Effect of systemic alpha-2 adrenergic blockade on the morning increase in platelet aggregation in normal subjects

The extended autonomic system, dyshomeostasis, and COVID-19

The pandemic viral illness COVID-19 is especially life-threatening in the elderly and in those with any of a variety of chronic medical conditions. This essay explores the possibility that the heightened risk may involve activation of the “extended autonomic system” (EAS). Traditionally, the autonomic nervous system has been viewed as consisting of the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system. Over the past century, however, neuroendocrine and neuroimmune systems have come to the fore, justifying expansion of the meaning of “autonomic.” Additional facets include the sympathetic adrenergic system, for which adrenaline is the key effector; the hypothalamic-pituitary-adrenocortical axis; arginine vasopressin (synonymous with anti-diuretic hormone); the renin-angiotensin-aldosterone system, with angiotensin II and aldosterone the main effectors; and cholinergic anti-inflammatory and sympathetic inflammasomal pathways. A hierarchical brain network—the “central autonomic network”—regulates these systems; embedded within it are components of the Chrousos/Gold “stress system.” Acute, coordinated alterations in homeostatic settings (allostasis) can be crucial for surviving stressors such as traumatic hemorrhage, asphyxiation, and sepsis, which throughout human evolution have threatened homeostasis; however, intense or long-term EAS activation may cause harm. While required for appropriate responses in emergencies, EAS activation in the setting of chronically decreased homeostatic efficiencies (dyshomeostasis) may reduce thresholds for induction of destabilizing, lethal vicious cycles. Testable hypotheses derived from these concepts are that biomarkers of EAS activation correlate with clinical and pathophysiologic data and predict outcome in COVID-19 and that treatments targeting specific abnormalities identified in individual patients may be beneficial.

Platelets and Cardiovascular Disease

Platelets play an important, but often under-recognized role in cardiovascular disease. For example, the normal response of the platelet can be altered, either by increased pro-aggregatory stimuli or by diminished anti-aggregatory substances to produce conditions of increased platelet activation/aggregation and occur in active cardiovascular disease states both on a chronic (e.g. stable angina pectoris) and acute basis (e.g. acute myocardial infarction). In addition, platelet hyperaggregability is also associated with the risk factors for coronary artery disease (e.g. smoking, hypertension, and hypercholesterolaemia). Finally, the utility of an increasing range of anti-platelet therapies in the management of the above disease states further emphasizes the pivotal role platelets play in the pathogenesis of cardiovascular disease. This paper provides a comprehensive overview of the normal physiologic role of platelets in maintain homeostasis, the pathophysiologic processes that contribute to platelet dysfunction in cardiovascular disease and the associated role and benefits of anti-platelet therapies.

Orthostatic hypertension-a new haemodynamic cardiovascular risk factor

Orthostatic hypertension-a condition characterized by a hyperactive pressor response to orthostatic stress-is an emerging risk factor for cardiovascular disease and is associated with hypertensive target-organ damage (resulting in silent cerebrovascular disease, left ventricular hypertrophy, carotid atherosclerosis and/or chronic kidney disease) and cardiovascular events (such as coronary artery disease and lacunar stroke). The condition is also considered to be a form of prehypertension as it precedes hypertension in young, normotensive adults. Orthostatic blood pressure changes can be assessed using orthostatic stress tests, including clinic active standing tests, home blood pressure monitoring and the head-up tilting test. Devices for home and for ambulatory blood pressure monitoring that are equipped with position sensors and do not induce a white-coat effect have increased the sensitivity and specificity of diagnosis of out-of-clinic orthostatic hypertension. Potential major mechanisms of orthostatic hypertension are sympathetic hyperactivity (as a result of hypersensitivity of the cardiopulmonary and arterial baroreceptor reflex) and α-adrenergic hyperactivation. Orthostatic hypertension is also associated with morning blood pressure surge and extreme nocturnal blood pressure dipping, both of which increase the pulsatile haemodynamic stress of central arterial pressure and blood flow in patients with systemic haemodynamic atherothrombotic syndrome.

