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Am J Physiol Heart Circ Physiol 291: H223-H230, 2006. First published February 17, 2006; doi:10.1152/ajpheart.00058.2006
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Effects of carbon monoxide and heme oxygenase inhibitors in cerebral vessels of rats and mice

Jon J. Andresen,1 Nadeem I. Shafi,2 William Durante,3 and Robert M. Bryan, Jr.1

1Department of Anesthesiology and 2Section of Pediatric Critical Care, Baylor College of Medicine, Houston, Texas; and 3Department of Medical Pharmacology and Physiology, School of Medicine, University of Missouri-Columbia, Columbia, Missouri

Submitted 13 January 2006 ; accepted in final form 12 February 2006


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Carbon monoxide (CO) has been postulated to be a signaling molecule in many tissues, including the vasculature. We examined vasomotor responses of adult rat and mouse cerebral arteries to both exogenously applied and endogenously produced CO. The diameter of isolated, pressurized, and perfused rat middle cerebral arteries (MCAs) was not altered by authentic CO (10–6 to 10–4 M). Mouse MCAs, however, dilated by 21 ± 10% at 10–4 M CO. Authentic nitric oxide (NO·, 10–10 to 10–7 M) dilated both rat and mouse MCAs. At 10–8 M NO·, rat vessels dilated by 84 ± 4%, and at 10–7 M NO·, mouse vessels dilated by 59 ± 9%. Stimulation of endogenous CO production through heme oxygenase (HO) with the heme precursor {delta}-aminolevulinic acid (10–10 to 10–4 M) did not dilate the MCAs of either species. The metalloporphyrin HO inhibitor chromium mesoporphyrin IX (CrMP) caused profound constriction of the rat MCA (44 ± 2% at 3 x 10–5 M). Importantly, this constriction was unaltered by exogenous CO (10–4 M) or CO plus 10–5 M biliverdine (both HO products). In contrast, exogenous CO (10–4 M) reversed CrMP-induced constriction in rat gracilis arterioles. Control mouse MCAs constricted by only 3 ± 1% in response to 10–5 M CrMP. Magnesium protoporphyrin IX (10–5 M), a weak HO inhibitor used to control for nonspecific effects of metalloporphyrins, also constricted the rat MCA to a similar extent as CrMP. We conclude that, at physiological concentrations, CO is not a dilator of adult rodent cerebral arteries and that metalloporphyrin HO inhibitors have nonspecific constrictor effects in rat cerebral arteries.

cerebral arteries; chromium mesoporphyrin; endothelium-derived hyperpolarization factor; gracilis arteriole

HEME OXYGENASE (HO) is the major enzyme responsible for degrading heme (29). Three isoforms of HO (HO-1, -2, and -3) catalyze the conversion of heme to carbon monoxide (CO), biliverdine IX{alpha}, and iron (29, 35, 54). CO, in turn, has been postulated to be a signaling molecule in many tissues, including the vasculature (22, 29, 40–42, 48, 54, 55, 58). HO-1 is not normally expressed in most tissues, although it may be upregulated by a wide variety of stimuli, including subarachnoid hemorrhage (45) and ischemia-reperfusion (38). HO-2 is constitutively expressed and is found in most tissues, including the brain, vascular smooth muscle, and endothelium (8, 29, 30, 38, 47, 57). HO-3 may be constitutively expressed in the rat, although recent work suggests that it is a splice variant of HO-2 transcripts (16, 31, 54).

CO binds to heme moieties such as those found in soluble guanylyl cyclase, nitric oxide (NO·) synthase, and cytochrome P-450 enzymes. Binding of CO to heme moieties can either increase or decrease enzymatic activity (7, 17, 22, 29, 40–43, 46). Large-conductance calcium-dependent K+ channels can also be activated by CO (18, 24, 28, 34, 53), and CO is a dilator in some arteries. CO dilates rat and rabbit aortas (4, 10, 11), rat tail arteries (52), rat gracilis arterioles (20), rat mesenteric arteries (33, 34), and cerebral arterioles in the piglet (18, 24, 28). The effect of CO on adult cerebral arteries has been studied sparingly and with conflicting results. In one study, CO caused dilation of canine basilar arteries (23), whereas another study found that CO did not dilate the basilar arteries of rabbits or dogs (4). With the exception of pial arterioles in newborn pigs (28), vasodilation to CO in the aforementioned studies usually occurs at relatively high concentrations (10–5 M or greater).

