Effects of adenosine on autonomic control of heart rate in man

The association between heart rate reserve and impaired coronary flow velocity reserve: a study based on adenosine stress echocardiography

This study was to explore the correlation between heart rate reserve (HRR) to coronary flow velocity reserve (CFVR), using adenosine stress echocardiography (SE), in patients with angina and no obstructive coronary artery disease (ANOCA). 111 ANOCA patients underwent adenosine SE were enrolled, which were divided into two groups, impaired CFVR group (CFVR < 2) and control groups (CFVR ≥ 2). The relationships between HRR and impaired CFVR were explored in total and subgroup to sex. A reduced HRR during adenosine infusion was seen in ANOCA patients with impaired CFVR (25.73 ± 8.39 vs. 34.30 ± 19.93, P < 0.001). Compared to respective controls, the blunted HRR to adenosine was more pronounced in female patients (women: 27.21 ± 8.01 vs. 39.48 ± 10.57, P < 0.001; men: 24.05 ± 8.70 vs. 29.12 ± 8.69, P = 0.041). A strong association between CFVR and a blunted HRR was observed in women (r = 0.46, P < 0.001), while no association in men (r = 0.18, P = 0.199). In female, a multivariate logistic regression identified HRR as the strongest negative predictor of impaired CFVR [HR (95% CI) = 0.854 (0.764-0.956), P = 0.006]. Based on the ROC curve, HRR < 35% was a strong indicator of impaired CFVR, with AUC of 0.838, sensitivity of 70%, and specificity of 87% in females. A blunted HRR was seen in patients with impaired CFVR, with a most pronounced effect being seen in female patients. The blunted HRR < 35% is intricately linked to impaired CFVR in women with ANOCA beyond the value of traditional risk factors, which could ultimately contribute to risk stratification of impaired CFVR in such patients.

Reduced Sympathetic Reserve Detectable by Heart Rate Response after Dipyridamole in Anginal Patients with Normal Coronary Arteries

Background. Patients with ischemia and normal coronary arteries (INOCA) may show abnormal cardiac sympathetic function, which could be unmasked as a reduced heart rate reserve (HRR) during dipyridamole stress echocardiography (SE). Objectives. To assess whether HRR during dipyridamole SE predicts outcome. Methods. Dipyridamole SE was performed in 292 patients with INOCA. HRR was measured as peak/rest heart rate and considered abnormal when ≤1.22 (≤1.17 in presence of permanent atrial fibrillation). All-cause death was the only endpoint. Results. HRR during SE was normal in 183 (63%) and abnormal in 109 patients (37%). During a follow-up of 10.4 ± 5.5 years, 89 patients (30%) died. The 15-year mortality rate was 27% in patients with normal and 54% in those with abnormal HRR (p < 0.0001). In a multivariable analysis, a blunted HRR during SE was an independent predictor of outcome (hazard ratio 1.86, 95% confidence intervals 1.20–2.88; p = 0.006) outperforming inducible ischemia. Conclusions. A blunted HRR during dipyridamole SE predicts a worse survival in INOCA patients, independent of inducible ischemia.

Prognostic Value of Heart Rate Reserve during Dipyridamole Stress Echocardiography in Patients With Abnormal Chronotropic Response to Exercise

Heart rate reserve (HRR) during physical or pharmacological stress is a sign of cardiac autonomic function and sympathetic reserve, but it can be reduced during exercise for confounders such as poor motivation, drugs or physical fitness. In this study we sought to assess the prognostic meaning of HRR during dipyridamole stress echocardiography (DSE) in patients with abnormal chronotropic response to exercise. From 2004 to 2019, we prospectively acquired and retrospectively analyzed 379 patients (age 62 ± 11 years; ejection fraction 60 ± 5%) with suspected (n = 243) or known (n = 136) chronic coronary syndromes, referred to DSE for chronotropic incompetence during upright bicycle exercise-electrocardiography test defined as HRR used [(peak HR - rest HR) / (220 - age) - rest HR] ≤80% in patients off and ≤62% in patients on beta-blockers. All patients were in sinus rhythm and underwent DSE (0.84 mg/kg) within 3 months of exercise testing. During DSE, age-independent HRR (peak/rest HR) ≤1.22 was considered abnormal. All patients were followed-up. All-cause death was the only outcome measure. HRR during DSE was normal in 275 (73%) and abnormal in 104 patients (27%). During a follow-up of 9.0 ± 4.2 years, 67 patients (18%) died. The 15-year mortality rate was 23% in patients with normal and 61% in patients with abnormal HRR (p < 0.0001). At multivariable analysis a blunted HRR during DSE was an independent predictor of outcome (hazard ratio 2.01, 95% confidence intervals 1.23-3.29; p = 0.005) with age and diabetes, while neither inducible ischemia nor ongoing beta-blocker therapy were significant predictors. In conclusion, a blunted HRR during DSE predicts a worse survival in patients with chronotropic incompetence during exercise test. HRR during DSE is an appealingly simple biomarker of cardiac autonomic dysfunction independent of imaging, exercise and beta-blocker therapy.

