Sodium (2-sulfonatoethyl) methanethiosulfonate prevents S-nitroso-L-cysteine activation of Ca2+-activated K+ (BKCa) channels in myocytes of the guinea-pig taenia caeca

An Update on Thiol Signaling: S-Nitrosothiols, Hydrogen Sulfide and a Putative Role for Thionitrous Acid

Long considered vital to antioxidant defenses, thiol chemistry has more recently been recognized to be of fundamental importance to cell signaling. S-nitrosothiols—such as S-nitrosoglutathione (GSNO)—and hydrogen sulfide (H₂S) are physiologic signaling thiols that are regulated enzymatically. Current evidence suggests that they modify target protein function primarily through post-translational modifications. GSNO is made by NOS and other metalloproteins; H₂S by metabolism of cysteine, homocysteine and cystathionine precursors. GSNO generally acts independently of NO generation and has a variety of gene regulatory, immune modulator, vascular, respiratory and neuronal effects. Some of this physiology is shared with H₂S, though the mechanisms differ. Recent evidence also suggests that molecules resulting from reactions between GSNO and H₂S, such as thionitrous acid (HSNO), could also have a role in physiology. Taken together, these data suggest important new potential targets for thiol-based drug development.

Gasotransmitter Heterocellular Signaling

SIGNIFICANCE

The family of gasotransmitter molecules, nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S), has emerged as an important mediator of numerous cellular signal transduction and pathophysiological responses. As such, these molecules have been reported to influence a diverse array of biochemical, molecular, and cell biology events often impacting one another. Recent Advances: Discrete regulation of gasotransmitter molecule formation, movement, and reaction is critical to their biological function. Due to the chemical nature of these molecules, they can move rapidly throughout cells and tissues acting on targets through reactions with metal groups, reactive chemical species, and protein amino acids.

CRITICAL ISSUES

Given the breadth and complexity of gasotransmitter reactions, this field of research is expanding into exciting, yet sometimes confusing, areas of study with significant promise for understanding health and disease. The precise amounts of tissue and cellular gasotransmitter levels and where they are formed, as well as how they react with molecular targets or themselves, all remain poorly understood.

FUTURE DIRECTIONS

Elucidation of specific molecular targets, characteristics of gasotransmitter molecule heterotypic interactions, and spatiotemporal formation and metabolism are all important to better understand their true pathophysiological importance in various organ systems. Antioxid. Redox Signal. 26, 936-960.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

Potassium channels importantly contribute to the regulation of vascular smooth muscle (VSM) contraction and growth. They are the dominant ion conductance of the VSM cell membrane and importantly determine and regulate membrane potential. Membrane potential, in turn, regulates the open-state probability of voltage-gated Ca²⁺ channels (VGCC), Ca²⁺ influx through VGCC, intracellular Ca²⁺, and VSM contraction. Membrane potential also affects release of Ca²⁺ from internal stores and the Ca²⁺ sensitivity of the contractile machinery such that K⁺ channels participate in all aspects of regulation of VSM contraction. Potassium channels also regulate proliferation of VSM cells through membrane potential-dependent and membrane potential-independent mechanisms. VSM cells express multiple isoforms of at least five classes of K⁺ channels that contribute to the regulation of contraction and cell proliferation (growth). This review will examine the structure, expression, and function of large conductance, Ca²⁺-activated K⁺ (BK_Ca) channels, intermediate-conductance Ca²⁺-activated K⁺ (K_Ca3.1) channels, multiple isoforms of voltage-gated K⁺ (K_V) channels, ATP-sensitive K⁺ (K_ATP) channels, and inward-rectifier K⁺ (K_IR) channels in both contractile and proliferating VSM cells.

Ion channels and transporters as therapeutic targets in the pulmonary circulation

Pulmonary circulation is a low pressure, low resistance, high flow system. The low resting vascular tone is maintained by the concerted action of ion channels, exchangers and pumps. Under physiological as well as pathophysiological conditions, they are targets of locally secreted or circulating vasodilators and/or vasoconstrictors, leading to changes in expression or to posttranslational modifications. Both structural changes in the pulmonary arteries and a sustained increase in pulmonary vascular tone result in pulmonary vascular remodeling contributing to morbidity and mortality in pediatric and adult patients. There is increasing evidence demonstrating the pivotal role of ion channels such as K(+) and Cl(-) or transient receptor potential channels in different cell types which are thought to play a key role in vasoconstrictive remodeling. This review focuses on ion channels, exchangers and pumps in the pulmonary circulation and summarizes their putative pathophysiological as well as therapeutic role in pulmonary vascular remodeling. A better understanding of the mechanisms of their actions may allow for the development of new options for attenuating acute and chronic pulmonary vasoconstriction and remodeling treating the devastating disease pulmonary hypertension.

