Articles

Reduced Pulmonary Artery Vasoconstriction to Methacholine in Cholesterol-Fed Rabbits

Sandra L. Pfister, William B. Campbell
https://doi.org/10.1161/01.HYP.27.3.804
Hypertension. 1996;27:804-810
Originally published March 1, 1996

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Abstract

Abstract Alterations in vascular tone are well documented in hypercholesterolemia, yet little is known about the role of dietary cholesterol in endothelium-dependent contractions of pulmonary arteries. Methacholine and arachidonic acid cause endothelium-dependent contractions in normal rabbit pulmonary artery that are mediated by thromboxane A2. We tested the effect of these agonists on pulmonary arteries from rabbits fed standard rabbit chow or chow supplemented with 2% cholesterol for 2 weeks. Arachidonic acid–induced contractions did not differ in the groups. However, methacholine-induced contractions were significantly depressed in cholesterol-fed rabbits. Vascular thromboxane A2 production was similar in normal and cholesterol-fed rabbits. Pretreatment with the nitric oxide synthase inhibitor nitro-l-arginine had no effect on contractions observed with methacholine in normal rabbits but enhanced methacholine-induced contractions in cholesterol-fed rabbits. In norepinephrine-precontracted vessels, methacholine caused a small relaxation response in normal rabbits. In contrast, in cholesterol-fed rabbits, methacholine produced enhanced relaxations, suggesting that cholesterol feeding augments relaxations and decreases contractions by increasing nitric oxide. However, nitric oxide synthase activity in pulmonary arteries from cholesterol-fed and normal rabbits was not different between the two groups. In an additional experiment, the calcium-dependent potassium channel blocker charybdotoxin had little effect on methacholine-induced contractions in cholesterol-fed rabbits. In summary, the present study demonstrates that hypercholesterolemia alters pulmonary artery vascular contractions and relaxations to methacholine. This effect is not mediated by a decreased production of thromboxane A2 or by an increased production of nitric oxide. Although the mechanism mediating the altered vascular responses is still unknown, the results from this study clearly indicate that the regulation of vascular tone is different in normal and hypercholesterolemic vessels.

Hypercholesterolemia is considered a major risk factor in the development of atherosclerotic disease.1 2 Damage to the endothelial cells lining the lumen of the blood vessel as well as an increased adherence of other cell types, such as monocytes, to the endothelial surface characterize the progression of atherosclerosis.3 The endothelium is the source of a number of vasoactive compounds that may be altered by the disease process.4 For example, endothelium-derived relaxing factor has been identified as NO, and numerous studies have demonstrated decreased NO-mediated relaxations in experimental models of atherosclerosis.5 6 7 8 In addition to NO, the endothelium synthesizes a number of vasodilator arachidonic acid metabolites. PGI2 is a cyclooxygenase metabolite that causes both relaxation of vascular smooth muscle and inhibition of platelet aggregation.9 The production of PGI2 is attenuated in atherosclerosis.10 11 We have shown that in rabbits fed a 2% cholesterol diet for 2 weeks, synthesis of the cytochrome P-450 epoxygenase metabolites of arachidonic acid was enhanced in aortas.12 Using cultured human vein endothelial cells, Pritchard and coworkers13 found a similar enhancement of epoxyeicosatrienoic acid production by low-density lipoprotein cholesterol. The epoxyeicosatrienoic acids possess a variety of biological actions,14 including relaxation of rabbit aorta12 and canine and bovine coronary arteries.15 16

Contracting factors are also produced by the vascular endothelium.4 Yanagisawa and Masaki17 identified the potent peptide vasoconstrictor endothelin. We and others have identified the arachidonic acid metabolite TXA2 as the endothelium-dependent contracting factor in rabbit pulmonary vessels.18 19 Many studies have reported augmented contractions to serotonin and depressed contractions to norepinephrine in coronary arteries and aortas obtained from atherosclerotic compared with normal animals.20 21 22 However, less is understood about the role of endothelium-dependent contracting factor or endothelin in atherosclerosis. It is accepted that the platelet production of TXA2 is enhanced in atherosclerosis.11 In addition, very little is known about the regulation of vascular tone in the pulmonary vasculature of cholesterol-fed animals. The reason for the lack of studies is unclear because it is known that human pulmonary hypertension is often associated with pulmonary artery atherosclerotic lesions.23 24 25

