The effect of experimental diabetes on cholinergic neurotransmission in rat trachea: role of nitric oxide

We investigated the effect of nitric oxide (NO) on the responses of isolated tracheas to acetylcholine and to electrical field stimulation in streptozotocin-diabetic and controls rats. The contractile responses to acetylcholine were neither different nor affected by the NO synthase blocker, N(omega)-nitro-L-arginine methyl ester (L-NAME), in the two groups. Diabetic rat tracheas were supersensitive to field stimulation. L-NAME enhanced field stimulation-induced contractions at low frequencies in control rat tracheas, but had no effect in diabetic rat tracheas. After L-NAME treatment, there was no difference in sensitivity to field stimulation between the groups. The relaxation responses to sodium nitroprusside in acetylcholine-precontracted tracheas were not different between the groups. However, diabetic rat trachea was supersensitive to the relaxant effect of sodium nitroprusside on contractile responses to field stimulation. These results suggested that the increase in sensitivity to field stimulation in tracheas from diabetic rats might be due to impairment in the production and/or release of an endogenous NO-like factor.     

Lipoxin A4 inhibits cholinergic neurotransmission through nitric oxide generation in the rabbit trachea

The effect of lipoxin A₄ and lipoxin B₄ on cholinergic neurotransmission in rabbit tracheal segments was studied under isometric conditions in vitro. Lipoxin A₄ attenuated the contractile responses to electrical field stimulation and caused a rightward shift of the frequency-response curves, so that the stimulus frequency required to produce a half-maximal effect (ES₅₀) increased from 8.1 ± 0.8 to 25.7 ± 1.9 Hz (P < 0.001), whereas lipoxin B₄ had no effect. In contrast, lipoxin A₄ did not alte contractile responses to acetylcholine. Pretreatment of tissues with N^G-nitro- -arginine methylester inhibited the effect of lipoxin A₄ on electrical field stimulation, but N^G-nitro- -arginine methylester did not. This inhibition by N^G-nitro- -arginine methylester was reversed by -arginine but not by -arginine. These results suggest that lipoxin A₄ prejunctionally reduces the vagal nerve-mediated contraction of airway smooth muscle, probably by inhibiting the release of acetylcholine, and that this effect may be exerted through stimulation of nitric oxide generation.     

Modulation of cholinergic neurotransmission in guinea-pig trachea by neuropeptide Y.

1. Neuropeptide Y (NPY) is localized to adrenergic nerves in guinea-pig airways but its function is not known. 2. NPY (1 X 10(-10)-3 X 10(-7) M) had no direct effect on guinea-pig tracheal smooth muscle in vitro. 3. NPY produced a concentration- and frequency-dependent inhibition of the cholinergic component of responses elicited by electrical field stimulation (EFS) whilst having no effect on the contractile response to exogenously applied acetylcholine (ACh). 4. Yohimbine was able to reverse significantly the inhibitory effect of noradrenaline on the cholinergic component to EFS without having any significant effect on the inhibition produced by NPY. 5. Neither blockade of beta-adrenoceptors by propranolol, nor depletion of adrenergic nerves by incubation with 6-hydroxydopamine caused any significant alteration in the response to EFS in the presence of 3 X 10(-7) M NPY. Bretylium tosylate incubation to prevent noradrenaline release produced a small but significant enhancement of the inhibitory effect of NPY on EFS at high frequencies. 6. NPY appears to reduce the cholinergic component to EFS via a prejunctional mechanism, acting directly on receptors on cholinergic nerve terminals, rather than affecting adrenergic mechanisms. NPY released by adrenergic nerves may modulate cholinergic neurotransmission in guinea-pig airways.     

Modulation of cholinergic and substance P-like neurotransmission by nitric oxide in the guinea-pig ileum.

