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(Hypertension. 1997;30:962-967.)
© 1997 American Heart Association, Inc.

Articles

Role of Endogenous Carbon Monoxide in Central Regulation of Arterial Pressure

Robert A. Johnson; Eduardo Colombari; Debora S. A. Colombari; Manuel Lavesa; William T. Talman; ; Alberto Nasjletti

From the Department of Pharmacology (R.A.J., M.L., A.N.), New York Medical College, Valhalla; the Department of Physiology (E.C., D.S.A.C.), UNIFESP-EPM, Sao Paulo, Brazil; and the Department of Neurology, University of Iowa and VA Medical Center (W.T.T.), Iowa City.

Correspondence to Robert A. Johnson, PhD, Department of Pharmacology, New York Medical College, Valhalla, NY 10595. E-mail RobertJ393{at}aol.com

	Abstract

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Abstract We investigated the contribution of neural mechanisms to the arterial pressure increase produced by zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG), an inhibitor of endogenous carbon monoxide synthesis. The arterial baroreceptor reflex control of heart rate was examined in rats with and without ZnDPBG pretreatment (45 µmol/kg IP) by analysis of the arterial pressure–heart rate relationship during infusions of phenylephrine or sodium nitroprusside to vary arterial pressure. ZnDPBG increased arterial pressure from 110±3 to 126±2 mm Hg without eliciting bradycardia. The maximum gain of the heart rate response to changes in arterial pressure was attenuated by ZnDPBG treatment (-1.9±0.3 versus -4.8±1.0 bpm/mm Hg). The possibility that ZnDPBG elevates arterial pressure by attenuating baroreceptor reflex function was addressed by comparing the pressor response to ZnDPBG (45 µmol/kg IP) in rats with and without sinoaortic denervation. The pressor effect of ZnDPBG was similar in rats with and without arterial baroreceptor deafferentation, implying that the increase in pressure is not simply the consequence of attenuated baroreceptor reflex function per se. The possibility that ZnDPBG increases arterial pressure via an effect on the nucleus tractus solitarii (NTS) also was investigated. ZnDPBG (1 nmol in 10 0 nL) injected into the NTS of rats increased arterial pressure from 111±4 to 126±5 mm Hg, and this effect was reversed by an ipsilateral microinjection of carbon monoxide into the NTS. Accordingly, the pressor effect of ZnDPBG may rely on inhibition of carbon monoxide production in the NTS. This implies that carbon monoxide formed by brain heme oxygenase plays a role in the central regulation of arterial pressure.

Key Words: carbon monoxide • heme oxygenase • blood pressure • reflex, baroreceptor • solitary tract nucleus • hypertension

	Introduction

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The metabolism of heme by heme oxygenase(s) yields biliverdin and carbon monoxide.^{1 2 3} Two distinct heme oxygenase isoforms have been identified.⁴ Both isoenzymes, one inducible (heme oxygenase-1) and the other constitutive (heme oxygenase-2), are expressed in various tissues, including neural and cardiovascular tissues.^{2 3} Recent studies have suggested that carbon monoxide, arising from heme via metabolism by heme oxygenase, stimulates soluble guanylate cyclase activity and promotes elevation of cGMP in neural and cardiovascular tissues.^{4 5 6 7 8 9} This has prompted consideration of the heme oxygenase–carbon monoxide system as a potential regulator of various neural^{6 7} and cardiovascular^{5 8 9} functions.

