsis are not fully understood. We aimed to characterize the neurotransmitters involved in the origin and modulation of circular smooth muscle esophageal body (EB) contractions. Responses of porcine EB strips to electrical stimulation of motor neurons (MNs) were assessed in organ baths and with microelectrodes. The effect of antagonists of inhibitory (L-NAME 1 mmol L(-1) , MRS2179 10 μmol L(-1) ) and excitatory neurotransmitters (atropine 1 μmol L(-1) ; SR140333 1 μmol L(-1) -NK(1) ra-, GR94800 1 μmol L(-1) -NK(2) ra-) and of ganglionic neurotransmitters (hexamethonium 100 μmol L(-1) , ondansetron 1 μmol L(-1) , NF279 10 μmol L(-1) ) were characterized. Electrical field stimulation (EFS) induced a frequency-dependent off-contraction (16.8 ± 0.8 g) following a latency period. Latency was significantly reduced by L-NAME (-66.1 ± 4.1%) and MRS2179 (-25.9 ± 5.6%), and strongly increased by atropine (+36.8 ± 5.8%). Amplitude was reduced by L-NAME (-69.9 ± 10.4%), MRS2179 (-34.1 ± 6.0%), atropine (-42.3 ± 4.7%), hexamethonium (-18.9 ± 3.3%), NF279 (-20.7 ± 3.5%), ondansetron (-16.3 ± 3.2%), GR94800 (-28.0 ± 4.8%) SR140333 (-20.9 ± 7.1%), and α-chymotrypsin (-31.3 ± 7.0%). The EFS induced a monophasic nitrergic inhibitory junction potential. Our results suggest that timing (latency) and amplitude of esophageal contractions are determined by a balance of complex interactions between excitatory and inhibitory MNs. Latency depends on the activation of inhibitory MNs releasing NO and a minor purinergic contribution through P2Y(1) receptors, and excitatory MNs releasing ACh. Amplitude depends on a major contribution of excitatory MNs releasing ACh and tachykinins, and also on inhibitory MNs releasing NO, ATP or related purines, and peptidergic neurotransmitters acting as strong modulators of the excitatory neuroeffector transmission.

Z-338, a newly synthetized carboxyamide derivative, stimulates gastric motility through enhancing the excitatory neurotransmission.

We studied the effects of Z-338, a newly synthesized carboxyamide derivative, on autonomic neuroeffector transmission in the guinea-pig stomach in relation to its gastrointestinal prokinetic action, by use of tension recording and microelectrode methods. Z-338 (>10(-8) M) dose-dependently enhanced the amplitude of twitch-like contractions and excitatory junction potentials (EJPs) evoked by single or repetitive electrical field stimulation (EFS) without affecting the non-adrenergic non-cholinergic (NANC) relaxation and inhibitory junction potentials (IJPs) in the circular muscle strips of the guinea-pig stomach. Similar to the action of Z 338, pirenzepine (> 10(-8) M), a muscarinic M1-antagonist, enhanced the EJP amplitude, although AF-DX116 (M2-) and 4-DAMP (M3-antagonists) reduced the EJP amplitude, dose-dependently. The EC50 of the Z-338 and pirenzepine concentration response curves on EJPs were 4.7 x 10(-8) M and 1.0 x 10(-8) M, and Hill coefficients were 0.96 and 0.94 respectively. In addition, Z-338 slightly but significantly enhanced the amplitude of slow waves with or without initial spike. These results provide the first evidence that Z-338 exerts its gastrointestinal prokinetic action mainly through enhancing excitatory neuro effector transmission with no effect on NANC inhibitory neuro-effector transmission in the guinea-pig stomach.

Possible role of nitric oxide in NANC relaxation of the airway and in airway hyperreactivity in airway disease

In the lung, there is increasing evidence that endogenous nitric oxide (NO) plays multiple roles in physiological control of airway functions, immune responses and host defense against infection, and it is also implicated in inflammatory disease of the airways. As elsewhere, it is generally accepted that NO derived from constitutive NO synthase (cNOS) is involved in physiological regulation of airway function, whereas NO derived from inducible NO synthase (iNOS) is involved in immune responses and inflammatory diseases of the airway. In this mini review, first we describe evidence showing the possible roles of NO and NO-related compounds in nonadrenergic noncholinergic (NANC) relaxation and inhibitory action on excitatory neuro-effector transmission, thereby indicating that NO derived from neuronal cNOS plays a role to provide "double braking" in bronchoconstrictions. Secondly, we will discuss the possible involvement of epithelial damage due to excessive NO production through iNOS in the genesis of airway hyperreactivity and airway inflammation.