Depression and coronary heart disease

There are exciting findings in the field of depression and coronary heart disease. Whether diagnosed or simply self-reported, depression continues to mark very high risk for a recurrent acute coronary syndrome or for death in patients with coronary heart disease. Many intriguing mechanisms have been posited to be implicated in the association between depression and heart disease, and randomized controlled trials of depression treatment are beginning to delineate the types of depression management strategies that may benefit the many coronary heart disease patients with depression.

Effect of live-fire training drills on firefighters' platelet number and function

BACKGROUND

The leading cause of line-of-duty death among firefighters is sudden cardiac events. Platelets play a critical role in the formation of an occlusive thrombus during an ischemic event.

OBJECTIVE

The purpose of this study was to examine the acute effect of firefighting on platelet number and aggregability.

METHODS

Apparently healthy male firefighters (N = 114; age 29.4 ± 7.8 years) participated in 18 minutes of simulated firefighting activity in a training structure that contained live fires. Blood samples were obtained before and after simulated firefighting activity and analyzed for complete blood cell count (CBC), chemistry, and platelet number and function. Platelet function was measured using a PFA-100 analyzer to assess platelet aggregability.

RESULTS

As expected, performing firefighting activity resulted in significant increases in heart rate (75 b·min(-1)) and core temperature (0.7 °C), and significant changes in blood chemistry values. The most important finding in this study is that 18 minutes of simulated firefighting caused a 24% increase in platelet number and a significant increase in platelet aggregability.

CONCLUSIONS

Firefighting resulted in a significant increase in platelet number and aggregability, indicating that even short bouts of firefighting can increase thrombotic potential in apparently healthy firefighters.

Diurnal variation in platelet inhibition by clopidogrel

Morning increase in the occurrence of myocardial infarction, stroke and sudden cardiac death is a well-recognized phenomenon, which is in line with a morning enhancement of platelet aggregation. We investigated whether platelet inhibition during clopidogrel and aspirin therapy varies during the day. Fifty-nine consecutive patients (45 men and 14 women) with first ST-segment elevation myocardial infarction (STEMI) treated with primary percutaneous coronary interventions (pPCI) on dual antiplatelet therapy were prospectively enrolled into the study. Blood samples were collected 4 days after start of clopidogrel treatment at 6.00 a.m., 10.00 a.m., 2.00 p.m. and 7.00 p.m. Arachidonic acid and adenosine diphosphate (ADP)-induced platelet aggregation were assessed by impedance aggregometry. Platelet inhibition by clopidogrel was lowest in the midmorning: median ADP-induced platelet aggregation was 55%, 17% and 27% higher at 10.00 a.m. compared to 6.00 a.m., 2.00 p.m. and 7.00 p.m., respectively (p < 0.002). Nonresponsiveness to clopidogrel defined according to the device manufacturer was 2.4-fold more frequent in the midmorning than in the early morning. We observed a more pronounced midmorning increase in ADP-induced platelet aggregation in diabetic patients when compared to non-diabetics. In contrast, no diurnal variation in the antiplatelet effect of aspirin was observed. In conclusion, in patients presenting with STEMI undergoing pPCI, platelet inhibition by clopidogrel is less strong in the midmorning hours. This periodicity in platelet aggregation in patients on dual antiplatelet therapy should be taken into consideration when assessing platelet function in clinical studies.

Predictors of exercise-induced platelet reactivity in patients with chronic stable angina

OBJECTIVE

Previous studies have shown that exercise increases platelet reactivity in patients with coronary artery disease (CAD). However, the response of platelet reactivity to exercise is considerably variable and its predictors are poorly known.

METHODS

We studied 214 consecutive patients (age 61.9 +/- 9 years, 167 men) with stable angina and obstructive coronary artery disease. All patients underwent a symptom-limited treadmill exercise stress test. Venous blood samples were collected before and at peak exercise. Platelet reactivity was assessed by the platelet function analyzer system as the time for flowing whole blood to occlude a collagen-adenosine diphosphate ring (closure time: shorter times = higher reactivity). Both closure time at peak exercise and the exercise-induced change in closure time from rest were assessed as an expression of exercise-related platelet reactivity.