Light-induced vs. bradykinin-induced relaxation of coronary arteries: do S-nitrosothiols act as endothelium-derived hyperpolarizing factors?

BACKGROUND

Light-induced relaxation depends on S-nitrosothiols. S-Nitrosothiols may also serve as endothelium-derived hyperpolarizing factors, mediating the relaxant response of porcine coronary arteries (PCAs) to bradykinin. Here we compared the mechanism of light-induced and bradykinin-induced PCA relaxation.

METHODS

PCAs were mounted in organ baths in the dark, preconstricted and exposed to polychromatic light (5 min) or 100 nmol/l bradykinin.

RESULTS

Light relaxed PCAs by maximally 71 +/- 1%. S-Nitrosothiol depletion abolished this relaxation. Relaxations diminished following repetitive light exposures, particularly if the dark periods between the light exposures were less than 10 min, and increased following endothelium removal or nitric oxide synthase blockade with N(omega)-nitro-L-arginine methyl ester (L-NAME), despite the prevention of guanosine-3',5'-cyclic monophosphate generation by the latter two procedures. Thus, reloading of the storage pools occurs in the dark, endothelial nitric oxide inhibits this process and photorelaxation does not depend on guanosine-3',5'-cyclic monophosphate. Bradykinin relaxed PCAs by 69 +/- 3%. The nitric oxide scavenger hydroxocobalamin and the Na+-K+ ATPase inhibitor ouabain abolished the responses to bradykinin and light. The guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one abolished the response to light, and, like L-NAME, blocked the response to bradykinin by more than 50%. On top of L-NAME, intermediate and small conductance Ca2+-dependent K+ channel (IKCa/SKCa) blockade further reduced the response to bradykinin and enhanced photorelaxation.

CONCLUSION

Photorelaxation depends on stored S-nitrosothiols and their release/synthesis is negatively affected by endothelial nitric oxide and IKCa/SKCa. S-Nitrosothiols activate endothelial IKCa/SKCa and, via guanylyl cyclase, smooth muscle Na+-K+ ATPase. Thus, they possess all properties of a bradykinin-induced endothelium-derived hyperpolarizing factor.

Calcium-activated potassium channels and endothelial dysfunction: therapeutic options?

The three subtypes of calcium-activated potassium channels (K(Ca)) of large, intermediate and small conductance (BK(Ca), IK(Ca) and SK(Ca)) are present in the vascular wall. In healthy arteries, BK(Ca) channels are preferentially expressed in vascular smooth muscle cells, while IK(Ca) and SK(Ca) are preferentially located in endothelial cells. The activation of endothelial IK(Ca) and SK(Ca) contributes to nitric oxide (NO) generation and is required to elicit endothelium-dependent hyperpolarizations. In the latter responses, the hyperpolarization of the smooth muscle cells is evoked either via electrical coupling through myo-endothelial gap junctions or by potassium ions, which by accumulating in the intercellular space activate the inwardly rectifying potassium channel Kir2.1 and/or the Na(+)/K(+)-ATPase. Additionally, endothelium-derived factors such as cytochrome P450-derived epoxyeicosatrienoic acids and under some circumstances NO, prostacyclin, lipoxygenase products and hydrogen peroxide (H(2)O(2)) hyperpolarize and relax the underlying smooth muscle cells by activating BK(Ca). In contrast, cytochrome P450-derived 20-hydroxyeicosatetraenoic acid and various endothelium-derived contracting factors inhibit BK(Ca). Aging and cardiovascular diseases are associated with endothelial dysfunctions that can involve a decrease in NO bioavailability, alterations of EDHF-mediated responses and/or enhanced production of endothelium-derived contracting factors. Because potassium channels are involved in these endothelium-dependent responses, activation of endothelial and/or smooth muscle K(Ca) could prevent the occurrence of endothelial dysfunction. Therefore, direct activators of these potassium channels or compounds that regulate their activity or their expression may be of some therapeutic interest. Conversely, blockers of IK(Ca) may prevent restenosis and that of BK(Ca) channels sepsis-dependent hypotension.

