Potentiation of nitroglycerin-induced coronary dilatation by N-acetylcysteine

Transcription

1 THERAPY AND PREVENTION PHARMACOLOGY Potentiation of nitroglycerin-induced coronary dilatation by N-acetylcysteine MICHAEL D. WINNIFORD, M.D., PATRICK L. KENNEDY, M.D., PETER J. WELLS, M.D., AND L. DAVID HILLIs, M.D. ABSTRACT Previous studies have suggested that (1) nitroglycerin causes vasodilatation by interacting with sulfhydryl groups in vascular smooth muscle, thereby activating guanylate cyclase and increasing the intracellular concentration of cyclic GMP, and (2) N-acetylcysteine, a source of sulfhydryl groups, potentiates the peripheral vasodilatory effect of nitroglycerin. This study was performed to explore the influence of N-acetylcysteine on nitroglycerin-induced coronary dilatation. In 18 patients (13 men and five women, 30 to 76 years old), coronary sinus blood flow (by thermodilution) was measured before and during intracoronary administration of nitroglycerin, 25,g, both before and 5 min after a 15 min intravenous infusion of (1) 5% dextrose in water (n = 8, control) or (2) 100 mg/kg N-acetylcysteine (n = 10). Nitroglycerin caused no change in heart rate or systemic arterial pressure. In the control patients, coronary sinus blood flow behaved similarly during the two injections: it was ml/min (mean ± SD) before and ml/min during injection No. 1 (average increase, ml/min; average percent increase, %); and it was 131 ± 34 ml/min before and ml/min during injection No. 2 (average increase, ml/min; average percent increase, %) (NS compared with injection 1). In the patients who received N-acetylcysteine, coronary sinus blood flow was ml/min before and ml/min during injection 1 (average increase, ml/min; average percent increase, %) (NS compared with eight control values). After N- acetylcysteine, coronary sinus blood flow was similar before nitroglycerin (145 ± 44 ml/min), but it rose markedly with nitroglycerin ( ml/min) (average increase, ml/min; average percent increase, 50 20%) (p <.01 compared with the values obtained in the same patients during injection 1). Thus, N-acetylcysteine, a source of sulfhydryl groups, potentiates the coronary vasodilative effect of nitroglycerin. Circulation 73, No. 1, , SEVERAL STUDIES have suggested that nitroglycerin causes vasodilatation by interacting with sulfhydryl groups in vascular smooth muscle, thereby activating guanylate cyclase and increasing the intracellular concentration of cyclic GMP. 1-3 According to this hypothesis, any agent that reduces sulfhydryl group availability may diminish the vasodilatory effect of nitroglycerin; in turn, any maneuver that increases sulfhydryl group availability may augment the magnitude of nitroglycerin-induced vasodilatation. In support of this, Needleman et al.' have shown that etha- From the Department of Internal Medicine (Cardiovascular Division), the University of Texas Health Science Center, Dallas. Supported in part by Ischemic SCOR grant HL from the National Institutes of Health, Bethesda, and by a grant from the Texas affiliate of the American Heart Association. Address for correspondence: Dr. L. David Hillis, Room L5.134, University of Texas Health Science Center, 5323 Harry Hines Blvd., Dallas, TX Received Aug. 6, 1985; revision accepted Sept. 26, Dr. Hillis is an Established Investigator of the American Heart Association, Dallas. 138 crynic acid (which alkylates sulfhydryl groups) inhibits nitrate-induced relaxation of rabbit aortic strips, and Horowitz et al.4 have demonstrated that N- acetylcysteine, a sulfhydryl group "donor," potentiates the peripheral hemodynamic responses to intravenous nitroglycerin. However, no previously published report has evaluated the influence of modulators of sulfhydryl group availability on the coronary vascular response to nitroglycerin. Therefore, the present study was performed to explore the effect of N-acetylcysteine on the vasodilative response to small amounts of intracoronary nitroglycerin. Materials and methods Patient population. The studies were performed in 18 patients (13 men and five women, 30 to 76 year old) undergoing cardiac catheterization at Parkland Memorial Hospital for the evaluation of chest pain. The protocol was approved by the Human Subjects Review Committee of the University of Texas Health Science Center, and all patients gave written informed consent. Antianginal medications (/3-adrenergic-blocking