Modulatory mechanisms involved in the parasympathetic excitatory neuro-effector transmission in the airway and gastrointestinal tracts--perspective for a new drug creation

It is generally considered that dominant excitatory control of airway and gastrointestinal tract is exerted by the parasympathetic nervous system, and the transmitter output from the nerve terminals, mainly acetylcholine (ACh), is modulated in many ways for example by ACh itself, prostaglandins E series (PGE) neuropeptides and nitric oxide (NO). These modulations are probably due to the suppression of high-voltage-activated calcium channels in the nerve terminals, since, for example, ACh or prostaglandin E1 & E2 selectively suppressed both the N- and R-type Ca2+ currents, through M2 muscarinic and EP3-receptors, respectively in paratracheal ganglion cells. Parasympathetic nervous system also releases nonadrenergic-noncholinergic (NANC) inhibitory neurotransmitters in addition to ACh. The threshold level for ACh and NO release, for example, is almost identical, thereby suggesting the possible interactions between NO and ACh at the post-junctional cites, in addition to the prejunctional inhibitory action of NO to suppress ACh release from the nerve terminal. In this context, NO-donors, including SNAP or Cys-NO, reduced the amplitude of carbachol-induced Ca(2+)-dependent Cl- currents (I infinity h, I alpha(Ca)) dose-dependently (IC50: about 10 microM) in the cat tracheal myocytes, and similar inhibition was also exerted by dibutyryl cGMP (db-cGMP). However, two structurally distinct types of G-kinase inhibitor, H-8 (2.5 microM) and KT5823 (1 microM) failed to counteract the inhibitory effects of SNAP or db-cGMP. Another G-kinase-specific inhibitor, (Rp)-8-pCPT-cGMPS, itself caused a marked reduction in I infinity h, thereby indicating that inhibition of I infinity h by NO donors involves a cGMP-dependent but G-kinase-independent mechanism. All these experimental facts taken together indicate that there are pre- and post-junctional "brakings" in the excitatory parasympathetic neuro-effector transmission. Furthermore, in the gastrointestinal tract, Z-338, a newly synthetized gastroprokinetic agent, stimulates gastrointestinal motility through inhibition of the pre-junctional braking system through inhibition of M1 receptors. Such a drug may be useful to control the respiratory and gastrointestinal smooth muscles.

The possible role of nitric oxide in relaxations and excitatory neuroeffector transmission in the cat airway.

1. To study the possible role of nitric oxide (NO free radical; NO) or NO-containing compounds in the non-adrenergic, non-cholinergic (NANC) relaxations, we observed the effects of carboxy-2-phenyl-4, 4, 5, 5-tetramethyl-imidazoline-1-oxyl-3-oxide (C-PTIO; a newly synthesized NO scavenger) on NANC relaxations in the cat airway. In addition, we also observed the effects of C-PTIO on excitatory junction potentials (EJPs), since NO has a prejunctional action on transmitter release. 2. Nitrosocystine (Cys-NO) (10(-7)-10(-3) M) dose-dependently relaxed the bronchial tissue in the presence of 5-HT, atropine and guanethidine and C-PTIO (10(-4) M) shifted the concentration-response curve of the Cys-NO to the right. 3. Electrical field stimulation (EFS) evoked biphasic NANC relaxations in the small bronchi of the cat. In general, C-PTIO suppressed non-selectively both the first and second components of the NANC relaxations to a similar extent. However, in some bronchial preparations C-PTIO (10(-4) M) selectively suppressed the first component of the NANC relaxation to approximately 50% of the initial value, enhancing the amplitude of the second component of the NANC relaxations. 4. After pretreatment of the bronchial tissues with alpha-chymotrypsin (1 unit ml-1) for 30 min in order to inhibit any response to peptides, EFS evoked monophasic NANC relaxation. C-PTIO (10(-5) - 10(-4) M) dose-dependently suppressed, and at a concentration of 10(-4) M almost halved, the amplitude of NANC relaxation. Additional application of L-NAME further reduced the C-PTIO-resistant NANC relaxation to 20-30% of the initial value. 5. C-PTIO (10(-4) M) enhanced the EJP amplitude evoked by single EFS in the trachea but not in the bronchi. However, C-PTIO enhanced the summation of the EJPs to repeated stimulation to a similar extent in the tracheal and bronchial tissues. Simultaneous application of C-PTIO and L-NAME did not further enhance the summation. 6. These results indicate that NO. and NO-containing compounds are involved in the L-NAME-sensitive NANC relaxation in the cat airway, and that only NO. has a prejunctional action which inhibits excitatory neuroeffector transmission.

