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Making Complex Arrhythmias from Simple Mechanisms: Exploring Anti- and Proarrhythmic Effects of Na Channel Blockade with the Guarded Receptor Paradigm.

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Presentation on theme: "Making Complex Arrhythmias from Simple Mechanisms: Exploring Anti- and Proarrhythmic Effects of Na Channel Blockade with the Guarded Receptor Paradigm."— Presentation transcript:

1 Making Complex Arrhythmias from Simple Mechanisms: Exploring Anti- and Proarrhythmic Effects of Na Channel Blockade with the Guarded Receptor Paradigm C. Frank Starmer Medical University of South Carolina

2 shock tachycardiafibrillation Dynamics of transmembrane potential (monophasic cathodal truncated exponential shock, -100 V, 8 ms)

3 How To Initiate Reentry or Fibrillation: The cardiac vulnerable period Refractory: s1s2 = 2.1 Vulnerable: s1s2 = 2.2 Excitable: s1s2 = 2.3 refractory conduction Partial Conduction (arrhythmia)

4 Ion Channel Blockade Reduces Excitability (Anti- effect) and Slows Conduction (Pro- effect) Historical observations that provided a foundation for a model of ion channel blockade: Johnson and McKinnon (1957) (memory) West and Amory (1960) (use-dependence) Armstrong (1967) (open channel block) Heistracher (1971) (frequency-dependence) Carmeliet (1988) (trapping)

5 Steady-state Frequency-dependent AP Alterations: Quinidine Johnson and McKinnon JPET , 1957 dV/dt(max) decreases with increased stim rate AP amplitude decreases with increased stim rate

6 Freq-dependent Quinidine Block: Alteration of AP Duration West and Amory: JPET 130: ,1960 Increased stim rate slows repolarization

7 An Early Model of Use-dependent Blockade West and Amory: JPET 130: ,1960

8 Frequency- as well as Use-dependence: Detailed Characterization of Ajmaline Blockade Heistracher. Naunyn-Schmeidebergs Archiv Fur Pharmakologie 269: , 1971 dV/dt(max) reduced with repeated stimulation: note approx exponential decrease with stimulation number Steady-state dV/dt(max) Reduced with faster stimulation

9 Voltage and Time-dependent TEA Block of K + Channels Armstrong. J. Gen Physiol 54: , mV -46 mV CP Control: no inactivation + TEA: Apparent inactivation IKIK

10 Once a Drug Molecule Blocks the Channel, Can it Escape? i.e. is it possible to trap it in the channel

11 Is use-dependent channel blockade a special process or is it simply a variant of ordinary ligand-receptor interactions? If its a variant - what variant? From These Observations, One Wonders:

12 Ordinary (not use-dependent) Chemistry: Reacting with a Continuously Accessible Site No possibility of use- or frequency dependence Ligand + Receptor LR-Complex b = b(t) = b + (b 0 - b ) e - t (b - b 0 )/2 = K d =

13 How to Build a Model that Displays use- and frequency dependence? Unblocked + Drug Blocked (V) A necessary condition: Either a Real or Apparent Voltage-dependent Equilibrium Dissociation Constant: K d = (V) / (V)

14 Modeling Apparent Voltage Dependence Of the Equilibrium Dissociation Constant Voltage-dependent Access to the Binding Site InaccessibleBlocked kD l

15 Hypothesis: Control of Binding Site Access by Channel Conformation accessibleinaccessible

16 Blockade During Accessible and Inaccessible Intervals: Channel + D Blocked Channel + D Blocked Accessible ConformationInaccessible Conformation

17 Characterization of Access Control: Guarded Receptor Model (when channel transition time << drug binding time) Unblocked Channel + Drug Blocked Channel where G and T act as switches that control binding site accessibility G*k l G = guard function controls drug ingress: e.g. h, m, m 3 h, d, n, n 4 T = trap function controls drug egress: e.g. m 3 h, h In reality, the guard and trap functions are hypothesized to reflect specific channel protein conformations, and not arbitrary model parameters Starmer, Grant, Strauss. Biophys J 46:15-27, 1984 Starmer and Grant. Mol Pharm 28: ,1985 Starmer. Biometry 44: , 1989

18 Combining Gated Access with Repetitive Stimulation makes Use-dependent Blockade: Switched Accessibility to a Binding Site b recov = r ss - (b 0 - r ss ) e - n b activated = a ss - (a 0 - a ss ) e - n b(t) = b - (b 0 - b ) e - k + l t = a t a + r t r t r tata U B a r Starmer and Grant. Mol. Pharm 28: , 1985