2 agents, calcium antagonists, and long-acting nitrates) were discontinued more than 24 hr, and sublingual niroglycerin was not given for more than 4 hr before study. The experimental protocol was performed in subjects in the fasting state after premedication with 10 mg oral diazepam. All measurements were obtained before the introduction of iodinated contrast material. Experimental protocol. A No. 8F femoral arterial sheath was inserted percutaneously, through which a No. 7F Judkins catheter was advanced to the ostium of the left coronary artery. Its position was confirmed by the occurrence of transient T wave alterations during the injection of saline. Systemic arterial pressure was measured through the sheath's side-port extension, and heart rate was determined by electrocardiographic monitoring. A thermodilution catheter (model CCS-7U-90B, Wilton Webster Laboratories, Altadena, CA) was advanced to the coronary sinus from a basilic vein, and its position was verified fluoroscopically and oximetrically. Subsequent determinations of coronary sinus blood flow were performed by the thermodilution technique.5 After catheter placement, mean systemic arterial pressure, heart rate, and coronary sinus blood flow were recorded before, during, and for 30 sec after the intracoronary administration of 25 gg nitroglycerin (in 5 ml of solution). Then, each patient was assigned to receive an intravenous infusion of (1) 200 ml 5% dextrose in water or (2) 100 mg/kg N-acetylcysteine (kindly provided by Mead-Johnson Pharmaceuticals, Evansville, IN) in 200 ml 5% dextrose in water, given over 15 min. Five minutes after the infusion was completed, values for all variables again were recorded before, during, and for 30 sec after intracoronary 25 gg nitroglycerin in an identical manner to that followed 20 min previously. Thus, in each patient, the response of mean systemic arterial pressure, heart rate, and coronary sinus blood flow to intracoronary nitroglycerin was assessed before and after (1) 5% dextrose in water (n = 8, control) or (2) 100 mg/kg N-acetylcysteine (n = 10). Statistical analysis. All results are reported as mean + 1 SD. In each patient, the value for coronary sinus blood flow before nitroglycerin was subtracted from the value during nitroglycerin administration, yielding the net increase in coronary sinus blood flow (in ml/min). In turn, the net increase was divided by the value for coronary sinus blood flow before nitroglycerin to yield the percent increase. Changes occurring from before to after 5% dextrose in water or N-acetylcysteine within each group were evaluated with a paired t test.6 The two groups were compared with Student's t test.7 For all analyses, a p value <.05 was considered significant. Results Selective coronary arteriography (performed after the experimental protocol) revealed that three patients had no significant coronary artery disease (defined as.50% luminal diameter narrowing of the.large epicardial coronary arteries), six had one-vessel disease, seven had two-vessel disease, one had three-vessel involvement, and one had narrowing of the left main coronary artery (table 1). Six patients (three controls and three to whom N-acetylcysteine was given) received oral isosorbide dinitrate, 20 to 100 mg/day in divided doses, for more than 1 week before study, whereas the other 12 had not received a long-acting nitrate preparation for more than 1 month (table 1). Intracoronary nitroglycerin, 25,ug, caused no Vol. 73, No. 1, January 1986 THERAPY AND PREVENTION-PHARMACOLOGY change in heart rate or systemic arterial pressure. No patient noted angina during its administration. In the control patients, coronary sinus blood flow was m/min before and ml/min during the initial injection of nitroglycerin (average increase, ml/min; average percent increase, 38 ± 21%). Twenty minutes later, coronary sinus blood flow was ml/minute before and 178 ± 45 ml/min during nitroglycerin (average increase, 47 ± 23 ml/min; average percent increase, 37 ± 20%) (all NS compared with initial nitroglycerin administration) (table 1; figure 1). In those who received N-acetylcysteine, coronary sinus blood flow was mllmin before and ml/min during the initial nitroglycerin injection (average increase, ml/min; average percent increase, 30 ± 12%). After N-acetylcysteine was given, coronary sinus blood flow was ml/min before nitroglycerin (NS compared with the values obtained before the initial nitroglycerin injection). However, it rose markedly to 218 ± 68 ml/min