Block of some non-adrenergic inhibitory responses of smooth muscle by a substance from haemolysed erythrocytes

Robert F. Furchgott, Nobel laureate (1916-2009)--a personal reflection

Robert F. Furchgott, pharmacologist and joint winner of the Nobel Prize for Medicine or Physiology (1998) died on the 12th of May 2009 aged 92. By unlocking the astonishingly diverse biological actions of nitric oxide, Furchgott leaves behind a rich legacy that has both revolutionized our understanding of human physiology and stimulated new and exciting opportunities for drug development in a wide range of pathological conditions. In this article, William Martin, who worked with Furchgott for 2 years (1983-1985), following the exciting discovery of endothelium-derived relaxing factor/nitric oxide, pays tribute to his close friend and colleague.

Characterization of the rat isolated retractor penis muscle as a model for the study of nitrergic transmission

INTRODUCTION

The anococcygeus and retractor penis muscles are part of the erectile machinery in male rodents. The rat anococcygeus muscle is a widely used smooth muscle preparation for the study of the effects of test substances on adrenergic, nitrergic, and cholinergic transmission. There is, however, little information available on the process of autonomic transmission in the rat retractor penis muscle, although its autonomic innervation has generally been assumed to be similar to that of the anococcygeus muscle because of the contiguous nature of the two muscles. The present study investigated the involvement of nitrergic transmission in mediating relaxant responses of the rat retractor penis muscle to electrical field stimulation.

METHODS

The retractor penis muscle was isolated from Sprague-Dawley rats and mounted in Krebs solution. Phentolamine (5 microM) was added to the bath to block the adrenergic responses of the muscle, which was then precontracted with carbachol (10 microM).

RESULTS

Electrical field stimulation (20-30 V, 1 ms pulse width, at 0.5-20 Hz for 10 s) of the carbachol precontracted muscle elicited frequency-dependent relaxant responses (0.9-68%). Tetrodotoxin (1 microM), N(G)-nitro-L-arginine (L-NOARG) (50 microM), N(G)-nitro-L-arginine methylester (L-NAME) (100 microM), and haemoglobin (100 microM) inhibited these relaxant responses by 99.3%, 93.9%, 86.9%, and 77.5%, respectively. L-Arginine (250 microM) (but not its D-isomer) reversed the blockade produced by L-NOARG (72.7%) and L-NAME (81.5%).

DISCUSSION

Our results provide clear evidence that the inhibitory (relaxant) responses of the rat retractor penis muscle to electrical field stimulation are mediated by nitric oxide involving the L-arginine-nitric oxide synthase-nitric oxide pathway. The rat retractor penis muscle is a versatile preparation that can replace the cumbersome preparations from the pig, ox, and horse, hitherto used as pharmacological models for the study of the retractor penis muscle.

Biology of the anococcygeus muscle

The anococcygeus is a smooth muscle tissue of the urogenital tract which, in the male, runs on to form the retractor penis. The motor innervation is classically sympathetic with noradrenaline as transmitter, but the relaxant parasympathetic transmitter has only recently been identified as nitric oxide. Indeed, the anococcygeus has provided an extremely useful model with which to probe the mechanisms underlying this novel nitrergic system, including the importance of physiological antioxidants in maintaining the potency of nitric oxide as a neurotransmitter. The cellular mechanisms of contraction and relaxation are slowly being clarified, with particular interest in the contribution of capacitative calcium entry and the guanylyl cyclase/cyclic GMP system. Many questions remain unanswered, however, including the precise physiological role of the muscle, the identity of substances released from subcellular vesicles of nitrergic nerves, the unusual sensitivity of the tissue to certain peptides (oxytocin and urotensin II), and the nature of store-operated channels through which calcium enters the cell to maintain contraction.

Evidence that nitric oxide acts as an inhibitory neurotransmitter supplying taenia from the guinea-pig caecum

Nitric oxide synthase-containing nerve fibres are abundant within taenia of the guinea-pig caecum, but there is little previous evidence supporting a direct role for nitric oxide (NO) in responses to enteric inhibitory nerve stimulation. In this study we have attempted to identify an NO-dependent component of inhibitory transmission in isolated taenia coli. Isometric tension was recorded in the presence of atropine and guanethidine (both 1 microM). Tone was raised with histamine (1 microM), and intrinsic inhibitory neurons stimulated using either a nicotinic agonist (1,1-dimethyl-4-phenylpiperazinium iodide; DMPP) or electrical field stimulation (EFS). DMPP (1-100 microM) produced concentration-dependent biphasic relaxations, comprising an initial peak relaxation followed by a sustained relaxation. Responses to DMPP were antagonized by tetrodotoxin (1 microM) or apamin (0.3 microM) and abolished by hexamethonium (300 microM). L-nitro-arginine (L-NOARG; 100 microM) and oxyhaemoglobin (2%) both significantly reduced sustained relaxations produced by DMPP. EFS (5 Hz, 30 s) also produced biphasic relaxations. Both L-NOARG and an inhibitor of soluble guanylate cyclase (ODQ, 1-10 microM) reduced the sustained component of EFS responses. Two NO donors, sodium nitroprusside (SNP) and diethylenetriamine-nitric oxide adduct (DENO), produced concentration-dependent relaxations. Responses to SNP and DENO were antagonized by ODQ (1 microM) and by apamin (0.3 mM). These results suggest that NO contributes directly to a component of inhibitory transmission in guinea-pig taenia coli. The actions of NO appear to be mediated via cyclic GMP synthesis, and may involve activation of small conductance calcium activated K+ channels. A role for NO is most evident during sustained relaxations evoked by longer stimulus trains or chemical stimulation of intrinsic neurons.

Parallel pathways mediate inhibitory effects of vasoactive intestinal polypeptide and nitric oxide in canine fundus

1. The gastric adaptation reflex is activated by the release of non-adrenergic, non-cholinergic (NANC) inhibitory transmitters, including nitric oxide (NO) and vasoactive intestinal polypeptide (VIP). The role of NO in this reflex is not disputed, but some investigators suggest that NO synthesis is stimulated by VIP in post-junctional cells or in nerve terminals. We investigated whether the effects of these transmitters are mediated by independent pathways in the canine gastric fundus. 2. VIP and NO produced concentration-dependent relaxation of the canine fundus. Nomega-nitro-L-arginine (L-NNA) reduced relaxation induced by electrical field stimulation (EFS; 0.5-8 Hz), but had no effect on responses to exogenous VIP and sodium nitroprusside (SNP, 10 microM). 3. Oxyhaemoglobin reduced relaxations produced by EFS and SNP. Oxyhaemoglobin also reduced relaxation responses to low concentrations of VIP (<10 nM), but these effects were non-specific and mimicked by methaemoglobin which had no effect on nitrergic responses. 4. A blocker of guanylyl cyclase, 1H-[1,2,4]oxidiazolo [4,3,-a]quinoxalin-1-one, (ODQ) inhibited responses to EFS, SNP and DETA/NONOate (an NO.donor), but had no effect on responses to VIP. cis-N-(2-phenylcyclopentil)-azacyclotridec-1en-2-amine monohydrochloride (MDL 12,330A), a blocker of adenylyl cyclase, reduced responses to EFS, VIP and forskolin, but did not affect responses to SNP. 5. Levels of cyclic GMP were enhanced by the NO donor S-nitroso-n-acetylpenicillamine (SNAP) but were unaffected by VIP (1 microM). The increase in cyclic GMP in response to SNAP was blocked by ODQ. 6. The results suggest that at least two transmitters, possibly NO and VIP, mediate relaxation responses in the canine fundus. NO and VIP mediate responses via cyclic GMP- and cyclic AMP-dependent mechanisms, respectively. No evidence was found for a serial cascade in which VIP is coupled to NO-dependent responses.

A comparison of the effects of capsaicin with inhibitory nerve stimulation in the rat anococcygeus muscle in vitro

Capsaicin was used to test whether centrifugal activation of sensory fibres in the rat anococcygeus muscle can contribute to non-adrenergic non-cholinergic (NANC) relaxation of the muscle. In a solution containing 0.5 mM Ca2+ and in the presence of carbachol (10 microM) capsaicin evoked a fast concentration-dependent relaxation of the muscle that was usually followed by a smaller, slower, relaxant response. The fast relaxant response was reduced when extracellular Ca2+ was raised to 2.5 mM, desensitized after a single application of capsaicin and was blocked by tetrodotoxin (1 microM) or ruthenium red (10 microM). The fast response was greatly reduced by haemoglobin, by cold storage of the muscles or by N-monomethyl-L-arginine (100 microM) in the absence but not in the presence of L-arginine (100 microM). It is concluded that centrifugal activation of sensory fibres evokes a nitric oxide-mediated relaxation of the anococcygeus muscles that probably contributes to electrically evoked NANC relaxation.