An important point to be emphasized about the abovementioned studies relates to the period of cholesterol feeding. In most cases, animals were fed the diet for a period long enough to produce atherosclerotic lesions, at which point vascular changes were assessed. Because cholesterol is believed to be a major risk factor in cardiovascular disease, we are interested in investigating the early changes that occur in blood vessels before the development of the atherosclerotic lesion. In the present study, we characterized the role of endothelium-derived factors in the responses to methacholine in pulmonary arteries from cholesterol-fed rabbits.

Methods

Animals

NZW rabbits were purchased from New Franken Rabbitry (New Franken, Wis) at 2 months of age. Rabbits in group 1 were fed a standard laboratory rabbit chow, and rabbits in group 2 were fed the standard diet to which 2% cholesterol was added. The rabbits received the respective diets for 2 weeks. Plasma cholesterol concentrations were determined enzymatically in samples from both normal and cholesterol-fed rabbits by the catalase method from Boehringer Mannheim Diagnostics.

Vascular Reactivity

Pulmonary artery tissue was isolated as previously described.19 Briefly, normal and cholesterol-fed rabbits were killed (sodium pentobarbital, 120 mg/kg IV) and the heart and lungs were removed as a unit and placed immediately in a Krebs’ bicarbonate buffer of the following composition (mmol/L): NaCl 118, KCl 4, CaCl2 3.3, NaHCO3 24, KH2PO4 1.4, MgSO4 1.2, and glucose 11, pH 7.4. The main pulmonary artery was identified at its origin from the right ventricle, and both left and right pulmonary arteries were dissected to their most distal end. The pulmonary artery distal to the first branching of the left or right pulmonary artery was used and is referred to as the intrapulmonary artery. After dissection, the tissue was cleaned of adherent lung parenchyma and connective tissue, with care taken to not disturb the endothelial layer. Rings of intrapulmonary artery were obtained (2 to 3 mm) and suspended in 15-mL organ baths containing Krebs’ bicarbonate buffer that was warmed to 37°C and continuously aerated with 95% O2/5% CO2. Isometric tension was measured with force-displacement transducers (Grass Instrument Co) and recorded with a polygraph (Grass model 7D). Resting tension was adjusted to 1 g, the length-tension maxima, and the vessels were allowed to equilibrate for 1 hour. Contractions were produced by raising the KCl concentration of the baths to 40 mmol/L. After the vessels had reproducible, stable contractions to KCl, cumulative concentration-response curves to methacholine (10−8 to 10−4 mol/L), arachidonic acid (10−8 to 10−5 mol/L), the TXA2 mimetic U46619 (10−11 to 10−7 mol/L), or norepinephrine (10−8 to 10−5 mol/L) were determined. In some experiments, the vessels were pretreated with the NO synthase inhibitor LNA (3×10−5 mol/L) for 10 minutes before methacholine administration. These experiments were repeated in the presence of l-arginine (10−4 mol/L). A separate experiment examined the effect of pretreatment with the calcium-dependent potassium channel blocker charybdotoxin (5×10−9 mol/L) for 45 minutes before methacholine administration. To standardize for any minor differences in the size of the tissue, we expressed contractile responses as a percentage of the maximal response to 40 mmol/L KCl. In some experiments, the vascular responses were investigated in precontracted vessels. Isolated rings of pulmonary arteries from normal and cholesterol-fed rabbits were suspended in organ baths as described above. Norepinephrine (10−7 mol/L) was added to precontract the vessels. Once a stable response was obtained, cumulative concentration-response curves to methacholine were measured. Results are expressed as the percent relaxation of the norepinephrine contraction.