Melatonin ameliorates hippocampal nitric oxide production and large conductance calcium-activated potassium channel activity in chronic intermittent hypoxia

Melatonin protects against hippocampal injury induced by intermittent hypoxia (IH). IH-induced oxidative stress is associated with decreases in constitutive production of nitric oxide (NO) and in the activity of large conductance calcium-activated potassium (BK) channels in hippocampal neurons. We tested the hypothesis that administration of melatonin alleviates the NO deficit and impaired BK channel activity in the hippocampus of IH rats. Sprague-Dawley rats were injected with melatonin (10 mg/kg, i.p.) or vehicle before daily IH exposure for 8 hr for 7 days. The NO and intracellular calcium ([Ca2+]i) levels in the CA1 region of hippocampal slices were measured by electrochemical microsenor and spectrofluorometry, respectively. The activity of BK channels was recorded by patch-clamping electrophysiology in dissociated CA1 neurons. Malondialdehyde levels were increased in the hippocampus of hypoxic rats and were lowered by the melatonin treatment. Levels of NO under resting and hypoxic conditions, and the protein expression of neuronal NO synthase (nNOS) were significantly reduced in the CA1 neurons of hypoxic animals compared with the normoxic controls. These deficits were mitigated in the melatonin-treated hypoxic rats with an improved [Ca2+]i response to acute hypoxia. The open probability of BK channels was decreased in the hypoxic rats and was partially restored in the melatonin-treated animals, without alterations in the expression of channel subunits and unitary conductance. Acute treatment of melatonin had no significant effects on the BK channel activity or on the [Ca2+]i response to hypoxia. Collectively, these results suggest that melatonin ameliorates the constitutive NO production and BK channel activity via an antioxidant mechanism against an IH-induced down-regulation of nNOS expression in hippocampal neurons.

Nitric oxide deficit in chronic intermittent hypoxia impairs large conductance calcium-activated potassium channel activity in rat hippocampal neurons

Sleep apnea associated with chronic intermittent hypoxia (IH) impairs hippocampal functions but the pathogenic mechanisms involving dysfunction of nitric oxide (NO) and ionic channels remain unclear. We examined the hypothesis that hippocampal NO deficit impairs the activity of large conductance calcium-activated potassium (BK) channels in rats with chronic IH, mimicking conditions in patients with sleep apnea. A patch-clamp study was performed on hippocampal CA1 neurons acutely dissociated from IH and control rats. The levels of endogenous NO and intracellular calcium in the CA1 region of the hippocampal slices were measured respectively by electrochemical microsensors and spectrofluorometry. We found that the open probability of BK channels remarkably decreased in the CA1 pyramidal neurons in a time-dependent manner with the IH treatment, without changes in the unitary conductance and reversal potential. NO donors, SNP or DETA/NO, significantly restored the activity of BK channels in the IH neurons, which was prevented by blockade of S-nitrosylation with NEM or MTSES but not by inhibition of the cGMP pathway with ODQ or 8-bromo-cGMP. Endogenous NO levels were substantially lowered in the IH hippocampus during resting and hypoxia. Also, the level of protein expression of neuronal NO synthase was markedly lessened in the IH neurons with decreased intracellular calcium response to hypoxia. Collectively, the results suggest that the IH-induced NO deficit mediated by a down-regulation of the expression of neuronal NO synthase plays a causative role in the impaired activity of BK channels, which could account for the hippocampal injury in patients with sleep apnea.

New perspectives on the mechanisms through which nitric oxide may affect learning and memory processes

Nitric oxide (NO) has been well established as a molecule necessary for memory consolidation. Interestingly, the majority of research has focused on only a single mechanism through which NO acts, namely the up-regulation of guanylate cyclase (GC). However, since NO and NO-derived reactive nitrogen species are capable of interacting with a broad array of enzymes, ion channels and receptors, a singular focus on GC appears short-sighted. Although NO inhibits the action of a number of molecules there are four, in addition to GC, which are up-regulated by the direct presence of NO, or NO-derived radicals, and implicated in memory processing. They are: cyclic nucleotide-gated channels; large conductance calcium-activated potassium channels; ryanodine receptor calcium release (RyR) channels; and the enzyme mono(ADP-ribosyl) transferase. This review presents evidence that not only are these four molecules worthy of investigation as GC-independent mechanisms through which NO may act, but that behavioural evidence already exists suggesting a relationship between NO and the RyR channel.

