Adenosine spares platelets during cardiopulmonary bypass in man without causing systemic vasodilatation

Effect of diltiazem on plasma concentrations of oxypurines and uric acid

To determine the clinical effect of diltiazem on the metabolism of adenosine, and its importance in ischemic heart disease, arterial plasma concentrations of the purine metabolites were determined in 21 healthy volunteers (10 female and 11 male) and 19 patients with effort angina (8 female and 11 male) before, during, and immediately after standard treadmill exercise tests conducted before and after they had taken 60 mg diltiazem (Cardizem; Hoechst Marion Roussel, Laval, QC, Canada) four times a day for 1 week. The results showed that the cardiac patients had significantly lower mean plasma concentrations of uric acid (46.82 +/- 25.51 versus 95.47 +/- 35.41 micrograms/ml, p 0.05), inosine (0.25 +/- 0.19 versus 0.84 +/- 0.17 microgram/ml, p < 0.05), and hypoxanthine (0.28 +/- 0.35 versus 0.50 +/- 0.27 microgram/ml, p < 0.05). Diltiazem decreased the mean resting plasma concentrations of uric acid in patients (uric acid 43.47 +/- 22.26 versus 46.82 +/- 25.51 micrograms/ml, p < 0.05) and healthy volunteers (uric acid 85.68 +/- 26.71 versus 95.47 +/- 35.41 micrograms/ml, p < 0.05). There was no statistically significant change in the plasma concentrations of the purine metabolites during exercise (p < 0.05). Female subjects had significantly lower plasma concentrations of uric acid than males (patients, 34.87 +/- 26.93 versus 55.78 +/- 21.25 micrograms/ml; healthy volunteers, 84.79 +/- 32.07 versus 104.22 +/- 37.05 micrograms/ml; p < 0.05 for both). Results of the study suggest that normal therapeutic doses of diltiazem may modulate the metabolism of adenosine and that some of the purine metabolites may be useful markers for specific types of ischemic heart disease.

Adenosine reduces postbypass transfusion requirements in humans after heart surgery

OBJECTIVE

The objective of this study was to determine the effect, if any, of adenosine blood cardioplegia on blood component usage after heart surgery.

SUMMARY BACKGROUND DATA

The most common cause of nonsurgical postcardiopulmonary bypass bleeding is platelet dysfunction. For this reason, pharmacologic agents are under investigation in an effort to reduce the need for transfusion in this setting.

METHODS

A posthoc analysis of blood product usage was performed in data obtained from a Phase I, single center, open label, randomized study performed in 63 patients. The trial was designed to test the safety and tolerance of adenosine when added to blood cardioplegia in increasing doses to enhance myocardial protection. The database provided information regarding the effect of adenosine cardioplegia on venous plasma adenosine concentrations, the amount of platelets, fresh frozen plasma and packed erythrocytes used, and the association between the adenosine dose and postoperative thoracic drainage.

RESULTS

The postoperative thoracic drainage at 6 hours, 24 hours, and at the time of chest tube removal in the high-dose adenosine cardioplegia group was 68%, 76%, and 75% of the placebo and low-dose adenosine cardioplegia group (p < 0.05). The highest dose of adenosine studied increased baseline adenosine venous plasma levels 360-fold, from 0.17 +/- 0.09 mumol/L to 42.30 +/- 11.20 mumol/L (p < 0.05). This marked increase was associated with a 68%, 56%, and 58% reduction in platelet, fresh frozen plasma, and packed erythrocyte usage, respectively (p < 0.05).

CONCLUSIONS

In addition to enhancing the heart's tolerance to ischemia, adenosine-supplemented cardioplegic solution also may reduce bleeding after cardiopulmonary bypass.

