(Hypertension. 1996;28:347-353.)
© 1996 American Heart Association, Inc.
Articles |
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
The Cardiovascular Center and Department of Pharmacology and the Department of Psychology (A.K.J.), The University of Iowa, Iowa City.
Correspondence to Stephen J. Lewis, PhD, Department of Pharmacology, 2-272 Bowen Science Bldg, The University of Iowa, Iowa City, IA 52242.
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Abstract |
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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
-nitro-L-arginine methyl ester (L-NAME, 25 µmol/kg IV). In saline-treated rats, air-jet stress produced alerting behavior, minor changes in blood pressure, pronounced mesenteric vasoconstriction, 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.
Key Words: vasodilation • nitric oxide • stress • rats
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Introduction |
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The interaction of nitrosonium ions (NO+) with free thiols or iron thiols leads to the formation of S-nitrosothiols1 and dinitrosyl iron (II)–thiol complexes.2 There is now substantial evidence that these or related NOFs can be formed and released from vascular endothelial cells,2 3 bronchial tissue,4 and activated macrophages.5 It is generally accepted that these NOFs are released immediately on their formation.6 However, Ignarro6 has postulated that preformed pools of S-nitrosothiols may exist within acidic storage sites in vascular endothelial cells and that endothelium-dependent agonists may induce the Ca2+-dependent exocytotic release of these stores. Although there is no direct evidence to support this postulate, the existence of preformed pools of NOFs may explain why NO synthase inhibitors do not block the initial endothelium-dependent fall in vascular resistance in vivo but markedly diminish the duration of this vasodilation.7 8 9 More specifically, endothelium-dependent vasodilation may be initiated by the release of preformed pools of NOFs, whereas the vasodilation may be sustained by the subsequent release of de novo synthesized NO or NOFs.
Exposure of the rat to a noxious environmental stimulus such as AJS elicits a defense reaction that includes behavioral phenomena and hemodynamic changes, such as a modest increase in arterial blood pressure, tachycardia, vasoconstriction in the renal and mesenteric vascular beds, and marked vasodilation in the hindquarter bed.10 11 12 The mechanisms responsible for the AJS-induced hindlimb vasodilation, which plays a vital role in preparing the animal for flight, have not been fully established. The fall in hindquarter resistance may result from a withdrawal of sympathetic drive, the action of circulating adrenal catecholamines, or the activation of a sympathetic neurogenic vasodilator system.11 12 13 We have recently demonstrated that repeated episodes of medium-intensity electrical stimulation of the lumbar sympathetic chain produced pronounced and equivalent reductions in HLR in pentobarbital-anesthetized rats.14 After administration of the NO synthesis inhibitor L-NAME, the first episode of electrical stimulation produced a pronounced vasodilation. However, subsequent episodes produced progressively and markedly smaller vasodilator responses. These findings raise the possibility that sympathetic neurogenic vasodilation may be mediated by the release of preformed pools of NOFs. The progressive diminution of the vasodilation would be consistent with the gradual "use-dependent" depletion of these pools of NOFs that could not be replenished in the absence of NO synthesis. We also demonstrated that postganglionic lumbar sympathetic nerves innervating the hindlimb vasculature of the rat contain NADPH diaphorase,15 which is a marker for NO synthase in paraformaldehyde-treated tissues.16 As such, this active neurogenic vasodilation may involve the release of these preformed pools of NOFs from sympathetic NO synthase–containing vasodilator nerves15 or may result from sympathetic nerve–derived norepinephrine17 18 19 or ATP20 releasing these factors from the vascular endothelium.
We have demonstrated that AJS produces an active sympathetic neurogenic hindlimb vasodilation in the conscious rat that is reduced but not abolished by L-NAME.15 We found that L-NAME did not significantly affect the initial (1 to 5 seconds) AJS-induced hindlimb vasodilator response, whereas it virtually abolished the sustained phase of the response (5 to 30 seconds). In this previous study, we tested the hemodynamic effects of only one episode of AJS before and after L-NAME administration. It occurred to us that if this active neurogenic vasodilation is initiated by the release of preformed stores of NOFs, then we should be able to demonstrate a progressive loss of vasodilation in the absence of NO synthesis due to the use-dependent depletion of these preformed stores.