during administration of nitroglycerin (average increase, mni/min; average percent increase, %) (p <.01 compared with the initial nitroglycerin injection in the same group) (table 1; figure 1). The response of coronary sinus blood flow to nitroglycerin after N-acetylcysteine was similar in the three patients (Nos. 9, 1 1, and 18) who received oral isosorbide dinitrate until the day before study (average percent increase, 52 ± 29%) and the seven patients who had not received a long-acting nitrate for more than 1 month (average percent increase, 50 ± 18%) (NS) (table 1). If the data from patients 11 and 16 (in whom N-acetylcysteine induced the greatest augmentation of coronary sinus blood flow with nitroglycerin) are excluded, those from the remaining eight patients demonstrate that N-acetylcysteine still potentiated nitroglycerin's vasodilative effect: the average increase in coronary sinus blood flow rose from 37 ± 11 ml/min before to 58 ± 17 ml/min after N-acetylcysteine (p <.01), and the average percent increase in coronary sinus blood flow increased from 27 ± 1 1% before to 42 ± 10% after N-acetylcysteine (p <.01) (table 1). N-acetylcysteine produced no adverse effects. Discussion Nitroglycerin relieves or prevents myocardial ischemia by relaxing the smooth muscle of arteries and veins. As a result, it (1) diminishes myocardial oxygen demand by dilating peripheral arteries and veins, with a resultant decline in left ventricular afterload and preload,>'0 and (2) augments myocardial oxygen sup- 139

3 WINNIFORD et al. TABLE 1 Individual responses to intracoronary nitroglycerin Baseline Patient CSBF CSBF No. Age Sex HR mean BP pre-tng with TNG ACSBF % T CSBF Infusion Control 1 30 F D5W 2 40 M D5W 3 47 M D5W 4 45 M D5W 5 58 M D5W 6 62 M D5W 7 47 M D5W 8 76 F D5W Mean SD N-Acetylcysteine 9 49 M NAC M NAC F NAC F NAC M NAC M NAC M NAC M NAC F NAC M NAC Mean SD HR = heart rate (beats/min); BP blood pressure (mm Hg); CSBF coronary sinus blood flow (ml/min); TNG = nitroglycerin; LAD - left anterior descending; LCx = left circumflex; RCA = right coronary artery; LM = left main coronary artery; D5W = 5% dextrose in water; NAC = N- acetylcysteine; ISDN = isosorbide dinitrate. Ap <.01; Bp <.05 compared with the same patients before NAC infusion. ply by dilating the epicardial coronary arteries' I-" and by increasing collateral and subendocardial blood flow."'" Although nitroglycerin is known to exert its salutary effect by dilating coronary and peripheral vessels, the manner in which it causes such vasodilatation is not well understood. The present study was performed to explore the mechanism of nitroglycerin-induced coronary dilatation. Several authors have provided data in support of the hypothesis that nitroglycerin causes vasodilatation by combining with available sulfhydryl groups in vascular smooth muscle, leading to the production of S- nitrosothiols. In turn, these compounds activate guanylate cyclase9' 20 and increase the intracellular concentration of cyclic GMP.2'1 Of the various S-nitrosothiols, S-nitroso-cysteine is extremely effective in activating guanylate cyclase. 19 Needleman et al. ' have shown that ethacrynic acid, a sulfhydryl alkylating agent (which reduces sulfhydryl group availability), diminishes the magnitude of smooth muscle relaxation caused by nitroglycerin, and Horowitz et al.4 have 140 demonstrated that N-acetylcysteine, a source of sulfhydryl groups, potentiates the peripheral vascular effects of nitroglycerin. Our data complement these observations, in that N-acetylcysteine potentiated the coronary vasodilative effect of a small intracoronary injection of nitroglycerin. N-Acetylcysteine is extensively hydrolyzed to cysteine in vivo.2' It is safe and effective in the emergency treatment of acetaminophen overdose.22 In the present study, it was administered at a dosage and rate similar to that previously described.4' 22 As noted in these studies, a total dose of 100 mg/kg, given over 15 min, caused no change in systemic arterial pressure or heart rate, and coronary sinus blood flow was not altered (table 1). This study has the following limitations. First, it assesses the effect of nitroglycerin on total coronary sinus blood flow. The thermodilution technique used in this study does not allow a measurement of regional myocardial blood flow, and, therefore, we are unable to speculate about the effect of N-acetylcysteine on

