Mechanism of nitroglycerin-induced coronary dilatation: Lack of relation to intracoronary thromboxane concentrations

Mechanism of Nitroglycerin-Induced Coronary Dilatation: Lack of Relation to Intracoronary Thromboxane Concentrations ROGER B. REHR, MD, JAY A. JACKSON, MD, MICHAEL D. WINNIFORD, MD, WILLIAM B. CAMPBELL, PhD, and L. DAVID HILLIS, MD

Intracoronary nitroglycerin (NTG) increases coronary blood flow and NTG inhibits thromboxane (Tx) A~, production and release. However, whether an alteration in TxA~, is the mechanism by which NTG increases coronary blood flow is not known. Coronary sinus (CS) blood flow (BF) (by thermodiluUon) and the concentration of TxB2 (the stable metabolite of TxA2) in CS blood were measured in 23 patients (16 men and 7 women, aged 26 to 65 years) with coronary artery disease before, during and after injection of normal saline solution (n = 5, control subjects) or NTG, 100/zg (n = 18), into the left coronary artery. In the 5 control subjects, saline solution caused no change in CSBF or the concentration of TxB~,in CS blood. Ten of the 18 patients to whom NTG was given had received no cyclooxygenase inhibitors for 10 days. In these patients, NTG caused a marked increase in CSBF (from 112 -I- 64

to 152 -I- 70 ml/min, p <0.01) but no consistent change in the concentration of TxB2 in CS blood (141 + 132 to 160 -I- 155 pg/ml, difference not significant [NS]). The remaining 8 patients to whom NTG was given received aspirin before the study. In these patients, NTG caused a marked increase in CSBF (from 111 -I- 39 to 180 -I- 63 ml/min, p <0.01), even though the concentration of TxB2 in CS blood (8 -I- 10 to 0 -I- 0 pg/ml, NS) was lower (p <0.05) than that in control subjects and patients not receiving aspirin. Thus, although NTG induces a marked increase in CSBF, it exerts no effect on the concentration of TxB2 in CS blood, and its influence on CSBF is not affected by inhibition of TxA2 production and release. Therefore, the coronary vasodilatory action of NTG is not mediated by alterations in intracoronary TxA2. (Am J Cardiol 1984;54:971-974)

Although nitroglycerin (NTG) is used to relieve or prevent myocardial ischemia, how it does so is not well understood. NTG diminishes myocardial oxygen demands by dilating peripheral arteries and veins, with a resultant decline in left ventricular afterload and preload.I-3 It also augments myocardial oxygen supply by dilating the epicardial coronary arteries4-s and by increasing collateral and subendocardial blood flow (BF). 9-II Both Mehta 12 and Schafer Iz and their coworkers suggested that N T G inhibitsthe production of

thromboxane (Tx) A2, the powerful vasoconstrictor and promoter of platelet aggregation produced by aggregating platelets. 14 One could speculate that the dilatory effect of NTG o n t h e coronary arteries is mediated by alterations in thromboxane production and release. Therefore, the present study was performed to assess the influence of intracoronary N T G o n coronary BF and simultaneous concentrations of TxB2 (the stable metabolite of TxA2) in coronary sinus (CS) blood, and to determine if the augmentation in coronary BF induced by intracoronary NTG is abolished or attenuated by aspirin, a cyclooxygenase inhibitor known to impair TxA2 production.

From the Departments of Internal Modlclne (Cardiovascular Division) and Pharmacology, the University of Texas Health Science Center, Dallas, Texas. This study was supported in part by Ischemlc SCOR Grants HL-17669 and HL-25471 from the National Institutes of Health, Bethesda, Maryland; by Research Career Development Award KO 4HL-00801 (Dr. Campbell) from the National Head, Lung, and Blood Institute, Bethesda, Maryland; and by a grant from the Texas affiliate of the American Heart Association, Austin, Texas. Manuscript received April 23, 1984; revised manuscript received June 20, 1984, accepted June 26, 1984. Address for reprints: L. David HIIIIs, MD, Room L5.134, University of Texas Health Science Center, 5323 Harry Hines Boulevard, Dallas, Texas 75235.

