Electrical Activity in ventricular cells Flashcards
Describe the currents involved in a ventricular action potential
Currents responsible for ventricular action potential
- Na+ current (INa)
- Ca2+ current (ICaL)
- K+ currents (IKr, IKs, Ito, IK1)
- Exchanger currents (INaCa, INaK)
Describe action potentials in cardiac cells - are they one shape or not?
- Different shapes and lengths depending on location
- Due to difference in expression of ion channels
What channel is responsible for Phase 0 and what is that?
Phase 0 – Ventricular Upstroke
- INa
- Nav1.2 main channel
What is the structure of Nav1.2 channel?
The alpha subunit
What does each domain have?
- Alpha subunit
o Glycoprotein of 4 transmembrane domains, I, II, III, IV.
o 3 inter-domain linkers I-II, II-III, III-IV
o One channel consisting of the 4 domains - Each domain has
o Six helices, all hydrophobic, S1, S2, S3, S4, S5, S6
o Voltage sensor (S4), rich in +ve charged amino acids, lysine & arginine at every third position
o Pore loop (P) between S5 and S6
What components are involved in gating? 4
- Gating Components
o S4 voltage sensors: 4 x + charged regions from the domains
o M gate (activation): 4 x P loops from the domains
o H gate (inactivation): one N-terminal constituting ‘ball & chain’
o III-IV linker (inactivation): one III-lV linker region – ‘the lid’
Describe the gating mechanism of the Na channel
Resting, Activated, Inactivated, Deactivated
o Resting state (-80mV) M gate closed H gate open III-IV linker open o Activated state (early depolarisation to +20mV) S4 moves M gate quickly opens H gate and III-IV linker remains open o Inactivated state (Maintained depolarisation) M gate remains open H gate closes v.slowly II-IV linker closes slowly. o Deactivated state (Repolarisation) S4 returns m gate rapidly closes h gate recovers & opens III-IV linker recovers & opens.
Give the evidence for the properties of the Na channel - IV curve - Current and reversal potential - Sensitivity to TTX 1
- Brown et al., 1981
- Isolated cells from rat ventricular myocardium by a double-suction-pipette voltage clamp technique
- IV curve does not follow electrochemical driving force as currents were both time- and voltage-dependent, had a threshold between -70 and -60 mV and increased in magnitude in the inward direction as the potential became more positive
- Decreased current after peak due to decreased driving force as you near the reversal potential, according to the Nernst equation
- Reversal potential was around +30mV which was due to conditions being 50% normal sodium conc at room temp, hence ENa=30 mV, remember Nernst equation ENa=~60 mV
- Not as highly sensitive to TTX as INa in nerve or skeletal muscle
- Both inactivation processes are blocked by TTX
What evidence was there for determining NA current in Phase 0?
What species and how did this caution be overcome?
2
- Carmeliet & Vereecke, 1969
- To determine whether Na was responsible for the phase 0 upstroke
- At moderate concentrations of TTX the maximum rate of depolarization is decreased
- At higher concentrations all-or-nothing depolarization and conduction of electrical activity is blocked
- Confirmation that the Na+ current is responsible for Phase 0 of the ventricular action potential
- Cow Purkinje fibers were used, caution should be taken as there is marked heterogeneity in the heart and across species. This result was backed up by further evidence in human hearts however so more solid conclusions can be made
What current is responsible for Phase 1 and what is that?
Phase 1 – Initial fast Ventricular Repolarisation
- Ito is responsible
- Transient Outward Current
- Kv4.3 potassium channel gene
Who showed the heterogeneity of Ito channel in the heart?
1
- Antzelevitch et al (2011)
o Showed the heterogeneity of ventricular cells in a canine left ventricular multicellular (wedge) preparation
o Epicardial APs have a larger phase 1 repolarisation and are shorter than Endocardial APs
What evidence did this further support? - Ito in Epi and Endo - Blocked by what - Implications 2
- Liu, Gintant & Antzelevitch (1993)
o Whole-cell patch-clamp techniques from free wall of the canine left ventricle
o Recorded voltage-dependent activation of the transient outward current, Ito
o A prominent ‘spike and dome’ was apparent and Ito was significantly greater in myocytes from epicardium and the M region but not in myocytes from endocardium
o Showed Ito largely abolished by exposure of the myocytes to 5 mM 4-AP
o Implications are that the presence of a prominent Ito in epicardium but not endocardium also contributes to differences in the time and rate dependence of APD and refractoriness
What is the effective refractory period?
- During the ‘effective refractory period’ a cardiac myocyte cannot respond to a second stimulus
- The refractory period lasts most of the duration of the first AP. It is caused by the inactivation and recovery of INa
What is the purpose of the refractory period?
- Prevents sustained tetanic contraction of cardiac muscle. The muscle must undergo a relaxation period, allowing venous blood to refill the heart during the cardiac cycle, essential to normal pumping action.
What happens if EPI repolarisation occurs outside ENDO refractory period?
the depolarised EPI tissue may re-activate the ENDO causing a cyclic re-entry arrhythmia (Ventricular Tachycardia).
What causes shorter APD in Epi tissue?
- The shorter EPI APD is due to a larger Ito ensures that the ENDO is still refractory during EPI repolarisation and cannot be activated by the EPI
