Nifedipine prevents sympathetic vasoconstriction distal to severe coronary stenoses

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

Contribution of electromechanical coupling between Kv and Ca v1.2 channels to coronary dysfunction in obesity

Previous investigations indicate that diminished functional expression of voltage-dependent K(+) (KV) channels impairs control of coronary blood flow in obesity/metabolic syndrome. The goal of this investigation was to test the hypothesis that KV channels are electromechanically coupled to CaV1.2 channels and that coronary microvascular dysfunction in obesity is related to subsequent increases in CaV1.2 channel activity. Initial studies revealed that inhibition of KV channels with 4-aminopyridine (4AP, 0.3 mM) increased intracellular [Ca(2+)], contracted isolated coronary arterioles and decreased coronary reactive hyperemia. These effects were reversed by blockade of CaV1.2 channels. Further studies in chronically instrumented Ossabaw swine showed that inhibition of CaV1.2 channels with nifedipine (10 μg/kg, iv) had no effect on coronary blood flow at rest or during exercise in lean swine. However, inhibition of CaV1.2 channels significantly increased coronary blood flow, conductance, and the balance between coronary flow and metabolism in obese swine (P < 0.05). These changes were associated with a ~50 % increase in inward CaV1.2 current and elevations in expression of the pore-forming subunit (α1c) of CaV1.2 channels in coronary smooth muscle cells from obese swine. Taken together, these findings indicate that electromechanical coupling between KV and CaV1.2 channels is involved in the regulation of coronary vasomotor tone and that increases in CaV1.2 channel activity contribute to coronary microvascular dysfunction in the setting of obesity.

Cardioprotection by Ivabradine Through Heart Rate Reduction and Beyond

The present review summarizes the experimental and clinical evidence for the anti-ischemic action of the selective bradycardic agent ivabradine. Improvements in myocardial blood flow and contractile function during experimental myocardial ischemia and the clinical anti-anginal effect are largely mediated by heart rate reduction. However, a significant reduction in infarct size by ivabradine persists in the absence of heart rate reduction, and such protection can also be recruited when ivabradine is given only during early reperfusion. The mechanisms for such pleiotropic action of ivabradine remain to be resolved.

Design and synthesis of alkyl 7,7-dihalo-3-methyl-5-(nitrophenyl)-2-azabicyclo[4.1.0]hept-3-ene-4-carboxylates with calcium channel antagonist activity

A group of alkyl 7,7-dihalo-3-methyl-5-(2- or 3-nitrophenyl)-2-azabicyclo[4.1.0]hept-3-ene-4-carboxylates were prepared by reaction of dihalocarbenes (:CX(2), X=Br, Cl) with alkyl 2-methyl-4-(2- or 3-nitrophenyl)-1,4-dihydropyridine-3-carboxylates. In vitro calcium channel antagonist activities were determined using a guinea pig ileum longitudinal smooth muscle assay. The title compounds exhibited weaker CC antagonist activity (10(-5) to 10(-7)M range) than the reference drug nifedipine (1.4 x 10(-8)M). Structure-activity relationships showed that the position (ortho or meta) of the nitro-substituent on the C-5 phenyl ring, the size (van der Waal's radius for Br and Cl are 1.95 and 1.80A, respectively) and/or electronegativity (Cl>Br) of the C-7 geminal halogen atoms do not appear to have a significant effect on CC antagonist activity. In contrast, the effect of the alkyl ester substituent was more pronounced where compounds having a Me or Et alkyl ester group showed superior potency (IC(50) in the 10(-7)M range) relative to the reference drug nifedipine (IC(50)=1.40 x 10(-8)M). Replacement of a 2-methyl-3-methoxycarbonylvinyl moiety present in nifedipine by a bioisosteric geminal-dihalocyclopropyl moiety provided a novel class of calcium channel antagonists that do not exhibit any inotropic effect on guinea pig atria.

Design and synthesis of methyl 2-methyl-7,7-dihalo-5-phenyl-2-azabicyclo[4.1.0]hept-3-ene-4-carboxylates with calcium channel antagonist activity

A group of methyl 2-methyl-7,7-dihalo-5-(substituted-phenyl)-2-azabicyclo[4.1.0]hept-3-ene-4-carboxylates were prepared by reaction of dihalocarbenes (:CX2, X = Br, Cl) with methyl 1-methyl-4-(substituted-phenyl)-1,4-dihydropyridine-3-carboxylates. In vitro calcium channel antagonist activities were determined using a guinea pig ileum longitudinal smooth muscle assay. The title compounds exhibited weaker CC antagonist activity (10(-5)-10(-6)M range) than the reference drug nifedipine (1.4 x 10(-8)M). Structure-activity relationship studies showed that the position of a nitro substituent on the C-5 phenyl ring where the relative potency order was ortho > meta > para, and the size and/or electronegativity of the C-7 geminal-dihalo substituents (Br > Cl), were determinants of calcium channel antagonist activity. This class of compounds did not exhibit any inotropic effect on guinea pig left atria. A dihalocyclopropyl moiety is a potential bioisostere for the 2-methyl-3-methoxycarbonylvinyl moiety present in the calcium channel antagonist nifedipine.

Blood pressure elevation after phenylephrine infusion may adversely affect myocardial perfusion in patients with coronary artery disease

BACKGROUND

Although blood pressure is a major determinant of myocardial oxygen-demand, little information is currently available regarding the changes in blood pressure (BP) during myocardial ischemia. Since BP elevation may cause left ventricular (LV) wall stress and an increase in oxygen demand, infusion of an alpha-adrenergic agonist, such as phenylephrine (PH), may provoke changes in myocardial perfusion in coronary artery disease (CAD) patients. As the effects of BP changes alone on myocardial perfusion have never been assessed by thallium-201 (Tl) scintigraphy, we investigated the effects of BP elevation after PH infusion, in order to study the hypothesis that pressure loading alone without increases in heart rate, may provoke transient impairment of regional myocardial perfusion, in CAD patients.