Microvascular dysfunction and sympathetic hyperactivity in women with supra-normal left ventricular ejection fraction (snLVEF)

BACKGROUND

Recently, a new disease phenotype characterized by supra-normal left ventricular ejection fraction (snLVEF) has been suggested, based on large datasets demonstrating an increased all-cause mortality in individuals with an LVEF > 65%. The underlying mechanisms of this association are currently unknown.

METHODS

A total of 1367 patients (352 women, mean age 63.1 ± 11.6 years) underwent clinically indicated rest/adenosine stress ECG-gated ¹³N-ammonia positron emission tomography (PET) between 1995 and 2017 at our institution. All patients were categorized according to LVEF. A subcohort of 698 patients (150 women) were followed for major adverse cardiac events (MACEs), a composite of cardiac death, non-fatal myocardial infarction, cardiac-related hospitalization, and revascularization.

RESULTS

The prevalence of a snLVEF (≥ 65%) was higher in women as compared to that in men (31.3% vs 18.8%, p < 0.001). In women, a significant reduction in coronary flow reserve (CFR, p < 0.001 vs normal LVEF) and a blunted heart rate reserve (% HRR, p = 0.004 vs normal LVEF) during pharmacological stress testing-a surrogate marker for autonomic dysregulation-were associated with snLVEF. Accordingly, reduced CFR and HRR were identified as strong and independent predictors for snLVEF in women in a fully adjusted multinomial regression analysis. After a median follow-up time of 5.6 years, women with snLVEF experienced more often a MACE than women with normal (55-65%) LVEF (log rank p < 0.001), while such correlation was absent in men (log rank p = 0.76).

CONCLUSION

snLVEF is associated with an increased risk of MACE in women, but not in men. Microvascular dysfunction and an increased sympathetic tone in women may account for this association.

Sex and age differences in the association of heart rate responses to adenosine and myocardial ischemia in patients undergoing myocardial perfusion imaging

BACKGROUND

In light of growing cardiovascular mortality rates observed in young women, sexual dimorphism in cardiac autonomic nervous control is gaining increasing attention. Heart rate responses to adenosine mirror autonomic activity and may carry important prognostic information.

METHODS AND RESULTS

Hemodynamic changes during adenosine stress were retrospectively analysed in a propensity-matched cohort of 1932 consecutive patients undergoing myocardial perfusion single-photon-emission computed tomography (MPI-SPECT). Heart rate (HR) and systolic blood pressure (SBP) increased during adenosine infusion (P < 0.001). The increase in SBP and HR (heart rate reserve, HRR), was significantly more pronounced in women compared with men (P < 0.05). Patients ≤ 55 years had a higher HRR compared with patients > 55 years (46.8% vs 37.5%, P = 0.015). Women ≤ 55 years with a reversible perfusion defect on MPI-SPECT exhibited the highest HRR (89.2%), while age-matched men showed a blunted HR response to adenosine (26.4%, P = 0.01). Accordingly, age and an interaction term of female sex and increased HRR were identified as significant predictors of myocardial ischemia in a multiple regression analysis (OR 1.4, 95% CI 1.02-1.9, P = 0.038).

CONCLUSION

HRR during adenosine infusion is influenced by age and sex. Our data suggest a stronger, sympathetic-driven, hemodynamic response to adenosine in younger women with myocardial ischemia.

Usefulness of Blunted Heart Rate Reserve as an Imaging-Independent Prognostic Predictor During Dipyridamole Stress Echocardiography

