Differential effects of timolol and metoprolol on platelet function at rest and during exercise

Differential associations of beta-blockers with hemorrhagic events for chronic heart failure patients on warfarin

PURPOSE

Beta-blockers have many different physiologic effects that could potentially influence the risk of hemorrhagic events in chronic heart failure patients (CHF) on warfarin. We examined how different beta-blockers vary in their associated risk of a hemorrhagic event.

METHODS

We used databases from the Department of Veterans Affairs (VA) that contain information on medications prescribed, diagnoses, and hospitalizations. We identified patients with CHF on warfarin and either metoprolol, carvedilol, atenolol, or no beta-blocker during 1999-2001. We modeled time to first hemorrhagic event using a Cox proportional hazards model, adjusting for age, ethnicity, comorbidities, and other factors. INR levels were examined in a subsample of 3546 patients.

RESULTS

We identified 66,988 CHF patients on warfarin. Hemorrhagic events occurred in 15.3% of the sample and, in 3.8% of the sample, the hemorrhage was considered severe. Compared to patients on carvedilol, the hazards ratio for a new hemorrhagic event was 1.25 (1.17, 1.34) for no beta-blocker, 1.27 (1.18, 1.38) for atenolol, and 1.38 (1.28, 1.48) for metoprolol. No differences in INR levels were evident among the four groups.

CONCLUSIONS

The risk for a hemorrhagic event among CHF patients on warfarin may be affected by beta-blocker use and varies depending on which beta-blocker is prescribed.

Modulation of beta-adrenergic receptor subtype activities in perioperative medicine: mechanisms and sites of action

This review focuses on the mechanisms and sites of action underlying beta-adrenergic antagonism in perioperative medicine. A large body of knowledge has recently emerged from basic and clinical research concerning the mechanisms of the life-saving effects of beta-adrenergic antagonists (beta-AAs) in high-risk cardiac patients. This article re-emphasizes the mechanisms underlying beta-adrenergic antagonism and also illuminates novel rationales behind the use of perioperative beta-AAs from a biological point of view. Particularly, it delineates new concepts of beta-adrenergic signal transduction emerging from transgenic animal models. The role of the different characteristics of various beta-AAs is discussed, and evidence will be presented for the selection of one specific agent over another on the basis of individual drug profiles in defined clinical situations. The salutary effects of beta-AAs on the cardiovascular system will be described at the cellular and molecular levels. Beta-AAs exhibit many effects beyond a reduction in heart rate, which are less known by perioperative physicians but equally desirable in the perioperative care of high-risk cardiac patients. These include effects on core components of an anaesthetic regimen, such as analgesia, hypnosis, and memory function. Despite overwhelming evidence of benefit, beta-AAs are currently under-utilized in the perioperative period because of concerns of potential adverse effects and toxicity. The effects of acute administration of beta-AAs on cardiac function in the compromised patient and strategies to counteract potential adverse effects will be discussed in detail. This may help to overcome barriers to the initiation of perioperative treatment with beta-AAs in a larger number of high-risk cardiac patients undergoing surgery.

Haemodynamics and tissue specificity with isradipine

The significant effects of isradipine in producing arterial vasodilation and mild negative chronotropic effects without significant negative inotropic effects suggests that this agent should provide excellent antihypertensive efficacy in the treatment of perioperative hypertension. Isradipine may prove to be a safe antihypertensive treatment in patients with impaired ventricular function (cardiac failure), impaired myocardial perfusion (ischaemia), and in cases of selected conduction abnormalities or arrhythmias. The demonstration that its effects are limited to vascular resistance rather than vascular capacitance is an important distinguishing feature of isradipine compared with other antihypertensive agents. Finally, the potential application of this dihydropyridine calcium antagonist for cytoprotection and its effects on atherosclerosis remain exciting therapeutic prospects.

Platelet activation and prostacyclin release in essential hypertension

To evaluate platelet activation thromboxane A2 (TxA2) and beta-thromboglobulin (beta TG) were used as markers and in addition we studied the biosynthesis of prostacyclin. Synthesis of TxA2 and prostacyclin was assessed by measurement of urinary metabolites. Fifteen untreated hypertensive patients (HT) and 15 age-matched normotensive controls (NT) were investigated at rest, during and after exercise. HT patients were re-examined after 3 months on enalapril. During basal conditions there was no difference in the excretion of Tx-M, PGI-M or beta TG between the groups. During strenuous exercise HT exhibit a significantly higher increase in prostacyclin synthesis (162%) compared to NT (76%). The levels of beta TG increased with 82% in the HT and 24% in the NT group, Tx-M increased with 27% and 23% respectively. Treatment with the ACE-inhibitor enalapril did not significantly alter these findings. These results indicate that there is no evidence of basal platelet activation in early essential hypertension. Strenuous exercise leads to some increase in Tx-M in both groups, with no pronounced differences between the groups. Hypertensive patients exhibit a significantly increased prostacyclin response to exercise which could be due to differences in vessel-wall reactivity. Enalapril seems to exert no effect on platelet activation or on prostacyclin biosynthesis.