PATIENTS AND METHODS

Forty-one (41) patients with angiographically documented CAD, aged 54+/-8 years, were included in our study. Each patient was given, without any complications, a PH (0.1 mg/ml) dose infused at a rate of 0.8 ml/mm, determined by a standardisation procedure and producing a mean blood pressure elevation of approximately 30% above baseline levels and a heart rate response to levels of no less than 50 bpm. One minute after the desired blood pressure and heart rate responses were reached, 2 mCi of Tl were injected and the PH infusion continued until the termination of the test. Tl scintigraphy was performed both 2 min after Tl injection and 4 h later, while the results were correlated to coronary angiography findings.

RESULTS

PH scintigraphy produced 152 total defects. The mean perfusion defect size (%) was 14+/-12 and was directly related to the number of diseased vessels, i.e., 2% for one-vessel disease, 15% for two-vessel disease and 25% for three-vessel disease (P<0.05). The lowest percentage Tl activity values were 56+/-14 and were inversely related to the number of diseased vessels (P<0.01). The mean Tl lung counts/pixel values were 25+/-8 while it increased as the number of diseased vessels increased (P<0.01). The mean lung/heart ratio values were 0.31+/-0.08 while it increased as the number of diseased vessels increased (P<0.01).

CONCLUSION

BP elevation after PH loading, produces a significant impairment of myocardial perfusion, that correlates well with the extend of angiographic findings.

[Anti-ischemic effect of verapamil within the scope of interventional recanalization]

We report on a 46-year-old male with unstable angina pectoris due to a total proximal occlusion of the left circumflex artery. At the side of a plaque rupture there was a thrombotic occlusion. With PTCA recanalization was possible, but a dissection occurred, therefore coronary stents were implanted. Besides an optimal morphological result and recurrent applications of nitroglycerin, the baseline blood flow velocity declined from initial 16 cm/s to 11 cm/s after PTCA down to 8.4 cm/s after stent implantation. Because peak flow velocity remained almost unchanged, the low baseline velocity ("slow flow phenomenon") did not lead to an impaired coronary flow velocity reserve (CFVR). Only after application of 1 mg verapamil, a sustained flow velocity on a higher baseline level was reached (17 cm/s), at the same time typical signs of ischemia in the ECG (ST-segment depressions) improved. Additionally, in the non-treated LAD there was an increase in coronary blood flow velocity from 10 cm/s up to 25 cm/s. The reduction in coronary blood flow velocity with increasing manipulations might be due to an impairment of the coronary microcirculations with increasing alpha-adrenergic vasoconstriction, a distribution of vasoactive agents and peripheral microembolizations. Only after administration of verapamil, a calcium channel blocker with non-specific anti-adrenergic effects, the slow flow was removed and ECG signs of ischemia improved. The blood flow velocity in the non-treated LAD was low at baseline and improved after verapamil. This phenomenon leads to the conclusion that mechanisms with vasoconstrictive effect are present in the whole coronary system, but these mechanisms are less pronounced in non-treated vessels.

Comparison of perioperative myocardial protection with nifedipine versus nifedipine and metoprolol in patients undergoing elective coronary artery bypass grafting

A randomized study was performed on 70 patients undergoing elective coronary bypass grafting to examine whether the combined infusion of the calcium channel blocker nifedipine (10 micrograms/kg per hour) and the beta 1-blocker metopropol (12 micrograms/kg per hour, n = 34) reduces the prevalence of perioperative myocardial ischemia and arrhythmias. The control group received nifedipine alone (n = 36). In both groups the infusion was started from the onset of extracorporal circulation and maintained over a period of 24 hours. Repeated 12-lead electrocardiographic and 3-channel Holter monitor recordings for 48 hours were used to define perioperative myocardial ischemia (transient ischemic event, myocardial infarction) and arrhythmias (sinus tachycardia, supraventricular tachycardia, atrial flutter/fibrillation, ventricular tachycardia). Hemodynamic parameters were repeatedly assessed for 24 hours and serum enzyme levels (creatine kinase, MB isoenzyme of creatine kinase) for up to 36 hours after the operation. The two groups did not differ significantly with respect to preoperative anamnestic and surgical data. No signs of perioperative myocardial infarction were detected in either group. However, a significantly lower incidence of transient ischemic episodes was observed in the nifedipine-metoprolol group than in the nifedipine group (3% vs 11%; p < 0.05). In addition, there was a tendency toward lower creatine kinase MB levels and peak values of creatine kinase and creatine kinase MB in the nifedipine-metoprolol group. With regard to perioperative arrhythmias, there was a significantly lower incidence of sinus tachycardia and atrial flutter/fibrillation in the nifedipine-metoprolol group (9% and 6%) than in the nifedipine group (33% and 27%, p < 0.05). In addition, postoperative heart rate was lower in the nifedipine-metoprolol group starting from the sixth hour after release of the aortic crossclamp (p < 0.05 and p < 0.01, respectively). No other hemodynamic parameters showed significant differences between the two groups and all returned to preoperative levels within 24 hours. In conclusion, perioperative application of nifedipine and metoprolol in patients undergoing elective coronary bypass grafting reduces the prevalence of perioperative myocardial ischemia and arrhythmias without significant negative inotropic effects. The combined infusion of the two drugs appears superior to nifedipine alone in preventing perioperative myocardial ischemia and reducing reperfusion-induced arrhythmias.