A blunted heart rate (HR) response during dipyridamole myocardial perfusion imaging has been associated with a poor outcome. To assess the value of HR response in patients who underwent high-dose dipyridamole stress echocardiography (SE), we retrospectively selected a sample of 3,059 patients (none with pacemakers or atrial fibrillation; mean age 66 ± 11 years). All underwent high-dose (0.84 mg/kg) dipyridamole SE for evaluation of known or suspected coronary artery disease and/or heart failure in 2 laboratories of Pisa-IFC and Lucca. HR (with 12-lead ECG) was obtained each minute and recorded at rest and peak stress. HR reserve (HRR) was calculated as the peak/rest HR ratio. All patients were followed up. Patients were randomly divided into the modeling and validation group of equal size. During a median follow-up time of 1,004 days, 321 hard events occurred: 231 deaths and 90 nonfatal myocardial infarctions. HRR ≤ 1.22 identified by receiver operating characteristic analysis in the modeling group was an independent predictor of infarction-free survival in the modeling (hazard ratio 1.83, 95% confidence interval [CI] 1.30 to 2.60, p = 0.001), in the validation (hazard ratio 1.47, 95% CI 1.08 to 2.01, p = 0.02), and in the overall group (hazard ratio 1.60, 95% CI 1.27 to 2.02, p <0.0001), either off- or on-β blockers. Five-year event rate increased from 8% to 24 % from the highest (≥1.41) to the lowest (≤1.14) HRR quartile. In conclusion, blunted HRR is a useful nonimaging predictor of adverse events during high-dose dipyridamole SE, independent of inducible ischemia, and beta-blocker therapy.

Heart rate reserve is a long-term risk predictor in women undergoing myocardial perfusion imaging

BACKGROUND

Although women with cardiovascular disease experience relatively worse outcomes as compared to men, substantial knowledge gaps remain regarding the unique female determinants of cardiovascular risk. Heart rate (HR) responses to vasodilator stress mirror autonomic activity and may carry important long-term prognostic information in women.

METHODS AND RESULTS

Hemodynamic changes during adenosine stress were recorded in a total of 508 consecutive patients (104 women) undergoing clinically indicated ¹³N-ammonia Positron-Emission-Tomography (PET) at our institution. Following propensity matching, 202 patients (101 women, mean age 61.3 ± 12.6 years) were analyzed. During a median follow-up of 5.6 years, 97 patients had at least one cardiac event, including 17 cardiac deaths. Heart rate reserve (% HRR) during adenosine infusion was significantly higher in women as compared to men (23.8 ± 19.5 vs 17.3 ± 15.3, p = 0.009). A strong association between 10-year cardiovascular endpoints and a blunted HRR was observed in women, while this association was less pronounced in men. Accordingly, in women, but not in men, reduced HRR was selected as a strong predictor for adverse cardiovascular events in a Cox regression model fully adjusted for imaging findings and traditional risk factors (HR 2.41, 95% CI 1.23-4.75, p = 0.011). Receiver operating characteristics (ROC) curves revealed that a blunted HRR <21% was a powerful predictor for MACE in women with a sensitivity of 77% and a specificity of 68%.

CONCLUSION

Blunted HRR to adenosine stress adds incremental prognostic value for long-term cardiovascular outcomes in women beyond that provided by traditional risk factors and imaging findings.

Heart rate reserve during pharmacological stress is a significant negative predictor of impaired coronary flow reserve in women

PURPOSE

Evidence to date has failed to adequately explore determinants of cardiovascular risk in women with coronary microvascular dysfunction (CMVD). Heart rate responses to adenosine mirror autonomic activity and may carry important prognostic information for the diagnosis of CMVD.

METHODS

Hemodynamic changes during adenosine stress were analyzed in a propensity-matched cohort of 404 patients (202 women, mean age 65.9 ± 11.0) who underwent clinically indicated myocardial perfusion ¹³N-ammonia Positron-Emission-Tomography (PET) at our institution between September 2013 and May 2017.

RESULTS

Baseline heart rate (HR) was significantly higher in patients with abnormal coronary flow reserve (CFR, p < 0.001 vs normal CFR). Accordingly, a blunted HR response to adenosine (=reduced heart rate reserve, %HRR) was seen in patients with abnormal CFR, with a most pronounced effect being observed in female patients free of myocardial ischemia (45.9 ± 34.9 vs 26.5 ± 18.0, p < 0.001 in women and 29.1 ± 16.9 vs 24.3 ± 21.7, p = 0.15 in men). Hence, a fully-adjusted multivariate logistic regression model identified HRR as the strongest negative predictor of reduced CFR in women free of myocardial ischemia, but not in men. Accordingly, receiver operating characteristics (ROC) curves for the presence of reduced CFR revealed that a %HRR <35 was a powerful predictor for abnormal CFR with a sensitivity of 81% and a specificity of 60% in women.

CONCLUSION

A blunted HRR <35% is associated with abnormal CFR in women. Taking into account HR responses during stress test in women may help to risk stratify the heterogeneous female population of patients with non-obstructive coronary artery disease (CAD).

