Effects of hydroxocobalamin and carboxy-PTIO on nitrergic transmission in porcine anococcygeus and retractor penis muscles

A novel role for HNO in local and spreading vasodilatation in rat mesenteric resistance arteries

Nitric oxide-mediated vasodilatation has previously been attributed to the uncharged form of the molecule (NO(•)), but increasing evidence suggests that nitroxyl (HNO) may play a significant role in endothelium-dependent relaxation. The aim of this study was to investigate the mechanisms underlying HNO-mediated vasodilatation in phenylephrine pre-constricted pressurized (70 mmHg) mesenteric arteries, and on membrane currents in isolated smooth muscle cells using whole cell and perforated patch clamp recordings. Angeli's salt (AS: nitroxyl donor), evoked concentration-dependent vasodilatation that was insensitive to the NO(•) scavengers carboxy-PTIO and hydroxocobalamin (HXC), but sensitive to either the HNO scavenger L-cysteine, K-channel blockers (4-AP and iberiotoxin), raised [K(+)](o), or inhibition of soluble guanylyl cyclase (ODQ). AS-evoked smooth muscle hyperpolarization significantly augmented K(V) current in an ODQ sensitive manner, and also increased the BK(Ca) current. Importantly, 30 μM AS initiated conducted or spreading vasodilatation, and following blockade of endothelial K-channels (TRAM-34 and apamin), ACh was able to evoke similar L-cysteine-sensitive conducted dilatation. These data show that vasodilatation induced by HNO is mediated by both K(V) and BK(Ca) channels, and suggest a physiological role in vasodilatation through the vasculature.

Nitric Oxide Scavenging by Hydroxocobalamin May Account for Its Hemodynamic Profile

BACKGROUND

Antidotal doses of hydroxocobalamin are associated with transient increases in blood pressure in some animals and humans. These studies in anesthetized rabbits were undertaken to explore the possible mechanisms underlying the hemodynamic effects of hydroxocobalamin by investigating 1) possible hemodynamic effects of cyanocobalamin, which is formed on a molar-to-molar basis when hydroxocobalamin binds cyanide, and 2) the interference of hydroxocobalamin with the endothelial nitric oxide system.

METHODS

Study 1 investigated the hemodynamic effects of cyanocobalamin. This study included two treatment arms: 1) cyanocobalamin (75 mg/kg, IV) followed by saline (n = 7) and 2) saline followed by cyanocobalamin (n = 7). Study 2 assessed the hemodynamic effects of hydroxocobalamin (75 mg/kg, IV) in the presence and absence of the nitric oxide synthase inhibitor L-Nomega-nitro-L-arginine methyl ester (L-NAME; 30 mg/kg, IV). Nitric oxide synthase inhibition itself increases blood pressure. Thus, as part of Study 2, the hemodynamic effects of hydroxocobalamin were also investigated in the presence of an equipressor dose of angiotensin II (ANGII; 0.05 microg/kg/min, IV) in order to determine whether elevated blood pressure per se could interfere with hydroxocobalamin's hemodynamic effects. This study included six treatment arms (designated as first treatment + second treatment): saline + saline (n = 5), L-NAME + saline (n = 7), saline + hydroxocobalamin (n = 7), L-NAME + hydroxocobalamin (n = 7), ANGII + hydroxocobalamin (n = 7), and ANGII + saline (n = 7).

RESULTS

In Study 1, the effects of cyanocobalamin on hemodynamic parameters were indistinguishable from those of saline. In Study 2, hydroxocobalamin infusion was associated with moderate hemodynamic effects, including an increase in systemic vascular resistance, an increase in blood pressure, and a decrease in cardiac output. Administration of L-NAME abolished the effects of hydroxocobalamin on all hemodynamic parameters. ANGII at a dose producing a pressor response comparable to that of L-NAME did not influence the hydroxocobalamin-associated hemodynamic changes.

CONCLUSION

These studies in anesthetized rabbits demonstrate that the moderate pressor effect of hydroxocobalamin is not related to the formation of cyanocobalamin but is very likely related to the scavenging of nitric oxide by hydroxocobalamin.

