Effects of caffeine on baroreflex activity in humans

Diagnostic Ultrasound-Based Investigation of Central vs. Peripheral Arterial Changes Consequent to Low-Dose Caffeine Ingestion

Caffeine is present in various foods and medicines and is highly accessible through various routes, regardless of age. However, most studies on caffeine have focused on the effects of high-dose caffeine ingestion based on the recommended daily amount for adults. In this study, we examined the physiological changes in the central and peripheral vessels that may occur when ingesting low-dose caffeine due to its high accessibility, with the aim of creating an environment of safe caffeine ingestion. This study included 26 healthy participants in their 20s. Peak systolic velocity (PSV), heart rate (HR), and pulse wave velocity (PWV) for vascular stiffness assessment were measured at 0, 30, and 60 min after caffeine ingestion using diagnostic ultrasound to determine the physiological changes in the blood vessels, common carotid artery (CCA) and radial artery (RA). In addition, percutaneous oxygen saturation (SpO2), blood pressure (BP), and accelerated photoplethysmography (APG) were measured. In comparison with before ingestion, the HR tended to decrease and showed a significant difference at 30 and 60 min (p = 0.014 and p = 0.031, respectively). PSV significantly decreased in both vessels at 30 and 60 min (p < 0.001 and p < 0.001, respectively). APG showed a decreasing trend until 60 min after ingestion, with a significant difference at 30 and 60 min (p = 0.003 and p = 0.012, respectively). No significant difference was observed in SpO2, BP, or PWV; however, they showed a tendency to increase after ingestion. Decreased HR may occur because of the baroreflex caused by an increase in BP. The RA has many branches and a smaller diameter; therefore, the PSV was lower in the RA than that in the CCA. This effect can occur because of the difficulty in the smooth expansion of blood vessels, which leads to a decrease in blood flow. In addition, an increase in intracellular calcium concentration can prevent vasodilation and increase the propagation velocity of pulse waves. The reflected waves can increase systolic blood pressure but reduce PWV and vascular elasticity. These results suggest that even low-dose caffeine can improve blood vessel health by providing temporary stimulation to the blood vessels; however, it can also cause changes in blood flow and blood vessel elasticity, which can lead to serious diseases such as stroke and high blood pressure. Therefore, caution should be exercised when caffeine consumption is indiscriminate.

Immediate effect of caffeine on sympathetic nerve activity: why coffee is safe? A single-centre crossover study

PURPOSES

Habitual coffee drinking is ubiquitous and generally considered to be safe despite its transient hypertensive effect. Our purpose was to determine the role of the sympathetic nervous system in the hypertensive response.

METHODS

In a single-centre crossover study, medical caregivers were studied after consumption of standard coffee (espresso), water and decaffeinated coffee (decaff) given in random order at least 1 month apart. Plasma caffeine levels, mean arterial pressure, heart rate, total peripheral resistance and muscle sympathetic activity were recorded. Baroreflex activity was assessed using burst incidence and RR interval changes to spontaneous blood pressure fluctuations.

RESULTS

A total of 16 subjects (mean [± standard error] age 34.4 ± 2 years; 44% female) were recruited to the study. Three agents were studied in ten subjects, and two agents were studied in six subjects. Over a 120-min period following the consumption of standard coffee, mean (± SE) plasma caffeine levels increased from 2.4 ± 0.8 to 21.0 ± 4 µmol/L and arterial pressure increased to 103 ± 1 mmHg compared to water (101 ± 1 mmHg; p = 0.066) and decaff (100 ± 1 mmHg; p = 0.016). Peripheral resistance in the same period following coffee increased to 120 ± 4% of the baseline level compared to water (107 ± 4; p = 0.01) and decaff (109 ± 4; p = 0.02). Heart rate was lower after both coffee and decaff consumption: 62 ± 1 bpm compared to water (64 bpm; p = 0.01 and p = 0.02, respectively). Cardio-vagal baroreflex activity remained stable after coffee, but sympathetic activity decreased, with burst frequency of 96 ± 3% versus water (106 ± 3%; p = 0.04) and decaff (112 ± 3%; p = 0.001) despite a fall in baroreflex activity from - 2.2 ± 0.1 to - 1.8 ± 0.1 bursts/100 beats/mmHg, compared to water (p = 0.009) and decaff (p = 0.004).

CONCLUSION

The hypertensive response to coffee is secondary to peripheral vasoconstriction but this is not mediated by increased sympathetic nerve activity. These results may explain why habitual coffee drinking is safe.

The Influence of Caffeine Supplementation on Resistance Exercise: A Review

This paper aims to critically evaluate and thoroughly discuss the evidence on the topic of caffeine supplementation when performing resistance exercise, as well as provide practical guidelines for the ingestion of caffeine prior to resistance exercise. Based on the current evidence, it seems that caffeine increases both maximal strength and muscular endurance. Furthermore, power appears to be enhanced with caffeine supplementation, although this effect might, to a certain extent, be caffeine dose- and external load-dependent. A reduction in rating of perceived exertion (RPE) might contribute to the performance-enhancing effects of caffeine supplementation as some studies have observed decreases in RPE coupled with increases in performance following caffeine ingestion. However, the same does not seem to be the case for pain perception as there is evidence showing acute increases in resistance exercise performance without any significant effects of caffeine ingestion on pain perception. Some studies have reported that caffeine ingestion did not affect exercise-induced muscle damage, but that it might reduce perceived resistance exercise-induced delayed-onset muscle soreness; however, this needs to be explored further. There is some evidence that caffeine ingestion, compared with a placebo, may lead to greater increases in the production of testosterone and cortisol following resistance exercise. However, given that the acute changes in hormone levels seem to be weakly correlated with hallmark adaptations to resistance exercise, such as hypertrophy and increased muscular strength, these findings are likely of questionable practical significance. Although not without contrasting findings, the available evidence suggests that caffeine ingestion can lead to acute increases in blood pressure (primarily systolic), and thus caution is needed regarding caffeine supplementation among individuals with high blood pressure. In the vast majority of studies, caffeine was administered in capsule or powder forms, and therefore the effects of alternative forms of caffeine, such as chewing gums or mouth rinses, on resistance exercise performance remain unclear. The emerging evidence suggests that coffee might be at least equally ergogenic as caffeine alone when the caffeine dose is matched. Doses in the range of 3-9 mg·kg^-1 seem to be adequate for eliciting an ergogenic effect when administered 60 min pre-exercise. In general, caffeine seems to be safe when taken in the recommended doses. However, at doses as high as 9 mg·kg^-1 or higher, side effects such as insomnia might be more pronounced. It remains unclear whether habituation reduces the ergogenic benefits of caffeine on resistance exercise as no evidence exists for this type of exercise. Caution is needed when extrapolating these conclusions to females as the vast majority of studies involved only male participants.

