Influence of prostaglandin synthesis inhibitors on pulmonary vasodilatory effects of hydralazine in dogs with hypoxic pulmonary vasoconstriction

Diuretic Resistance Treated with Low-Dose Hydralazine: A Case Report and Review of the Literature

We present a case of severe diuretic resistance and edema from acute cardiorenal syndrome complicating heart failure with preserved ejection fraction (HFpEF) and mild alcoholic liver disease. High doses of intravenous (iv) furosemide plus iv doses of chlorothiazide failed to increase the daily urine output (UV) above 1,500 mL or the fractional excretion of sodium (FE_Na) above 2%. The addition of a relatively low dose of hydralazine (10 mg thrice daily PO) during 5 days of constant iv infusion of furosemide plus iv bolus chlorothiazide doubled the UV and FE_Na while reducing the serum creatinine concentration from 3.3 to 2.0 mg/dL. Hydralazine may have restored a response to the diuretics by increasing the renal blood flow and thereby the renal diuretic delivery, or by reducing the filtration fraction or reducing the renal congestion and thereby reducing the proximal reabsorption during blockade of distal reabsorption with diuretics. Further mechanistic studies of low-dose hydralazine for diuretic resistance are warranted.

An Analysis of Responses to Defibrotide in the Pulmonary Vascular Bed of the Cat

Defibrotide is a polydisperse mixture of single-stranded oligonucleotides with many pharmacologic properties and multiple actions on the vascular endothelium. Responses to defibrotide and other vasodepressor agents were evaluated in the pulmonary vascular bed of the cat under conditions of controlled pulmonary blood flow and constant left atrial pressure. Lobar arterial pressure was increased to a high steady level with the thromboxane A2 analog U-46619. Under increased-tone conditions, defibrotide caused dose-dependent decreases in lobar arterial pressure without altering systemic arterial and left atrial pressures. Responses to defibrotide were significantly attenuated after the administration of the cyclooxygenase inhibitor sodium meclofenamate. Responses to defibrotide were also significantly attenuated after the administration of both the adenosine 1 and 2 receptor antagonists 8-cyclopentyl-1,3-dimethylxanthine and 8-(3-chlorostyryl)caffeine. Responses to defibrotide were not altered after the administration of the vascular selective adenosine triphosphate-sensitive potassium channel blocker U-37883A, or after the administration of the nitric oxide synthase inhibitor L-N-(1-iminoethyl)-ornithine. These data show that defibrotide has significant vasodepressor activity in the pulmonary vascular bed of the cat. They also suggest that pulmonary vasodilator responses to defibrotide are partially dependent on both the activation of the cyclooxygenase enzyme and adenosine 1 and 2 receptor pathways and independent of the activation of adenosine triphosphate-sensitive potassium channels or the synthesis of nitric oxide in the pulmonary vascular bed of the cat.

Controversies in heart failure. Are beneficial effects of angiotensin-converting enzyme inhibitors attenuated by aspirin in patients with heart failure?

To whatever extent the improvement in symptoms and survival rendered by treatment with ACE inhibitors is attributable to their effects on the circulation and the kidneys, this benefit can be rescinded by concomitant administration of aspirin. Although some useful prostaglandin-independent actions may persist, shutting down the entire prostaglandin system and trading off a substantial portion of the potential risk reduction with forfeit of salutary hemodynamic and renal effects is a high price to pay just to suppress production of TXA2. In patients requiring treatment for heart failure, if possible, aspirin should be avoided and the integrity of prostaglandin metabolism respected; the severer the heart failure the more compelling. There are other ways to inhibit platelet aggregation, some equally effective or even better than aspirin. Orally active platelet glycoprotein IIb/IIIa receptor antagonists, which may be more efficient than aspirin, have been developed and are now in clinical testing. Ticlopidine and clopidogrel, although more expensive than aspirin, are as easy to use and at least as effective as aspirin. Finally, because patients with severer heart failure are likely to be those with very low ejection fractions, these patients are good candidates for oral anticoagulation even though this treatment requires additional monitoring.

The effect of interstitial hyperthermia on local pulmonary blood flow and lung parenchyma

Hyperthermia has shown promise as an anticancer therapy, but its application to lung neoplasms has been limited by whole body hyperthermia complications and the intrathoracic location of lung neoplasms. Previous studies have shown that interstitial hyperthermia could be performed through a thoracotomy approach and that animals tolerated the procedure without biochemical or hematologic abnormalities. The normal lung's local blood flow pattern and parenchymal changes due to hyperthermia of various temperatures and durations were studied. The experimental protocol applied hyperthermia through interstitial electrodes at temperatures of 39 degrees, 41 degrees, 43 degrees, and 45 degrees C. Tissue blood flow was measured with radioactive microspheres at 20, 40, and 60 minutes of hyperthermia. Histologic examination was performed of biopsy specimens taken from the heated area, from 2 and 5 cm from the heated area, and from distant parenchyma. These initial studies demonstrate that interstitial hyperthermia in the normal lung does not cause a decrease in pulmonary vascular resistance, that interstitial hyperthermia in the normal lung at temperatures of 43 degrees or 45 degrees C for durations of longer than 20 minutes decreases local pulmonary blood flow, that interstitial hyperthermia causes mild vascular disruption at temperatures of 39 degrees C or greater in the heated area but does not affect adjacent or distant lung parenchyma, and that lung hyperthermia causes generalized vascular disruption with severe widening of the pulmonary interstitium and severe hemorrhage in the heated areas when temperatures of 43 degrees or 45 degrees C are applied. Because interstitial hyperthermia affected only local lung damage, it may be applicable in the treatment of localized lung tumors.