1. The role of endogenous nitric oxide (NO) as a modulator of enteric neurotransmission was investigated in longitudinal muscle myenteric plexus (LMMP) preparations of guinea-pig isolated ileum. 2. In tissues previously incubated with [3H]-choline, exogenous NO inhibited electrically-evoked [3H]-choline overflow as well as responses to exogenous agonists, indicating that NO has the potential of neuromodulation both pre- and postjunctionally. 3. A series of NO synthase inhibitors enhanced contractile responses to nerve stimulation indicating inhibitory neuromodulation by endogenous NO. 4. The potency order of the NO synthase inhibitors and their consistent effects after dexamethasone, on responses to nerve stimulation, indicate action on a constitutive NO synthase. 5. Responses enhanced by NO synthase inhibitors were inhibited by the substance P receptor antagonist, spantide, suggesting a neuromodulatory influence on substance P-like neurotransmission by the endogenous NO. 6. NO synthase inhibition did not modify contractile responses to application of acetylcholine or substance P, or [3H]-choline overflow, indicating that endogenous NO mainly has a prejunctional inhibitory action on substance P-like neurotransmission. Nor did it modify responses to direct electrical muscle stimulation in the presence of tetrodotoxin. This suggests a prejunctional enhancing effect by NO synthesis inhibition. 7. Evidence for endogenous NO modulation of acetylcholine release was obtained when NO synthase inhibition modified atropine-sensitive, nerve-mediated contractile responses. However, [3H]-choline overflow was unaltered by NO synthase inhibition. 8. NO synthase inhibition did not modify responses to inhibitory neurotransmission. 9. The findings suggest that endogenous NO inhibits substance P-like motor neurotransmission, probably via prejunctional mechanisms. Cholinergic transmission may also be reduced by endogenous NO, acting prejunctionally.     

Involvement of nitric oxide in the mediation of NANC inhibitory neurotransmission of guinea- pig trachea

1 The involvement of nitric oxide (NO) in the non-adrenergic non-cholinergic inhibitory (NANC-i) neurotransmission was evaluated in guinea-pigs anaesthetized with chloralose–urethane, using a tracheal pouch preparation. 2 The tracheal pouch, a surgically isolated segment of trachea with intact nerve and blood supply, is an in situ method to demonstrate NANC-i response after complete cholinergic and adrenergic blockade using atropine (5 mg kg^??1) and propranolol (1 mg kg^??1), respectively. Cervical vagi and sympathetic trunks were isolated and cut cranially. The distal ends of the vagi were positioned on bipolar electrodes for subsequent stimulation with 5 V pulses for 2 ms duration at 15 Hz for a total of 90 s. The relaxation response was measured as a pressure drop (cm of H₂O) in the pouch. Each experimental group was composed of six animals. 3 NANC-i responses to two consecutive nerve stimulations at 25 min apart were reproducible. 4 Pouch relaxation responses to electrical nerve stimulations were determined before and after incubation of the pouch with N^ω-nitro-l -arginine methyl ester (l -NAME; 10^??5 m ), a NO synthase (NOS) inhibitor, for 30 min. l -NAME significantly, but not completely, inhibited the NANC-i response of the pouch, suggesting involvement of NO in the NANC-i neurotransmission. 5 The pouch relaxations to vagal stimulations were inhibited significantly after incubation with oxyHb indicating that NO was released. 6 The amount of methaemoglobin (metHb) formed from oxyhaemoglobin (oxyHb) during vagal stimulation was measured by spectrophotometry. Comparison of the values between the control and after nerve stimulation indicated a trend (P = 0.07) toward greater metHb formation in the pouch perfusate after nerve stimulation. 7 NANC-i responses were not significantly inhibited by incubation of the pouch with either of the guanylate cyclase inhibitors, methylene blue or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). However, a trend toward significance (P ≤ 0.07) was observed. 8 This study demonstrated that NO is involved in NANC-i neurotransmission. However, the findings did not conclusively support the contention that NO is the sole neurotransmitter of NANC inhibition. It is possible that NO produced relaxation of guinea-pig trachea through a cGMP-independent mechanism.     

The role of prostaglandins in cholinergic neurotransmission in the guinea pig.

