Stimulation of lumbar sympathetic nerves may produce hindlimb vasodilation via the release of pre-formed stores of nitrosyl factors

Hypoxia releases S-nitrosocysteine from carotid body glomus cells-relevance to expression of the hypoxic ventilatory response

We have provided indirect pharmacological evidence that hypoxia may trigger release of the S-nitrosothiol, S-nitroso-L-cysteine (L-CSNO), from primary carotid body glomus cells (PGCs) of rats that then activates chemosensory afferents of the carotid sinus nerve to elicit the hypoxic ventilatory response (HVR). The objective of this study was to provide direct evidence, using our capacitive S-nitrosothiol sensor, that L-CSNO is stored and released from PGCs extracted from male Sprague Dawley rat carotid bodies, and thus further pharmacological evidence for the role of S-nitrosothiols in mediating the HVR. Key findings of this study were that 1) lysates of PGCs contained an S-nitrosothiol with physico-chemical properties similar to L-CSNO rather than S-nitroso-L-glutathione (L-GSNO), 2) exposure of PGCs to a hypoxic challenge caused a significant increase in S-nitrosothiol concentrations in the perfusate to levels approaching 100 fM via mechanisms that required extracellular Ca2+, 3) the dose-dependent increases in minute ventilation elicited by arterial injections of L-CSNO and L-GSNO were likely due to activation of small diameter unmyelinated C-fiber carotid body chemoafferents, 4) L-CSNO, but not L-GSNO, responses were markedly reduced in rats receiving continuous infusion (10 μmol/kg/min, IV) of both S-methyl-L-cysteine (L-SMC) and S-ethyl-L-cysteine (L-SEC), 5) ventilatory responses to hypoxic gas challenge (10% O2, 90% N2) were also due to the activation of small diameter unmyelinated C-fiber carotid body chemoafferents, and 6) the HVR was markedly diminished in rats receiving L-SMC plus L-SEC. This data provides evidence that rat PGCs synthesize an S-nitrosothiol with similar properties to L-CSNO that is released in an extracellular Ca2+-dependent manner by hypoxia.

S-Nitroso-L-Cysteine Stereoselectively Blunts the Deleterious Effects of Fentanyl on Breathing While Augmenting Antinociception in Freely-Moving Rats

Endogenous and exogenously administered S-nitrosothiols modulate the activities of central and peripheral systems that control breathing. We have unpublished data showing that the deleterious effects of morphine on arterial blood-gas chemistry (i.e., pH, pCO2, pO2, and sO2) and Alveolar-arterial gradient (i.e., index of gas exchange) were markedly diminished in anesthetized Sprague Dawley rats that received a continuous intravenous infusion of the endogenous S-nitrosothiol, S-nitroso-L-cysteine. The present study extends these findings by showing that unanesthetized adult male Sprague Dawley rats receiving an intravenous infusion of S-nitroso-L-cysteine (100 or 200 nmol/kg/min) markedly diminished the ability of intravenous injections of the potent synthetic opioid, fentanyl (10, 25, and 50 μg/kg), to depress the frequency of breathing, tidal volume, and minute ventilation. Our study also found that the ability of intravenously injected fentanyl (10, 25, and 50 μg/kg) to disturb eupneic breathing, which was measured as a marked increase of the non-eupneic breathing index, was substantially reduced in unanesthetized rats receiving intravenous infusions of S-nitroso-L-cysteine (100 or 200 nmol/kg/min). In contrast, the deleterious effects of fentanyl (10, 25, and 50 μg/kg) on frequency of breathing, tidal volume, minute ventilation and non-eupneic breathing index were fully expressed in rats receiving continuous infusions (200 nmol/kg/min) of the parent amino acid, L-cysteine, or the D-isomer, namely, S-nitroso-D-cysteine. In addition, the antinociceptive actions of the above doses of fentanyl as monitored by the tail-flick latency assay, were enhanced by S-nitroso-L-cysteine, but not L-cysteine or S-nitroso-D-cysteine. Taken together, these findings add to existing knowledge that S-nitroso-L-cysteine stereoselectively modulates the detrimental effects of opioids on breathing, and opens the door for mechanistic studies designed to establish whether the pharmacological actions of S-nitroso-L-cysteine involve signaling processes that include 1) the activation of plasma membrane ion channels and receptors, 2) selective intracellular entry of S-nitroso-L-cysteine, and/or 3) S-nitrosylation events. Whether alterations in the bioavailability and bioactivity of endogenous S-nitroso-L-cysteine is a key factor in determining the potency/efficacy of fentanyl on breathing is an intriguing question.

NADPH diaphorase detects S-nitrosylated proteins in aldehyde-treated biological tissues

NADPH diaphorase is used as a histochemical marker of nitric oxide synthase (NOS) in aldehyde-treated tissues. It is thought that the catalytic activity of NOS promotes NADPH-dependent reduction of nitro-blue tetrazolium (NBT) to diformazan. However, it has been argued that a proteinaceous factor other than NOS is responsible for producing diformazan in aldehyde-treated tissues. We propose this is a NO-containing factor such as an S-nitrosothiol and/or a dinitrosyl-iron (II) cysteine complex or nitrosated proteins including NOS. We now report that (1) S-nitrosothiols covalently modify both NBT and TNBT, but only change the reduction potential of NBT after modification, (2) addition of S-nitrosothiols or β- or α-NADPH to solutions of NBT did not elicit diformazan, (3) addition of S-nitrosothiols to solutions of NBT plus β- or α-NADPH elicited rapid formation of diformazan in the absence or presence of paraformaldehyde, (4) addition of S-nitrosothiols to solutions of NBT plus β- or α-NADP did not produce diformazan, (5) S-nitrosothiols did not promote NADPH-dependent reduction of tetra-nitro-blue tetrazolium (TNBT) in which all four phenolic rings are nitrated, (6) cytoplasmic vesicles in vascular endothelial cells known to stain for NADPH diaphorase were rich in S-nitrosothiols, and (7) procedures that accelerate decomposition of S-nitrosothiols, markedly reduced NADPH diaphorase staining in tissue sections subsequently subjected to paraformaldehyde fixation. Our results suggest that NADPH diaphorase in aldehyde-fixed tissues is not enzymatic but is due to the presence of NO-containing factors (free SNOs or nitrosated proteins such as NOS), which promote NADPH-dependent reduction of NBT to diformazan.