The major goal of the present study was to determine the effect of CO on adult cerebral arteries by using an in vitro model that resembles, as closely as possible, in vivo conditions. The effect of CO on vessel diameter was determined in isolated, pressurized, and perfused middle cerebral arteries (MCAs) of rats and mice by applying authentic CO exogenously or by stimulating endogenous production of CO through use of the heme precursor {delta}-aminolevulinic acid ({delta}-ALA). Endogenous production of CO was eliminated by using metalloporphyrin inhibitors of HO. Thus changes in vessel diameter were investigated both in the presence and absence of CO in resistance-sized adult cerebral vessels.


   METHODS
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 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Animals

Male Long-Evans rats (250–350 g) and male C57BL/6J mice (20–25 g) were used in the present study. Rats were purchased from Charles River (Wilmington, MA), and mice were purchased from Jackson Laboratory (Bar Harbor, ME). Rats and mice were anesthetized with 3% isoflurane and decapitated, and their brains were removed for study. All procedures were approved by the Animal Protocol Review Committee at Baylor College of Medicine.

Immunoblotting

Tissue samples were homogenized in a buffer containing 1% SDS (Bio-Rad, Hercules, CA), 1 x 10–2 M EDTA (Sigma, St. Louis, MO), and a protease inhibitor cocktail (Complete, mini, Roche, Basel, Switzerland). Samples were boiled for 15 min and centrifuged at 15,000 g for 15 min. Samples were diluted 1:1 with 4x Laemmli buffer and boiled again for 15 min before loading 30–62 µg into wells of 4–20% SDS gels (Ready Gel, Bio-Rad). After electrophoresis at a constant 150 V for 1 h at room temperature, proteins were transferred to supported nitrocellulose membranes (Bio-Rad) at a constant 250 mA for 2.5 h at 4°C. After transfer, equal loading was confirmed by staining the nitrocellulose blots using 0.1% Ponceau S (Sigma). The blots were blocked for 1 h in cold blocking solution [5% nonfat milk and 1% bovine serum albumin (Sigma)] and then incubated overnight at 4°C with primary antibodies to either HO-1 (SPA-895) or HO-2 (OSA-200), diluted 1:1,000 (rabbit polyclonals, Stressgen Bioreagents, Victoria, British Columbia, Canada).

Blots were then rinsed in PBS (Invitrogen, Carlsbad, CA), blocked for 1 h at room temperature, and exposed to a horseradish peroxidase-conjugated secondary antibody (1:10,000, anti-rabbit) for 1 h at room temperature. After being rinsed in PBS, immunoreactive protein bands were detected by chemiluminescence (Supersignal West Femto Maximum Sensitivity; Pierce, Rockford, IL).

Vessel Studies

After decapitation, brains were rapidly removed and placed in ice-cold Krebs solution (in M: 1.19 x 10–1 NaCl, 4.7 x 10–3 KCl, 2.4 x 10–2 NaHCO3, 1.18 x 10–3 KH2PO4, 1.19 x 10–3 MgSO4, 5.5 x 10–5 glucose, and 1.6 x 10–3 CaCl2; all from Sigma). Beginning at the Circle of Willis, the middle cerebral artery (MCA) was carefully removed from the pial surface using a dissecting microscope and microdissection instruments. A branch-free section of the MCA was then mounted between two glass pipettes (150-µm diameter pipettes for rat MCAs and 80-µm pipettes for mouse MCAs) in the well of a water-jacketed vessel chamber containing Krebs solution saturated with 5% CO2-20% O2 with balance N2 (Messer, Malvern, PA) at 37°C as previously described (5, 56). Vessels were securely affixed to the pipettes with 11-0 suture (Microsurgery Instruments, Bellaire, TX) and pressurized by columns of Krebs to ~80 mmHg for rats and ~75 mmHg for mice. Luminal flow of Krebs was adjusted by altering the heights of the inflow and outflow columns.

Vessels were allowed to equilibrate for 1 h before the start of the experiments. Ca2+-free Krebs (in M: 1.19 x 10–1 NaCl, 4.7 x 10–3 KCl, 2.4 x 10–2 NaHCO3, 1.18 x 10–3 KH2PO4, 1.19 x 10–3 MgSO4, 5.5 x 10–5 glucose, and 1 x 10–3 EGTA; all from Sigma) was added luminally and abluminally at the end of each experiment to determine the maximum vascular diameter. Some vessels were treated with N{omega}-nitro-L-arginine methyl ester (L-NAME, 10–5 M, Sigma) and indomethacin (10–5 M, Sigma) luminally and abluminally for 30 min. Endothelial denudation was accomplished by passing 10 ml of air through the vessel lumen over 10 min at 80 mmHg. Denudation was confirmed by absence of dilation to luminally applied 10–5 M ATP (Sigma). During blackout conditions, the vessel apparatus was surrounded by heavy drapes, the room lights were turned off, and the vessel was illuminated only before and after drug administration. Vessel experiments were recorded onto VHS tapes with a charge-coupled device camera mounted on a microscopic tube. An image of the vessel was also displayed on a video screen with a final magnification of x500. Changes in the outer diameter were quantified with Optimus image analysis software (version 5.1, Optimus, Bothell, WA).