Methods Patients: The studies were performed in 23 patients (16 men and 7 women, aged 26 to 65 years) undergoing cardiac catheterization for the evaluation of chest pain. All patients gave informed consent. Nitrate treatment was discontinued 12 hours before the study. There was no attempt to control or alter other antianginal medications, but careful records were maintained of ~-adrenergic blocking agents and calcium an971

972

NITROGLYCERINAND INTRACORONARY THROMBOXANE

TABLE I

Clinical, Arterlographlc and Serologic Characteristics of Patients % Luminal Diameter Narrowing

Pt. No.

Concomitant Medications (mg/day)

LAD

LC

Coronary Sinus TxB2 (pg/ml)

Right

Before NTG

After NTG

Intracoronary Saline (Control Subjects)

1 2 3 4 5

Propranolol (80) Metoprolol (200) Metoprolol (100) Nlfedlplne (160) None Diltiazem (240)

90 99 50

75 99 0

75 90 100

28 225 114

38 217 109

100 99

0 90

0 50

190 14 114 84

199 23 117 80

Mean :J:SD Intracoronary NTG Without Aspirin Pretreatment

6 7 8 9 •10 11 12 13 14 15 Mean -I-SD

Diltiazem (360) Atenolol (100) None Propranolol (80) Nifedipine (60) None Propranolol (160) DIItiazem (270) Atenolol (100) Nifedipine (30) Atanolol (100) Verapamil (320) Propranoiol (160)

90 75 60 100

75 0 99 100

99 0 100 100

39 26 56 22

34 28 52 71

90 99

100 100

100 50

117 50

103 107

100

0

100

345

198

99 0 50

90 0 0

90 0 100

319 349 89 141 -1-132

492 417 100 160 -I-155

Intracoronary Nitroglycerin with Aspirin Pretreatment 16 17 18 19 20

21 22 23 Mean -I-SD

Nifedipine (30) None None Propranolol (120) Nifedipine (60) Propranolol (160) Diltiazem (180) Nifedipine (40) Diltiazem (120) None

75 90 99 99

90 100 25 60

100 100 25 25

2 32 0 1

0 10 0 7

75

90

100

6

16

50 99 99

0 90 0

0 100 25

5 12 6 8 -1-10

0 5 10 6 -I-6

LAD = left anterior descending; LC = left circumflex; NTG -- nitroglycerin; SD = standard deviation; TxB2 = thromboxane B2.

tagonists given within I week of the study (Table I). The experimental protocol was performed with the patient in the fasting state after premedication with oral diazepam, 10 mg. All hemodynamic measurements and blood sampling were performed before systemic administration of heparin and the introduction of iodinated contrast material. In 15 patients (9 men and 6 women, aged 48 ± 9 years [mean standard deviation]), the experimental protocol was performed at a time when they had not received cyclooxygenase inhibitors for 10 days. In 8 patients (7 men and I woman, aged 45 • 7 years), 650 mg of aspirin was administered orally 10 hours and again 2 to 3 hours before catheterization. Experimental protocol: In each patient, a femoral arterial cannula was inserted percutaneously to measure systemic arterial pressure, and heart rate was determined by electrocardiographic monitoring. A thermodilution catheter (model CCS-7U-90B, Wilton Webster Laboratories) was advanced to the CS from a basilic vein, and its position was verified oximetrically and fluoroscopically. Subsequent determinations of CSBF were performed by the thermodilution technique, 15 and coronary vascular resistance was calculated by dividing mean systemic arterial pressure by CSBF. A No. 7Fr Goodale-Lubin woven Dacron ® catheter was advanced from the femoral vein to the coronary sinus, and its position also was confn~med oximetrically and fluoroscopically. A Judkins