Recently we reported that systemic administration of the heme oxygenase inhibitor zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG)^{10 11} elicits peripheral vasoconstriction and produces sustained elevation of blood pressure in rats.¹² These effects of ZnDPBG were attributed to blunting of a vasodepressor mechanism mediated by endogenous carbon monoxide.¹² The notion that the heme oxygenase–carbon monoxide system subserves a vasodepressor function received additional support from reports that carbon monoxide of vascular origin inhibits endothelial cell expression of endothelin-1¹³ and that treatment with heme lowers blood pressure in hypertensive rats via a heme oxygenase–dependent mechanism.^{14 15}

Two observations suggest the contribution of neural mechanisms to the elevation of blood pressure caused by systemic administration of ZnDPBG in rats. First, pretreatment with agents that block the autonomic ganglia or ₁-adrenoceptors prevents the pressor response to the inhibitor of heme oxygenase.¹² Second, the pressor response to ZnDPBG is not accompanied by reflex bradycardia.¹² Accordingly, the increase in blood pressure elicited by the inhibitor of heme oxygenase appears to rely on the activity of the sympathetic nervous system and may be linked to an abnormality of baroreceptor reflex function.

Since ZnDPBG is capable of inhibiting brain heme oxygenase when administered systemically,¹² the possibility arises that the pressor effect of ZnDPBG is prompted by inhibition of heme oxygenase at central nervous system sites concerned with blood pressure regulation. Accordingly, the present study was undertaken with two objectives in mind: first, to determine whether the increase in blood pressure produced by systemic administration of the heme oxygenase inhibitor ZnDPBG is linked to abnormal baroreceptor function; and second, to examine the blood pressure response to microinjections of ZnDPBG or carbon monoxide in the nucleus tractus solitarii (NTS), the site of termination of afferent nerve fibers from peripheral cardiovascular baroreceptors and chemoreceptors.¹⁶

	Methods

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Materials
ZnDPBG was obtained from Porphyrin Products, carbon monoxide from Airgas, and all other drugs from Sigma Chemical Co. ZnDPBG was dissolved in 50 mmol/L Na₂CO₃ (15 µmol drug/mL) immediately before use. All other drugs were dissolved with normal saline on the day of the experiment. To prepare carbon monoxide–saturated saline, 5 mL of saline was placed in a 10-mL vial fitted with a small aperture and bubbled at 4°C with carbon monoxide gas for 20 minutes. The vial was tightly sealed and then warmed to room temperature before use.

Animals
Male Sprague-Dawley rats (275 to 350 g; Charles River, Wilmington, Mass) were used in these studies approved by the Institutional Animal Care and Use Committee. Rats were individually housed in a controlled environment at 27°C and with automatic lighting programmed with a 12-hour on/off cycle. Rats had free access to commercial rat chow (Ralston-Purina) and tap water. Four experimental protocols were implemented.

Experimental Design
Protocol 1
Experiments were designed to investigate the effect of systemic administration of ZnDPBG on arterial baroreflex–mediated control of heart rate. Rats were anesthetized with sodium pentobarbital (60 mg/kg IP) and instrumented with indwelling vascular polyethylene cannulas (PE-50) filled with heparinized saline. One cannula was introduced through a femoral artery and advanced into the lower abdominal aorta, and another cannula was introduced into a femoral vein and advanced into the inferior vena cava. Both cannulas were sealed with steel pins and tunneled subcutaneously to an exit point at the nape of the neck. All animals received ampicillin (30 mg/kg SC) after surgery. The arterial cannula was connected to a pressure transducer (model P23XL, Statham Division, Gould Inc) for recording of arterial pressure on a polygraph (model 7D, Grass Instruments Co). Mean arterial pressure was determined by feeding the signal into the model-7D driver amplifier with the half-amplitude frequency adjusted to 0.5 Hz. Heart rate was monitored by means of a cardiotachometer (model 7P44C, Grass Instruments Co) triggered by the systolic pressure rise. The venous cannula was used for administration of drugs. Experiments were conducted after a 4-day recovery period.