P-564 - Effects of NOS inhibitor and NO-scavenger on the excitatory neuroeffector transmission in the cat airway.

P-564 - Effects of NOS inhibitor and NO-scavenger on the excitatory neuroeffector transmission in the cat airway.

Effect of histamine and substance P on the rabbit and human iris sphincter muscle

In an attempt to clarify the functional action of histamine and substance P on atropine-resistant miosis, we isolated rabbit and human iris sphincter muscles and investigated their mechanical properties using the isometric tension recording method. Substance P dose-dependently contracted the rabbit iris sphincter, but had no effect on the human iris sphincter. In the rabbit iris sphincter, histamine reduced the amplitude of twitch contraction evoked by field stimulation but had no effect on carbachol-induced contraction. Thioperamide, but not mepyramine or cimetidine, partially antagonized the histamine-induced reduction in the amplitude of twitch contractions. In the human iris sphincter, on the other hand, histamine dose-dependently provoked contraction and the amplitude of histamine-induced contraction was affected neither by atropine nor by indomethacin. These results provide evidence that histamine has strong contractile effect on the human iris sphincter muscle; the rabbit iris sphincter muscle, however, apparently lacks functional histamine receptors. In rabbits, exogenously applied histamine only activates H3 receptors located on the cholinergic nerve terminal, hence the excitatory neuro-effector transmission is suppressed. Thus, histamine may have an important roles in atropine-resistant miosis in humans, but not in rabbits.

Role of nitric oxide in non‐adrenergic, non‐cholinergic relaxation and modulation of excitatory neuroeffector transmission in the cat airway.

1. The effects of nitrosocysteine (cys-NO), L-N omega-nitroarginine (L-NNA) and L-N omega-nitro-L-arginine methylester (L-NAME), oxyhaemoglobin and Methylene Blue were observed on the resting membrane potential, muscle tone and excitatory junction potentials (EJPs) of cat tracheal smooth muscle tissue. 2. Cys-NO (10(-9) to 10(-6) M) showed no effect on the resting membrane potential of smooth muscle cells of the cat trachea but it dose-dependently relaxed the tracheal tissue in the presence of 5-HT, atropine and guanethidine. 3. Electrical field stimulation (EFS) applied during contraction evoked by 5-HT in the presence of atropine and guanethidine evoked non-adrenergic, non-cholinergic (NANC) muscle relaxation. L-NNA (10(-4) M) and L-NAME (10(-4) M) completely suppressed the relaxation when single or short repetitive stimuli were applied, but suppression was incomplete with repetitive stimuli of 4 ms pulse duration applied at 20 Hz. A substantial part of the L-NNA- or L-NAME-insensitive relaxation was abolished by tetrodotoxin. 4. Cys-NO dose-dependently suppressed the EJPs without changing the resting membrane potential, and L-NNA, L-NAME, Methylene Blue and oxyhaemoglobin enhanced the amplitude of the EJP to 1.2-1.5 times the control value. 5. EJPs showed some summation when repetitive field stimulation was applied at 20 Hz. L-NNA or L-NAME enhanced the summation, and the mean slopes were increased from 0.61 +/- 0.22 to 2.0 +/- 0.3, or 1.9 +/- 0.2 mV per stimulus. Vasoactive intestinal polypeptide (VIP) antiserum and VIP antagonists further enhanced the summation in the presence of L-NNA. 6. These results indicate that NANC relaxation can be classified into two different components according to the threshold for activation, and nitric oxide is involved in one. The present results also suggest that endogenous or exogenous nitric oxide has a prejunctional action in inhibiting excitatory neuroeffector transmission in addition to a direct action on the smooth muscle cells, presumably by suppressing transmitter release from the vagus nerve.