Differential effects of ouabain on the vasodilator actions of nitric oxide and S-nitrosothiols in vivo: Relevance to the identity of EDRF/EDHF

OBJECTIVES

This study examined the role of Na+/K+-ATPase in the vasodilator actions of nitric oxide (NO), S-nitrosothiols and the endothelium-dependent agonist, acetylcholine.

METHODS

The vasodilator responses elicited by intravenous injections of (i) the NO-donors, sodium nitroprusside and MAHMA NONOate, (ii) the S-nitrosothiols, L-S-nitrosocysteine and S-nitrosocoenzyme A, and (iii) acetylcholine, in urethane-anesthetized rats.

RESULTS

The NO-donors, S-nitrosothiols and acetylcholine elicited dose-dependent depressor responses and reductions in hindquarter (HQR) and mesenteric (MR) vascular resistances. The depressor responses and associated reductions in HQR elicited by NO-donors were markedly attenuated after injection of ouabain. In contrast, the depressor responses and reductions in HQR elicited by the S-nitrosothiols and acetylcholine were not affected. The reductions in MR elicited by all vasodilator agents were exaggerated after injection of ouabain. Finally, the decomposition of sodium nitroprusside, MAHMA NONOate, L-S-nitrosocysteine and S-nitrosocoenzyme A to NO upon addition to rat blood or vascular preparations was not affected by ouabain.

CONCLUSION

This study demonstrates that ouabain has opposing effects on NO-mediated vasodilation in resistance arteries in the hindquarter and mesenteric beds of the rat. The similarity of effects of ouabain on the vasodilator actions of acetylcholine, L-S-nitrosocysteine and S-nitrosocoenzyme A as opposed to the NO-donors supports the possibility that endothelium-derived relaxing factor released by acetylcholine in resistance arteries is an S-nitrosothiol.

ACE inhibition restores the vasodilator potency of the endothelium-derived relaxing factor, L-S-nitrosocysteine, in conscious Spontaneously Hypertensive rats

OBJECTIVE

The major aim of this study was to determine whether the angiotensin converting enzyme (ACE) inhibitors, captopril or enalapril, restore the diminished vasodilator potency of the endothelium-dependent agonist, acetylcholine (ACh), and the endothelium-derived relaxing factor (EDRF), L-S-nitrosocysteine (L-SNC), in conscious Spontaneously Hypertensive (SH) rats.

METHODS

The hemodynamic responses elicited by i.v. injections of ACh, L-SNC, and nitric oxide donors such as MAHMA NONOate, were determined in SH rats treated for 7 days with captopril, enalapril, or the direct vasodilator hydralazine. The effects of captopril, enalapril or hydralazine on oxidant stress levels in blood serum and aorta of WKY and SH rats were also determined.

RESULTS

Captopril, enalapril and hydralazine elicited equivalent falls in mean arterial pressure and systemic vascular resistances in SH rats. ACh- and L-SNC-induced vasodilation were increased in captopril- or enalapril-treated SH rats such that the responses were equal to those in normotensive Wistar Kyoto rats. The attenuated responses of ACh and L-SNC in SH rats were not improved by hydralazine. The vasodilator effects of MAHMA NONOate, which were substantially augmented in SH rats, were not affected by captopril, enalapril or hydralazine. The levels of oxidant stress were markedly reduced in captopril- or enalapril-treated but not hydralazine-treated SH rats.

CONCLUSIONS

The finding that the ACE inhibitors improved the vasodilator potencies of L-SNC and the EDRF released by ACh in SH rats, suggests that the diminished vasodilator potency of these compounds was due to augmented ACE activity, which increased oxidant stress levels. This study provides the first evidence to support the concept that ACE inhibition lowers arterial pressure in SH rats, at least in part, by restoring the vasodilator potency of endothelium-derived L-SNC.