Purine metabolism in the heart. Strategies for protection against myocardial ischaemia

Adenosine has recently received much attention for the protection it provides against the deleterious effects of ischaemia reperfusion. Whenever the demand for oxygen exceeds its supply, adenosine triphosphate in myocytes is rapidly dephosphorylated to adenosine. Adenosine may then protect the myocardium against ischaemia-reperfusion damage. However, the accumulation of adenosine is limited by its rapid uptake and catabolism in the endothelium and in red blood cells. The strict compartmentalization of the enzyme pathways involved in the metabolism of adenosine, e.g. adenosine production by myocytes, its pharmacological action in the interstitium, its catabolism in the endothelium and in red blood cells, and its carrier-mediated transport across membranes, provides a unique target for pharmacological interventions. Blockade of adenosine uptake may indeed result in prolonged adenosine accumulation specifically in those regions of the heart where it is produced. In recent years considerable evidence has been gathered on the adenosine-mediated cardioprotective actions of nucleoside transport inhibitors.

Effects of adenosine in combination with calcium channel blockers in patients with primary pulmonary hypertension

OBJECTIVES

The purpose of this study was to evaluate the effects of vasodilator combination therapy in patients with primary pulmonary hypertension.

BACKGROUND

Calcium channel blockers and adenosine have each been shown to be effective in reducing pulmonary artery pressure and pulmonary vascular resistance in patients with primary pulmonary hypertension. However, the effects of combining these vasodilators have not been studied.

METHODS

To test the combination, 12 patients were placed on oral nifedipine and 3 on diltiazem therapy, using a dose titrated to maximal effect (mean nifedipine dose 103 +/- 24 mg, mean diltiazem dose 300 +/- 49 mg). Patients were then given maintenance doses of the calcium channel blocker at half the cumulative loading dose at 6-h intervals. One hour after the maintenance dose of calcium blocker, all patients received an infusion of adenosine, starting with 50 micrograms/kg per min and increasing by 50 micrograms/kg per min at 2-min intervals to a maximally tolerated dose (180 +/- 63 micrograms/kg per min).

RESULTS

Ten patients responded to calcium channel blockers (defined as a > or = 20% decrease in pulmonary vascular resistance), with a 16% decrease in mean pulmonary artery pressure (p = 0.057), a 39% decrease in pulmonary vascular resistance (p = 0.002) and a 24% increase in stroke volume (p = 0.007). Five patients were nonresponders, with no significant changes in pulmonary artery pressure, pulmonary vascular resistance, cardiac index or stroke volume. In the calcium channel blocker responders, the combination of adenosine and calcium blocker reduced pulmonary vascular resistance by 49%, increased stroke volume by 33% and decreased mean pulmonary artery pressure by 14% compared with drug-free baseline values. In nonresponders, combination therapy resulted in nonsignificant changes in pulmonary artery pressure and pulmonary vascular resistance.

CONCLUSIONS

Adenosine has the ability to further decrease pulmonary artery pressure and pulmonary vascular resistance in patients with primary pulmonary hypertension who respond to calcium channel blockers. Those who fail to respond to these agents have little added effect from adenosine.

Comparison of the effects of adenosine and nifedipine in pulmonary hypertension

The hemodynamic effects of intravenously administered adenosine, a potent vasodilator, were examined in 15 patients with pulmonary hypertension. All patients were given adenosine, 50 micrograms/kg per min, increased by 50 micrograms/kg per min at 2 min intervals to a maximum of 500 micrograms/kg per min or until the development of untoward side effects. The patients were then given oral nifedipine, 20 mg every hour, until a greater than or equal to 20% decrease in pulmonary vascular resistance or systemic hypotension occurred. The administration of maximal doses of adenosine, 256 +/- 46 micrograms/kg per min, produced a 2.4% reduction in pulmonary artery pressure (p = NS), a 37% decrease in pulmonary vascular resistance (p less than 0.001) and a 57% increase in cardiac index (p less than 0.001). The administration of maximally effective doses of nifedipine (91 +/- 36 mg) produced a 15% reduction in the mean pulmonary artery pressure (p less than 0.05), a 24% decrease in pulmonary vascular resistance (p less than 0.01) and an 8% increase in cardiac index (p = NS). There was a significant correlation (r = 0.714, p = 0.01) between the reduction in pulmonary vascular resistance that resulted from adenosine administration and that achieved with the administration of nifedipine. Six patients had substantial reductions in pulmonary vascular resistance with adenosine but not with nifedipine. Thus, adenosine is an effective vasodilator in patients with pulmonary hypertension and can be used for safe and rapid assessment of vasodilator reserve in these patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacodynamic interactions between infused adenosine and oral theophylline