Hypoxia-induced vagal withdrawal is independent of the hypoxic ventilatory response in men

Hypoxia increases heart rate (HR) in humans by sympathetic activation and vagal withdrawal. However, in anaesthetized dogs hypoxia increases vagal activity and reduces HR if pulmonary ventilation does not increase and we evaluated whether that observation applies to awake humans. Ten healthy males were exposed to 15 min of normoxia and hypoxia (10.5% O2), while respiratory rate and tidal volume were volitionally controlled at values identified during spontaneous breathing in hypoxia. End-tidal CO2 tension was clamped at 40 mmHg by CO2 supplementation. β-Adrenergic blockade by intravenous propranolol isolated vagal regulation of HR. During spontaneous breathing, hypoxia increased ventilation by 3.2 ± 2.1 l/min ( P = 0.0033) and HR by 8.9 ± 5.5 beats/min ( P < 0.001). During controlled breathing, respiratory rate (16.3 ± 3.2 vs. 16.4 ± 3.3 breaths/min) and tidal volume (1.05 ± 0.27 vs. 1.06 ± 0.24 l) were similar for normoxia and hypoxia, whereas the HR increase in hypoxia persisted without (8.6 ± 10.2 beats/min) and with (6.6 ± 5.6 beats/min) propranolol. Neither controlled breathing ( P = 0.80), propranolol ( P = 0.64), nor their combination ( P = 0.89) affected the HR increase in hypoxia. Arterial pressure was unaffected ( P = 0.48) by hypoxia across conditions. The hypoxia-induced increase in HR during controlled breathing and β-adrenergic blockade indicates that hypoxia reduces vagal activity in humans even when ventilation does not increase. Vagal withdrawal in hypoxia seems to be governed by the arterial chemoreflex rather than a pulmonary inflation reflex in humans.

NEW & NOTEWORTHY Hypoxia accelerates the heart rate of humans by increasing sympathetic activity and reducing vagal activity. Animal studies have indicated that hypoxia-induced vagal withdrawal is governed by a pulmonary inflation reflex that is activated by the increased pulmonary ventilation in hypoxia. The present findings, however, indicate that humans experience vagal withdrawal in hypoxia even if ventilation does not increase, indicating that vagal withdrawal is governed by the arterial chemoreflex rather than a pulmonary inflation reflex.

Assessment of Myocardial Perfusion at Rest and During Stress Using Dynamic First-Pass Contrast-Enhanced Magnetic Resonance Imaging in Healthy Dogs

Objective: To assess the feasibility of myocardial perfusion analysis in healthy dogs using dynamic contrast-enhanced cardiac magnetic resonance (DCE-MR) imaging at rest and during simulated stress with two doses of adenosine.

Animals: Ten healthy beagle dogs.

Procedures: Dogs were anesthetized and positioned in dorsal recumbency in a 3.0 Tesla MR scanner. Electrocardiogram-triggered dynamic T1-weighted ultrafast gradient echo images of three slices in short-axis orientation of the heart were acquired during breath holds and the first pass of gadolinium contrast. Image acquisition was performed after 4 min infusion of 140 μg/kg/min and 280 μg/kg/min adenosine and, after a washout period, without adenosine, respectively. Images were processed by dividing each slice into 6 radial segments and perfusion analysis was performed from signal intensity-time data.

Results: No differences in perfusion parameters were found between segments within any of the slices, but significant differences were found between slices for peak enhancement, accumulated enhancement, and the maximum upslope. In addition, significant differences were found within each slice between data at rest and during adenosine-induced stress for the relative and absolute maximum upslope, relative peak enhancement, time to peak, and accumulated enhancement although inter-individual variation was large and no difference was found between the two stress tests for some parameters.

Conclusion and Clinical Relevance: Results of this study showed that rest and stress myocardial perfusion can be assessed using DCE-CMR in dogs using the methods described. Both, adenosine dose and slice appear to affect perfusion parameters in healthy dogs and individual response to adenosine was variable.

Delayed heart rate recovery after adenosine stress testing with supplemental arm exercise predicts mortality

BACKGROUND

Delayed heart rate (HR) recovery after treadmill exercise testing predicts mortality. Patients with suspected ischemic heart disease who cannot perform adequate treadmill exercise are typically evaluated with pharmacological stress myocardial perfusion imaging (MPI) studies, but little prognostic significance has been attributed to the hemodynamic response to vasodilator stress testing with low-level exercise. We hypothesized that a delay in HR recovery after adenosine stress testing with arm exercise is associated with increased mortality.