Prostaglandins have contrasting effects on neurotransmission at different cholinergic nerve endings. This is a report on the role of prostaglandins in a number of cholinergic preparations from the guinea pig. In the isolated ileum PGE1 (2 X 10(-10) to 5 X 10(-8) M) potentiated the response to electrical stimulation of the cholinergic nerves. PGE1 (10(-7 M) caused an increase in tone followed by a period of transient inhibition of twitch height. Responses to simulation of the ileum with drugs were not potentiated by PGE1. Responses of atropinized or plexus-free muscle to electrical stimulation were also not potentiated by PGE1. Acetylsalicylic acid (2.5 X 10(-4) M) diminished the twitch response and the output of acetylcholine from the ileum. Both effects were reversed by PGE1. Qualitatively similar observations were made on the trachea. It is concluded that prostaglandins facilitate acetylcholine release in the ileum and trachea. PGE1 diminished the effect of vagal stimulation on the heart rate. The response to stimulation of the phrenic nerve was not affected.     

The role of prostacyclin in modulating cholinergic neurotransmission in guinea-pig ileum.

The mechanism of action of prostacyclin (PGI2) on isolated segments of guinea-pig terminal ileum was studied by recording the changes in isometric tension. In these preparations PGI2 (1 nM-1 microM) caused a concentration-dependent increase in muscle tension. This effect was rapid and short-lasting. PGI2-induced contractions were inhibited by atropine and potentiated by physostigmine. Hemicholinium-3 reduced the response to PGI2 and the inhibition was quantitatively comparable at any PGI2 concentration tested. Tetrodotoxin as well as low temperature (20 degrees C) abolished and beta-bungarotoxin reduced the effect of PGI2. Hexamethonium decreased the response to submaximal, but not to maximal PGI2 concentrations. PGI2 potentiated the twitch response of the ileum to electrical stimulation. In the presence of tetrodotoxin, PGI2 did not alter the effect of a sub-maximal concentration of acetylcholine (ACh). The present results give indirect evidence for the ability of PGI2 to facilitate ACh release from intramural nerves possibly by increasing the excitability of cholinergic cell bodies.     

The role of nitric oxide in inhibitory non-adrenergic non-cholinergic neurotransmission in the canine lower oesophageal sphincter.

1. The role of nitric oxide (NO) in non-adrenergic non-cholinergic (NANC) neurotransmission was studied on circular muscle strips of the canine lower oesophageal sphincter (LOS). Electrical field stimulation evoked frequency-dependent relaxations, which were resistant to adrenergic and cholinergic blockade and abolished by tetrodotoxin. 2. Exogenous administration of NO induced concentration-dependent and tetrodotoxin-resistant relaxations which mimicked those in response to electrical stimulation. 3. NG-nitro-L-arginine (L-NNA), a stereospecific inhibitor of NO-biosynthesis, inhibited the relaxations induced by electrical stimulation but not those by exogenous NO or vasoactive intestinal polypeptide (VIP). 4. The effect of L-NNA was prevented by L-arginine, the precursor of the NO biosynthesis but not by its enantiomer D-arginine. 5. Haemoglobin abolished the NO-induced responses and reduced those evoked by electrical stimulation. 6. Cumulative administration of VIP induced concentration-dependent relaxations, which were slow in onset and sustained. A complete relaxation to VIP was not achieved and the relaxations were not affected by L-NNA. 7. In conclusion, our results provide evidence that NANC relaxations are mediated by NO, suggesting NO or a NO releasing substance as the final inhibitory NANC neurotransmitter in the canine LOS.     

Influence of ouabain on the cholinergic neurotransmission in the canine trachea

The effect of ouabain on the cholinergic neurotransmission of the trachea was investigated using isolated tracheal strips in dogs. Tracheal strips without epithelium were suspended in organ chambers filled with modified Krebs-Henseleit solution. Ouabain (3 X 10(-7)-10(-5) M) concentration-dependently caused a slow sustained tracheal contraction. The contractile response was significantly inhibited by 10(-6) M atropine and was enhanced by 10(-8) M physostigmine. The ouabain-induced tracheal contraction was unaffected by 10(-7) M tetrodotoxin, but was significantly reduced by 10(-3) M hemicholinium-3. In superfusion experiments, ouabain (10(-5) M) produced an increase in the ACh release. Superfusion with Ca(++)-free solution almost eradicated the ACh release and abolished the tracheal contraction induced by ouabain. omega-Conotoxin (5 X 10(-8) M), but not nicardipine (10(-6) M), inhibited significantly the increase in ACh release induced by ouabain. These results suggest that the ouabain-induced tracheal contraction may be mainly due to acceleration of presynaptic ACh release by enhancing the influx of Ca++, and the Ca(+)+ entry may occur through the N-type Ca(+)+ channels in the canine airway presynaptic site.     