Therefore, we examined the effects of repeated episodes of AJS (six episodes given 5 minutes apart) on MAP, mesenteric resistance, and resistances in intact and sympathetically denervated hindlimbs of conscious rats after the administration of saline or L-NAME (25 µmol/kg IV). The first AJS was given either 30 or 60 minutes after L-NAME administration to ensure that any loss of vasodilation in response to the subsequent episodes of AJS was not due to the progressively greater L-NAME–induced inhibition of NO synthesis. We examined the effects of AJS in the sympathetically denervated hindlimb vasculature to confirm that the vasodilation was a sympathetic neurogenically mediated response and also to remove possible confounding neurohumoral influences on sympathetic terminals. We now report that successive episodes of AJS produce progressively smaller reductions in resistance in the intact hindlimb vascular beds of L-NAME– but not saline-treated rats. Evidence will be presented that this loss of hindlimb vasodilation may be due to the progressive depletion of preformed pools of NOFs within the hindlimb vasculature.
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Animals
The protocols were approved by the University of Iowa Institutional Animal Care and Use Committee. All experiments were performed on conscious, freely moving male Sprague-Dawley rats (Harlan, Madison, Wis) weighing 300 to 400 g (n=18). The rats were individually housed in Plexiglas cages and maintained in a room with a 12-hour light/dark cycle. Food (Purina Mills) and tap water were freely available at all times except during testing procedures.
Surgical Procedure
One week before the experiments, the rats were anesthetized with ketamine (120 mg/kg IP) and acepromazine maleate (12 mg/kg IP). Catheters (PE-50) were implanted into the left common carotid artery for measurement of pulsatile arterial pressure, MAP, and heart rate and into the right jugular vein for administration of drugs. Immediately after catheterization, a midline laparotomy was performed, and miniature pulsed Doppler flow probes were placed on the superior mesenteric artery for measurement of mesenteric blood flow and vascular resistance and on the left and right iliac arteries for measurement of HLF and HLR. The probes were sutured in place and the leads and catheters were tunneled subcutaneously and exteriorized between the scapulae. At this point, blood flows in both limbs were measured. One group of rats (n=12) then underwent selective sympathetic denervation of the left hindlimb. The left lumbar sympathetic chain was isolated, cut, and removed caudally to the bifurcation of the left common iliac artery and vein. The right sympathetic chain was left intact. The blood flows and vascular resistances in the innervated bed of these rats are referred to as HLFi and HLRi, respectively. These parameters in the denervated bed are referred to as HLFd and HLRd, respectively. Another group of rats (n=6) underwent sham surgery in which the left sympathetic chain was exposed and isolated but not cut. Again, the right sympathetic chain was left intact. The wounds were then closed, and the free ends of the catheters and Doppler leads were led through a stainless steel skin button–spring swivel assembly that was mounted to a ring stand clamp and suspended above the cage. This apparatus protected probe wires and polyethylene tubing while allowing rats unrestricted movement during recovery and experimental testing. The skin button was attached to the skin incision in the scapular region with stainless steel sutures. Details of the Doppler technique, including construction of the probes, the reliability of the method for estimation of flow velocity (measured as Doppler shift in kilohertz), and determination of vascular resistances (MAP/blood flow in millimeters of mercury per kilohertz), have been described in detail.14 15 21
Experimental Procedure
After a 7-day recovery period, rats remained in their home cages and were connected to a Beckman Dynograph–coupled pressure transducer (Cobe Laboratories) and Doppler flowmeter (Department of Bioengineering, The University of Iowa, Iowa City) for recording of hemodynamic parameters. A 90-minute stabilization period after connection to the recording equipment was allowed for all rats. One subgroup of the denervated rats (n=6) was used for comparison of changes in HLRi and HLRd produced by the selective 
1-adrenoceptor antagonist prazosin (100 µg/kg IV). This was done to check for the completeness of the surgical sympathectomy. The second subgroup of denervated rats (n=6) was used for the AJS studies. AJS consisted of a 1-second standardized burst of compressed air (Tech Duster, Techni-Tool) directed to the top of the rat's head. Each rat received a total of six air-jets, each given 5 minutes apart, on each of 2 consecutive days. Three of these rats received an injection of saline (0.9% NaCl IV) on day 1 and then after 30 minutes were exposed to the six episodes of AJS. On the next day, these rats again underwent a 90-minute stabilization period after connection to the recording equipment and then subsequently received L-NAME (25 µmol/kg IV). After 30 minutes, the rats were exposed to the repeated AJS protocol. As such, the AJS episodes were given between 30 and 60 minutes after L-NAME. The other three rats were treated identically except that they received L-NAME on the first day and saline on the second day of testing. Since the behavioral and hemodynamic responses in both rat groups were identical, the results were pooled for statistical analysis. We designed this study to ensure that habituation to AJS did not occur from one day to the next. Because the maximal loss of the hindlimb vasodilator responses in the L-NAME–treated rats occurred with the third AJS, the responses during these first three episodes of AJS are summarized in the figures.