Neuronal control of coronary blood flow

Controversies on acetylcholine-induced increases or decreases in coronary blood flow arise from obvious species differences, the role of endothelium in mediating vascular smooth muscle responses, and the marked negative chronotropic and inotropic effects of acetylcholine. In man, there appears to be a predominant dilation of intact epicardial coronary arteries and a constriction of artherosclerotic segments. However, at present there is no evidence for a vagal initiation of myocardial ischemia. Coronary vascular beta-adrenergic receptors mediate dilation, but appear to be functionally insignificant during sympathetic activation. The beta-adrenergic mechanism contributing to myocardial ischemia are indirect, mediated by a tachycardia-related redistribution of blood flow away from the ischemic myocardium. alpha-Adrenergic receptors mediating epicardial coronary artery constriction in experimental studies appear not to be responsible for the initiation of ischemia in patients with angina at rest. However, alpha-adrenergic constriction of coronary resistance vessels resulting in the precipitation of post-stenotic myocardial ischemia was demonstrated in experimental studies and recently confirmed in patients with effort angina. Non-adrenergic, non-cholinergic neurotransmitters exist; however, their role in regulating coronary blood flow remains entirely unclear.

Local and neurohumoral control of coronary blood flow

The powerful local metabolic regulation adjusting coronary blood flow to myocardial oxygen consumption under normal conditions is beyond doubt. However, despite substantial experimental efforts the responsible mediators are still largely unknown. Adenosine, a purported mediator of local metabolic control of coronary blood flow, is probably only involved in transient flow adaptations, but not in steady-state coronary autoregulation. Even below the autoregulatory range a substantial vasodilator reserve persists. Recruitment of such vasodilator reserve results in improved regional myocardial blood flow and attenuated regional ischemic dysfunction. beta-adrenergic coronary dilation is of minor functional importance. alpha-adrenergic coronary constriction acts to attenuate increases in coronary blood flow during sympathetic activation under normal conditions, such that myocardial oxygen extraction increases to match the increased oxygen consumption. alpha-adrenergic coronary constriction remains operative in ischemic myocardium, thus precipitating or contributing to acute myocardial ischemia during sympathetic activation and exercise in experimental animals as well as in patients with stable angina. The vagal transmitter acetylcholine--upon exogenous intracoronary infusion--induces critical constriction of epicardial coronary arteries with endothelial dysfunction and atherosclerosis. However, a vagal initiation of coronary spasm or myocardial ischemia has not been documented so far. Similarly, peptide hormones/transmitters such as NPY, vasopressin, and angiotensin can induce myocardial ischemia upon exogenous administration. Their pathophysiological role in myocardial ischemia and reperfusion, however, remains to be established.

Nifedipine reduces the incidence of myocardial infarction and transient ischemia in patients undergoing coronary bypass grafting

A randomized study was performed on 104 patients undergoing elective coronary artery bypass grafting to examine whether the infusion of nifedipine (n = 53) reduces the incidence of perioperative myocardial ischemia and necrosis in the early postoperative period. Continuous hemodynamic and three-channel Holter monitoring was performed for 24 hours and serial assessment of serum enzymes and 12-lead electrocardiography were performed for 36 hours postoperatively. Nifedipine (minimum dose, 10 micrograms/kg/hr for 24 hours) was applied from the onset of extracorporal circulation. The control group (n = 51) received nitroglycerin (minimum dose, 1 micrograms/kg/min for 24 hours). Using the combined analyses of electrocardiography and Holter recordings, myocardial ischemia was defined as being either a transient ischemic event (TIE), transient coronary spasm (TCS), or myocardial infarction (MI). The two groups did not differ with respect to preoperative New York Heart Association classification, age, history of myocardial infarction, extracorporal circulation and aortic cross-clamp time, number of distal anastomoses, or systemic and pulmonary hemodynamics. The incidence of perioperative myocardial ischemia was substantially lower in the nifedipine than in the nitroglycerin group [TIE: three of 53 patients (6%) versus nine of 50 patients (18%), p less than 0.001; MI: two of 53 patients (4%) versus six of 50 patients (12%), p less than 0.001; and TCS: none of 53 patients (0%) versus two of 50 patients (4%), p = NS].(ABSTRACT TRUNCATED AT 250 WORDS)

Prevention of alpha-adrenergic coronary constriction by calcium-antagonists

This manuscript reviews the experimental evidence for a functional antagonism of Ca-antagonists against alpha-adrenoceptor-mediated increases in coronary vasomotor tone. In studies on anesthetized dogs, intravenous nifedipine effectively prevented the alpha 1-adrenoceptor-mediated increase in epicardial coronary resistance, as well as the increase in end-diastolic resistance mediated by both alpha 1- and alpha 2-adrenoceptors during cardiac sympathetic nerve stimulation. Both intracoronary and intravenous administration of nifedipine also prevented the alpha 2-adrenoceptor-mediated increase in coronary resistance distal to severe stenoses, as well as the resulting ischemic dysfunction and net lactate production during cardiac sympathetic nerve stimulation. Felodipine was equally effective as nifedipine in preventing an alpha 2-adrenoceptor-mediated increase in coronary resistance and the resulting contractile dysfunction distal to severe coronary stenoses. alpha 1- and alpha 2-Adrenergic coronary constriction also contribute to the severity of myocardial ischemia in conscious dogs during treadmill exercise. Again, nifedipine improved regional myocardial blood flow and attenuated regional contractile dysfunction during exercise-induced ischemia in conscious dogs with a chronic coronary stenosis. This beneficial effect of nifedipine was attributed to a recruitment of coronary dilator reserve and not to a reduction in heart rate or afterload. In conclusion, there is solid experimental evidence for a functional antagonism of Ca-antagonists against alpha-adrenergic coronary constriction and its contribution to myocardial ischemia.