Counteraction of the vasodilator effects of enalapril by aspirin in severe heart failure

OBJECTIVES

This study was undertaken to determine if a standard dose of aspirin interacts relevantly with the circulatory effects of enalapril in severe heart failure.

BACKGROUND

The frequent association of heart failure with coronary artery disease confers potential for combined treatment with an angiotensin-converting enzyme inhibitor and the prostaglandin synthesis inhibitor aspirin, the pharmacodynamic actions of which are, in part, mutually opposed.

METHODS

In 18 patients, on 3 consecutive days, hemodynamic measurements were performed at baseline and 4 h after administration of a double placebo, enalapril (10 mg) plus placebo and enalapril plus aspirin (350 mg) according to a double-blind, randomized, crossover protocol.

RESULTS

Enalapril given before aspirin led to significant decreases in systemic vascular resistance, left ventricular filling pressure and total pulmonary resistance together with a significant increase in cardiac output. When given with or on the day after aspirin, enalapril did not elicit significant changes in any of these variables. There was a clear tendency to lower values for pulmonary artery pressure on all regimens, and slowing of the heart rate was incurred whether or not aspirin had been given. Chi-square analysis of the individual responses showed that the probability of effecting a decrease in systemic vascular resistance > or = 300 dynes.s.cm-5 was six times greater when enalapril was given without aspirin (p < 0.01).

CONCLUSIONS

In severe heart failure, the prostaglandin synthesis inhibition by aspirin counteracts the systemic arterial vasodilation of angiotensin-converting enzyme inhibition with enalapril and substantiates its dependence on the integrity of prostaglandin metabolism. Trends toward reductions of pulmonary artery pressure and slowing of the heart rate were still observed, presumably subsequent to lowered norepinephrine concentrations indicating maintenance of prostaglandin-independent actions of angiotensin-converting enzyme inhibition.

Effect of urethane on hydralazine-induced tachycardia in rats

1. In conscious normotensive rats, hydralazine (5-10 mg kg-1 p.o.) produced a dose-related fall in systolic blood pressure, accompanied by a pronounced increase in heart rate. 2. The tachycardia induced by hydralazine (10 mg kg-1 p.o.) in conscious normotensive rats was strongly inhibited after anaesthesia with urethane (1.26 g kg-1 i.p.). 3. In anaesthetized normotensive rats, hydralazine (1 mg kg-1 i.v.) caused a fall in mean blood pressure, accompanied by irregular effects on the heart rate that consisted in a combination of initial tachycardia followed by bradycardia. 4. In pithed rats, hydralazine (1 mg kg-1 i.v.) did not affect mean arterial blood pressure but produced a significant decrease in heart rate. 5. In rat isolated atria, hydralazine (2 mM) produced a positive inotropic/negative chronotropic effect. 6. These results suggest that urethane inhibits the cardiovascular reflex that causes the tachycardia induced by hydralazine in conscious normotensive rats. For this reason, in anaesthetized normotensive rats appear the direct effect of the drug on the heart.

Halothane inhibits hypoxic pulmonary vasoconstriction in the presence of cyclooxygenase blockade

Using an isolated lung the effects of halothane on hypoxic pulmonary vasoconstriction (HPV) were studied in the presence of cyclooxygenase blockade. The pulmonary vasculature can be divided into arterial, middle and venous segment resistances. Analysis of the vascular pressure-flow relationship further separates resistance into a flow dependent resistance (1/slope) and a zero-flow pressure intercept (PCRIT). We ventilated six lobes with control (35 per cent O2) and hypoxic (three per cent O2) gas mixtures with the addition of either 0, 0.5, 1.0, or 2.0 per cent halothane. We found that after addition of indomethacin (5 mg.kg-1), ventilation with three per cent O2 increased total resistance by 87 per cent over baseline with the increase primarily in the middle vascular segment. During normoxic ventilation PCRIT was 7.9 cm H2O and this increased significantly with hypoxia to 11.5 cm H2O). Only 2.0 per cent halothane blocked the increases in middle segment resistance and in PCRIT. We conclude that following cyclooxygenase blockade, halothane inhibits HPV by acting on middle segment vessels.

Hydralazine does not stimulate prostacyclin biosynthesis in hypertensive patients

