Function and Dysfunction of Human Sinoatrial Node

Sinoatrial node (SAN) automaticity is jointly regulated by a voltage (cyclic activation and deactivation of membrane ion channels) and Ca2+ clocks (rhythmic spontaneous sarcoplasmic reticulum Ca2+ release). Using optical mapping in Langendorff-perfused canine right atrium, we previously demonstrated that the beta-adrenergic stimulation pushes the leading pacemaker to the superior SAN, which has the fastest activation rate and the most robust late diastolic intracellular calcium (Cai) elevation. Dysfunction of the superior SAN is commonly observed in animal models of heart failure and atrial fibrillation (AF), which are known to be associated with abnormal SAN automaticity. Using the 3D electroanatomic mapping techniques, we demonstrated that superior SAN served as the earliest atrial activation site (EAS) during sympathetic stimulation in healthy humans. In contrast, unresponsiveness of superior SAN to sympathetic stimulation was a characteristic finding in patients with AF and SAN dysfunction, and the 3D electroanatomic mapping technique had better diagnostic sensitivity than corrected SAN recovery time testing. However, both tests have significant limitations in detecting patients with symptomatic sick sinus syndrome. Recently, we reported that the location of the EAS can be predicted by the amplitudes of P-wave in the inferior leads. The inferior P-wave amplitudes can also be used to assess the superior SAN responsiveness to sympathetic stimulation. Inverted or isoelectric P-waves at baseline that fail to normalize during isoproterenol infusion suggest SAN dysfunction. P-wave morphology analyses may be helpful in determining the SAN function in patients at risk of symptomatic sick sinus syndrome.

Cervical Vagal Nerve Stimulation Activates the Stellate Ganglion in Ambulatory Dogs

BACKGROUND AND OBJECTIVES: Recent studies showed that, in addition to parasympathetic nerves, cervical vagal nerves contained significant sympathetic nerves. We hypothesized that cervical vagal nerve stimulation (VNS) may capture the sympathetic nerves within the vagal nerve and activate the stellate ganglion. MATERIALS AND METHODS: We recorded left stellate ganglion nerve activity (SGNA), left thoracic vagal nerve activity (VNA), and subcutaneous electrocardiogram in seven dogs during left cervical VNS with 30 seconds on-time and 30 seconds off time. We then compared the SGNA between VNS on and off times. RESULTS: Cervical VNS at moderate (0.75 mA) output induced large SGNA, elevated heart rate (HR), and reduced HR variability, suggesting sympathetic activation. Further increase of the VNS output to >1.5 mA increased SGNA but did not significantly increase the HR, suggesting simultaneous sympathetic and parasympathetic activation. The differences of integrated SGNA and integrated VNA between VNS on and off times (DeltaSGNA) increased progressively from 5.2 mV-s {95% confidence interval (CI): 1.25-9.06, p=0.018, n=7} at 1.0 mA to 13.7 mV-s (CI: 5.97-21.43, p=0.005, n=7) at 1.5 mA. The difference in HR (DeltaHR, bpm) between on and off times was 5.8 bpm (CI: 0.28-11.29, p=0.042, n=7) at 1.0 mA and 5.3 bpm (CI 1.92 to 12.61, p=0.122, n=7) at 1.5 mA. CONCLUSION: Intermittent cervical VNS may selectively capture the sympathetic components of the vagal nerve and excite the stellate ganglion at moderate output. Increasing the output may result in simultaneously sympathetic and parasympathetic capture.

The Role of the Calcium and the Voltage Clocks in Sinoatrial Node Dysfunction

Recent evidence indicates that the voltage clock (cyclic activation and deactivation of membrane ion channels) and Ca2+ clocks (rhythmic spontaneous sarcoplasmic reticulum Ca2+ release) jointly regulate sinoatrial node (SAN) automaticity. However, the relative importance of the voltage clock and Ca2+ clock for pacemaking was not revealed in sick sinus syndrome. Previously, we mapped the intracellular calcium (Cai) and membrane potentials of the normal intact SAN simultaneously using optical mapping in Langendorff-perfused canine right atrium. We demonstrated that the sinus rate increased and the leading pacemaker shifted to the superior SAN with robust late diastolic Cai elevation (LDCAE) during beta-adrenergic stimulation. We also showed that the LDCAE was caused by spontaneous diastolic sarcoplasmic reticulum (SR) Ca2+ release and was closely related to heart rate changes. In contrast, in pacing induced canine atrial fibrillation and SAN dysfunction models, Ca2+ clock of SAN was unresponsiveness to beta-adrenergic stimulation and caffeine. Ryanodine receptor 2 (RyR2) in SAN was down-regulated. Using the prolonged low dose isoproterenol together with funny current block, we produced a tachybradycardia model. In this model, chronically elevated sympathetic tone results in abnormal pacemaking hierarchy in the right atrium, including suppression of the superior SAN and enhanced pacemaking from ectopic sites. Finally, if the LDCAE was too small to trigger an action potential, then it induced only delayed afterdepolarization (DAD)-like diastolic depolarization (DD). The failure of DAD-like DD to consistently trigger a sinus beat is a novel mechanism of atrial arrhythmogenesis. We conclude that dysfunction of both the Ca2+ clock and the voltage clock are important in sick sinus syndrome.