Pain and myocardial ischemia: the role of sympathetic activation

In a first series, we tested whether the relative ischemia distal to a severe stenosis on the left circumflex coronary (CX) artery increases the activity of cardiac sympathetic (CS) nerves which, in turn, may result in a poststenotic vasoconstriction and an aggravation of ischemia. In 23 anesthetized, vagotomized dogs, an acute stenosis that reduced CX blood flow to 50% of control was produced and maintained for 20 min. The activity of postganglionic CS nerves increased by 23 +/- 4% within 20 min. In parallel, poststenotic coronary resistance increased from 0.48 +/- 0.03 (SEM) to 0.61 +/- 0.03 mm Hg.min.100 g/ml, resulting in a net lactate production after 15 min. The selective alpha 2-adrenoceptor antagonist rauwolscine (0.2 mg/kg i.v.; n = 6) and the calcium antagonist nifedipine (10 micrograms/kg i.v.; n = 6) prevented the progressive increase in poststenotic resistance and the net lactate production, but still permitted an increase in CS activity. Segmental anesthesia of CS nerves with epidural infiltration of procaine at segments C7-T6 (n = 6) prevented the sympathetic activation, the progressive increase in poststenotic resistance and the net lactate production. In six additional dogs with intact vagus nerves, CS activation and a concomitant increase in poststenotic resistance resulting in myocardial ischemia were also found. These data suggest a vicious cycle between poststenotic coronary vasoconstriction and CS activation, resulting in severe myocardial ischemia. In a second series, stimulation of high-threshold somatic afferents (= nociceptive stimulation: NCS) was used to cause reflex CS activation. The superficial peroneal nerve was electrically stimulated in 14 anesthetized, vagotomized dogs. With intact CX arteries, a 1 min stimulation resulted in a pronounced increase in CX blood flow and perfusion pressure. In contrast, NCS in the presence of a severe stenosis on the CX artery increased end-diastolic poststenotic coronary resistance by 96 +/- 15% due to a reflex activation of CS nerve fibers. This activation was markedly reduced after injection of fentanyl (27 micrograms/kg i.v.; n = 6). Injection of naloxone (60 micrograms/kg) restored the original effect. Systolic wall thickening (WT; sonomicrometry) in the CX artery-perfused myocardium was increased during NCS (10.9 +/- 3.9 (SD) vs. 13.6 +/- 5.0%) in additional five dogs with intact coronary arteries. In the presence of a stenosis on the CX artery, systolic WT was reduced to 7.0 +/- 2.5% and was further decreased to 4.6 +/- 2.3% during NCS. The additional deterioration of systolic regional function during NCS was prevented after i.v. injection of fentanyl, as was the increase in poststenotic coronary resistance.(ABSTRACT TRUNCATED AT 400 WORDS)

Felodipine prevents the poststenotic myocardial ischemia induced by alpha 2-adrenergic coronary constriction

Alpha 2-adrenoceptor-mediated coronary constriction contributes to the precipitation of myocardial ischemia during sympathetic activation. Felodipine is a novel dihydropyridine calcium-channel antagonist with vascular selectivity. In this study, the effect of felodipine on alpha 2-adrenoceptor-mediated poststenotic coronary constriction was investigated. In ten open-chest dogs, the selective alpha 2-adrenoceptor agonist BHT 933 (200 micrograms IC) was infused before and after production of a severe stenosis on the left circumflex coronary artery. BHT 933 increased calculated resistance of the intact left circumflex coronary artery from 1.16 +/- 0.30 (SD) to 2.00 +/- 0.70 mmHg*min*100 g/ml (p less than 0.05) without changing posterior systolic wall thickening (sonomicrometry) (14.2 +/- 2.8% vs. 14.1 +/- 2.7%). In the presence of a severe stenosis, BHT 933 increased poststenotic coronary resistance from 1.59 +/- 0.54 to 2.88 +/- 1.16 mmHg*min*100 g/ml (p less than 0.05) and decreased posterior systolic wall thickening from 11.9 +/- 2.7% to 8.2 +/- 3.1% (p less than 0.05). In contrast, after intravenous pretreatment with felodipine (4 micrograms/kg), intracoronary infusion of BHT 933 did not change coronary resistance (1.69 +/- 0.61 vs. 1.61 +/- 0.64 mmHg*min*100 g/ml) and posterior systolic wall thickening (12.1 +/- 3.0% vs. 12.6 +/- 2.9%). In conclusion, felodipine prevents alpha 2-adrenoceptor-mediated coronary constriction and ischemic regional myocardial dysfunction distal to a severe coronary stenosis.

Vasodilatory effects of nisoldipine on coronary arteries--correlation with plasma levels

Vasomotion of angiographically normal and stenotic epicardial coronary arteries was analyzed up to 15 minutes after the onset of an intravenous infusion (4 minutes) of 0.5 mg (13 patients, group A) or 1 mg nisoldipine (13 patients, group B). After both doses the maximal increase of the mean diameters of normal coronary segments was achieved not before the 15th minute, averaging 11 +/- 6% in group A (p less than 0.001) and 18 +/- 9% in group B (p less than 0.001). Eleven of 15 and 8 of 9 coronary stenoses in groups A and B dilated to 5-80% and 15-70%, respectively. The nisoldipine concentration reached maximal levels at the end of the infusion (fourth minute) with an average of 8 +/- 4 ng/ml and 17 +/- 7 ng/ml in groups A and B, respectively. A significant correlation between nisoldipine plasma levels and dilation of normal coronary segments was obtained only with the individual maxima of these parameters and only in group A (p less than 0.01). The hysteresis of the coronary dilation in relation to the drug plasma levels may be due to the high receptor affinity of nisoldipine. In either group nisoldipine provoked a persistent increase in coronary sinus oxygen saturation (p less than 0.01) and a substantial and prolonged drop in systolic and diastolic aortic pressure (p less than 0.001). Both doses of nisoldipine induced a rise in heart rate (p less than 0.01) and a slight drop in the rate-pressure product (p less than 0.05).