The hypothesis that vascular prostacyclin synthesis is stimulated by the oral administration of hydralazine and may account for part of its vascular effect was tested. Eight white patients with mild essential hypertension were studied in a randomized, double-blind design to assess the effects of indomethacin on hydralazine's ability to lower blood pressure, elevate pulse, and alter the vascular prostacyclin biosynthesis as assessed by the urinary excretion of the major enzymatically produced metabolite of prostacyclin, 2,3-dinor-6-keto-prostaglandin F1 alpha (PGF1 alpha) measured by gas chromatography-mass spectrometry. Administration of hydralazine at either 50 mg bid or 100 mg bid for a week, doses commonly administered in clinical settings, was not associated with a statistically significant fall in mean blood pressure, although there was a tendency towards a decrease but did result in an increase in heart rate. Administration of indomethacin had no effect on the hemodynamic parameters secondary to hydralazine. Administration of indomethacin resulted in a slight but significant weight gain compared to placebo, but the addition of hydralazine did not result in a further increase in weight. Neither dose of hydralazine resulted in an increase in the urinary excretion of 2,3-dinor-6-keto-PGF1 alpha. The excretion rate was 85 +/- 16 ng/g of creatinine during placebo, 88 +/- 16 ng/g of creatinine during hydralazine, 50 mg bid, and 65 +/- 8 ng/g of creatinine during hydralazine, 100 mg bid. Administration of indomethacin, 50 mg bid, resulted in a significant decrease in 2,3-dinor-6-keto-PGF1 alpha from 65 +/- 6 ng/g to 37 +/- 8 ng/g of creatinine.(ABSTRACT TRUNCATED AT 250 WORDS)

Megakaryocyte function and dysfunction

More than a hundred years have passed since platelets were recognized as cells and their haemostatic functions discovered. However, the process of platelet production is still not understood. The location, the mechanism and the regulation of thrombopoiesis remain elusive. Megakaryocytes are known to be the source of platelets. Investigations of megakaryocytes have revealed their normal functions and some of the abnormalities present in various diseases which affect platelets. In recent years, new techniques of cell isolation and tissue culture have been developed and have made possible advances in characterizing megakaryocyte precursors and differentiation. The primary function of megakaryocytes is to synthesize and assemble platelet components and organelles. Although debated for a long time, new data seems to indicate that the lung may be a central locus of platelet production. The new techniques for megakaryocyte investigations have barely begun to be of use in the study of abnormal platelet production in disease.

Effect of diazoxide and hydralazine on intestinal transit and blood pressure in the rat

This study examines the effects of diazoxide and hydralazine on gastrointestinal and vascular smooth muscle. The effect of the drugs administered individually and in combination was determined on small intestinal transit and blood pressure in adult male Sprague-Dawley rats. Both diazoxide and hydralazine produced dose-dependent decreases in the ratio of intestinal transit to total length. Additionally, combinations of various doses of hydralazine and diazoxide produced a greater than simple summation of the inhibitory effects on intestinal transit. The combination of diazoxide and hydralazine produced only a simple additive effect on mean arterial pressure.

Dissociation of the prostaglandin and renin angiotensin systems during captopril therapy for chronic congestive heart failure secondary to coronary artery disease

Neurohumoral systems are activated as compensatory mechanisms in congestive heart failure (CHF). A close correlation has been reported between the renin angiotensin and prostaglandin systems in CHF. Furthermore, serum sodium concentration provided an excellent index of hormonal status. In this study, these relations were examined after acute and chronic blockade of renin angiotensin system with captopril. Eight patients with advanced CHF (New York Heart Association III or IV) were studied. Before captopril treatment, all hormone levels were elevated. Mean plasma renin activity was 24 +/- 7 ng Al/ml/hour, angiotensin concentration was 221 +/- 11 pg/ml and aldosterone concentration was 82 +/- 17 pg/ml. Plasma PGE2 metabolite was 1,425 +/- 321 pg/ml. A close correlation was observed between plasma angiotensin II and PGE2 metabolite levels (r = 0.7); inverse correlations existed between serum sodium concentration and PGE2 metabolite levels (r = -0.9) and with plasma renin activity (r = -0.6). Captopril therapy reduced the plasma angiotensin II level to 38 +/- 6 pg/ml and aldosterone concentration to 15 +/- 3 pg/ml, but did not affect plasma renin activity (31 +/- 10 ng Al/ml/hour) when measured in 1 week. Paradoxically, PGE2-metabolite levels increased further (to 3,031 +/- 346 pg/ml) despite blockade of the renin angiotensin system. Serum sodium concentration no longer correlated with hormone levels. These effects were sustained at 2 months of follow-up. Thus, captopril caused a dissociation between the renin angiotensin system and prostaglandin. The activation of prostaglandin is probably due to captopril's effect on prostaglandin biosynthesis and may contribute to captopril's sustained efficacy in CHF.

The role of prostaglandins in mediating the effects of angiotensin converting enzyme inhibitors and other antihypertensive drugs

This review considers the hypothesis that angiotensin converting enzyme (ACE) inhibitors and other antihypertensive drugs may reduce blood pressure by altering the balance of vasoconstrictor and vasodilator hormones. Most cases of essential hypertension are characterized by increased vascular resistance. Since vascular resistance can be viewed as a net balance of offsetting vasoconstrictor and vasodilator neurohumoral forces, and since angiotensin II and norepinephrine can stimulate the synthesis of vasodilator prostaglandins that can in turn counteract peripheral vasoconstriction, we measured changes in vasoconstrictor (angiotensin II) and vasodilator (PGE2) hormones after giving different antihypertensive drugs. We found that the sulfhydryl containing converting enzyme inhibitor captopril increased vasodilator prostaglandin production (PGE2-metabolite) both acutely and chronically. This increase in PGE2-metabolite was not seen with the non-sulfhydryl converting enzyme inhibitor, enalapril.

Pulmonary megakaryocytes: "missing link" between cardiovascular and respiratory disease?