catheter was advanced from the femoral arteryto the ostium of the leftcoronary artery. After catheter placement was accomplished, systemic arterialpressure,heart rateand C S B F were measured; CS blood was obtained for measurement of TxB2. Blood sampling from C S was begun 15 to 20 seconds after the measurement of C S B F to ensure that the normal salinesolutionrequired for the measurement of flow had dissipatedcompletely from CS. After baselinerecordings and sampling, I of 2 agents was injected into the coronary artery.In 5 patients (none of w h o m received aspirinbefore the study),salinesolution (5 ml) was given, and these patients served as control subjects. In 18 patients (10 who had not received aspirinand 8 who had received it),100 #g of N T G (in 5-ml solution)was administered. Systemic arterialpressure, heart rate, and C S B F were recorded throughout the injectionand for 30 seconds thereafter, at which time CS blood again was obtained to measure TxB2. Thus, in each patient,systemic arterialpressure,heart rate, CSBF, and the concentration of TxB~ in CS blood were measured before and immediately after the intracoronary administration of normal salinesolution(n = 5) or N T G , 100 #g (n = 18, 10 of w h o m had not and 8 of w h o m had received aspirin). Chemical analyses: The blood specimens for TxB2 analysis were drawn into heparinized plasticsyringesand trans-

November 1, 1984 THE AMERICAN JOURNAL OF CARDIOLOGY Volume 54

T A B L E II

973

H e m o d y n a m l c and Serologic Responses to Intracoronary Saline Solution or Nitroglycerin Intracoronary Saline (Controls, n -- 5)

Variables

Baseline

HR (beats/min) Mean BP (mm Hg) CSBF (ml/mln) CVR (mm Hg/ml/min) CS TxB2 (pg/ml)

84 89 115 0.94 114

Intracoronary N T G ~ No ASA Pretreatment (n = 10)

Repeat

-I- 14 -I- 16 4- 50 -I- 0.47 4- 84

82 87 117 0.93 117

Baseline

-I- 15 4- 16 4- 54 -1- 0.49 -I- 80

76 94 112 1.04 141

Repeat

-I- 16 4- 9 4- 64 4- 0.47 4- 132

77 87 152 0.66 160

-1- 15 -t- 13 • 4- 70 t 4- 0.251" 4- 155

Intracoronary NTG--ASA Pretreatment (n = 8) Baseline 78 91 111 0.93 8

-I- 18 4- 15 -1- 39 -I- 0.44 4- 10 t

Repeat 79 87 180 0.55 6

-I- 17 4- 16" 4- 63 t 4- 0.271" 4- 6$

• p < 0.05, t p < 0.01 In comparison with baseline value. t p <0.05 in comparison with values In the other 2 groups. ASA = aspirin; BP = blood pressure; CS = coronary sinus; CSBF -- CS blood flow; CVR = coronary vascular resistance; HR = heart rate; other abbreviations as in Table I.

ferred quickly into iced 10-ml tubes that contained indomethacin (10/~g) and heparin (1,000 units). They were immediately centrifuged at 2,000 g for 15 minutes at 4°C. The supernatants were separated and stored at -20°C for subsequent analysis. TxB2 was measured by radioimmunoassay. 16 Statistical analyses: All results are reported as mean + standard deviation. Changes that occurred from baseline to repeat measurements within each group (control subjects, NTG without aspirin and NTG with aspirin) were evaluated using a paired t test. 17The 3 groups were compared with one another with an analysis of variance. For all analyses, a p value <0.05 was considered significant.

Results

Normal saline solution (control subjects): For the 5 subjects who received saline solution, the systemic arterial pressure, heart rate, CSBF, coronary vascular resistance, and the concentration of TxB2 in CS blood were similar at baseline and after administration of

300

saline solution (Tables I and II, Fig. 1 and 2). Table I provides detailed information on the patients' coronary anatomy and concomitant antianginal medications. Intracoronary nitroglycerin without aspirin pretreatment: For the 10 patients not pretreated with aspirin, NTG induced a modest decline in systemic arterial pressure, a dramatic increase in CSBF, and a marked decrease in coronary vascular resistance (Table II and Fig. 1 and 2). Although CSBF increased 46 ± 41%, the concentrations of TxB2 in CS blood were not consistently altered (Tables I and II). Intracoronary nitroglycerin with aspirin pretreatment: In the patients pretreated with aspirin, NTG induced a modest decrease in systemic arterial pressure, a marked increase in CSBF and, as a result, a substantial diminution in coronary vascular resistance (Table II, Fig. 1 and 2). CSBF increased 64 + 27%, not significantly different from that in the 10 patients who received NTG without aspirin pretreatment. Although pretreatment with aspirin exerted no effect on the NTG-induced augmentation of CSBF, it caused a substantial reduction (p <0.05) in the concentration of TxB2 in CS blood (Tables I and II).