Mouse MCAs. Constriction was determined to abluminal KCl (4 x 10–2 and 8 x 10–2 M) and phenylephrine (PE, 10–6 to 10–4 M, Sigma). Papaverine (3 x 10–5 M, Sigma) was added abluminally after PE. In vessels preconstricted with PE (3 x 10–6 M), dilatory responses were determined to UTP (10–5 M and 10–4 M, Sigma), the heme precursor {delta}-ALA (10–10 to 10–4 M, Frontier Scientific, Logan, UT), authentic NO· (10–10 to 10–7 M, Airgas, Radnor, PA), and authentic CO (10–7 to 10–4 M, Airgas). UTP was applied luminally, whereas {delta}-ALA, NO·, and CO were all applied abluminally. The response to abluminal application of the HO inhibitor chromium (III) mesoporphyrin IX chloride (CrMP, 10–5 M, Frontier Scientific) was also determined in the mouse MCA.

Rat MCAs. Unlike the mouse MCA, rat MCAs did not require PE preconstriction because they developed active tone. Responses to CrMP (3 x 10–7 to 3 x 10–5 M) were determined in control vessels, in those treated with L-NAME and indomethacin, and in vessels denuded of endothelium. Vascular responses were also determined to the HO inhibitors cobalt (III) protoporphyrin IX chloride and magnesium (II) protoporphyrin IX disodium (CoPP and MgPP, 10–5 M, Frontier Scientific). Stock solutions of metalloporphyrins were prepared at 10–2 M using 0.1 N NaOH. The 10–2 M stock solutions were diluted by at least 103-fold in the vessel chamber; pH was not affected by the addition of metalloporphyrins. In the rat MCA, metalloporphyrins were applied both luminally and abluminally. Responses to the heme precursor {delta}-ALA (10–9 to 10–4 M), authentic NO· (10–10 to 10–8 M), and authentic CO (10–6 to 10–4 M) were also examined. Biliverdine (Frontier Scientific) was also applied luminally and abluminally. {delta}-ALA was dissolved in H2O at 10–2 M, and biliverdine was dissolved in DMSO at 10–1 M. After the addition of biliverdine, the DMSO concentration in the bath was 0.001%.

Rat gracilis arterioles. In some experiments, first-order rat gracilis arterioles were isolated as previously described (21, 44) and studied in vitro as described above. The gracilis arteriole was pressurized to ~80 mmHg and perfused. The HO inhibitor CrMP (10–5 M) was applied luminally and abluminally and was followed 30 min later by abluminal application of authentic CO (10–4 M).

Delivery of authentic NO· and CO gases. Krebs was deoxygenated by vigorous bubbling with 100% N2 (Airgas) for >20 min in a septum port gas sampling tube. Deoxygenated Krebs was saturated with either NO· or CO gases by vigorously bubbling for 20 min. NO· and CO saturated at ~10–2 and ~10–3 M, respectively. NO·- or CO-saturated Krebs buffer was injected into the abluminal compartment of the vessel chamber using a gas-tight syringe (Hamilton, Reno, NV) to achieve the desired concentration. Gas-permeable tubing submerged inside of the vessel chamber allowed the luminal perfusate to equilibrate with the abluminal compartment. Thus the vessel lumen was exposed to the same gas concentrations as the abluminal bath.

Statistical Analysis

Data are expressed as means ± SE. Changes in vessel diameter were calculated as follows: %constriction = [(baseline diameter – constricted diameter)/baseline diameter] x 100; %dilation = [(dilated diameter – baseline diameter)/(Ca2+-free diameter – baseline diameter)] x 100.