Pulmonary megakaryocytes were quantitated in a series of 30 consecutive hospital necropsies using a two stage immunoperoxidase stain for factor VIII related antigen. In all 30 cases they were found with a mean density of 14.65 megakaryocytes/cm2 in lung sections of 5 micron in thickness. The maximum concentration of intrapulmonary megakaryocytes was consistently found to be in the central zone of the right upper lobe. Less than 22% of the observed cells possessed abundant cytoplasm, the rest appearing as effete, naked, and seminaked nuclei. The mean megakaryocyte count was found to be increased in association with both respiratory pathology (positive smoking history and impaired lung function) and cardiovascular disease states--shock; thromboembolism; myocardial infarction; and severe atheroma in the abdominal aorta, the coronary circulation, and the circle of Willis. Pulmonary megakaryocytes probably embolise from bone marrow. This may reflect stimulated thrombopoiesis, caused by increased platelet consumption in association with atherosclerotic disease, but it cannot be taken to confirm that the lung is the principal site of platelet production.

Does prostacyclin enhance the selective pulmonary vasodilator effect of oxygen in children with congenital heart disease?

We have obtained dose-response curves for the effects of prostacyclin on the pulmonary and systemic circulations in 20 children (median age 3 years) with pulmonary hypertension complicating congenital heart disease. Results were obtained with the children breathing both air and 100% oxygen. Under both sets of conditions, remote respiratory mass spectrometry was used to measure oxygen consumption and hence cardiac output by the direct Fick principle. When the subjects breathed air, prostacyclin caused a dose-dependent fall in pulmonary vascular resistance (measured in mm Hg . liter-1 . min . m2) (11.12 to 8.07, standard error of difference [SED] = 0.5, p less than .01). The level of the pulmonary vascular resistance when the subjects breathed air during the infusion of 20 ng/kg/min prostacyclin was not significantly different from that found when they breathed 100% oxygen and did not receive the drug (8.67 vs 8.93, SED = 0.55, p = NS). When infused while the subjects breathed 100% oxygen, prostacyclin caused additional dose-dependent pulmonary vasodilation (pulmonary vascular resistance 8.93 to 7.23, SED = 0.3, p less than .01). Unlike 100% oxygen, prostacyclin was not selective, and caused tachycardia and systemic hypotension at the higher doses. These results suggest that in children with congenital heart disease 100% oxygen does not maximally vasodilate the pulmonary circulation, and further pulmonary vasodilatation can be obtained with a blood-borne agent.

Long-term effects of nitrendipine on hemodynamics and oxygen transport in patients with cor pulmonale

Recent studies have suggested that vasodilators may acutely improve pulmonary hemodynamics in patients with chronic obstructive pulmonary disease and cor pulmonale, but the effects of long-term therapy have not been assessed. We evaluated the hemodynamic and gas exchange effects of nitrendipine, a calcium channel blocker with a cardiovascular profile similar to nifedipine, acutely and after five days and six weeks of therapy in eight patients with stable COPD and cor pulmonale. After six weeks, nitrendipine significantly decreased both mean pulmonary artery pressure (40.4 +/- 10.3 to 31.2 +/- 6.6 mm Hg, p less than 0.01) and pulmonary vascular resistance (6.8 +/- 3.9 to 3.0 +/- 1.1 units, p less than 0.01), while cardiac index increased (2.4 +/- 0.8 to 3.6 +/- 1.0 L/min/m2, p less than 0.001). Despite a fall in arterial PO2 (53.1 +/- 18.7 to 45.5 +/- 11.3 mm Hg, p = NS), systemic oxygen transport increased by over 30 percent (843 +/- 284 to 1,111 +/- 373 ml/min, p less than 0.05). Systemic arterial pressure, pulmonary capillary wedge pressure, and heart rate were unchanged. Despite these hemodynamic changes, three patients died from complications of their underlying disease while receiving long-term therapy after eight, nine, and ten months. These preliminary findings suggest that long-term vasodilator therapy can result in persistent hemodynamic improvement in some patients with cor pulmonale secondary to COPD, although the impact of this form of therapy on survival remains to be clarified.

A comparison of the acute hemodynamic effects of prostacyclin and hydralazine in primary pulmonary hypertension

In patients with primary pulmonary hypertension the administration of a vasodilating drug is often used to test pulmonary vasoreactivity. Hydralazine has been employed as a test drug, but because of its long duration of action there is a risk of sustained systemic arterial hypotension in patients with a fixed pulmonary vascular resistance. In this study we compared the acute hemodynamic effects of intravenous prostacyclin, a potent, short-acting vasodilator, with the effects of oral or intravenous hydralazine. Both prostacyclin and hydralazine increased cardiac output and decreased systemic pressure without changing pulmonary arterial pressure in seven patients with primary pulmonary hypertension. The average decrease in total pulmonary resistance with prostacyclin (-46% +/- 5%) was more than that with hydralazine (-32% +/- 6%). The respective decreases in total systemic resistance were -50% +/- 4% vs -43% +/- 6%. The percent changes in individual responses to the two agents were correlated (p less than 0.05) for pulmonary arterial pressure, systemic arterial pressure, total pulmonary resistance, and total systemic resistance. We concluded that the pulmonary hemodynamic effects of prostacyclin resembled those of hydralazine. Prostacyclin may predict the acute pulmonary hemodynamic effects of hydralazine in primary pulmonary hypertension and because of its prompt, brief action may provide greater patient safety.