一氧化氮在大鼠模拟心肌缺血/再灌注损伤中的作用

目的探讨一氧化氮(NO)在大鼠心肌缺血/再灌注损伤(IRI)中的作用。方法20只Wistar雄性大鼠随机分成假手术组,缺血30min再灌注180min组(IRI组)。检测:①心肌组织及血清中NO2-+NO3-值;②心功能指标:心率(HR)和平均动脉压(MAP);③血清肌酸激酶同工酶(CK-MB)含量;④以缺口末端标记法(TUNEL)检测心肌细胞凋亡的变化。结果心肌细胞缺血再灌注后:①心肌组织NO2-+NO3-IRI组较假手术组增高26.7%(P<0.05),血清NO2-+NO3-IRI组较实验后较实验前升高41.1%(P<0.01);②HR和MAPIRI组显著低于假手术组;③血清CK-MB含量IRI组较假手术组升高51%(P<0.01);④心肌组织TUNEL染色IRI组检测到大量阳性凋亡细胞,凋亡细胞阳性指数(AI)IRI组较假手术组升高89.4%(P<0.01)。结论NO升高可能是诱导细胞凋亡,加重心肌细胞缺血/再灌注损伤的重要原因之一。     

The role of nitric oxide in iron-induced rat renal injury

Iron overload and enhanced hydroxyl radical (*OH) formation have been implicated as the causative factors of oxidative stress in different organs. Both pro-oxidant and anti-oxidant properties have been reported for nitric oxide (NO) in iron-mediated tissue injury. To determine the contribution of NO to iron-induced renal injury, eight groups of rats (eight in each group) were studied as follows: control (normal saline), L-Arg (L-arginine as a substrate of NO synthase, 400 mg/kg), L-NAME (an inhibitor of NO synthase, 8 mg/kg), Fe (iron dextran, 600 mg/kg), DFO (deferroxamine as a chelator of iron, 150 mg/kg), Fe+L-Arg, Fe+L-NAME, DFO+L-Arg. Twenty-four hours after the injections, blood samples were taken and kidneys removed for biochemical analysis. Plasma creatinine and urea were used to stimulate renal function. Renal tissue and plasma vitamin E levels, the most important endogenous fat soluble antioxidant, were measured by HPLC and UV detection. In this study, renal function was markedly reduced in the Fe group compared to controls (creatinine, 1.02+/-0.05 mg/dL versus 0.78+/-0.04 P <0.05; urea, 49.59+/-1.69 mg/dL versus 40.75+/-0.86, P <0.01). Vitamin E levels were significantly lower in the Fe group compared to controls (plasma P <0.01; renal tissue P <0.05). Administration of L-Arg to Fe-treated groups prevented these reductions. L-NAME increased iron-induced toxicity significantly, demonstrated by further reduction in the vitamin E levels and renal function compared to the Fe group alone. We concluded that NO plays an important role in protecting the kidney from iron-induced nephrotoxicity. NO synthase blockade enhances iron-mediated renal toxicity in this model.     

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.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of nitric oxide in inhibitory neurotransmission in the middle cerebral artery of the sheep