Effects of intracerebroventricular injections of 5-HT on systemic vascular resistances of conscious rats

The aims of this study were to determine (i) the effects of intracerebroventricular (i.c.v.) injections of 5-hydroxytryptamine (5-HT, 10μg) on mean arterial blood pressure (MAP), heart rate (HR) and mesenteric (MR), renal (RR) and hindquarter (HQR) vascular resistances of conscious rats, (ii) the central 5-HT receptor subtype which mediates these effects, and (iii) the role of nitric oxide (NO) in the expression of these responses. The i.c.v. injection of 5-HT had minor effects on MAP but produced a decrease in HR (-18±4%), which lasted for 20min. The i.c.v. injection of 5-HT elicited marked increases in MR (+50±7%) and reductions in HQR (-31±3%). These responses occurred promptly and lasted for 25-35min. 5-HT also produced a transient decrease in RR (-26±8% at 10min). All of these responses were prevented by the prior i.c.v. injection of the 5-HT1/5-HT2-receptor antagonist, methysergide (10μg). The intravenous injection of the NO synthesis inhibitor, L-NAME (25μmol/kg), produced a sustained pressor response, bradycardia and increases in MR, RR and HQR. Subsequent i.c.v. injection of 5-HT produced a minor pressor response (+7±2%), bradycardia (-18±3%), an increase in MR (+52±8%) but no decreases in RR or HQR. This study demonstrates that i.c.v. 5-HT differentially affects peripheral vascular resistances by activation of central 5-HT1/5-HT2-receptors. It appears that L-NAME did not interfere with the central actions of 5-HT as it did not prevent the 5-HT-induced bradycardia or mesenteric vasoconstriction. Since the 5-HT-induced falls in RR and HQR were abolished by L-NAME, it is possible that these responses are mediated by an active neurogenic process involving the release of NO within the vasculature.

Hemodynamic responses elicited by systemic injections of isotonic and hypertonic saline in hemorrhaged rats

PURPOSE

The objectives of this study were (i) to characterize the hemodynamic responses caused by controlled hemorrhage (HEM) in pentobarbital-anesthetized rats, and (ii) to determine the responses elicited by systemic bolus injections of isotonic saline (0.15M) or hypertonic saline (3M) given 5min after completion of HEM.

RESULTS

Controlled HEM (4.3±0.2ml/rat at 1.5ml/min) resulted in a pronounced and sustained fall in mean arterial blood pressure (MAP) to about 40mmHg. The fall in MAP was associated with a reduction in hindquarter vascular resistance (HQR) but no changes in renal (RR) or mesenteric (MR) vascular resistances. Systemic injections of isotonic saline (96-212μmol/kg i.v., in 250-550μl) did not produce immediate responses but promoted the recovery of MAP to levels below pre-HEM values. Systemic injections of hypertonic saline (750-3000μmol/kg, i.v., in 250-550μl) produced immediate and pronounced falls in MAP, RR, MR and especially HQR of 30-120s in duration. However, hypertonic saline prompted a full recovery of MAP, HQR and RR to pre-HEM levels and an increase in MR to levels above pre-HEM values.

CONCLUSIONS

This study demonstrates that (i) HEM induced a pronounced fall in MAP which likely involved a fall in cardiac output and HQR, (ii) isotonic saline did not fully normalize MAP, and (iii) hypertonic saline produced dramatic initial responses, and promoted normalization of MAP probably by restoring blood volume and cardiac output through sequestration of fluid from intracellular compartments.

S-Nitrosylation signaling regulates cellular protein interactions

BACKGROUND

S-Nitrosothiols are made by nitric oxide synthases and other metalloproteins. Unlike nitric oxide, S-nitrosothiols are involved in localized, covalent signaling reactions in specific cellular compartments. These reactions are enzymatically regulated.

SCOPE

S-Nitrosylation affects interactions involved in virtually every aspect of normal cell biology. This article is part of a Special Issue entitled Regulation of Cellular Processes by S-nitrosylation.

MAJOR CONCLUSIONS AND SIGNIFICANCE

S-Nitrosylation is a regulated signaling reaction.

Flavin adenine dinucleotide may release preformed stores of nitrosyl factors from the vascular endothelium of conscious rats

This study determined whether flavin adenine dinucleotide (FAD) may elicit vasodilation in conscious rats via release of preformed endothelium-derived nitrosyl factors. Injections 1-6 (inj(1-6)) of FAD (2.5 micromol/kg, IV) elicited pronounced and equivalent vasodilator responses in saline-treated rats. Inj(1) of FAD elicited pronounced vasodilation in L-NAME-treated rats pretreated with the nitric oxide (NO) synthesis inhibitor, NG-nitro-L-arginine (L-NAME; 50 micromol/kg, IV), whereas Inj(2-6) elicited progressively smaller responses such that inj(6) elicited minor responses. The vasodilator responses elicited by the endothelium-dependent agonist, acetylcholine, were markedly attenuated in L-NAME-treated rats that had received inj(1-6) of FAD but not in saline-treated rats that had received inj(1-6) of FAD. The vasodilator actions of L-S-nitrosocysteine and the NO donor, sodium nitroprusside, were not diminished after the injections of FAD in saline- or in L-NAME-treated rats. Binding studies demonstrated that the densities of muscarinic M3 receptors were increased in thoracic aorta endothelium of rats treated with L-NAME + inj(1-6) of saline or L-NAME + inj(1-6) of FAD as compared to rats treated with saline + inj(1-6) of saline or saline + inj(1-6) of FAD. The progressive loss of response to injections of FAD in L-NAME-treated rats coupled with the loss of response to acetylcholine suggests that FAD elicits the use-dependent depletion of vesicular pools of nitrosyl factors in endothelial cells that cannot be replenished in the absence of NO synthesis.