The clinical toxicology of caffeine: A review and case study

Caffeine is a widely recognized psychostimulant compound with a long history of consumption by humans. While it has received a significant amount of attention there is still much to be learned with respect to its toxicology in humans, especially in cases of overdose. A review of the history of consumption and the clinical toxicology of caffeine including clinical features, pharmacokinetics, toxicokinetics, a thorough examination of mechanism of action and management/treatment strategies are undertaken. While higher (i.e., several grams) quantities of caffeine are known to cause toxicity and potentially lethality, cases of mainly younger individuals who have experienced severe side effects and death despite consuming doses not otherwise known to cause such harm is troubling and deserves further study. An attempted case reconstruction is performed in an effort to shed light on this issue with a focus on the pharmacokinetics and pharmacodynamics of caffeine.

The influence of CYP1A2 genotype in the blood pressure response to caffeine ingestion is affected by physical activity status and caffeine consumption level

This study aimed to investigate whether the influence of CYP1A2 genotype in the blood pressure (BP) response to caffeine ingestion was affected by physical activity status and habitual caffeine consumption. Thirty-seven participants (19-50 years old) took place in the study and were categorized according to i) genotype: CYP1A2 (AA) "fast metabolizer", and CYP1A2 (AC) "slow metabolizer"; ii) physical activity level: sedentary (S) and physically active (A); and iii) caffeine consumption level: non-habitual caffeine consumer (NC) and habitual heavy caffeine consumer (C). All groups had BP assessed before (basal) and 1 hourh after (post) caffeine ingestion (6 mg·kg^-1). It was observed that AC genotype individuals had increased basal-DBP and post-caffeine SBP when compared to AA individuals. Additionally, acute caffeine ingestion increased SBP only in the AC group. It was also found that physical activity only modulated the BP responses to acute caffeine ingestion in AC individuals. Furthermore, the results indicated that the habitual heavy caffeine consumers AC individuals had increased basal-DBP when compared to the AA ones. Our results suggest that the influence of CYP1A2 genotype in the basal and post-caffeine BP response to caffeine ingestion is modified by physical activity status and caffeine consumption level.

Caffeine and cardiovascular diseases: critical review of current research

Caffeine is a most widely consumed physiological stimulant worldwide, which is consumed via natural sources, such as coffee and tea, and now marketed sources such as energy drinks and other dietary supplements. This wide use has led to concerns regarding the safety of caffeine and its proposed beneficial role in alertness, performance and energy expenditure and side effects in the cardiovascular system. The question remains "Which dose is safe?", as the population does not appear to adhere to the strict guidelines listed on caffeine consumption. Studies in humans and animal models yield controversial results, which can be explained by population, type and dose of caffeine and low statistical power. This review will focus on comprehensive and critical review of the current literature and provide an avenue for further study.

A Review of Cardiovascular Autonomic Control in Cluster Headache

OBJECTIVE

This review aims to evaluate existing literature concerning cardiovascular autonomic function and CH. Suggestions about future research are offered and known difficulties in investigating the autonomic nervous system in cluster headache are discussed.

BACKGROUND

Little is known of the pathophysiological mechanisms behind cluster headache. Cranial autonomic features are an inherent and diagnostic feature; however, a number of studies and clinical observations support the involvement of systemic autonomic control in its pathophysiology. Further, cluster headache attacks are apparently more easily triggered during periods of parasympathetic dominance. A better understanding of this interaction may provide insight into central autonomic regulation and its role in cluster headache.

METHODS

A PubMed search was performed in April 2015 using the search terms "cluster headache," "cardiovascular," "autonomic nervous system," and "cardiac." References of identified articles were also searched for relevant articles. Studies were included if they contained data on cardiovascular or autonomic responses to autonomic tests, induced or spontaneous attacks.

RESULTS

In total, 22 studies investigating cardiac autonomic control in cluster headache were identified. Three overall categories of investigations exist: (1) Those studying changes in heart rate, blood pressure, and electrocardiographic changes; (2) those employing various clinical autonomic tests; and finally (3) those using spectral and nonlinear analysis of heart rate variability. Although not completely congruent, overall, results suggest ictal hyperactivation of the parasympathetic branch and a sympathetic deficit. Subclinical autonomic dysregulation is also present in the pain-free state.

CONCLUSION

Cardiac autonomic control is subclinically affected in cluster headache. The changes could be attributed to the suggested central dysregulation present in this disorder.

Effects of energy drinks on blood pressure, heart rate, and electrocardiographic parameters: An experimental study on healthy young adults

Objective:

To evaluate the effects of the consumption of energy drinks on cardiovascular parameters in a group of healthy young individuals.

