J Holtz

Comparative effects of chronic angiotensin-converting enzyme inhibition and angiotensin II type 1 receptor blockade on cardiac remodeling after myocardial infarction in the rat.

BACKGROUND After myocardial infarction, the noninfarcted left ventricle develops reactive hypertrophy associated with a depressed coronary flow reserve, myocardial interstitial fibrosis, and reduced capillary density. The present study investigated the comparative cardiac effects of chronic angiotensin-converting enzyme (ACE) inhibition and selective angiotensin II type 1 receptor (AT1) blockade in the rat model of myocardial infarction and failure. METHODS AND RESULTS Seven days after coronary ligation (MI), rats were randomized to enalapril (n = 8; 500 micrograms.kg-1.d-1), losartan (n = 9; 3 mg.kg-1.d-1), or placebo (n = 8) and treated for 6 weeks. Sham-operated rats (n = 10) served as controls. Coronary blood flow was measured with radiolabeled microspheres during baseline and maximal coronary dilation induced by dipyridamole (2 mg.kg-1.min-1 over 10 minutes). Right and left ventricular (LV) weight was increased in infarcted rats compared with sham-operated animals and enalapril- and losartan-treated MI rats. Minimal LV and right ventricular coronary vascular resistance was increased in MI rats but normalized with enalapril and losartan (LV:sham, 8.9; MI-placebo, 12.7; MI-enalapril, 9.2; MI-losartan, 8.8 mm Hg.mL-1.min-1.g-1, all P < .05 versus MI-placebo). Interstitial fibrosis determined from perfusion-fixed hearts was increased in infarcted rats but reduced by both enalapril and losartan. Myocardial capillary density improved with enalapril and losartan. In separate groups treated as above, plasma and tissue ACE activity was determined and demonstrated significantly higher ACE activity in noninfarcted LV tissue of MI-placebo rats compared with sham (0.64 vs 0.27 nmol.mg protein-1.min-1, P < .05). Enalapril and losartan reduced LV ACE activity (0.39 and 0.29 nmol.mg protein-1.min-1, P < .05 versus MI-placebo). CONCLUSIONS The present study demonstrates that both chronic ACE inhibition and AT1 receptor blockade (1) reduces cardiac hypertrophy, (2) restores minimal coronary vascular resistance in postinfarction reactive hypertrophy, and (3) attenuates the development of myocardial interstitial fibrosis in the noninfarcted LV. These results suggest that inhibition of generation of angiotensin II and AT1 receptor blockade are equally effective in preventing important features of ventricular remodeling after myocardial infarction.

Nitrate tolerance in epicardial arteries or in the venous system is not reversed by N-acetylcysteine in vivo, but tolerance-independent interactions exist.

N-acetylcysteine is assumed to reverse nitrate tolerance by replenishing depleted intracellular sulfhydryl groups, but data on interactions of N-acetylcysteine and nitrates in patients with stable angina are controversial and disappointing. Therefore, we studied the effect of N-acetylcysteine on nitrate responsiveness of epicardial arteries and of the venous system (assessed as changes in effective vascular compliance) in dogs (n = 12) during long-term nitroglycerin treatment (1.5 micrograms/kg/min i.v. for 5-6 days). In dogs with nitroglycerin-specific tolerance (shift of venous or epicardial artery dilation to 15-17-fold higher dosages), N-acetylcysteine (100 mg/kg i.v.) had no dilator effect and did not alter the dose-response relations of nitroglycerin. Yet, in nontolerant dogs (n = 17), N-acetylcysteine augmented (1.5-2.0-fold) the dilation of epicardial arteries and the reduction of peripheral vascular resistance induced by 0.5-1.5 micrograms/kg/min nitroglycerin. In vitro, the augmentation of purified guanylate cyclase activity by nitroglycerin (10-100 microM) was potentiated by N-acetylcysteine (0.01-1.0 mM) in saline or in canine plasma, but N-acetylcysteine alone was ineffective. We conclude that 1) N-acetylcysteine does not restore nitroglycerin responsiveness in tolerant epicardial arteries or veins in vivo, 2) a small, tolerance-independent augmentation of nitroglycerin-induced dilation may result from N-acetylcysteine-induced extracellular formation of a stimulant of guanylate cyclase from nitroglycerin.