We also examined the hypotensive and vasodilator effects of the NO donor SNP (2 µg/kg IV) and L-SNC (100 nmol/kg IV) in the saline- and L-NAME–treated rats. These vasodilators were administered after the AJS protocol was completed.
Drugs
All drugs used in this study were obtained from Sigma Chemical Co, except for SNP, which was from Abbott Laboratories, and L-SNC, which was synthesized as described previously.3
Statistics
Data are expressed as mean±SE. SE was determined by the formula (EMS/n)½, where EMS is the error mean square term from the ANOVA, and n is the number of rats per group.22 Data were analyzed by repeated measures ANOVA22 followed by Student's modified t test with Bonferroni correction for multiple comparisons between means using the modified EMS term from the ANOVA.23
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Baseline Hemodynamic Parameters
Baseline blood flow values were equivalent in the left and right hindlimbs of the anesthetized rats on the day of surgery (3.2±0.4 versus 3.4±0.2 kHz, P>.05, n=12). Seven days later, the conscious sham-operated controls had equivalent resting vascular resistances in the left and right hindlimbs (36±4 versus 39±6 mm Hg/kHz, P>.05, n=6). The Table
summarizes the effects of surgical sympathectomy on HLR and the effects of the selective 1-adrenoceptor antagonist prazosin or the NO synthesis inhibitor L-NAME on baseline hemodynamic parameters of conscious rats. Seven days after surgery, the resting HLRd was substantially higher than the HLRi (pre-HLRi versus pre-HLRd values). Prazosin produced a marked and sustained reduction in HLRi but had no effect on HLRd, suggesting that our surgical procedure produced a complete interruption of the sympathetic supply to the hindlimb. L-NAME injection produced a pressor response, a marked increase in HLRi, and a smaller but significant increase in HLRd.
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Effects of Repeated AJS on Hemodynamic and Behavioral Parameters
Fig 1 shows typical examples of the effects of repeated administration (three episodes given 5 minutes apart) of AJS on hemodynamic parameters in a conscious rat pretreated with either saline or the NO synthesis inhibitor L-NAME. Saline injection did not change any of the resting parameters. L-NAME injection produced an increase in MAP of 15 mm Hg and substantial decreases in HLFi at the time the AJS was applied. Because the initial resting MAP of the rat that received saline was approximately 10 mm Hg higher than that of the rat which received L-NAME, the postsaline and post–L-NAME MAP values were not substantially different between these two rats. However, the resting resistance in the innervated hindlimb (ie, MAP/HLF) was markedly greater in the L-NAME–treated rat. The first AJS produced minor changes in blood pressure, an immediate and marked fall in mesenteric blood flow, and an immediate and pronounced increase in HLFi. The increase in HLFd was considerably smaller. In addition, the first AJS produced typical "defense response" behaviors, including alerting, piloerection, hunching, and claw extension. Most importantly, the immediate increases in HLF were not due to movement of the hindlimbs. The rats did not move their hindlimbs during the first moments of the AJS response, and only minor movements of the hindlimbs were observed thereafter. The observation that the rapid increase in HLF in the sympathetically denervated hindlimb was markedly smaller than in the intact hindlimb demonstrates that the rapid increase in blood flow in the intact limb is not an artifact of movement. The rats maintained full motor control of the sympathetically denervated hindlimb. Each successive AJS produced virtually identical changes in these hemodynamic and behavioral parameters. After L-NAME administration, the first AJS produced behavioral and hemodynamic responses that were equivalent to those after saline injection. However, each subsequent AJS produced progressively smaller increases in HLFi but not HLFd. In contrast, each successive AJS caused decreases in mesenteric blood flow and behavioral responses that were equivalent to those produced by the first AJS.