On the day of the experiments, awake rats were given an intraperitoneal injection of ZnDPBG (45 µmol/kg) or drug vehicle only (50 mmol/L Na₂CO₃). Starting 5 minutes later, the control of heart rate by the arterial baroreflex was examined in both groups of animals as follows. Mean arterial pressure and heart rate were measured before and during intravenous infusions (SAGE model 341B) of phenylephrine at 0.5, 0.8, 1.1, 1.7, and 2.3 µg · kg^-1 · min^-1 to increase arterial pressure and infusions of sodium nitroprusside at 0.7, 1.1, 3.5, and 7.7 µg·kg^-1·min^-1 to lower arterial pressure. For both agents, infusion periods at each dose lasted 1 minute and were separated by 5-minute intervals to allow restoration of arterial pressure to within 5% of the preinfusion level. In this manner, the mean arterial pressure was varied over the range of 73±4 to 153±4 mm Hg in rats pretreated with vehicle and 73±5 to 146±4 mm Hg in rats pretreated with ZnDPBG.

To analyze the relationship between mean arterial pressure and heart rate, corresponding values of arterial pressure and heart rate were fitted for each rat to a logistic function curve based on the following mathematical model¹⁷ : HR=P₁/{1+exp[P₂(MAP–P₃)]}+P₄. In this equation, HR is the heart rate, MAP is the mean arterial pressure, P₁ is the range of heart rate responses, P₂ is the slope coefficient, P₃ is the mean arterial pressure at the midpoint of the range of heart rate, and P₄ is the minimum heart rate value. Data points were fitted to the logistic function by using a nonlinear regression program (SYSTAT version 4.1, NONLIN module). The first derivative of the fitted curve is taken to reflect the gain of the heart rate response to changes in arterial pressure,¹⁸ and maximum gain (G_max) was estimated as –P₁xP₂/4. These constants were calculated independently for each animal, and the results were expressed as the mean±SEM.

Protocol 2
Experiments were designed to investigate whether the pressor response to systemic administration of the heme oxygenase inhibitor ZnDPBG is affected after disruption of the baroreceptor reflex by sinoaortic denervation. Rats anesthetized with ketamine (80 mg/kg IP) and xylazine (12 mg/kg IP) were instrumented with femoral arterial cannulas to measure arterial pressure as described in protocol 1. After premedication with atropine (1.5 mg/kg SC), bilateral sinoaortic denervation was performed using the method of Krieger¹⁹ as modified by Schreihofer and Sved.²⁰ The procedure included sectioning of the carotid sinus nerve near the glossopharyngeal nerve, removal of the superior cervical ganglion, sectioning of the aortic depressor nerve at its junction with the superior laryngeal nerve, and stripping of all neural and connective tissues from the common carotid artery, the carotid bifurcation, and the internal and external carotid arteries, followed by swabbing of the areas with 10% phenol in ethanol. Experiments were conducted 1 week after surgery. On the day of the experiment, the completeness of baroreceptor reflex disruption in rats with sinoaortic deafferentation was ascertained by establishing that heart rate did not change when mean arterial pressure was acutely increased (71±4 mm Hg, 0±0 bpm) or decreased (-86±7 mm Hg, 0±0 bpm) by bolus intravenous injections of phenylephrine (2 µg/kg) and sodium nitroprusside (5 µg/kg), respectively. After a 30-minute stabilization period, ZnDPBG (45 µmol/kg IP) or 50 mmol/L Na₂CO₃ vehicle (3 mL/kg IP) was administered to awake animals while monitoring arterial pressure and heart rate. The effects on arterial pressure and heart rate caused by treatment with ZnDPBG or with vehicle alone also were examined in control rats that did not undergo sinoaortic deafferentation.