Nafazatrom (Bay g 6575) blunts canine hypoxic pulmonary vasoconstriction: evidence for a prostaglandin-mediated mechanism

The eicosanoids are circulating vasoactive substances which may serve as modulators of pulmonary vasomotor tone. Nafazatrom (Bay g 6575) has been reported to raise circulating levels of prostaglandin I2 (PGI2) either by stimulating its synthesis or inhibiting its metabolism, and may also decrease leukotriene synthesis by inhibiting lipoxygenase. We evaluated the hemodynamic effects of nafazatrom administered intravenously in doses of 2,5, and 10 mg/kg in intact, anesthetized dogs ventilated with 21% and 10% oxygen. The hypoxia-induced increase in pulmonary vascular resistance was blunted by nafazatrom in an inverse dose-dependent fashion. Pretreatment with the cyclooxygenase inhibitor indomethacin (5 mg/kg subcutaneously twice daily for two days) abolished the response to low-dose nafazatrom. The pulmonary vasodilator response to the infusion of arachidonic acid (100 micrograms/kg/min) during hypoxic ventilation was blocked by the 10 mg/kg dose of nafazatrom. We conclude that nafazatrom blunts hypoxic pulmonary vasoconstriction in a manner consistent with a drug-induced increase in PGI2 activity, and that the inverse dose-dependent effects of the drug which we observed are due to an inhibition of cyclooxygenase at higher doses.

Influence of prostaglandin inhibition on dihydralazine induced acute effects in patients with essential hypertension

In order to determine whether vasodilator prostaglandins (PG) contribute to the acute vascular and endocrine responses to intravenously administered dihydralazine, we examined its effects on systolic and diastolic blood pressure (BP), heart rate (HR), plasma renin activity (PRA) and the plasma catecholamines (CA) noradrenaline (NA), adrenaline (A) and dopamine (DA) as well as on sodium, potassium and creatinine clearance (CNa, CK and CCr respectively), urinary flow rate, urinary catecholamines (NA, A, DA) and iPGE2 and i6-keto-PGF1 alpha excretion rate in six patients (three females, three males) with essential hypertension before and after PG synthesis inhibition by the nonsteroidal, anti-inflammatory agent diclofenac. Diclofenac, which reduced urinary iPGE2 and i6-keto-PGF1 alpha excretion by 62% (P = 0.026) and 45% (P = 0.037), respectively, antagonized dihydralazine induced diastolic BP reduction (P = 0.0009), HR increase (P = 0.01), noradrenaline and adrenaline increase in plasma (P = 0.02 and P = 0.05, respectively), increase in urine flow rate (P = 0.03), sodium clearance (P = 0.01) and tended to reduce PRA. We conclude that dihydralazine-mediated changes can be reduced by cyclo-oxygenase inhibition, most likely on the basis of a reduction of its effect on peripheral resistance, thereby leading to less reflex activation of the sympathetic nervous and renin-angiotensin systems.

Vasodilative effect of hydralazine in awake dogs: the roles of prostaglandins and the sympathetic nervous system

The relative roles of prostaglandins and the sympathetic nervous system in mediating the hypotensive effects of hydralazine were studied in awake dogs with and without pretreatment with indomethacin, propranolol, and phentolamine. In normal dogs, mean aortic pressure decreased 23 +/- 4 mm Hg after administration of hydralazine (cumulative dose of 0.8 mg/kg). This hypotensive effect of hydralazine was potentiated by phentolamine but was abolished by propranolol. Indomethacin caused a paradoxic pressor response (11 +/- 3 mm Hg) to hydralazine, which also was abolished by addition of phentolamine. Hydralazine produced vasodilation in the coronary, skeletal muscle (quadriceps), splanchnic, and renal circulations in normal dogs. The increase in coronary blood flow was associated with increased cardiac oxygen consumption and narrowed arteriovenous oxygen difference across the heart. Propranolol reduced the increases in cardiac oxygen consumption and coronary blood flow, but only indomethacin abolished the narrowed arteriovenous oxygen difference, suggesting that the increase in coronary blood flow was related to both the increased cardiac oxygen demand and prostaglandin-mediated active coronary vasodilation. The decrease in skeletal muscle vascular resistance after hydralazine was abolished by propranolol. Skeletal muscle vascular resistance actually increased after administration of hydralazine in dogs pretreated with both propranolol and indomethacin. These effects were blocked by the addition of phentolamine. Unlike the normal response, renal and splanchnic vascular resistances increased after administration of hydralazine in dogs pretreated with indomethacin. The splanchnic vasoconstriction was abolished by phentolamine, but the renal vascular change was affected by neither phentolamine nor propranolol. The results indicate that hydralazine does not produce uniform vasodilation in all organs and that the cardiovascular actions of hydralazine involve both prostaglandins and the sympathetic nervous system.

Naloxone: effects on hypoxic pulmonary vasoconstriction

To determine the effect of naloxone on the hypoxic pulmonary vasoconstrictive response, six mongrel dogs were rendered hypoxic with 10% oxygen and were given either saline or naloxone. Following hypoxia all dogs had significant increases in mean pulmonary artery pressure (PAP) and pulmonary arterial resistance index (PARI) without changes in cardiac output or systemic blood pressure. Beta endorphins did not change at any time following hypoxia. Dogs receiving naloxone had significant lowering of PAP and PARI without changes in plasma beta endorphin levels. We conclude that naloxone attenuates hypoxic pulmonary vasoconstriction without measurable alterations of plasma beta endorphin levels.