4 THERAPY AND PREVENTION-PHARMACOLOGY TABLE 1 (Continued) Repeat CSBF CSBF % Diameter narrowing ISDN dose HR mean BP pre-tng with TNG ACSBF % $ CSBF LAD LCx RCA LM (mg/day) B 73A 5OA nitroglycerin-induced alterations in regional flow. For example, this technique provides no information on nitroglycerin-induced changes in subendocardial and subepicardial flow, nor does it allow one to determine if nitroglycerin dilates the larger conductance vessels (on the epicardial surface), the smaller precapillary resistance vessels (within the myocardium), or both. It is possible that N-acetylcysteine potentiates the vasodilative effect of nitroglycerin only in normal segments of the coronary vasculature, producing no change or even a decrease in flow through diseased arterial segments. However, no patient noted angina while receiving nitroglycerin before or after N-acetylcysteine. Second, we evaluated the coronary vasodilative response to a single dose of nitroglycerin (25 gg), but we did not examine the effect of N-acetylcysteine with a range of doses of nitroglycerin. In the patients who manifested only modest N-acetylcysteine-induced potentiation of nitroglycerin's vasodilative effect (i.e., patients 9, 14, 17, 18), it is conceivable that different doses of nitroglycerin may have more clearly delineated N-acetylcysteine's ability to augment the effect of nitroglycerin. Third, although it is attractive to speculate that N-acetylcysteine potentiates the coronary vasodilative Vol. 73, No. 1, January 1986 effect of nitroglycerin by enhancing sulfhydryl group availability in vascular smooth muscle, our data do not allow us to define the mechanism of such N-acetylcysteine-induced potentiation. Nitroglycerin is metabolized both by the liver (via the enzyme hepatic glutathione organic nitrate reductase) as well as by arteries and veins As a result, NV-acetylcysteine's potentiation of nitroglycerin's coronary vasodilative effect may be caused by its ability to prevent or. at least slow the metabolism of nitroglycerin within the coronary vasculature. The findings of Horowitz et al.4 and of the present study suggest that sulfhydryl group availability in vascular smooth muscle may, at least in part, explain the substantial interpatient variability in the hemodynamic and clinical responses to nitrate therapy. Long-term exposure to long-acting nitrates may diminish sulfhydryl group availability, thereby attenuating the vasodilative effect of nitroglycerin. In the present study, previous treatment with isosorbide dinitrate did not systematically alter the magnitude of nitroglycerin-induced coronary dilatation or the degree to which N- acetylcysteine potentiated this effect. In addition, our data, in conjunction with various assessments, in vi- 141

5 WINNIFORD et al. 3: 0 Z11 w8 z 0DL Wa: U) 0-4Z 60 F k 80 r QL #1 #2 #1 #2 5% DEXTROSE IN WATER N-AC ETYLCYSTEINE FIGURE 1. The percent increase in coronary sinus blood flow caused by intracoronary nitroglycerin before (labeled 1) and after (2) a 15 min infusion of 5% dextrose in water (left; n = 8, controls) or N-acetylcysteine (right, n = 10). Each line represents the data from one patient, and the mean ± 1 SD is displayed on either side of each set of lines. In the control patients, nitroglycerin caused a similar increase in coronary sinus blood flow before and after 5% dextrose in water. In contrast, the 10 patients who received N-acetylcysteine manifested a greater response to nitroglycerin after N-acetylcysteine. *p <.01 compared with the values obtained before N-acetylcysteine. tro 13, 19, 20 may offer insight into the mechanism of nitrate tolerance. Thus, some patients may develop tolerance to nitrates because sulfhydryl group availability is diminished, thereby reducing the production of S-nitrosothiols and the subsequent activation of guanylate cyclase.5'26 If this hypothesis is correct, a sulfhydryl group source, such as N-acetylcysteine, may alleviate nitrate tolerance by restoring sulfhydryl group availability. These possibilities merit further study. We acknowledge the skilled technical assistance of Nancy Smith, Randy Christian, Debbie Nichols, Martha Solodyna, and Nayer El Ashram, M.D. References 1. Needleman P, Jakschik B, Johnson EM Jr: Sulfhydryl requirement for relaxation of vascular smooth muscle. J Pharmacol Exp Ther 187: 324, Kobayashi A, Suzuki Y, Kamikawa T, Hayashi H, Yamazaki N: The effects of nitroglycerin on cyclic nucleotides in the coronary artery in vivo. 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