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.

Sympathectomy Causes Aggravated Lesions and Dedifferentiation in Large Rabbit Atherosclerotic Arteries without Involving Nitric Oxide

Previously [Histochem J 1997;29:279-286], we found that sympathectomy induced neointima formation in ear but not cerebral arteries of genetically hyperlipidemic rabbits. To clarify the influence of sympathetic nerves in atherosclerosis, and whether their influence involves vascular NO activity, we studied groups of normocholesterolemic intact (NI) and sympathectomized (NS), and hypercholesterolemic intact (HI) and sympathectomized (HS) rabbits (diet/6-hydroxydopamine for 79 days). Segments of basilar (BA) and femoral (FA) arteries were studied histochemically, to evaluate differentiation (anti-desmin, anti-vimentin, anti-h-caldesmon, and nuclear dye), by confocal microscopy, and by in vitro myography. In BAs, staining of NI and NS groups was similar. In hypercholesterolemic groups, a small neointima developed, more frequently in HS segments where smooth muscle cells (SMCs) positive for all antibodies appeared to be migrating into the neointima. In FAs, SMCs stained for the three antibodies in the NI group, but we observed desmin- and h-caldesmon-negative, vimentin-positive cells in some external medial layers of the NS, HI and HS groups, identical to adventitial fibroblasts. Large neointimas of the HS group contained vimentin-positive and largely desmin- and h-caldesmon-negative cells. Relaxation of BA or FA segments to acetylcholine was not decreased by sympathectomy. Sympathectomy increased the contraction of resting FAs to nitro-L-arginine (p = 0.0379). Thus, sympathectomy aggravates the tendency for FA SMCs to migrate and dedifferentiate, increasing atherosclerotic lesions, without decreasing NO activity, but has only minor effects on BAs.

Central nitric oxide modulates hindquarter vasodilation elicited by AMPA receptor stimulation in the NTS of conscious rats

Microinjection of S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) in the nucleus of the solitary tract (NTS) of conscious rats causes hypertension, bradycardia, and vasoconstriction in the renal, mesenteric, and hindquarter vascular beds. In the hindquarter, the initial vasoconstriction is followed by vasodilation with AMPA doses >5 pmol/100 nl. To test the hypothesis that this vasodilation is caused by activation of a nitroxidergic pathway in the NTS, we examined the effect of pretreatment with the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 10 nmol/100 nl, microinjected into the NTS) on changes in mean arterial pressure, heart rate, and regional vascular conductance (VC) induced by microinjection of AMPA (10 pmol/100 nl in the NTS) in conscious rats. AMPA increased hindquarter VC by 18 +/- 4%, but after pretreatment with L-NAME, AMPA reduced hindquarter VC by 16 +/- 7% and 17 +/- 9% (5 and 15 min after pretreatment, P < 0.05 compared with before pretreatment). Pretreatment with L-NAME reduced AMPA-induced bradycardia from 122 +/- 40 to 92 +/- 32 beats/min but did not alter the hypertension induced by AMPA (35 +/- 5 mmHg before pretreatment, 43 +/- 6 mmHg after pretreatment). Control injections with D-NAME did not affect resting values or the response to AMPA. The present study shows that stimulation of AMPA receptors in the NTS activates both vasodilatatory and vasoconstrictor mechanisms and that the vasodilatatory mechanism depends on production of nitric oxide in the NTS.

Effects of Thiol Chelation on α1-Adrenoceptor-Induced Vasoconstriction In Vivo

The aims of this study were to determine whether systemic injections of the lipophobic thiol chelator, para-hydroxymercurobenzoic acid (PHMBA) would reduce the vasoconstrictor responses elicited by the alpha1-adrenoceptor agonist, phenylephrine, in urethane-anesthetized rats by chelation of thiol residues in alpha1-adrenoceptors in vascular smooth muscle rather than voltage-sensitive Ca(2+)-channels (Ca(2+)VERSUS-channels). The magnitudes and durations of the vasoconstrictor responses elicited by phenylephrine were markedly reduced after the injections of PHMBA. In contrast, the maximal phenylephrine-induced responses were not affected whereas the durations of these responses were markedly attenuated after injection of the Ca(2+)VERSUS-channel blocker, nifedipine. Nifedipine elicited pronounced and sustained falls in mean arterial blood pressure and vascular resistances in PHMBA-treated rats. Moreover, the vasodilator actions of the nitric oxide-donor, sodium nitroprusside were minimally attenuated by PHMBA whereas they were markedly attenuated by nifedipine. These findings support evidence that the vasoconstrictor responses due to activation of alpha1-adrenoceptors are initiated by mobilization of intracellular pools of Ca(2+) whereas they are sustained by opening of Ca(2+)VERSUS-channels. These findings also suggest that PHMBA diminishes the vasoconstrictor effects of phenylephrine by chelation of thiol residues in alpha1-adrenoceptors rather than by blockade of Ca(2+)VERSUS-channels, and that chelation of these thiol residues prevents agonist occupation and/or activation of these receptors and subsequent mobilization of intracellular pools of Ca(2+).

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.

Role of nitrosyl factors in the hindlimb vasodilation elicited by baroreceptor afferent nerve stimulation

This study determined whether electrical stimulation (ES) of the baroreceptor afferent fibers in the aortic depressor nerve (ADN) produces hindlimb vasodilation in pentobarbital-anesthetized rats via the release of nitric oxide (NO)-containing (nitrosyl) factors from NO synthase-positive lumbar sympathetic nerve terminals. ES of the ADN (1-10 Hz for 15 s) produced frequency-dependent reductions in mean arterial blood pressure (MAP) and mesenteric and hindlimb vascular resistance (MR and HLR, respectively). The falls in resistance were substantially smaller in hindlimb beds in which the ipsilateral lumbar sympathetic chain had been transected 7-10 days previously. The maximal falls in MR and hindquarter vascular resistance (HQR) produced by 1- to 10-Hz ES of the ADN were unaffected by the specific inhibitor of neuronal NO synthase 7-nitroindazole (7-NI, 45 mg/kg iv). However, the total falls in HQR (mmHg.kHz(-1).s) produced by these stimuli were significantly diminished by 7-NI, whereas the total falls in MR were not affected. Four successive episodes of 10-Hz ES produced equivalent reductions in MAP, MR, and HQR. The peak changes in these parameters were not affected by 7-NI. However, the total falls in HQR progressively diminished with each successive stimulus, whereas the total falls in MR remained unchanged. These results provide evidence that the hindlimb vasodilation produced by ES of baroreceptor afferents within the ADN may involve the activation of postganglionic lumbar sympathetic vasodilator fibers, which release newly synthesized and preformed nitrosyl factors.

