Cellular Electrophysiology. Cardiac Electrophysiology

Transcription

1 Part 1: Resting and Action Potentials Cardiac Electrophysiology Theory Simulation Experiment Scale The membrane: structure, channels and gates The cell: resting potential, whole cell currents, cardiac cell types The tissue: myocardial structure, propagation The heart: conduction system, extracellular electrograms ECG and the volume conductor: the heart in the thorax

2 Membrane Composition Proteins and the Membrane

3 Membrane Functions Control of solutes movement Compartmentalization Electrical Activity Theory Simulation Experiment Background Physics = Theory

4 Current and Ohm s Law Without potential difference there is no current! Without conductance, there is no current. Ohm s law: linear relationship between current and voltage not universal, especially not in living systems I = 1 R V = GV v(0) j x 0 x L v(x) Electricity Basics: Resistance = Req = R1+ R2 + R3 R1 R2 R3 = 1/Req =1/R1+ 1/R2 + 1/R3 Geq = G1 + G2 + G3 IV Curve I Slope = 1/R = G A V V I = V/R = VG

5 Electricity Basics: Rectification IV Curve I A V Slope = 1/R = G V I = V/R = VG Equilibrium Net Forces Equal Zero No change over time

6 Ion Channel Permeability An electron has a unitary charge of x1019 C Cation+ has a charge of qo= zeo where z is the valence The attractive force between ions is given by Coulombs law: ε is the Dielectric constant, a parameter related to the properties of the material capable of separating charge Ion Transport Active transport (pump) Passive cotransport (symporter) Channels

7 Ion Transport Net K + Forces Diffusive Force J = Dr2 c Chemical Potential µ = µ 0 + RT ln(c) Electrical Force Electrical Potential F e = k e q 1 q 2 r 2 = zf

8 Resting Potential Equilibrium Potential a) Membrane is impermeable b) Membrane becomes permeable to potassium only (semipermeable) c) Equilibrium established when electrostatic and chemical gradients balance.

9 Example Nernst Potentials Ion External Internal Nernst Potential (mv) Resting Potential V m + + Electrostatic (V m = 80mV) Chemical (V eq = 94mV) 61.5 log([k + ] i / [K + ] o ) [K + ] o [K + ] i Net Gradient V m + + Electrostatic (V m = 80mV) Chemical (V eq = 70mV) 61.5 log([na + ] o / [Na + ] i ) [Na + ] o [Na + ] i Net Gradient What determines resting potential? K + Nernst Potential

10 Resting Potential All ions contribute GoldmanHodgkinKatz Equation E m = RT F ln N i P M + i N i P M + i [M + i ] out + N i P A + [A + i ] in i [M + i ] in + N i P A + [A + i ] out i Cardiac Action Potentials

11 [K + ]o [K + ]i Vm Veq Driving Force Vm = 80 mv (electrical) Veq = 94 mv (chemical) VD = Vm Veq = 14 mv (net) Sign convention is inside relative to outside. Driving Force = Vm Veq is the potential available to drive ions across the membrane. Membrane resistance = Rm is the resistance of the membrane through a specific channel for a specific ion. I = V m R m V eq Ohm s Law: links these parameters and describes the membrane current. Action PotentialsPositive Feedback Depolarization Increase in g Na Na flux What starts the positive feedback? What stops the positive feedback?

12 Cardiac Action Potential 0 Ca threshold (35mV) mv Na threshold (65mV) 80 Na + current K + current Ca ++ current ms = depolarizing = repolarizing Cardiac Cell Currents 0 T2 Chemical K + Chemical Ca ++ Chemical Na + Vm mv Electrostatic 80 T1 T3 Time [Na + ]o [Ca ++ ]o [Na + ]i [Ca ++ ]i [K + ]o [Ca ++ ]o + + [K + ]i + [Ca ++ ]i + + [K + ]o [K + ]i T1 T2 T3 Note: Only includes relevant currents, i.e., for which G > 0

13 At AP peak: [Na + ]o Driving Force: Sodium [Na + ]i Chemical Veq = 70 mv Electrical Vm= 10 mv Net Vd= 60 mv Is there ever a time when the Net Gradient (driving force) = 0? 0 What stops the Nacurrent? mv 80 Sodium Channel Behavior Depolarization (voltage) Inactivation (time) Repolarization (voltage, time) Recovery (voltage, time) Note: voltage and time dependence

14 Summary: Membrane Channel Voltage dependence Time, voltage, and ligand dependent Cardiac Ion Currents Na + Ca 2+ 2K + 3Na + 3Na + Ca 2+ K + NaK pump NaCa exchanger Ca pump Ion channels Passive ion movement Driven by concentration and electrostatic gradients Channels are selective Gates control opening Carrier mediated ion transport NaK and Ca pumps require ATP Capable of driving against concentration gradient NaCa exchange does not require ATP

15 Summary: Cardiac Action Potential Resting potential depends almost entirely on [K+]. Na channels require time at potentials more negative than 65 mv in order to recovery. Without it, they will remain inactive. Slow (Ca++) channels have a threshold of 35 mv The plateau represents balance between Ca++ and K+ currents. Some cardiac cells depolarize spontaneously; most do not. Nature Cell Biology 6, (2004) Thomas J. Jentsch, Christian A. Hübner & Jens C. Fuhrmann Theory Simulation Experiment Measurement = Experiment

16 Measuring Membrane Potential 1x Vm Unity Gain High Impedance Amplifier 80mV Ground Optical Methods

17 Whole Cell Currents (Voltage Clamp) For each ion type: Vc Vm Outward A activation 0 Iv Iv Inward inactivation 10 Vc [mv] 40 Voltage Clamp in HH [Na]e Simulation of Cell EP

18 Voltage Clamp Results Note use of Na Channel blocker to isolate Na current IV curve is nonlinear Vc Single Channel (Unitary) Currents 0 pa Channel current (pa) closed open 2 channels open Time Membrane voltage (mv) 10 mv 80 mv

19 Membrane Patch Clamp Single Channel Examples Two different channels Current/ Voltage characteristic Current as function of voltage

20 Theory Simulation Experiment Simulations Membrane Equivalent Circuit Lipid Bilayer Channel Charged Polar Head Rm + Em Cm

21 HodgkinHuxley Formalism Qualitative concepts Quantitative formulation and simulation (see next lecture) Sir Alan Hodgkin Sir Andrew Huxley brother of Aldous Huxley Nobel Prize: 1963 Single Channel Model Active γ α At Rest β Recovering α, β, γ: state transition probabilities, (functions of v and t)

22 HH Derivation and Homework Assignment Control of Heart Rate Pacemaking

23 Pacemaker Cells in the Heart Note difference in basic AP shape: why? Note unstable (depolarizing baseline): why? Pacemaker and ECC Regulation of Heart Rate SA Node Epi, NE ACh Epi/NE; increases I f (β 1 adrenergic receptor) Pacemaker and ECC ACh: increases p K reduces I f (muscarinic receptor)