Metoprolol does not reduce platelet aggregability during sympatho-adrenal stimulation

The possibility that beta-adrenoceptor blockers, especially beta 1-selective agents might inhibit platelet function is of considerable interest, as this might be of pathophysiological importance in cardiovascular diseases. Platelet function, however, is difficult to assess and in vivo related data are scarce. The effect of one week of treatment with metoprolol 200 mg/day on platelet aggregability during mental stress (colour word conflict test; CWT) and low and high dose adrenaline infusions has been evaluated in a double-blind, placebo-controlled, cross-over study in 10 healthy male volunteers. Platelet function in vivo was assessed using ex vivo filtragometry, and the urinary excretions of beta-thromboglobulin (HMW beta-TG) and 11-dehydro-TxB2 (a thromboxane metabolite). Conventional in vitro aggregometry and the urinary levels of 2,3-dinor-6-keto-PGF1 alpha (a prostacyclin metabolite) were also studied. During the interventions there was increased platelet aggregability in vivo, as filtragometry readings were shortened by 41 +/- 11% during high dose adrenaline infusion, urinary HMW beta-TG levels increased and urinary 11-dehydro-TxB2 tended to increase. In contrast, platelet sensitivity to ADP in vitro was reduced. The urinary 2,3-dinor-6-keto-PGF1 alpha levels were increased during the interventions. Despite the cardiovascular and biochemical signs of beta-adrenoceptor blockade at rest and during the interventions, metoprolol failed to influence platelet function in vivo, as measured by ex vivo filtragometry, or urinary HMW beta-TG or 11-dehydro-TxB2 levels. It tended rather to enhance the stress response measured by ex vivo filtragometry. Platelet aggregability in vitro and urinary 2,3-dinor-6-keto-PGF1 alpha levels were not altered by metoprolol.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect on platelet behaviour of treatment with atenolol and the combination of atenolol and nifedipine in healthy volunteers

Two double blind crossover studies have been carried out in healthy volunteers to determine the effect on platelet behaviour and on blood coagulation of treatment with atenolol and with a fixed combination of atenolol and nifedipine. The first study compared atenolol with placebo, whilst the second compared atenolol alone with the combination of atenolol and nifedipine. Treatment with atenolol alone had no effect on platelet behaviour. In contrast, treatment with the combination of the beta blocker and the calcium antagonist gave rise to a small but significant increase in platelet dense granule release ex vivo in response to a wide range of agonists. This was not associated with any change in serum levels of thromboxane B2 nor with any alteration in plasma concentrations of platelet factor 4 and beta-thromboglobulin. Neither treatment had any effects on coagulation parameters. The mechanism and clinical significance of the observed increase in the extent of the platelet release reaction during combined atenolol/nifedipine treatment remain speculative.

Dose-response aggregometry--contribution to the precise platelet function evaluation

A method for the determination of blood platelet aggregability as the function of ADP concentration is described. Individual platelet aggregability was characterised by means of parameters of the sigmoidal relationship between the dose of the aggregating reagent and the aggregation curve amplitude. The method was applied to establish ADP-induced aggregability of various mammalian species. The aggregation curves of rabbit and rat platelets reached significantly lower maximum amplitudes than those of human and dog platelets. The eman concentration of ADP was significantly higher in dog platelets and the slope of the dose-response curve was significantly steeper in rat platelets compared to the other species studied. Some of the causes of individual intra-species ADP-induced aggregation variability revealed by means of dose-response aggregometry are discussed.

Platelet P1, P4-Di (adenosine-51) tetraphosphate (AP4A) in migraine patients before and during beta-adrenoceptor blockade

P1,P4-Di(adenosine-51) tetraphosphate (AP4A) is a metabolically inactive nucleotide which can be released from platelet dense granules. This study was designed firstly, to investigate whether platelet content of AP4A was decreased in patients with classical migraine and secondly, whether the content of AP4A was changed by beta-adrenoceptor blockade. No significant difference in platelet dense granule content of AP4A, was observed between 10 migraine patients and 10 normal controls. Both metoprolol, a beta 1-selective blocker and propranolol, a non-selective beta-blocker significantly decreased the migraine attack rate. However, while propranolol significantly reduced the platelet content of AP4A, metoprolol did not. Therefore, the present data suggest that platelet dense granule release, as estimated by the content of AP4A is not of major importance in migraine.