Endogenous nitrosyl factors may inhibit the desensitization of 5-HT3 receptors on vagal cardiopulmonary afferents

The pronounced tachyphylaxis to the Bezold-Jarisch reflex (BJR) responses elicited by systemic injections of the 5-HT(3) receptor (5-HT(3)R) agonists such as phenylbiguanide (PBG) may involve desensitization and/or reduced rate of resensitization of 5-HT(3)Rs on vagal cardiopulmonary afferents. The presence of nitric oxide synthase (NOS) in vagal afferents raises the possibility that endogenous nitrosyl factors regulate the status of 5-HT(3)Rs in these afferents. Accordingly, the aim of this study was to determine whether the inhibition of NOS alters the development of tachyphylaxis to the BJR responses elicited by PBG in conscious rats. The first injection of PBG (100 microg/kg, i.v.) elicited robust reductions in heart rate (HR), diastolic arterial blood pressure (BP(D)), and cardiac output (CO) but minor changes in total peripheral resistance in saline-treated rats. Subsequent injections elicited progressively smaller responses such that the sixth injections elicited minor responses only. The first injection of PBG (100 microg/kg, i.v.) in rats treated with the NOS inhibitor, L-NAME (25 micromol/kg, i.v.) elicited similar reductions in HR, BP(D), and CO as in saline-treated rats. However, the rate of development of tachyphylaxis to PBG was markedly faster in the L-NAME-treated rats. The BJR responses elicited by 5-HT (40 microg/kg, i.v.) were markedly attenuated after the development of tachyphylaxis to PBG in saline- and in L-NAME-treated rats whereas the BJR responses elicited by the S-nitrosothiol, L-S-nitrosocysteine (5 micromol/kg, i.v.), were not attenuated in either group. These findings suggest that tachyphylaxis to PBG was not due to the loss of central or efferent processing of the BJR. Taken together, these findings suggest NOS exists in vagal cardiopulmonary afferents mediating the BJR and that nitrosyl factors influence 5-HT(3)R function.

Rostral ventrolateral medulla: an integrative site for muscle vasodilation during defense-alerting reactions

1. Evidence gathered over the last 30 years has firmly established that the rostral ventrolateral medulla (RVLM) is a major vasomotor center in the brainstem, harboring sympathetic premotor neurons responsible for generating and maintaining basal vasomotor tone and resting levels of arterial blood pressure. Although the RVLM has been almost exclusively classified as a vasopressor area, in this report we review some evidence suggesting a prominent role of the RVLM in muscle vasodilation during defense-alerting responses. 2. Defense-alerting reactions are a broad class of behavior including flexion of a limb, fight/flight responses, apologies, etc. They comprise species-distinctive motor and neurovegetative adjustments. Cardiovascular responses include hypertension, tachycardia, visceral vasoconstriction, and muscle vasodilation. Since defense-alerting reactions generally involve intense motor activation, muscle vasodilation is regarded as a key feature of these responses 3. In anesthetized or unanesthetized-decerebrate animals, natural or electrical stimulation of cutaneous and muscle afferents produced hypertension, tachycardia, and vasodilation restricted to the stimulated limb. 4. Unilateral inactivation of the RVLM contralateral to the stimulated limb abolished cardiovascular adjustments to stimulation of cutaneous and muscle afferents. Within the RVLM glutamatergic synapses mediate pressor responses, whereas GABAergic synapses mediates muscle vasodilation. 5. In urethane-anesthetized rats, electrical stimulation of the hypothalamus elicited hypertension, tachycardia, visceral vasoconstriction, and hindlimb vasodilation. The hindlimb vasodilation induced by hypothalamic stimulation is a complex response, involving reduction of sympathetic vasoconstrictor tone, release of catecholamines by the adrenal medulla, and a still unknown system that may use nitric oxide as a mediator. 6. Blockade of glutamatergic transmission within the RVLM selectively blocks muscle vasodilation induced by hypothalamic stimulation. 7. The results obtained suggest that, besides its role in the generation and maintenance of the sympathetic vasoconstrictor drive, the RVLM is also critical for vasodilatory responses during defense reactions. The RVLM may contain several, distinctive mechanisms for muscle vasodilation. Anatomical and functional characterization of these pathways may represent a breakthrough in our understanding of cardiovascular control in normal and/or pathological conditions.

Comparative regional haemodynamic effects of the nitric oxide synthase inhibitors, S-methyl-L-thiocitrulline and L-NAME, in conscious rats

1. The regional haemodynamic effects of the putative nNOS inhibitor, S-methyl-L-thiocitrulline (SMTC), were compared with those of the nonselective NOS inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), in conscious, male Sprague-Dawley rats. 2. SMTC (0.3 mg kg(-1) bolus) produced a significant, short-lived, pressor effect associated with renal, mesenteric and hindquarters vasoconstriction; the same dose of L-NAME did not affect mean blood pressure (BP), although it caused bradycardia and mesenteric vasoconstriction. 3. At the highest dose tested (10 mg kg(-1)), L-NAME produced a significantly greater bradycardia and fall in mesenteric vascular conductance than SMTC, although the initial pressor response to SMTC was greater, but less sustained, than that to L-NAME. 4. Infusion of SMTC or L-NAME (3 mg kg(-1) h(-1)) induced rises in BP and falls in renal, mesenteric and hindquarters vascular conductances, but the effects of L-NAME were greater than those of SMTC, and L-NAME also caused bradycardia. 5. The renal vasodilator response to acetylcholine was markedly attenuated by infusion of L-NAME, but unaffected by SMTC. The hindquarters vasodilatation induced by salbutamol was attenuated by L-NAME, but not by SMTC. The mesenteric vasodilator response to bradykinin was modestly enhanced by SMTC, but not by L-NAME. The depressor and renal, mesenteric and hindquarters vasodilator responses to sodium nitroprusside were enhanced by L-NAME, whereas SMTC modestly enhanced the hypotensive and renal vasodilator effects of sodium nitroprusside, but attenuated the accompanying tachycardia. 6. The results are consistent with the cardiovascular effects of low doses of SMTC being attributable to nNOS inhibition.