Differences were analyzed with one- or two-factor ANOVA as appropriate, followed by Bonferroni’s multiple comparisons or Tukey’s posttest. Two-tailed unpaired or paired t-tests were also performed where appropriate. In addition, nonlinear curve fitting was used to determine log EC50 values. In all cases, data were fitted to sigmoidal dose-response (variable slope) curves with appropriate constraints on the tops, bottoms, and hill slopes of the curves. Statistical analyses were computed using Prism 4 (version 4.03, GraphPad Software, San Diego, CA). Significance was accepted at P ≤ 0.05.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 GRANTS
 REFERENCES
 
Immunoblotting

HO-1 and -2 were present in tissues sampled from rats and mice (Fig. 1). A ~32-kDa band corresponding to the molecular mass of HO-1 was observed in samples of brain and spleen from both rats and mice. A ~36-kDa band corresponding to the molecular mass of HO-2 was observed in all rat tissues examined, including aorta, cerebral arteries (MCA and basilar), brain, heart, liver, lung, muscle, and spleen. In mice, HO-2 was observed in the brain but not in the spleen. HO expression was not examined in samples of mouse cerebral arteries because of their small size.


Figure 1
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  		Fig. 1. Western blots of heme oxygenase (HO)-1 and -2 in tissues of rats and mice. Top: in rats, a band corresponding to molecular mass of HO-2 (36 kDa) was present in all tissues examined, including aorta, cerebral arteries [middle cerebral arteries (MCA) and basilar arteries], brain, heart, liver, lung, skeletal muscle, and spleen. A band corresponding to molecular mass of HO-1 (32 kDa) was found only in the brain and spleen in the rat. Thirty micrograms of protein were loaded into each lane. Bottom: in mice, expression of HO-1 and -2 was examined in the brain (B) and spleen (S). In mice, both HO-1 and -2 were found in the brain, whereas in the spleen, only HO-1 was detected. Sixty-two micrograms of protein were loaded into each lane.

 
Vessel Studies

Characterization of mouse MCA. The rat MCA has been studied extensively in our laboratory (5, 56); however, we have not previously examined the mouse MCA. Therefore, we determined the response of the mouse MCA to various vasoactive substances. Unlike the rat MCA, which developed 23 ± 3% tone during the 1-h equilibration period, the mouse MCA did not develop active tone and remained at or near the maximum diameter. Mouse MCAs constricted to KCl (n = 32) in a concentration-dependent fashion (P < 0.0001), reaching 20 ± 1% constriction at 8 x 10–2 M KCl (Fig. 2A). Constriction of the mouse MCA to PE (n = 16) was also concentration dependent, reaching a maximum of 33 ± 2% at 10–4 M PE with a log EC50 of –5.9 ± 0.11 M (Fig. 2B). After the last concentration of PE (10–4 M), mouse MCAs (n = 15) dilated by 95 ± 2% in response to 3 x 10–5 M papaverine (not shown). After a plateau was reached, constriction to PE remained stable in the mouse MCA, and thus 3 x 10–6 M PE was used to preconstrict mouse vessels before application of dilator substances.


Figure 2
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  		Fig. 2. Response of C57BL/6J mouse MCA to KCl, phenylephrine (PE), and UTP. A: abluminal application of KCl caused concentration-dependent constriction of mouse MCA (P < 0.0001; n = 32). B: abluminal application of PE resulted in concentration-dependent constriction of mouse MCA with a log EC50 of –5.9 ± 0.11 M (n = 16). C: in vessels preconstricted with 3 x 10–6 M PE, luminal application of UTP caused concentration-dependent dilation (P < 0.001) of both control MCAs (solid bars) and those treated with 10–5 M N{omega}-nitro-L-arginine methyl ester and indomethacin (L-NAME and Indo, open bars; n = 5 each). At 10–4 M UTP, dilation in control vessels was significantly greater than in those treated with L-NAME and Indo (*P = 0.0257). Data are means ± SE.

 
Preconstricted mouse MCAs dilated to the P2Y2 receptor agonist UTP in a concentration-dependent manner (P < 0.001) under control conditions and in the presence of L-NAME and indomethacin (Fig. 2C). In control vessels (n = 5), 10–4 M UTP caused dilations of 89 ± 3%, whereas dilations were reduced to 69 ± 7% in vessels pretreated with L-NAME and indomethacin (P = 0.0257; n = 5). Pretreatment with L-NAME and indomethacin constricted the mouse MCA by 12 ± 1% (not shown) and did not alter the preconstriction to PE (control preconstriction, 19 ± 1%, vs. L-NAME and indomethacin preconstriction, 26 ± 2%; P = 0.0902, not shown).

