HBF-ii_2024-L6_pt1_notes-chung.pdf

Human Body Foundations II: Cardiac Electrophysiology and Cardiac Function (Physiology) 2024 Faculty Charles S Chung PhD Associate Professor of Physiology [email protected] Note: In most cases, if a statement text is in a gray color, it is either an exercise or additional information. Throughout...

Human Body Foundations II: Cardiac Electrophysiology and Cardiac Function (Physiology) 2024 Faculty Charles S Chung PhD Associate Professor of Physiology [email protected] Note: In most cases, if a statement text is in a gray color, it is either an exercise or additional information. Throughout the notes, essential concepts are underlined, but studying all components is important to prepare for additional topics. Lecture 6-Cardiac Cellular Electrophysiology Part 1: Cardiac Action Potentials Learning Objectives Describe the five phases of cardiac action potentials Describe whether sodium, calcium, and/or potassium channels are active during each phase Describe how changing ion channel’s activity will change an action potential Define automaticity and understand which ion channel(s) modify automaticity Define escape and understand how a non-pacemaker cell might undergo an escape event 1 Review Resources This lecture deals with ion exchange, ion channels, and action potentials. The following concepts should be reviewed: Voltage Gated Ion Channels Potassium ions move out Sodium ions move in Calcium ions move in Secondary Active Transport (Na+-K+-ATPase) Potassium ions move in Sodium ions move out Na+/Ca2+ exchanger (NCX) Sodium ions move in Calcium ions move out Channels generally have an Open, Inactive, and Closed state Inactivation can be voltage gated also Closed and inactive both mean no current If you feel you need a review or want a deeper reading of these concepts, consider these resources: DeGracia HBFI Lectures 3, 4 o Basic Electrophysiology o Cellular Electrophysiology If you want to go deeper and understand more about channels and electrophysiology, refer to: Boron and Boulpaep, Medical Physiology o https://elibrary.wayne.edu/record=b5156592~S47 sections: o 6. Electrophysiology of the Cell Membrane o 7. Electrical Excitability and Action Potentials o 8. Synaptic Transmission Other reading, primarily what channels and ion exchangers exist in the myocardium, their gating, etc: o Varro et al Physiol Rev 2021 https://pubmed.ncbi.nlm.nih.gov/33118864/ o Nattel and Carlsson Nat Rev Drug Discov 2006 https://pubmed.ncbi.nlm.nih.gov/17139288/ Additional Notes/Cautions: This lecture doesn’t address all of the different voltage gated potassium or calcium channels. There are at least 6 types of calcium channels that are characterized biochemically with various gating voltages, etc. (See the additional resources, such as the Varro reference on the first page, for details.) I make this point because cardiac pharmacology and pathophysiology may be channel specific. (It can get more complicated since most channels are multi-mers. If one component is mutated or a different isoform, its properties may change. 2 Ventricular Myocyte Action Potential The myocyte action potential was introduced in HFBI-Cell Electrophysiology. There are 5 phases of the action potential Phase 0: Depolarization Na+ in increases voltage Phase 1: Early repolarization Ca2+ in, but K+ out is fast Phase 2: Plateau Ca2+ in balances with K+ out Phase 3: Rapid Repolarization K+ out decreases voltage Phase 4: Resting Potential High K+ permeability (IK1) In general, these properties are sufficient in understanding what controls each phase of the action potential. For example, even though there are multiple potassium channels, if one says that potassium flow is altered, it won't change Phase 0 of a ventricular myocyte. However, the following sections provide more detail. 1 0 2 Voltage [mV] -40 3 0 -80 4 4 Ito Gated Currents 0- ← in – out → IKr 0- IKs 0- IK1 0- ICaL 0- INa 0- FIGURE: Alignment of the myocyte action potential (with phases numbered) and schematic timing of voltage gated ion currents. Channel specific descriptions of ion exchange that drives the myocyte action potential Phase 0: the depolarization (upstroke) of the action potential Triggers around -60 mV due to opening of fast voltage gated sodium channel, creating INa Sodium current moves in 3  The rapid spike in myocytes is due to INa ICaL also opens, but does not exchange ions as fast as INa Phase 1: a brief rapid early repolarization INa slows, i.e. the fast voltage gated sodium