RESULTS

Closure time decreased significantly with exercise in the whole population (from 95.9 +/- 22 to 81.2 +/- 18 s, P < 0.001). The only variable significantly associated with closure time at peak exercise was hematocrit (P = 0.003). Basal systolic blood pressure (P = 0.023) and lack of nitrate use (P = 0.03), on the contrary, were the only variables significantly associated with increased exercise-induced closure time change. Peak hematocrit maintained an independent association with peak closure time in multivariable analysis, although the correlation was mild. No variable, on the contrary, was associated with exercise-induced platelet reactivity after correction for basal closure time values at multivariable analyses.

CONCLUSION

Among stable coronary artery disease patients, platelet reactivity after exercise cannot be reliably predicted by several common clinical and laboratory variables.

Circadian Variation in Stroke Onset: Identical Temporal Pattern in Ischemic and Hemorrhagic Events

Stroke is the culmination of a heterogeneous group of cerebrovascular diseases that is manifested as ischemia or hemorrhage of one or more blood vessels of the brain. The occurrence of many acute cardiovascular events--such as myocardial infarction, sudden cardiac death, pulmonary embolism, critical limb ischemia, and aortic aneurysm rupture--exhibits prominent 24 h patterning, with a major morning peak and secondary early evening peak. The incidence of stroke exhibits the same 24 h pattern. Although ischemic and hemorrhagic strokes are different entities and are characterized by different pathophysiological mechanisms, they share an identical double-peak 24 h pattern. A constellation of endogenous circadian rhythms and exogenous cyclic factors are involved. The staging of the circadian rhythms in vascular tone, coagulative balance, and blood pressure plus temporal patterns in posture, physical activity, emotional stress, and medication effects play central and/or triggering roles. Features of the circadian rhythm of blood pressure, in terms of their chronic and acute effects on cerebral vessels, and of coagulation are especially important. Clinical medicine has been most concerned with the prevention of stroke in the morning, when population-based studies show it is of greatest risk during the 24 h; however, improved protection of at-risk patients against stroke in the early evening, the second most vulnerable time of cerebrovascular accidents, has received relatively little attention thus far.

Influence of the Circadian System on Disease Severity

The severity of many diseases varies across the day and night. For example, adverse cardiovascular incidents peak in the morning, asthma is often worse at night and temporal lobe epileptic seizures are most prevalent in the afternoon. These patterns may be due to the day/night rhythm in environment and behavior, and/or endogenous circadian rhythms in physiology. Furthermore, chronic misalignment between the endogenous circadian timing system and the behavioral cycles could be a cause of increased risk of diabetes, obesity, cardiovascular disease and certain cancers in shift workers. Here we describe the magnitude, relevance and potential biological basis of such daily changes in disease severity and of circadian/behavioral misalignment, and present how these insights may help in the development of appropriate chronotherapy.

Activity-adjusted 24-hour ambulatory blood pressure and cardiac remodeling in children with sleep disordered breathing

Questions remain as to whether pediatric sleep disordered breathing increases the risk for elevated blood pressure and blood pressure-dependent cardiac remodeling. We tested the hypothesis that activity-adjusted morning blood pressure surge, blood pressure load, and diurnal and nocturnal blood pressure are significantly higher in children with sleep disordered breathing than in healthy controls and that these blood pressure parameters relate to left ventricular remodeling. 24-hour ambulatory blood pressure parameters were compared between groups. The associations between blood pressure and left ventricular relative wall thickness and mass were measured. 140 children met the inclusion criteria. In children with apnea hypopnea index <5 per hour, a significant difference from controls was the morning blood surge. Significant increases in blood pressure surge, blood pressure load, and in 24-hour ambulatory blood pressure were evident in those whom the apnea hypopnea index exceeded 5 per hour. Sleep disordered breathing and body mass index had similar effect on blood pressure parameters except for nocturnal diastolic blood pressure, where sleep disordered breathing had a significantly greater effect than body mass index. Diurnal and nocturnal systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure predicted the changes in left ventricular relative wall thickness. Therefore, sleep disordered breathing in children who are otherwise healthy is independently associated with an increase in morning blood pressure surge, blood pressure load, and 24-hour ambulatory blood pressure. The association between left ventricular remodeling and 24-hour blood pressure highlights the role of sleep disordered breathing in increasing cardiovascular morbidity.