Alpha-adrenergic mechanisms in myocardial ischemia

alpha-Adrenoceptor-mediated effects of sympathetic activation on the heart and coronary circulation are reviewed with emphasis on the pathophysiology of myocardial ischemia. A classification of alpha-adrenoceptor subtypes is presented, and the effects of alpha-adrenoceptor activation on presynaptic sympathetic nerve terminals, cardiomyocytes, endothelium, platelets, and coronary smooth muscle cells are discussed. alpha-Adrenergic coronary vasoconstriction at rest and during situations of sympathetic activation such as exercise and excitement is analyzed for the segmental, transmural, and regional distribution of coronary blood flow. Evidence for a significant contribution of alpha-adrenergic coronary vasoconstriction to experimental and clinical myocardial ischemia is provided. Cardiomyocyte alpha-adrenoceptor activation may be involved in ischemic and reperfusion arrhythmias. The participation of presynaptic and postsynaptic alpha-adrenoceptors, as well as of alpha 1- and alpha 2-adrenoceptors, in experimental and clinical myocardial ischemia will require further investigation.

Infusion of nifedipine after coronary artery bypass grafting decreases the incidence of early postoperative myocardial ischemia

We performed a randomized study on patients undergoing elective coronary bypass grafting to examine whether postoperative infusion of nifedipine (n = 25) could reduce the incidence of isolated transient myocardial ischemia, myocardial infarction, or both. The control group (n = 25) received nitroglycerin. Hemodynamic and Holter monitoring and serial assessment of enzymatic and electrocardiographic changes were performed for all patients. Both groups showed comparable preoperative and operative data. The incidence of myocardial infarction was significantly lower in the nifedipine group (n = 1) as compared with the control group (n = 4), whereas the number of patients with isolated transient myocardial ischemia was similar in both groups (nifedipine, 3; control, 4). At the time of peak activity, levels of creatine kinase (350 +/- 129 versus 511 +/- 287 IU/mL), creatine kinase-MB (8.4 +/- 5.4 versus 17.1 +/- 11.0 IU/mL), and glutamate-oxaloacetate-transaminase (30.4 +/- 4.4 versus 41.0 +/- 7.9 IU/mL) were markedly lower in the nifedipine group (p less than 0.05). We conclude that infusion of nifedipine after elective coronary artery bypass grafting effectively decreases the incidence of myocardial infarction and the extent of myocardial necrosis during the early postoperative period.

Pharmacology of acute effort angina

From the pharmacologic point of view, each of the major types of antianginal agents--calcium antagonists, beta-blockers, and nitrates--seem to act at least in part by an improvement of the myocardial blood supply. The recently elucidated mechanism of action of nitrates, acting on a common pathway with the endothelium-derived relaxation factor (EDRF), suggests an important role for guanylate cyclase and cyclic GMP in maintaining coronary artery patency in patients with coronary atheroma. The efficacy of calcium antagonists, even in effort-induced angina, is in accord with a current hypothesis that physical exercise in the presence of coronary stenosis can cause relative coronary vasoconstriction, or at the least, failure of full dilation. Therefore, calcium antagonists all act, at least in part, on the "supply" side of the supply-demand equation. Beta-adrenergic blockers appear to have as their major mode of action a reduction of heart rate, which not only reduces the oxygen demand but, through an anti-ischemic effect, also appears to improve the endocardial blood supply (in relation to the heart rate). Thus beta-blockade indirectly enhances the supply side of the equation. The intriguing situation arises whereby all three major types of antianginal compounds may also act by a common mechanism of anginal relief, namely, improvement in the coronary blood supply, in addition to the diverse mechanisms specific to each type of compound. That conclusion does not mean the the "demand" side of the equation can be ignored. Rather, the critical importance of a reduced myocardial blood supply in the production of anginal syndromes is highlighted.

Mechanisms of regional ischemia and antianginal drug action during exercise

Several mechanisms involved in the production of regional exercise-induced ischemia are described. Each offers the potential for modification using different types of antianginal drugs operating to alter regional O2 demands, improve regional perfusion, or both, leading to reduced ischemia and increased contractile function in the ischemic zone. Evidence is presented for matching of regional subendocardial myocardial blood flow and flow per beat with regional myocardial contraction at various levels of ischemia at rest, during steady-state exercise, and after antianginal drugs, signifying a particularly important role for heart rate control. In addition to reducing myocardial O2 demand per minute, beta-blockers and bradycardic drugs cause improvement of absolute subendocardial blood flow and particularly flow per beat by producing vasoconstriction in the epicardial region of the ischemic zone, with improvement of transmural blood flow distribution. Vasodilator drugs can act at several locations to increase regional blood flow and also to decrease O2 demands. A recruitable vasodilator reserve has been shown to exist during exercise-induced ischemia either in native resistance vessels, collateral channels, or both, which appears to be due at least in part to reduction of increased alpha-adrenergic constrictor tone to the coronary vessels during exercise, even in the presence of severe ischemia. The potential for additive effects using combinations of bradycardic and vasodilating agents are described within a framework relating regional subendocardial blood flow to regional systolic contraction. The experimental findings described suggest some potential new directions for antianginal therapy and, along with recent clinical observations, support the use of combinations of antianginal agents that act by different mechanisms.