Protocol 3
Experiments were designed to investigate the effects of the heme oxygenase inhibitor ZnDPBG and carbon monoxide injected into the NTS on the arterial pressure and on heart rate of awake rats. Animals under halothane anesthesia (2% to 4%) were placed in a stereotaxic apparatus and instrumented, unilaterally or bilaterally, with guide cannulas implanted in the direction of the intermediate NTS using the stereotaxic coordinates of Paxinos and Watson²¹ and a published technique.²² Briefly, a small window was opened caudal to lambda, through which a 15-mm-long stainless steel guide cannula (22 gauge) was introduced in a perpendicular way 14.0 mm caudal to bregma, 0.5 mm lateral to midline, and 7.9 mm below the skull surface of the bregma. The tip of the guide cannula was placed in the cerebellum 1.0 mm above the dorsal surface of the brain stem. The cannula guide was secured to the skull using methacrylate and watch screws and closed with an occluder until the time of the experiments. The 33-gauge needle used for microinjections was 1.5 mm longer than the guide cannula and was connected by PE-10 tubing to a 1-µL Hamilton syringe.

Three days after implantation of cannulas for microinjection into the NTS, rats were again anesthetized with halothane and instrumented with an arterial cannula for measurement of arterial pressure as described in protocol 1. One day later, the arterial pressure and heart rate were monitored in awake rats before and after unilateral or bilateral microinjections of ZnDPBG (1 nmol in 100 nL) or of vehicle alone (50 mmol/L Na₂CO₃, 100 nL) in the NTS. In additional experiments, the arterial pressure and heart rate of awake rats were monitored after unilateral microinjections of saline saturated with carbon monoxide (100 nL) or of saline vehicle alone (100 nL) into the NTS of untreated rats or of rats given (15 minutes earlier) an injection of ZnDPBG (1 nmol in 100 nL) into the ipsilateral NTS.

After completion of the experiments, the animals were anesthetized and perfused transcardially with 10% buffered formalin. Subsequently, the brain was removed and placed in the fixative solution for 2 days. Serial coronal sections of the brain (40 µm) were stained with Giemsa for histological verification of sites of injection.

Protocol 4
Animals were fitted bilaterally with guide cannulas implanted in the direction of the NTS and with chronic arterial catheters as described in protocol 3. On the day of the experiment, animals were injected with ZnDPBG (45 µmol/kg IP) followed 20 minutes later with bilateral microinjections of carbon monoxide to the NTS (100 nL). Heart rate and blood pressure were measured throughout the experiment. After completion of the experiment the site of microinjection was confirmed histologically as described in protocol 3.

Statistics
Results are expressed as mean±SEM. Data were analyzed by ANOVA, with a value of P<.05 being significant; this was followed by orthogonal contrasts.²³

	Results

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Fig 1 (top) depicts the relationship between data on heart rate and mean arterial pressure in awake rats pretreated with ZnDPBG (45 µmol/kg IP; n=10) or vehicle only (50 mmol/L Na₂CO₃, 3 mL/kg IP; n=10) and subsequently infused intravenously with phenylephrine (0.5 to 2.3 µg kg^-1 min^-1) or sodium nitroprusside (0.7 to 7.7 µg kg^-1 min^-1). Vehicle pretreatment did not affect mean arterial pressure (109±2 versus 109±2 mm Hg) or heart rate (382±12 versus 384±14 bpm). ZnDPBG pretreatment increased (P<.05) mean arterial pressure from 110±3 to 126±2 mm Hg but did not affect heart rate (346±31 versus 397±26 bpm). In both groups of animals, phenylephrine-induced increases and sodium nitroprus- side–induced decreases in mean arterial pressure were associated with decreases and increases of heart rate, respectively. Analysis of the mean arterial pressure–heart rate relationship revealed that the range of heart rate responses (P₁) in rats pretreated with ZnDPBG (135±16 bpm) was reduced relative to the range of responses in vehicle controls (297±37 bpm, P<.05), whereas the minimum heart rate values were increased (P₄, 372±11 versus 324±13 bpm, P<.05). On the other hand, no significant differences were noted in the slope coefficient (P₂, 0.06±0.01 versus 0.06±0.01) or the mean arterial pressure at midrange of heart rate responses (P₃, 94±12 versus 93±8 mm Hg). As shown in Fig 1 (bottom), the gain of the heart rate responses to changes in arterial pressure was decreased in rats pretreated with ZnDPBG; the maximum gain (G_max) was -1.9±0.3 and -4.8±1.0 bpm/mm Hg (P<.05) in rats pretreated with ZnDPBG and vehicle, respectively.