g IL

g-

200

Z

~

7<

_

t00

~

1.0

8 o~:m

o

7<- o I B

I R SALINE (Controls)

I I B R NITROGLYCERIN (Without Aspirin)

I

I

B R NITROGLYCERIN ( With Aspirin )

FIGURE 1. Coronary sinus blood flow (ml/min) before (B) and after (R) intracoronary administration of saline solution (left), niVoglycarin without aspirin pretreatment (middle) or nitroglycerin with aspirin pretreatment (right). Each line represents the data from 1 patient, and the mean 4standard deviation is shown on either side of each set of lines. Coronary sinus blood flow increased significantly during intracoronary nitroglycerin, irrespective of whether aspirin was administered. ° p <0.01 compared with baseline value.

~

o

k:

"~ I

I

B

R SALINE (Controls)

I

I

B R B R NITROGLYCERIN NITROGLYCERIN (Without Aspirin) ( With Aspirin )

FIGURE 2. Coronary vascular resistance (mm Hg/ml/min) before (B) and after (R) intracoronary saline solution (left), nitroglycerin without aspirin pretreatment (middle) or nitroglycerin with aspirin pretreatment (right). Each line represents the data from 1 patient, and the mean -Istandard deviation is shown on either side of each set of lines. Coronary vascular resistance decreased significantly during intracoronary nitroglycerin irrespective of whether aspirin was given. ° p <0.01 compared with baseline value.

974

NITROGLYCERINAND INTRACORONARY THROMBOXANE

Discussion Previous studies have shown that the intracoronary administration of NTG causes a transient but marked increase in CSBF. is Other studies have shown that sublingual NTG diminishes the TxB2 concentration 12,13 in the blood of patients with atherosclerotic coronary artery disease. Therefore, it is attractive to speculate that the NTG-induced augmentation of CSBF is caused by this alteration in'TxA2 production and release. To test this hypothesis, we measured CSBF and concentrations of TxB2 (the stable metabolite of TxA2) in CS blood during the intracoronary administration of NTG. In response to NTG, CSBF increased and coronary vascular resistance decreased dramatically (Table II and Fig. 1 and 2), but there was no consistent change in the concentrations of TxB2 in CS blood (Tables I and II). Thus, NTG's coronary dilatory effect is not mediated by alterations in intracoronary TxA2. Both Levin 19 and Mehta 2° and their co-workers demonstrated that human endothelial cells exposed to NTG in vitro produce small amounts of prostacyclin (PGI2), a powerful endogenous vasodilator and inhibitor of platelet aggregation, 21,22 and other investigators have shown that NTG in vivo causes a small increase in CS2a and peripheral venous24 concentrations of PGI2's stable metabolite, 6 keto-PGFl~. However, it is unknown if the NTG-induced alterations in PGI2 production and release shown in these studies are of sufficient magnitude to exert a substantial vasodilatory effect. In the present study, we did not attempt to quantitate the concentrations of 6 keto-PGFl~ in CS blood, because there is concern among many investigators about the reliability of these measurements. 25 In the 8 patients to whom it was administered, aspirin was given in a dose sufficient to inhibit the production and release of PGI2 by the coronary artery endothelium, 26 but the coronary vasodilatory effect of NTG was neither abolished nor attenuated, suggesting that NTG does not exert its effect on the coronary arteries by stimulating PGI2 production and release. However, it is conceivable that only a small NTG-induced change in the concentration of PGI2 within the coronary arteries is needed to cause dilatation. Numerous studies have shown that NTG relieves or prevents myocardial ischemia through several mechanisms, including dilatation of peripheral veins and arteries, 1-3 an increase in collateral BF to areas of myocardium supplied by narrowed or occluded coronary arteries, zl a redistribution of BF from nonischemic to ischemic areas,9,1° and dilatation of coronary arteries. 4-8 In all probability, the antiischemic effect of NTG is the result of all these mechanisms, and it is difficult to determine the relative importance of'each one. In this study, we administered NTG into the left coronary artery to minimize its systemic hemodynamic effects and to maximize its concentration within the coronary vasculature. Although it caused a marked increase in CSBF, it induced no consistent alteration in the concentration of TxB2 in CS blood, and its influence on coronary BF was not affected by a dose of aspirin suf-