Norepinephrine constricts the canine coronary bed via postsynaptic α2-adrenoceptors

Abstract The effect of α2-blockade (0.3 mg/kg i.v. rauwolscine) and α1-blockade (1.2 mg/kg i.v. prazosin) on coronary constrictions induced by intracoronary injections of azepexole (B-HT 933, α2-agonist, 0.1–10 μg/kg), phenylephrine (0.3–3 μg/kg) and norepinephrine (0.001–0.1 μg/kg) were studied in dog hearts perfused in situ under β-blockade. Constrictions by azepexole (antagonized by rauwolscine, yet resistant to prazosin and methylsergide) demonstrated coronary α2-adrenoceptors. Norepinephrine-induced constrictions were more attenuated (22-fold) by α2-blockade than by α1-blockade (2.6-fold) and thus were mediated mainly by activation of postsynaptic α2-receptors.

Altered spectrum of nitroglycerin action in long-term treatment: nitroglycerin-specific venous tolerance with maintenance of arterial vasodepressor potency.

The study of venodilator tolerance to nitroglycerin has been complicated by reflex compensation and by problems in analyzing venous tone in the presence of multiple determinants of venous pressure. We assessed venous tone as total effective vascular compliance (TEVC) under autonomic blockade in six dogs, in the nontolerant state, and during a 5 day infusion of nitroglycerin (1.5 micrograms/kg/min). Under long-term treatment, baseline TEVC was unaffected and the nitroglycerin dose-response relationship for TEVC was shifted to greater than 10-fold higher doses, whereas baseline mean arterial pressure (MAP) was lowered by 17 +/- 3 mm Hg without any shift in nitroglycerin responsiveness. This lowering of MAP was observed only after autonomic blockade. In six additional dogs instrumented with aortic flow probes, nitroglycerin (1.5 micrograms/kg/min) induced a 15 +/- 1% decline in peripheral vascular resistance (PVR) under autonomic blockade, but with reflexes intact these dogs showed no change in PVR and a 21 +/- 10% increase in norepinephrine release rate. We conclude that modest long-term exposure to nitroglycerin results in tolerance to its venodilating effects, whereas arteriolar action is maintained. This tolerance-induced shift in action from venous toward arteriolar dilation is normally masked by compensatory reflexes.

Neurohormonal inhibition and hemodynamic unloading during prolonged inhibition of ANF degradation in patients with severe chronic heart failure.

BackgroundThe purpose of this study was to investigate the therapeutic potential of prolonged inhibition of atrial natriuretic factor (ANF) degradation in patients with severe chronic heart failure. Methods and ResultsThe effects of repeated doses of the endopeptidase inhibitor candoxatrilat (150 mg i.v.) were examined over a 24-hour period in patients with severe chronic heart failure (New York Heart Association class III-IV). Plasma α-hANF(99–126) was elevated at baseline (235±59 pg/ml), increased 2.5-fold at 2 hours after the first dose, and remained significantly elevated throughout the 24-hour protocol. In contrast, pro-hANF(31–67) decreased from 3,151±616 to 2,072±362 pg/ml (p < 0.05). Cardiac index (CI) increased only transiently after the first dose of candoxatrilat (CI, 2.11±0.2 to 2.67±0.28 I/min/m2, p < 0.05). Sodium excretion increased sixfold (p < 0.05) 2 hours after the first dose of candoxatrilat and remained significantly elevated throughout the protocol. Degree of natriuresis and diuresis in response to candoxatrilat was closely related to baseline cardiac output. Glomerular filtration rate and volume excretion did not change significantly. Pulmonary capillary wedge pressure fell from 23±3 to 18±3 mm Hg (p < 0.05) and remained below baseline throughout the 24 hours. Arterial pressure, heart rate, and total peripheral resistance did not change significantly during the 24-hour period. Urinary cGMP excretion increased fivefold (p < 0.05), whereas urinary ANF immunoreactivity and plasma cGMP levels remained unchanged. Excretion of prostacyclin metabolite 6-keto-PGF-1α increased 3.3-fold (p < 0.05). Plasma norepinephrine and epinephrine levels decreased significantly after candoxatrilat and remained suppressed over the 24-hour period. There was also a transient reduction in plasma vasopressin, aldosterone levels, and plasma renin activity. Hematocrit, total protein content, and plasma albumin concentrations did not change, indicating that no fluid shift into the extravascular space had occurred. Conclusions1) The inhibition of ANF degradation causes sustained drop in left and right atrial pressures that appears to be mediated by an inhibition of neurohumoral activity; 2) concomitant inhibition of bradykinin breakdown (which in turn stimulates renal prostacyclin synthesis) contributes to natriuresis; 3) the close correlation between renal response and baseline cardiac index indicates that an inadequate renal perfusion secondary to low cardiac output diminishes the efficacy of this treatment modality. This spectrum of action would be advantagous for a first-line diuretic agent early in the course of disease rather than in patients with advanced chronic heart failure.