VIP antagonists enhance excitatory cholinergic neurotransmission in the human airway.

It has been reported that a low concentration of exogenously applied vasoactive intestinal peptide (VIP) suppresses the release of acetylcholine (ACh) from vagus nerve terminals in the ferret and feline trachea. There has been, however, no documentation of the prejunctional action of VIP in the human airway. We observed the effects of VIP and VIP antagonists on cholinergic excitatory neuro-effector transmission in the human bronchus to study the possible role of endogenous VIP on excitatory neurotransmission. In the human bronchus, VIP (10(-10) to 10(-7) M) showed no effect on either the contractions evoked by electrical field stimulation (EPS) or those evoked by ACh. To investigate the possible role of endogenous VIP on the human bronchus, we observed the effects of the VIP antagonists [4-Cl-D-Phe6,Leu17]-VIP and [Ac-Tyr1,D-Phe2]-GRF(1-29)-NH2 on excitatory neuro-effector transmission. Both VIP antagonists (10(-8) M) significantly enhances the contractions evoked by EFS without affecting the ACh sensitivity of smooth muscle cells. These results indicate that VIP antagonists have a prejunctional action that enhances excitatory neurotransmission. This study suggests that endogenous VIP may suppresses ACh release from the vagus nerve terminals in the human airway. It is also suggested that exogenously applied VIP may be inactivated by some mechanism in the human airway.

Effects of epithelial cell supernatant on membrane potential and contraction of dog airway smooth muscles.

We investigated the effects of cultured epithelial cells and supernatants on resting membrane potential and excitatory neuroeffector transmission in smooth muscle cells of dog trachea and bronchioles. The mean resting membrane potential of the mucosa-free tracheal smooth muscle cells was -59.5 +/- 1.5 mV (+/- SD). Application of cultured epithelial cells (> 2.5 x 10(5) cells/ml) hyperpolarized the membrane, resulting in a potential of -64.5 +/- 1.7 mV. The supernatant of the cultured epithelial cells also increased the resting membrane potential of the mucosa-free tracheal smooth muscle cells by 4 to 9 mV. These hyperpolarizing actions were not modified by indomethacin (10(-5) M), l-NG-nitroarginine (10(-5) M), or oxyhemoglobin (10(-5) M), but were inhibited by glibenclamide (10(-6) M). The supernatants of the cultured epithelial cells completely or partially suppressed the contractile response of epithelium-denuded bronchioles to electrical field stimulations and suppressed the amplitude of excitatory junction potentials of the trachealis evoked by electrical field stimulations. Indomethacin prevented the inhibitory effect of supernatants on the amplitude of twitch contractions and excitatory junction potentials and markedly suppressed supernatant-associated inhibition of the excitatory neuroeffector transmission. Furthermore, indomethacin with AA861, a lipoxygenase inhibitor, completely suppressed this effect. Our findings suggest that cultured airway epithelial cells spontaneously release at least two factors. One factor selectively modulates the resting membrane potential, and the other inhibits the excitatory neuroeffector transmission.

Functional innervation of bovine ophthalmic artery.

To investigate the functional innervation pattern in smooth muscle of bovine ophthalmic artery, we studied the effect of electrical stimulation and various drugs on the mechanical responses of this muscle using isometric tension recording methods. Electrical stimulation evoked phasic contractions which were abolished by guanethidine and tetrodotoxin (TTX), suggesting that the responses were neurogenic in origin. Phentolamine (10(-5) M) and 10(-5) M timolol did not affect this contractions, but 10(-5) M phentolamine and 10(-6) M prazosin markedly reduced the amplitude of contraction evoked by exogenously applied phenylephrine (10(-5) M). Electrical stimulation applied during sustained contraction evoked by 5-hydroxytryptamine in the presence of guanethidine and atropine evoked relaxation which was markedly suppressed by TTX. These results indicate that bovine ophthalmic artery is innervated by adrenergic nerve fibers, and an unidentified neurotransmitter, other than noradrenaline, may play an important role in the excitatory neuroeffector transmission, and that the tissue is also innervated by non-adrenergic non-cholinergic inhibitory nerve fibers.

O-261 - effects of nitrosocystine (NCYS) and NOS inhibitor on excitatory neuro-effector transmission in the cat trachea.