Ca2+ sensitization and the regulation of contractility in rat anococcygeus and retractor penis muscle

Stimulation of the RhoA/Rho-kinase (ROK) signaling represents a key step in the maintenance of agonist-induced contraction of smooth muscle. We aimed to demonstrate Ca(2+) sensitization in rat anococcygeus and retractor penis muscles and to identify the molecular expression of major components of this pathway. Both anococcygeus and retractor penis showed a similar expression of RhoA, ROKalpha, and ROKbeta at the protein level as well as the mRNA for RhoGEFs. Cumulative addition of the ROK inhibitors H-1152 (0.001-3 microM), Y-27632 (0.01-30 microM) or HA-1077 (0.01-30 microM) caused sustained relaxations of precontracted smooth muscle strips. Ca(2+) sensitization induced by phenylephrine, norepinephrine and carbachol was markedly antagonized by all three ROK inhibitors. In addition, the contractile response to KCl-induced depolarization was highly sensitive to these ROK inhibitors. H-1152 was approximately 8-20 more potent than Y-27632 and HA-1077 to inhibit contraction. Electrical field stimulation (EFS, 1-32 Hz) caused transient contractions in both anococcygeus and retractor penis muscle, which were blocked by tetrodotoxin (1 microM), phentolamine (1 microM) or bretylium tosylate (30 microM). Similarly, H-1152 (0.1-1 microM), Y-27632 (1-10 microM) or HA-1077 (1-10 microM) significantly reduced EFS-evoked contractions in a concentration-dependent manner. The results indicate that the RhoA/ROK-mediated Ca(2+) sensitization pathway is expressed in anococcygeus and retractor penis muscles and enhances contractions produced by receptor-dependent and independent mechanisms.

Bradykinin-induced relaxation of coronary microarteries: S-nitrosothiols as EDHF?

1. To investigate whether S-nitrosothiols, in addition to NO, mediate bradykinin-induced vasorelaxation, porcine coronary microarteries (PCMAs) were mounted in myographs. 2. Following preconstriction, concentration-response curves (CRCs) were constructed to bradykinin, the NO donors S-nitroso-N-penicillamine (SNAP) and diethylamine NONOate (DEA-NONOate) and the S-nitrosothiols L-S-nitrosocysteine (L-SNC) and D-SNC. All agonists relaxed PCMAs. L-SNC was approximately 5-fold more potent than D-SNC. 3. The guanylyl cyclase inhibitor ODQ and the NO scavenger hydroxocobalamin induced a larger shift of the bradykinin CRC than the NO synthase inhibitor L-NAME, although all three inhibitors equally suppressed bradykinin-induced cGMP responses. 4. Complete blockade of bradykinin-induced relaxation was obtained with L-NAME in the presence of the large- and intermediate-conductance Ca(2+)-activated K(+)-channel (BK(Ca), IK(Ca)) blocker charybdotoxin and the small-conductance Ca(2+)-activated K(+)-channel (SK(Ca)) channel blocker apamin, but not in the presence of L-NAME, apamin and the BK(Ca) channel blocker iberiotoxin. 5. Inhibitors of cytochrome P450 epoxygenase, cyclooxygenase, voltage-dependent K(+) channels and ATP-sensitive K(+) channels did not affect bradykinin-induced relaxation. 6. SNAP-, DEA-NONOate- and D-SNC-induced relaxations were mediated entirely by the NO-guanylyl cyclase pathway. L-SNC-induced relaxations were partially blocked by charybdotoxin+apamin, but not by iberiotoxin+apamin, and this blockade was abolished following endothelium removal. ODQ, but not hydroxocobalamin, prevented L-SNC-induced increases in cGMP, and both drugs shifted the L-SNC CRC 5-10-fold to the right. 7. L-SNC hyperpolarized intact and endothelium-denuded coronary arteries. 8. Our results support the concept that bradykinin-induced relaxation is mediated via de novo synthesized NO and a non-NO, endothelium-derived hyperpolarizing factor (EDHF). S-nitrosothiols, via stereoselective activation of endothelial IK(Ca) and SK(Ca) channels, and through direct effects on smooth muscle cells, may function as an EDHF in porcine coronary microarteries.

The role of nitric oxide in vascular headache

Shortly after the invention of nitroglycerin (NTG), it was noticed that this substance is capable of inducing a violent headache. Only recently, it became known that this was due to the release of nitric oxide (NO) by NTG. As the molecular mechanism of migraine pain remains to be determined, NTG, being pro-drug for NO, has been used to study the aetiology and pathophysiology of migraine. Such studies with NTG- and also histamine-induced headaches, have led to propose that NO may be the causative molecule in migraine pain. The evidence supporting the role of NO in migraine is discussed, e.g. substances capable of inducing experimental vascular headache do so with NO as the common mediator, while drugs with antimigraine activity inhibit NO and the cascade of intracellular reactions triggered by NO. The importance of NO as a potential initiator of the migraine attack opens new directions for the pharmacological treatment of migraine and other vascular headaches.