Hypotensive effect of ANG II and ANG-(1-7) at the caudal ventrolateral medulla involves different mechanisms

The objective of the present study was to determine the contribution of the autonomic nervous system and nitric oxide to the depressor effect produced by unilateral microinjection of ANG-(1-7) and ANG II into the caudal ventrolateral medulla (CVLM). Unilateral microinjection of ANG-(1-7), ANG II (40 pmol), or saline (100 nl) was made into the CVLM of male Wistar rats anesthetized with urethane before and after intravenous injection of 1) methyl-atropine, 2.5 mg/kg; 2) prazosin, 25 microg/kg; 3) the nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), 5 mg/kg; or 4) the specific inhibitor of neuronal NOS, 7-nitroindazole (7-NI), 45 mg/kg. Arterial pressure and heart rate (HR) were continuously monitored. Microinjection of ANG-(1-7) or ANG II into the CVLM produced a significant decrease in mean arterial pressure (MAP; -11 +/- 1 mmHg, n = 12 and -10 +/- 1 mmHg, n = 10, respectively) that was not accompanied by consistent changes in HR or in cardiac output. The effect of ANG-(1-7) was abolished after treatment with methyl-atropine (-3 +/- 0.6 mmHg, n = 9) or L-NAME (-2.3 +/- 0.5 mmHg, n = 8) or 7-NI (-2.8 +/- 0.6 mmHg, n = 5). In contrast, these treatments did not significantly interfere with the ANG II effect (-10 +/- 2.6 mmHg, n = 8; -8 +/- 1.5 mmHg, n = 8; and -12 +/- 3.6 mmHg, n = 6; respectively). Peripheral treatment with prazosin abolished the hypotensive effect of ANG-(1-7) and ANG II. Microinjection of saline did not produce any significant change in MAP or in HR. These results suggest that the hypotensive effect produced by ANG II at the CVLM depends on changes in adrenergic vascular tonus and, more importantly, the hypotensive effect produced by ANG-(1-7) also involves a nitric oxide-related mechanism.

Regional haemodynamic responses to activation of the medial prefrontal cortex depressor region

Electrical or chemical stimulation of the medial prefrontal cortex (MPFC) produces depressor and sympathoinhibitory responses. To characterise the MPFC depressor response more fully, we determined the regional haemodynamic changes which occurred in response to stimulation of the MPFC. In halothane-anaesthetised rats, we recorded arterial blood pressure and renal, superior mesenteric, and iliac arterial vascular conductance using miniaturised Doppler flow probes. Electrical stimulation of the MPFC (50-100 microA) was used to map the location of the depressor region. Increases in vascular conductance (or increases in blood flow) were recorded from the renal (+2.3+/-0.5 kHz/mmHgx10(3)), mesenteric (+4.4+/-0.4 kHz/mmHgx10(3)), and iliac (+8.3+/-1.0 kHz/mmHgx10(3)) vascular beds in response to stimulation of the MPFC depressor region coinciding with the ventral infralimbic (IL) and dorsal peduncular (DP) cortical areas. Similar responses were obtained after microinjection of the chemical excitant L-glutamate (n=3, 100 nl, 100 mM), indicating that the responses were due to excitation of cell bodies and not due to axons traversing the area. Administration of the nitric oxide synthesis inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 25 micromol/kg, i.v., n=5) significantly reduced the MPFC depressor response (51%, 12.5+/-1.2 to 6.1+/-2.5 mmHg). The increases in conductance in the hindquarter and mesenteric vascular beds were significantly reduced after L-NAME treatment (mesenteric by 77%, iliac by 70%), but there was no significant reduction of renal flow (35%). These observations indicate that the depressor region of the MPFC is localised to ventral regions (IL and DP) and that the depressor response is mediated by increased conductance in the hindquarters and mesenteric vascular beds. Furthermore, the depressor response may be mediated, in part, by release of nitric oxide in these vascular beds.

Blockade of voltage-sensitive Ca(2+)-channels markedly diminishes nitric oxide- but not L-S-nitrosocysteine- or endothelium-dependent vasodilation in vivo

The aim of this study was to determine the hemodynamic responses elicited by systemic injections of (i) the nitric oxide (NO)-donors, sodium nitroprusside (10 nmol/kg, i.v.) and (Z)-1-(N-methyl-N-(6(N-methylammoniohexyl)amino))diazen-1-ium-1, 2-diolate (MAHMA NONOate, 25 nmol/kg, i.v.), (ii) the endothelium-derived S-nitrosothiol, L-S-nitrosocysteine (100 nmol/kg, i.v.), and (iii) the endothelium-dependent agonist, acetylcholine (1.0 microg/kg, i.v.), in anesthetized rats, before and after injection of the voltage-sensitive Ca(2+)-channel (Ca(VS)(2+)-channel) blocker, nifedipine (500 nmol/kg, i.v.). Before injection of nifedipine, the agents produced similar falls in mean arterial blood pressure, and in hindquarter and mesenteric vascular resistances. The depressor and vasodilator responses elicited by sodium nitroprusside and MAHMA NONOate were markedly attenuated by nifedipine. The falls in mean arterial blood pressure and mesenteric resistance elicited by L-S-nitrosocysteine and acetylcholine were not attenuated but the falls in hindquarter resistance were slightly attenuated by nifedipine. The cyclooxygenase inhibitor, indomethacin (10 mg/kg, i.v.), did not affect the actions of sodium nitroprusside, MAHMA NONOate, L-S-nitrosocysteine or acetylcholine or the effects of nifedipine on the hemodynamic actions of these compounds. The decomposition of sodium nitroprusside (0.2 nmol/ml), MAHMA NONOate (0.5 nmol/ml) and L-S-nitrosocysteine (2 nmol/ml) to NO upon addition to rat blood was not affected by nifedipine (10 microM). These findings suggest that (i) exogenously applied NO relaxes resistance arteries in vivo by inhibition of Ca(VS)(2+)-channels whereas L-S-nitrosocysteine and the non-prostanoid endothelium-derived relaxing factor (EDRF) released by acetylcholine acts by additional mechanisms, and (ii) this EDRF may be an S-nitrosothiol which acts independently of its decomposition to NO.