METHODS AND RESULTS

Technetium 99m-Sestamibi MPI was performed in 1,455 consecutive patients (70 +/- 12 years, 50.2% men) with adenosine stress and supplemental arm exercise. HRs were recorded at rest, continuously during infusion, and then 5 minutes post-infusion. Delayed HR recovery was defined as a decline of < or = 12 bpm from peak HR at 5 minutes after discontinuation of the infusion. Of 1,356 patients during 5 years of follow up, there were 135 deaths (10%). Delayed HR recovery was strongly predictive of all-cause mortality (16.5% vs 5.3% in those with normal HR recovery, P < .001) with an adjusted hazard ratio of 2.5 (95% CI, 1.7-3.6; P < .001).

CONCLUSION

Delayed HR recovery after adenosine stress testing with arm exercise is a readily available and powerful predictor of all-cause mortality.

Heart rate response to adenosine in patients with diabetes mellitus and normal myocardial perfusion imaging

Adenosine infusion, as is used in stress single photon-emission computed tomographic (SPECT) imaging, produces an increase in heart rate (HR) and a decrease in blood pressure (BP). The increase in HR is most likely due to direct sympathetic stimulation rather than a reflex to the decrease in BP. In this study, it was hypothesized that the HR response to adenosine is different in patients with versus without diabetes mellitus (DM) in the setting of normal SPECT imaging results. We studied 60 patients with DM (53% women, mean age 62 +/- 10 years) and 60 controls (50% women, mean age 61 +/- 12 years) (p = NS). All patients underwent adenosine SPECT imaging (140 mug/kg/min for 5 minutes) for clinical indications and had normal perfusion and systolic left ventricular function. HR and BP were measured at baseline and during adenosine infusion. HR ratio was defined as peak HR divided by baseline HR and the change as peak HR minus baseline HR. The change in HR (17 +/- 12 vs 22 +/- 14 beats/min, p = 0.034) and the ratio (1.24 +/- 0.20 vs 1.33 +/- 0.25, p = 0.048) were significantly lower in patients with DM compared with those without DM. The baseline and change in mean BP were not significantly different between the 2 groups. In a stepwise regression analysis model, DM was identified as a predictor of the change in HR (p = 0.022). In conclusion, HR response to adenosine infusion is diminished in patients with DM with normal SPECT imaging results. This is likely due to cardiovascular autonomic neuropathy and may carry important prognostic information.

Molecular mechanisms and therapeutical implications of intramembrane receptor/receptor interactions among heptahelical receptors with examples from the striatopallidal GABA neurons

The molecular basis for the known intramembrane receptor/receptor interactions among G protein-coupled receptors was postulated to be heteromerization based on receptor subtype-specific interactions between different types of receptor homomers. The discovery of GABAB heterodimers started this field rapidly followed by the discovery of heteromerization among isoreceptors of several G protein-coupled receptors such as delta/kappa opioid receptors. Heteromerization was also discovered among distinct types of G protein-coupled receptors with the initial demonstration of somatostatin SSTR5/dopamine D2 and adenosine A1/dopamine D1 heteromeric receptor complexes. The functional meaning of these heteromeric complexes is to achieve direct or indirect (via adapter proteins) intramembrane receptor/receptor interactions in the complex. G protein-coupled receptors also form heteromeric complexes involving direct interactions with ion channel receptors, the best example being the GABAA/dopamine D5 receptor heteromerization, as well as with receptor tyrosine kinases and with receptor activity modulating proteins. As an example, adenosine, dopamine, and glutamate metabotropic receptor/receptor interactions in the striatopallidal GABA neurons are discussed as well as their relevance for Parkinson's disease, schizophrenia, and drug dependence. The heterodimer is only one type of heteromeric complex, and the evidence is equally compatible with the existence of higher order heteromeric complexes, where also adapter proteins such as homer proteins and scaffolding proteins can exist. These complexes may assist in the process of linking G protein-coupled receptors and ion channel receptors together in a receptor mosaic that may have special integrative value and may constitute the molecular basis for some forms of learning and memory.

Caffeine abstinence augments the systolic blood pressure response to adenosine in humans

Blood pressure and heart rate responses to adenosine infusion (35, 70, and 140 microg/kg/min, intravenously) were studied in 7 healthy men after 6, 30, 78, 150, and 318 hours of abstinence from regular caffeine use. The finding that caffeine abstinence augmented the systolic pressor response (from -1 +/- 2 mm Hg at 6 hours to +9 +/- 2 mm Hg at 318 hours; p = 0.01) but not the tachycardic response to adenosine has implications for current clinical and research applications of this purine.