Mediation of nitric oxide from photosensitive stores in the photorelaxation of the rabbit corpus cavernosum

Isolated rabbit corpus cavernosum relaxed in response to ultraviolet (UV) light (365 nm). The UV light-induced relaxation (photorelaxation) was diminished on repeated UV irradiation from 30.5+/-4.0% (the first photorelaxation) to 15.5+/-2.7% (the last photorelaxation). Hydroxocobolamine of 100 microM and hemoglobin (Hb) of 10 microM, which are nitric oxide (NO) scavengers, and 10 microM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a soluble guanylyl cyclase inhibitor, markedly reduced photorelaxation. However, 300 microM 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (carboxy-PTIO) failed to inhibit photorelaxation. NaNO(2) and N(G)-nitro-L-arginine (L-NA) but not 3-nitro-L-tyrosine (3-NT) were found to be photosensitive in that these compounds are photolysed to release NO, as demonstrated by use of an amperometric NO probe; NO signals produced by 500 microM NaNO(2), and 500 microM L-NA were 133.3+/-28.9 and 54.4+/-10.4 pA, respectively. Not 3-NT but the other compounds (all 200 microM) also enhanced photorelaxation of the cavernosal tissue. Based on these findings, the substance, which mediates photorelaxation, could be NO released from putative stores in the rabbit corpus cavernosum, and L-NA as well as NaNO(2) but not 3-NT produce NO under the influence of UV light.

Hydroxocobalamin, a nitric oxide scavenger, in the prophylaxis of migraine: an open, pilot study

Drugs which directly counteract nitric oxide (NO), such as endothelial receptor blockers, NO-synthase inhibitors, and NO-scavengers, may be effective in the acute treatment of migraine, but are also likely to be effective in migraine prophylaxis. In the underlying pilot study the prophylactic effect of the NO scavenger hydroxocobalamin after intranasal administration in migraine was evaluated. Twenty patients, with a history of migraine of > 1 year and with two to eight migraine attacks per month, were included in an open trial. A baseline period was followed by an active treatment period of 3 months with 1 mg intranasal hydroxocobalamin daily. Patients were instructed to complete a diary in which details of each attack were described. A reduction in migraine attack frequency of >/ or = 50% was seen in 10 of 19 patients, which corresponds to 53% of the patients (responders). A reduction of > or = 30% was noted in 63% of the patients. The mean attack frequency in the total study population showed a reduction from 4.7 +/- 1.7 attacks per month to 2.7 +/- 1.6 (P < 0.001). For the responders the migraine attack frequency was reduced from 5.2 +/- 1.9 (baseline) to 1.9 +/- 1.3 attacks per month (P < 0.005), while for those who did not respond a non-significant reduction was found: 4.1 +/- 1.4 to 3.7 +/- 1.5 (P > 0.1). A reduction was also observed for the total duration of the migraine attacks per month, the total number of migraine days per month and the number of medication doses for acute treatment used per month. This is the first prospective, open study indicating that intranasal hydroxocobalamin may have a prophylactic effect in migraine. As a percentage of responders in prophylactic trials of > 35-40% is unlikely to be a placebo effect, a double-blind study is warranted.

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.

Negative inotropic effect of bradykinin in porcine isolated atrial trabeculae: role of nitric oxide

OBJECTIVES

To investigate whether bradykinin affects cardiac contractility independently of its effects on coronary flow and noradrenaline release, and whether such inotropic effects, if present, are mediated via nitric oxide (NO).

METHODS

Right atrial trabeculae were obtained from 35 pigs, suspended in organ baths and attached to isometric transducers. Resting tension was set at approximately 750 mg and tissues were paced at 1.5 Hz. Tissue viability was checked by constructing a concentration response curve (CRC) to noradrenaline. Next, CRCs were constructed to bradykinin, either under baseline conditions or after pre-stimulation with the positive inotropic agent forskolin (1 or 10 micromol/l), in the absence or presence of the bradykinin type 2 (B2) receptor antagonist D-Arg [Hyp3-Thi5, d-Tic7, Oic8]-bradykinin (Hoe 140) (1 micromol/l), the NO synthase inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME) (100 micromol/l) and/or the NO scavenger hydroxocobalamin (200 micromol/l).