channel inactivates The L-type calcium channel, which opened already, creates ICaL moves more positive ions in Potassium channels (transient outward potassium channel (Ito) and delayed rectifier channels) move positive ions out Phase 2: a plateau phase INa and ICaL are less active, but can continue to push ions in Potassium currents, especially the IKr (and other delayed rectifier currents like IKs) move charges out Sodium Calcium Exchanger (NCX) and Na+-K+-ATPase exchanges ions Net exchange means total ion movement is balanced Phase 3: the repolarization Sodium and Calcium channels now inactivated or closed (Voltage gated) Delayed Rectifier Potassium channels drive IKs and IKr currents out reducing the voltage Phase 4: the diastolic or resting potential Inward rectifying potassium channel (IK1) induces high K+ permeability (current moves out) Na+-K+-ATPase, NCX balance ions further Generally constantly active “Clarity” about the Inward rectifying potassium channel (IK1) Inward rectification means that the fastest current measured goes inwards Inward rectifying potassium channel (IK1) moves potassium OUT in myocytes Myocytes (usually) do not reach voltages low enough to move potassium inwards 2 Rectifying outwards current 1 0 Current -150 -100 -50 0 50 100 150 -1 -2 Fast inwards current -3 Voltage [mV] FIGURE: The Inward rectifying potassium channel is named this because the fastest current is inward at very low voltages. These voltages are not normally achieved in the heart. So for cardiac cells, potassium moves out of its channels.. 4 Regulating the Action Potentials is possible Sodium There are a number of mechanisms to alter the sodium current, most of which will be detailed in cardiac pharmacology, but examples include: PKC Phosphorylation reduces current Sodium Channel Blockers reduce current Tetrodotoxin (TTX) Lidocaine Ranolazine Quinine The example below shows what happens if the fast sodium current is suppressed. 1 0 2 Voltage [mV] -40 3 0 -80 4 4 Ito Gated Currents 0- ← in – out → IKr 0- IKs 0- IK1 0- ICaL 0- INa 0- FIGURE: if the sodium current is reduced or delayed, (solid blue to dashed blue), then the phase 0 of the action potential will be slowed- i.e. the cell won’t be able to depolarize as quickly. Calcium There are a number of mechanisms to alter the calcium current, most of which will be detailed in cardiac pharmacology, but examples include: PKA increases current Isoproterenol Calcium Channel Blockers decrease current Verapamil Nifedipine The example below shows what happens if the L-type calcium current is enhanced or suppressed. 5  1 0 2 Voltage [mV] -40 3 0 -80 4 4 Ito Gated Currents 0- ← in – out → IKr 0- IKs 0- IK1 0- ICaL 0- INa 0- FIGURE: if the calcium current is enhanced, (red to dashed purple), then phase 1 and 2 may be altered. If the calcium current is reduced or delayed (red to dashed yellow), then the phase 2 plateau may be reduced. Remember that the L-type calcium transient contributes to the early repolarization and its flux balances the potassium fluxes to create a plateau during phase 2. Non-voltage gated channels Another example is suppressing the Na+-K+-ATPase If ATP is less available in the heart, the Na+-K+-ATPase may not exchange as quickly During what phase(s) is the Na+-K+-ATPase most active? 0 Voltage [mV] -40 -80 FIGURE: if the Na+-K+-ATPase is impaired, then the resting membrane potential (Phase 4) won’t be maintained properly. Tl;dr: if an ion current is altered, one can consider the phases where it is most active (most open for channels) to understand which phases of the action potential are changed. To understand the direction of change, think about which way the ions are flowing (potassium out polarizes, sodium and calcium in de-polarizes). 6 Self-practice exercise: What happens if you modify the potassium currents? What happens if you inhibit IK1? Remember that the inward rectifying potassium channel maintains the resting potential What phases would change if you inhibit another delayed rectifier, IKs? What phases would change if you enhance Ito? Not all Myocytes are Identical The focus of the above section was the ventricular myocyte, but within a ventricle, there is variation in how much of some ions are in each cell. A specific example is the transmural gradient Expression of potassium channels varies transmurally from the epicardium (outer wall) to the endocardium (inner wall) The epicardial action potentials are shorter than the endocardial action potentials Endocardial action potentials are longer than epicardial Actually, mid-myocardial action potentials are longest! One can consider what this means. Since the major change is in phase 3, delayed rectifier (IKs, IKr) and inward rectifying currents (IK1) are likely the cause of this difference. 