Methods:

In a quasi-experimental study, 44 healthy adult participants aged between 15 and 30 years were evaluated. The blood pressure (BP) as well as electrocardiographic indices, including heart rate (HR), PR interval, QRS duration, corrected QT (QTc) interval, and ST-T changes were recorded before consumption of a caffeine-containing energy drink and at the specific time points over a 4-h test duration.

Results:

We found statistically significant HR decline (p=0.004) and more frequent ST-T changes (p=0.004) after the participants consumed the energy drink. However, readings for systolic BP (p=0.44), diastolic BP (p=0.26), PR interval (p=0.449), QRS duration (p=0.235), and QTc interval (p=0.953) showed no significant change post-consumption.

Conclusion:

In conclusion, we demonstrated that the consumption of energy drinks could contribute to HR decline and ST-T change in healthy young adults.

Caffeine delays autonomic recovery following acute exercise

BACKGROUND

Impaired autonomic recovery of heart rate (HR) following exercise is associated with an increased risk of sudden death. Caffeine, a potent stimulator of catecholamine release, has been shown to augment blood pressure (BP) and sympathetic nerve activity; however, whether caffeine alters autonomic function after a bout of exercise bout remains unclear.

METHODS

In a randomized, crossover study, 18 healthy individuals (26 ± 1 years; 23.9 ± 0.8 kg·m(-2)) ingested caffeine (400 mg) or placebo pills, followed by a maximal treadmill test to exhaustion. Autonomic function and ventricular depolarization/repolarization were determined using heart rate variability (HRV) and corrected QT interval (QTc), respectively, at baseline, 5, 15, and 30 minutes post-exercise.

RESULTS

Maximal HR (HRmax) was greater with caffeine (192 ± 2 vs. 190 ± 2 beat·min(-1), p < 0.05). During recovery, HR, mean arterial pressure (MAP), and diastolic blood pressure (DBP) remained elevated with caffeine (p < 0.05). Natural log transformation of low-to-high frequency ratio (LnLF/LnHF) of HRV was increased compared with baseline at all time points in both trials (p < 0.05), with less of an increase during 5 and 15 minutes post-exercise in the caffeine trial (p < 0.05). QTc increased from baseline at all time points in both trials, with greater increases in the caffeine trial (p < 0.05).

CONCLUSIONS

Caffeine ingestion disrupts post-exercise autonomic recovery because of increased sympathetic nerve activity. The prolonged sympathetic recovery time could subsequently hinder baroreflex function during recovery and disrupt the stability of autonomic function, potentiating a pro-arrhythmogenic state in young adults.

A prospective placebo controlled randomized study of caffeine in patients with supraventricular tachycardia undergoing electrophysiologic testing

INTRODUCTION

Patients with cardiac arrhythmias are generally instructed to avoid caffeine intake. A comprehensive evaluation of the electrophysiological effects of caffeine on atrial and ventricular tissues in humans has not previously been performed.

METHODS AND RESULTS

Eighty patients (31 men, mean age 49 ± 14 years) with symptomatic supraventricular tachycardia (SVT) undergoing an electrophysiologic study (EPS) prior to catheter ablation were randomized to receive oral caffeine or placebo. Caffeine at a dosage of 5 mg/kg (moderate intake) or placebo tablets were administered orally at a mean time of 57 ± 13 minutes prior to the EPS. The median (IQR) caffeine level in patients receiving caffeine was 7.4 μg/mL (4.7-8.7), as compared with 0.15 (0.00-0.61) in patients receiving placebo, P < 0.0001. Caffeine was associated with a significant increase in resting systolic and diastolic blood pressures as compared with placebo, while the resting heart rate was not significantly different between both groups. Caffeine was not associated with significant effects on the effective refractory period of the atrium or ventricle, as well as on AV node conduction. SVT was induced in all but 3 patients; there was no significant difference between groups receiving placebo or caffeine on SVT inducibility or the cycle length of induced tachycardias.

CONCLUSIONS

Caffeine, at moderate intake, was associated with significant increases in systolic and diastolic blood pressures, but had no evidence of a significant effect on cardiac conduction and refractoriness. Furthermore, no effect of caffeine on SVT induction or more rapid rates of induced tachycardias was found.

Caffeinated coffee consumption, cardiovascular disease, and heart valve disease in the elderly (from the Framingham Study)

The relation between caffeinated coffee consumption and heart disease morbidity and mortality is of great interest given the extensive use of this beverage. A recent prospective epidemiologic study found a strong protective association in elderly subjects without moderate to severe hypertension in the NHANES. To test this association in the Framingham Heart Study population, in which cardiovascular risk factors and health behaviors were carefully documented, Cox regression analyses were conducted for 1,354 subjects aged 65.4 to 96.6 years at study entry. There were 210 deaths from cardiovascular disease and 118 from coronary heart disease (CHD) during 10.1 years of follow-up. A significant negative association between caffeinated coffee consumption and CHD mortality was observed for subjects with systolic blood pressure (BP) <160 mm Hg and diastolic BP <100 mm Hg. The decrease in risk of CHD mortality for any caffeinated coffee versus none was 43% (95% confidence interval 9 to 64). This decreased risk appeared to be caused primarily by an inverse prospective relation between caffeinated coffee consumption and the development or progression of heart valve disease. The decrease in risk of heart valve disease for subjects with systolic BP <160 mm Hg and diastolic BP <100 mm Hg for any caffeinated coffee versus none was 43% (95% confidence interval 4 to 66). In conclusion, caffeinated coffee consumption was associated with lower risk of CHD mortality and heart valve disease development or progression in older Framingham subjects without moderate or severe hypertension.

Low doses of caffeine reduce heart rate during submaximal cycle ergometry

Background

The purpose of this study was to examine the cardiovascular effects of two low-levels of caffeine ingestion in non habitual caffeine users at various submaximal and maximal exercise intensities.