Endothelium-dependent relaxation and hyperpolarization in guinea-pig coronary artery: role of epoxyeicosatrienoic acid

1. Acetylcholine (ACh) elicits an endothelium-dependent relaxation and hyperpolarization in the absence of nitric oxide (NO) and prostaglandin synthesis in the guinea-pig coronary artery (GPCA). This response has been attributed to a factor termed endothelial-derived hyperpolarizing factor (EDHF). Recently it has been suggested that EDHF may be a cytochrome P450 product of arachidonic acid (AA) i.e., an epoxyeicosatrienoic acid (EET). The present study investigated whether this pathway could account for the response to ACh observed in the GPCA in the presence of 100 microM N(omega)-nitro-L-arginine and 10 microM indomethacin. 2. ACh, AA and 11,12-EET each produced concentration-dependent relaxations in arteries contracted with the H1-receptor agonist AEP (2,2-aminoethylpyridine). The AA-induced relaxation was significantly enhanced in the presence of the cyclo-oxygenase/lipoxygenase inhibitor, eicosatetranynoic acid (30 microM). 3. The cytochrome P450 inhibitors proadifen (10 microM) and clotrimazole (10 microM) inhibited ACh, lemakalim (LEM) and AA-induced relaxation, whereas 17-octadecynoic acid (100 microM) and 7-ethoxyresorufin (10 microM) were without effect on all three vasodilators. Proadifen and clotrimazole also inhibited ACh (1 microM) and LEM (1 microM)-induced hyperpolarization. 4. The ability of various potassium channel blockers to inhibit relaxation responses elicited with ACh, AA and 11,12-EET was also determined. Iberiotoxin (IBTX; 100 nM) was without effect on responses to ACh but significantly reduced responses to both AA and 11,12-EET. In contrast, 4-aminopyridine (4-AP; 5 mM) significantly reduced response to ACh but not responses to AA and 11,12-EET. Combined IBTX plus (4-AP) inhibited the ACh-induced relaxation to a greater extent than 4-AP alone. Apamin (1 microM), glibenclamide (10 microM) and BaCl2 (50 microM) had no significant effect on responses to ACh, AA and 11,12-EET. 5. IBTX (100 nM) significantly reduced both 11,12-EET (33 microM) and AA (30 microM) hyperpolarization without affecting the ACh (1 microM)-induced hyperpolarization. In contrast, 4-AP significantly reduced the ACh-induced hyperpolarization without affecting either AA or 11,12-EET-induced hyperpolarizations. 6. In summary, our results suggest that the coronary endothelium releases a factor upon application of AA which hyperpolarizes the smooth muscle. The similarity of pharmacology between AA and 11,12-EET suggests that this factor is an EET. However, the disparity of pharmacology between responses to ACh versus responses to 11,12-EET do not support the hypothesis that EETs represent the predominant factor which ACh releases from the endothelium that leads to NO- and prostaglandin-independent hyperpolarization and relaxation in the GPCA.

Basal release of nitric oxide induces an oscillatory motor pattern in canine colon

1. The consequences of intrinsic, basal nitric oxide release on electrical and contractile activity of canine proximal colon were examined. Membrane potential and contraction were simultaneously recorded from the circular muscle in the presence of drugs to block adrenergic and cholinergic responses. 2. Electrical slow waves were recorded from muscle cells near the submucosal surface of the circular layer. Spontaneous contractions were initiated by each slow wave. Contractile amplitude increased 1.9-fold when nerves were blocked with tetrodotoxin (TTX, 1 microM). 3. Muscle cells near the myenteric surface displayed myenteric potential oscillations (MPOs) averaging 16 cycles per minute (c.p.m.) in frequency and 10 mV in amplitude. Twenty-five per cent of muscles displayed an additional slow, neurogenic oscillation (mean frequency, 1 c.p.m.; amplitude, 14 mV) superimposed upon the MPO rhythm. 4. The nitric oxide (NO) synthase inhibitor N omega -nitro-L-arginine (L-NA, 100 microM; n = 16) abolished neurogenic oscillations, depolarized cells, and increased MPO upstroke velocity, amplitude and frequency. The actions of L-NA were mimicked by N omega-nitro-L-arginine methylester (L-NAME, 100 microM) and oxyhaemoglobin (3%). 5. Spontaneous contractions were increased 2.3-fold by L-NA, and TTX had no effect on contractions after addition of L-NA. 6. The NO-donor sodium nitroprusside (SNP, 1 microM) reversed the electrical and mechanical effects of L-NA and initiated slow oscillations similar to the neurogenic oscillations. Slow oscillations were also evoked with S-nitroso-N-acetylpenicillamine (SNAP, 1 microM). The effects of NO donors were blocked by oxyhaemoglobin. 7. Slow electrical oscillations could not be elicited by SNP after removal of a thin strip of circular muscle along the myenteric edge. 8. These data suggest that the spontaneous electrical and contractile activity of the proximal colon is tonically suppressed by basal release of NO. Basal NO causes an oscillatory pattern of electrical and mechanical activity. This activity does not require patterned firing of nerves; rather a continuous, low level release of NO would be capable of producing the neurogenic oscillatory behaviour. The slow oscillatory activity depends upon the presence of the myenteric region of the circular muscle layer, which contains cell bodies of enteric neurons and interstitial cells of Cajal.

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

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 omega-conotoxin GVIA. In the presence of guanethidine (5 microM) and atropine (2 microM), transmural nerve stimulation of precontracted artery rings elicited an endothelium-independent vasodilator response that could be abolished by tetrodotoxin. 2. The magnitude of the vasodilator response was virtually abolished by NG-nitro-L-arginine-p-nitroanilide (L-NAPNA; 100-500 microM) and significantly reduced by NG-nitro-L-arginine (50 microM) or haemolysate (1 microliter ml-1). NG-nitro-D-arginine (50 microM) had no effect. In the presence of the NO synthase inhibitors, addition of L-arginine (300 microM) produced either no effect or a partial, transient restoration of inhibitor responses following electrical field stimulation (EFS). L-NAPNA (100 microM) 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. 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. omega-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.

Hyperpolarization and inhibition of contraction mediated by nitric oxide released from enteric inhibitory neurones in guinea-pig taenia coli

1. Inhibition of nitric oxide synthase by NG-nitro-L-arginine (L-NNA) reduced the neurogenic relaxation of precontracted taenia coli only in the absence of atropine. The membrane hyperpolarization associated with the neurogenic relaxation was also reduced by inhibition of NOS only when atropine was absent. 2. The membrane hyperpolarization associated with the neurogenic relaxation of the taenia coli was inhibited by oxyhaemoglobin only in the absence of atropine. In the presence of atropine, oxyhaemoglobin did not reduce the i.j.p. or nerve evoked relaxation. 3. Inhibition of NOS by L-NNA did not affect the overflow of [3H]-ACh in response to electrical field stimulation (EFS), suggesting that, under the conditions of our experiments, endogenous NO did not modulate release of ACh. Sodium nitroprusside also had no effect on the neurogenic overflow of [3H]-ACh; however, noradrenaline significantly reduced [3H]-ACh overflow. 4. In summary, the postjunctional effects of neurally-released NO are not apparent in guinea-pig taenia coli when atropine is present. This implies muscarinic regulation of NO release or muscarinic regulation of another excitatory substance, such as tachykinin(s), that, when blocked, masks the postjunctional effects of NO. These data, together with previous studies, suggest a possible regulatory role for NO in enteric neurotransmission that may be more prominent in some species or tissues than others.

Blockade of nitrergic transmission by hydroquinone, hydroxocobalamin and carboxy-PTIO in bovine retractor penis: role of superoxide anion

1. The effects of inhibiting endogenous Cu/Zn superoxide dismutase (SOD) with diethyldithiocarbamate (DETCA) were examined on the ability of hydroquinone, hydroxocobalamin and carboxy-PTIO to block nitrergic relaxation in the bovine retractor penis (BRP) muscle. 2. Incubation of strips of BRP with DETCA (3 mM) for 2 h reduced SOD activity from 73.1 +/- 15.7 to 8.2 +/- 1.9 units mg-1 protein. 3. Hydroquinone (10 microM--1 mM) produced weak inhibition of nitrergic (4 Hz, 10 s) relaxation in control strips of BRP, but powerful inhibition in strips treated with DETCA (3 mM, 2 h). Exogenous SOD (250 units ml--1) produced a partial blockade of the ability of hydroquinone to inhibit nitrergic relaxation in DETCA-treated strips. 4. In an assay of SOD-inhibitable reduction of cytochrome C, hypoxanthine (0.1 mM)/xanthine oxidase (16 munits ml-1) and pyrogallol (10 microM), led to the rapid generation of superoxide anion. Hydroquinone (10 microM) also led to the generation of the free radical, although the rate of generation was slower. 5. Two NO-scavenging agents, hydroxocobalamin (0.1 microM--1 mM) and carboxy-PTIO (0.1-1 mM), produced concentration-dependent blockade of nitrergic relaxation of the BRP. The magnitude of the blockade induced by these agents was unaffected following treatment with DETCA or SOD. 6. The findings with hydroquinone support our previous proposal that endogenous Cu/Zn SOD plays a vital role in protecting nitrergic neurotransmission from inactivation by superoxide anion. Results with hydroxocobalamin and carboxy-PTIO are consistent with the known ability of these agents to scavenge NO. The nitrergic neurotransmitter in the BRP thus appears to have the properties of NO.