RESULTS

Bradykinin exerted a negative inotropic effect, both with and without forskolin pre-stimulation, reducing contractility by maximally 22 +/- 3.6% (mean +/- SEM) and 23 +/- 3.6%, respectively (pEC50 8.37 +/- 0.23 and 8.62 +/- 0.22, respectively). L-NAME reduced this effect in pre-stimulated, but not in unstimulated, trabeculae. Hoe 140 and hydroxocobalamin fully blocked the inotropic effect of bradykinin.

CONCLUSIONS

Bradykinin induces a modest negative inotropic effect in porcine atrial trabeculae that is mediated via B2 receptors and NO. The inconsistent results obtained with L-NAME suggest that it depends on NO synthesized de novo and/or NO from storage sites.

Involvement of nitric oxide and vasoactive intestinal peptide in the nonadrenergic-noncholinergic relaxation of the porcine retractor penis muscle

Neurotransmitters mediating nonadrenergic-noncholinergic (NANC) relaxation were investigated in strips of porcine retractor penis muscle (RPM). Muscle tone was raised by phenylephrine (1 microM) in the presence of atropine (1 microM) and guanethidine (50 microM). Upon electrical field stimulation (1 ms, 80 V, 1-32 Hz for 10 s), the initial fast relaxation was followed by the slow relaxation. Although the fast and the slow relaxation were completely abolished by tetrodotoxin (1 microM), they showed different pharmacological sensitivities to the nitric oxide (NO) synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 0.1 mM). The fast relaxation was markedly inhibited by L-NAME in an L-arginine reversible manner and by oxyhemoglobin (50 microM), while the slow relaxation was hardly blocked by L-NAME. L-NAME and alpha-chymotrypsin (alpha-CT, 3 U/ml) selectively inhibited the fast and the slow relaxation, respectively. Alpha-CT abolished L-NAME-resistant slow relaxation, and L-NAME completely abolished the alpha-CT-resistant fast relaxation. Alpha-CT-resistant relaxation was not significantly different from the digitally calculated L-NAME-sensitive component, and L-NAME-resistant relaxation was similar to the digitally calculated alpha-CT-sensitive component. Vasoactive intestinal peptide (VIP, 0.003-0.1 microM) relaxed porcine RPM in a concentration-dependent manner. The effect of a VIP was partially inhibited by a VIP receptor antagonist, VIP(10-28) (1 and 3 microM). L-NAME-resistant relaxation was also reduced by VIP(10-28) (3 microM) and by another putative antagonist, VIP(6-28) (1 microM), although the effects of the two antagonists were somewhat inconsistent. From the histochemical staining, it was verified that nerve bundles that showed VIP-like immunoreactivities were also positive for the NADPH diaphorase reaction. These results suggest that NO and peptide neurotransmitter(s) including VIP mediate the NANC relaxation in porcine RPM.

L-NAME-resistant bradykinin-induced relaxation in porcine coronary arteries is NO-dependent: effect of ACE inhibition

1. NO synthase (NOS)inhibitors partially block bradykinin (BK)-mediated vasorelaxation. Here we investigated whether this is due to incomplete NOS inhibition and/or NO release from storage sites. We also studied the mechanism behind ACE inhibitor-mediated BK potentiation. 2. Porcine coronary arteries (PCAs) were mounted in organ baths, preconstricted, and exposed to BK or the ACE-resistant BK analogue Hyp(3)-Tyr(Me)(8)-BK (HT-BK) with or without the NOS inhibitor L-NAME (100 microM), the NO scavenger hydroxocobalamin (200 microM), the Ca(2+)-dependent K(+)-channel blockers charybdotoxin+apamin (both 100 nM), or the ACE inhibitor quinaprilat (10 microM). 3. BK and HT-BK dose-dependently relaxed preconstricted vessels (pEC(50) 8.0+/-0.1 and 8.5+/-0.2, respectively). pEC(50)'s were approximately 10 fold higher with quinaprilat, and approximately 10 fold lower with L-NAME or charybdotoxin+apamin. Complete blockade was obtained with hydroxocobalamin or L-NAME+ charybdotoxin+apamin. 4. Repeated exposure to 100 nM BK or HT-BK, to deplete NO storage sites, produced progressively smaller vasorelaxant responses. With L-NAME, the decrease in response occurred much more rapidly. L-Arginine (10 mM) reversed the effect of L-NAME. 5. Adding quinaprilat to the bath following repeated exposure (with or without L-NAME), at the time BK and HT-BK no longer induced relaxation, fully restored vasorelaxation, while quinaprilat alone had no effect. Quinaprilat also relaxed vessels that, due to pretreatment with hydroxocobalamin or L-NAME+charybdotoxin+apamin, previously had not responded to BK. 6. In conclusion, L-NAME-resistant BK-induced relaxation in PCAs depends on NO from storage sites, and is mediated via stimulation of guanylyl cyclase and/or Ca(2+)-dependent K(+)-channels. ACE inhibitors potentiate BK independent of their effect on BK metabolism.