Methods

Nine male subjects (19–25 yr; 83.3 ± 3.1 kg; 184 ± 2 cm), underwent three testing sessions administered in a randomized and double-blind fashion. During each session, subjects were provided 4 oz of water and a gelatin capsule containing a placebo, 1.5 mg/kg caffeine, or 3.0 mg/kg caffeine. After thirty minutes of rest, a warm-up (30 Watts for 2 min) the pedal rate of 60 rpm was maintained at a steady-state output of 60 watts for five minutes; increased to 120 watts for five minutes and to 180 watts for five minutes. After a 2 min rest the workload was 180 watts for one minute and increased by 30 watts every minute until exhaustion. Heart rate (HR) was measured during the last 15-seconds of each minute of submaximal exercise. Systolic blood pressure (BP) was measured at rest and during each of the three sub-maximal steady state power outputs. Minute ventilation (V_E), Tidal volume (V_T), Breathing frequency (Bf), Rating of perceived exertion (RPE), Respiratory exchange ratio (RER), and Oxygen consumption (VO₂) were measured at rest and during each minute of exercise.

Results

Caffeine at 1.5 and 3.0 mg/kg body weight significantly lowered (p < 0.05) HR during all three submaximal exercise intensities compared to placebo (range – 4 to 7 bpm lower) but not at rest or maximal exercise. BP was significantly higher (p < 0.05) at rest and after the 3 mg/kg caffeine vs placebo (116 ± 13 vs 123 ± 10 mm Hg). Neither dose of caffeine had any effect on BP during submaximal exercise. Caffeine had no effect on V_E, V_T, VO₂, RPE, maximal power output or time to exhaustion.

Conclusion

In non habitual caffeine users it appears that consuming a caffeine pill (1.5 & 3.0 mg/kg) at a dose comparable to 1–3 cups of coffee lowers heart rate during submaximal exercise but not at near maximal and maximal exercise. In addition, this caffeine dose also only appears to affect systolic blood pressure at rest but not during cycling exercise.

Caffeinated beverage intake and the risk of heart disease mortality in the elderly: a prospective analysis

BACKGROUND

Motivated by the possibility that caffeine could ameliorate the effect of postprandial hypotension on a high risk of coronary events and mortality in aging, we hypothesized that caffeinated beverage consumption decreases the risk of cardiovascular disease (CVD) mortality in the elderly.

OBJECTIVE

The objective of the study was to use prospective cohort study data to test whether the consumption of caffeinated beverages exhibits this protective effect.

DESIGN

Cox regression analyses were conducted for 426 CVD deaths that occurred during an 8.8-y follow-up in the prospective first National Health and Nutrition Examination Survey Epidemiologic Follow-up Study. The analysis involved 6594 participants aged 32-86 y with no history of CVD at baseline.

RESULTS

Participants aged >or=65 y with higher caffeinated beverage intake exhibited lower relative risk of CVD and heart disease mortality than did participants with lower caffeinated beverage intake. It was a dose-response protective effect: the relative risk (95% CI) for heart disease mortality was 1.00 (referent), 0.77 (0.54, 1.10), 0.68 (0.49, 0.94), and 0.47 (0.32, 0.69) for <0.5, 0.5-2, 2-4, and >or=4 servings/d, respectively (P for trend = 0.003). A similar protective effect was found for caffeine intake in mg/d. The protective effective was found only in participants who were not severely hypertensive. No significant protective effect was found in participants aged <65 y or in cerebrovascular disease mortality for those aged >or=65 y.

CONCLUSION

Habitual intake of caffeinated beverages provided protection against the risk of heart disease mortality among elderly participants in this prospective epidemiologic analysis.

Effects of Caffeine on Physiological Responses to Exercise in Young Boys and Girls

PURPOSE

To describe the influence of caffeine on physiological responses to exercise in young children and determine whether sex differences in these responses exist.

METHODS

Twenty-six healthy 7- to 9-yr-old boys and 26 healthy 7- to 9-yr-old girls volunteered to participate in a double-blind, randomized, double crossover study design. Each child randomly received both the placebo (PL) and caffeinated (5 mg x kg(-1)) drink (CAF) twice each on four separate days. Following a 1-h wash-in period and resting measures, each child rode a cycle ergometer at 25 and then 50 W for 8 min each, while HR, blood pressure (BP), and oxygen consumption (VO2) were measured.

RESULTS

HR (bpm) was significantly (P < 0.05) lower at rest and at 25 and 50 W in CAF versus PL in both boys and girls. Diastolic BP (mm Hg) was significantly (P </= 0.05) higher at rest, 25 W in both boys and girls, and at 50 W in boys, in CAF versus PL. Systolic BP (mm Hg) was significantly (P </= 0.05) higher at rest in both boys and girls, at 25 W in boys, and at 50 W in girls. During exercise, VO2 (L x min(-1) or mL x kg(-1) x min(-1)) and RER were not different in CAF versus PL in either boys or girls.

CONCLUSIONS

A moderate dose of caffeine (5 mg.kg) does not affect metabolism (VO2 or RER) in young children at low-moderate intensities of exercise. However, CAF causes a significantly lower HR (bpm) and higher BP (mm Hg) in both young boys and girls.

Absence of QTc-Interval-Prolonging or Hemodynamic Effects of a Single Dose of Bitter-Orange Extract in Healthy Subjects

STUDY OBJECTIVE

To evaluate the hemodynamic and electrocardiographic effects of a single dose of commercially available bitter-orange dried-fruit extract, which is increasingly being used in dietary supplements.

DESIGN

Randomized, double-blind, placebo-controlled, crossover study.

SETTING

University of Connecticut, Storrs Campus.

SUBJECTS

Eighteen healthy volunteers aged 18 years or older.

INTERVENTION

Subjects were given either placebo or bitter-orange dried-fruit extract (450 mg standardized to 27 mg of m- or p-synephrine) in phase 1. The opposite treatment was given during phase 2 after a washout period of at least 7 days.

