Ion Channel Structure and the Ionic Basis of the Resting Potential

1. Ion Channel Structure and the Ionic Basis of the Resting Potential 10 Feb 2012

2. Gibbs-Donnan Equilibrium Rule • http://entochem.tamu.edu/Gibbs-Donnan/index.html • Gibbs-Donnan rule: The ratio of ion movement across the membrane will be equal at equilibrium: = [Cl]i - X [K]o - X [K]i + X [Cl]o + X

3. Voltage-gated Na+ channels • Consist of a single, large protein(approx 1830 AA) • Protein has four Domains (I-IV) reminiscent of subunits • Each domain consists of 7 hydrophobic regions • Each S4 region contains several charged AAs

4. Voltage-gated Ca2+ Channels

5. Voltage-gated Ca2+ Channels • High similarity to voltage-gated Na+ channels • Exhibit many, tissue-specific subtypes • Characterized by channel properties • Voltage sensitvity • Conductance • Can be coexpressed with other subunits (b, g, d)

6. Transmembrane regions M3 * * * * * * * *site of charged amino acid (Glu-, Asp-, Gly-)

7. Voltage-gated K+Channels • Very similar to Na+ and Ca2+ Channels except: • Much Shorter • Subunits

8. What we know about the K+ channel • K+ current can be blocked by charybdotoxin (CTX) • Binding to a single subunit is sufficient to block current • Mutant subunits from Drosophila are unaffected by CTX What we don’t know about the K+channel • How many subunits form a functional channel?

9. Two superfamilies of ion channels • Ligand-gated receptor superfamily • AChr • GABA • Glycine • 5-HT3 • Voltage-activated superfamily • Na+ • Ca2+ • K+

10. Other “passive” ion channels • Mechanically-gated ion channels, e.g.: • Membrane stretch • “tip-links”

11. Active transport of Ions • Sodium-potassium cotransporter (ATPase) • Moves 2K+ ions into and 3Na+ ions out of the cell

12. Na/K ATPase • The transport rate for a single Na+/K+ pump is about 195 Na+ ions and 130 K+ ions per second • There are about 1000 Na+/K+ pumps per micron2 of membrane surface • There are about one million Na+/K+ pumps in a small neuron • 195,000,000 Na+ ions/second • Na+/K+ pumps account for about 1/3 of the body’s ATP usage!

13. Other active transporters • Na+/Ca2+ exchanger

14. Other transporters

15. Other ion channels • May be over 300 different kinds of ion channel in a single cell • Transient receptor potential (Trp) channels • Light-activated ion channels • Proton channels • Cyclic-nucleotide-gated channels • IP3-gated channels

16. Ionic Basis for Membrane Potential • At rest, the membrane is permeable to K+ • The membrane is also permeable to Na+ • (And Cl-) • The resting membrane potential can be determined from the steady-state current for each ion. • Current for Na+(INa) = gNa(Vm – ENa) • IK = gK(Vm – EK) • ICl = gCl(Vm – ECl) “Driving Force”

17. Ionic Basis for Membrane Potential

18. How do we measure electrical potential across the membrane? Inside of cell is more negative than outside

19. Ion concentrations in a cell What is the RMP? What is EK? What is ENa? What is ECl?

20. Potassium and the RMP Changing extracellular [K+] changes the resting membrane potential Extracellular [K+] Intracellular [K+] = 400mM

21. Extracellular K+ and the RMP ?

22. Contribution of Na+ to the RMP • At rest, neurons are permeable to Na+ • Permeability to Na+ is approximately 5% of the permeability to K+ • When do K and Na ions stop moving?

23. Donnan Equilibrium • http://entochem.tamu.edu/Gibbs-Donnan/index.html • Donnan rule: The ratio of ion movement across the membrane will be equal at equilibrium: = [Cl]i - X [K]o - X [K]i + X [Cl]o + X

24. Ionic basis for neuronal RMP • Assuming Cl is at equilibrium, ICl= 0 • For the neuron to be at rest, IK = INa: gK(Vm – EK) = gNa(Vm – ENa) • Are gKand gNasimilar? • No! So what must be true? • The Driving Force on K must be smaller than on Na • The membrane potential must be closer to the equilibrium for K than for Na

25. Resting Membrane Potential • Described by the Goldman-Hodgkin-Katz (GHK) equation (or Constant Field Equation): • Factors determining the contribution of an ion to the RMP: • Permeability (P) and Concentration (C) Vm

26. Resting Membrane Potential • Constant field equation assumes no net movement of ions • This ignores Na+ current and the Na+/K+ pump • The pump moves 3 Na ions out and 2 K in: Or, INa gK (Vm – EK) = = -1.5 gNa (Vm – ENa) IK 1.5 gK (EK) + gNa (ENa) Vm = 1.5 gK + gNa

27. Resting Membrane Potential • Modified GHK equation: Where r is the absolute value of the ratio of IK to INa Vm r PK [K]o + PNa [Na]o Vm = 58 log r PK [K]i + PNa [Na]i

28. How much does the Na/K pump contribute to the RMP? • Permeability ratio Na:K = 0.04:1 • With Na/K pump (r = 1.5) (3:2 pumping ratio) • Without Na/K pump (r = 1) • The Na/K pump contributes about -10 mV to the RMP in neurons r PK [K]o + PNa [Na]o Vm = 58 log r PK [K]i + PNa [Na]i

29. Summary: Ionic basis of the RMP • The RMP is due to ionic imbalance, sustained by differential permeability of the neuronal membrane • K+ permeability is far greater than Na+ permeability • Na/K pump contributes a small amount to the RMP • At rest, there is no net ion movement across the membrane

30. Ion Concentrations at Rest(no ionic movement illustrated)

31. II • http://www.youtube.com/watch?v=VSdxqIBfEAw