O-261 - effects of nitrosocystine (NCYS) and NOS inhibitor on excitatory neuro-effector transmission in the cat trachea.

Possible roles of airway epithelial cells in the control of airway smooth muscle tone

While the airway epithelium provides a diffusion barrier to the access of inhaled stimulants, it has recently been considered to have a more active physiological and pharmacological role in the regulation of the airway smooth muscle tone. The initial evidence for this comes from the fact that mechanical removal of the airway epithelium increases the in vitro responsiveness of airway smooth muscle to various spasmogens 3-5 times, although the nature of these regulatory mechanisms on the smooth muscle by the epithelium remains largely unresolved. One of the possible explanations is that the epithelium a generates factor(s) that inhibits the responsiveness of the underlying smooth muscle cells (epithelium-derived inhibitory or relaxing factor EpDIF/EpDRF). Furthermore, recent investigations revealed that airway epithelial cells also release a factor(s) that modulates the vascular smooth muscle tone, excitatory-neuroeffector transmission and resting membrane potential of the airway smooth muscle cells (epithelium-derived hyperpolarizing factor: EpDHF). This review will present and discuss the evidence indicating that the epithelium generates and releases a factor(s) that modulates smooth muscle tone, resting membrane potential of airway smooth muscle cells (EpDHF) and excitatory neuro-effector transmission in the airway.

&lt;b&gt;BIPHASIC ACTIONS OF PINACIDIL ON EXCITATORY NEURO-EFFECTOR TRANSMISSION IN DOG &lt;/b&gt;&lt;b&gt;TRACHEA &lt;/b&gt;

Effects of pinacidil were investigated on the resting membrane potential and excitatory neuro-effector transmission in the smooth muscle tissue of dog trachea using tension recording, microelectrode and double sucrose gap methods. Pinacidil (>5×10 -7 M) increased the ionic conductance and hyperpolarized smooth muscle cell membranes in dog trachea, in a dose-dependent manner. This pinacidil-induced hyperpolarization was antagonized by glibenclamide (2×10 -6 M)

Effects of PAF on excitatory neuro-effector transmission in dog airways.

1. Effects of PAF on excitatory neuro-effector transmission in smooth muscle cells of mucosa-free trachea and epithelium-intact bronchiole of the dog were investigated, by isometric tension recording, microelectrode and double sucrose-gap methods. 2. PAF (10(-11)-10(-7) M) dose-dependently enhanced the amplitude of contraction evoked by repetitive field stimulations (10 stimuli at 20 Hz) in both tracheal and bronchiolar tissues. At higher concentrations PAF (> 10(-8) M) increased the amplitude of contraction to a greater extent in the bronchiole than in the trachea. 3. In both muscle tissues, in parallel to the amplitude of contraction, PAF markedly enhanced the amplitude of excitatory junction potentials (e.j.ps) evoked by a single field stimulation in a dose-dependent manner, with no change in the resting membrane potential or input membrane resistance of the smooth muscle cells. PAF (5 x 10(-7) M) enhanced the amplitude of e.j.p. to a greater extent in the bronchiole than in the trachealis. In contrast, lyso-PAF (10(-10)-10(-7) M) showed no effect on e.j.p. amplitude in bronchiolar tissues. At a high concentration (10(-7) M) lyso-PAF slightly enhanced the e.j.p. amplitude in tracheal tissue, however the lyso-PAF induced stimulation of e.j.p. amplitude in the trachea was small compared to that of PAF. 4. PAF (10(-7) M) had no effect on the membrane depolarization induced by acetylcholine (ACh, 10(-9)-10(-5) M) and carbachol (10(-9)-10(-5) M) in tracheal smooth muscle cells. 5. The PAF-antagonists CV3988 (5 x i0-7 M) or WEB2086 (5 x 10-7 M) significantly enhanced the e.j.p. amplitude themselves, PAF (5 x 10-8 M) further enhanced the ej.p. amplitude in the presence of WEB2086 (5 x l0-7 M) but not CV3988 (5 x 10-7 M). In contrast, the new PAF-antagonist, E 6123(5 x l0-8 M), did not affect the ej.p. amplitude itself, and completely inhibited the increase in ej.p. amplitude caused by 5 x 10-8 M PAF. On the other hand, in the presence of the Hi-antagonist,mepyramine, PAF (5 X 10-8 M) further enhanced the ej.p. amplitude.6. The leukotriene synthesis inhibitor AA-861 (10-6 M) or leukotriene antagonist ONO1078 (10-7 M)inhibited the increase in ej.p. amplitude caused by 5 X 10-8 M PAF, respectively.7. In the presence of AA-861 (10-6 M), leukotriene B4 (LTB4, 10-' M) or LTD4 (10-8 M) slightly, and LTC4 (10- M) markedly enhanced the ej.p. amplitude. In contrast, LTE4 (10-8 M) significantly suppressed the e.j.p. amplitude.8. PAF (5 x 10-8 M) attenuated the depression phenomena of ej.ps observed during double stimulus experiments at different time intervals (5-10 s), but had no effect on the summation of ej.ps during repetitive field stimulation at a high frequency (20 Hz) in the trachealis.9. These results indicate that PAF potentiates excitatory neuro-effector transmission mainly through stimulating the release of lipoxygenase products, mainly LTC4 in the dog airway smooth muscle tissues.