MEASUREMENTS AND MAIN RESULTS

The rate-corrected QT (QTc) interval and blood pressure were measured before dosing and at 1, 3, 5, and 8 hours after dosing. Mean+/-SD values of the maximum postdose values were compared between groups. Subjects receiving bitter-orange extract versus those receiving placebo had similar postdose QTc intervals (402+/-29 vs 403+/-24 msec, p=0.653), systolic blood pressure (114+/-10 vs 115+/-8 mm Hg, p=0.686) and diastolic blood pressure (68+/-9 vs 68+/-8, p=0.879).

CONCLUSION

Bitter-orange dried-fruit extract standardized to m- or p-synephrine 27 mg did not significantly alter the QTc interval or blood pressure after a single dose was administered. Future studies are necessary to ensure the safety of this herbal product with multiple doses.

Respiratory sinus arrhythmia during speech production

The amplitude of the respiratory sinus arrhythmia (RSA) was investigated during a reading aloud task to determine whether alterations in respiratory control during speech production affect the amplitude of RSA. Changes in RSA amplitude associated with speech were evaluated by comparing RSA amplitudes during reading aloud with those obtained during rest breathing. A third condition, silent reading, was included to control for potentially confounding effects of cardiovascular responses to cognitive processes involved in the process of reading. Calibrated respiratory kinematics, electrocardiograms (ECGs), and speech audio signals were recorded from 18 adults (9 men, 9 women) during 5-min trials of each condition. The results indicated that the increases in respiratory duration, lung volume, and inspiratory velocity associated with reading aloud were accompanied by similar increases in the amplitude of RSA. This finding provides support for the premise that sensorimotor pathways mediating metabolic respiration are actively modulated during speech production.

Baroreceptor reflex pathways and neurotransmitters: 10 years on

The central nervous system plays a critical role in the management of blood flow to the tissues and its return to the heart and lungs. This is achieved by a complex interplay of neural efferent pathways, humoral mechanisms and afferent pathways. In this review, we focus on recent progress (within the past 10 years) that has been made in the sympathetic control of arterial blood pressure with a special emphasis on the role of baroreceptor mechanisms and central neurotransmitters. In particular, we focus on new features since 1991, such as neurotransmission in the nucleus tractus solitarius, the role of neurons in the most caudal part of the ventrolateral medulla oblongata and the increasing understanding of the exquisite control of different sympathetic pathways by different neurotransmitter systems.

Role of endogenous adenosine as a modulator of syncope induced during tilt testing

BACKGROUND

Previous reports that used head-up tilt testing and adenosine administration have suggested that adenosine may be an important endogenous mediator that may trigger a vasovagal response in susceptible patients. However, little is known regarding endogenous adenosine plasma levels (APLs) during vasovagal syncope provoked by tilt testing. The aim of this study was to determine whether APLs differ in patients with a positive head-up tilt test compared with those with a negative test and whether APLs are modified during tilt-induced vasovagal syncope.

METHODS AND RESULTS

APLs (mean+/-SEM) were measured during head-up tilt test in 26 patients who presented with unexplained syncope. In the 15 patients with a negative test, APLs were 0.39+/-0.03 micromol/L at baseline, 0.22+/-0.03 micromol/L immediately after tilting, and 0.44+/-0.03 micromol/L after 45 minutes. APLs were significantly higher in the 11 patients with a positive test (2.66+/-0.67 micromol/L at baseline and 3.22+/-0.85 micromol/L immediately after tilting) than in those with a negative test. During tilt testing-induced syncope, APLs increased to reach 4.03+/-0.66 micromol/L (ie, a 52% increase compared with baseline levels; P<0.02). Furthermore, we observed that the higher the APL during syncope, the shorter the time to appearance of symptoms.

CONCLUSIONS

This study showed that APLs were higher in patients with a positive tilt test than in patients with a negative test and that they increased during tilt testing-induced syncope. These observations suggest that adenosine release may be involved in the triggering mechanism of syncope induced during tilt testing.

Baroreflex buffering and susceptibility to vasoactive drugs

BACKGROUND

The overall effect of vasoactive drugs on blood pressure is determined by a combination of the direct effect on vascular tone and an indirect baroreflex-mediated effect, a baroreflex buffering of blood pressure. Differences in baroreflex function affect the responsiveness to vasoactive medications, particularly baroreflex buffering of blood pressure; however, the magnitude is not known.

METHODS AND RESULTS

We characterized baroreflex function and responses to vasoactive drugs in patients with idiopathic orthostatic intolerance, patients with essential hypertension, patients with monogenic hypertension and brachydactyly, patients with multiple system atrophy, and control subjects. We used phenylephrine sensitivity during ganglionic blockade as a measure of baroreflex buffering. Phenylephrine (25 microg) increased systolic blood pressure 6+/-1.6 mm Hg in control subjects, 6+/-1.1 mm Hg in orthostatic intolerance patients, 18+/-3.9 mm Hg in patients with essential hypertension, 31+/-3.4 mm Hg in patients with monogenic hypertension, and 25+/-3.4 mm Hg in patients with multiple system atrophy. Similar differences in sensitivities between groups were observed with nitroprusside. The sensitivity to vasoactive drugs was highly correlated with baroreflex buffering function and to a lesser degree with baroreflex control of heart rate. In control subjects, sensitivities to nitroprusside and phenylephrine infusions were correlated with baroreflex heart rate control and sympathetic nerve traffic.

CONCLUSIONS

Our findings are consistent with an important effect of baroreflex blood pressure buffering on the sensitivity to vasoactive drugs. They suggest that even moderate changes in baroreflex function may have a substantial effect on the sensitivity to vasoactive medications.