Chronobiology of hemostasis and inferences for the chronotherapy of coagulation disorders and thrombosis prevention

The hemostatic system in its multiple components displays an intricate organization in time which is characterized by circadian (approximately 24-hour), circaseptan (approximately 7-day), menstrual (approximately monthly), and circannual (approximately yearly) bioperiodicities. The interaction of the rhythms of the variables participating in hemostasis determine transient risk states of thromboembolic events, including myocardial infarction and stroke, and of hemorrhage and hemorrhagic events, each with a unique timing. The circadian staging of the rhythms in vascular, cellular, and coagulation factors that favors blood coagulation and thrombosis coincides with the daily minimum in fibrinolytic activity; as a result there is elevated risk in the morning of acute myocardial infarction and stroke. Similar hemostatic rhythms may determine the epidemiology of thromboembolic and hemorrhagic events during the week, month and year. This article focuses on the large-amplitude circadian rhythms operative in the hemostatic system. Their implication for preventive and curative pharmacotherapy of hemostatic disorders is presented, with discussion of related problems.

Aggregation and Activation of Blood Platelets in Exercise and Training

This article presents an overview of the progress that has been made in recent years in our understanding of the interaction between exercise and platelets in health and disease. Although platelets are important in normal haemostasis, recent evidence emphasises the pivotal role of abnormal platelet function in acute coronary artery diseases, myocardial infarction, unstable angina and stroke. In light of the positive health benefits of exercise, interest has been heightened on the association between exercise and platelet aggregation and function, not only in normal healthy subjects but also in patients. However, the study of exercise effects on blood platelets are highly contentious because of the fact that the analytical methods employed to study platelets are bedevilled by numerous methodological problems. While exercise effects on platelet aggregation and function in healthy individuals have been extensively examined, the evidence reported has been conflicting. Somewhat less contradictory are the results generated from studies in patients with coronary heart disease, as the preponderance of evidence available would strongly suggest that platelet aggregation and function are increased with exercise. Several drugs are known to influence platelet aggregation and function, the most examined among these medications is aspirin (acetylsalicylic acid). However, aspirin appears to be ineffective to attenuate exercise-induced increases in platelet aggregation and activation. Few studies are available on the effect of training on blood platelets and the exact effects of exercise training on platelet activation and function is not as yet known. This lack of information makes further studies particularly important, in order to clarify whether there are favourable effects of exercise training on platelet aggregation and function in health and disease.

Regulation of platelet alpha 2A-adrenoceptors, Gi proteins and receptor kinases in major depression: effects of mirtazapine treatment

Major depression is associated with the upregulation of alpha(2A)-adrenoceptors in brain tissue and blood platelets. The homologous regulation of these receptors by G-protein-coupled receptor kinases (GRKs) might play a relevant role in the pathogenesis and treatment of depression. This study was designed to assess the status of the complex alpha(2A)-adrenoceptor/Galphai/GRK 2 in the platelets of depressed patients (n=22) before and after treatment with the antidepressant mirtazapine, an antagonist at alpha(2A)-adrenoceptors (30-45 mg/day for up to 6 months). A second series of depressed suicide attempters (n=32) were also investigated to further assess the status of platelet GRK 2 and GRK 6. Platelet alpha(2A)-adrenoceptors and Galphai protein immunoreactivities were increased in depressed patients (49 and 35%) compared with matched controls. In contrast, GRK 2 content was decreased in the two series of depressed patients (27 and 28%). GRK 6 (a GRK with different properties) was found unchanged. In drug-free depressed patients, the severity of depression (behavioral ratings with two different instruments) correlated inversely with the content of platelet GRK 2 (r=-0.46, n=22, p=0.032, and r=-0.55, n=22, p=0.009). After 4-24 weeks of treatment, mirtazapine induced downregulation of platelet alpha(2A)-adrenoceptors (up to 34%) and Galphai proteins (up to 28%), and the upregulation of GRK 2 (up to 30%). The results indicate that major depression is associated with reduced platelet GRK 2, suggesting that a defect of this kinase may contribute to the observed upregulation of alpha(2A)-adrenoceptors. Moreover, treatment with mirtazapine reversed this abnormality and induced downregulation of alpha(2A)-adrenoceptor/Galphai complex. The results support a role of supersensitive alpha(2A)-adrenoceptors in the pathogenesis and treatment of major depression.

