Lecture 4: Ionic basis of the cardiac action potential

Question 1

What is the Nernst equation, describe reversal potentials for K+, Na+, Ca+, If, and NCX

Answer 1

Nernst
Eion=61.5 log [out]/[in]
K=-90, Na=65, Ca=120 at rest, 90 while contracting, If=-20, NCX=0

Question 2

Describe the shape of the cardiac myocyte action potential

Answer 2

4= resting membrane potential due to Ik1 (-70mV)
0= rapid depolarization due to Ina (threshold around -55)
1= rapid repolarization due to Ikto1 and Ikto2
2= plateau due to Ica and NCX - where contraction is regularted, variable duration
3= slow repolarization due to IkDR (Ikudr, Ikrdr, Iksdr)

Question 3

Describe Ik1

Answer 3

inward rectifier, only active between -80 and -20, at -20 blocked by polyamines, below that, it is not in physiological range

Question 4

List the alternative names for the Ik currents and calcium current

Answer 4

Ik1 = inward rectifier
Iks = KvLQT1
Ikur = Kv1.5
Ikr= hERG
ICa= Cav1.2

Question 5

Describe the shape of the ion channels

Answer 5

Na = 1 alpha subunit composed of 24 transmembrane domains, 2 beta subunits
K = 4 alpha subunits composed of 6 transmembrane domains, one beta subunit
Ca = 1 alpha, 1 beta, 1 gamma, 1 delta-alpha2