Response to authentic NO· and CO. MCAs from both rats and mice dilated to authentic NO· (10–10 to 10–7 M). At 10–8 M NO·, rat vessels dilated 84 ± 4% (n = 7, Fig. 3A). At 10–7 M NO·, mouse MCAs dilated 59 ± 9% (n = 6; Fig. 3C). In contrast to NO·, CO (10–6 to 10–4 M) did not dilate rat MCAs (Fig. 3B; n = 6, P = 0.3763). CO (10–7 to 10–4 M) did not elicit dilation of mouse MCAs until the CO concentration reached 10–4 M. This concentration of CO resulted in 21 ± 10% dilation (Fig. 3D; n = 6, P < 0.001).


Figure 3
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  		Fig. 3. Response of rat and mouse MCAs to authentic nitric oxide (NO·) and carbon monoxide (CO) gases. Rat MCAs dilate in response to authentic NO· (A; n = 7) but not to authentic CO (B; n = 6). Mouse MCAs dilate in response to authentic NO· (C; n = 6) but not to authentic CO except at 10–4 M CO (D; n = 6; *P < 0.001). Data are means ± SE.

 
Response to {delta}-ALA. Stimulation of endogenous CO production using the heme substrate {delta}-ALA (10–9 to 10–4 M) did not elicit concentration-dependent dilations (P = 0.3827) in either control (n = 3) or L-NAME- and indomethacin-treated (n = 2) rat vessels (Fig. 4). The response of rat MCAs was similar in control and L-NAME- and indomethacin-treated vessels (P = 0.1237). Mouse MCAs (n = 6) also did not dilate to {delta}-ALA (10–10 to 10–4 M; Fig. 4).


Figure 4
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  		Fig. 4. Response of rat and mouse MCAs to heme precursor {delta}-aminolevulinic acid ({delta}-ALA). Left: heme precursor {delta}-ALA had no significant effect on diameter of either control (n = 3) or L-NAME- and indomethacin-treated rat MCAs (n = 2). Right: mouse MCAs preconstricted with 3 x 10–6 M PE (n = 6) also did not dilate in response to heme precursor {delta}-ALA. Data are means ± SE.

 
Effect of HO Inhibitors

Rat MCA. Figure 5A is a tracing of a rat MCA treated with increasing concentrations of CrMP and depicts two salient features of the effect of CrMP on rat cerebral vessels: profound vasoconstriction and reduction of the maximum attainable diameter under Ca2+-free conditions. Constrictions to CrMP in control vessels, those pretreated with L-NAME and indomethacin, and in denuded vessels are summarized in Fig. 5B. The constrictions were not light sensitive because they occurred with either normal room light or during blackout conditions. The constriction to 3 x 10–5 M CrMP was 44 ± 2% in control, 36 ± 2% in L-NAME and indomethacin, and 38 ± 4% in denuded vessels (n = 4 each). The log EC50 for denuded vessels (–6.1 ± 0.08 M) was, however, significantly (P = 0.0061) shifted to the left of that of the control vessels (log EC50 = –5.8 ± 0.08 M) and those treated with L-NAME and indomethacin (log EC50 = –5.8 ± 0.08 M). Interestingly, the structurally related HO inhibitors CoPP and MgPP also constricted the rat MCA (Fig. 5C) even though MgPP is a weak inhibitor of HO. A single concentration of CrMP (10–5 M) constricted rat MCAs by 37 ± 2% (n = 9), whereas 10–5 M CoPP (n = 3) and MgPP (n = 5) constricted the rat MCA by a similar 35 ± 5 and 28 ± 3%, respectively (P = 0.155). In addition to causing profound constriction, 10–5 M CrMP significantly reduced the Ca2+-free diameter of the rat MCA from 315 ± 6 µm under control conditions to 266 ± 6 µm in the presence of CrMP (P = 0.0002; n = 8 each; Fig. 5, A and D).


Figure 5
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  		Fig. 5. Effect of metalloporphyrin HO inhibitors on rat MCA. A: tracing of a rat MCA treated with increasing concentrations of chromium (III) mesoporphyrin IX chloride (CrMP) depicts both profound constriction and reduction in Ca2+-free diameter caused by CrMP. B: summary data of concentration response to CrMP in control (squares), L-NAME- and indomethacin-treated (triangles), and denuded (diamonds) rat MCAs (n = 4 for each). Log EC50 for control and L-NAME- and indomethacin-treated vessels was –5.8 ± 0.08 M for each. In denuded vessels, log EC50 was significantly shifted to the left (–6.1 ± 0.08 M; *P = 0.0061) compared with either control or L-NAME- and indomethacin-treated vessels. C: at 10–5 M, metalloporphyrins CrMP, cobalt protoporphyrin IX chloride (CoPP), and magnesium protoporphyrin IX disodium (MgPP) constricted rat MCAs by a similar amount (P = 0.155). D: Ca2+-free diameter of the rat MCA was reduced from 315 ± 6 to 266 ± 6 µm by 10–5 M CrMP (*P = 0.0002). Data are means ± SE.