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Figs 2 through 4 summarize the effects of the first three episodes of AJS given 5 minutes apart on hemodynamic parameters in conscious rats. Repeated AJS produced transient (5 to 10 seconds) but consistent pressor responses and increases in mesenteric resistance in saline-treated controls (Fig 2). After L-NAME administration, the AJS-induced pressor responses were reduced. This is most likely because of the L-NAME–induced elevation in baseline MAP (Table
). The AJS-induced increase in mesenteric vascular resistance was similar to that observed in saline-treated rats. In addition, each AJS produced quantitatively similar mesenteric vasoconstrictor responses.
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P<.05, AJS-induced changes in intact and denervated hindlimb; *P<.05, subsequent episodes of AJS vs first episode of AJS.
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Fig 3 summarizes the effects of AJS on HLF and HLR in saline- and L-NAME–treated rats. In saline-treated controls, AJS produced immediate (within 5 seconds) and sustained (up to 60 seconds) increases in HLFi and decreases in HLRi. Significantly smaller AJS-induced increases in HLFd and decreases in HLRd were observed. In these saline-treated rats, each successive AJS caused highly reproducible changes in flow and resistance in both hindlimbs. After L-NAME administration, the first AJS produced increases in flow and falls in resistance in the innervated and denervated hindlimbs that were similar to those produced by the first AJS in saline-treated rats. However, each successive AJS produced progressively smaller increases in flow and decreases in resistance in the innervated bed. In contrast, the smaller AJS-induced decreases in HLRd were not altered on repeated exposure to the AJS stimulus. The first episode of AJS produced pronounced and quantitatively similar vasodilator responses in the sympathetically intact hindlimb beds when applied either 30 or 60 minutes after L-NAME administration. Moreover, each subsequent AJS produced progressively smaller responses in each case. In addition, the first episode of AJS produced less pronounced but similar vasodilator responses in the sympathetically denervated hindlimb beds when applied either 30 or 60 minutes after L-NAME administration. These findings are reflected by the relatively small standard errors of the data, which represent the mean±SE of the 30- to 60-minute and 60- to 90-minute data (see Figs 2 and 3).
Effects of SNP and L-SNC on Hemodynamic Parameters
Fig 4 summarizes the effects of SNP (2 µg/kg IV) and L-SNC (100 nmol/kg IV) on MAP, HLRi, and HLRd after injection of saline and L-NAME. This dose of SNP produces maximal changes in hemodynamic parameters in these conscious rats. Larger doses of SNP (4 to 64 µg/kg IV) do not produce significantly greater responses. The dose of L-SNC was chosen because it produces a fall in MAP relatively similar to that produced by the 2 µg/kg dose of SNP. Higher doses of L-SNC (200 to 1600 nmol/kg IV) produce greater hypotensive and vasodilator effects. The injection of SNP in saline-treated rats produced a fall in MAP, a relatively minor fall in HLRi, and a more substantial fall in HLRd. L-SNC produced a depressor response and pronounced and similar reductions in HLRi and HLRd. The hypotensive and vasodilator effects of SNP were augmented in the L-NAME–treated rats. This L-NAME–induced potentiation of the effects of SNP was most evident in the innervated bed, in which the fall in resistance went from -8±3% to -43±5%. L-NAME also augmented the hypotensive and vasodilator effects of L-SNC in the innervated bed. In contrast, this NO synthesis inhibitor did not affect the vasodilator effects of this dose of L-SNC in the denervated bed.