Mechanisms involved in the response to prolonged infusion of atrial natriuretic factor in patients with chronic heart failure.

We examined the mechanisms involved in the cardiovascular and renal response to prolonged infusion of atrial natriuretic factor (ANF) in patients with chronic heart failure. ANF infusion was titrated to produce a 30% decrease in pulmonary capillary wedge pressure or a 20% increase in cardiac output, and this dose (average, 75 ± 4 ng/kg/min) was then administered for 20 hours. The short-term response to ANF included significant reductions in central filling pressures, increases in cardiac output, modest increases in diuresis and glomerular filtration rates, significant reduction in plasma aldosterone levels, and a 3.6-fold increase in plasma cyclic GMP levels. During prolonged infusion, plasma cGMP levels and cardiac output gradually returned to baseline. Similarly, the initially increased diuretic effects were completely abolished during prolonged ANF infusion, although plasma a-hANF levels remained consistently elevated above baseline values (control, 198 ± 38; titration, 2,760 ± 596; 20 hours, 3,499 ± + 659 pg/ml). Four hours after beginning the ANF infusion, marked increases in hematocrit levels were noted (42.5 ± 1.0% versus 45.3 ± 1.4%, control and infusion, respectively, p < 0.05); during this time, no change in total plasma protein concentration occurred, indicating extravascular shift of fluid and plasma proteins. No evidence was noted for activation of vasoconstrictor hormones during prolonged ANF infusion, although mean arterial pressure was significantly reduced throughout the infusion period. Plasma pro-ANF (31–67) levels, determined as a marker for endogenous ANF secretion, were significantly suppressed as were the reductions of central filling pressures. After ANF discontinuation, heart rate and pulmonary capillary wedge pressure increased significantly above baseline values without evidence for sympathetic stimulation. We conclude that 1) prolonged infusion of ANF causes only transient increases in plasma cGMP levels but a sustained reduction of the cardiac release of ANF and that 2) the beneficial hemodynamic effects of ANF, that is, unloading of the ventricles, may be associated with or, in part, may be secondary to a shift of plasma constituents into the extravascular space. The latter may limit the therapeutic potential of ANF for long-term treatment.

Long-term nitroglycerin treatment: effect on direct and endothelium-mediated large coronary artery dilation in conscious dogs.

We examined the effect of nitroglycerin (GTN) tolerance on an important determinant of nitrate-antianginal action, large coronary artery dilation, in 11 chronically instrumented conscious dogs. In addition, endothelium-mediated coronary artery dilation was studied because this shares a common dilator pathway with the nitrates, i.e., activation of soluble guanylate cyclase. With long-term GTN (1.5 micrograms/kg/min iv for 5 days) the diameters of the left circumflex and anterior descending coronary arteries showed an initial increase of 8.2 +/- 0.3% and 10.8 +/- 0.9%, respectively, returning to control levels by the second to third day of treatment. On days 4 and 5, the dose-response relations for GTN-induced epicardial artery dilation were shifted (p less than .01) to 17- to 20-fold higher doses. However, there was no attenuation of epicardial artery dilation induced by SIN-1 (n = 7), another activator of guanylate cyclase, or of endothelium-mediated dilation assessed both as flow-dependent dilation (n = 7) and as direct intra-arterial acetylcholine-induced dilation (n = 4). In addition, there was no clear tolerance to the peripheral vascular actions of GTN responsible for reflex tachycardia and increased coronary flow. We conclude that a moderate degree of nitrate tolerance to epicardial artery dilation does not affect the responsiveness to other exogenous or endogenous activators of guanylate cyclase. However, this tolerance to epicardial artery dilation, together with the maintenance of peripheral vascular actions that can induce reflex tachycardia, result in a potentially unfavorable balance of GTN effects.