 
Neither coadministration of 10–4 M CO nor 10–4 M CO plus 10–5 M biliverdine (both products of the HO reaction) lessened the constriction caused by CrMP (Fig. 6). Constriction to 10–5 M CrMP was 37 ± 2% in control MCAs (n = 9) and a similar 37 ± 3 and 45 ± 5% in vessels cotreated with either 10–4 M CO (n = 12) or a combination of 10–4 M CO and 10–5 M biliverdine (n = 3), respectively (P = 0.3619). Only in the absence of Ca2+ did CrMP not cause constriction of rat MCAs (n = 3, not shown). Rat MCAs treated with 10–5 M CrMP (n = 4) dilated by 88 ± 4% to the NO· donor sodium nitroprusside (not shown). In rat gracilis arterioles treated with 10–5 M L-NAME and indomethocin (n = 4), 10–5 M CrMP caused 21 ± 3% constriction after 30 min (Fig. 6). Unlike in the MCA, however, 10–4 M CO reversed the constriction caused by CrMP, resulting in only 5 ± 3% constriction after CO administration in the presence of CrMP (P = 0.0003).


Figure 6
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  		Fig. 6. Effect of CO and biliverdine on constriction caused by CrMP in rat MCA and gracilis arteriole. Left: administration of 10–4 M CO or 10–4 M CO plus 10–5 M biliverdine (both products of HO reaction) along with CrMP did not reduce constriction caused by CrMP (P = 0.3619). Control vessels treated with 10–5 M CrMP constricted by 37 ± 2% (n = 9). CO- and CrMP-treated vessels constricted by 37 ± 3% (n = 12), and rat MCAs treated with CO, biliverdine, and CrMP constricted by 45 ± 5% (n = 3). Right: CrMP constricted rat gracilis arterioles by 21 ± 3% and administration of 10–4 M CO reversed constriction caused by CrMP (*P = 0.0003). Data are means ± SE.

 
Mouse MCA. In contrast to the rat MCA, the mouse MCA constricted very little to 10–5 M CrMP (Fig. 7). Relative to the diameter when CrMP was administered, the maximum constriction observed over 30 min was 3 ± 1% in control vessels (P = 0.0006; n = 11) and 5 ± 1% in vessels treated with L-NAME and indomethacin (P = 0.0138; n = 4). There was no difference in the constrictions to CrMP in the presence or absence of L-NAME and indomethacin (P = 0.3345). Unlike in the rat MCA, the Ca2+-free diameter of the mouse MCA was unaffected by CrMP treatment (Fig. 7). The maximum diameter of control vessels was 186 ± 2 µm (n = 24). Similarly, in vessels treated with 10–5 M CrMP, the maximum diameter was 182 ± 8 µm (P = 0.1946 vs. control; n = 11).


Figure 7
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  		Fig. 7. Response of C57BL/6J mouse MCA to CrMP. Left: 10–5 M CrMP caused very modest constriction of control (3 ± 1%; n = 11) and L-NAME- and indomethacin-treated (5 ± 1%; n = 4) mouse MCAs, which was similar in both groups (P = 0.3345). Right: Ca2+-free diameter of mouse MCAs was similar (P = 0.1946) in control MCAs (186 ± 2 µm; n = 24) and vessels treated with 10–5 M CrMP (182 ± 8 µm; n = 11). Data are means ± SE.

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
There are two major findings in the present study. Adult rat and mouse cerebral arteries did not dilate to CO at concentrations that would be considered physiological, and the metalloporphyrin HO inhibitors exhibited nonspecific effects in rat cerebral arteries. We have demonstrated that high concentrations of exogenous CO did not dilate rat vessels and only dilated mouse MCAs at a concentration of 10–4 M. Additionally, although HO is present in rat cerebral arteries and in the mouse brain (Fig. 1), stimulation of endogenous CO production with the heme substrate {delta}-ALA did not dilate MCAs from either species. The HO inhibitors CrMP and CoPP as well as the control metalloporphyrin MgPP (a weak inhibitor of HO) profoundly constricted the rat MCA. The constriction to metalloporphyrins in the rat MCA could not be prevented, attenuated, or reversed by CO or CO plus biliverdine (both products of HO-mediated heme degradation).