Pharmacological mechanisms to attenuate sympathetically induced myocardial ischemia

Distal to a coronary stenosis, resting myocardial blood flow and function can be maintained by a compensatory dilation of the poststenotic vascular bed and an increased collateral blood flow from adjacent coronary vessels. Under this condition, electrical stimulation of cardiac sympathetic nerves, as well as their activation during sympathoexcitatory reflexes and exercise, induces a poststenotic alpha 2-adrenoceptor-mediated coronary constriction and a beta-adrenoceptor-mediated, tachycardia-related redistribution of blood flow away from the ischemia myocardium. Thus, activation of cardiac sympathetic nerves can precipitate poststenotic myocardial ischemia. In experimental studies in anesthetized, vagotomized dogs, as well as in conscious, chronically instrumented dogs, selective alpha 2-adrenoceptor antagonists and calcium-channel blockade with nifedipine were able to attenuate the sympathetically induced poststenotic myocardial ischemia. Beta-adrenoceptor blockade with atenolol was only proven beneficial as long as there was a heart-rate reduction. Conversely, a specific bradycardic agent (ULFS-49) also exerted beneficial effects. Myocardial ischemia can activate cardiac sympathetic afferents and then, by a spinal reflex, can in turn activate sympathetic efferents and aggravate the severity of myocardial ischemia. This vicious cycle could be interrupted by segmental epidural anesthesia with procaine as well as by blockade of sympathoexcitation at the central nervous level with clonidine in anesthetized dogs.

Effects of gallopamil, diltiazem and nifedipine on the loss of K+ from ischaemic pig hearts

K+ release into the extracellular space was investigated during repeated 6-min coronary occlusions before and after the intravenous administration of cardiovascular active doses of gallopamil (0.02; 0.05 mg/kg), diltiazem (1.0; 2.0 mg/kg) or nifedipine (0.01; 0.05 mg/kg) to anaesthetized pigs. [K+]e was measured epicardially using silver valinomycin electrodes calibrated in vivo. During control occlusions [K+]e- rose steeply in all groups, from a pre-ischaemic baseline value of about 3.5 mmol/l reaching a plateau value within the ischaemic period. This response was reproducible in an untreated control group. Gallopamil reduced the ischaemic K+ efflux dose dependently and significantly 10 min after injection; the higher dose also did 60 min after injection. Diltiazem had less effect on K+ efflux 10 min after administration and an effect was no longer detectable after 60 min. Nifedipine did not significantly inhibit the ischaemic K+ loss. Besides these differences in the direct protection of the ischaemic myocardium, the Ca2+ antagonists also had the following effects on the haemodynamic profile. Diltiazem and gallopamil significantly prolonged PQ intervals whereas nifedipine caused a shortening accompanied by a significant increase in heart rate. Blood pressure and LV dP/dtmax were significantly reduced by all compounds, but to a different degree. Diltiazem reduced blood pressure to a greater extent than did nifedipine and gallopamil. LV dP/dtmax was comparably reduced by gallopamil and diltiazem, while nifedipine had less effect. Thus, gallopamil exerted pronounced protective effects on the ischaemic pig heart.

Coronary collateral reserve during exercise induced ischemia in swine

We determined coronary collateral vasodilator reserve during exercise-induced ischemia in 17 mini-swine. We induced coronary collateral development in the left circumflex bed by placing an ameroid occluder on that artery. Four weeks later we studied the animals at rest and during exercise (EX) eliciting heart rates (HR) of 240 and 265 beats/min. We measured myocardial blood flow with microspheres and myocardial function by wall thickness sonomicrometry gauges. At matched exercise HRs we treated the animals with nifedipine (10 micrograms/kg IV) (EXN 10), nifedipine (100 micrograms/kg IV), (EXN 100), and adenosine infusion (1.2 mg/min/kg) EXAD. EXN 10 did not significantly alter hemodynamics compared to EX but EXN 100 and EXAD both decreased blood pressure significantly (p less than 0.05). Ischemic endocardial/nonischemic endocardial flow ratios and collateral resistance served as indices of vasodilator reserve. In the ischemic zone exercise reduced vasodilator reserve to 24 +/- 3% in the endocardium and 64 +/- 7% in the epicardium. Neither EXN 10 nor EXAD improved exercise-induced ischemia measured either as flow or function. However EXN 100 improved function during exercise-induced ischemia without improving coronary collateral flow. We conclude there is no additional coronary flow reserve during exercise-induced ischemia in the collateral dependent bed of the pig a few days after occlusion that can be recruited. Large doses of nifedipine improve function by direct action on the myocardium or by reducing afterload. The lack of development and deep myocardial distribution of the coronary collateral vessels in the pig may be an important factor of why these nifedipine responses differ from those reported in species which have primarily large epicardial coronary collaterals.

Ventricular fibrillation induced by the interaction between acute myocardial ischemia and sympathetic hyperactivity: effect of nifedipine

Sympathetic hyperactivity plays a major role in the genesis of malignant arrhythmias during acute myocardial ischemia. An experimental model in which life-threatening arrhythmias are specifically and consistently induced by the interaction between acute myocardial ischemia and left stellate ganglion stimulation has been developed in alpha-chloralose anesthetized cats. In this preparation, drugs that share antiischemic, antiadrenergic, and specific electrophysiologic effects, such as verapamil, diltiazem, and amiodarone, were most effective. To evaluate the relative role of these different properties in mediating the effect of antiarrhythmic drugs, we used this same model to test nifedipine, a calcium channel blocker that is able to counteract the consequences of sympathetic stimulation on coronary circulation but has no electrophysiologic properties at concentrations relevant in the clinical setting. Nifedipine (15 micrograms/kg) prevented the occurrence of ventricular fibrillation in 10 of 13 animals (77%). Its efficacy was independent of changes in the peripheral hemodynamics. Plasma concentrations of nifedipine were within the therapeutic range in humans. To evaluate if this rather striking protective effect was specifically related to the prevention of the deleterious consequences of sympathetic stimulation, the effect of nifedipine on ventricular fibrillation threshold was studied in an additional group of 13 cats in the nonischemic state, during acute myocardial ischemia and during ischemia plus sympathetic stimulation. Nifedipine did not modify ventricular fibrillation threshold in nonischemic or in ischemic conditions. However, nifedipine specifically prevented the further reduction in ventricular fibrillation threshold occurring when sympathetic stimulation was superimposed on acute ischemia. These data suggest that the extension of ischemic damage by sympathetic stimulation is an important progenitor of arrhythmogenic action during acute ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Nisoldipine and perfusion of post-stenotic myocardium in conscious pigs with different degrees of concentric stenosis