Radioimmunoassay

Synthesis of TXA2 and PGI2 was compared in cholesterol-fed and normal rabbit pulmonary arteries. Segments of intrapulmonary arteries were obtained from both normal and cholesterol-fed rabbits and incubated for 30 minutes at 37°C in HEPES buffer containing methacholine (10−5 mol/L). Synthesis of TXB2, the stable metabolite of TXA2, and 6-keto-PGF1α, the stable metabolite of PGI2, was measured in the buffer by specific radioimmunoassays with the method of Campbell and Ojeda.26 The antibody for TXB2 and 6-keto-PGF1α was produced in rabbits. The sensitivity of the assay is 1 pg/0.3 mL for TXB2 and 5 pg/0.3 mL for 6-keto-PGF1α. The cross-reactivity of the antisera with known arachidonic acid metabolites is less than 0.1%.

NO Synthase Activity

Pulmonary arteries were obtained from cholesterol-fed rabbits and rabbits fed a standard rabbit chow as described above. The vessels were minced with a razor blade and washed with HEPES buffer. Vessels were lysed with a buffer (pH 7.4) of the following composition: 11 mmol/L HEPES, 350 mmol/L sucrose, 0.1 mmol/L EDTA, 1 mmol/L dithiothreitol, 10 μg/mL leupeptin, 2 μg/mL aprotinin, 10 μg/mL soybean trypsin inhibitor, 10 μg/mL phenylmethylsulfonyl fluoride, 1% NP-40, and 10% glycerol. Total protein content was determined in the extracts by the Bradford technique (Bio-Rad) with albumin as a standard. NO synthase activity in the pulmonary artery lysates was determined by monitoring the conversion of [3H]arginine to [3H]citrulline according to the method of Hevel and Marletta.27 For the NO synthase assay, samples were incubated in a 100-μL reaction volume of buffer containing 50 mmol/L Tris-HCl, 0.1 mmol/L EDTA, 0.1% mercaptoethanol, 100 μmol/L leupeptin, 1 mmol/L phenylmethylsulfonyl fluoride, 10 μg/mL soybean trypsin inhibitor, 2 μg/mL aprotinin, 10 nmol/L calmodulin, 1 mmol/L NADPH, 3 μmol/L tetrahydrobiopterin, 10 μmol/L l-arginine, 0.2 μCi [3H]l-arginine (66 Ci/mmol), 2.5 mmol/L calcium chloride, and 100 μg protein. Samples were incubated for 15 minutes at 37°C, and the reaction was stopped by addition of 1 mL stop buffer (20 mmol/L HEPES and 2 mmol/L EGTA at pH 5.5). In some samples, LNA (3×10−5 mol/L) was included. [3H]Citrulline was separated from [3H]arginine by applying the entire sample over a Dowex cation-exchange column (Na+ form) that had been preequilibrated with the stop buffer. [3H]Citrulline was eluted with distilled water and radioactivity determined with a liquid scintillation counter. NO synthase activity is expressed as femtomoles of [3H]citrulline produced per milligram protein per minute.

Nitrite/Nitrate Determination

For these experiments, isolated rings of pulmonary arteries from normal and cholesterol-fed rabbits were incubated with methacholine (10−5 mol/L) in HEPES buffer at 37°C for 60 minutes. Incubation buffer was saved and stored at −20°C until assayed. The production of nitrate (NO3−) and nitrite (NO2−) in the incubation buffer was determined by a QuickChem AE Automated Ion Analyzer (Lachat Instruments) with the method of Pratt et al.28 The samples were automatically injected into this two-channel detector; one channel is for NO2− and the other for NO3−. The NO3− channel has a cadmium column that reduces the NO3− to NO2−. The NO2− species is reacted with Greiss reagent to form an azo compound.29 The absorption of both channels is detected with a 520-nm filter. The minimal detectable concentration is 25 nmol/L. Standard curves are constructed from serial dilutions of sodium nitrate in HEPES buffer.

Statistical Analysis

Data are expressed as mean±SE. Statistical analysis of the data was performed with ANOVA to determine differences within groups and Student’s t test to determine differences between groups.