Lack of significant effect of therapeutic propranolol on measurable platelet function in healthy subjects

Propranolol, a non-selective beta blocker, was administered orally in therapeutic doses. The effects of a single dose (160 mg) and one week's treatment (80 mg twice a day) on platelet function were compared in healthy young subjects. There were no significant changes in circulating platelet aggregates, template bleeding time, platelet factor 3 availability and thromboxane beta 2 (TX beta 2) generation. Platelet aggregation responses as assessed by angle of slope and maximal percentage aggregation (all agonists) and lag phase (collagen) showed no changes of biological importance, although minor changes reaching significance were observed with some agonists. These findings suggest that propranolol does not significantly affect platelet function when taken at doses commonly encountered in clinical practice.

Differential effects of non-specific beta-blockade and calcium antagonism on blood-clotting mechanisms

The effects of non-selective beta-blockade (timolol, 5 mg twice daily) and calcium antagonism (isradipine, 2.5 mg twice daily) on heart rate, blood pressure, platelet aggregation, fibrinolytic activity, and platelet cyclic adenosine monophosphate content were investigated in 10 patients with mild hypertension in a randomized, placebo-controlled, double-blind study. Each patient served as his or her own control, taking each drug in turn for two weeks. Both drugs lowered blood pressure to the same degree. During timolol treatment, however, platelet aggregation increased whereas isradipine resulted in a shortening of the euglobulin clot lysis time (p less than 0.05), indicating increased fibrinolytic activity. Platelet aggregation and fibrinolytic activity are modified by cyclic adenosine monophosphate. Since beta-adrenoceptors are present on platelets and endothelial cells, the differences in platelet behavior and fibrinolytic activity may reflect a decreased cyclic adenosine monophosphate production caused by non-selective beta-adrenoceptor blockade.

Effect of felodipine, a new calcium channel antagonist, on platelet function and fibrinolytic activity at rest and after exercise

Fourteen healthy male volunteers (age 21-29 years, mean 23.7 years) were given placebo for 14 days followed immediately by felodipine 5 mg b.d. for a further 14 days. After each period of treatment blood for analysis was taken at rest and after exercise on a cycle ergometer. Platelet aggregation, plasma beta-thromboglobulin (B-TG), platelet factor 4 (PF-4), adrenaline and noradrenaline, and serum thromboxane B2 (TXB2), 6-keto-prostaglandin (F1 alpha (6-keto-PGF1 alpha), and euglobulin clot lysis time (ECLT), were measured on each occasion. The ECG, heart rate and blood pressure were monitored during the exercise tests. After felodipine therapy there was a significant decrease in the plasma level of PF-4 and B-TG at rest. This effect was maintained during exercise. There was no significant change in platelet aggregation, ECLT, TXB2 or 6-keto-PGF1 alpha at rest or during exercise. Irrespective of therapy, the plasma levels of adrenaline and noradrenaline were increased and the ECLT was decreased when measured immediately after exercise. Thus, felodipine in modest therapeutic dosage decreases the plasma levels of B-TG and PF-4, indicating an inhibitory effect on platelet on release. The effect also occurred during exercise. Felodipine did not change fibrinolytic activity or the production of TXB2 or 6-keto-PGF1 alpha.

Effects of three beta-blockers with different pharmacodynamic properties on platelet aggregation and platelet and plasma cyclic AMP

The effects of three different types of beta-adrenoceptor blocking agents on platelet aggregation and on platelet and plasma cyclic AMP content have been studied in 14 patients with mild hypertension given each drug in turn for two weeks. The drugs were a non-selective blocking agent with high intrinsic sympathomimetic activity, pindolol, the nonspecific blocking agent propranolol, and the beta 1-selective metoprolol. The threshold values of ADP and adrenaline for irreversible platelet aggregation were significantly higher for pindolol and metoprolol than for propranolol. The cyclic AMP content of platelets was higher during pindolol and metoprolol than during propranolol treatment. Pindolol produced a substantial increase in plasma cyclic AMP relative to the other two drugs. Thus, platelet aggregation and cyclic AMP formation are influenced by beta-adrenoceptor blockade in proportion to intrinsic sympathomimetic activity and affinity for different beta-adrenoceptor subtypes.