• 1.1. The involvement of nitric oxide (NO) as a mediator of inhibitory neurotransmission and its potential release mechanism in sheep isolated middle cerebral artery rings was investigated using NO synthase inhibitors, haemolysate, superoxide dismutase (SOD) and ω-conotoxin GVIA. In the presence of guanethidine (5 μM) and atropine (2 μM), transmural nerve stimulation of precontracted artery rings elicited an endothelium-independent vasodilator response that could be abolished by tetrodotoxin.
• 2.2. The magnitude of the vasodilator response was virtually abolished by N^G-nitro-l-arginine-p-nitroanilide (l-NAPNA; 100–500 μM) and significantly reduced by N^G-nitro-l-arginine (50 μM) or haemolysate (1 μl ml⁻¹). N^G-nitro-d-arginine (50 μM) had no effect. In the presence of the NO synthase inhibitors, addition of l-arginine (300 μM) produced either no effect or a partial, transient restoration of inhibitor responses following electrical field stimulation (EFS). l-NAPNA (100 μM) did not affect the relaxant response to the NO donor SIN-1. These results suggest that NO is involved in the relaxation elicited by transmural nerve stimulation.
• 3.3. Superoxide dismutase (SOD; 150 Uml⁻¹) did not produce any significant changes in the magnitude of the EFS-induced vasodilation. Thus, superoxide anions appear not to be a limiting factor for NO-mediated neurogenic vasodilation in sheep MCA.
• 4.4. ω-Conotoxin GVIA (100 nM) caused an almost immediate abolition of the EFS-induced vasoconstrictor response at resting tension, but had no effect on the vasodilator response at all frequencies of stimulation (0.5–8 Hz) tested. Thus, the neurotransmission process mediating this vasodilator response does not appear to involve Ca²⁺ entry via N-type Ca²⁺ channels.

     

The role of nitric oxide in aloe-induced diarrhoea in the rat

The role of nitric oxide (NO) on aloe-induced diarrhoea was studied in the rat. Nine hours after oral administration, aloe produced diarrhoea at doses of 5 g kg(-1)(20% rats with diarrhoea) and 20 g kg(-1) (100% of rats with diarrhoea). Lower doses of aloe (0.1 and 1 g kg(-1) did not produce a diarrhoeal response. Pre-treatment (i.p.) of rats with the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME 2.5-25 mg kg(-1) reduced the diarrhoea induced by aloe (20 g kg(-1) 9 h after its oral administration. L-NAME (25 mg kg(-1)) also reduced the increase in faecal water excretion produced by aloe (20 g kg(-1). L-arginine (1500 mg kg(-1), i.p.), administered to rats pre-treated with L-NAME (25 mg kg(-1), drastically reduced the effect of L-NAME on diarrhoea and increase in faecal water excretion induced by aloe (20 g kg(-1). Given alone, L-arginine did not modify aloe-induced diarrhoea. Basal Ca2+ -dependent NO synthase activity in the rat colon was dose-dependently inhibited by aloe (0.1-20 g kg(-1)) and by aloin (0.1-1 g kg(-1)), the active ingredient of aloe. These results suggest that endogenous NO modulates the diarrhoeal effect of aloe.     

Hyperbaric oxygen increases plasma exudation in rat trachea: involvement of nitric oxide

This study investigates the microvascular permeability changes in tracheal tissue of rats exposed to hyperbaric oxygen (HBO). Rats, following exposure to HBO or ambient air (control animals) for 1.5, 3 and 6 h, were prepared for recording of nitric oxide exhaled (FENO) in air using a chemiluminescence analyser. The level of FENO was not statistically different in the two groups. Plasma exudation, evaluated by measuring the leakage of Evans blue (EB) dye into the tracheal tissue, was significantly elevated (48, 86 and 105% at 1.5, 3 and 6 h, respectively) in HBO-treated rats. Plasma exudation in the trachea of control rats was significantly increased (42%, P<0.05) by NG-nitro-L-arginine methyl ester (L-NAME), whereas it was significantly reduced (31%, P<0.05) in rats exposed to HBO for 3 h. N-acetylcysteine (NAC) and flunisolide significantly prevented the increase in plasma leakage in HBO-treated rats. In contrast, indomethacin was devoid of anti-exudative activity in these experiments. Western immunoblot showed a significant increase in the level of inducible nitric oxide synthase (iNOS) protein in the tracheal homogenates of HBO-treated rats, as compared to basal levels. These results indicate that nitric oxide (NO) is involved in the maintenance of microvascular permeability in tracheal tissue of rats. The protective effect observed with the steroid seems to support this hypothesis. Furthermore, the beneficial action of NAC underlines that reactive oxygen species participate in the microvascular permeability changes observed in tracheal tissue of rats exposed to HBO.     

Inducible nitric oxide synthase in rat neutrophils: role of insulin.