Cardiovascular effects of nitric oxide and adenosine in the nucleus tractus solitarii of rats

It has been shown that nitric oxide (NO) is synthesized in the central nervous system as well as in vascular endothelial cells. We recently reported that NO was involved in central cardiovascular regulation, modulated the baroreflex, and was involved in a reciprocal release with excitatory amino acids in the nucleus tractus solitarii (NTS) of rats. We also reported previously that adenosine increased the release of glutamate in the NTS. The purpose of the present study was to investigate the possible interaction of NO and adenosine in the NTS. Male Sprague-Dawley rats were anesthetized with urethane, and blood pressure was monitored intra-arterially. Unilateral microinjection of L-arginine (3.3 nmol/60 nL) into the NTS produced decreases in blood pressure and heart rate. Microinjection of adenosine (2.3 nmol/60 nL) also produced depressive and bradycardic effects. These cardiovascular effects were attenuated by prior administration of the specific adenosine receptor antagonist DPSPX (0.92 nmol). Similarly, prior administration of NO synthase inhibitor NG-monomethyl-L-arginine or NG-nitro-L-arginine methyl ester significantly attenuated the depressive and bradycardic effects of adenosine. These results demonstrate a reciprocal attenuation of adenosine receptor antagonist and NO synthase inhibitor on L-arginine and adenosine responses, respectively, in the NTS and implicate an interaction between NO and adenosine in central cardiovascular regulation.

Effects of caffeine on blood pressure, heart rate, and forearm blood flow during dynamic leg exercise

This study examined the acute effects of caffeine on the cardiovascular system during dynamic leg exercise. Ten trained, caffeine-naive cyclists (7 women and 3 men) were studied at rest and during bicycle ergometry before and after the ingestion of 6 mg/kg caffeine or 6 mg/kg fructose (placebo) with 250 ml of water. After consumption of caffeine or placebo, subjects either rested for 100 min (rest protocol) or rested for 45 min followed by 55 min of cycle ergometry at 65% of maximal oxygen consumption (exercise protocol). Measurement of mean arterial pressure (MAP), forearm blood flow (FBF), heart rate, skin temperature, and rectal temperature and calculation of forearm vascular conductance (FVC) were made at baseline and at 20-min intervals. Plasma ANG II was measured at baseline and at 60 min postingestion in the two exercise protocols. Before exercise, caffeine increased both systolic blood pressure (17%) and MAP (11%) without affecting FBF or FVC. During dynamic exercise, caffeine attenuated the increase in FBF (53%) and FVC (50%) and accentuated exercise-induced increases in ANG II (44%). Systolic blood pressure and MAP were also higher during exercise plus caffeine; however, these increases were secondary to the effects of caffeine on resting blood pressure. No significant differences were observed in heart rate, skin temperature, or rectal temperature. These findings indicate that caffeine can alter the cardiovascular response to dynamic exercise in a manner that may modify regional blood flow and conductance.

Uncoupling of the baroreflex by N(N)-cholinergic blockade in dissecting the components of cardiovascular regulation

Systemic administration of adrenergic agonists and nitric oxide donors is used extensively to determine cardiovascular receptor sensitivity. Conclusions regarding receptor sensitivity in the presence of the baroreflex may be misleading. In 8 normal volunteers, we determined the heart rate and blood pressure changes after incremental bolus doses of isoproterenol, phenylephrine, and sodium nitroprusside before and during neuronal nicotinic cholinergic (N(N)-cholinergic) blockade with trimethaphan. Results are given as median (25th/75th percentile). With trimethaphan, the baroreflex slope (as determined by bolus doses of nitroprusside and phenylephrine) decreased from 24 (22/26) to 0.00 (0.00/0.09) ms/mm Hg (P<0.01). The dose of isoproterenol that decreased systolic blood pressure (SBP) 12.5 mm Hg changed from 0.61 (0.51/5.3) to 0.17 (0.12/0.21) microg (P<0.01); the dose required to increase heart rate 12.5 bpm changed from 0.22 (0.17/0.41) to 0.74 (0.33/2.3) microg (P<0.01). The dose of nitroprusside required to decrease SBP 12.5 mm Hg changed from 2.3 (1.3/3.4) to 0.18 (0.14/0.24) microg/kg (P<0.01). The dose of phenylephrine required to increase SBP 12.5 mm Hg changed from 135 (110/200) to 16 (10/30) microg (P<0.01). We conclude that the efferent arc of the baroreflex can be completely interrupted with N(N)-cholinergic blockade. Estimation of adrenoreceptor sensitivity and sensitivity to nitric oxide donors by systemic administration of agonists is severely confounded by baroreflexes. Uncoupling of the baroreflex by N(N)-cholinergic blockade may be a useful method to obtain an integrated measure of adrenergic receptor sensitivity and sensitivity to nitric oxide donors in humans. This approach would permit the comparison of normal and abnormal physiological states without the "noise" of baroreflex buffering.

Differential alteration of neuronal and cardiovascular responses to adenosine microinjected into the nucleus tractus solitarius of spontaneously hypertensive rats