View larger version (22K): [in this window] [in a new window]   Figure 1. Top, Graph illustrates the relationship between mean arterial pressure (MAP) and heart rate (HR) in awake rats pretreated with ZnDPBG (45 µmol/kg IP) or vehicle only and subsequently infused intravenously with phenylephrine or sodium nitroprusside to vary arterial pressure over a wide range. Lines illustrate the average ideal nonlinear trends (n=10 animals for each condition). Bottom, Graph depicts the gain of the heart rate responses to changes in arterial pressure obtained from the first derivative of the average ideal logistic functions.

Fig 2 displays data on mean arterial pressure and heart rate before and after intraperitoneal administration of ZnDPBG (45 µmol/kg) or vehicle (50 mmol/L Na₂CO_3, 3 mL/kg) in awake normal rats and rats with bilateral sinoaortic deafferentation. The mean arterial pressure of rats 1 week after bilateral sinoaortic deafferentation (136±4 mm Hg) exceeded that of unoperated rats (108±5 mm Hg) (P<.05). ZnDPBG elicited sustained elevations of blood pressure that were comparable in normal rats (n=7) and in rats with bilateral sinoaortic deafferentation (n=7). ZnDPBG did not affect the heart rate in either group of animals. Mean arterial pressure and heart rate remained unchanged after vehicle treatment in both normal rats (n=7) and rats with sinoaortic deafferentation (n=7).

View larger version (30K): [in this window] [in a new window]   Figure 2. Line graphs show mean arterial pressure and heart rate before and after administration of ZnDPBG (45 µmol/kg IP) or vehicle alone (3.0 mL/kg IP) to awake unoperated rats and rats 1 week after bilateral sinoaortic deafferentation. Symbols represent the mean±SEM. *P<.05 from pretreatment values.

Fig 3 shows data on mean arterial pressure and heart rate before and after microinjections of ZnDPBG (1 nmol in 100 nL) or vehicle alone (50 mmol/L Na₂CO₃, 100 nL) into the NTS. Mean arterial pressure increased within 15 minutes, from 111±5 to 126±4 mm Hg (P<.01) following unilateral administration (n=5) and from 111±3 to 142±6 mm Hg (P<.01) following bilateral administration (n=3) of ZnDPBG into the NTS. Heart rate was not affected by either unilateral or bilateral administration of ZnDPBG. Microinjection of vehicle into the NTS did not change mean arterial pressure or heart rate. ZnDPBG did not affect the blood pressure of rats (n=7) in which microinjections were unintentionally placed in the hypoglossal nucleus (119±3 and 120±4 mm Hg before and 15 minutes after unilateral ZnDPBG microinjection, respectively; P>.05).

View larger version (22K): [in this window] [in a new window]   Figure 3. Line graphs show mean arterial pressure and heart rate before and after unilateral or bilateral microinjection of ZnDPBG (1 nmol) or vehicle alone (100 nL) into the nucleus tractus solitarii of awake rats. Symbols represent the mean±SEM. *P<.05 from pretreatment value.

Fig 4 shows data on mean arterial pressure and heart rate before and after unilateral microinjections of saline saturated with carbon monoxide (100 nL) or of saline alone (100 nL) into the NTS of untreated rats and of rats given a unilateral injection of ZnDPBG (1 nmol) into the NTS 15 minutes earlier. Neither carbon monoxide (n=5) nor vehicle alone (n=4) affected the arterial pressure or heart rate of rats without ZnDPBG pretreatment. In contrast, carbon monoxide (n=5) injected into the NTS of rats pretreated with ZnDPBG decreased mean arterial pressure from 135±5 to 119±6 mm Hg (P<.01) within 15 minutes without affecting the heart rate. Neither arterial pressure nor heart rate was changed by the administration of saline alone (n=5) in rats pretreated with ZnDPBG.