1. The effects of oral nisoldipine on the perfusion and wall function of a myocardial segment distal to a fixed coronary artery stenosis were studied in 2 groups of conscious pigs with different degrees of stenosis. In group 1 (n = 8) systolic wall thickening (SWT) of the post-stenotic segment was more than 15% (27 +/- 4%); in group 2 (n = 7) SWT was less than 10% (7 +/- 1%). 2. The systemic haemodynamic profiles at baseline and during nisoldipine were similar in both groups. Dose-titrations of nisoldipine (0.24 +/- 0.02 mg kg-1 and 0.47 +/- 0.04 mg kg-1) were performed to obtain increases in heart rate of 25% and 50%, respectively. These increases were accompanied by increases in cardiac output (up to 50%) and left ventricular (LV)dP/dt max (60%), while systemic vascular resistance (35%) and mean arterial blood pressure (10%) were reduced. Left ventricular systolic and end-diastolic blood pressure and stroke volume were not affected. 3. In both groups, nisoldipine caused increases in blood flow to the non-stenotic area which favoured the subepicardium more than the subendocardium. Blood flow to the post-stenotic area of group 1 was normal at baseline and was only slightly enhanced (preferentially to the subepicardium) by nisoldipine. In the post-stenotic area of group 2 transmural and subendocardial blood flow were lower at baseline compared to the control area. Nisoldipine did not affect subepicardial blood flow but reduced subendocardial blood flow. 4. In spite of the reflex-mediated positive chronotropic actions of nisoldipine, the acute poststenotic systolic wall thickening was not affected by nisoldipine in either group. 5. We conclude that, under the experimental conditions employed (concentric stenosis, no coronary collaterals and acute drug administration), nisoldipine does not have a useful effect on post-stenotic myocardial blood flow, particularly in animals with severe stenosis. In view of a possible resetting of the baroreceptors (subsiding of the tachycardia) with chronic treatment and the presence of eccentric stenosis in many patients, additional studies are warranted.

Role of calcium channel blockers in experimental exercise-induced ischemia

Calcium channel blockers, which induce vasodilation by relaxing vascular smooth muscle cells, have proven effective in the treatment of angina pectoris. To study mechanisms of calcium blockade in ischemic heart disease, conscious chronically instrumented dogs with a single coronary artery ameroid constrictor were studied during steady-state treadmill runs which induced regional myocardial ischemia. During exercise-induced ischemia, regional systolic wall thickening and subendocardial blood flow were both significantly reduced in the ischemic zone. Calcium channel blockade with verapamil, diltiazem, or nifedipine enhanced regional systolic wall thickening. Regional subendocardial blood flow in the ischemic region, measured during diltiazem and nifedipine experiments, improved during exercise. Reduced coronary artery resistance in the native vessels and/or recruitment of collaterals appears to largely explain the increased total myocardial blood supply in the jeopardized area and the increased function. However, after diltiazem, reduced exercise heart rates as well as reduced left ventricular end-diastolic pressure also contributed to the improvement in the oxygen-supply imbalance in the ischemic myocardium. These data provide a basis for understanding the efficacy of calcium channel blocker treatment in patients with coronary artery disease.

Mechanism of beneficial effect of beta-adrenergic blockade on exercise-induced myocardial ischemia in conscious dogs

We examined the importance of decreased heart rate in the beneficial effect of beta-adrenergic blockade on exercise-induced regional myocardial ischemia and contractile dysfunction in conscious dogs with single vessel coronary stenosis (ameroid constrictor). Studies were performed during control treadmill exercise, which produced regional myocardial ischemia (blood flow measured with microspheres) and wall dysfunction (measured using sonomicrometers). A second run was performed after the administration of atenolol (0.3-1.0 mg/kg i.v.), and the reduced heart rate caused by atenolol during early steady-state running was then prevented by atrial pacing during the latter portion of the run. Atenolol reduced the exercise heart rate from 217 +/- 25 beats per minute (SD, n = 9) to 166 +/- 15, and ischemic zone wall thickening during systole improved from 27 +/- 22% of the resting value in the control run to 50 +/- 25% of the resting value in the atenolol run (p less than 0.01). Atrial pacing then increased heart rate to 217 +/- 23 beats per minute, and regional wall thickening deteriorated to 15 +/- 25% of the resting value. Regional subendocardial blood flow in the ischemic zone during atrial pacing with atenolol was slightly less than that observed in the control run, in both ischemic and control zones, indicating no remaining beneficial effect of atenolol when heart rate reduction was eliminated. We conclude that the only significant mechanism for the improvement in exercise-induced ischemia and wall motion produced by atenolol is a reduction in the exercise heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Elimination of exercise-induced regional myocardial dysfunction by a bradycardiac agent in dogs with chronic coronary stenosis