Comparison of the effects of hydroxocobalamin and oxyhaemoglobin on responses to NO, EDRF and the nitrergic transmitter

1. The effects of ranges of concentrations of oxyhaemoglobin (0.01-30 microM) and hydroxocobalamin (1-100 microM) were compared for their abilities to reduce relaxant responses to EDRF released by acetylcholine in endothelium-intact rat aortic rings, the nitergic transmitter in rat anococcygeus muscles, and NO in aqueous solution in both tissues (aortic rings were denuded of endothelium). 2. The concentrations of oxyhaemoglobin producing 50% reduction of responses to EDRF and NO in rat aorta correspond closely, the IC50 values being 0.13 +/- 0.02 microM and 0.11 +/- 0.02 microM respectively. 3. Oxyhaemoglobin was equally effective in inhibiting responses to NO in anococcygeus muscles and in aortic rings with an IC50 of 0.14 +/- 0.05 microM. However, responses to the nitrergic transmitter were considerably less sensitive to inhibition by oxyhaemoglobin, the IC50 being 19.7 +/- 5.1 microM. 4. The IC50 values for hydroxocobalamin in inhibiting responses to EDRF and NO in aorta were 3.4 +/- 0.2 microM and 8.4 +/- 0.63 microM, respectively, but it was less effective against responses to NO in anococcygeus muscles the IC50 being 46 +/- 9.6 microM. However, even in the highest concentration used (100 microM), it did not reduce responses to the nitrergic transmitter. 5. The findings are compatible with the views that EDRF is NO, but suggest that the nitergic transmitter in the rat anococcygeus muscle does not behave like free NO.

Relaxation of sheep cerebral arteries by vasoactive intestinal polypeptide and neurogenic stimulation: inhibition by L-NG-monomethyl arginine in endothelium-denuded vessels

1. Perivascular nerves of the sheep middle cerebral artery show immunoreactivity for both vasoactive intestinal polypeptide (VIP) and calcitonin gene-related peptide (CGRP). 2. Rings of endothelium-denuded sheep middle cerebral artery precontracted with 5-hydroxytryptamine were relaxed by CGRP (maximum relaxation = 87.8 +/- 8.1%, pD2 = 7.81 +/- 0.12, n = 12) and by VIP (maximum relaxation = 55.1 +/- 4.1%, pD2 = 7.65 +/- 0.04, n = 18). Rings of endothelium-denuded cat middle cerebral artery precontracted with U46619 were also relaxed by vasoactive intestinal polypeptide (maximum relaxation = 53.1 +/- 6.1%, pD2 = 7.82 +/- 0.11, n = 6). 3. Haemolysate (1 microliters ml-1) inhibited VIP-induced relaxation in endothelium-denuded sheep and cat middle cerebral artery (n = 6) but had no effect on the CGRP-induced relaxation of the sheep middle cerebral artery (n = 6). 4. The relaxant response to VIP in endothelium-denuded sheep middle cerebral artery was inhibited by methylene blue (10 microM) and augmented by either M&B 22948 (10 microM) or superoxide dismutase (150 units ml-1). Indomethacin (1 microM) had no effect. 5. The addition of L-NG-monomethyl arginine (100 microM) inhibited both neurogenic and VIP-induced relaxation of endothelium-denuded sheep MCA by 56 +/- 6% and 60 +/- 6% (n = 5) respectively. The CGRP-induced relaxation was unaffected. 6. It is concluded that neurally mediated vasodilatation in the sheep middle cerebral artery is mediated largely by VIP through a direct action on smooth muscle through a cyclic-GMP-mediated mechanism that appears to involve synthesis of nitric oxide from L-arginine. Vasodilatation by CGRP, which is also contained in perivascular nerves, does not utilize this pathway.

Relaxation of the bovine retractor penis muscle by a small K+ excess and the role of K+ in its neurogenic relaxation

The mechanism of the relaxation of the bovine retractor penis muscle induced by 6.7 mM K+ as well as the role of K+ in the neurogenic relaxation of this muscle induced by nicotine, acetylcholine or electrical field stimulation, was studied. The relaxation induced by 6.7 mM K+ was, contrary to that induced by nicotine or electrical field stimulation, abolished by 10(-7) M ouabain. 15 min exposure to 10(-5) M NG-nitro-L-arginine, 3.2 x 10(-6) M tetrodotoxin, 5.0 x 10(-4) M hexamethonium, 5.3 x 10(-4) M methylene blue or hypoxia, all known to inhibit the neurogenic relaxation, did not affect the relaxation induced by 6.7 mM K+, which was also unaffected by 10(-5) M apamin, 3 x 10(-3) M 4-aminopyridine, 2.6 x 10(-2) M tetraethylammonium and 7.3 x 10(-4) M Ba2+. Exposure to K(+)-free solution reversibly abolished the neurogenic relaxations. The relaxations caused by 5.0 x 10(-7) M cromakalim and 2.0 x 10(-6) M pinacidil were totally blocked by 10(-5) M glibenclamide. Glibenclamide and apamin did not affect the tone of the muscle or its neurogenic relaxations. 4-Aminopyridine 4.0 x 10(-5) to 3.0 x 10(-3) M and tetraethylammonium 10(-4) to 2.6 x 10(-2) M raised the tone and enhanced the relaxations elicited by electrical field stimulation. The results indicate that the relaxation induced by 6.7 mM K+ is partly mediated by activation of Na(+)-K(+) ATPase and that its mechanism is thoroughly different from that of the neurogenic relaxations.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclic GMP-independent relaxation and hyperpolarization with acetylcholine in guinea-pig coronary artery

1. The effects of acetylcholine (ACh) on membrane potential, relaxation and cyclic GMP levels were compared to the NO donor L-nitrosocysteine (Cys-NO) in segments of guinea-pig coronary artery. 2. ACh and Cys-NO produced concentration-dependent relaxations of muscles contracted with the H1 receptor agonist, 2-(2-aminoethyl)pyridine (AEP, 0.35 mM). The relaxation to ACh was unchanged in the presence of NG-monomethyl-L-arginine (L-NMMA; 350 microM) or indomethacin (3 microM). 3. Oxyhaemoglobin (HbO; 20 microM) alone or in combination with L-NMMA increased the EC50 for ACh-induced relaxation whereas relaxation with Cys-NO was almost completely abolished with HbO. 4. Scorpion venom (SV; 8.7 micrograms ml-1) increased the EC50 for relaxation with ACh but not Cys-NO. Combined L-NMMA, HbO and SV produced nearly complete abolition of ACh-induced relaxations. 5. Basal cyclic GMP levels (i.e., 20 pmol mg-1 protein) were significantly increased following addition of either ACh (190 pmol mg-1 protein) or Cys-NO (240 pmol mg-1 protein). L-NMMA significantly reduced the rise of cyclic GMP with ACh but not Cys-NO. In contrast, SV did not significantly reduce the rise in cyclic GMP produced with ACh. In the combined presence of L-NMMA and HbO neither ACh nor Cys-NO produced a significant increase in cyclic GMP levels. 6. ACh gave rise to significantly greater membrane hyperpolarization than Cys-NO both in the presence and absence of AEP. Combined L-NMMA and HbO did not reduce the amplitude of hyperpolarization with ACh. 7. These data indicate that some but not all of the actions of ACh in the coronary artery can be mimicked by the NO donor, Cys-NO, suggesting that ACh releases NO as well as a second hyperpolarizing factor (i.e., EDHF). Release of NO results in a large increase in tissue cyclic GMP levels and minimal change in membrane potential whereas release of EDHF results in a large membrane hyperpolarization which is independent of changes in tissue cyclic GMP levels. Both of these pathways appear to contribute to relaxation throughout the entire ACh concentration-relaxation relationship.

Localization of nitric oxide synthase in canine ileocolonic and pyloric sphincters

The distribution of neurons containing NAD-PH-diaphorase (NADPH-d) activity and nitric oxide synthase-like immunoreactivity (NOS-LI) in the canine pyloric and ileocolonic sphincters was studied. Cells within the myenteric and submucosal ganglia were positive for NADPH-d. These cells generally had the morphology of Dogiel type-I enteric neurons, however, there was some diversity in the morphology of NADPH-d-positive neurons in the myenteric plexus of the pylorus. Intramuscular ganglia were observed in both sphincters, and NADPH-d was found in a sub-population of neurons within these ganglia. Dual staining with an antiserum raised against nitric oxide synthase (NOS) demonstrated that almost all cells with NOS-LI were also NADPH-d positive. Varicose fibers within ganglia and within the circular and longitudinal muscle layers also possed NOS-LI and NADPH-d activity. Dual staining with anti-VIP antibodies showed that some of the NADPH-d-positive cells in the myenteric and submucosal ganglia also contained VIP-LI, but all VIP-LI-positive cells did not express NADPH-d activity. These data are consistent with recent physiological studies suggesting that nitric oxide serves as an inhibitory neurotransmitter in the pyloric and ileocolonic sphincters. The data also suggest that VIP is expressed in a sub-population of NADPH-d-positive neurons and may therefore act as a co-transmitter in enteric inhibitory neurotransmission to these specialized muscular regions.