View larger version (23K): [in this window] [in a new window]   Figure 4. Line graphs show mean arterial pressure and heart rate before and after unilateral microinjection of carbon monoxide–saturated saline (100 nL) or saline alone (100 nL) into the nucleus tractus solitarii of untreated rats and rats given a microinjection of ZnDPBG (1 nmol) into the ipsilateral nucleus tractus solitarii 15 minutes earlier. Symbols represent the mean±SEM. *P<.05 from pretreatment value.

Fig 5 shows data on mean arterial pressure and heart rate before and after bilateral microinjection of saline saturated with carbon monoxide (100 nL) into the NTS of rats (n=5) given an intraperitoneal injection of ZnDPBG (45 µmol/kg) 20 minutes earlier. Systemic administration of ZnDPBG increased arterial pressure from 107±2 to 121±2 mm Hg (P<.05). Subsequent bilateral microinjection of carbon monoxide into the NTS caused blood pressure to decrease to 108±1 mm Hg, thus reversing the pressor response to ZnDPBG administered systemically.

View larger version (18K): [in this window] [in a new window]   Figure 5. Line graphs show mean arterial pressure and heart rate before and after administration of ZnDPBG (45 µmol/kg IP) followed by bilateral microinjection of carbon monoxide–saturated saline (100 nL) into the nucleus tractus solitarii (NTS) of awake rats. Symbols represent the mean±SEM. *P<.05 from time=0 minutes; P<.05 from times=5 to 20 minutes.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
In accordance with a previous report,¹² awake rats injected intraperitoneally with the heme oxygenase inhibitor ZnDPBG displayed a sustained elevation of arterial pressure without bradycardia. The present study demonstrates that in these animals both the range and the gain of the heart rate responses obtained during phenylephrine-induced increases and sodium nitroprusside–induced decreases of arterial pressure are reduced. These findings suggest that treatment with ZnDPBG results in attenuation of baroreceptor reflex control of heart rate. However, our study offers no information on the precise mechanism underlying such an effect.

Disruption of arterial baroreflex function by severing the nerve afferents arising from the aortic arch and the carotid sinus causes elevation of arterial pressure due to increased sympathetic outflow.^{16 19} The increase in arterial pressure produced by the heme oxygenase inhibitor ZnDPBG in awake rats also relies on the sympathetic nervous system, because it can be prevented by pretreatment with prazosin or chlorisondamine.¹² One possibility to consider is that ZnDPBG elicits a sympathetically driven elevation of arterial pressure by attenuating the arterial baroreflex control of sympathetic outflow, which impacts importantly on vascular tone. This notion is challenged by our findings that ZnDPBG is equally effective in increasing arterial pressure in unoperated controls and in rats with disrupted arterial baroreceptor reflex function due to sinoaortic deafferentation. Hence, the pressor effect of ZnDPBG in rats is not readily attributable to an alteration in arterial baroreceptor reflex control of cardiovascular function produced by the inhibitor of heme oxygenase.

Disruption of nerve afferents arising from the aortic arch and carotid sinus does not interrupt afferent nerve input from cardiopulmonary baroreceptors into the NTS,²⁰ the site of termination of visceral afferent nerves from various cardiovascular and noncardiovascular mechano- and chemoreceptors.¹⁶ Accordingly, after sinoaortic denervation, cardiopulmonary baroreceptor input into the NTS may provide inhibitory influences on sympathetic outflow to cardiovascular structures. Our study does not allow exclusion of the possibility that the pressor effect of ZnDPBG in rats results from impairment in the control of arterial pressure by cardiopulmonary baroreceptors.