Redox regulation of S-nitrosocysteine-mediated vasodilation in vivo

This study examined the effects of the lipophobic electron acceptor, nitroblue tetrazolium (2x5 micromol/kg, i.v.) on the vasodilation produced by the putative endothelium-derived S-nitrosothiol, L-S-nitrosocysteine (400 nmol/kg, i.v.), and the nitric oxide (NO) donor, (Z)-1-N-methyl-N-[6(N-methylammoniohexyl)amino]&z. sfnc;diazen-1-ium-1,2-diolate (MAHMA NONOate, 25 nmol/kg, i.v.), in anesthetized rats. The administration of nitroblue tetrazolium resulted in delayed but long-lasting increases in vascular resistances. The L-S-nitrosocysteine-induced vasodilator responses were markedly diminished whereas the MAHMA NONOate-induced responses were not affected by nitroblue tetrazolium. These results support the possibility that L-S-nitrosocysteine interacts with membrane thiols that are subject to nitroblue tetrazolium-induced oxidation (i.e., disulfide-bridge formation) and that nitroblue tetrazolium-induced vasoconstriction may involve a loss of potency of endothelium-derived S-nitrosothiols.

Beta-adrenoceptor dysfunction after inhibition of NO synthesis

G(s) protein-coupled beta-adrenoceptors rapidly desensitize on exposure to agonists in reconstituted membrane preparations, whereas rapid tachyphylaxis to beta-adrenoceptor-mediated vasodilation does not readily occur in vivo. This study examined the possibility that endothelium-derived nitrosyl factors prevent the rapid desensitization of beta-adrenoceptors in the vascular smooth muscle of resistance arteries in pentobarbital-anesthetized rats. The fall in mean arterial blood pressure and in hindquarter vascular resistance produced by the beta-adrenoceptor agonist isoproterenol (ISO, 0.1 to 10 microg/kg IV) was slightly but significantly smaller in rats treated with the NO synthase inhibitor N:(G)-nitro-L-arginine methyl ester (L-NAME, 100 micromol/kg IV) than in saline-treated rats. The ISO-induced fall in mesenteric resistance was similar in L-NAME-treated and in saline-treated rats. The fall in hindquarter vascular resistance and in mesenteric resistance produced by ISO (8 x 10 microg/kg IV) was subject to tachyphylaxis on repeated injection in rats treated with L-NAME (100 micromol/kg IV) but not in rats treated with saline. Injections of L-S:-nitrosocysteine (1200 nmol/kg IV), a lipophobic S:-nitrosothiol, before each injection of ISO (10 microg/kg IV) prevented tachyphylaxis to ISO in L-NAME-treated rats. The vasodilator effects of ISO (0.1 to 10 microg/kg IV) in L-NAME-treated rats that received 8 injections of ISO (10 microg/kg IV) were markedly smaller than in L-NAME-treated rats that received 8 injections of saline. These results indicate that (1) the vasodilator actions of ISO in pentobarbital-anesthetized rats only minimally involve the release of endothelium-derived nitrosyl factors, (2) the effects of ISO are subject to development of tachyphylaxis in L-NAME-treated rats, and (3) tachyphylaxis to ISO is prevented by L-S:-nitrosocysteine. These findings suggest that endothelium-derived nitrosyl factors may prevent desensitization of beta-adrenoceptors in vivo.

Absence of cholinergic sympathetic innervation from limb muscle vasculature in rats and mice

Although the existence of cholinergic sympathetic vasodilatory innervation in limb muscle vasculature is well established for some species, previous pharmacological studies have failed to reveal the presence of such innervation in rats. Recently, Schafer and colleagues [Schafer, M.K., Eiden, L.E., Weihe, E., 1998. Cholinergic neurons and terminal fields revealed by immunohistochemistry for the vesicular acetylcholine transporter. II. The peripheral nervous system. Neuroscience 84(2), 361-376] reported that vesicular acetylcholine transporter immunoreactivity (VAChT-IR), a marker for cholinergic terminals, is present in the innervation of the microvasculature of rat hindlimb skeletal muscle and concluded that rats possess cholinergic sympathetic innervation of limb muscle vasculature. Because of our interest in identifying targets of cholinergic sympathetic neurons, we have analyzed the transmitter properties of the innervation of muscle vessels in rat and mouse limbs. We found that the innervation of vasculature in muscle is noradrenergic, exhibiting robust catecholamine histofluorescence and immunoreactivity for tyrosine hydroxylase (TH) and the peptide transmitters, neuropeptide Y (NPY) and occasionally vasoactive intestinal peptide (VIP). In contrast, cholinergic phenotypic markers,VAChT-IR and acetylcholinesterase (AChE) activity, are absent. Neuron cell bodies in sympathetic ganglia, retrogradely labeled with injections of tracer into limb muscles, also lacked VAChT but contained TH-IR. The innervation of large extramuscular feed arteries in hindlimbs was also devoid of cholinergic markers, as were the cell bodies of sympathetic neurons innervating extramuscular femoral arteries. These results, like those of previous physiological studies, provide no evidence for the presence of cholinergic sympathetic innervation of muscle vasculature in rats or mice.