Enteric inhibitory neural regulation of human colonic circular muscle: role of nitric oxide

BACKGROUND

Nitric oxide and an apamin-sensitive transmitter may both contribute to neural inhibition in the human colon. The present study investigated the role of NO in regulating spontaneous rhythmic contractions and examined NO-dependent and independent components of neurally evoked hyperpolarization in the human colon.

METHODS

Mechanical and electrical activity were recorded from isolated circular muscle strips.

RESULTS

Rhythmic contractions were inhibited by nerve stimulation. This response was reduced by apamin, oxyhemoglobin, and L-NG-nitro arginine methyl ester (L-NAME). Electrical recording revealed two components of neurally evoked hyperpolarization: a fast hyperpolarization resulting from a single stimulus and a sustained hyperpolarization that developed with repetitive stimulation. Fast hyperpolarization was not affected by L-NAME or oxyhemoglobin but was significantly reduced by apamin. The sustained hyperpolarization was reduced by L-NAME or apamin. Exogenous NO and the P2y receptor agonist 2-methylthio adenosine 5'-triphosphate (2-MATP) inhibited spontaneous contractions and produced hyperpolarization. Apamin reduced the effects of 2-MATP but not those of NO.

CONCLUSIONS

The results support the concept that the inhibitory neurotransmission in the human colon involves two transmitters. A single stimulus results in an apamin-sensitive response. With multiple stimuli, a NO-dependent response develops and sums with the apamin-sensitive mechanism, producing sustained hyperpolarization and inhibition of contractions.

NADPH diaphorase innervation of the rat anococcygeus and retractor penis muscles

NADPH diaphorase histochemistry was used to determine whether the rat anoocccygeus (AC) and retractor penis (RP) muscles are innervated by nerves capable of synthesizing nitric oxide. In both tissues, muscle fascicles were enclosed by a varicose plexus of NADPH diaphorase positive (ND+) fibers. Perikarya of neurons on the surface of the AC muscle were also intensely stained for NADPH diaphorase. Many AC-RP ganglion cells in the pelvic plexus, located by the retrograde tracer Fluorogold, also stained for the enzyme. However, a significant population of AC-RP neurons in this location remained unstained. These results provide further evidence that nitric oxide may be an important neurotransmitter in these tissues.

Evidence for nitric oxide as an inhibitory neurotransmitter in rabbit isolated anococcygeus

The role of nitric oxide (NO) and ATP as putative inhibitory non-adrenergic, non-cholinergic (NANC) neurotransmitters was investigated in rabbit isolated anococcygeus after block of adrenergic and cholinergic responses, and raising tone with histamine. NANC nerve stimulation produced rapid relaxations which were completely abolished by tetrodotoxin. The magnitude of the NANC inhibitory responses was significantly reduced by the nitric oxide synthase inhibitors NG-nitro-L-arginine (NO-Arg) and NG-nitro-L-arginine methyl ester (L-NAME). This effect could be partially reversed by L-arginine but not by D-arginine. Oxyhaemoglobin inhibited NANC nerve responses and sodium nitroprusside mimicked the effects of NANC nerve stimulation. NO-Arg also reduced the magnitude of the inhibitory junction potentials recorded from the smooth muscle cells during NANC nerve stimulation. Exogenously applied ATP and adenosine each produced concentration dependent relaxations which were unaffected by the NO-synthase inhibitor NO-Arg. Relaxations to adenosine were virtually abolished by the P1 purinoceptor antagonist 8-(p-sulphophenyl)theophylline. Relaxations to ATP were also significantly reduced, indicating that part of the response to exogenous ATP is due to its breakdown to adenosine and subsequent action on P1 purinoceptors. Relaxations of the tissue to ATP and adenosine were unaffected by the P2 purinoceptor antagonist suramin. NANC nerve mediated responses were not significantly changed by either 8-(p-sulphophenyl)theophylline or suramin. These results suggest that NO is involved in inhibitory NANC neurotransmission in the rabbit isolated anococcygeus, but do not support a role for ATP as a NANC neurotransmitter in this tissue.

Role of nitric oxide in non-adrenergic, non-cholinergic inhibitory junction potentials in canine ileocolonic sphincter

1. Electrical field stimulation causes neurally-mediated relaxation of the ileocolonic sphincter that is due to activation of non-adrenergic and non-cholinergic (NANC) nerves. Recent studies have suggested that nitric oxide (NO) is the neurotransmitter that mediates relaxation. 2. Using intracellular recording techniques, we have tested whether NANC inhibitory junction potentials (i.j.ps) in the canine ileocolonic sphincter are also mediated by NO. 3. Electrical field stimulation elicited excitatory and inhibitory junction potentials: e.j.ps were blocked by atropine (10(-6) M) and tetrodotoxin (TTX; 10(-6) M); i.j.ps were also blocked by TTX and partially blocked by apamin (10(-6) M). I.j.ps were unaffected by atropine, phentolamine and propranolol (all at 10(-6) M). 4. The arginine analogues, L-NG-nitroarginine methyl ester (L-NAME) and NG-monomethyl-L-arginine (L-NMMA), decreased the amplitude of i.j.ps and L-arginine, but not D-arginine, partially restored the i.j.ps. 5. I.j.ps were also inhibited by oxyhaemoglobin (1%), but not by methaemoglobin. 6. Exogenous NO (10(-7) M to 3 x 10(-5) M) caused concentration-dependent hyperpolarizations that were similar in amplitude to the NANC nerve-evoked i.j.ps. Hyperpolarizations to NO were unaffected by L-NAME, but were blocked by oxyhaemoglobin. 7. Tetrodotoxin, L-NAME and oxyhaemoglobin all caused depolarization of resting membrane potential. 8. The specific guanosine 3':5'-cyclic monophosphate phosphodiesterase inhibitor, M&B 22948, caused hyperpolarization, increased the maximum level of hyperpolarization reached during i.j.ps, and increased the duration of i.j.ps. 9. These data further support the hypothesis that NANC neurotransmission in the ileocolonic sphincter is mediated by NO or an NO-releasing compound. The data also suggest that tonic release of NO, possibly from spontaneous firing of NANC nerves, may regulate resting membrane potential and tone in this sphincter.

Cerebrovascular effects of substance P after experimental subarachnoid haemorrhage

The vasoactive effects of substance P (SP), as well as the content of cyclic guanine monophosphate (cGMP), were determined in the rabbit basilar artery after subarachnoid haemorrhage (SAH). Out of 47 rabbits, 24 were subjected to a SAH, induced by injecting 5 ml of autologous arterial blood into the cisterna magna; 23 were used as controls. In 20 animals (10 SAH and 10 controls), isometric tension recording of isolated rings of the basilar artery--dissected 2 days after SAH--was employed to assess the dose-dependent vasodilatation to SP (10(-10) to 10(-6) M) after precontraction with serotonin (10(-8) to 10(-5) M). In 15 animals (8 SAH and 7 controls), the basal cGMP content was measured in the basilar artery 2 days after SAH. In the other 12 animals (6 SAH and 6 controls), the increase in cGMP content was measured in the basilar artery after a 10-minute incubation with SP (10(-6) M). SP caused significantly less dilatation in animals subjected to SAH than in controls, especially for concentrations between 10(-9) and 10(-6) M (p < 0.001). The cGMP content in the arteries 2 days after SAH was significantly lower than in control arteries (31.5 +/- 7.3 against 57.3 +/- 4.3 pmoles/g tissue). In the preparations incubated with SP, the increase of cGMP was 440 +/- 115% in the control arteries, and only 97 +/- 30% in the arteries after SAH. It is concluded that the vasodilator activity of SP is significantly impaired after SAH.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitrergic transmission: nitric oxide as a mediator of non-adrenergic, non-cholinergic neuro-effector transmission

1. The possibility that transmission at some non-adrenergic, non-cholinergic (NANC) neuro-effector junctions is mediated by nitric oxide (NO) arose from the discoveries that NO mediated the effects of nitrovasodilator drugs and that endothelium-derived relaxing factor (EDRF) was NO or a NO-yielding substance. 2. NO donated by nitrovasodilator drugs or formed by endothelial cells activates soluble guanylate cyclase in smooth muscle and the consequent increase in cyclic guanosine monophosphate (cGMP) results in relaxation. The relaxations produced by stimulation of some NANC nerves are also due to a rise in cGMP. 3. The biosynthesis of NO by oxidation of a terminal guanidino nitrogen of L-arginine is inhibited by some NG-substituted analogues of L-arginine. These substances block EDRF formation by NO synthase and endothelium-dependent vasodilatation, and the blockade is overcome by L-arginine 4. NANC relaxations in some tissues are blocked by NG-substituted analogues of L-arginine and restored by L-arginine. Other agents that affect endothelium-dependent vasodilator responses produce corresponding changes in responses to stimulation of these NANC nerves. Such observations indicate that transmission is mediated by NO: we have termed this mode of transmission nitrergic. 5. There is evidence for nitrergic innervation of smooth muscle in the gastrointestinal tract, genito-urinary system, trachea and some blood vessels (penile and cerebral arteries). 6. The recognition of a mediator role for NO in neurotransmission calls for reconsideration of previously accepted generalizations about mechanisms of transmission. 7. Studies on nitrergic transmission will provide new insights into physiological control mechanisms and pathophysiological processes and may lead to new therapeutic developments.