0 -40 -80 FIGURE: Schematic differences of the transmural gradient in action potentials. The endocardial action potential (blue) is depolarized longer than the epicardial action potential (purple dash). Not only is the epicardial action potential shorter in duration, but it actually triggers later (see cardiac conduction). 7 Atrial myocyte action potentials differ from ventricular myocyte action potentials Similar to ventricular, but different combinations of channels Atrial Ito slower to turn off Ultra-rapid delayed rectifier current IKur is larger, changing Phases 2 and 3 0 Voltage [mV] -40 -80 FIGURE: Schematic differences in the ventricular (blue) and atrial (red dash) action potential. Since Ito stays on longer in the atrial myocyte, it drives a steeper decline in phase 1. Differences in the delayed rectifier channels impact phases 2 and 3. Pacemaker Cell Action Potentials There are two major pacemaker cell types Sino-atrial (SA) Node Primary pacemaker of the heart Atrio-ventricular (AV) Node Contains bundle of His that delays propagation of the depolarization from the atria to the ventricles The SA Node (Pacemaker) Action Potential differs from the myocyte action potential: No voltage gated sodium channel Hyperpolarization-activated cyclic nucleotide–gated (HCN) channel (If); moves sodium in T-type (transient) calcium channel (ICaT) present T-type channel gates at lower voltage than L-type channel, both move calcium in L-type channel still present Acetylcholine activated potassium channel (IK,Ach) present Ligand gated potassium channel, moves potassium out 8 NOTE: In the schematic and text below, many channels are not shown. Even though the schematics and descriptions don’t discuss the inward rectifying potassium channel etc, they are still present at different concentrations. If a specific channel is relevant to clinical care (pharmacology), the physician must check to see if the channels are actually present in a cell type. 0 Voltage [mV] 0 3 -40 4 4 -80 If Gated Currents 0- ← in – out → IKr 0- IK, ACh 0- ICaL 0- ICaT 0- FIGURE: the action potential and major currents of a pacemaker cell The THREE phases of the pacemaker cell action potential Phase 0: depolarization ICaL comes in to the cell to drive depolarization Unlike myocytes, essentially no voltage gated sodium current Phase 3: repolarization highly regulated by potassium current out of the cell (IKr, etc) Phase 4 Unlike myocytes, the diastolic phase is not constant Potassium goes out via Ikr and the ligand gated acetylcholine activated potassium channel (IK,Ach) Sodium continuously comes in via HCN (If) Calcium transiently comes in (ICaT) as threshold voltage approaches There is no early repolarization (Phase 1) or plateau (Phase 2) in SA Node cells Differences from myocyte: No Phase 1, 2 Depolarization by L-type calcium channel, not voltage gated sodium channel Has T-type calcium channel Has HCN (funny channel) 9 Regulation of pacemaker cell action potentials The HCN (funny channel, If) can be inhibited by drugs (such as ivabradine, see cardiac pharmacology). This reduces the If current and prolongs Phase 4 0 0 3 Voltage [mV] -40 4 4 -80 If Gated Currents 0- in ← in – out → IKr 0- ICaL 0- ICaT 0- FIGURE: if the funny channel current is suppressed, then less ions will enter the cell during phase 4. This will reduce the rate of the depolarization of the action potential. Note that this changes the rate of reaching the full depolarization threshold (how long it takes to open the L-type calcium channel), but not the voltage of the threshold. The HCN can be activated Beta-1 adrenergic receptor (Norepinepherine) Self Test: what does activation (increasing activity) of HCN do to Phase 4? Other Self Test exercises for pacemaker cell action potentials: What will happen if you add a calcium channel blocker? Does it need to be selective? What happens when you increase acetylcholine and enhance the outwards muscarinic- gated K+-current, IK,ACh 10 Purkinje Cell/Fiber action potential The Purkinje cells are a special cell type. First, they are considered the most neuron-like. They have microarchitecture that makes them more similar to a thick axon (long tube), but they contain some contractile elements. Second, they contain the ion channels of myocytes, but ALSO contain the HCN/funny channel and T-type calcium channels. 