Hemodynamic response to adenosine infusion before and after coronary artery bypass surgery

This study describes the hemodynamic dose-response characteristics of a titrated, continuous adenosine infusion before and 1 h (anesthetized), 1 week, and 1 year after coronary artery bypass graft (CABG) surgery. Average tolerated adenosine infusion rates were less 1 h and 1 week after surgery (128 +/- 23 and 118 +/- 27 microg/kg/min, respectively) than before (156 +/- 29 microg/kg/min) and 1 year after surgery (156 +/- 24 microg/kg/min). Heart rate (HR) increased with a 120-microg/kg/min adenosine infusion rate both preoperatively (21 +/- 11%) and 1 year postoperatively (16 +/- 8%). Systolic blood pressure (BP) decreased 26 +/- 11%, 14 +/- 7%, and 9 +/- 6% with 120 microg/kg/min adenosine for the three postoperative examinations. The integral of the outflow tract velocity with 120 microg/kg/min adenosine increased 49 +/- 22% and 29 +/- 12% after 1 h and 1 week, respectively, whereas its product with HR increased equally for all examinations (40 +/- 22%, 62 +/- 27%, 46 +/- 13%, and 39 +/- 11%). The average preoperative left ventricular area shortening was 45 +/- 10% and neither it nor end-diastolic left ventricular area (preload) changed with surgery, time after surgery, or with adenosine. A titrated adenosine infusion is well suited to patients requiring a pharmacologic provocation to expose reversible myocardial ischemia during the first hours or days after CABG surgery. The anesthetized and anemic patient are particularly unsuited for the commonly used fixed-infusion-rate protocol.

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.

Receptor-receptor interactions as an integrative mechanism in nerve cells

Several lines of evidence indicate that interactions among transmission lines can take place at the level of the cell membrane via interactions among macromolecules, integral or associated to the cell membrane, involved in signal recognition and transduction. The present view will focus on this last subject, i.e., on the interactions between receptors for chemical signals at the level of the neuronal membrane (receptor-receptor interaction). By receptor-receptor interaction we mean that a neurotransmitter or modulator, by binding to its receptor, modifies the characteristics of the receptor for another transmitter or modulator. Four types of interactions among transmission lines may be considered, but mainly intramembrane receptor-receptor interactions have been dealt with in this article, exemplified by the heteroregulation of D2 receptors via neuropeptide receptors and A2 receptors. The role of receptor-receptor interactions in the integration of signals is discussed, especially in terms of filtration of incoming signals, of integration of coincident signals, and of neuronal plasticity.

Adenosine infusion for the reversal of pulmonary vasoconstriction in biventricular failure. A good test but a poor therapy

BACKGROUND

Elevation of pulmonary vascular resistance is an important determinant of right ventricular function in patients with end-stage biventricular heart failure. Vasodilator drug therapy directed at the pulmonary vasculature is used in the hemodynamic assessment of patients for orthotopic heart transplantation, and therapy aimed at decreasing pulmonary vascular resistance and transpulmonary pressure gradient has been advocated in patients awaiting heart transplantation. Adenosine infusion has been shown to cause selective pulmonary vasodilatation in normal subjects and in patients with primary pulmonary hypertension but has not been assessed in patients with biventricular heart failure.

METHODS AND RESULTS

Using two infusion doses, we studied the pulmonary and renal hemodynamic effects of adenosine on patients referred for heart transplantation (n = 21) and compared it with sodium nitroprusside (n = 18). Patients received 30% oxygen via face mask throughout the study. Adenosine at 100 micrograms/kg min achieved the same percentage fall in pulmonary vascular resistance as nitroprusside (41 +/- 6% versus 42 +/- 4%) and a greater and more consistent fall in transpulmonary pressure gradient (35 +/- 6% versus 9 +/- 30%, p less than 0.02). The mean arterial blood pressure fell by 16 mm Hg with nitroprusside but was unchanged by adenosine, indicating that in contrast to nitroprusside, adenosine acted as a selective pulmonary vasodilator. Despite this, cardiac index showed only a modest increase with adenosine (1.73 +/- 0.09 to 1.89 +/- 0.16 l.m-2, p less than 0.05), and there was a rise in pulmonary capillary wedge pressure from baseline at the higher dose (29.7 +/- 2.5 to 33.4 +/- 3.4 mm Hg, p less than 0.05). Renal blood flow was unchanged during adenosine infusion.

CONCLUSIONS

Adenosine is a potent selective pulmonary vasodilator in patients with biventricular heart failure and is preferable to sodium nitroprusside as a test for the reversibility of pulmonary vasoconstriction. However, its deleterious effects on left atrial pressure make it unsuitable as a therapeutic agent in patients awaiting heart transplantation.