Potassium channel activators and bronchial asthma.

The cromakalim-like KCOs relax airways smooth muscle by an action that is associated with the opening of plasmalemmal K(+)-channels. The K(+)-channel involved may be analogous to the ATP-sensitive K(+)-channel identified in pancreatic beta-cells. It is unlikely to be open under normal circumstances and plays little role in determining the strong outward rectifying behaviour of the plasmalemma of the airways smooth muscle cell. K(+)-channel opening may cause relaxation of the airways smooth muscle cell by mechanisms additional to inhibition of Ca2+ influx through L-type VOCs. The cromakalim-like KCOs have bronchodilator activity in vivo and can depress NANC excitatory neuroeffector transmission in the lung at concentrations smaller than those required to relax airways smooth muscle. The mechanism of action of cromakalim in alleviating nocturnal asthma may not involve direct relaxation of airways smooth muscle. It is possible that cromakalim may instead act to inhibit the mechanisms underlying airway hyper-reactivity.

Effects of vasoactive intestinal polypeptide antagonists on cholinergic neurotransmission in dog and cat trachea.

1. The effects of vasoactive intestinal polypeptide (VIP) antagonists [AC-Tyr1, D-Phe2]-GRF(1-29)-NH2 and [4-Cl-D-Phe6, Leu17]-VIP on excitatory neuroeffector transmission in the dog and cat trachea were investigated by use of microelectrode, double sucrose-gap and tension recording methods. 2. In the dog trachea, repetitive stimuli at high frequency (20 Hz) markedly enhanced the amplitude of contraction, the amplitude of contractions evoked by 50 stimuli at 20 Hz relative to that evoked by 5 stimuli being 14.2 +/- 3.8 times (n = 7, +/- s.d.). In the cat, the summation was much less marked, the amplitude of contractions evoked by 50 stimuli relative to that evoked by 5 stimuli being only 2.1 +/- 0.6 times (n = 5, +/- s.d.). Neither VIP antagonist had any effect on the relationship between the number of stimuli at 20 Hz and the relative amplitude of contraction in the dog trachea, but did enhance the amplitude of contractions to 1.1-1.5 times control in the cat trachea. 3. VIP antagonists dose-dependently enhanced the amplitude of excitatory junction potentials (e.j.ps) evoked by a single stimulus in the cat trachea, without changing the resting membrane potential or input membrane resistance of the smooth muscle cells. However, neither antagonist had any effect on the amplitude of the e.j.p. in the dog trachea. 4. Neither VIP antagonist had any effect on the post-junctional response of smooth muscle cells to exogenously applied acetylcholine (ACh; 10(-9)-10(-5) M) in the dog or cat trachea.5. In the cat trachea, VIP (10-11 M) suppressed the ej.p. amplitude to 0.74 +/- 0.09 times the control value (n = 6). However, after pretreatment of the tissue with the VIP antagonists [Ac-Tyr', D-Phe2]-GRF(1-29)- H2 (10-8M) and [4-Cl-D-Phe6, Leu17]-VIP (10-8M), VIP (10-11 M) did not suppress the ej.p. amplitude, indicating that VIP antagonists block the presynaptic inhibitory action of exogenous VIP.6. In parallel with the enhancement of contraction, ej.ps showed marked summation when repetitive field stimulations were applied at high frequency (20 Hz) in the dog trachea. The relationship between the relative amplitude of the ej.p. and number of stimuli at 20 Hz was linear and the slope was 2.2 +/- 0.3 mV/stimulation. VIP antagonists did not affect this relationship. However, in the cat trachea, summation of ej.ps was not at all marked and a linear relationship was not observed with the double sucrose-gap method. Incubation of the cat tracheal tissue with either of the VIP antagonists (10-8 or 10-7M) markedly enhanced the summation of ej.ps evoked by repetitive field stimulation at 20 Hz, and after the treatment a linear relationship between the number of stimuli and the amplitude of ej.ps was observed, the slopes being 0.6 +/- 0.1 (n = 8) and 0.55 +/- 0.1 mV/stimulation (n = 5), respectively.7. These results indicate that both VIP antagonists, [Ac-Tyr', D-Phe2]-GRF(1-29)-NH2 and [4-Cl-DPhe6, Leu17]-VIP, have a prejunctional action accelerating the excitatory neuroeffector transmission, presumably by enhancing transmitter release from the vagus nerves in the cat, but not in the dog trachea.