We have previously demonstrated that the beneficial effect of cardioselective beta-blockade on exercise-induced ischemia is due entirely to negative chronotropism. Therefore we studied the effect of a new bradycardiac agent (UL-FS 49) in 10 dogs with chronic coronary artery stenosis produced by an ameroid constrictor. Regional myocardial function (sonomicrometers, wall thickness) and blood flow (microspheres) were measured during a control treadmill exercise bout and an identical run 3 hr later after the administration of UL-FS 49 (1.0 mg/kg iv). In the control run, heart rate increased from 114 +/- 20 to 230 +/- 19 beats/min and systolic wall thickening (%WT) in the poststenotic myocardium decreased from 23.3 +/- 5.2% at rest to 9.3 +/- 5.0%, a 60% reduction. Subendocardial blood flow in the ischemic area decreased from 1.04 +/- 0.30 to 0.55 +/- 0.40 ml/min/g, blood flow per beat decreased from 9.1 X 10(-3) to 2.5 X 10(-3) ml/g, and mean transmural flow failed to increase (1.06 +/- 0.30 vs 1.08 +/- 0.39 ml/min/g). During exercise with UL-FS 49, heart rate increased from 89 +/- 10 to only 139 +/- 10 beats/min. End-diastolic left ventricular pressure was increased compared with that during the control run (35.7 +/- 3.0 vs 28.9 +/- 5.5 mm Hg) but left ventricular peak systolic pressure and dP/dt were unchanged. %WT in the ischemic zone did not change significantly during exercise with UL-FS 49 (23.3 +/- 7.9% at rest, 21.5 +/- 8.4% during the run), and in the nonischemic zone it increased to the same extent as during the control run.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuation of exercise-induced myocardial ischemia in dogs with recruitment of coronary vasodilator reserve by nifedipine

There is now evidence that under resting conditions coronary vasodilator reserve exists even in the presence of myocardial ischemia. Therefore, we tested the hypothesis that a vasodilator reserve may exist during exercise so that during exercise-induced ischemia a reduction in coronary constrictor tone can be produced that attenuates the decreases in regional myocardial blood flow and function distal to a severe coronary stenosis without changing the determinants of myocardial oxygen demand. Nine dogs were instrumented with an ameroid constrictor on the left circumflex coronary artery and were studied 2 to 3 weeks later. During a control treadmill run, heart rate increased from 119 +/- 20 to 225 +/- 20 beats/min and peak left ventricular pressure increased from 144 +/- 17 to 163 +/- 28 mm Hg. Poststenotic subendocardial blood flow (measured by a microsphere technique) fell from 1.19 +/- 0.36 to 0.51 +/- 0.30 ml/min X g and systolic wall thickening (by sonomicrometry) decreased from 24.3 +/- 5.8% to 6.0 +/- 6.1%. During an identical run after nifedipine (10 micrograms/kg iv), systemic hemodynamics were not significantly altered. However, subendocardial blood flow was increased to 0.85 +/- 0.51 ml/min X g (p less than .05) and systolic wall thickening to 11.4 +/- 7.8% (p less than .01). We conclude that in this study the amelioration of exercise-induced myocardial ischemia was due to the recruitment by nifedipine of coronary vasodilator reserve.

Alpha 2-adrenoceptor-mediated coronary vasoconstriction persists after exhaustion of coronary dilator reserve

The effect of intracoronary administration of the selective alpha 2-adrenoceptor agonist B-HT 920 on coronary resistance was tested in 14 open-chest dogs before and after exhaustion of coronary dilator reserve. Under both conditions B-HT 920 increased coronary resistance to the same extent: 27 +/- 4% before and 32 +/- 3% after. Administration of B-HT 920 did not affect cardiovascular haemodynamics significantly, although after exhaustion of coronary dilator reserve left ventricular dP/dtmax decreased slightly by 12 +/- 5%. While efferent cardiac sympathetic discharge remained constant during control conditions it increased by 27 +/- 7% when B-HT 920 was administered into a stenotic coronary artery. We conclude that, in contrast to observations made on alpha 1-adrenoceptors, activation of coronary alpha 2-adrenoceptors may have deleterious effects after exhaustion of coronary dilator reserve because these receptors mediate a comparable constriction in well- and under-perfused coronary vessels. Thus they are unlikely to favour a redistribution of coronary blood flow into under-perfused myocardial regions. In addition, this unfavourable effect of B-HT 920 on coronary perfusion after coronary dilator reserve has been exhausted may be further exacerbated by an increase in efferent sympathetic discharge.

Cardiac sympathetic nerve activity and progressive vasoconstriction distal to coronary stenoses: feed-back aggravation of myocardial ischemia

This study tested the hypothesis that the relative ischemia distal to a severe coronary stenosis increases the activity of cardiac sympathetic nerves which in turn results in poststenotic vasoconstriction and an aggravation of ischemia. An acute severe stenosis which reduced coronary blood flow to 50% of control was produced in 23 anesthetized, vagotomized dogs and maintained for 20 min. The activity of postganglionic cardiac sympathetic nerves increased by 23 +/- 4% within 20 min. In parallel, poststenotic coronary resistance increased from 0.48 +/- 0.03 to 0.61 +/- 0.03 mm Hg X min X 100 g/ml resulting in a net lactate production after 15 min. Pretreatment with aspirin (6 mg/kg i.v.; n = 5) was without any influence on these reactions. The selective alpha 2-adrenoceptor antagonist rauwolscine (0.2 mg/kg i.v.; n = 6) and the calcium antagonist nifedipine (10 micrograms/kg i.v.; n = 6) prevented the progressive increase in poststenotic resistance and lactate production, but still permitted an increase in sympathetic activity. Segmental anesthesia of cardiac sympathetic nerves by epidural infiltration of procaine at segments C7-T6 (n = 6) prevented the sympathetic activation, the progressive increase in poststenotic resistance and the resulting myocardial ischemia. Sympathetic activation and a concomitant increase in poststenotic resistance resulting in myocardial ischemia were also found in 6 dogs with intact vagus nerves. These data support the hypothesis of a vicious cycle between poststenotic coronary vasoconstriction and sympathetic activation resulting in severe myocardial ischemia.