Nitroglycerin therapy in the management of pulmonary hypertensive disorders

Vasodilator therapy has not been effective in patients with pulmonary hypertension because most of the drugs that have been utilized in treating this disorder do not exert selective effects on the pulmonary circulation. Nonselective agents may cause predominant systemic vasodilation and lead to severe hypotension; they may elicit reflex activation of the sympathetic nervous system and further elevate pulmonary artery pressures; or they may exert depressant effects on right ventricular function and aggravate right-sided heart failure. Nitroglycerin has theoretic appeal as a vasodilator drug in patients with pulmonary hypertension because it exerts a direct effect on the pulmonary circulation in doses that do not affect systemic resistance vessels or the myocardium and do not activate neurohumoral reflexes. Furthermore, the drug uniquely reduces pulmonary artery pressures in addition to pulmonary vascular resistance due to its ability to dilate venous capacitance vessels. Preliminary studies with sublingual and intravenous nitroglycerin in patients with pulmonary hypertension have shown that the drug produces marked hemodynamic improvement and that clinical benefits follow long-term therapy with transcutaneous or oral nitrates. However, treatment may provoke hypotensive events in some patients and systemic hypoxemia in others; still others may fail to benefit because the pulmonary vasculature is unresponsive to any vasodilator stimulus. Further work is needed to define the benefits and risks of nitroglycerin therapy in patients with pulmonary hypertension.

Role of platelet antagonists in coronary artery disease: implications in coronary artery bypass surgery and balloon-catheter dilatation

Platelets play an important role in regulation of hemostasis and maintenance of vascular tone. Endothelial disruption occurring during coronary artery bypass surgery and balloon-catheter dilatation may promote platelet adhesion, aggregation, and thrombus formation. Recent studies suggest that platelet-endothelial interaction is mediated in part through products of arachidonic acid metabolism. Understanding of the platelet interaction with blood vessels is important in pharmacologic interventions directed at prevention of thrombus formation in bypass grafts. Although it remains to be proved, use of platelet-suppressive drugs may also improve patency of coronary arteries after balloon-catheter dilatation.

Prostaglandins in severe congestive heart failure. Relation to activation of the renin--angiotensin system and hyponatremia

To determine whether prostaglandins are involved in circulatory homeostasis in congestive heart failure, we measured plasma levels of the metabolites of vasodilator prostaglandins I2 and E2 in 15 patients with severe chronic heart failure. Mean circulating levels of both metabolites were 3 to 10 times higher than those in normal subjects. Plasma levels of both metabolites correlated directly with plasma renin activity and plasma angiotensin II concentrations (r = 0.64 and 0.84, respectively). Individual serum sodium concentrations were inversely correlated with levels of prostaglandin E2 metabolites (r = -0.92, P less than 0.001) and plasma renin activity (r = -0.69, P less than 0.02). Of 23 patients with severe heart failure challenged with indomethacin (an inhibitor of prostaglandin synthesis), the 9 with hyponatremia had significant decreases in the cardiac index (1.99 +/- 0.12 to 1.72 +/- 0.13 liters per minute per square meter of body-surface area, P less than 0.001) and significant increases in the pulmonary capillary wedge pressure (17.4 +/- 2.0 to 24.0 +/- 1.9 mm Hg, P less than 0.001) and systemic vascular resistance (1882 +/- 239 to 2488 +/- 315 dyn x sec x cm-5, P less than 0.001), whereas the 14 patients with a normal serum sodium concentration had no significant hemodynamic changes. We conclude that both vasoconstrictor (renin-angiotensin) and vasodilator (prostaglandin) mechanisms are operative in patients with heart failure complicated by hyponatremia and that these mechanisms interact to modulate circulatory homeostasis.

Haemodynamic response to intravenous hydralazine in patients with pulmonary hypertension

The acute pulmonary and systemic haemodynamic response to low (0.15 mg/kg) and high (0.30 mg/kg) doses of intravenous hydralazine was evaluated in 26 consecutive patients with severe pulmonary hypertension due to cor pulmonale (nine patients), primary pulmonary hypertension (11 patients), or pulmonary embolism (six patients). Hydralazine did not cause a significant change in pulmonary arterial resistance or pressure in any group but produced a significant reduction in systemic resistance, which correlated with plasma concentration, and a significant increase in pulmonary blood flow index in all groups. Ten patients who experienced a reduction in pulmonary arterial resistance of at least 5 U X m2 after administration of hydralazine had higher initial values for pulmonary arterial resistance and systemic resistance and a lower pulmonary blood flow index than those who did not respond. Maintenance oral hydralazine treatment during nine to 36 months of follow up did not seem to affect symptoms or mortality. These results indicate that hydralazine has limited value in acutely reducing pulmonary arterial pressure or affecting clinical outcome in patients with pulmonary hypertension.