Role of nitric oxide in modulation of afferent impulses in cutaneous branches of somatic nerves by polarized light

Subcutaneous injection of L-NAME inhibited afferent impulse activity in n. ischiadicus and n. saphenus and abolished the increase in this activity induced by stimulation of mechanoreceptors after skin irradiation with polarized light with various spectral characteristics. Subsequent subcutaneous injection of sodium nitroprusside restored the pattern of afferent impulse activity in these nerves during repeated skin irradiation with polarized light.

Spontaneous photo-relaxation of urethral smooth muscle from sheep, pig and rat and its relationship with nitrergic neurotransmission

1. In the present work we have characterized the relaxant response induced by light stimulation (LS) in the lower urinary tract from sheep, pig and rat, establishing its relationship with nitrergic neurotransmission. 2. Urethral, but not detrusor, preparations showed pronounced photo-relaxation (PR) which declined progressively following repetitive LS. Sheep urethral PR was again restored either spontaneously or (to a greater extent) by exogenous nitric oxide (NO) addition and by electrical field stimulation (EFS) of intrinsic nitrergic nerves. 3. Greater NO generation was detected from sheep urethral than from detrusor homogenates following illumination. 4. Sheep urethral PR was inhibited by oxyhaemoglobin, but not by methaemoglobin, carboxy-PTIO, extracellular superoxide anion generators or superoxide dismutase. Guanylyl cyclase but not adenylyl cyclase activation mediates urethral relaxation to LS. 5. Urethral PR was more resistant to inhibition by L-thiocitrulline than EFS-induced responses, although this agent prevented PR restoration by high-frequency EFS. 6. Urethral PR was TTX insensitive and partially modified in high-K+ solutions. Cold storage for 24 h greatly impaired urethral PR, although it was restored by high-frequency EFS. 7. Repetitive exposure to LS, EFS or exogenous NO induced changes in the shape of the EFS-induced nitrergic relaxation, possibly by pre-synaptic mechanisms. 8. In conclusion, we suggest the presence of an endogenous, photo-labile, nitro-compound store in the urethra, which seems to be replenished by neural nitric oxide synthase activity, indicating a close functional relationship with the nitrergic neurotransmitter.

Effects of superoxide anion generators and thiol modulators on nitrergic transmission and relaxation to exogenous nitric oxide in the sheep urethra

The effects of superoxide anion generators, the nitric oxide (NO) scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoine-1-oxyl 3-oxide (carboxy-PTIO), the specific guanylate cyclase inhibitor 1H-[1,2,4]-oxadiazole-[4,3-a]-quinoxalin-1-one (ODQ), and thiol modulating agents were investigated on relaxations induced by nitrergic stimulation and exogenous NO addition in the sheep urethra. Methylene blue (MB, 10 microM), pyrogallol (0.1 mM) and xanthine (X, 0.1 mM)/xanthine oxidase (XO, 0.1 u ml(-1)) inhibited NO-mediated relaxations, without affecting those induced by nitrergic stimulation. This resistance was not diminished following inhibition of endogenous Cu/Zn superoxide dismutase (Cu/Zn SOD) with diethyldithiocarbamic acid (DETCA, 3 mM), which almost abolished tissue SOD activity. Carboxy-PTIO (0.1 - 0.5 mM) inhibited NO-mediated relaxations but had no effect on responses to nitrergic stimulation, which were not changed by treatment with ascorbate oxidase (2 u ml(-1)). Relaxations to NO were reduced, but not abolished, by ODQ (10 microM), while nitrergic responses were completely blocked. The thiol modulators, ethacrynic acid (0.1 mM), diamide (1.5 mM), or 5,5'-dithio-bis (2-nitrobenzoic acid) (DTNB, 0. 5 mM), and subsequent treatment with dithiothreitol (DTT, 2 mM) had no effect on responses to nitrergic stimulation or NO. In contrast, N-ethylmaleimide (NEM, 0.2 mM) markedly inhibited both relaxations. L-cysteine (L-cys, 0.1 mM) had no effect on responses to NO, while it inhibited those to nitrergic stimulation, in a Cu/Zn SOD-independent manner. Our results do not support the view that the urethral nitrergic transmitter is free NO, and the possibility that another compound is acting as mediator still remains open. British Journal of Pharmacology (2000) 129, 53 - 62