Pre-exercise administration of yohimbine may enhance the efficacy of exercise training as a fat loss strategy by boosting lipolysis

The natural alpha-2 antagonist yohimbine promotes sympathetic activity by central as well as peripheral mechanisms, and yet in moderate doses dose not usually raise heart rate, increase blood pressure, or induce anxiety (in contrast to sympathomimetic drugs such as ephedrine). Administered prior to exercise, it boosts lipolysis and serum FFA levels both during and following exercise; blockade of adipocyte alpha-2 adrenoreceptors makes at least a modest contribution to this pro-lipolytic activity. These considerations suggest that pre-exercise administration of yohimbine will lower the respiratory quotient during and following exercise, thus promoting fat loss. Since yohimbine can potentiate postprandial insulin secretion, its bariatric benefits should be greatest if administered on a schedule that minimizes postprandial yohimbine activity. A possible synergism of yohimbine and caffeine should be explored. Pre-exercise yohimbine administration has the potential to down-regulate the lipoprotein lipase activity of visceral adipocytes, increase lipolysis in refractory gynoid fat depots, and improve the impaired lipolytic response to exercise in the elderly.

Yohimbine: a clinical review

Although yohimbine (YOH) has been available for the treatment of male erectile dysfunction (ED) for longer than Viagra, there is a perception that little is known about the clinical performance of the drug. This review attempts, by comprehensive analysis of the literature, to cover the clinical, pharmacological, and therapeutic profiles of YOH, relevant to its potential utility in the management of patients with ED. Relatively few well-designed studies have been completed. From these, however, it can be concluded that YOH as monotherapy possesses only modest efficacy in ED patients. In acute and chronic (long-term) studies, YOH has been found to be relatively free of side effects over the dose range predicted to be effective in ED. At much higher doses, the most frequently observed effects, consistent with the primary pharmacological action of the drug, are elevation of blood pressure, a slight anxiogenic action, and increased frequency of urination. These side effects are all easily reversible on termination of YOH therapy. There is increasing evidence that the erectogenic action of YOH can be augmented by concomitant administration of agents that augment the release and/or action of nitric oxide in the corpus cavernosum. YOH has yet to be studied in female sexual dysfunction. Overall, the benefit risk profile of YOH would indicate that it has potential, more probably as part of a combination strategy, e.g., with a drug that enhances the nitric oxide pathway, in the treatment of ED.

Low-grade exercise enhances platelet aggregability in patients with obstructive coronary disease independently of myocardial ischemia

Moderate and strenuous exercise is known to enhance platelet aggregability in patients with obstructive coronary artery disease (CAD), but the effect of low-grade exercise is not known. We assessed shear-induced platelet aggregability before and after mild exercise (less than or equal to stage III of the modified Bruce protocol) in 27 patients with documented CAD who were receiving aspirin and in 12 subjects without CAD (controls). Ex vivo platelet aggregability was assessed in flowing whole blood as the time to occlude a collagen and adenosine diphosphate-coated ring; shorter times indicated greater aggregability. Aggregability, plasma von Willebrand factor (vWF) antigen, platelet and white cell counts, and hematocrit were measured at baseline, immediately after exercise (peak), and at 30 and 180 minutes after exercise. Exercise of similar workloads induced myocardial ischemia in 14 patients (group 1), but not in the other 13 (group 2) nor in controls. Both patient groups showed a reduction in aggregation time at peak exercise compared with baseline (group 1: 84+/-17 seconds at peak vs 96+/-22 seconds at baseline; group 2: 84+/-20 seconds at peak vs 99+/-20 seconds at baseline; p <0.03 for both comparisons), with a return to baseline values within 180 minutes. No significant variation occurred in controls (89+/-18 seconds at peak vs 85+/-21 second at baseline). Changes in vWF antigen did not differ significantly among groups. Aggregation times did not correlate with hematocrit or platelet and white cell counts. Thus, even low-grade exercise transiently enhances whole blood platelet aggregability in patients with obstructive CAD, but not in controls. The effect is independent of myocardial ischemia, occurs despite aspirin, and is likely dependent on hemodynamic factors interacting with coronary obstructions or dysfunctional endothelium.