Regulation of vascular prostaglandin synthesis by metabolites of arachidonic acid in perfused rabbit aorta

To address the hypothesis that metabolites of arachidonic acid are important regulators of prostaglandin (PG) synthesis in intact vascular tissue, we studied arachidonate metabolism in rabbit aortas in response to a continuous infusion of arachidonic acid, 10 micrograms/ml. Prostacyclin (PGI2; measured as 6-keto-PGF1 alpha) production rate accelerated during the first 2 min, reached peak velocity at 2 min, and then progressively decelerated. The velocity profile of PGI2 production was similar to that previously reported for cyclooxygenase holoenzyme assayed in vitro, and was consistent with progressive inactivation of the enzymes leading to PGI2 synthesis. We determined the specific inhibition of cyclooxygenase and prostacyclin synthetase by measuring PGI2 and PGE2 production rates and by infusing cyclic endoperoxides. Our results indicate preferential inactivation of cyclooxygenase during arachidonate metabolism, most likely due to cyclooxygenase-derived oxidative intermediates. This was a dose-dependent response and resulted in a progressive decrease in the 6-keto-PGF1 alpha/PGE2 ratio. Exogenously added 15-hydroperoxy eicosatetraenoic acid, on the other hand, actually stimulated cyclooxygenase activity at low doses, while markedly inhibiting prostacyclin synthetase. This finding, along with the accelerating nature of arachidonate metabolism, is consistent with the concept of "peroxide tone" as a mediator of cyclooxygenase activity in this system. These results demonstrate that arachidonate metabolites regulate PG synthesis in intact blood vessels. The progressive enzymatic inhibition intrinsic to arachidonate metabolism may be a model for similar changes occurring in states of enhanced lipid peroxidation. These metabolic alterations might greatly influence the numerous vascular functions known to involve arachidonic acid metabolism.

New perspectives on therapeutic application of nitrates as vasodilator agents for severe chronic heart failure

Although nitrates produce marked decreases in ventricular filling pressures in patients with severe heart failure, their therapeutic value has long been believed to be limited because they were thought to exert minimal arterial dilating effects. Recently, however, new conceptual approaches to vasodilator drugs have been developed that have challenged this traditional view. These new perspectives indicate that nitrates exert dilator actions on both the arterial and venous circulations, and reduce both preload and afterload; such balanced circulatory responses are particularly evident when large doses of these drugs are used. Cardiac output increases markedly with nitrates in patients with a greatly increased systemic vascular resistance before treatment or with significant mitral regurgitation. The major reason for the limited increases in cardiac output noted in previous studies is the inclusion of patients with heart failure whose pretreatment values for cardiac output were within normal limits; in these persons nitrates markedly activate neurohumoral vasoconstrictor mechanisms that counteract the arterial dilating actions of these drugs. Long-term nitrate therapy attenuates exercise-induced increases in pulmonary venous pressures, which permit patients to undergo repeated submaximal exercise with fewer symptoms; this improves physical conditioning and exercise capacity, even in the absence of drug-related changes in cardiac output. The long-term hemodynamic and clinical benefits of nitrates in heart failure have been confirmed by two independent randomized double-blind placebo-controlled clinical trials.

Nonadrenergic effects of isoproterenol in dogs with hypoxic pulmonary vasoconstriction. Possible role of prostaglandins

To determine whether the pulmonary vasodilation produced by isoproterenol is mediated solely by its beta adrenergic effects, we studied the hemodynamic responses to isoproterenol in three groups of dogs with pulmonary vasoconstriction produced by continuous ventilation with 10% oxygen: (a) hypoxia alone, (b) hypoxia and propranolol 0.3 mg/kg i.v. bolus followed by an infusion of 5 micrograms/kg per min, and (c) hypoxia after pretreatment with an inhibitor of cyclooxygenase, either indomethacin or meclofenamate 5 mg/kg s.c. twice daily for 2 d prior to study. All groups had similar values for mean pulmonary artery pressure (PAPm) and pulmonary vascular resistance (PVR) during room air and hypoxic ventilation. Isoproterenol in doses of 0.0025, 0.005, and 0.05 micrograms/kg per min produced a dose-related decline in PAPm and PVR during hypoxia in group 1. Despite beta-blockade with propranolol (group 2), isoproterenol at all three doses significantly reduced PAPm and PVR. The responses to isoproterenol were comparable in the presence or absence of propranolol; at 0.05 micrograms/kg per min the effects of isoproterenol were blunted, but not abolished, by propranolol. Similar results were observed even when five times the dose of propranolol was given. Isoproterenol at all three doses had no effect, however, on PAPm and PVR in the cyclooxygenase inhibitor-pretreated group. These data suggest that the pulmonary vasodilator effects of isoproterenol are not mediated solely by pulmonary vascular beta adrenergic receptors, and that vasodilator prostaglandins may play a role in the responses to this drug.

Abnormal intrapulmonary platelet production: a possible cause of vascular and lung disease

It is postulated that platelets are not produced by megakaryocyte budding within the bone marrow but by physical fragmentation in the pulmonary circulation. Alterations in the nature of the production site (the pulmonary vessels and their concomitant biochemical environment) or changes in the antecedent megakaryocyte cytoplasmic volumes with a concomitant alteration in protein structure can produce platelets with a mean volume larger than normal. If such platelets are more reactive then they may be involved in atherogenesis and arterial thrombosis. Furthermore several pulmonary pathological states may arise from dysfunction during pulmonary platelet production.