Effects of cromakalim on neurally‐mediated responses of guinea‐pig tracheal smooth muscle

1. The ability of cromakalim to modulate several different types of neuroeffector transmission has been assessed in guinea-pig isolated trachea. 2. In trachea treated with propranolol (10(-6) M) and indomethacin (2.8 x 10(-6) M), stimulation of the extrinsic vagal nerves evoked contractions which were blocked by hexamethonium (5 x 10(-4) M) or by tetrodotoxin (TTX; 10(-6) M). Cromakalim (10(-5) M) caused a two fold rightward shift of the frequency-response curve. 3. In carinal trachea treated with propranolol and indomethacin, transmural stimulation evoked an initial, rapid contraction followed by a more sustained secondary contraction. The initial, rapid contractile response was virtually ablated by atropine (10(-6) M) or by TTX but was resistant to hexamethonium. Cromakalim (10(-8)-10(-5) M) caused a concentration-dependent rightward shift of the frequency-response curve for the initial contraction. 4. In carinal trachea treated with atropine, propranolol and indomethacin, transmural stimulation evoked only the secondary (non-adrenergic, non-cholinergic (NANC] contractile responses. These were markedly reduced by TTX but were resistant to hexamethonium. Cromakalim (10(-8)-10(-5) M) suppressed the NANC contractile responses in a concentration-dependent manner. This action could be offset by glibenclamide (10(-6) M). 5. In trachea treated with atropine, histamine (10(-4) M), propranolol and indomethacin, transmural stimulation evoked NANC relaxant responses. Cromakalim (up to 10(-5) M) was without effect on the frequency-response curve for the stimulation of NANC inhibitory nerves. 6. Tested on trachea bathed by drug-free Krebs solution, cromakalim (10(-7)-10(-5) M) caused concentration-dependent suppression of tracheal tone. In trachea treated with propranolol and indomethacin, cromakalim (10- 7-1O- 5 M) caused concentration-dependent antagonism of acetylcholine (ACh). In trachea treated with atropine, propranolol and indomethacin, cromakalim (up to 10- 5M) failed to antagonize effects of either histamine or substance P.7. It is concluded that cromakalim can inhibit cholinergic (excitatory) neuroeffector transmission in the trachea but only at a concentration having demonstrable inhibitory activity against the action of exogenous ACh and the spontaneous tone of the airways smooth muscle. In contrast, cromakalim may depress NANC excitatory (putative peptidergic) neuroeffector transmission at a concentration below that exerting inhibitory activity on airways smooth muscle. Cromakalim does not concurrently depress NANC inhibitory neuroeffector transmission. Depression of NANC excitatory neuroeffector transmission could explain the ability of cromakalim to suppress airway hyperreactivity or bronchial asthma at doses lacking direct relaxant effect on airways smooth muscle.

Prejunctional Control of Excitatory Neuroeffector Transmission by Prostaglandins in the Airway Smooth Muscle Tissue

Prejunctional Control of Excitatory Neuroeffector Transmission by Prostaglandins in the Airway Smooth Muscle Tissue