Defective leukocyte-endothelial interactions are observed in experimental diabetes mellitus. Endogenous substances, including nitric oxide (NO), have anti-inflammatory effects within the vasculature by reducing leukocyte adherence to post-capillary venules. The purpose of this study was to examine the activity and expression of NO synthase in neutrophils from alloxan-induced diabetic rats. Glycogen-elicited peritoneal neutrophils were obtained from diabetic rats and matching controls 10, 30, and 180 days after alloxan (42 mg/kg, i.v.) or saline injection. NO synthase activity was determined by the [3H]L-citrulline assay method. Expression of the enzyme was investigated by western blot analysis. Relative to controls, neutrophils obtained from diabetic rats presented a 2-fold increase in the activity of inducible NO synthase (iNOS), accompanied by an increase in the expression of the enzyme depicted by western blot. Treatment of diabetic animals with NPH insulin (2 IU/day, for 3 days) reduced both the activity and expression of iNOS to normal levels. Results presented suggest that overexpression of the inducible isoform of NO synthase by neutrophils may be responsible, at least in part, for the defects in leukocyte-endothelial interactions in diabetes mellitus.     

Endothelial nitric oxide modulates perivascular sensory neurotransmission in the rat isolated mesenteric arterial bed

1. A possible role of nitric oxide (NO) as a modulator of capsaicin-sensitive sensory neurotransmission in blood vessels was investigated in the rat isolated mesenteric arterial bed. 2. Electrical field stimulation (EFS) of methoxamine-preconstricted mesenteric beds elicited frequency-dependent vasorelaxation mediated by capsaicin-sensitive sensory nerves. N(G)-nitro-L-arginine methyl ester (L-NAME, 10 and 300 microM) and 7-nitroindazole (7-NI, 100 microM), inhibitors of nitric oxide synthase (NOS), augmented sensory neurogenic vasorelaxation. D-NAME (300 microM), 6-aminoindazole (100 microM) and N(omega)-propyl-L-arginine (50 nM), a selective inhibitor of neuronal NOS, were without effect. The effect of 10 microM L-NAME was reversed by L-arginine (1 mM), the substrate for NOS. 3. L-NAME (300 microM) and 7-NI (100 microM) had no significant effect on vasorelaxations to calcitonin gene-related peptide (CGRP), the principal motor neurotransmitter of capsaicin-sensitive sensory nerves in rat mesenteric arteries, or to capsaicin, indicating a prejunctional action. The inhibitors of NOS had no effect on vasorelaxation to forskolin, but augmented vasorelaxation to sodium nitroprusside (SNP). 4. Removal of the endothelium augmented sensory neurogenic vasorelaxation, but did not affect vasorelaxation to CGRP, indicating a prejunctional action of endothelial NO. 5. In the absence of endothelium, L-NAME (300 microM) inhibited, and 7-NI (100 microM) caused no further augmentation of sensory neurotransmission. 6. SNP (100 nM), a nitric oxide donor, attenuated sensory neurogenic relaxations to EFS. 7. In rat isolated thoracic aortic rings, L-NAME (100 microM) and 7-NI (100 microM) attenuated concentration-dependent relaxations to acetylcholine. 8. These data show that NO modulates sensory neurotransmission evoked by EFS of the rat isolated mesenteric arterial bed, and that when NO synthesis is blocked sensory neurogenic relaxation is augmented. The source of NO is the vascular endothelium.     

Calcium channel subtypes for cholinergic and nonadrenergic noncholinergic neurotransmission in isolated guinea pig trachea