0 Voltage [mV] -40 -80 -Ventricular Myocyte -Atrial Myocyte -Purkinje FIGURE: comparison of action potentials from ventricular myocytes (blue), atrial myocytes (red dashed) and Purkinje cells (purple dashed). Note that the Purkinje cells have all of the phases (including phase 1 and 2) of a myocyte action potential, but it has a raising voltage during phase 4 due to the presence of HCN. Automaticity Intrinsic, spontaneous depolarization of a cell Recall from HBFI Cellular Electrophysiology that cells with HCN channels exhibit automaticity, or an intrinsic pacemaker rate. The mechanism of this is understood to be calcium pulses (probably associated with the T-type calcium channel, which brings the resting voltage up to the threshold of the L-type calcium channel. Varro et al, Physiol Rev. 2021 Jul 1;101(3):1083-1176. FIGURE: mechanism of automaticity. 11  Cells with HCN channels (producing If), have spontaneous depolarization Increased Na+ and Ca2+ leads to Phase 0, the opening of L-type Calcium Channels (ICaL) Produce intrinsic pacing rates TABLE: Pacemaker rates, i.e. the rate of a cell reaching the threshold to open fast sodium or L-type calcium channels and depolarize. Myocardial cells (myocytes) do not have an intrinsic rate because they don’t have HCN (funny channels), therefore they do not exhibit automaticity. Escape A type of ectopic depolarization Normally, the depolarization of one cell type will conduct along a regular pathway: SA Node → Atrial Myocytes → AV Node → Purkinje Fibers → Ventricular Myocytes. If a cell down the pathway depolarizes prior to one earlier in the path, that cell type is considered an escape. For example, myocytes do not have intrinsic pacing activity, but can depolarize (escape) If automaticity does not trigger depolarization, a cell may still depoliarize (Escape) Alter the Na+-K+-ATPase A spontaneous calcium release (calcium spark) Mechanical stretch Occurs during phase 4 Clinically: Trigger arrhythmias Escape that depolarizes myocytes may be called premature contractions 12 Cardiac Conduction: Normally in a neuron, the action potential will travel down the very long axon, meaning the neuron will depolarize along its length. Neurotransmitters conduct the signals SA Node is essentially the only place where efferent neurons release neurotransmitters to modulate cardiac function directly. Thus, there are no neurological signals to help the rest of the heart depolarize. Purkinje fibers are most ‘neuron’-like (diameter), but do not release neurotransmitters. How does the rest of the heart know when to depolarize? Gap Junctions Recall from HBFI Microanatomy of Muscle and HBFII Histology (Microanatomy of the heart), that myocytes are much smaller in length than neurons, but they link to other cells with Gap Junctions. Gap Junctions are pores created by Connexins. Localized at the intercalated disk (myocyte ends). They act as tubes that allow ions to pass between cells, passing the voltage change. Boron Baulpaep, Medical Physiology Fig 21-3 FIGURE: Schematic of how changes in ion concentration (i.e. current) would propogate (i.e. conduct) to nearby cells using gap junctions. Cells may be connected by gap junctions, but what stops the action potential from cycling back on itself- i.e. how do you stop “re-entry”? Refractory Period- a regulation of cardiac conduction/action potential propagation. Recall from HBFI Cell Electrophysiology Absolute refractory period of a neuron Region of membrane directly behind action potential wave still has a high positive voltage INa is inactive due to the high voltage More depolarizations just hold the voltage high, keeping the voltage gated sodium channels inactive Relative refractory period As voltage repolarizes, voltage gated sodium channels close, to allow the next depolarization 2 Myocyte Because the cells are connected, you don’t want to have a cell depolarize immediately after it depolarized earlier. One way to think of this is that you don’t want a heart in tetanus. I like to think of this as a “no-tag-back” rule. Once a cell passes on the change in action potential, it should wait longer before it depolarizes again. The refractory period enforces this so that the conduction doesn’t go in a problematic direction. Absolute Refractory Period (ARP) Sodium and calcium channels can’t reopen after inactivating Sometimes “Effective Refractory Period” Relative