The Calcium and Voltage Clocks in Sinoatrial Node Automaticity

Recent evidence indicates that the voltage (cyclic activation and deactivation of membrane ion channels) and Ca2+ clocks (rhythmic spontaneous sarcoplasmic reticulum Ca2+ release) jointly regulate sinoatrial node (SAN) automaticity. Since the intact SAN is a heterogeneous structure that includes multiple different cell types interacting with each other, the relative importance of the voltage and Ca2+ clocks for pacemaking may be variable in different regions of the SAN. Recently, we performed optical mapping in isolated and Langendorff-perfused canine right atria. We mapped the intracellular calcium (Cai) and membrane potentials of the intact SAN simultaneously. Using previously described criteria of the timing of the late diastolic Cai elevation (LDCAE) relative to the action potential upstroke to detect Ca2+ clock activity, we demonstrated that the sinus rate increased and the leading pacemaker shifted to the superior SAN with the robust LDCAE during beta-adrenergic stimulation. We also showed that the LDCAE was caused by spontaneous diastolic SR Ca2+ release and was closely related with heart rate changes. We conclude that the Ca2+ and voltage clocks work synergistically to generate SAN automaticity.

Arrhythmogenic Gene Change and Nerve Sprouting after Acute Myocardial Infarction in Mice

BACKGROUND AND OBJECTIVES: Myocardial infarction (MI) elicits nerve sprouting. However, the time course and spatial distribution of this nerve sprouting and its relationship to the expression of neurotrophic factors is unclear. The aim of this study was to identify the association of nerve sprouting with the expression of neurotrophic factors. MATERIALS AND METHODS: We induced MI in FVB mice by ligating the left coronary artery. The hearts were removed at 3 hours to 13 months after MI for growth associated protein 43 (GAP-43) immunostaining. The nerve density (micrometer2/mm2) was determined by ImagePro software. In another group of mice, their myocardial tissues were processed and analyzed with using an Affymetrix RG U74V2 array. RESULTS: The density of the nerve fibers that were immunopositive for GAP-43 was the highest 3 hours after MI in both the peri-infarct areas and the remote areas. The outer loop of the ventricle had a higher nerve density than that in the inner loop of the ventricle. The differences were at a peak 3 hours after MI, but they persisted for 2 months afterwards. The expressions of nerve growth factor, insulin-like growth factor, leukemia inhibitory factor, transforming growth factor-beta3 and interleukin-1alpha were increased for up to 2 months after MI as compared to the normal control. qRT PCR analyses showed increased mRNA for tyrosine hydroxylase, synaptophysin, nerve growth factor and leukemia inhibiting factor in the peri-infarct areas for up to 2 months after MI, but this occurred only for roughly 3 days after MI in the remote areas. CONCLUSION: We conclude that MI resulted in immediate upregulation of nerve growth factor, insulin-like growth factor, leukemia inhibitory factor, transforming growth factor-beta3 and interleukin-1alpha in the peri-infarct areas and this all occurred to a lesser extent in the remote areas. These changes persisted for at least 2 months, and they were associated with increased nerve sprouting activity, which was most active in the outer loop of the heart.

Wavebreak Mechanism During Ventricular Fibrillation in Isolated Swine Right Ventricle

BACKGROUND: Several different patterns of wavebreak have been described by mapping of the tissue surface during fibrillation. However, it is not clear whether these surface patterns are caused by multiple distinct mechanisms or by a single mechanism. METHODS: To determine the mechanism by which wavebreaks are generated during ventricular fibrillation, we conducted optical mapping studies and single cell transmembrane potential recording in 6 isolated swine right ventricles. RESULTS: Among 763 episodes of wavebreak (0.75 times/sec/cm2), optical maps showed 3 patterns: 80% due to a wavefront encountering the refractory waveback of another wave, 11.5% due to wavefronts passing perpendicularly each other and 8.5% due to a new (target) wave arising just beyond the refractory tail of a previous wave. Computer simulations of scroll waves in 3-D tissue showed that these surface patterns could be attributed to two fundamental mechanisms: head-to-tail interactions and filament break. CONCLUSION: We conclude that during sustained ventricular fibrillation in swine RV, surface patterns of wavebreak are produced by two fundamental mechanisms: head-to-tail interaction between waves and filament break.

The Effects of Excitation-Contraction Uncouplers on the Dynamics of Ventricular Fibrillation in Isolated Swine Right Ventricles

BACKGROUND: Whether or not the excitation-contraction (EC) uncoupler, diacetyl monoxime (DAM) and cytochalacin D (Cyto D) alter the ventricular fibrillation activation patterns is unclear. METHODS: We recorded single cell action potentials and performed optical mapping in isolated perfused swine right ventricles at different concentrations of DAM and cyto D during ventricular fibrillation and dynamic pacing. RESULTS: Increasing concentration of DAM results in progressively shortened action potential duration measured to 90% repolarization (APD90), reduced slope of the action potential duration restitution(APDR) curve, decreased Kolmogorov-Sinai entropy, and reduced number of ventricular fibrillation wavefronts. In all right ventricles, 15 to 20 mmol/l DAM converted ventricular fibrillation to ventricular tachycardia. The ventricular fibrillation could be reinduced after the DAM was washed out. In comparison, cyto D (10 to 40 mol/l) has no effects on APDR curve or the dynamics of ventricular fibrillation. The effects of DAM on ventricular fibrillation are associated with reduced number of wavefronts and dynamic complexities in ventricular fibrillation. CONCLUSION: These results are compatible with Restitution Hypothesis of ventricular fibrillation and suggest that DAM may be unsuitable as an EC uncoupler for optical mapping studies of ventricular fibrillation in the swine right ventricles.