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Discussion |
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This study demonstrates that AJS produced modest increases in arterial blood pressure, mesenteric vasoconstriction, and a marked vasodilation in sympathetically intact hindlimb beds but a much smaller vasodilation in sympathetically denervated hindlimbs of conscious rats. AJS also produced behavioral responses, including alerting, hunching, piloerection, and extension of the claws. This pattern of responses is referred to as the defense reaction.10 11 24 Since the AJS-induced vasodilator responses were markedly smaller in the sympathetically denervated bed, it is evident that the vasodilation depends on the lumbar sympathetic nerves.
Successive episodes of AJS caused similar hemodynamic and behavioral responses in the saline-treated rats, demonstrating that habituation to the AJS stimulus did not occur. It was necessary to establish that the AJS-induced responses were reproducible because habituation to other stressful stimuli such as foot shock can occur.25 26 The first episode of AJS produced similar behavioral and hemodynamic responses in the saline- and L-NAME–treated rats. This suggests that L-NAME did not interfere with the central processing of the AJS or the activation of the autonomic nervous system. Moreover, the finding that the first AJS produced a robust vasodilation in the innervated hindlimb suggests that this response was not mediated by the de novo synthesis of NO or newly formed NOFs.
The principal finding of this study is that subsequent episodes of AJS produced progressively smaller vasodilator responses in the innervated hindlimb of the L-NAME–treated rats. In contrast, each successive AJS produced equivalent behavioral and mesenteric vasoconstrictor responses. The first AJS produced similar and pronounced vasodilator responses in the sympathetically intact hindlimb beds when applied either 30 or 60 minutes after L-NAME. Moreover, each subsequent AJS produced progressively smaller responses in each case, suggesting that the progressive loss of the AJS-induced vasodilation between 30 and 60 minutes was not simply due to the progressively greater L-NAME–induced inhibition of NO synthesis. These findings raise the possibility that the hindlimb vasodilation in L-NAME–treated rats is due to the release of preformed NOFs within the vasculature. The use-dependent loss of hindlimb vasodilation in L-NAME–treated rats may be due to the depletion of these preformed stores of NOFs. A loss of vasodilator effectiveness of endogenous NO/NOFs is unlikely to be responsible for the progressive loss of AJS-induced hindlimb vasodilation because L-NAME potentiated the hindlimb vasodilator responses to SNP and L-SNC. The first AJS-induced vasodilation in the sympathetically denervated hindlimb bed was similar to that observed before L-NAME administration. Moreover, there was no progressive diminution of these responses with each subsequent AJS. These responses may be due to the actions of adrenomedullary catecholamines such as epinephrine.12 The systemic administration of epinephrine produces a hindlimb vasodilation that is not attenuated by L-NAME.15
The possibility that the use-dependent loss of this neurogenically mediated hindlimb vasodilation is due to the diminution of preformed stores of NOFs in vascular tissue is supported by our recent findings that the hindlimb vasodilation produced by the electrical stimulation of the lumbar sympathetic chain in anesthetized rats also diminishes in a use-dependent manner in the presence of L-NAME.14 These findings suggest that active sympathetic neurogenic hindlimb vasodilation is mediated by the release of preformed NOFs such as S-nitrosothiols1 3 4 27 28 or dinitrosyl iron (II) complexes.2 There is evidence that the vascular smooth muscle of arteries contains preformed pools of NOFs.29 30 31 32 In addition, the rabbit thoracic aorta is innervated by postganglionic sympathetic NO synthase–positive nerve terminals that may contain cytosolic-protected pools of NOFs.33 Ignarro6 has postulated that S-nitrosothiols may exist within plasmalemmal vesicles of vascular endothelial cells.34 The existence of preformed NOFs within plasmalemmal vesicles of endothelial cells or nerves may explain why L-NAME is only partially effective in inhibiting one episode of AJS-induced hindlimb vasodilation15 but there is a progressive loss of this response on repeated application of AJS in the presence of the NO synthesis inhibitor. This progressive loss of hindlimb vasodilation would be due to the use-dependent loss of these preformed pools of NOFs, which cannot be replenished in the absence of NO synthesis.