Ergonovine-induced constrictions of epicardial coronary arteries in conscious dogs: α-adrenoceptors are not involved

SummaryThe effect of i.v. ergonovine tartrate infusions (0.05–20 μg/kg/min, 12 minutes duration) on coronary arteries was studied in 14 conscious dogs instrumented to continuously measure vascular diameter by an ultrasonic dimension gauge using 10-MHz piezoelectric crystals.Ergonovine induced a biphasic coronary response: small, transient dilation during the first minutes of infusion, followed by slowly developing constriction reaching its maximum 5 to 15 minutes after the end of the infusion and persisting at this level for at least 10 minutes. The threshold dosage for significant constriction was 0.05 μg/kg/min. A dosage of 5 μg/kg/min (cumulative 60 μg/kg, corresponding to 35 μg/kg ergonovine maleate) caused a decline in mean left circumflex artery diameter by 137±15 μm (=4.6%) without significantly altering heart rate, plasma catecholamines or plasma renin activity. Coronary venous O2 saturation did not decline, indicating the absence of coronary resistance vessel constriction. The epicardial artery constriction was not attenuated by a vasopressin antagonist. Under adrenergic blockade (2 mg/kg phentolamine and 2 mg/kg nadolol) or under ganglionic blockade (5 mg/kg pentolinium tartrate), ergonovine (5 μg/kg/min) caused substantial elevation in mean arterial pressure, while the decline in coronary artery diameter was attenuated. When this increase in arterial pressure was prevented by appropriate bleeding, the ergonovine-induced coronary constriction was not diminished by adrenergic or ganglionic blockade. The serotonin antagonist methysergide (0.5 mg/kg) completely abolished the ergonovine-induced coronary artery vasomotion.It is concluded that ergonovine in dogs causes an epicardial coronary artery constriction comparable to the diffuse coronary artery narrowing in men not suffering from variant angina pectoris. These constrictions are not mediated by an adrenergic mechanism.ZusammenfassungDie Wirkung intravenöser Infusionen von Ergonovitartrat (0.05–20 μg/kg/min, 12 Minuten Dauer) auf die Koronararterien wurde in 14 wachen Hunden untersucht. Die Tiere waren für eine kontinuierliche Messung des Gefäßdurchmessers mit einem Ultraschall-Verfahren mittels implantierten piezoelektrischen 10-MHz-Kristallen ausgerüstet.Ergonovin verursachte eine biphasische Koronarreaktion: eine geringgradige, vorübergehende Dilatation während der ersten Minuten der Infusion, anschließend eine allmählich einsetzende Konstriktion, die ihr Maximum erst 5–15 Minuten nach Ende der Infusion erreichte und für mindestens 10 Minuten bestehen blieb. Die Schwellendosis für signifikante Konstriktion betrug 0,05 μg/kg/min. Die Dosis von 5 μg/kg/min (kumulativ 60 μg/kg, was 35 μg/kg Ergonovin-Maleat entspricht) verursachte eine Abnahme des mittleren Durchmessers der linken umschlingenden Koronararterie um 137±15 μm (=4,6%) ohne signifikante Wirkungen auf Herzfrequenz, Plasma-Katecholamine oder Plasma-Renin-Aktivität. Die koronarvenöse O2-Sättigung nahm nicht ab, demnach erfolgte also keine Konstriktion von koronaren Widerstandsgefäßen. Die Konstriktion der epikardialen Gefäße war nicht durch einen Vasopressin-Antagonisten abschwächbar. Unter adrenerger Blockade (2 mg/kg Phentolamin und 2 mg/kg Nadolol) oder unter Ganglienblockade (5 mg/kg Pentoliniumtartrat) verursachte Ergonovin (5 μg/kg/min) eine erhebliche Zunahme des arteriellen Mitteldruckes, während die Verminderung des Koronararterien-Durchmessers abgeschwächt war. Wurde dieser arterielle Druckanstieg durch entsprechende Blutentnahme verhindert, so war die Ergonovin-induzierte Koronarkonstriktion weder durch adrenerge Blockade noch durch Ganglienblockade abgeschwächt. Der Serotonin-Antagonist Methysergid (0,5 mg/kg) hob die ergonovininduzierten Koronarreaktionen vollständig auf.Ergonovin verursacht in Hunden eine Konstriktion epikardialer Koronararterien ähnlich der diffusen Koronarverengung in Menschen, die nicht an “Variant”-Angina-pectoris leiden. Diese Konstriktionen werden nicht durch einen adrenergen Mechanismus bewirkt.