Materials

Norepinephrine, arachidonic acid, LNA, and methacholine were from Sigma Chemical Co. U46619 was obtained from Caymen Chemical. Charybdotoxin was from Research Biochemicals International. Unless otherwise specified, drugs were dissolved in distilled water such that volumes of 0.05 mL were added to the tissue baths. Charybdotoxin and LNA were dissolved in Krebs’ bicarbonate buffer. Arachidonic acid and U46619 were prepared in ethanol previously sparged with nitrogen. The stock solution and dilutions were made fresh for each experiment and were kept on ice under a nitrogen atmosphere. The final ethanol concentration of the bath was less than 0.07%. Control experiments indicated that ethanol vehicle had no effect on basal tone or on the response of the vasoactive compounds. Cholesterol was obtained from ICN Biomedicals.

Results

Gross examination of pulmonary arteries from normal and cholesterol-fed rabbits appeared similar, with no evidence of lipid accumulation, fatty streaks, or plaques. KCl-induced contractions in vessels from cholesterol-fed rabbits and normal rabbits did not differ (1.5±0.1 versus 1.3±0.1 g, respectively). Norepinephrine-induced contractions also did not differ in the two groups (maximal contractile response, 70±4% versus 65±5%, cholesterol-fed versus normal; data not shown). Methacholine elicited concentration-related contractions of pulmonary arteries from both cholesterol-fed and normal rabbits (Fig 1⇓). However, methacholine-induced contractions were decreased in cholesterol-fed rabbits (Fig 1⇓). We have previously shown that in pulmonary artery obtained from normal NZW rabbits, endothelium removal inhibited contractions to methacholine.19 In cholesterol-fed denuded pulmonary arteries, there was a similar inhibition of methacholine-induced contractions (data not shown).

Figure 1.
Figure 1.

Methacholine-induced contractions of normal and cholesterol-fed rabbit pulmonary arteries. Data are expressed as percent contractile response of maximal KCl contraction and are shown as mean±SE; n=24. *P<.05, cholesterol-fed vs normal.

We measured the effect of arachidonic acid on vascular responses in the two groups and found that contractions to arachidonic acid were not different in cholesterol-fed and normal rabbits (Fig 2A⇓). The TXA2 mimetic U46619 also produced contractions in cholesterol-fed rabbit pulmonary arteries that were similar to those in normal rabbits (Fig 2B⇓). Vessels from normal and cholesterol-fed rabbits were incubated with methacholine, and the release of TXB2 and 6-keto-PGF1α was measured. Results are shown in Table 1⇓ and indicated that under both basal and stimulated conditions, production of TXB2 and 6-keto-PGF1α was similar in both groups.

Figure 2.
Figure 2.

Arachidonic acid–induced (A) and U46619-induced (B) contractions of normal and cholesterol-fed rabbit pulmonary arteries. Data are expressed as percent contractile response of maximal KCl contraction and are shown as mean±SE; n=18.

View this table:
Table 1.

TXB2 and 6-Keto-PGF1α Production in Normal and Cholesterol-Fed Rabbit Pulmonary Arteries

The next experiments investigated relaxation responses in pulmonary arteries from cholesterol-fed and normal rabbits. In norepinephrine-precontracted vessels, methacholine caused only a small dilation response in normal NZW rabbits (Fig 3⇓; maximal relaxation, 27±5%). In contrast, in pulmonary arteries from cholesterol-fed rabbits, methacholine produced an enhanced relaxation response (Fig 3⇓; maximal relaxation, 44±4%) compared with that in normal rabbits. An additional experiment examined the effect of the specific NO synthase inhibitor LNA on methacholine-induced contractions in cholesterol-fed and normal NZW rabbit pulmonary arteries. In normal NZW rabbits, pretreatment with LNA had no effect on the vascular contractions observed with methacholine (Fig 4A⇓). LNA pretreatment enhanced methacholine-induced contractions in cholesterol-fed rabbits (Fig 4B⇓). The maximal contractile response to methacholine in the presence of LNA was 106±10% compared with 77±9% in the absence of LNA. Interestingly, pretreatment of pulmonary arteries with l-arginine for 45 minutes before LNA and methacholine administration did not block the enhanced response observed in the presence of LNA alone (Fig 4B⇓).