19 Dissecting the Mechanism of Use-Dependent Blockade: Using Voltage Clamp Protocols to Amplify or Attenuate Blockade

20 Continuous Access Associated with Channel Inactivation (shift in apparent h) V(cond) Unblocked + Drug Blocked (1-h) Starmer et. al. Amer. J Physiol 259:H626-H634, 1990 block

21 Transient Access Associated with Channel Opening Pulse duration: 2 ms 2 ms ms 550 Gilliam et al Circ Res 65: , 1989

22 Shift in Apparent Activation: Evidence of Open (?) Channel Access Control 10 ms Starmer et. Al. J. Mol Cell Cardiol 23:73-83, 1991

23 Exploring a Model of Use-Dependent Blockade Are the Analytical Predictions Testable? Analytical Description: block associated with the n th pulse: b n = b ss + (b 0 - b ss ) e -( a t a + r t r )n Use-dependent rate = a t a + r t r Steady-state block: b ss = a + (r + a ) Steady-state slope (1 - e - r tr ) / (1 - e - )

24 Testing the Model Pulse-train stimulation evokes an exponential pattern of use- dependent block There is a linear relation between exponential rate and stimulus recovery interval There is a linear relation between steady-state block and a function of the recovery interval ( ) There is a shift in the midpoint of channel availability and / or activation (depending on the access control mechanism)

25 Test 1. Frequency-dependent Lidocaine Uptake: Exponential Pulse-to-pulse Blockade (50 ms) Gilliam et al Circ Res 65: ,

26 Test 2: Linear Uptake Rate, Linear Steady State Block t a constant and t r variable = a t a + r t r b ss = a + (r - a ) Linear Uptake Rate Linear Steady-State Block

27 Test 3: Shifting Apparent Inactivation (channel availability) Unblocked + Drug Blocked (1-h) V = s ln(1 + D/K D ) = mV K = 3940 /M/s l =.678 /s K D = 18.8 M Obs V = 9 mV

28 Test 4: Shifting Apparent Channel Activation Nimodipine Blockade of Ca ++ Channels Unblocked + Drug Blocked d V = 40.1 mV V = k (1 + D/K D ) = 43.4 mV K D =.38 nM

29 Exploiting the Therapeutic Potential of Use-dependent Blockade Cellular Antiarrhythmic Response Multicellular Proarrhythmic Response

30 Therapeutic Potential: Cellular Effects of Blockade (Antiarrhythmic) Prolonging Recovery of Excitability: Control and with Use-dependent Blockade

31 Therapeutic Potential: Multicellular Effects of Blockade (Proarrhythmic) Slowed Conduction, Increased Vulnerable Period Why? Propagation: Responses to Excitation 1) no response 2) front propagates away from stimulation site 3) front propagates in some directions and fails to propagate in other direction (proarrhythmic)

32 Premature Excitation: The Vulnerable Period Normal excitation: cells are in the rest state Premature excitation: Following a propagating wave is a refractory region that recovers to the resting state. Stimulation in the transition region can be proarrhythmic

33

34 The Dynamics of Vulnerability Using a simple 2 current model (Na: inward; K: outward) we can demonstrate role of introducing a stimulus within and outside the interval of vulnerability: We demonstrate the paradox of channel blockade: block extends the refractory period, slows conduction and increases the VP Here, we switch to Matlab, to demonstrate the dynamic events defining the Vulnerable PeriodMatlab

35 Demonstrating the Vulnerable Period: Control Refractory Period = 352 ms VP = 3 ms

36 Demonstrating Extension of the VP: Drug Refractory Period = 668 ms VP = 59 ms

37 Use-dependent Extension of the VP

38 2-D Responses to Premature Excitation: Note geometric distance between 1 st and 2 nd fronts (refractory, unidirectional conduction, bidirectional conduction) Refractory: s1s2 = 2.1 Vulnerable: s1s2 = 2.2 Excitable: s1s2 = 2.3 refractory conduction unidirectional conduction

39 Extending the VP with Na Channel Block: Fact or Fantasy? Starmer et. al. Amer. J. Physiol 262:H , 1992

40 More Apparent Complexity: Monomorphic and Polymorphic Reentry and ECG MonomorphicPolymorphic gNa = 2.25gNa = 4.5 Polymorphic gna = 2.3

41 Major Lessons Learned From Ideas Originating in Studies of Johnson, Heistracher and Carmaliet

42 Use caution when repairing channels that arent broken: Blockade of normal Na Channels Antiarrhythmic –Extended refractory interval and reduced excitability leading to PVC suppression Proarrhythmic –Extends the vulnerable period (increases the probability of a PVC initiating reentry) –Slowed conduction increase the probability of sustained reentry –Increases probability of wavefront fractionation