Refractory Period (RRP) Depolarization can again be triggered, but won’t be as rapid/high magnitude until sodium and calcium channels fully open, but some of these channels may be open 0 Voltage [mV] -40 -80 Voltage to high Hard to for INa to be reopen Can active depolarize ARP RRP again Total Refractory Period = ARP+RRP FIGURE: Schematic of cardiac action potentials and the impact of refractory periods. During the absolute refractory period (red), ion exchange or ion transport cannot trigger a new action potential. This is because channels are inactive or haven’t fully closed so they will not reopen. During the relative refractory period (yellow), a subset of channels may begin to close and be ready to reopen. However, not all are restored so it cannot create a full action potential (gold voltage). After waiting until all the channels complete their inactive period and close, they can all reopen again. A new excitation can then create another full action potential (green). Self Test: In the yellow depolarization that occurs during the relative refractory period, why is the Phase 0 smaller than a normal (blue, green) action potential? Are the ion channels open, closed, or inactivated during Phase 3? Are some different than others? What happens if you change any ion current? Importance of the cardiac action potential refractory period and differences from skeletal muscle The refractory period will be important in understanding the pathophysiology of cardiac electrophysiology. Some examples are how inactivation is modified and how conduction may re-enter, causing an arrhythmia. 3 Boron Baulpaep, Medical Physiology Fig 21-15 FIGURE: Left: The sodium current will be ready to reopen only at low membrane potentials. However, some cells undergoing pathologic changes may not fully recover, impacting pacing. Right: cell-cell conduction is an important factor in arrhythmias. Concepts like reentry become important. As a reminder, some major differences between cardiac and skeletal muscle action potentials include: Skeletal muscle can be tetanized Multiple depolarizations causing summation of forces Cardiac muscle cannot Differences in action potential (ion channels) Integration: What does this mean about skeletal ion channels?) FIGURE: Differences in skeletal vs cardiac action potentials. 4 Conduction The sequence of activation through the heart Atria and ventricles primarily composed of myocytes If connected by gap junctions, would trigger contraction Two upper chambers (atria) normally separated from the two lower chambers (ventricle) by annulus fibrosus Extracellular material (collagen) that prevents conduction TABLE: Activation sequence, conduction velocities, color key, and pacemaker rates for cardiac cell types. Modified from: Boron Baulpaep, Medical Physiology Fig 21-1 FIGURE: Conduction begins at the SA Node (red) and spreads outward through the atrial (yellow). The conduction later produces a depolarization in the AV Node (green), which slowly passes through the Bundle of His (labled AV Bundle, teal) before traveling down the two bundle branches (blue) that conduct to Purkinje cells (purple). Finally, the myocytes (dark purple) will use cell-cell conduction (gap junctions) to spread the action potential from the endocardium to the epicardium. 5 Modifying conduction: Decremental Conduction The AV Node has unusually slow conduction, which is associated with reduced connexin-45 and reduced cross-sectional area in the cells of the AV node. There is also changes in ion channels that will change the action potential. Review: recall from HBF1 that cross-sectional area is related to action potential conduction velocity. Do you remember why a smaller area can slow the velocity? Note that Decremental Conduction can be pathophysiologic Modifying conduction: Overdrive Suppression When a cell is paced frequently, the enhanced activity of the Na+-K+-ATPase causes hyperpolarization. This drives more sodium ions out, reducing the resting potential, making it more difficult to depolarize. https://www.cvphysiology.com/Arrhythmias/A018 https://thoracickey.com/electrophysiological-mechanisms-of-cardiac-arrhythmias-2/ FIGURE: Top: Schematic of ion exchange in a secondary pacemaker cell when high frequency depolarizations occur in a primary pacemaker cell, such as the SA Node. Bottom: an example of overdrive stimulation (pacing a cell faster than normal) and its effect on the resting membrane potential, which leads to a delay before spontaneous pacing occurs. 6