Figure 3.
Figure 3.

Methacholine-induced relaxations of normal and cholesterol-fed rabbit pulmonary arteries. Vessels were precontracted with norepinephrine (NE, 10−7 mol/L). Data are expressed as percent relaxation response of norepinephrine contraction and are shown as mean±SE; n=16. *P<.05, cholesterol-fed vs normal.

Figure 4.
Figure 4.

Effect of LNA (3×10−5 mol/L) on methacholine-induced contractions of normal (A) and cholesterol-fed (B) rabbit pulmonary arteries. The effect of l-arginine (10−4 mol/L) plus LNA is shown in B. Data are expressed as percent contractile response of maximal KCl contraction and are shown as mean±SE; n=24. *P<.01, control vs LNA treatment.

NO release was compared in vessels obtained from cholesterol-fed and normal NZW rabbits and stimulated with methacholine (10−5 mol/L). Cholesterol feeding had no effect on the production of total nitrite (NO2− plus NO3−) in pulmonary arteries compared with rabbits fed the standard rabbit chow (63.2±14.9 versus 60.5±20.4 nmol/mg). To further characterize NO synthase activity, we assayed protein extracts from cholesterol-fed and normal rabbit pulmonary arteries for the conversion of [3H]arginine to [3H]citrulline. Measurement of NO synthase activity again indicated that the activity in pulmonary artery homogenates from cholesterol-fed and normal NZW rabbits did not differ (Table 2⇓). The specificity for NO synthase activity was verified by the addition of the NO synthase inhibitor LNA to the protein extracts. Removal of calcium from the reaction buffer blocked NO synthase activity in both normal and cholesterol-fed rabbits.

View this table:
Table 2.

Nitric Oxide Synthase Activity in Normal and Cholesterol-Fed Rabbit Pulmonary Arteries

A final experiment examined the role of endothelium-derived hyperpolarizing factor in the decreased contractions observed in cholesterol-fed rabbits. Pretreatment with the calcium-dependent potassium channel blocker charybdotoxin produced a slight increase in methacholine-induced contractions in cholesterol-fed rabbits only at the highest concentration of methacholine (Fig 5⇓). Charybdotoxin had no effect on contractile responses in normal rabbits (data not shown).

Figure 5.
Figure 5.

Effect of charybdotoxin (10−4 mol/L) on methacholine-induced contractions of cholesterol-fed rabbit pulmonary arteries. Data are expressed as percent contractile response of maximal KCl contraction and are shown as mean±SE; n=16. *P<.05, control vs charybdotoxin treatment.

Discussion

Our previous studies in pulmonary arteries from rabbits maintained on a normal diet showed that arachidonic acid and methacholine elicited endothelium-dependent contractions that were mediated by the cyclooxygenase metabolite TXA2.19 In pulmonary arteries from cholesterol-fed rabbits, methacholine caused a decreased contractile response compared with that in arteries from normal NZW rabbits. When vessels were precontracted with norepinephrine, methacholine-induced relaxations were greater in the cholesterol-fed rabbits. We investigated a number of possible explanations for the observed responses.

The contractile response to methacholine in cholesterol-fed rabbits also depended on an intact endothelium. It is conceivable that the decreased response to methacholine was due to a decreased production of TXA2 by the cholesterol-fed rabbit pulmonary arteries. Production of TXB2, the stable metabolite of TXA2, did not differ in segments of pulmonary arteries obtained from cholesterol-fed and normal rabbits, either in the basal state or when the vessels were stimulated with methacholine. Because the endothelium is the source of the vasodilator prostaglandin PGI2, it was also possible that methacholine caused a greater release of PGI2 in the cholesterol-fed rabbits and that this vasodilator prostanoid contributed to the depressed contractile response. However, 6-keto-PGF1α production did not differ in vessels from the two groups of rabbits.