43 Repairing Channels that are Broken (e.g. SCN5A) may have Clinical Utility: Blockade of defective channels diminishes EADs in LQT Syndrome, Heart Failure, Epilepsy

44 Long QT Syndrome: Links to Mutant Na and K Channels Q T

45 Stable and Unstable Action Potentials Beeler-Reuter ModelHuman Ventricular Cells

46 Yet Another Variant: Epilepsy

47 Summary Use- and Frequency Na channel block are consistent with ordinary binding to a periodically accessible site Tonic block is compatible with block of inactivated channels at the rest potential. Tests are available to validate the applicability of the guarded-receptor paradigm to observations of drug- channel interactions

48 For individual cells: use-dependent Na channel block reduces excitability (prolongs the refractory period (antiarrhythmic effect) For connected cells (tissue): reduced excitability ALSO slows propagation which extends the vulnerable period (proarrhythmic effect) The guarded receptor paradigm is a tool for in numero exploration of channel blockade in both cellular and multicellular preparations and direct characterization of anti- and proarrhythmic effects

49

50 Apparent Trapping of Quinidine and Disopyramide Zilberter et. Al. Amer. J. Physiol 266:H2007-H2017, uM Diso 5 uM Quinidine

51 Demonstrating the Trap Zilberter et. Al. Amer. J. Physiol 266:H2007-H2017, 1994

52 Examples of Recent State- Transition Models Balser et al J. Clin Invest. 98: , 1996 Vedantham and Cannon J. Gen Physiol 113:7-16, 1999

53 Transforming a State-transition Model to a Macroscopic Model: The Importance of Rapid Equilibration Unblocked Channel + Drug Blocked Channel G

54 Reducing a Complex State-Transition Model to a Simple Macro GRH Model R I B kD l Differential Equation Description: Guard Function: 1-h Guarded Receptor Formulation:

55 Spontaneous Oscillation: Mutant KVLQT1 and HERG (K+) and SCN5A (Na+) Channels: Altering Electrical Stability with Channel Blockade

56 Use- and Frequency-Dependent Blockade: Central Features Degree of Blockade Depends on V clamp Degree of Blockade Depends on T clamp Degree of Blockade Depends on V hold V clamp V hold T clamp

57 1. Frequency-dependent Lidocaine Uptake: Exponential Pulse-to-pulse Blockade (2 ms) Test 1: Exponential UDP Block, t a = constant Gilliam et al Circ Res 65: , 1989

58 Recovery of Excitability: Drug

59 Evolution of a Spiral Wave T = 0 T = 1 T = 5T = 15

60 Monomorphic and Polymorphic EKGs Role of Wavefront Energy

61 Building a Model of Discontinuous (Use- dependent) Drug-Channel Interaction: Unblocked + Drug Blocked (V) Apparent Voltage-dependent Equilibrium Dissociation Constant: K d = (V) / (V)

62 Why Does the Guraded Receptor Model Work? Comparing State-Transition and Macro Models Macro Model: Unblocked + Drug Blocked G

63 Reduction in AP Duration: C L C Q Colatsky Circ Res 50:17-27, 1982

64 Altering the Equilibrium Stability of a Cell: Blockade of Na Current Can be reversed by Na blockade

65 EADs and Suppression via Na Channel Blockade Maltsev et al Circ 98: , 1998

66 Frequency-dependent Lidocaine Uptake: Access Controlled by Inactivation Pulse duration: 50 ms 50 ms ms Gilliam et al Circ Res 65: , 1989

67 Voltage-dependent Recovery from Blockade Starmer, et. al. J. Mol. Cell. Card 23;73-83, 1992

68 Two Modes of Na Channel Blockade: Test 3: Linearity with variations in both t a and t r t a = 50 ms t a = 10 ms t r = constant = a t a + r t r t clamp

69 A Conformation-dependent Blockade Model Closed Open Blocked Armstrong. J. Gen Physiol 54: , 1969

70 Binding to Accessible Sites at Sub-threshold V m A single mechanism for tonic and use-dependent block -80 mV, = 694 ms -20 mV, t = 373 ms Gilliam et al Circ Res 65: , 1989 : Channel Inactivation V (mV) (ms) Block independent of rate of inactivation but dependent on potential dependence of h Evidence that lidocaine does not compete with fast-inactivation and that slow recovery does not result from accumulated fast inactivated channels. Vedantham and Cannon J. Gen. Physiol 113:7-16, x 2x (no evidence of 2 exp) block % block

71 Test 4: Exponential Binding to a Continuously Accessible Site independent of inactivation Gilliam et al Circ Res 65: , mV -80 mV -120 mV tctc I = I + (I 0 - I ) e t


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