Since ZnDPBG is capable of inhibiting brain heme oxygenase activity when administered intraperitoneally,¹² the accompanying pressor response may emanate from a primary effect of the heme oxygenase inhibitor at central nervous system sites that are concerned with the regulation of arterial pressure. This notion is supported by our demonstration that microinjections of ZnDPBG into the NTS, unilaterally or bilaterally, increase the arterial pressure of awake rats. Conceivably, the pressor effect of ZnDPBG may be a consequence of decreased production of carbon monoxide within the NTS. Such a view is in accordance with our findings that microinjections of carbon monoxide into the NTS reverse the increase of blood pressure produced by ZnDPBG injected either systemically or into the NTS. Collectively, these results suggest that carbon monoxide formed within the NTS subserves a vasodepressor mechanism that is tonically active in awake rats. In fact, such a carbon monoxide–mediated vasodepressor mechanism appears to be maximally active in awake normotensive rats, since injections of carbon monoxide into the NTS of rats not pretreated with ZnDPBG are without effect on arterial pressure.

Previous studies established that experimentally induced lesions of the NTS increase blood pressure in rats with intact baroreceptor reflex function but not in rats with sinoaortic deafferentation.²⁰ That sinoaortic deafferentation did not affect the pressor response to ZnDPBG injected intraperitoneally may be taken to indicate that ZnDPBG administered systemically does not increase blood pressure by disrupting NTS function. However, one cannot exclude the possibility that the inhibitor of heme oxygenase affects discrete NTS neurons that are not totally dependent on input from baroreceptor afferents for their blood pressure regulatory activity.

Recent studies suggested stimulatory regulation of brain guanylate cyclase by carbon monoxide arising within brain structures via metabolism of heme by heme oxygenase.^{6 7} It was also reported that an inhibitor of heme oxygenase is effective in preventing the effects of metabotropic glutamate receptor activation in the NTS, presumably by arresting carbon monoxide–mediated stimulation of cGMP production.²⁴ Our study did not address the possibility that the vasodepressor mechanism subserved by carbon monoxide formed within the NTS utilizes cGMP as a second messenger. Other studies provide convincing evidence that nitric oxide donors and atrial natriuretic peptides, which respectively stimulate soluble and particulate guanylate cyclases, activate central vasodepressor mechanisms involving the NTS.^{25 26 27 28} However, carbon monoxide also binds to other heme-bearing proteins and can inhibit electron transport into mitochondria and thus to interfere with ATP production. While the current studies were designed to focus on potential sites for the blood pressure actions of ZnDPBG and carbon monoxide, they do not clarify the signaling mechanism(s) linked with a heme oxygenase–carbon monoxide system in the brain.

In summary, the present study demonstrates that the increase in arterial pressure displayed by awake rats injected intraperitoneally with the heme oxygenase inhibitor ZnDPBG is accompanied by a significant attenuation in the arterial baroreceptor reflex control of heart rate. However, the pressor effect of ZnDPBG is equally demonstrable in rats with sinoaortic deafferentation, suggesting that it is not the result of a dysfunctional arterial baroreceptor reflex. In our study, ZnDPBG also increased arterial pressure when microinjected into the NTS, and this effect was reversed by ipsilateral administration of carbon monoxide–saturated saline. Accordingly, the possibility arises that inhibition of carbon monoxide production by ZnDPBG within the NTS is a major determinant of the pressor effect of this agent. This would imply that carbon monoxide formed within the NTS subserves a vasodepressor mechanism that is tonically active in awake normotensive rats.

	Acknowledgments

 
This research was supported by grants HL-18579, 5POI HL-34300, HL-32205, and HL-14388 from the National Institutes of Health, Bethesda, Md, CNP (200 006/93-02), and VA Merit Review, VA Clinical Investigatorship (Dr Talman). We thank Jennifer Brown for secretarial assistance.

Received October 4, 1996; first decision October 25, 1996; accepted February 26, 1997.

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