Another possible explanation for the observed reduced contractile responses could be an alteration in the vascular smooth muscle TXA2 receptor or postreceptor events in cholesterol-fed rabbits. Evidence exists that other receptors are altered by atherosclerosis.30 Studies have also shown that the vascular TXA2 receptor appears to be regulated by various experimental conditions.31 32 33 For example, administration of dexamethasone to rabbits causes a decrease in TXA2 receptor density in aortic membranes.32 Dexamethasone-treated rabbit aortas also exhibit decreased contractions to the TXA2 mimetic U46619. We have also shown that a subgroup of normal NZW rabbits lack the TXA2 vascular receptor and fail to contract to methacholine, arachidonic acid, or U46619.33 In the present study, although methacholine-induced contractions were less in cholesterol-fed rabbit pulmonary arteries, contractions to arachidonic acid and U46619 were unaltered. Thus, it seems unlikely that a decrease in vascular TXA2 receptors or postreceptor events explains the depressed contractile response to methacholine.

An alternative explanation for the decreased contractions to methacholine was suggested by the enhanced relaxations to methacholine in cholesterol-fed rabbit pulmonary arteries compared with arteries of normal rabbits. Methacholine releases the vasodilator NO34 and the vasoconstrictor endothelium-derived contracting factor or TXA2.19 In pulmonary arteries, the effect of methacholine in precontracted vessels is a small relaxation at low concentrations and vasoconstriction at higher concentrations. If precontracted pulmonary arteries from normal NZW rabbits were pretreated with a specific TXA2 receptor antagonist, the major response to the administration of methacholine was relaxation.33 These results indicate that NO-induced relaxation is unmasked by blockade of the vasoconstrictor TXA2. In the present study, it is possible that the pulmonary arteries from cholesterol-fed rabbits have greater relaxations to methacholine compared with those from normal rabbits because cholesterol-fed rabbit pulmonary arteries have an increased production of NO. As support for this hypothesis, the NO synthase inhibitor LNA enhanced the contractions in cholesterol-fed pulmonary arteries but had no effect on normal pulmonary arteries. A recent report by Verbeuren and coworkers35 showed a similar effect in aortas of atherosclerotic rabbits. In their study, rabbits were fed an elevated cholesterol diet for 30 weeks compared with our study, in which the rabbits received the diet for only 2 weeks. In both studies, the rabbits developed hypercholesterolemia; however, the rabbits from the study by Verbeuren et al developed atherosclerotic lesions. They also found that the NO synthase inhibitor enhanced basal tone and norepinephrine-induced contractions. No effect of the inhibitor was observed in age-matched control rabbit aortas.

It should be noted that other studies have reported that hypercholesterolemia and atherosclerosis impair endothelium-dependent relaxations.5 6 7 8 In vessels from cholesterol-fed rabbits, NO synthesis was not impaired.36 Instead, the decreased relaxation response was due to an increased inactivation of NO by superoxide radicals formed in the intima of atherosclerotic arteries.37 The present study also indicates that NO production was not altered in hypercholesterolemia. First, the production of nitrite/nitrate did not differ between cholesterol-fed and normal rabbits. Second, NO synthase activity was not different in cholesterol-fed pulmonary artery homogenates. The inability of l-arginine to reverse the effect of LNA on the enhancement of methacholine-induced contractions was unexpected. This result suggests that the effect of LNA in isolated blood vessels is unrelated to its effect to inhibit NO synthase.