AJS-induced vasodilation may be mediated by a number of mechanisms, including the sympathetic nerve (norepinephrine or ATP)–mediated release of NOFs from the vascular endothelium18 19 20 or the direct release of NOFs from postganglionic sympathetic nerves themselves.15 The hindlimb vasodilator responses to endothelium-dependent agonists such as acetylcholine also progressively diminish on repeated injection in conscious rats treated with NO synthase inhibitors.35 Injection of acetylcholine (1.0 µg/kg IV) in L-NAME–treated rats (n=6) that have undergone a repeated AJS protocol produced a fall in HLRi of -49±6%. This acetylcholine-induced hindlimb vasodilation was similar to that observed in L-NAME–treated rats that were not exposed to the repeated AJS protocol (-42±5%, n=6, P<.05). Thus, at a time when the mediator of AJS-induced hindlimb vasodilation appears to have been depleted, endothelium-dependent hindlimb vasodilation remains essentially intact. Therefore, the loss of AJS-induced hindlimb vasodilation may be due to the use-dependent depletion of NOFs from the postganglionic sympathetic nerve terminals rather than from the endothelium.
Another possible explanation for our data is that the entry of L-NAME into the sympathetic terminals is use dependent; that is, more L-NAME enters these terminals each time the nerves are activated by AJS. The use-dependent entry of L-NAME would lead to a progressively greater inhibition of NO synthesis and therefore diminished AJS-induced vasodilation. However, L-NAME is a methyl ester and would be expected to be highly lipophilic. Indeed, L-NAME is highly soluble in organic solvents such as methanol (30 mg/mL; Sigma Chemical Co, personal communication, 1996). Therefore, the entry of L-NAME into the sympathetic terminals would probably occur by simple diffusion.
Our observation that chronic surgical sympathectomy resulted in a significant increase in baseline vascular resistance in the denervated hindlimb bed supports findings that the destruction of the sympathetic nerves by neonatal treatment with the sympathetic neurotoxin guanethidine results in an increase in hindlimb vascular resistance in adult anesthetized rats.36 In addition, chronic sympathectomy of the rabbit ear artery greatly reduces the vasorelaxant effects of the endothelium-dependent vasodilator methacholine.37 Our observation that L-NAME produced a minor increase in resistance in the denervated hindlimb of conscious rats supports findings that NO synthase inhibitors produce less vasoconstriction in sympathectomized feline hindlimbs.8 The sympathetic nerves play a vital role in the synthesis and release of endothelium-derived relaxing factor in anesthetized rats.17 Moreover, activation of the lumbar sympathetic trunk causes an NO synthase–dependent hindlimb vasodilation in conscious rats.15 This suggests that the increase in resistance produced by lumbar sympathectomy is due to (1) a loss of endothelium-dependent NO synthase activity and/or the release of NOFs, and (2) a loss of neurogenic vasodilator input. A loss of tonic NO/NOF-mediated vasodilator tone in the sympathetically denervated hindlimb may also explain the augmented vasodilator effects of SNP in this bed. The vasodilator effectiveness of NOFs markedly increases after NO synthesis inhibition because of an upregulation of soluble guanylate cyclase in vascular smooth muscle.38 It is unlikely that the L-NAME–induced potentiation of the vasodilator effects of SNP and L-SNC in the innervated hindlimb is due to the loss of baroreceptor reflex function.39 40 Indeed, NO synthesis inhibitors actually augment reflex vasoconstriction in various vascular beds, including the hindlimb.41
In summary, these findings suggest that physiological activation of the sympathetic nerves innervating the hindlimb vasculature produces a profound vasodilation that may be mediated by the release of preformed pools of NOFs. The release of these preformed pools of NOFs from the vascular endothelium and postganglionic NO synthase–containing sympathetic nerves would provide an important mechanism by which these cells could regulate vascular tone, especially under conditions in which NO synthesis is temporarily compromised.
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Received February 27, 1996; first decision March 26, 1996; accepted April 8, 1996.
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