Gene annotation for heart rhythm 1. 2. 3. 4. 5. Control of heart rate Action Potential Ion channels and transporters Arrhythmia EC coupling Control of heart rate Autonomic regulation of heart function Autonomic Regulation II • Central integration of blood pressure and respiratory control • Afferents via baroreceptors, chemoreceptors etc • Integrated in brainstem centres Autonomic Regulation III Effector arm Proteins involved in presynaptic vesicle release Proteins involved in signal transduction in the SA node Heart Rate Variability • The heart beat is not quite regular subject to small variations • e.g. sinus arrhythmia • Indicative of health. Correlates inversely with outcome after MI etc • Time domain:– Tachograms, SD of R-R or DR-R • Frequency domain:- Potentially more revealing. • HF=vagal\respiration, LF=sympathetic\BP control What ionic mechanisms are responsible? • Intrinsic rhythm set by SA node • Modulation of pacemaker depolarisation receptor activation M2 receptor activation Gs Adenylate cyclase Increased cAMP Adenylate cyclase Decreased cAMP Gi\o Gg liberation IKAch activation If activation If inhibition What is the intrinsic pacemaker? • Spontaneous activity in the absence of innervation (intrinsic heart rate) • Actually currently quite controversial • Two hypotheses – If current is centrally important and\or Ca2+ cycling If\HCN channels • • • • • • • Activated by hyperpolarisation Cation but otherwise nonselective Directly opened by cAMP HCN1-4, mainly HCN4 in heart Largely expressed in SA node Ivabradine used for the treatment of angina Action potential Cardiac Action Potential I Conduction system Anatomy AP in heart regions Cardiac action potential II – Kv1.5 • IKACh – Kir3.1\3.4 • IKATP – SUR1\Kir6.1\Kir6.2 vs SUR2A\Kir6.2 • Cx40 in atria. Cx43 in ventricle • SK channels • IKur Ion Channels and Transporters What is happening at the molecular level? Ion channels predominantly control membrane excitability Sodium channels SCN5A in the heart. Both beta subunits present. Potassium channels Lots of genes underlying K+ channels Current Molecular composition Channel structure Function Location Reference(s) Ito,f (Ito1) -subunit Kv4.3 and -subunit KChiP2. Octameric complex of a tetramer of 6 TMD subunits and 4 subunits. Provides the rapid component of the transient outward current that contributes to early rapid repolarization during Phase 1. Atrial and Ventricular. [23], [24], (a), (b) Ito,s (Ito1) -subunit Kv1.4 and possibly subunits (Kv1.2, Kv1.3 and Kv2) -subunit Kv1.5 and -subunit Kv1.2. -subunit Kv11.1 (HERG) and probably -subunit KCNE2. subunit Kv7.1 (KCNQ1) and subunit KCNE1. Kir2.1 and perhaps Kir2.2 and Kir2.3. Kir3.1 and Kir3.4. A tetramer of 6 TMD subunits may coassemble with 4 -subunits. Atrial and Ventricular. [25], [26], (a), (c) Atrial. [27], [28], (d), (e) Atrial and Ventricular. (f), (g), (h), (i) Atrial and Ventricular. [20], [21], [22], (h), (i) Ventricular and Atrial [30], [31], (j), (k), (l) Predominantly Atrial and nodal tissue expression. [32], [33], (j), (m), (n), (o), (p) Kir6.2 and SUR2A. Octameric complex formed by coassembly of 4 2 TMD pore subunits and 4 17 TMD SUR subunits. Octameric complex formed by coassembly of 4 2 TMD pore subunits and 4 17 TMD SUR subunits. Provides the slow component of the transient outward current that contributes to early rapid repolarization during Phase 1. Plays an important role in early phase (1-2) atrial repolarization. Repolarisation, outward rectifier during Phase 2 and 3. Repolarisation, outward rectifier during Phase 2 and 3. Contributes to late repolarisation, late phase 3, and helps to set membrane potential. During late phase 3 and phase 4 activation of IKACh by acetylcholine acts to hyperpolarise the membrane potential, slow the firing rate of pacemaker cells in the SA and AV nodes and delays AV conduction. During late phase 3 and phase 4 this channel acts to link cellular metabolism and membrane excitability. During late phase 3 and phase 4 this channel acts to link cellular metabolism and membrane excitability. Ventricular. [10], (q), (r), (s) Atrial. [9] , [10], (q), (r), (s) IKur IKr IKs IK1 IKACh IKATP (Ventricular) IKATP (Atrial) Kir6.2 and SUR1. (Kir6.1?) A tetramer of 6 TMD subunits associates with 4 -subunits to form an octameric complex. A tetramer of 6 TMD -subunits and an unknown number of 1 TMD -subunits. Tetramer of 6 TMD -subunits assembles with probably two 1 TMD -subunits. Tetramer of 2 TMD subunits. Tetrameric complex of 2 Kir3.1 and 2 Kir3.4 2 TMD subunits. (During development channel may be formed by a homotetramer of Kir3.4) • Also SK channels and twin pore channels K channels in Long QT Voltage-gated (6-TM) H5 Extracellular N Extracellular Intracellular Intracellular N alpha KCNQ1 (KvLQT1) HERG KCNE family C C beta current KCNE1 (IsK) Iks KCNE2 (MIRP1) Ikr Na+\K+ ATPase • Member of the P type ATPase pumps • 1, 2 and 3 subunits. 1 and 2 auxiliary subunits • Electrogenic 3Na+ for 2K+ but transport rate ~4 four fold less than the Na channel (100 ions\second) Arrhythmia This carefully orchestrated activity can go wrong Classification of arrhythmia • Site of origin e.g. atrial, nodal, ventricular • Rate e.g. bradycardia, tachycardia • Process\Substrate e.g. fibrillation, heart block, ectopic etc Electrocardiogram (ECG) The benchmark of clinical diagnosis is the ECG P wave= atrial depolarisation QRS= ventricular depolarisation T wave=ventricular repolarisation Examples Atrial Fibrillation Ventricular Tachycardia Repolarisation and K+ currents Excitation-contraction coupling Cardiac excitation-contraction coupling Calcium channels • Gene = CACNAx for alpha subunits (CACNA1C = Cav1.2) • Cav1 = L-type, Cav2 = N-, P\Q and R type and Cav3 = T type Ryanodine receptor • RyR2 in heart • Calcium induced calcium release • LTCC and RyR2 opposed in Ttubule • Large tetrameric complex • Protein interactions Sodium\calcium exchanger • • • • • Major mechanism for calcium extrusion from the heart Electrogenic – 3 Na+ for single Ca2+ Passive coupled counter transport system NCX1 in the heart (3 isoforms in total) Also P type ATPase Ca2+ pump present in heart which actively extrudes Ca2+ (PMCA) SERCA2a and phospholamban • Major mechanism for calcium uptake into SR • P-type ATPase that transports Ca2+ actively driven by ATP hydrolysis • Regulated by phospholamban