Mouse Vessels

Because we have not previously reported studies using isolated mouse vessels and because relatively few laboratories have experience studying mouse cerebral vessels in vitro, we characterized vasomotor responses of mouse MCAs as part of this study. In the present study, mouse MCAs did not develop myogenic tone in response to pressure and flow. In the absence of luminal flow, previous studies have found that mouse MCAs developed myogenic tone in response to pressure (2, 12, 13). For example, at 80 mmHg, <10% tone was observed in MCAs of C57BL/6J mice (12, 13), whereas CD-1 mouse MCAs developed 21% tone (2). Besides possible strain-related differences (39), perhaps luminal flow, and thus shear stress, on the endothelium altered the amount of myogenic tone observed in our studies. Resting tone of mouse MCAs has a large NO· and prostacyclin component. In the present study, as well as in previous studies (2, 12, 13), inhibition of NO· synthase and cyclooxygenase resulted in 12–20% constriction.

Strain-related differences exist among mice that affect vascular function (39). For instance, CD-1 mouse MCAs constrict much more to KCl than do the C57BL/6J MCAs used in the present study, although the constriction to PE is similar in the two strains (2). Endothelium-dependent dilation was elicited in CD-1 mouse MCAs by acetylcholine, bradykinin, and substance P (2). In the present study, the endothelium-dependent dilator UTP caused dilation of C57BL/6J mouse MCAs in control vessels and in the presence of L-NAME and indomethacin (Fig. 2). The latter finding suggests that mouse cerebral arteries have a large dilatory component attributable to endothelium-derived hyperpolarizing factor. In support of this, mouse pial arterioles dilated to ADP in both wild-type mice treated with a NO· synthase inhibitor and in endothelial NO· synthase knockout mice (9).

Effect of CO in Cerebral Vessels

The role of the HO/CO system in control of cerebrovascular function may be developmentally regulated. For example, piglet cerebral arteries dilate to CO by stimulating large-conductance calcium-dependent K+ channel activity, although soluble guanylyl cyclase (sGC) may play an additional, permissive role in CO-induced dilations in the piglet (3, 18, 24, 27, 28). In adult animals, there are conflicting reports of the effect of CO on cerebral arteries. One study (23) of canine basilar arteries found that exogenous CO (5.7 x 10–7 to 5.67 x 10–4 M) caused dilation beginning at 5.7 x 10–5 M, a concentration that would be considered supraphysiological (6, 14, 32, 51), whereas another study (4) using the same vessels found that CO did not elicit dilation even up to 3 x 10–4 M. In yet another study (17), blockade of NO· synthase by endogenously produced CO impaired NO·-dependent vasodilation of adult rat pial arterioles. Data from the present study convincingly demonstrate that adult cerebral arteries from rats do not dilate to CO (Fig. 3). Mouse MCAs also did not dilate to CO until the concentration of CO reached 10–4 M, which is only 10-fold less than the saturation limit for CO (Fig. 3). Such a high concentration of CO is probably not physiologically relevant (6, 14, 32, 51) and stands in stark contrast to NO·, which began to dilate the MCA at 10–10 M.

It is currently unknown why piglet cerebral arteries dilate to physiological concentrations of CO and adult cerebral arteries from a variety of species (i.e., dog, rat, rabbit, and mouse) do not. We are not aware of any studies examining the response of adult pig cerebral arteries to CO. Thus it cannot be said whether or not the differences are developmental or species related. While differences may exist between large and small cerebral arteries, both the MCA and pial arterioles of neonatal pigs dilate to CO by a similar mechanism (3). Dilation to CO can therefore be considered a general property of piglet cerebral arteries. On the basis of results from other species, including those with gyrencephalic cerebrums (i.e., dogs) like the pig, differences in the response of cerebral arteries to CO appear to be due to developmental changes that favor NO·-dependent dilation in the adult.

Great care was taken to ensure that CO was effectively delivered to the vessel chamber in the expected concentrations. As a control for effective CO delivery, constriction to CrMP in rat gracilis arterioles was readily reversed by exogenous CO (Fig. 5) as previously described (25). As an additional control of effective gas delivery, both rat and mouse vessels dilated to low concentrations of exogenous NO·, which is more volatile than CO in aqueous solution because it may react with dissolved oxygen.