We previously reported that adenosine elicited site-dependent neuronal and cardiovascular responses in two subareas of the nucleus tractus solitarius (NTS) of normotensive rats. Pressor and tachycardic responses were obtained from the rostral NTS (adenosine pressor system), and depressor and bradycardic responses were obtained from the caudal NTS (adenosine depressor system). In both areas, adenosine inhibited the firing rate of barosensitive neurons. The present study investigated whether spontaneously hypertensive rats (SHR) exhibit abnormal neuronal and cardiovascular responses mediated by the adenosine pressor and depressor systems within the NTS. Male SHR and Wistar-Kyoto rats (WKY) were anesthesized with urethane and prepared for blood pressure and heart rate recording, stereotaxic microinjection of adenosine into the NTS, and extracellular recording of single-unit neuronal activity of NTS neurons. Chemical identification of the targeted neuronal pool was made by L-glutamate (5 nmol) and confirmed by histology. SHR exhibited significantly higher mean arterial pressure and firing rate of caudal NTS neurons (45.0 +/- 4.5 versus 27.3 +/- 4.7 spikes per 2.5 seconds, P <.05) but similar heart rate and neuronal firing rate of rostral NTS neurons compared with WKY. Adenosine (0.1, 1, and 10 nmol) elicited dose-related neuronal and cardiovascular responses in both strains. However, SHR exhibited differential alterations in both adenosine systems. Compared with WKY, SHR exhibited attenuated pressor, tachycardic, and neuronal responses mediated by the adenosine pressor system and exaggerated depressor, bradycardic, and neuronal responses mediated by the adenosine depressor system. In both strains, the responses elicited by adenosine were virtually abolished by theophylline (10 mg/kg IV), suggesting that these responses were mediated by adenosine receptors in the NTS. Furthermore, the theophylline-evoked increase in blood pressure was twofold higher in SHR (15.0 +/- 1.7 versus 6.9 +/- 1.5 mm Hg, P <.05); larger but nonsignificant increases in heart rate and neuronal firing rate also were evident in SHR compared with WKY. These findings suggest differential alterations in adenosine pressor and depressor systems in the NTS of SHR, which may be implicated in the pathophysiology of this model of hypertension.

Attenuated cardiovascular response to adenosine in the brain stem nuclei of spontaneously hypertensive rats

We previously reported that adenosine has significant depressor effects in the nucleus tractus solitarii and area postrema of the rat. The purpose of this study was to determine whether the spontaneously hypertensive rat (SHR) has abnormalities in medullary sensitivity to adenosine. Male SHR and Wistar-Kyoto (WKY) rats (aged 12 to 15 weeks) were anesthetized with urethane, and blood pressure was monitored intraarterially. Stereotaxic microinjection (60 nL) of adenosine was made into the nucleus tractus solitarii and the area postrema and was confirmed histologically. Dose-related decreases in mean blood pressure and heart rate occurred in both strains tested, and this effect was completely abolished by 1,3-dipropyl- 8-p-sulfophenylxanthine (0.92 nmol), a potent adenosine receptor antagonist. However, there were significant differences between SHR and WKY rats in the magnitude of blood pressure and heart rate depression. A similar pattern of response was found in the area postrema. Thus, adenosine is a potent depressor agent in the nucleus tractus solitarii and area postrema of rats, and adenosine has significantly fewer depressor effects in SHR. These data suggest that alterations in purinergic mechanisms of central cardiovascular control exist in the SHR model.

Hemodynamic and neurohumoral effects of various grades of selective adenosine transport inhibition in humans. Implications for its future role in cardioprotection

In 12 healthy male volunteers (27-53 yr), a placebo-controlled randomized double blind cross-over trial was performed to study the effect of the intravenous injection of 0.25, 0.5, 1, 2, 4, and 6 mg draflazine (a selective nucleoside transport inhibitor) on hemodynamic and neurohumoral parameters and ex vivo nucleoside transport inhibition. We hypothesized that an intravenous draflazine dosage without effect on hemodynamic and neurohumoral parameters would still be able to augment the forearm vasodilator response to intraarterially infused adenosine. Heart rate (electrocardiography), systolic blood pressure (Dinamap 1846 SX; Critikon, Portanje Electronica BV, Utrecht, The Netherlands) plasma norepinephrine and epinephrine increased dose-dependently and could almost totally be abolished by caffeine pretreatment indicating the involvement of adenosine receptors. Draflazine did not affect forearm blood flow (venous occlusion plethysmography). Intravenous injection of 0.5 mg draflazine did not affect any of the measured hemodynamic parameters but still induced a significant ex vivo nucleoside-transport inhibition of 31.5 +/- 4.1% (P < 0.05 vs placebo). In a subgroup of 10 subjects the brachial artery was cannulated to infuse adenosine (0.15, 0.5, 1.5, 5, 15, and 50 micrograms/100 ml forearm per min) before and after intravenous injection of 0.5 mg draflazine. Forearm blood flow amounted 1.9 +/- 0.3 ml/100 ml forearm per min for placebo and 1.8 +/- 0.2, 2.0 +/- 0.3, 3.8 +/- 0.9, 6.3 +/- 1.2, 11.3 +/- 2.2, and 19.3 +/- 3.9 ml/100 ml forearm per min for the six incremental adenosine dosages, respectively. After the intravenous draflazine infusion, these values were 1.6 +/- 0.2 ml/100 ml forearm per min for placebo and 2.1 +/- 0.3, 3.3 +/- 0.6, 5.8 +/- 1.1, 6.9 +/- 1.4, 14.4 +/- 2.9, and 23.5 +/- 4.0 ml/100 ml forearm per min, respectively (Friedman ANOVA: P < 0.05 before vs after draflazine infusion). In conclusion, a 30-50% inhibition of adenosine transport significantly augments the forearm vasodilator response to adenosine without significant systemic effects. These results suggest that draflazine is a feasible tool to potentiate adenosine-mediated cardioprotection in man.

Cardiovascular, behavioral, and subjective effects of caffeine under field conditions

The effects of continuous and intermittent caffeine abstinence and their time course were investigated under field conditions. After 3 days with habitual coffee, subjects were switched for 9 days to regular instant coffee (n = 40), decaffeinated coffee (n = 40), or an intermittent regime (2 days decaff, 1 day caff, repeated, n = 40). Subjects were blind to the caffeine treatment. Motor activity was assessed continuously; subjective variables, blood pressure (BP), and heart rate (HR) were assessed by the subjects six times per day (electronic diary). Compliance was confirmed by the different caffeine concentrations in daily saliva samples. Continued caffeine consumption showed no effects. Caffeine abstinence resulted in increased HR, decreased motor activity, subjective wakefulness, and well-being, and in increased headaches and use of analgetics. The subjective effects and headaches were transient, i.e., they disappeared after a few days of abstinence and weakened over successive, separated abstinence periods. BP was not affected by the caffeine treatment. The intermittent onset of caffeine consumption resulted in increased wakefulness, whereas the other variables normalized to baseline level.