Role of nitric oxide and cyclic GMP as mediators of endothelium-independent neurogenic relaxation in bovine mesenteric artery

Electrical field stimulation (EFS) of phenylephrine-contracted bovine mesenteric arteries pretreated with guanethidine elicited a relaxation that amounted to roughly 40%. This relaxation was sensitive to tetrodotoxin pretreatment, suggesting a neurogenic origin. The EFS-induced relaxation was correlated to an increase in cGMP level, from 14.2 +/- 2.5 pmol/g wet wt in nonstimulated arteries to 31.6 +/- 3.4 pmol/g wet wt after 1 minute of EFS. cAMP values were not affected by EFS. Methylene blue (5 microM) and the compound LY 83583 (10 microM), inhibitors of soluble guanylate cyclase, inhibited the EFS-induced relaxation by 60% and 50%, respectively. Zaprinast (1 microM), a selective inhibitor of cGMP degradation, significantly (p = 0.005) potentiated the EFS-induced relaxation. The relaxation induced by EFS in bovine mesenteric arteries exhibits characteristics similar to the relaxations evoked by organic nitroesters and endothelium-dependent vasodilators, both of which are suggested to be mediated by cGMP and probably with nitric oxide as the common activator of the cGMP system. The possible involvement of nitric oxide as a mediator of EFS-induced relaxations was investigated with the use of known modulators of endogenous nitric oxide production. Preincubation of the arteries with 1 mM arginine or 1 mM N-alpha-benzoyl-L-arginine, both reported to potentiate endogenous nitric oxide production, or 5 mM L-canavanine, 0.25 mM NG-monomethyl-L-arginine, or 0.1 mM NG-nitro-L-arginine, alleged inhibitors of endogenous nitric oxide production, were without effect on the relaxation induced by EFS. However, pyrogallol, a generator of superoxide anions, was a potent inhibitor of relaxations induced by EFS in bovine mesenteric arteries.(ABSTRACT TRUNCATED AT 250 WORDS)

Vascular responsiveness to various vasoactive substances after exchange transfusion with pyridoxalated hemoglobin polyoxyethylene conjugate (PHP) solution in anesthetized rats

Vascular responsiveness to various vasoactive substances after exchange transfusion with a modified hemoglobin (pyridoxalated hemoglobin polyxyethylene conjugate; PHP) was examined in anesthetized rats. The hindlimb region of the rat was perfused through the abdominal aorta with autologous blood at a constant flow rate, using a perfusion pump. Perfusion pressure (PP), systemic blood pressure (SBP), and heart rate were measured. Exchange transfusion with a 6% PHP solution (20 ml/kg) was performed and effects of intra-arterial injections of norepinephrine (NE), angiotensin-II (A-II), serotonin (5-HT), acetylcholine (ACh), and nitroglycerin (NG) on PP and SBP were examined. The NE and A-II induced a dose-dependent increase in PP, while ACh and NG induced a decrease in PP. There was no difference in vascular responsiveness to ACh, NG, A-II, and 5-HT before and after exchange transfusion with PHP solution, but NE-induced responses were augmented after exchange transfusion with PHP. Results suggest that the presence of PHP moiety in circulation would not alter vascular responsiveness to ACh, NG, A-II, and 5-HT in a significant manner.

Lymphatic pumping in response to changes in transmural pressure is modulated by erythrolysate/hemoglobin

Red blood cells and lysate products (erythrolysate) are observed consistently in lymph draining acute and chronic inflammatory reactions and from tissues subjected to trauma or surgical procedures. Using hemoglobin as a marker for erythrolysate, we have measured hemoglobin in lymph up to the 10(-6) M range in a number of pathophysiological states. Data demonstrate that erythrolysate alters the pumping characteristics of lymphatic vessels. To test the effects of erythrolysate on lymphatic pumping, bovine lymphatics were suspended in an organ bath preparation with the vessels cannulated at both inflow and outflow ends. By raising the heights of the Krebs reservoir and the outflow catheters appropriately, a transmural pressure that stimulated pumping activity could be applied to the vessels. With a fixed transmural pressure of 6 cm H2O applied to the ducts, sheep erythrolysate depressed pumping activity between 40% and 100%, with dilutions containing between 10(-8) and 10(-5) M hemoglobin. Although the active principle in the red blood cells has not been characterized, evidence from precipitation purification experiments suggests that hemoglobin is an important component. Once suppressed, pumping could be restored in many but not all vessels (often to control levels) by elevating the distending pressure above 6 cm H2O. The relation between transmural pressure and fluid pumping is expressed as a bell-shaped curve, with pumping increasing up to a peak pressure (usually 8 cm H2O) and declining at pressures above this level. By comparing pressure/flow curves, we were able to ascertain that hemoglobin shifted the lymphatic function curve to the right and, on average, reduced the maximum pumping capability of the vessels. We speculate that the presence of erythrolysate/hemoglobin in lymph may modulate the ability of lymphatic vessels to drain liquid and protein from the tissue spaces.

L-NG-nitro-arginine and its methyl ester are potent inhibitors of non-adrenergic, non-cholinergic transmission in the rat anococcygeus

1. The effects of L-NG-nitro-arginine (L-NOARG) and some other arginine analogues on non-adrenergic, non-cholinergic (NANC) relaxations of the rat anococcygeus muscle were investigated. 2. L-NOARG (5-200 microM) produced concentration-related inhibition of the NANC response; 100 microM L-NOARG produced 90% inhibition. 3. L-Arginine (5-200 microM) produced a concentration-related reversal of the inhibitory effect of 20 microM L-NOARG; a five fold excess of L-arginine (100 microM) was required to obtain the maximum reversal of 90%. D-Arginine (100 microM) produced no such reversal, but significant reversal was produced by L-citrulline, L-arginine-L-aspartate, L-homoarginine and L-arginine-methyl-ester (all at 100 microM). 4. L-NG-nitro-arginine-methyl-ester (L-NAME; 5-200 microM) also reduced NANC relaxations, with a potency similar to that of L-NOARG; both L-NOARG and L-NAME were some ten times more potent than L-NG-monomethyl-arginine (L-NMMA). Like L-NOARG, the effects of L-NAME (20 microM) were reversed by 100 microM L- but not D-arginine. 5. Neither L-NOARG nor L-NAME (both 20 microM) affected submaximal relaxations induced by 10 microM sodium nitroprusside or 20 microM hydroxylamine. 6. D-NOARG, L-NG-tosyl-arginine and L-N alpha-(t-butyl-oxycarbonyl)-NG-nitro-arginine (all at 100 microM) had no effect on NANC relaxations. 7. Thus, in the rat anococcygeus, L-NOARG and L-NAME are more potent than L-NMMA as prejunctional inhibitors of NANC transmission. The reversibility of the effect of L-NOARG by arginine analogues suggests that the NANC system of the anococcygeus shows similarities to the endogenous nitrate system recently described in the brain.

Neurotransmission in the sheep middle cerebral artery: modulation of responses by 5-HT and haemolysate

In ring sections of the sheep middle cerebral artery, electrical field stimulation elicits a complex response due to the simultaneous release of vasodilator and vasoconstrictor neurotransmitters. Haemolysate abolishes the relaxant effects of the vasodilator neurotransmitter and causes a marked augmentation of the contractile response in both the presence (448 +/- 191%) and absence (409 +/- 134%) of an intact endothelium. The haemolysate also reverses relaxation induced by sodium nitroprusside or sodium nitrite but has no effect on relaxation induced by 8-Br-cGMP. The vasodilator neurotransmitter therefore appears to act directly on the smooth muscle to cause relaxation by the stimulation of guanylate cyclase. The vasoconstrictor neurotransmitters that are released are antagonised by prazosin (100 nM), ketanserin (100 nM) and atropine (100 nM), which suggests that the transmitters involved are noradrenaline, 5-hydroxytryptamine (5-HT), and acetylcholine, respectively. In the presence of these three antagonists at 10 microM, there was 86.9 +/- 4.8% inhibition. Incubation with 5-HT (10 microM) causes a marked augmentation of the contractile response (267 +/- 56%) to field stimulation that can be reduced by pretreatment with either desipramine or citalopram, inhibitors of noradrenergic and serotoninergic uptake mechanisms, respectively. The 5-HT appears to be taken up into noradrenergic nerves and released as an alternative neurotransmitter upon subsequent stimulation. These actions of haemolysate and 5-HT may be involved in the cerebral vasospasm observed following subarachnoid haemorrhage.