In addition to exogenous CO, endogenous CO production by HO can dilate some vessels. For instance, rat gracilis arterioles dilate to exogenous CO as well as to the heme precursor {delta}-ALA (20). Formation of {delta}-ALA is the major regulatory step in the heme biosynthetic pathway, and exogenous {delta}-ALA stimulates heme production and the subsequent synthesis of CO by HO (19, 26, 37). In the present study, administration of {delta}-ALA did not dilate either rat or mouse MCAs, a result consistent with the insensitivity to exogenous CO. Thus we conclude that CO is not a physiological dilator of adult rodent cerebral arteries.

Effect of Metalloporphyrins

The constriction of rat MCAs to metalloporphyrins probably involved a nonspecific effect on smooth muscle beyond inhibition of HO. This conclusion is based on the following lines of evidence. First, neither CO nor CO plus biliverdine attenuated or reversed constrictions to CrMP. If the depletion of CO or biliverdine after HO inhibition were responsible for the constriction, then exogenous replacement of these products should have reversed the constriction produced by CrMP. In this respect, the response in the rat MCA is different from the rat gracilis arteriole, where the addition of CO alone reversed the constriction elicited by CrMP (see Fig. 6 and Refs. 21 and 25). Second, MgPP, a weak inhibitor of HO that has been used as a control for nonspecific effects of metalloporphyrins, constricted rat MCAs to a similar degree as CrMP and CoPP (Fig. 5C). This response contrasts with rat gracilis arterioles, where CrMP and CoPP, but not MgPP, caused constriction (21, 25). Taken together, these observations strongly suggest that the constrictions to CrMP observed in rat MCAs were due to a mechanism(s) other than that of HO inhibition.

Nonspecific effects of metalloporphyrin HO inhibitors on blood vessels have been reported before (15, 36, 59). For instance, the metalloporphyrins zinc protoporphyrin IX (ZnPP) and tin protoporphyrin-IX (SnPP) and the simple porphyrin protoporphyrin-IX (PP) inhibited relaxation of the rat aorta (36). ZnPP and SnPP are HO inhibitors, whereas PP is not. All the porphyrin compounds, however, blocked relaxation of the rat aorta to vasoactive intestinal peptide and atrial natriuretic peptide. ZnPP also impaired dilation to acetylcholine. Dilation to direct activators of adenylyl cyclase and guanylyl cyclase was unaffected by ZnPP. Because several dilator pathways were inhibited and because the control compound PP had the same effects as ZnPP, it was concluded that ZnPP exhibited nonspecific effects in the rat aorta (36). Although CrMP is thought to be one of the more specific HO inhibitors (1), we now provide evidence that it has nonspecific effects in the rat MCA.

The reason(s) for the nonspecific actions of CrMP in the present study is not clear. Light sensitivity is not a likely explanation because CrMP is not light sensitive like other metalloporphyrin HO inhibitors, such as ZnPP (49, 50, 59). Indeed, we found that, even in darkness, rat MCAs constricted to CrMP. CrMP is a potent inhibitor of HO, and it is not metabolized (1, 6, 50). Nevertheless, depending on the concentration, it may inhibit sGC or NO· synthase. In a study of rat brain, neither basal- nor SNAP-induced sGC activity was altered by 10–5 M CrMP, although activity of NO· synthase was mildly inhibited (<5%) by CrMP (1). MCAs treated with 10–5 M CrMP in the present study dilated to sodium nitroprusside, and constriction to CrMP was unaltered by L-NAME. We believe that inhibition of NO· synthase or sGC by CrMP was not an important contributor to the nonspecific contractile effects observed in our study.

In summary, the present study demonstrates that, at physiological concentrations, CO is not a dilator of adult rodent cerebral arteries. In addition, at least in the rat MCA, use of the metalloporphyrin HO inhibitors may result in nonspecific effects unrelated to their actions on HO.


   GRANTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
This work was supported a Bugher Foundation Award from the American Heart Association (027011N, to R. M. Bryan) and National Institutes of Health Grants P01 NS-38660 (to R. M. Bryan), R01 NS-46666 (to R. M. Bryan), RO1 HL-59976 and R01 HL-74966 (to W. Durante), T32 HL-072754 and F32 HL-080916-01 (to J. J. Andresen), and T32 HL-07939-01A2 (to N. I. Shafi).


   ACKNOWLEDGMENTS
 
We thank Robert and Fruzsina Johnson at Tulane University for helpful advice and instruction in studies of rat gracilis arterioles.


   FOOTNOTES
 

Address for reprint requests and other correspondence: J. J. Andresen, Dept. of Anesthesiology, Baylor College of Medicine, One Baylor Plaza, Suite 434D, Houston, TX 77030 (e-mail: andresen{at}bcm.tmc.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
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 REFERENCES
 

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