Microneurographic evidence of sudden sympathetic withdrawal in carotid sinus syncope; treatment with ergotamine

A proportion of patients with carotid sinus syncope (CSS) remain symptomatic even after pacemaker implantation because of persistence of a vasodepressor component. We report a patient with CSS whose syncopal episodes could be reproduced by carotid sinus massage and were due to profound hypotension associated with sudden sympathetic withdrawal, based on direct measurements of sympathetic nerve traffic. A double-blind trial with inhaled ergotamine provided significant symptomatic relief.

Neuronal and cardiovascular responses to adenosine microinjection into the nucleus tractus solitarius

This study investigated neuronal, blood pressure, and heart rate responses to adenosine microinjection into caudal and rostral NTS of anesthetized rats. The site of recording and microinjection was verified chemically by observing the responses to a test dose of l-glutamate (5 nmol) and histologically at the conclusion of the experiment. Neuronal firing rate increased (+29.4 +/- 5.3%) and decreased (-48 +/- 9.4%) in response to l-glutamate microinjection into the rostral and caudal NTS, respectively. These opposite neuronal responses were followed by depressor (-32.4 +/- 8.3 vs. -36 +/- 5.5 mmHg) and bradycardic (-25.2 +/- 7.7 vs. -25.8 +/- 3.4 beats/min) responses to l-glutamate microinjection into the two subareas of the NTS. Microinjection of a submaximal dose (1 nmol) of adenosine into the NTS produced site-dependent cardiovascular responses which were preceded by similar inhibition of neuronal firing (-60 +/- 4 vs. -55.9 +/- 1.7%). Whereas adenosine microinjection into the rostral NTS elicited modest pressor (+10.1 +/- 2.1 mmHg) and tachycardic (+9 +/- 3.9 beats/min) responses, its microinjection into the caudal NTS produced depressor (-29.2 +/- 5.3 mmHg) and bradycardic (-14.6 +/- 1.7 beats/min) responses. These findings suggest that compared to l-glutamate, adenosine produces opposite (rostral) and similar (caudal) neuronal and cardiovascular effects in the two subareas of the NTS. In the caudal NTS, adenosine (0.1, 1, and 10 nmol) elicited dose-related inhibitory neuronal and cardiovascular responses that were attenuated by systemic theophylline but not 8-(p-sulfophenyl) theophylline (8-SPT) administration. The neuronal and cardiovascular responses to adenosine microinjection into the caudal NTS were also attenuated by microinjection of 8-SPT into the same area. Finally, single-unit activity inhibited by adenosine or l-glutamate microinjection into the caudal NTS was also inhibited by baroreceptor loading and excited by baroreceptor unloading. These findings suggest a) l-glutamate elicits opposite neuronal responses in the rostral and caudal NTS; b) the distinct hemodynamic responses elicited by adenosine in the two subareas may be related, at least in part, to their differing responses to l-glutamate; and c) the similarity between the neuronal responses to adenosine and l-glutamate microinjection into the caudal NTS and the response of the same neurons to baroreceptor activation support the hypothesis that adenosine plays a neuromodulatory role in the processing of baroreceptor information.

Contrasting excitatory and inhibitory effects of adenosine in blood pressure regulation

Administration of adenosine results in profound hypotension without the expected activation of reflex sympathetic and renin mechanisms in most animal models. This action can be explained by the vasodilatory and neuroinhibitory effects of adenosine. It is generally considered an inhibitory neuromodulator because it inhibits the release of virtually all neurotransmitters studied and produces hyperpolarization of neurons. In contrast, adenosine produces vasoconstriction of some vascular beds, including the renal and pulmonary circulations. Renal vasoconstriction is caused by activation of A1 receptors and involves an interaction with angiotensin II. In other vascular beds adenosine releases eicosanoids, including thromboxane, also resulting in vasoconstriction. Adenosine-induced vasoconstriction is transient and species dependent. Neither the receptor type, the molecular mechanisms of these actions, nor their significance to pathophysiological processes have been defined. Adenosine also has an apparent excitatory effect in the nucleus tractus solitarii. Microinjections of adenosine into this brain stem nucleus lead to decreased sympathetic tone and hypotension similar to those produced by the excitatory amino acid glutamate. The mechanism that explains this action has recently been explored and involves the release of glutamate by adenosine. Adenosine also stimulates afferent fibers mediating sympathetic activity, including renal and myocardial afferent nerves, and carotid and aortic chemoreceptors. Afferent nerve activation seems to be more pronounced in humans and may explain most of the cardiovascular and respiratory actions of adenosine in this species. Finally, animal studies suggest that endogenous adenosine plays a role in the regulation of the baroreceptor reflex and restrains the full expression of renin-dependent hypertension.

An update on research with coffee/caffeine (1989-1990)

The interest in research with coffee has been increasing in recent years, and this has resulted in a surge of publications dealing with a variety of pharmaco-physiological effects of coffee/caffeine. This review attempts to update the information on the research with coffee/caffeine, including epidemiological studies, laboratory investigations and tests with volunteers, published in 1989 and 1990. It groups published articles according to observed or investigated biological effects. The most significant findings and differences between studies are pointed out with brief commentaries on the results. The overall assessment for the safety of drinking coffee and the effect of coffee on human health, based on the literature published in 1989 and 1990, indicates that certain controversial issues are still unresolved.