The Ca²⁺ channel subtypes in the neurotransmission of isolated guinea pig trachea were elucidated by monitoring the effects of specific Ca²⁺ channel blockers on cholinergic contractions and nonadrenergic noncholinergic (NANC) relaxation elicited by electrical field stimulation (EFS). In isolated guinea pig trachea, cholinergic contractile responses to low- and high-frequency EFS were inhibited by the selective N-type calcium channel blocker, ω-conotoxin MVIIA. ω-Agatoxin IVA (a selective P-type blocker), ω-conotoxin MVIIC (a nonselective N-, Q-, and P-type blocker), and nifedipine (a selective L-type blocker) were ineffective, whereas Ni²⁺ (a T- and R-type blocker) facilitated cholinergic contractions and produced a late contracture when its concentration exceeded 30 μM. The more the concentration of Ni²⁺ increased, the greater the number of incidences and the late contracture areas which occurred. Late contracture did not seem to be due to the effects of acetylcholine, tachykinins, or other polypeptides, but disappeared in the absence of indomethacin. The NANC relaxant responses elicited by the low- and high-frequency EFS were inhibited by ω-conotoxin MVIIA or Ni²⁺, but unaffected by ω-Agatoxin IVA, ω-conotoxin MVIIC, and nifedipine. In the absence of indomethacin, Ni²⁺ did not alter the ω-conotoxin MVIIA (100 nM)-resistant component of cholinergic contraction, but significantly further inhibited that of NANC relaxation. These results suggest that in isolated guinea pig trachea, cholinergic contraction is regulated by N-type calcium channels which may mask T- and R-type calcium channels and may be co-modulated by both, while NANC relaxation is mainly and independently controlled by N-, T-, and R-type calcium channels.     

The role of nitric oxide in the altered vascular reactivity of pregnancy in the rat.

1. Pregnancy is characterized by a decrease in systemic vascular resistance and a blunting of the angiotensin II (AII) pressor response. We studied the role of nitric oxide (NO) and prostanoids in these vascular changes of pregnancy in anaesthesized, ganglion blocked non-pregnant and pregnant rats. 2. Inhibition of NO synthesis with NG-nitro-L-arginine methyl ester (L-NAME) led to an increase in mean arterial pressure (MAP) which was of a significantly greater magnitude in pregnant rats in late gestation than in non-pregnant rats, or rats in mid-gestation. 3. The pressor response to varying doses of AII was attenuated during late pregnancy, and this attenuation was partially reversed by L-NAME. 4. The pressor response to varying doses of a vasoconstrictor, phenylephrine (PE), was also attenuated in late pregnancy. However, this attenuation was not reversed by L-NAME. 5. Inhibition of prostanoid biosynthesis with meclofenamate did not alter basal MAP, nor the pressor response to varying doses of AII or PE in pregnant and non-pregnant animals. 6. It is concluded that (a) increased NO synthesis occurs during late gestation and contributes both to the decrease in systemic vascular resistance, as well as the blunting of the pressor response to AII during pregnancy, and (b) prostaglandins are not important in the maintenance of basal vascular tone, or the blunting of the pressor response to AII during pregnancy.     

Induction by endothelin-1 of epithelium-dependent relaxation of guinea-pig trachea in vitro: role for nitric oxide.

1. The purpose of the present experiments was to study the underlying mechanisms responsible for the relaxant action of endothelin-1 (ET-1) in the guinea-pig trachea in vitro. 2. In tracheal strips precontracted (60-70% of the maximum) with carbachol, ET-1 (1-100 nM) evoked slowly developing concentration-dependent relaxations. Preincubation of the tissues with the thromboxane A2/prostaglandin H2 receptor antagonist, BM 13505 (5 microM) significantly potentiated the relaxant response to ET-1. 3. Removal of the epithelium changed the response of precontracted tracheal preparations to ET-1 from a relaxation to a sustained contraction. 4. ET-1-induced relaxations were abolished by methylene blue (10 microM) and were almost completely attenuated by oxyhaemoglobin (5 microM) and NG-monomethyl-L-arginine (L-NMMA, 100 microM), an inhibitor of nitric oxide synthesis, but were not altered by indomethacin (10 microM). 5. In tracheal strips under passive tension, ET-1 (1-100 nM) elicited dose-dependent contractions. The sensitivity of tissues to ET-1 was significantly enhanced by removal of the epithelium (apparent EC50 values were 28.1 +/- 4.1 and 12.5 +/- 0.8 nM in intact and rubbed trachea, respectively, n = 7, P < 0.01). 6. Preincubation of intact tracheal strips with methylene blue, oxyhaemoglobin or L-NMMA did not mimic the effect of epithelium removal on ET-1-induced contractions. 7. There was a concentration-dependent increase in thromboxane A2 but not in PGE2 and prostacyclin release from intact tracheal strips following stimulation with ET-1 (5-100 nM).(ABSTRACT TRUNCATED AT 250 WORDS)