Recent evidence in hypercholesterolemic rabbit carotid arteries showed that relaxations to acetylcholine are mediated by an endothelium-derived hyperpolarizing factor38 that appears to be distinct from NO. Although the identity of this factor is presently unknown, it is proposed that it opens potassium channels on vascular smooth muscle cells, causing hyperpolarization and relaxation. In the study by Najibi and coworkers,38 relaxations to acetylcholine were not impaired in carotid arteries from cholesterol-fed rabbits compared with those from normal rabbits. The calcium-dependent potassium channel blocker charybdotoxin reduced acetylcholine-induced relaxations in carotid arteries obtained from cholesterol-fed rabbits but had no effect on relaxations in normal rabbit carotid arteries. The authors concluded that although the relaxations in the cholesterol-fed and normal rabbit carotid arteries were similar, the response was mediated by different mechanisms. Our study investigated the role of endothelium-derived hyperpolarizing factor in the altered vascular responses to methacholine in cholesterol-fed rabbit pulmonary arteries. We found that charybdotoxin increased contractions in cholesterol-fed rabbit pulmonary arteries only at the highest concentration of methacholine. Although further characterization studies are necessary, our initial results do not indicate that endothelium-derived hyperpolarizing factor mediates the altered contractile responses in hypercholesterolemic blood vessels.

A limited number of studies have investigated vascular responses in pulmonary arteries from cholesterol-fed animals. The reason for this lack of studies is unclear because it is known that human pulmonary hypertension is often associated with pulmonary artery atherosclerotic lesions.23 24 25 However, Ibengwe and Suzuki39 did investigate pulmonary artery responses in rabbits fed a 1% cholesterol diet for 12 weeks. The authors reported enhanced contractions to KCl and decreased contractions to norepinephrine. Pulmonary arteries from the cholesterol-fed rabbits also had diminished relaxations to acetylcholine. Although these studies did not evaluate pathological changes in the pulmonary arteries, Zhu et al40 showed that rabbits fed a 0.3% cholesterol diet for 10 weeks had increased pulmonary artery lesions as assessed by Sudan IV staining. Therefore, despite limited studies in rabbit pulmonary artery, there is evidence that these vessels develop atherosclerotic lesions and have altered vascular responses. In all cases, however, rabbits were fed the cholesterol diet for a period long enough to produce atherosclerotic lesions. In contrast, our study investigated the early changes that occur in blood vessels with hypercholesterolemia before the development of the atherosclerotic lesion.

In conclusion, the present study demonstrates that hypercholesterolemia decreases pulmonary artery vascular contractions and increases relaxations to methacholine. This effect is not mediated by a decreased production of the endothelium-dependent constrictor TXA2 or by an increased production of the endothelium-derived relaxing factor NO. Our results also indicate that in hypercholesterolemic vessels, activation of potassium channels by endothelium-derived hyperpolarizing factor does not modulate vascular tone. It is well recognized that in humans the pulmonary circulation is not the primary vascular bed that develops atherosclerosis.41 However, it is also known that pulmonary arteries do become atherosclerotic even though the manifestation of the disease is less severe than what is observed in the coronary arteries.23 42 Interestingly, in pulmonary hypertension, either primary or secondary forms, atherosclerosis is much more pronounced and plaque formation is observed in both large- and small-diameter vessels.24 Therefore, this study gives important new information regarding the vascular changes that occur in pulmonary vessels during the development of atherosclerosis. Although the mechanism mediating the altered vascular responses is still unknown, the results from this study clearly indicate that the regulation of vascular tone is different in normal and hypercholesterolemic vessels.

Selected Abbreviations and Acronyms

6-keto-PGF1α=6-ketoprostaglandin F1α
LNA=nitro-l-arginine
NO=nitric oxide
NZW=New Zealand White
PGI2=prostaglandin I2 (prostacyclin)
TXA2, B2=thromboxane A2, B2

Acknowledgments

Support was provided by grants from the American Heart Association (No. 92009440) and National Heart, Lung, and Blood Institute (HL-37981). We thank Jennifer Yauck, Joseph James, and Donna Kotulock for skillful technical assistance and Gretchen Barg for excellent secretarial assistance.

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  • Reduced Pulmonary Artery Vasoconstriction to Methacholine in Cholesterol-Fed Rabbits
    Sandra L. Pfister and William B. Campbell
    Hypertension. 1996;27:804-810, originally published March 1, 1996
    https://doi.org/10.1161/01.HYP.27.3.804
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