1. Five healthy human subjects were given, in single-blind fashion, either (a) 625 mg theophylline orally, followed 4 h later, by a 40 min infusion of adenosine (40 micrograms kg-1 min-1 for 5 min, 60 micrograms kg-1 min-1 for 5 min and 80 micrograms kg-1 min-1 for 30 min), or (b) 625 mg theophylline orally followed by 0.9% sodium chloride infusion, or (c) placebo theophylline tablets followed by adenosine infusion. 2. All five subjects experienced adverse effects during adenosine infusion, mainly at the higher infusion rates; two subjects also experienced chest pain but not during combined treatment with theophylline and adenosine. 3. Diastolic blood pressure (DBP) rose by 16.5 mmHg (P less than 0.001) following treatment with theophylline only, fell by 24.5 mmHg (P less than 0.001) during the adenosine infusion after placebo theophylline and remained unchanged during the adenosine infusion following theophylline. Pulse rate rose by 12 min-1 (P less than 0.01) during adenosine infusion following placebo, but not after theophylline alone or theophylline and adenosine combined. 4. The respiratory rate fell by 6 min-1 (P less than 0.01) during treatment with adenosine only, being lower than for the two treatments containing theophylline (P less than 0.05). 5. Plasma potassium and magnesium fell by 0.25 mmol l-1 (P less than 0.001) and 0.037 mmol l-1 (P less than 0.05), respectively, during treatment with theophylline only, but these effects were not altered by infusion of adenosine. 6. This study has demonstrated interactions between theophylline and adenosine on diastolic blood pressure and respiratory rate, but no interaction on metabolic parameters.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma adenosine changes during cardiac surgery

Changes in plasma adenosine and inosine were measured during high-dose narcotic anesthesia and surgery for coronary artery bypass grafting (CABG), and mitral or aortic valve replacement (V). Arterial and mixed venous blood samples were obtained for measurement of adenosine and inosine at eight sampling intervals ranging from preanesthesia induction to discontinuation of cardiopulmonary bypass (CPB). Arterial but not mixed venous adenosine was markedly elevated in blood samples 10 minutes after intubation, but the fourfold elevation was significant only in the CABG patient group. Mixed venous inosine and adenosine were most consistently elevated in post-CPB samples. In a separate study of arterial adenosine changes during induction, a uniform drug administration protocol was used, and again adenosine was significantly increased immediately after intubation. It is possible that adenosine and perhaps inosine may contribute to cardiovascular responses following induction-intubation and also after discontinuing CPB.

Effect of infused adenosine on cardiac output and systemic resistance in normal subjects

1. The purine nucleoside adenosine relaxes smooth muscle in vitro and is a vasodilator in animals, but its effects on cardiac output and systemic vascular resistance have not been measured in normal conscious human subjects. 2. We have studied the effects of infused adenosine in doses of 0.005, 0.03 and 0.07 mg kg-1 min-1 on pulmonary blood flow and systemic vascular resistance in eight healthy volunteers, using a non-invasive, inert gas method and mass spectrometry. 3. At a dose of 0.07 mg kg-1 min-1, there was a rise in effective pulmonary blood flow (which is approximately equivalent to cardiac output) of 0.52 +/- 0.08 l min-1 m-2 (mean +/- s.e. mean) and a fall in estimated systemic vascular resistance of 357 +/- 44 dyn s cm-5. Despite this marked systemic vasodilation, there was no significant change in mean heart rate. 4. The effects of this dose of adenosine were maximal 2 min after starting the infusion, and had disappeared within 5 min of stopping it. 5. Adenosine may be therapeutically useful in the reduction of left ventricular afterload, where the absence of reflex tachycardia may be advantageous. We suggest that adenosine in doses of 0.03 mg kg-1 min-1 should be evaluated as a selective pulmonary vasodilator.