Pharmacological characterisation of postjunctional 5-HT receptors in cerebral arteries from the sheep

In the sheep cerebral vasculature 5-hydroxytryptamine (5-HT) caused a contraction of which ketanserin was found to be an effective antagonist (Basilar artery, pA2 = 9.33 +/- 0.16; middle cerebral artery, pA2 = 9.19 +/- 0.16; pial artery, pA2 = 9.47 +/- 0.12). Sumatriptan (GR 43175), a selective 5-HT1-like receptor agonist, was also found to have a small contractile effect on the sheep cerebral vasculature (Basilar artery, pD2 = 6.26 +/- 0.11; middle cerebral artery, pD2 = 6.25 +/- 0.10; pial artery, pD2 = 6.13 +/- 0.15). The contractile effect of sumatriptan was not antagonised by either ketanserin (1 microM) or MDL 72222 (1 microM). 5-HT therefore appears to cause contraction by stimulation of a mixed receptor population of 5-HT1-like and 5-HT2 receptors. In the sheep middle cerebral artery the addition of haemolysate was found to cause a contractile response and also to augment the contractile effects of both noradrenaline and 5-HT but only in the presence of a functional endothelium. However, 5-HT was never found to relax precontracted rings of the middle cerebral artery in either the presence or absence of a functional endothelium. These results indicate a basal release of EDRF in cerebral arteries that attenuates the effects of various constrictor agents.

L-NG-monomethyl arginine and L-NG-nitro arginine inhibit non-adrenergic, non-cholinergic relaxation of the mouse anococcygeus muscle

1. The effects of L-NG-monomethyl arginine (L-NMMA) and L-NG-nitro arginine (L-NOARG) on non-adrenergic, non-cholinergic (NANC) relaxations of the mouse anococcygeus were investigated. 2. L-NMMA (10-200 microM) produced a concentration-related inhibition of the NANC response; the inhibitory effect of 50 microM L-NMMA was completely reversed by L-arginine but not D-arginine (both 100 microM). 3. L-NOARG (1-50 microM) also produced a concentration-related inhibition of the NANC response and was some 30-50 times more potent than L-NMMA; again, the effects of 10 microM L-NOARG were reversed by 100 microM L-, but not D-, arginine. By itself 100 microM L-arginine did not relax the tissue, but did cause a slight potentiation of the NANC response. 4. Sodium nitroprusside (0.01-10 microM), hydroxylamine (0.1-100 microM), sodium azide (1-100 microM) and nitric oxide (3-120 microM) all relaxed carbachol-induced tone; relaxations to submaximal concentrations of these nitrovasodilators were unaffected by either 50 microM L-NMMA or 10 microM L-NOARG. 5. L-NOARG 10 microM did not inhibit, but rather potentiated, contractions of the mouse anococcygeus due to stimulation of its sympathetic nerves. 6. The inhibitory effects of 10 microM L-NOARG on NANC relaxations were reversed by L-arginine (by 131%), L-citrulline (by 75%), L-arginine methyl ester (by 46%) and L-homoarginine (by 22%), but were unaffected by a variety of other amino acids and their derivatives (all at 100 microM). 7. The results provide strong evidence that NANC relaxations of the mouse anococcygeus are mediated by an endogenous nitrate material, probably derived from L-arginine, and confirm that L-NOARG provides a very useful and potent drug for the investigation of endogenous nitrate function.

The effects of pyrogallol and hydroquinone on the response to NANC nerve stimulation in the rat anococcygeus and the bovine retractor penis muscles

1. The effects of pyrogallol and hydroquinone on the bovine retractor penis (BRP) and rat anococcygeus muscles to non-adrenergic, non-cholinergic (NANC) nerve stimulation have been examined. Both drugs at a concentration of 10(-4) M significantly reduced the response in the rat anococcygeus muscle but had no effect in the BRP muscle. 2. The inhibition of the NANC response in the rat anococcygeus muscle by pyrogallol was completely reversed by superoxide dismutase suggesting it was due to the generation of superoxide anions. 3. Pyrogallol inhibited the response to nitric oxide (NO) in the rat anococcygeus muscle but not that to 3-isobutyl-1-methyl-xanthine (IBMX) which confirmed a selective action. 4. These results suggest that the NANC neurotransmitter in the rat anococcygeus muscle is susceptible to superoxide anions and may be NO or a substance that can liberate NO.

A comparison of haemoglobin and erythrocytes as inhibitors of smooth muscle relaxation by the NANC transmitter in the BRP and rat anococcygeus and by EDRF in the rabbit aortic strip

1. The inhibitory effect of erythrocyte suspensions and haemoglobin solutions on the response of the bovine retractor penis muscle (BRP) and the rat anococcygeus to field stimulation of their non-adrenergic non-cholinergic (NANC) nerves has been compared. Haemoglobin 3 microM greatly reduced the relaxant response in both tissues whereas a haemoglobin-equivalent suspension of erythrocytes was without effect. 2. A similar comparison of erythrocytes and haemoglobin on the response of the rabbit aortic strip to EDRF liberated by acetylcholine (ACh) showed that both reduced EDRF-mediated relaxation, though haemoglobin was significantly more effective. 3. These results suggest that the NANC transmitter may not be as freely diffusible through the erythrocyte membrane as EDRF and may therefore not be nitric oxide.

N-methylhydroxylamine inhibits and M&B 22948 potentiates relaxations of the mouse anococcygeus to non-adrenergic, non-cholinergic field stimulation and to nitrovasodilator drugs

1. The effects of N-methylhydroxylamine (NMH) and of M&B 22948 on relaxations of the mouse anococcygeus to non-adrenergic, non-cholinergic (NANC) field stimulation and to a number of smooth muscle relaxant drugs were investigated. 2. Relaxations to NANC field stimulation (10 Hz; 60 s train) were reversibly blocked by NMH (1-5 mM), which also caused weak, transient reductions of carbachol (50 microM)-induced tone. N,N-dimethylhydroxylamine (2 mM) and hydroxylamine (5 microM) reduced tone to the same extent as NMH, but neither produced any inhibition of NANC relaxations. 3. M&B 22948 10 microM, which by itself reduced tone by 12%, potentiated submaximal but not maximal relaxations to NANC field stimulation; overall the log frequency-response curve was displaced to the left by a factor of 2. 4. Sodium nitroprusside (0.01-1 microM), hydroxylamine (0.5-100 microM), and nitric oxide (2-200 microM) all relaxed carbachol-induced tone; relaxations to submaximal concentrations of these nitrovasodilators were reduced in the presence of 2 mM NMH, and potentiated in the presence of 10 microM M&B 22948. 5. Neither NMH (2 mM) nor M&B 22948 (10 microM) affected relaxations induced by submaximal concentrations of vasoactive intestinal peptide (VIP; 1 microM), papaverine (10 microM), 3-isobutyl-1-methyl-xanthine (10 microM), or 8-bromo-cyclic guanosine monophosphate (100 microM); relaxations to adenosine 5'-triphosphate (ATP, 2 mM) were unaffected by M&B 22948, but were potentiated by NMH. 6. The selective inhibition by NMH, and potentiation by M&B 22948, of NANC and nitrovasodilator-induced relaxations of the mouse anococcygeus suggests that the NANC transmitter is neither VIP nor ATP, but resembles the nitrovasodilator drugs in its mode of action. The NANC transmission system is therefore similar to that recently described in the bovine retractor penis.

The lack of involvement of cyclic nucleotides in the smooth muscle relaxant action of BRL 34915

1. The relaxant effect of BRL 34915 has been examined on four isolated preparations, the bovine retractor penis (BRP), guinea-pig taenia coli, guinea-pig trachea and rabbit aortic strip contracted by either histamine, carbachol, noradrenaline or 10 mM KCl. Even though the probability of the involvement of external calcium entering through voltage-operated channels in these tissues varied, there was little corresponding variation in sensitivity to BRL 34915. 2. The relaxant effect of BRL 34915 on the BRP and guinea-pig taenia coli was unaffected by haemoglobin 3.3 microM or Apamin 0.5 microM, concentrations which blocked completely the relaxant effect of non-adrenergic, non-cholinergic (NANC) nerve stimulation in these tissues. 3. BRL 34915 in doses causing maximum relaxation did not increase the levels of either cyclic AMP or cyclic GMP in the BRP, although the appropriate enzymes were present and could be stimulated by forskolin or sodium nitroprusside. 4. In the BRP isoprenaline 30 microM acting through beta-receptors caused maximum relaxation but did not raise the levels of cyclic AMP, even though lower doses of 2 microM did raise the levels of this nucleotide in the rabbit uterus. 5. These results provide some indirect evidence that membrane hyperpolarization may not be the only cause of the smooth muscle relaxation induced by BRL 34915. However, neither a rise in cyclic AMP nor cyclic GMP are satisfactory alternative mechanisms.