Mechanisms of interaction between the sulfhydryl precursor L-methionine and glyceryl trinitrate.

BackgroundL-Methionine potentiates systemic hemodynamic effects of intravenous glyceryl trinitrate (GTN) in tolerant and nontolerant patients to a similar extent as N-acetylcysteine (NAC). This potentiation of GTN action by L-methionine has been attributed to enhanced intracellular formation of nitrosothiols, known to be potent stimulators of soluble guanylyl cyclase. This study was performed to analyze directly the effects of L-methionine on GTN-induced dilation of large epicardial arteries and the venous capacitance system of the dog in the tolerant and nontolerant states. Cultured rat aortic vascular smooth muscle cells and purified guanylyl cyclase were used to study potential intracellular and extracellular mechanisms responsible for this interaction. Methods and ResultsIn awake nontolerant dogs, L-methionine (100 mg/kg) potentiated the tachycardic response to GTN (5.0 and 15 μg/kg/min) and enhanced the hypotensive action of GTN (1.5 and 5.0 μg/kg/min) in anesthetized, nonreflexic dogs. In nontolerant and tolerant dogs, however, L-methionine did not alter the dose-response of large epicardial artery dilation to intravenous GTN challenges and did not modify nitrate tolerance of the low pressure system of the dog. The infusion of L-methionine (100 mg/kg) significantly increased plasma methionine levels (from 52±12 to 1,141±239 μM), cystine levels (from 12±4 to 26±7 gM), but not homocystine levels. In vitro, the L-methionine conversion product L-cysteine (0.1–1.0 mM) but not homocysteine significantly enhanced the augmentation of purified guanylyl cyclase activity by GTN (100 μM). Incubation of cultured rat aortic smooth muscle cells with L-methionine (10 μM or 1 mM) did not result in a significant increase of free intracellular sulfhydryl group content. ConclusionsThe L-methionine conversion product L-cysteine mediates tolerance independent the potentiation of GTN action. This may result from an L-cysteine-induced formation of a vasoactive metabolite of GTN (nitric oxide) or nitrosothiol. This effect occurs primarily in the resistance vessel circulation, not in large epicardial arteries and veins. The lack of effect of L-methionine on sulfhydryl group content in large conductance vessels indicates that hepatic L-methionine metabolism constitutes the significant source of L-cysteine. These findings strongly suggest that administration of sulfhydryl-group precursor L-methionine does not represent a therapeutic alternative to a nitrate-free interval to restore nitrate sensitivity in tolerant large epicardial arteries and veins.

Preferential venoconstriction by cyclooxygenase inhibition in vivo without attenuation of nitroglycerin venodilation.

Because prostacyclin is a rather potent venodilator in vivo, we analyzed the effect of cyclooxygenase inhibition on venous tone in 14 anesthetized dogs during ganglionic and beta-adrenergic blockade and atraumatic conditions. Effective vascular stiffness (a reciprocal of effective vascular compliance) as a variable of integrated venous tone was 0.30 +/- 0.01 mm Hg.kg/ml (n = 35) and was augmented up to twofold by diclofenac (1, 3, and 10 mg/kg i.v.), ibuprofen (6 and 60 mg/kg), or indomethacin (5 mg/kg) parallel to augmentations in central venous pressure, while the rise in arterial pressure was less than half of the increase induced by equivenoconstrictor dosages of norepinephrine. After preconstriction by indomethacin or diclofenac, nitroglycerin (1.5 micrograms/kg/min) lowered effective vascular stiffness (by 24 +/- 2% or 23 +/- 5%, respectively), similarly as during preconstriction by norepinephrine (by 24 +/- 4%). Long-term cyclooxygenase inhibition (diclofenac 2 x 1 mg/kg/day for 4 days) did not modify arterial pressure, heart rate, or hematocrit levels in conscious dogs at rest, but it lowered plasma volume to 52.5 +/- 1.9 ml/kg (sham treatment: 59.1 +/- 1.6 ml/kg, p less than 0.05, n = 4). In conclusion, venoconstriction by clinical dosages of cyclooxygenase inhibitors does not interfere with the venodilator action of nitroglycerin and is compensated chronically by adjustments of plasma volume.