CS 2015 Modulation of the Cardiac Action Potential and Contraction Christian Stricker Associate Professor for Systems Physiology ANUMS/JCSMR - ANU [email protected]

CS 2015 Modulation of the Cardiac Action Potential and Contraction Christian Stricker Associate Professor for Systems Physiology ANUMS/JCSMR - ANU [email protected] http://stricker.jcsmr.anu.edu.au/AP_Modulation.pptx [email protected]://stricker.jcsmr.anu.edu.au/AP_Modulation.pptx THE AUSTRALIAN NATIONAL UNIVERSITY

CS 2015

Aims At the end of this lecture students should be able to sketch the elements involved in EC coupling in heart; explain Ca 2+ clearance from plasma membrane; recognise the receptors involved autonomic control of the heart, incl. which path is normally stronger; apply the different forms of tropisms to myocytic function; outline the signalling cascades and the downstream targets after receptor activation; and show how Ca 2+ channel blockers affect the heart.

CS 2015 Contents I.EC coupling in the heart and Ca 2+ clearance. Tropisms in the heart. II.Innervation of the heart and autonomic control. III.Sympathetic effect on SAN and myocyte incl. downstream targets of signalling. IV.Parasympathetic effect on SAN incl. downstream targets of signalling. V.AP under different conditions Calcium channel blockers. How hyperkalaemia and hypoxia alter AP.

CS 2015 The description of the ionic currents until here may already have exhausted the reader, but it certainly does not exhaust the mechanisms that have been found. Denis Noble 1936 - ; Oxford Why so many different ionic/metabolic currents? Complexity may result in stability / robustness (?). Evolutionary artefact (?)

CS 2015 I. EC Coupling & Ca 2+ Clearance

CS 2015 EC-Coupling and Relaxation Obligatory Ca 2+ flow via L-type channels: requires extracellular Ca 2+ ; heart stops beating in Ca 2+ -free solute (cardioplegic solute). Ca 2+ release from SR stores: electrochemical coupling. Ca 2+ causes shortening of contractile proteins. Pumps clear sarcoplasma from Ca 2+ (ER and plasma membrane).

CS 2015 Contractility and ANS Sympathetic stimulation of 1 -receptors via isoproterenol causes a much larger Ca 2+ influx and bigger force with a faster rate of shortening and relaxation. Targets of modulation by 2 nd messengers: Action potential (see next). Muscle L-type Ca 2+ channels, Cytosolic Ca 2+ concentration, Store refilling via SERCA/PLB, and Contractile proteins (troponin I). When phosphorylated via PKA, stronger inhibition of troponin C force, faster cross-bridge cycling and relaxation. Berne & Levy, 2008

CS 2015 Cardiac Tropisms Neg & Pos ICS Chronotropism: in regard to heart rate Bathmotropism: in regard to AP threshold Dromotropism: in regard to AVN conduction delay Myocyte Inotropism: in regard to contractile force Lusitropism: in regard to rate of relaxation

CS 2015 II. Autonomic Innervation

CS 2015 Autonomic Innervation of Heart VA: restricted innervation (nodes only); right: SA node; left: AV node. SY: diffuse innervation (whole heart); right: SA & VA node; left: ventricle. Levick, 5 th ed., 2010 Parasympathetic (X = vagal; VA): via ACh. Effect: inhibitory action (HR; SV) and fast. Sympathetic (T 1 -T 5 ; SY): via noradrenaline. Effect: excitatory action (HR; SV; TPR) and slow.

CS 2015 Autonomic Control of Heart Rate Effect directly and mostly on SA node: VA: via muscarinic receptors (M 2 -receptor) blocked by atropine. SY: via -adrenergic receptors ( 1 -adrenoceptor): blocked by *olol. In modulating HR, in normal subjects at rest, VA output is larger and typically predominates over SY output. Both, VA and SY outputs are tonically active. Berne & Levy, 2008

CS 2015 III. Sympathetic Signalling

CS 2015 Sympathetic Stimulation of SAN Over several beats, HR, AV conduction, AP shortening, AP amplitude, rate of pacemaker decay (positive -tropisms) no additional depol. Recovery from stimulation slow: terminated by diffusion of NA in ECF and neuronal re-uptake. Action via 1 -R - and 2 -R are there, but outnumbered 4:1. Hutter & Trautwein, J Gen Physiol 39 (1956):715

CS 2015 Sympathetic Effect in Myocyte HR doubles in this case AP shortens, no change in AP height or RMP, phase 2 larger, phase 3 faster (spike and dome AP). Sarcoplasma: faster Ca 2+ rise and decay and larger [Ca 2+ ]-amplitude. Contraction: both contractility and rate of relaxation (positive ino- and lusotropic). Consequence: diastolic filling considerably curtailed (later). Levick, 5 th ed., 2011

CS 2015 Sympathetic Signalling Linked to G s signalling cascade: G INCREASES cAMP by adenylyl cyclase activation (direct). Mimicked by caffeine, etc., which block cAMP breakdown. PKA activation (downstream). ICS: faster pacing HR . I f (direct; open probability ) faster decay of PMP HR . I K via PKA phosphorylation faster repolarisation HR . I CaL from PKA phosphorylation faster decay of PMP HR . Myocyte: bigger force production I CaL from PKA phosphorylation (open probability & MOT): plateau current bigger force. PLB phosphorylated: disinhibits SERCA pump faster clearance. RyR phosphoryl.: store release . Levick, 5 th ed., 2010 Signalling turned off via phosphatase activity (PP2A) around macromolecular complexes (SERCA, RyR). Chronic stimulation results in CaMK II activation with different downstream signalling.