ficient to inhibit TxA2 production and release (Tables I and II). The other actions of NTG, such as its effects on peripheral veins and arteries, may be produced by alterations in the circulating concentrations of TxA2 and PGI2, but the slight reduction in systemic arterial pressure induced by NTG was not attenuated by aspirin pretreatment (Table II). Acknowledgment: v~e acknowledge the skilled technical assistance of Sarah Hawkins, Randy Christian, Nancy Smith, Dr. Nayer Elashram, Milton Brady, and Mary Beth Santowski. References 1. Warren SE, Francis GS. Nitroglycerin and nitrate esters. Am J Med 1978;65:53-62. 2. Abrams J. Nitroglycerin and long-acting nitrates. N Engl J Med 1980;302: 1234-1237. 3. Vatner SF, Hlgglns CB, MIIlard RW, Franklin D. Direct and'reflex effects of nitroglycerinon coronary and left venViculardynamics in consciousdogs. J Clin Invest 1972;51:2872-2882. 4. Feldman RL, Peplne CJ, Curry RC Jr, Conll CR. Coronaryarterial responses to graded doses of nitroglycerin. Am J Cardiol 1979;43:91-97. S. Brown BG, Bolson E, Petemen RB, Pierce CD, Dodge HT. The mechanisms of nitroglycerin action: stenosis vasodilatation as a major component of the drug response. Circulation 1981;64:1089-1097. 6. Feldman RL, Peplne CJ, Contl CR. Magnitudeof dilatationof large and small coronary arteries by nitroglycerin. Circulation 1981;64:324-333. 7. Feldman RL, Marx JD, Peplne CJ, Contl CR. Analysis of coronary responses to various doses of intracoronary nitroglycerin. Circulation 1982;66:321-327. 8. Hood WP Jr, Amends I, Simon R, LIcMlen PR. The effects of intracoronary nitroglycerin on left ventricular systolic and diastolic function in man. Circulation 1980;61:1098-1104. 9. Wlnbury MM, Howe BB, Weiss HR. Effect of nitroglycerin and dlpyrldamole on eplcardial and endocardlal oxygen tension--further evidence for redistribution of myocardial blood flow. J Pharmacol Exp Ther 1971;176: 184-199. 10. Swale JL, Parker JP, Mcl.lale PA, Greenfield JC Jr. Effects of nitroglycerin and propranolol on the distribution of transmural myocardial blood flow during Ischemla in the absence of hemodynamic changes in the unanesthetized dog. J Clin Invest 1979;63:947-953. 11. Mehta J, Peplne CJ. Effect of sublingual nitroglycerin on regional flow in patients with and withoutcoronarydisease. Circulation 1978;58:803-807. 12. Mehta J, MeMa P, Feldman RL, ConU CR. Influence of tachycardla stress and nitroglycerin on transcardlac tromboxane levels in coronary disease (abstr). Am J Cardiol 1982;49:902. 13. Schafer AI, Alexander RW, Handln RI. Inhibition of platelet function by organic nitrate vasodllators. Blood 1980;55:649-654. 14. Hamberg M, Svensson J, Samuelsson B. Thromboxanes: a new group of biologically active compoundsderived from prostaglendinendoperoxides. Proc Natl Acad Sci USA 1975;72:2994-2998. 15. Ganz W, Tamura K, Marcus HS, DOnoso R, Yoehlda S, Swan HJC. Measurement of coronary sinus bloodflow by continuousthermedllutlon in man. Circulation 1971;44:181-195. 16. Hlrsh PD, HIIlla LD, Campbell WB, Firth BG, Wlllerson Jr. Release of prostaglandins and thromboxane into the coronary circulation in patients with Ischemic heart disease. N Engl J Med 1981;304:685-691. 17. Zar JH. BIostatistlcal Analysis. Englewood Cliffs, NJ: Prentice-Hall, 1974:173-177. 18. Ganz W, Marcus HS. Failure of Intracoronary nitroglycerin to alleviate pacing-induced angina. Circulation 1972;46:880-889. 19. Levln RI, Jaffe EA, Weksler BB, Tack-Goldman K. Nitroglycerin stimulates synthesis of prostacyclln by cultured human endothelial cells. J CIIn Invest

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