Role of adenine compounds in autonomic neurotransmission

Clearly adenine compounds exert numerous effects throughout the autonomic nervous system. The responses of various peripheral tissues to purines are summarized in Table 2. The evidence supporting a possible excitatory neurotransmitter function for ATP is very good in the vas deferens and good in both the bladder detrusor and certain blood vessels. ATP may also be an excitatory neurotransmitter in the colon, hepatocytes and frog atrium. These responses appear to be mediated by P2x-purinoceptors. There is good evidence supporting a role for ATP as an inhibitory neurotransmitter in the taenia coli and duodenum, and some support in the anal sphincter and possibly the rabbit portal vein; these responses appear to be mediated by P2y-purinoceptors. There is good evidence against ATP being an inhibitory neurotransmitter in the stomach fundic muscle and ileum. ATP (or more likely its metabolite adenosine) may act as an inhibitory neurotransmitter by interacting with postsynaptic P1-purinoceptors in cultured sympathetic neurones and also in the parasympathetic vesicle ganglion of the cat. It seems likely that ATP released from heart, platelets or vascular endothelium could be an endogenous relaxant of blood vessels through its actions on the endothelium. Although the addition of exogenous adenosine affects many tissues, evidence supporting modulatory functions for endogenous extracellular adenosine has only been clearly demonstrated in the ileum, gallbladder, vas deferens, fallopian tubes, kidney, blood vessels, carotid sinus, heart and adipose tissue. Both ATP and adenosine, released during periods of hypoxia or ischemia, could exert negative inotropic, chronotropic and dromotropic actions in the heart. In many cases, the potential sources of extracellular purines have not been established. This is particularly important when attempting to establish a neurotransmitter function for ATP in a tissue. For instance, the one outstanding piece of evidence required to confirm that ATP is an excitatory neurotransmitter released from sympathetic nerves in blood vessels is the unequivocal demonstration that it is, in fact, released from the sympathetic nerves when they are stimulated. To date, only the release of radiolabeled metabolites of ATP, possibly from post- rather than presynaptic sites, has been detected. Studies of the release of ATP are complicated by its rapid degradation extracellularly by ecto-ATPase. Unfortunately, there are no specific inhibitors of ecto-ATPase available at present, but one hopes that a suitable inhibitor will be developed shortly.(ABSTRACT TRUNCATED AT 400 WORDS)

Endothelium-dependent relaxation of canine basilar arteries. Part 2: Inhibition by hemoglobin and cerebrospinal fluid from patients with aneurysmal subarachnoid hemorrhage

The effects of hemoglobin and cerebrospinal fluid from patients with subarachnoid hemorrhage (CSF-SAH) on endothelium-dependent relaxation were studied. At 10(-6) M, hemoglobin somewhat inhibited the endothelium-dependent relaxation induced by A23187 in rings of canine basilar artery. At 3 X 10(-6) M, it almost completely inhibited the same response. At 3 X 10(-6) M, hemoglobin did not significantly inhibit smooth muscle relaxation mechanisms as papaverine-induced relaxation was not inhibited by hemoglobin. It was also demonstrated that pretreatment of arterial rings with CSF-SAH resulted in a dose-dependent inhibition of relaxation induced by A23187. The inhibitory effect of CSF-SAH was prominent in the case in which a high oxyhemoglobin concentration was measured by spectrophotometry. Normal CSF from patients without SAH did not affect endothelium-dependent relaxation. These results suggest that hemoglobin released from lysed erythrocytes inhibits endothelium-dependent relaxation of canine basilar arteries and may also play an important role in the pathogenesis of cerebral vasospasm after aneurysmal subarachnoid hemorrhage.

Innervation of airway smooth muscle. Efferent mechanisms

The classical view, with one excitatory (cholinergic) and one inhibitory (noradrenergic) component, of the innervation of airway smooth muscle is incomplete and at least two other, possibly peptidergic, types of innervation must be included when the innervation of airways is considered. A summary of these neuronal components is given in Fig. 1 and their possible origin is outlined. Besides the inhibitory noradrenergic innervation of the airways observed in some species, an inhibitory NANC (i-NANC) innervation has been demonstrated. The polypeptide, VIP, seems to be the most likely candidate for the neurotransmitter in the i-NANC innervation of the airways. The excitatory cholinergic innervation is present in the airways from the trachea down to the peripheral bronchi. In the guinea-pig bronchi an excitatory NANC (e-NANC) innervation has been demonstrated as well. The e-NANC nerves may correspond to chemosensitive primary afferent nerves with substance P or a related tachykinin as transmitter. When the innervation of airway smooth muscle of different mammalian species is compared it is evident that all nerve components except the cholinergic, show a considerable variability among species. The cholinergic innervation seems to be present in all mammalian species whereas the other components may be completely absent from some species. Distinct regional variations in the innervation of the airways may occur, which is exemplified by the distribution of the autonomic innervation in the guinea-pig tracheo-bronchial tree. Cholinergic neurotransmission in for example the guinea-pig and human airways can be modulated by NA via prejunctional inhibitory alpha 2-adrenoceptors. Furthermore, the e-NANC neurotransmission in the guinea-pig airways may be modulated by NA or by selective alpha 2-adrenoceptor agonists, acting via prejunctional inhibitory alpha 2-adrenoceptors. The clinical importance of the NANC innervation in relation to asthma is discussed. The i-NANC nerves may exert a modulating effect on bronchoconstriction, and a functional defect would presumably lead to an exaggerated response to constrictor stimuli. The e-NANC nerves in the airways may also be clinically relevant since the transmitter (tachykinins) from these nerves can produce bronchoconstriction and promote inflammation of the airway epithelium, either by direct mechanisms or indirectly by activation of mast cells, and thus contribute to the features of asthma.(ABSTRACT TRUNCATED AT 400 WORDS)

Atriopeptin II-induced relaxation of rabbit aorta is potentiated by M&B 22,948 but not blocked by haemoglobin

We examined the effects of haemoglobin (which inhibits the vascular responses to stimulation of soluble guanylate cyclase) and of M&B 22,948 (which selectively inhibits cyclic GMP phosphodiesterase) on the relaxation induced in rabbit aorta by the atrial natriuretic peptide, atriopeptin II (which stimulates particulate guanylate cyclase). Pretreatment with M&B 22,948 (100 microM) produced a 2.3 fold potentiation of atriopeptin II-induced relaxation of endothelium-denuded rings of rabbit aorta. Pretreatment with haemoglobin (10 microM) had no effect on the relaxation or the 10.9 fold increase in cyclic GMP content induced by atriopeptin II in endothelium-denuded rings of rabbit aorta. The potentiation by M&B 22,948 suggests a causal role for cyclic GMP in mediating atriopeptin II-induced vasodilatation of rabbit aorta. The inability of haemoglobin to block the atriopeptin II-induced rise in cyclic GMP suggests that it does not block stimulation of particulate guanylate cyclase. Thus, it is unlikely that a ferrous haem-containing receptor site is involved in the activation of the particulate form of guanylate cyclase as it is with soluble guanylate cyclase.

A comparison of the action of the endothelium-derived relaxant factor and the inhibitory factor from the bovine retractor penis on rabbit aortic smooth muscle

The dependence of relaxation of rabbit aortic strips by carbachol and by the inhibitory factor from the bovine retractor penis (BRP) on the presence of endothelium has been compared. Carbachol-induced relaxation is abolished by removing the endothelium, inhibitory factor-induced relaxation is unimpaired. The inhibitory factor, therefore, does not act by releasing an endothelium-derived relaxing factor (EDRF). The effect of inhibitors of eicosanoid metabolism on relaxation was examined. Quinacrine and nordihydroguaiaretic acid abolished the relaxant effect of carbachol and flurbiprofen had no effect. The relaxation produced by the inhibitory factor was unaffected by quinacrine and flurbiprofen while nordihydroguaiaretic acid potentiated the response. No eicosanoid appears, therefore, to be involved in the relaxant effect of the inhibitory factor from the BRP. Methylene blue, a drug reported to inhibit guanylate cyclase, in a concentration of 10 microM selectively abolished the relaxation produced by carbachol. However, at the higher concentration of 30 microM it abolished almost completely the response to inhibitory factor from the BRP and reduced inhibition by sodium nitroprusside. It is not possible from these results to exclude the possibility that the EDRF and the inhibitory factor from the BRP are chemically related.

The effect of hypoxia on neuroeffector transmission in the bovine retractor penis and rat anococcygeus muscles

The effects of reducing the PO2 of the bathing fluid were studied on non-adrenergic non-cholinergic (NANC) transmission in isolated preparations of the bovine retractor penis muscle, the rat anococcygeus muscle, the guinea-pig taenia caeci and the guinea-pig urinary bladder. Hypoxia rapidly and reversibly impaired NANC transmission in the bovine retractor penis and rat anococcygeus muscles but did not affect transmission in the guinea-pig taenia caeci or bladder, suggesting that different NANC mechanisms are involved. Although neurally-evoked relaxation of the bovine retractor penis was impaired by hypoxia, relaxations produced by vasoactive intestinal peptide, prostaglandin E1, sodium nitroprusside or an inhibitory factor isolated from the bovine retractor penis were unaffected. Since the inhibitory factor is similar to, and may actually be the NANC transmitter, the results suggest that the site of action of hypoxia in impairing transmission is prejunctional at the inhibitory nerve endings.