Adenosine causes bradycardia in pacing-induced cardiac failure

BACKGROUND

In normal, conscious dogs, systemic injection of adenosine causes arterial hypotension and a baroreceptor reflex tachycardia mediated in part by withdrawal of vagal tone from the sinoatrial node. After vagal section or muscarinic receptor blockade, however, adenosine injection causes bradycardia via a direct sinoatrial node inhibition. Because cardiac failure is marked by a loss of vagal tone, we hypothesized that adenosine injection in dogs with failing hearts would reduce heart rate.

METHODS AND RESULTS

Mongrel dogs were instrumented with indwelling catheters, manometers, and ventricular pacing electrodes. After the dogs had recovered from the surgery, the ventricles were paced continuously at 210 beats per minute for 3 weeks, followed by pacing at 240 beats per minute for an additional week. This regimen caused mild ventricular and more striking atrial hypertrophy and a gradual onset of physiological and clinical signs of congestive heart failure. Adenosine injections that caused large tachycardias before the pacing regimen began caused progressively smaller increments in heart rate during the first 2 weeks of pacing. After 3 and 4 weeks, adenosine injections caused overt reductions in heart rate despite the concomitant arterial depressor response.

CONCLUSIONS

We conclude that the loss of vagal tone associated with the development of cardiac failure unmasks the direct negative chronotropic effect of exogenous adenosine on the sinoatrial node.

Adenosine and the bradycardiac response to vagus nerve stimulation in rats

The effects of intracardiac infusions of adenosine on the changes in heart rate (HR), electrocardiogram (ECG) and arterial blood pressure (BP) induced by both vagal stimulation and exogenous acetylcholine (ACh) were studied in anesthetized rats. Adenosine inhibited the bradycardia induced by vagal nerve stimulation, an effect antagonized by theophylline, decreased the elongation caused by vagal stimulation of the R-R intervals of the ECG, and caused a small but consistent decrease in the hypotensive effect of vagus nerve stimulation. At the dose that reduced the bradycardiac responses to vagal stimulation, adenosine enhanced the bradycardiac effect of exogenous ACh, increased R-R intervals and the number of P waves not followed by the ECG and had little or no effect on the inhibition induced by ACh on BP. The effects of adenosine on the bradycardiac responses to vagal nerve stimulation or to ACh administration were similar in both non-reserpinized and reserpinized animals. These results suggest that exogenous adenosine can modify the vagal influences on the heart by exerting pre-junctional inhibition of the vagus nerve and post-junctional enhancement of the ACh actions, and that the adrenergic system does not contribute to these effects of adenosine.

Myocardial effects of adenosine- and sodium nitroprusside-induced hypotension: a comparative study in patients anaesthetized for abdominal aortic aneurysm surgery

The effects of adenosine and sodium-nitroprusside (SNP) on central and myocardial haemodynamics and metabolism were evaluated during fentanyl anaesthesia (100 micrograms.kg-1) in six patients with peripheral vascular disease. The investigation was performed during stable anaesthesia, before scheduled abdominal aortic graft surgery. Adenosine and SNP were infused intravenously in random order over 20 min, leaving a 30-min control period in between. The vasodilators were titrated in order to reduce mean arterial pressure by approximately 25%. Adenosine (90 +/- 20 micrograms.kg-1.min-1) reduced mean arterial pressure from 10.9 +/- 0.3 to 8.4 +/- 0.4 kPa (82 +/- 3 to 63 +/- 3 mmHg), and SNP (0.7 +/- 0.1 micrograms.kg-1.min-1) from 11.0 +/- 0.2 to 8.4 +/- 0.3 kPa (83 +/- 3 mmHg to 63 +/- 3 mmHg), during the hypotension period. Cardiac index remained unaffected during induced hypotension with both vasodilators, while heart rate increased during SNP infusion (8 +/- 3%) and remained unaffected with adenosine. Left ventricular stroke work index and myocardial oxygen consumption decreased during SNP infusion (33 +/- 3% and 17 +/- 5%, respectively), while these parameters were unchanged with adenosine. Adenosine hypotension increased coronary sinus flow 1-2 fold (128 +/- 26%), together with increased coronary sinus oxygen content (96 +/- 11%). In contrast, coronary sinus flow decreased during SNP hypotension (-15 +/- 4%) with unaffected coronary sinus oxygen content. It is concluded that adenosine, in contrast to SNP, is associated with a hyperkinetic myocardial circulation.