CS 2015 IV. Parasympathetic Signalling

CS 2015 Parasympath. Stimulation of SAN Instantaneous drop in HR: release of ACh broken down by AChE. Fast beat by beat In humans, effect restricted ONLY to ICS cells NOT on myocyte. Nerve stimulation generates PMP decay (large) with AP amplit. and hyperpolarisation: M 2 -R. Nature of hyperpolarisation: controversial. IJPs, hyperpolarising MP (a few mV). Mediated by synaptic M 2 -AChR (I bNa ). In most textbooks: ACh superfusion activates I KACh hyperpolarisation due to GIRK activation via G (fast): M 2 -AChR (likely extrasynaptic ). Wrong as not experim. blocked by Cs + (GIRK). Nerve stimulation ACh perfusion ! (in most textbooks not differentiated) Bolter et al., Autonom Neurosci 94 (2001):93

CS 2015 Parasympathetic Signalling Mostly exerted via M 2 activation. Linked to G i signalling cascade: G DECREASES cAMP () by adenylyl cyclase inhibition (direct). Converse of 1 activation. Downstream signalling: PKA activity. ICS: slower pacing I f (direct; open probability slower decay of PMP. I K via PKA activity slower repolarisation, longer AP. I CaL from PKA activity slower decay of PMP. Only at high stimulus rates, I K ACh via direct G activation hyperpolarisation (not physiol.). Levick, 5 th ed., 2011

CS 2015 Summary of Signalling in ICS Yin & yan around AC (agonist / antagonist) Sympathetic: cAMP Global effect on heart Faster decay of PMP Faster and larger Ca 2+ influx Shorter interval: I K No additional depolarisation Parasympathetic: cAMP Effect only on ICS Slower decay of PMP Slower Ca 2+ influx Longer interval: I K Small hyperpolarisation (IJP) Raff & Levitzky, 1 st ed., 2011

CS 2015 V. AP under different conditions

CS 2015 L-Type Ca 2+ Channel Blockers Verapamil, nifedipine & ipine Dose-dependent block of I CaL in clinically relevant doses. Verapamil better on cardiac tissue. ipine(s) better on vessels (vary). SAN Prolongs PMP decay and amplitude (small): HR - neg. chronotropic. AVN Reduces amplitude and shortens AP: currents for depolarisation of surrounding cells: neg. dromotropic. Cardiac myocyte Shoulder and amplitude (small): I CaL Force: neg. inotropic. Hirth et al., J Mol Cell Cardiol 15(1983):799

CS 2015 AP in Hyperkalaemia and Hypoxia Hyperkalaemia: RMP (E K ) Inactivation of I Na APs start to resemble those in ICS. Loss of phase 0 and 1. Hyp-/anoxia causes due ATP drop in Na + /K + -ATPase I KATP shorter AP less Ca 2+ influx contractility sympathetic activation: HR. store overload: delayed afterdepolarisation Mechanisms causing overload Sympathetic reflex activation: I CaL (see later). ATP transporter activity (Na/K-ATPase activity and Na/H-exchanger): Depolarisation (arrhythmia) [Na + ] NCX in reverse Ca 2+ influx: RyR activation (spontaneous discharge). Ca 2+ clearance into stores: overload. Levick (2010), 5 th Ed. Berne & Levy, 2008

CS 2015 Take-Home Messages Ca 2+ influx via L-type channels triggers EC coupling. Contractility and clearance of plasma modulated by 2 nd messenger systems. Autonomic innervation is diffuse (SYM) and specific (VA). Normally, parasympathetic activity predominates in heart. 1 -AR signal via cAMP to upregulate I f, I K and I CaL. M 2 -AChR signal via cAMP to downregulate I K and I CaL. There is a difference between action of synaptic & extrasynaptic AChR. Ca 2+ channel blockers are neg. ino-, dromo- and chronotrop.

CS 2015 MCQ Victor Helms, a 26 year-old is admitted to the Emergency Department (ED) after being stabbed in his neck. The ED physician diagnoses that his left vagal nerve may likely have been damaged? Which of the following statements is most consistent with this diagnosis? A.Sinus tachycardia. B.Increased cardiac contractility. C.Tachycardia with shortened PR interval. D.Ventricular bradycardia with large P waves. E.Sinus bradycardia with prolonged QRS complex.

CS 2015 Thats it folks

CS 2015 MCQ Victor Helms, a 26 year-old is admitted to the Emergency Department (ED) after being stabbed in his neck. The ED physician diagnoses that his left vagal nerve may likely have been damaged? Which of the following statements is most consistent with this diagnosis? A.Sinus tachycardia. B.Increased cardiac contractility. C.Tachycardia with shortened PR interval. D.Ventricular bradycardia with large P waves. E.Sinus bradycardia with prolonged QRS complex.

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CS 2015 Modulation of the Cardiac Action Potential and Contraction Christian Stricker Associate Professor for Systems Physiology ANUMS/JCSMR - ANU [email protected]