Tachyphylaxis to PACAP-27 after inhibition of NO synthesis: a loss of adenylate cyclase activation

The vasodilator effects of pituitary adenylate cyclase activating polypeptide (PACAP-27) are subject to tachyphylaxis in rats treated with the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). This study examined whether this tachyphylaxis is due to the loss of vasodilator potency of cAMP generated by activation of the G(s) protein-coupled PACAP receptors. Five successive treatments with PACAP-27 (2 nmol/kg iv) produced pronounced vasodilator responses in saline-treated rats that were not subject to tachyphylaxis. The first injection of PACAP-27 (2 nmol/kg iv) in L-NAME (50 micromol/kg iv)-treated rats produced vasodilator responses of similar magnitude to those in saline-treated rats, whereas four subsequent injections produced progressively and markedly smaller responses. The hemodynamic effects of the membrane-permeable cAMP analog 8-(4-chlorophenylthiol)-cAMP (8-CPT-cAMP; 5-15 micromol/kg iv) were similar in L-NAME-treated rats and in L-NAME-treated rats that had received the five injections of PACAP-27. In addition, five injections of 8-CPT-cAMP (10 micromol/kg iv) produced pronounced vasodilator responses in saline- and L-NAME-treated rats that were not subject to the development of tachyphylaxis. These results suggest that a loss of biological potency of cAMP is not responsible for tachyphylaxis to PACAP-27 in L-NAME-treated rats. This tachyphylaxis may be due to the inability of the G(s) protein-coupled PACAP receptor to activate adenylate cyclase.

Hemodynamic actions of systemically injected pituitary adenylate cyclase activating polypeptide-27 in the rat

The aims of this study were (1) to characterize the hemodynamic mechanisms underlying the hypotensive effects of pituitary adenylate cyclase activating polypeptide-27 (PACAP-27 0.1-2.0 nmol/kg, i.v.) in pentobarbital-anesthetized rats, and (2) to determine the roles of the autonomic nervous system, adrenal catecholamines and endothelium-derived nitric oxide (NO) in the expression of PACAP-27-mediated effects on hemodynamic function. PACAP-27 produced dose-dependent decreases in mean arterial blood pressure and hindquarter and mesenteric vascular resistances in saline-treated rats. PACAP-27 also produced pronounced falls in mean arterial blood pressure in rats treated with the ganglion blocker, chlorisondamine (5 mg/kg, i.v.). The hypotensive and vasodilator actions of PACAP-27 were not attenuated by the beta-adrenoceptor antagonist, propranolol (1 mg/kg, i.v.), or the NO synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME 50 micromol/kg, i.v.). PACAP-27 produced dose-dependent increases in heart rate whereas the hypotensive response produced by the nitrovasodilator, sodium nitroprusside (10 microg/kg, i.v.), was associated with a minimal tachycardia. The PACAP-27-induced tachycardia was unaffected by chlorisondamine, but was virtually abolished by propranolol. These results suggest that the vasodilator effects of PACAP-27 are due to actions in the microcirculation rather than to the release of adrenal catecholamines and that this vasodilation may not involve the release of endothelium-derived NO. These results also suggest that PACAP-27 produces tachycardia by directly releasing norepinephrine from cardiac sympathetic nerve terminals rather than by direct or baroreceptor reflex-mediated increases in sympathetic nerve activity.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Pathophysiology of nitric oxide and related species: free radical reactions and modification of biomolecules

Since its initial discovery as an endogenously produced bioactive mediator, nitric oxide (.NO) has been found to play a critical role in the cellular function of nearly all organ systems. Furthermore, aberrant production of .NO or reactive nitrogen species (RNS) derived from .NO, has been implicated in a number of pathological conditions, such as acute lung disease, atherosclerosis and septic shock. While .NO itself is fairly non-toxic, secondary RNS are oxidants and nitrating agents that can modify both the structure and function of numerous biomolecules both in vitro, and in vivo. The mechanisms by which RNS mediate toxicity are largely dictated by its unique reactivity. The study of how reactive nitrogen species (RNS) derived from .NO interact with biomolecules such as proteins, carbohydrates and lipids, to modify both their structure and function is an area of active research, which is lending major new insights into the mechanisms underlying their pathophysiological role in human disease. In the context of .NO-dependent pathophysiology, these biochemical reactions will play a major role since they: (i) lead to removal of .NO and decreased efficiency of .NO as an endothelial-derived relaxation factor (e.g. in hypertension, atherosclerosis) and (ii) lead to production of other intermediate species and covalently modified biomolecules that cause injury and cellular dysfunction during inflammation. Although the physical and chemical properties of .NO and .NO-derived RNS are well characterised, extrapolating this fundamental knowledge to a complicated biological environment is a current challenge for researchers in the field of .NO and free radical research. In this review, we describe the impact of .NO and .NO-derived RNS on biological processes primarily from a biochemical standpoint. In this way, it is our intention to outline the most pertinent and relevant reactions of RNS, as they apply to a diverse array of pathophysiological states. Since reactions of RNS in vivo are likely to be vast and complex, our aim in this review is threefold: (i) address the major sources and reactions of .NO-derived RNS in biological systems, (ii) describe current knowledge regarding the functional consequences underlying .NO-dependent covalent modification of specific biomolecules, and (iii) to summarise and critically evaluate the available evidence implicating these reactions in human pathology. To this end, three areas of special interest have been chosen for detailed description, namely, formation and role of S-nitrosothiols, modulation of lipid oxidation/nitration by RNS, and tyrosine nitration mechanisms and consequences.