Comparison of the redox forms of nitrogen monoxide with the nitrergic transmitter in the rat anococcygeus muscle

1. A sustained tone was produced in rat isolated anococcygeus muscles with guanethidine and clonidine and relaxant responses were elicited by electrical stimulation of its nitrergic nerves and by the three redox forms of nitrogen monoxide. 2. The nitroxyl anion (NO ) was donated by dissociation of Angeli's salt; the free radical (NO*) was from an aqueous solution of nitric oxide gas; the nitrosonium cation (NO+) was donated by dissociation of nitrosonium tetrafluoroborate. 3. The concentrations producing approximately 50% relaxations of the anococcygeus muscle were 0.3 microM for Angeli's salt (nitroxyl), 0.5 microM for NO* and 100 microM for nitrosonium tetrafluoroborate. Nitrergic nerve stimulation at 1 Hz for 10 s produced equivalent relaxant responses. 4. The superoxide generator pyrogallol (100 microM) had no effect on responses to nitrergic nerve stimulation or Angeli's salt but significantly reduced responses to NO* and nitrosonium tetrafluoroborate. 5. The NO* scavenger carboxy-PTIO (100 microM) had no effect on responses to nitrergic nerve stimulation or Angeli's salt but significantly reduced responses to NO* and nitrosonium tetrafluoroborate. 6. Hydroxocobalamin (30 microM) had no significant effect on responses to the nitrergic transmitter, enhanced the response to Angeli's salt, and significantly reduced responses to NO* and nitrosonium tetrafluoroborate. 7. The findings suggest that the nitroxyl anion donated by Angeli's salt is a better candidate than NO* to serve as the nitrergic transmitter in the rat anococcygeus muscle, although it still does not behave exactly like the transmitter.

Effects of hydroxocobalamin and carboxy-PTIO on nitrergic transmission in porcine anococcygeus and retractor penis muscles

The effects of carboxy-PTIO and hydroxocobalamin were studied on nitrergic transmission in anococcygeus and retractor penis muscles taken during post mortem examination from young male pigs. In both muscles under resting conditions, electrical field stimulation (EFS) caused contractions that were sensitive to tetrodotoxin (1 microM) and were greatly inhibited by prazosin (1 microM) and guanethidine (10-30 microM), but were not significantly affected by atropine (1 microM). In the anococcygeus muscle, but not in the retractor penis muscle, guanethidine produced a prolonged contraction. After tone was raised by guanethidine in the anococcygeus or by phenylephrine (1 microM) in the presence of guanethidine in the retractor penis, EFS caused tetrodotoxin-sensitive relaxations. The EFS-induced relaxations were abolished by the NO synthase inhibitor N(G)-L-nitro-arginine methyl ester (L-NAME; 100 microM) and its effect was partly overcome by L-arginine (1 mM), indicating it was mediated by nitrergic nerves. Carboxy-PTIO (0.1-1 mM) had no significant effect in reducing stimulation-induced nitrergic relaxations in either muscle. However, hydroxocobalamin (0.1-1 mM) caused concentration-dependent reductions of nitrergic relaxations in both muscles. Relaxations to exogenous nitric oxide (1 microM) in both muscles were abolished by carboxy-PTIO (0.3 mM) and hydroxocobalamin (0.1 mM). There were no differences in reactivity to carboxy-PTIO or hydroxocobalamin between anococcygeus and retractor penis muscles from the same species (pig). The finding also confirms earlier observations that the nitrergic transmitter is generally resistant to the NO-scavenger carboxy-PTIO.