The vasodilator potency of the endothelium-derived relaxing factor, L-S-nitrosocysteine, is impaired in conscious spontaneously hypertensive rats

OBJECTIVE

This study compared the hemodynamic responses elicited by the endothelium-derived relaxing factor (EDRF), L-S-nitrosocysteine (L-SNC), the non-prostanoid EDRF released by acetylcholine (ACh) and nitric oxide (NO)-donors such as MAHMA NONOate, in conscious spontaneously hypertensive (SH) and normotensive Wistar-Kyoto (WKY) rats.

METHODS

The depressor and/or vasodilator responses elicited by intravenous injections of ACh, L-SNC and MAHMA NONOate were determined in adult WKY and SH rats before and after intravenous injection of the NO synthesis inhibitor, N(G)-nitro-L-arginine methylester (L-NAME), or the cyclooxygenase inhibitor, indomethacin.

RESULTS

The responses elicited by ACh and L-SNC were smaller in SH than in WKY rats whereas the responses elicited by MAHMA NONOate were augmented in SH rats. The ACh-induced responses were not diminished after injection of L-NAME in WKY or SH rats. Indomethacin did not affect the responses to any of the vasodilator agents in WKY or SH rats. Addition of L-SNC to whole blood or thoracic aortae from SH rats yielded similar amounts of NO to those of WKY rats.

CONCLUSIONS

The vasodilator potencies of ACh and L-SNC were diminished whereas that of NO was augmented in SH rats. The loss of potency of L-SNC in SH rats was not obviously due to differences in decomposition to NO or the overactivity of cyclooxygenase factors. This study provides the first evidence that diminished endothelium-dependent vasodilation in SH rats may involve a loss of vasodilator potency of endogenous L-SNC.

Role of voltage-sensitive calcium-channels in nitric oxide-mediated vasodilation in spontaneously hypertensive rats

This study demonstrates that the vasodilator potencies of nitric oxide (NO) donors such as sodium nitroprusside are increased in conscious Spontaneously Hypertensive (SH) as compared to Wistar Kyoto (WKY) rats. For example, the NO donors do not dilate hindlimb resistance arteries in WKY rats whereas they elicit pronounced vasodilator responses in SH rats. This study also demonstrates that the NO-mediated vasodilator responses in WKY and SH rats were markedly diminished after blockade of voltage-sensitive Ca2+-channels (CaVS2+-channels) with nifedipine, diltiazem or verapamil. These findings suggest that NO dilates resistance arteries in vivo via direct and/or hyperpolarization-induced closure of CaVS2+-channels and that the increased potency of NO in SH rats may be due to the augmented CaVS2+-channel activity reported in this strain.

S-nitrosocysteine elicits hemodynamic responses similar to those of the Bezold-Jarisch reflex via activation of stereoselective recognition sites

We report here that L-S-nitrosocysteine elicits rapid dose-dependent reductions in mean arterial blood pressure, heart rate, cardiac output and renal sympathetic nerve activity in conscious rats whereas D-S-nitrosocysteine elicits minor responses. The reductions in mean arterial blood pressure, heart rate and cardiac output elicited by L- and D-S-nitrosocysteine were markedly diminished after blockade of cardiac muscarinic cholinergic receptors whereas the reductions in renal sympathetic nerve activity were not affected. The Bezold-Jarisch reflex-like pattern of responses elicited by the stereoisomers, suggests that (i) L- and L-S-nitrosocysteine may activate stereoselective recognition sites on vagal cardiopulmonary afferents, which elicit hemodynamic responses via increases in cardiovagal efferent nerve activity and decreases in sympathetic nerve activity, and (ii) L-S-nitrosocysteine is a more potent agonist of these recognition sites than is D-S-nitrosocysteine.

Vasodilator actions of the endothelium‐derived relaxing factor l ‐ S ‐nitrosocysteine in anaesthetized rats are markedly diminished by peroxynitrite

The aim of the present study was to assess the effects of the potent oxidant and nitrating agent peroxynitrite on the haemodynamic actions of the endothelium-derived S-nitrosothiol L-S-nitrosocysteine. The haemodynamic actions of L-S-nitrosocysteine (12.5-100 nmol/kg, i.v.) were determined in pentobarbital-anaesthetized rats before and after the induction of tachyphylaxis to peroxynitrite achieved by giving 10 intravenous injections of a 10 micromol/kg dose. L-S-Nitrosocysteine elicited dose-dependent reductions in mean arterial blood pressure and in hindquarter and mesenteric vascular resistance. The L-S-nitrosocysteine-induced responses were substantially attenuated after administration of peroxynitrite. We have reported previously that nitric oxide-mediated vasodilation is not diminished after the induction of tachyphylaxis to peroxynitrite. Taken together, these findings support the concept that peroxynitrite reduces the vasodilator actions of L-S-nitrosocysteine via oxidation and/or nitration of putative membrane-bound S-nitrosothiol recognition sites.