Indomethacin does not attenuate the effects of hydralazine in normal subjects

Indomethacin 100 mg/day or matching placebo was given for 2.5 days to 9 healthy volunteers in a double-blind cross-over study, followed by an intravenous injection of 0.2 mg/kg of hydralazine. Compared to placebo, indomethacin produced no statistically significant change in pulse rate or blood pressure in both standing and supine positions. Hydralazine injection was followed by a statistically significant fall in lying and standing diastolic pressure and a rise in lying and standing pulse after both indomethacin and placebo pretreatments. There were no significant differences in these effects following indomethacin compared to placebo pretreatment. These results do not support the hypothesis that endogenous prostaglandins are involved in the mechanism of action of hydralazine.

Thromboxane and prostacyclin (epoprostenol) during exercise in diffuse pulmonary fibrosis

Plasma thromboxane A2, a vasoconstrictor, and plasma prostacyclin (epoprostenol), a vasodilator, were assessed by double-antibody radioimmunological assay of their respective stable circulating metabolites, thromboxane B2 (TxB2) and 6-ketoprostaglandin F1 alpha, in 9 patients with severe diffuse pulmonary fibrosis (DPF), who were known to become hypo-oxaemic during exercise, and in 9 healthy volunteers. In the 7 patients with the most severe DPF, mean arterial PO2 fell from 68 mm Hg at rest to 51 mm Hg at peak aerobic exercise, and mean TxB2 increased to twice the value at rest. The 9 controls remained oxygen saturated throughout exercise; their mean TxB2 did not change during aerobic exercise, but during anaerobic exercise increased to twice the value at rest, and increased further during recovery. There were no significant changes in 6-keto-prostaglandin F1 alpha in either group. The selective release of TxB2 during aerobic exercise in hypo-oxaemic patients suggests that thromboxane mediates hypoxic pulmonary vasoconstriction. Its release in normal man during anaerobic exercise may reflect a more general response to the metabolic changes of tissue hypoxia.

Prostacyclin-induced acute pulmonary vasodilation in primary pulmonary hypertension

To evaluate the effects of prostacyclin (prostaglandin I2) on pulmonary vascular tone in primary pulmonary hypertension (PPH), we performed right-heart catheterization on seven patients with PPH and made hemodynamic measurements before and after infusing incremental doses of prostacyclin. In maximal doses of 2-12 mg/kg/min (mean 5.7 +/0 3.1 ng/kg/min), prostacyclin reduced mean pulmonary arterial pressure from 62 +/- 15 to 55 +/- 16 mm Hg (p less than 0.05) and total pulmonary resistance from 17.1 +/- 8.7 to 9.7 +/- 5.9 units (p less than 0.005), and increased cardiac output from 4.22 +/- 1.64 to 6.57 +/- 2.04 l/min (p less than 0.01). Heart rate increased from 83 +/- 13 to 94 +/- 11 beats/min (p = 0.1) and mean systemic arterial pressure decreased from 90 +/- 12 to 77 +/- 4 mm Hg (p = 0.055). Three patients who received a continuous infusion of prostacyclin for 24-48 hours had sustained reductions in total pulmonary resistance during the infusion period. These data demonstrate that prostacyclin can increase cardiac output and reduce pulmonary arterial pressure and resistance in PPH.

Deleterious effects of hydralazine in patients with pulmonary hypertension

Thirteen consecutive patients with primary and secondary pulmonary hypertension who had normal left ventricular function were treated with hydralazine in an effort to reduce pulmonary vascular resistance and clinical symptoms. Despite marked decreases in systemic vascular resistance (40 per cent; P less than 0.001), hydralazine produced only moderate decreases in pulmonary arteriolar resistance (21 per cent), without improving stroke volume or pulmonary-artery pressure. Instead, mean arterial pressure fell markedly (17.5 mm Hg, P less than 0.01) in association with a reflex increase in heart rate (11 beats per minute, P less than 0.01). Four patients became symptomatically hypotensive within 24 hours of the initiation of treatment; two of these four required pressors for circulatory support, and one died. Progressive renal insufficiency developed in one patient, and a symptomatic decrease in systemic arterial oxygen saturation occurred in another; both changes were reversed upon discontinuation of the drug. In conclusion, hydralazine fails to produce consistent hemodynamic and clinical benefits in patients with primary and secondary pulmonary hypertension, and it frequently causes serious adverse reactions.

Prostaglandins and ischemic heart disease

There is an abundance of information suggesting that prostaglandins are involved in the development and clinical expression of atherosclerosis. Many studies demonstrate a relationship between prostaglandins and the risk factors for peripheral and coronary artery disease. Thus, part of the mechanism by which hyperlipidemia, diabetes mellitus, smoking, hypertension, sex hormones, age, heredity, emotional stress and diet contribute to the development and progression of atherosclerosis may be through an imbalance between thromboxane A2 and prostaglandin I2. Recent studies show a temporal relationship between acute ischemic events (specifically, unstable angina) and a transcardiac increase in thromboxane B2, while others demonstrate a salutary effect of disaggregatory and vasodilatory prostaglandins in such patients. If prostaglandins and thromboxane prove important in ischemic vascular disease, attention will be directed at the correction of their pathologic imbalance. This may be accomplished by dietary manipulation as well as by the development of prostaglandin receptor antagonists or inhibitors of specific prostaglandin pathways.