Neurochemical distinction between skeletal muscle vasodilator neurons and pelvic vasodilator neurons in guinea-pigs

This study sets out to compare the combinations of potential vasodilator transmitters expressed by sympathetic and pelvic vasodilator neurons of guinea-pigs. Triple-labelling fluorescence immunohistochemistry was used to examine immunoreactivity (IR) to vasoactive intestinal peptide (VIP), nitric oxide synthase (NOS) and calcitonin gene-related peptide (CGRP) in lumbar sympathetic ganglia, and in perivascular axons supplying hindlimb skeletal muscles or pelvic viscera. Only 0.2% of VIP-IR nerve cell bodies in lumbar sympathetic ganglia (n = 4632 VIP-IR nerve cell profiles) contained NOS-IR, and one VIP-IR neuron contained CGRP-IR. The VIP-IR perivascular axons along the common and external iliac arteries, femoral artery and arteries to hindlimb muscles lacked NOS-IR and CGRP-IR. In contrast, all VIP-IR perivascular axons projecting from pelvic ganglia to the main uterine artery, and half of the VIP-IR axons along the internal iliac artery, contained NOS-IR and CGRP-IR. Thus, the neurochemical content of sympathetic vasodilator neurons to skeletal muscle arteries was clearly distinguishable from that of pelvic vasodilator neurons to the uterine vasculature. Furthermore, the autonomic dilation in each vascular bed is likely to be qualitatively different, and matched to the functional requirements of each target organ.

Hemodynamic effects of L-glutamate in NTS of conscious rats: a possible role of vascular nitrosyl factors

This study examined peripheral mechanisms responsible for changes in mean arterial blood pressure, heart rate, and renal, mesenteric, and hindquarter vascular resistances produced by microinjections of L-glutamate (L-Glu) into the nucleus tractus solitarii (NTS) of conscious rats. Microinjection of L-Glu produced an initial pressor response, bradycardia, and vasoconstriction in each vascular bed. Subsequent hindquarter vasodilation was observed. After prazosin was administered, L-Glu produced initial hypotension that was probably due to reduced cardiac output. This hypotension was followed by hindquarter vasodilation. Inhibition of nitric oxide synthesis did not affect the initial hypotension or bradycardia in rats treated with prazosin, but the first microinjection of L-Glu after administration of prazosin and NG-nitro-L-arginine methyl ester (L-NAME) produced significantly greater hindquarter vasodilation than after administration of prazosin alone. Second and third microinjections of L-Glu produced significantly smaller hindquarter vasodilation. We conclude that 1) hemodynamic effects produced by microinjection of L-Glu into the NTS of conscious rats involves activation of the sympathetic nervous system and 2) release of preformed nitrosyl factors may mediate vasodilation in the hindquarter vascular bed.

Use-dependent loss of acetylcholine- and bradykinin-mediated vasodilation after nitric oxide synthase inhibition. Evidence for preformed stores of nitric oxide-containing factors in vascular endothelial cells

In the present study, we examined the possibility that the endothelium-dependent vasodilators acetylcholine and bradykinin release preformed pools of nitric oxide-containing factors. Successive injections of selected doses of acetylcholine (1.18 +/- 0.3 micrograms/kg IV) or bradykinin (5 micrograms/kg IV) caused reproducible hypotensive and vasodilator responses within sympathetically intact and sympathetically denervated hindlimbs of conscious rats. After administration of the nitric oxide synthesis inhibitor N omega-nitro-L-arginine methyl ester (L-NAME, 25 mumol/kg IV), the first injection of acetylcholine or bradykinin produced pronounced depressor and vasodilator responses that, in the case of bradykinin, were greater than those observed before L-NAME administration. However, each successive injection of acetylcholine and bradykinin produced progressively smaller responses, such that the later injections elicited a markedly diminished hypotension and vasodilation. This "use-dependent" loss of endothelium-dependent vasodilation was not due to the diminished vasorelaxant potency of nitric oxide-containing factors because the vasodilator effects of the nitric oxide donor sodium nitroprusside (32 micrograms/kg IV) and the S-nitrosothiol compound S-nitro-socysteine (200 nmol/kg IV) were augmented in the presence of L-NAME. These results suggest that the use-dependent loss of the hemodynamic effects of acetylcholine and bradykinin in L-NAME-treated rats may be due to the release and subsequent depletion of a factor whose synthesis depends on the bioavailability of nitric oxide. Taken together, these results suggest that preformed pools of nitric oxide-containing factors exist within the endothelium of resistance vessels and that endothelium-dependent agonists exert their vasorelaxant effects at least in part by the mobilization of these performed pools.

Use-dependent loss of active sympathetic neurogenic vasodilation after nitric oxide synthase inhibition in conscious rats. Evidence for the presence of preformed stores of nitric oxide-containing factors

In this study, we examined whether air-jet stress-induced active sympathetic hindlimb vasodilation in conscious rats involves the release of preformed stores of nitric oxide-containing factors. We determined the effects of repeated episodes of air-jet stress (six episodes given 5 minutes apart) on mean arterial pressure and vascular resistances in the mesenteric bed and intact and sympathetically denervated hindlimb beds of conscious rats treated with saline or the nitric oxide synthesis inhibitor N omega-nitro-L-arginine methyl ester (L-NAME, 25 mumol/kg IV). In saline-treated rats, air-jet stress produced alerting behavior, minor changes in blood pressure, pronounced mesenteric vaso-constriction, and immediate and marked vasodilation in the sympathetically intact hindlimb but a minor vasodilation in the sympathetically denervated hindlimb. Each air-jet stress produced virtually identical responses. In L-NAME-treated rats, the first air-jet stress produced vasodilator responses in the sympathetically intact and sympathetically denervated hindlimbs that were similar to those in the saline-treated rats. However, each subsequent air-jet stress produced progressively smaller vasodilator responses in the sympathetically intact but not the sympathetically denervated hindlimb. There was no loss of air-jet stress-induced alerting behavior or mesenteric vasoconstriction, suggesting that L-NAME did not interfere with the central processing of the air-jet or the resultant changes in autonomic nerve activity. The progressive diminution of air-jet stress-induced vasodilation in the intact hindlimb of L-NAME-treated rats may be due to the use-dependent depletion of preformed stores of nitric oxide-containing factors that cannot be replenished in the absence of nitric oxide synthesis.