Differentiation of L- and D-S-Nitrosothiol Recognition Sites In Vivo

The main aim of this study was to determine the effects of the lipophobic electron acceptor, nitroblue tetrazolium (NBT), on the vasodilator responses elicited by femoral vein injections of L- and D-S-nitrosocysteine (L- and D-SNC), L- and D-S-nitroso-beta,beta-dimethylcysteine (L- and D-SNPEN) and the nitric oxide (NO) donor, MAHMA NONOate, in pentobarbital-anesthetized rats. L- and D-SNC, L- and D-SNPEN, and MAHMA NONOate elicited dose-dependent falls in mean arterial blood pressure (MAP), and hindquarter (HQR), renal (RR), and mesenteric (MR) vascular resistances. The L-SNC- and L-SNPEN-induced depressor and vasodilator responses were markedly attenuated after injection of NBT. The D-SNC- and D-SNPEN-induced falls in mean arterial pressure, hindquarter, and mesenteric vascular resistances were also reduced after injection of nitroblue tetrazolium whereas the falls in renal resistances were not affected. However, nitroblue tetrazolium inhibited the L-SNC and L-SNPEN responses much more profoundly than the D-SNC and D-SNPEN responses in each vascular bed. In contrast, the MAHMA NONOate-induced responses were not attenuated by nitroblue tetrazolium. This study demonstrates that nitroblue tetrazolium attenuates L- and D-SNC-and L- and D-SNPEN- mediated but not NO-mediated vasodilation. The lack of effects of NBT on the NO responses suggests that NBT does not interfere with the intracellular mechanisms by which NO relaxes vascular smooth muscle. The more pronounced effects of NBT on the vasodilator effects of L-SNC and L-SNPEN than D-SNC and D-SNPEN suggests that these stereoisomers differentially interact with stereoselective S-nitrosothiol recognition sites in the vasculature and that these sites (or their signaling elements) contain thiol residues that may be susceptible to occupation and/or oxidation (ie, disulfide-bond formation) by nitroblue tetrazolium.

Peroxynitrite Elicits Dysfunction of Stereoselective S-Nitrosocysteine Recognition Sites

The aim of this study was to determine whether induction of tachyphylaxis to peroxynitrite (induced by giving 10 intravenous injections of a 10-micromol/kg dose) differentially affects the vasodilator responses elicited by systemic injections of the L- and D-isomers of S-nitrosocysteine (L-SNC and D-SNC), in pentobarbital-anesthetized rats. L- and D-SNC (12.5-200 nmol/kg, iv) elicited dose-dependent reductions in hindquarter, mesenteric, and renal vascular resistances. The L-SNC-induced vasodilator responses in the hindquarter and renal vascular beds were virtually abolished whereas the vasodilator responses in mesenteric bed were markedly diminished after administration of peroxynitrite. The D-SNC-induced vasodilator responses in the hindquarter and renal beds were slightly attenuated whereas the vasodilator responses in the mesenteric bed were not diminished after administration of peroxynitrite. The vasodilator responses elicited by the nitric oxide donor, MAHMA NONOate (5-50 nmol/kg, iv), were not attenuated by peroxynitrite. The finding that induction of tachyphylaxis to peroxynitrite diminishes the effects of L- and D-SNC but not MAHMA NONOate suggests that the stereoisomers exert their vasodilator effects by mechanisms other than their decomposition to nitric oxide. Moreover, the finding that induction of tachyphylaxis to peroxynitrite causes a more pronounced attenuation of the vasodilator effects of L- than D-SNC supports evidence that the stereoisomers differentially interact with stereoselective S-nitrosothiol recognition sites in the vasculature. Taken together, these novel results support the possibility that peroxynitrite diminishes the vasodilator potencies of L- and D-SNC by oxidation and/or nitration of amino acids in these recognition sites.