Effect of aprotinin on need for blood transfusion after repeat open-heart surgery

Of 22 patients undergoing repeat open-heart surgery through a previous median sternotomy wound 11 were randomised to receive the serine proteinase inhibitor aprotinin in high dosage (about 700 mg intravenously from the start of anaesthesia to the end of operation, depending on the length of the surgical procedure). Their mean blood loss was 286 ml compared with 1509 ml in the 11 control patients (p less than 0.001), and mean haemoglobin losses were 8.3 g and 78 g, respectively (p less than 0.001). Blood transfusion requirements were eightfold higher in the control group than in the aprotinin group, 7 of whom received only the single unit of their own blood taken before cardiopulmonary bypass.

The disappearance of adenosine from blood and platelet suspension in relation to the platelet cyclic AMP content

Adenosine exerts anti-aggregatory effects on human platelets in vitro, probably by increasing intraplatelet levels of cyclic AMP. In addition, adenosine prevents platelet loss in vivo. We have studied the relationship between the concentration of adenosine in the platelet media and the level of cAMP. In PRP, exogenous adenosine (2-16 microM) was eliminated with a half-life close to 5 min. Approximately half of the added adenosine was deaminated (blocked by 1-2 microM EHNA), and half was eliminated by uptake into platelets (blocked by 2 microM dipyridamole). In whole blood the half-life for adenosine was much shorter, about 15 s. Addition of adenosine deaminase (0.3 microgram ml-1) to PRP resulted in a measured half-life for adenosine approximating that of whole blood. In PRP where adenosine was eliminated as quickly as in whole blood, the adenosine-mediated stimulation of cAMP was 35% lower than in PRP, and the cAMP response lasted 2 min versus 15 min in normal PRP. These results suggest that the magnitude and duration of adenosine's effect on platelets are markedly overestimated by studying platelet suspensions. In blood, the effect of adenosine is smaller in magnitude and very transient. The possibility is discussed that the action of adenosine in vivo on blood platelets can therefore be quite local.

Cardiovascular effects of adenosine

The results, briefly summarized above, indicate that adenosine could be a physiologically important modulator of several aspects of cardiovascular regulation. Most cells are equipped with adenosine receptors. These receptors are of at least two subtypes which can be defined by the relative agonist potency. At these adenosine receptors, methylxanthines, including caffeine and theophylline, act as competitive antagonists. The role of adenosine antagonism, as a mechanism behind the cardiovascular effects of these xanthines, was recently reviewed (Fredholm, 1984). The concentrations of adenosine are low during resting conditions, but may be raised substantially by, for example, hypoxia, ischaemia and increased mechanical or biochemical work. The adenosine levels can also be raised by drugs, including uptake inhibitors such as dipyridamole. Already the concentrations of adenosine that occur during basal conditions are sufficient to produce significant effects, for example, on blood-flow. When the concentrations are raised the importance of endogenous adenosine becomes even greater. Adenosine may not only be of physiological significance but may also be pharmacologically important. First, there are several drugs that may act by affecting the levels of adenosine or by influencing its receptors. Second, the possibility exists that adenosine itself could be used clinically. For example, adenosine may be an attractive alternative to sodium nitroprusside or nitroglycerin when controlled hypotension is to be achieved. Adenosine may also be used to preserve blood platelets during extracorporal circulation or to produce selective regional vasodilatation. Both the physiological and pharmacological aspects are subject to intense study in several laboratories.

Cardiovascular effects of adenosine in man; possible clinical implications

The results summarized above indicate that adenosine is a physiologically relevant modulator of the cardiovascular system in man. The levels of adenosine are low during resting conditions, but may increase during conditions of oxygen and/or substrate deficiency. Already the basal concentration seems to be sufficient to affect regional flow in vital organs such as the heart. Several drugs may act by increasing the levels of adenosine or by influencing its receptors. In addition, adenosine may be used in many clinical situations as a vasodilator, antiaggregatory compound as well as an antiarrythmic agent. Its effect is easy to control due to the extremely short plasma half-life. The dose range for the clinical effects are summarized in Table 6. Both the physiological and pharmacological aspect of adenosine are subject to intense study in several laboratories.