Mechanisms underlying electrical and mechanical responses of the bovine retractor penis to inhibitory nerve stimulation and to an inhibitory extract

The response of the bovine retractor penis (BRP) to stimulation of non-adrenergic, non-cholinergic (NANC) inhibitory nerves and to an inhibitory extract prepared from this muscle have been studied using intracellular microelectrode, sucrose gap and conventional mechanical recording techniques. Both inhibitory nerve stimulation and inhibitory extract hyperpolarized the membrane potential and relaxed spontaneous or guanethidine (3 X 10(-5) M)-induced tone. These effects were accompanied by an increase in membrane resistance. Following membrane potential displacement from an average value of -53 +/- 7 mV (n = 184; Byrne & Muir, 1984) inhibitory potentials to nerve stimulation were abolished at approximately -30 mV; there was no evidence of reversal. Displacement by inward hyperpolarizing current over the range -45 to -60 mV increased the inhibitory response to nerve stimulation and to inhibitory extract; at more negative potential values (above approximately -60 mV) the inhibitory potential decreased and was abolished (approximately -103 mV). There was no evidence of reversal. Removal of [K+]o reversibly reduced hyperpolarization to nerve stimulation and inhibitory extract. No enhancement was observed. Increasing the [K+]o to 20 mM reduced the inhibitory potential to nerve stimulation but this was restored by passive membrane hyperpolarization. Inhibitory potentials were obtained at membrane potential values exceeding that of the estimated EK (-49 mV). [Cl-]o-free or [Cl-]o-deficient solutions reduced and abolished (after some 20-25 min) the hyperpolarization produced by inhibitory nerve stimulation or inhibitory extract. The inhibitory potential amplitude following nerve stimulation was not restored by passive displacement of the membrane potential from -26 to -104 mV approximately. Ouabain (1-5 X 10(-5) M) reduced then (45-60 min later) abolished the inhibitory potential to nerve stimulation. The effects of this drug on the extract were not investigated. It is concluded that the inhibitory response to nerve stimulation and extract in the BRP may involve several ionic species. However, unlike that in gastrointestinal muscles the NANC response in the BRP is accompanied by an increased membrane resistance and does not primarily involve K+. The underlying mechanisms for the inhibitory response to both NANC nerve stimulation and inhibitory extract appear to be similar, compatible with the view that the latter may contain the inhibitory transmitter released from these nerves in this tissue.

Electrical and mechanical responses of the bovine retractor penis to nerve stimulation and to drugs

The responses of the bovine retractor penis muscle to field stimulation of intramural nerves in vitro was investigated using micro-electrode and extracellular (sucrose gap) techniques. In the absence of tone single pulses or trains of stimuli (1-50 Hz 0.1-0.5 m.sec) produced e.j.p.s and a decrease in membrane resistance; spike potentials were not observed. E.j.p.s often small in amplitude (3-5 mV to single pulse) and accompanied by contractions in almost all preparations were noradrenergic, abolished by guanethidine (1-3 x 10(-5) M) and tetrodotoxin (3.5 x 10(-6) M) but not by prazosin (0.05 - 1.4 x 10(-6) M). Prazosin abolished the depolarization and contraction produced by added NA (0.02 - 2 x 10(-8) moles). TEA (10(-2) M) depolarized the membrane and initiated spontaneous activity; e.j.p.s and contractions were enhanced and prolonged but no spikes were observed. Atropine (0.5 x 10(-6) M) increased and physostigimine (1-5 x 10(-6) M) decreased e.j.p.s and contractions indicating a cholinergic regulatory component in the release of the excitatory transmitter. In the presence of tone, nerve stimulation produced i.j.p.s. and relaxations which were unaffected by apamin (5 x 10(-7) M), atropine (3 x 10(-6) M), guanethidine (3 x 10(-5) M), phentolamine (5 x 10(-6) M) and propranolol (4 x 10(-6) M) but were abolished by tetrodotoxin (3.5 x 10(-7) M) suggesting their mediation by non-adrenergic non-cholinergic (NANC) nerves. Sodium nitroprusside (10(-10) - 10(-8) moles), which increases cyclic GMP, also hyperpolarized the membrane and relaxed the BRP. Those responses and those to inhibitory nerve stimulation were antagonized by oxyhaemoglobin which inhibits guanylate cyclase. 2-O-propoxyphenyl-8-azapurin-6-one (M & B 22948 3-9 x 10(-6) M) which inhibits cGMP-specific phosphodiesterase, enhanced the relaxation but not the i.j.p. TEA (10(-2) M) initially depolarized the membrane potential and raised tone. In the sucrose gap inhibitory potentials were abolished; the mechanical relaxation was not and a small contractile component emerged. Electrical and mechanical inhibitory components in the bovine retractor penis may not be correlated.

Cyclic GMP mediates neurogenic relaxation in the bovine retractor penis muscle

Field stimulation of the non-adrenergic, non-cholinergic inhibitory nerves to the bovine isolated retractor penis muscle evoked a relaxation that was preceded by a rise in the tissue content of cyclic GMP. There was no change in the content of cyclic AMP. The selective cyclic GMP phosphodiesterase inhibitor, 2-o- propoxyphenyl -8- azapurin -6-one (M&B 22948), elevated the tissue's cyclic GMP content, and potentiated both the relaxation and the rise in cyclic GMP produced by inhibitory nerve stimulation. Sodium nitroprusside and an inhibitory factor extracted from the bovine retractor penis muscle mimicked the effects of inhibitory nerve stimulation in that they each produced relaxation associated with a selective rise in cyclic GMP concentration. Haemoglobin (in the form of erythrocyte haemolysate) and N- methylhydroxylamine , which are known to block guanylate cyclase, blocked the relaxation and the rise in cyclic GMP content produced by inhibitory nerve stimulation, inhibitory factor and sodium nitroprusside. Haemoglobin itself caused a rise in muscle tone and at the same time reduced the cyclic GMP content of the tissue. 8-Bromocyclic GMP, a permeant derivative of cyclic GMP, produced a relaxation of the muscle that, as expected, was not blocked by haemoglobin. Vasoactive intestinal polypeptide, prostaglandin E1 and forskolin each produced relaxation associated with a selective rise in cyclic AMP content. Their effects were not blocked by haemoglobin or N- methylhydroxylamine . It is concluded that inhibitory nerve stimulation in the bovine retractor penis muscle produces a relaxation that is mediated by cyclic GMP, although some substances relax the muscle without affecting cyclic GMP levels. The results are also compatible with the view that the extracts of muscle contain the inhibitory neurotransmitter.

Neurogenic vasodilatation in isolated bovine and canine penile arteries

Field stimulation of isolated, perfused bovine or canine penile arteries produced dilatation, after the adrenergic motor component of the response had been blocked with guanethidine and the vessels had developed a background tone. The vasodilatation was blocked by tetrodotoxin but not by atropine. The vasodilator responses to field stimulation were compared with those produced by ATP, by vasoactive intestinal peptide (VIP), and by the inhibitory factor extracted from the bovine retractor penis muscle. Of the three putative transmitters, the inhibitory factor produced responses that most closely resembled those to field stimulation. Haemoglobin, which blocks non-adrenergic, non-cholinergic inhibitory transmission in the bovine and canine retractor penis muscles, did not impair the vasodilatations produced by ATP or VIP, but slowly reduced or abolished those produced by field stimulation or by the inhibitory factor. Haemoglobin itself produced a powerful constriction of the isolated penile arteries. The results are compatable with the possibility that the inhibitory factor from the bovine retractor penis muscle (which may be the inhibitory transmitter in that muscle) is, or closely resembles, the transmitter of non-adrenergic, non-cholinergic vasodilator fibres in the penile arteries of dog and ox.

Biphasic non-adrenergic, non-cholinergic relaxations of the mouse anococcygeus muscle

Trains of field stimulation of 60 s duration caused a biphasic relaxation of carbachol (50 microM)-induced tone in the mouse anococcygeus. The optimal pulse frequency and width were 10 Hz and 1 ms respectively. Tetrodotoxin (31, 124, and 310 nM) caused a dose-dependent reduction in the magnitude of both phases. Neither phase was affected by (+/-)-propranolol (1 microM), neostigmine (1 microM), (+)-tubocurarine (100 microM), or apamin (500 nM). Biphasic relaxations were observed in muscles from 6-hydroxydopamine pretreated mice. Haemolysed blood (10, 40, and 100 microliter/ml) reduced the magnitude of the first phase of nerve-induced relaxation to a greater extent than the second. This effect was reversible. Following a prolonged train of inhibitory nerve stimulation (10 Hz; 10 min) the magnitude of the first phase was reduced only slightly, but the second markedly. The possible relationships between the biphasic relaxation to field stimulation and putative non-adrenergic, non-cholinergic transmitters in the mouse anococcygeus are discussed.