Angiotensin II type 2 receptor-mediated vasodilation. Focus on bradykinin, NO and endothelium-derived hyperpolarizing factor(s)

Angiotensin (Ang) II type 1 (AT(1)) receptors account for the majority of the cardiovascular effects Ang II, including vasoconstriction and growth stimulation. Recent evidence, mainly obtained in animals, suggests that Ang II type 2 (AT(2)) receptors counteract some or all of these effects. This review summarizes the current knowledge on the vasodilator effects induced by AT(2) receptors in humans and animals, focussing not only on the mediators of this effect, but also on the modulatory role of age, gender, and endothelial function. It is concluded that AT(2) receptor-mediated vasodilation most likely depends on the bradykinin-bradykinin type 2 (B(2)) receptor-NO-cGMP pathway, although evidence for a direct link between AT(2) and B(2) receptors is currently lacking. If indeed B(2) receptors are involved, this would imply that, in addition to NO, also the wide range of non-NO 'endothelium-derived hyperpolarizing factors' (EDHFs) that is released following B(2) receptor activation (e.g., K(+), cytochrome P450 products from arachidonic acid, H(2)O(2) and S-nitrososothiols), could contribute to AT(2) receptor-induced vasodilation.

Redox Modulation of Vascular Tone

Opening of potassium channels on vascular smooth muscle cells with resultant hyperpolarization plays a central role in several mechanisms of vasodilation. For example, in the arteriolar circulation where tissue perfusion is regulated, there is an endothelial derived hyperpolarizing factor that opens vascular smooth muscle calcium-activated potassium channels, eliciting dilation. Metabolic vasodilation involves the opening of sarcolemmal ATP-sensitive potassium channels. Adrenergic dilation as well as basal vasomotor tone in several vascular beds depend upon voltage-dependent potassium channels in smooth muscle. Thus hyperpolarization through potassium channel opening is a fundamental mechanism for vasodilation. Disease states such as coronary atherosclerosis and its risk factors are associated with elevated levels of reactive oxygen (ROS) and nitrogen species that have well-defined inhibitory effects on nitric oxide-mediated vasodilation. Effects of ROS on hyperpolarization mechanisms of dilation involving opening of potassium channels are less well understood but are very important because hyperpolarization-mediated dilation often compensates for loss of other dilator mechanisms. We review the effect of ROS on potassium channel function in the vasculature. Depending on the oxidative species, ROS can activate, inhibit, or leave unaltered potassium channel function in blood vessels. Therefore, discerning the activity of enzymes regulating production or degradation of ROS is important when assessing tissue perfusion in health and disease.

L-S-nitrosothiols: endothelium-derived hyperpolarizing factors in porcine coronary arteries?

OBJECTIVE

Bradykinin-induced, endothelium-derived hyperpolarizing factor (EDHF)-mediated responses depend on Ca-dependent K-channels (KCa) of small (SKCa) and intermediate (IKCa) conductance, inwardly rectifying K (KIR) channels and/or Na-K-ATPase. Here we investigated in porcine coronary arteries (PCAs) whether S-nitrosothiols can act as EDHF.

METHODS

Preconstricted PCAs were exposed to bradykinin, the NO donor S-nitroso-N-penicillamine (SNAP), or the S-nitrosothiols L-S-nitrosocysteine (L-SNC), D-SNC and L-S-nitrosoglutathione (L-SNG), with or without KCl, the NO scavenger hydroxocobalamin, the S-nitrosothiol-depleting agent p-hydroxymercurobenzoic acid (PHMBA) and/or inhibitors of NO synthase (L-NAME), guanylyl cyclase (ODQ), SKCa channels (apamin), KCa channels of large conductance (BKCa) (iberiotoxin), IKCa + BKCa channels (charybdotoxin), KIR channels (BaCl2) or Na-K-ATPase (ouabain).

RESULTS

All agonists concentration-dependently relaxed PCAs. L-NAME, charybdotoxin + apamin, KCl, and ouabain shifted the bradykinin concentration-response curve (CRC) approximately 10-fold to the right. BaCl2 did not exert additional effects on top of ouabain. Full blockade of bradykinin was obtained when combining L-NAME with charybdotoxin + apamin, KCl or ouabain + BaCl2. PHMBA reduced the maximum effect of bradykinin. Iberiotoxin + apamin, alone or on top of L-NAME, did not affect bradykinin, SNAP or L-SNC. ODQ and hydroxocobalamin shifted the SNAP, L-SNC, D-SNC, and L-SNG CRCs approximately 10-fold to the right, and, in combination, fully blocked SNAP-induced effects. Charybdotoxin + apamin shifted the L-SNC and L-SNG CRCs, but not the D-SNC or SNAP CRCs, approximately 5-fold to the right. KCl and ouabain (but not BaCl2) shifted the SNAP, L-SNC and L-SNG CRCs 5-10 fold to the right.