Plasma adenosine concentrations during adenosine-induced respiratory stimulation in man

Intravenous infusion of the nucleoside adenosine stimulates respiration, probably at least partly by an action in the carotid bodies, and also potentiates the ventilatory response to hypoxia, suggesting that it might be involved in the control of breathing. Whether the effects of adenosine occur at concentrations likely to be achieved in vivo is unknown and was investigated in 7 patients with arterial catheters inserted for diagnostic purposes. During intravenous infusion of adenosine (Maximum dose per min: mean 130 micrograms kg-1) mean minute ventilation increased from 5.5 to 10.9 l min-1 while mean plasma adenosine concentration in the aortic arch increased from 0.07 to 1.2 microM. In 3 patients ventilation first increased without a detectable increase in aortic adenosine concentration, suggesting a possible intra-pulmonary effect of adenosine, although increased concentrations were apparent at higher doses. Micromolar concentrations of adenosine are probably achieved in vivo in tissues during hypoxia. The present results show that at such concentrations adenosine stimulates respiration and are consistent with the suggestion that adenosine release may mediate or modulate the ventilatory response to hypoxia. A possible intra-pulmonary effect of adenosine merits further study.

Species-dependent effects of adenosine on heart rate and atrioventricular nodal conduction. Mechanism and physiological implications

This study 1) compares the negative chronotropic and dromotropic actions of adenosine in guinea pig, rat, and rabbit hearts; 2) investigates the mechanism(s) for the different responses; and 3) determines the physiological implications. Isolated perfused hearts were instrumented for measurement of atrial rate and atrioventricular (AV) nodal conduction time. Differences in metabolism of adenosine were determined in the absence and presence of dipyridamole (nucleoside uptake blocker) and erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA, adenosine deaminase inhibitor). Dipyridamole plus EHNA decreased adenosine's EC50 for the negative dromotropic effect by 14-fold in guinea pig heart and 1.6-fold in rat heart. This is consistent with the greater number of [3H]nitrobenzylthioinosine binding sites measured in membranes from guinea pig (1,231 +/- 68 fmol/mg protein) compared with rat (302 +/- 31 fmol/mg protein) and rabbit (260 +/- 28 fmol/mg protein) atria. The potency of adenosine to slow atrial rate and prolong AV nodal conduction time was greater in guinea pig than in rat or rabbit hearts. This rank order of potency correlated well with the number of binding sites for the specific adenosine receptor radioligand 125I-aminobenzyladenosine in guinea pig (102 +/- 13 fmol/mg protein), rat (11 +/- 0.5 fmol/mg protein), and rabbit (8 +/- 1 fmol/mg protein) atrial membranes. Hypoxia increased the rate of adenosine release by severalfold and caused slowing of heart rate and AV block. In spontaneously beating hearts, the main effect of hypoxia was a slowing of ventricular rate, which in the guinea pig heart was due to AV block and in the rat heart to atrial slowing. In atrial paced hearts, hypoxia caused a marked prolongation of AV nodal conduction time in guinea pig (39 +/- 4 msec) and rabbit (29 +/- 5 msec) hearts, but only small effect in rat hearts (10 +/- 2 msec). The differences in response to hypoxia could be accounted for by the species-dependent differences in the 1) amount of adenosine released and metabolized, 2) sensitivity of the hearts to adenosine, and 3) dependency of AV nodal conduction on atrial rate. The findings indicate that the results from physiological or pharmacological studies on adenosine in one species may not be applicable to others, and the ultimate effect of adenosine and hypoxia is to slow ventricular rate.

Contrasting effects of two xanthines, theophylline and enprofylline, on the cardio-respiratory stimulation of infused adenosine in man

Six normal male subjects, ages 28-40 years, were studied on separate days during increasing infusions with adenosine, 40-120 micrograms kg-1 min-1, before and during infusions of two xanthine derivatives, theophylline (mean plasma concentration 9 mg l-1) and enprofylline (mean plasma concentration 3 mg l-1). The study was double-blind, randomized, placebo controlled. Cardio-respiratory variables were measured non-invasively. Adenosine by itself increased heart rate (P less than 0.05), skin temperature (P less than 0.05), resting minute ventilation (P less than 0.01) and decreased estimated Pa, CO2 (P less than 0.01). Compared with placebo enprofylline increased heart rate (P less than 0.05) and shifted the heart rate and ventilation dose-response curves of adenosine upwards (P less than 0.05 and P less than 0.02, respectively). Theophylline did not by itself affect heart rate but significantly (P less than 0.05) reduced the heart rate response to adenosine. Compared with placebo theophylline caused a small increase in minute ventilation (P less than 0.05) and flattened the dose-response curves of the effects of adenosine on ventilation (P less than 0.01) and Pa, CO2 (P less than 0.01). Theophylline also reduced abdominal and chest discomfort caused by adenosine permitting significantly (P less than 0.05) higher infusion rates of adenosine. These findings suggest that, with equipotent bronchodilating plasma concentrations, theophylline can inhibit while enprofylline augments some cardio-respiratory stimulant effects of infused adenosine in man.