CONCLUSIONS

L-S-nitrosothiols activate SKCa + IKCa channels in a stereoselective manner, whereas NO activates Na-K-ATPase. Since S-nitrosothiols decompose to NO, stored L-S-nitrosothiols may mediate bradykinin-induced, EDHF-dependent relaxation.

Nitric oxide increases the spontaneous firing rate of rat medial vestibular nucleus neurons in vitro via a cyclic GMP-mediated PKG-independent mechanism

The effects of nitric oxide (NO) on the discharge rate of medial vestibular nucleus neurons (MVNn) were investigated in rat brainstem slices. The NO-donor sodium nitroprusside (SNP, 200 microM) caused a marked enhancement (+36.7%) of MVNn spontaneous firing rate, which was prevented by the NO-scavenger, carboxy-PTIO (300 microM). The SNP effects were not modified (+37.4%) by synaptic uncoupling, suggesting that NO influences intrinsic membrane properties of MVNn rather than the synaptic input they receive. The excitatory action of SNP was virtually abolished by slice pretreatment with the soluble guanylyl cyclase inhibitor, ODQ (10 microM), and it was mimicked (+33.1%) by the cGMP analogue 8-Br-cGMP (400 microM). Protein kinase G (PKG) and cAMP/protein kinase A (PKA) were both excluded as downstream effectors of the NO/cGMP-induced excitation. However, the cyclic nucleotide-gated (CNG) channel blockers, L-cis-diltiazem (LCD, 100 microM) and Sp-8-Br-PET-cGMPS (100 microM), significantly reduced the firing rate increase produced by 8-Br-cGMP. Moreover, LCD alone decreased spontaneous MVNn firing (-19.7%), suggesting that putative CNG channels may contribute to the tonic control of resting MVNn discharge. 8-Br-cAMP (1 mM) also elicited excitatory effects in MVNn (+40.8%), which occluded those induced by 8-Br-cGMP, indicating that the two nucleotides share a common target. Finally, nested-polymerase chain reaction assay revealed the expression of CNG channel alpha subunit transcript in MVNn. Our data provide the first demonstration that NO/cGMP signalling modulates MVNn spontaneous firing through a mechanism that is independent of PKG or PKA and probably involves activation of CNG channels.

CHARACTERIZATION OF THE ION CHANNEL CURRENTS IN SINGLE MYOCYTES OF THE GUINEA PIG PROSTATE

PURPOSE

We characterized membrane ionic currents underlying the action potential in single myocytes freshly isolated from the stroma of the guinea pig prostate.

MATERIAL AND METHODS

Whole cell and single channel currents were recorded in single stromal smooth muscle cells using standard patch clamp techniques.

RESULTS

: A rapidly activating, nifedipine (1 microM) sensitive Ca current was recorded in CsCl (130 mM) filled myocytes at potentials positive to -50 mV This current was half maximally activated at -22 mV and half maximally inactivated at -53 mV. In KCl (130 mM) filled myocytes membrane depolarization evoked a complex set of K selective outward currents, consisting of a rapidly activating transient outward current (IKto) followed by a more slowly developing transient outward current (IP2), which decayed to a steady state current (ISS). Tetraethylammonium (1 mM), a blocker of large conductance, Ca activated K channels, substantially blocked IP2 and ISS. Initial IKto was half maximally activated at -5 mV, half maximally inactivated at -65 mV and blocked by 4-aminopyridine (IC50 0.8 mM). IP2 and ISS were decreased by ryanodine (10 microM) or cyclopiazonic acid (10 microM) and increased by caffeine (1 mM), suggesting that Ca release from internal stores participates in the activation of these large conductance, Ca activated K channel currents.

CONCLUSIONS

We speculate that membrane currents characterized in stromal myocytes underlie the generation of simple action potentials triggered during the slow wave recorded in the intact guinea pig prostate and pharmacological manipulation of IKto and IP2 may well provide a selective avenue of modulating stromal excitability and muscle tone.

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.