Session 4-Electrical And Molecular Mechanisms In CVS Flashcards
True or false: Na+ permeability sets the RMP
FALSE - K+ permeability
What is the intracellular K+ concentration?
~140mM
What is the extracellular K+ concentration?
~4mM
Do K+ ions move into or out of the cell?
OUT
True or false: at equilibrium potential, concentration gradient and electrical gradients are equal
TRUE
Why is RMP not exactly equal to Ek at rest?
Very small permeability to other ion species eg. Na+
Which cells in the heart are electrically active and fire AP?
Cardiac myocytes
True or false: AP triggers a decrease in cytosolic [Ca2+]
FALSE - increase
Why is a rise in calcium required?
To allow actin and myosin interaction (contraction)
What is phase 0 of the ventricular AP?
Opening of v-gated Na+ channels, causing an upstroke. These channels open on depolarisation and then inactivate
What happens in phase 1 of the ventricular AP?
Transient outward K+ current - initial fast repolarisation
What happens in phase 2 of the ventricular AP?
Plateau phase-opening of v-gated Ca2+ channels (some K+ channels also open)
What happens in phase 3 of ventricular AP?
Repolarisation-Ca2+ channels inactivate and v-gated K+ channels open
How much longer is diastole than systole?
2/3
True or false: cardiac myocytes have lots of different types of K+ channels
TRUE
Complete the sentence:
The SA node is a specialised __________ with not much contractile machinery but they are ________________ active.
Myocyte
Spontaneously
Which channels make up the funny current?
HCN channels
What are HCN channels activated by?
Hyperpolarisation
Which ions are HCN channels permeable to?
Na+
K+
What contributes to the initial slope to threshold in the pacemaker potential?
Funny current
When is the pacemaker potential activated?
At membrane potentials more negative than -50mV
What does ‘HCN’ channels stand for?
Hyperpolarisation-activated Cyclic Nucleotide-gated channels
What causes the upstroke of the SA node AP?
Opening of voltage-gated Ca2+ channels
What causes the downstroke (repolarisation) of SA node AP?
Opening of voltage-gated K+ channels
True or false: SA node is fastest to depolarise so it sets the rhythm
TRUE
What can occur if AP are fired to slowly in the heart?
Bradycardia
What can occur if AP fail in the heart?
Asystole
What can occur if AP fire too quickly in the heart?
Tachycardia
What happens if electrical activity becomes random in the heart?
Fibrillation
What is the normal plasma K+ concentration range?
3.5-5.5mmol/L-1
What plasma K+ concentration constitutes as hyperkalaemia?
> 5.5 mmol/L-1
What plasma concentration constitutes as hypokalaemia?
<3.5 mmol/L-1
Why are cardiac myocytes so sensitive to changes in [K+]?
K+ permeability dominates resting membrane potential
Complete the sentences:
Hyperkalaemia ______________ the myocytes and __________ _______ the upstroke of the action potential. If plasma K+ is raised then Ek gets ______ negative so membrane potential depolarises a bit and this inactivates some of the voltage-gated ___ channels.
Depolarises
Slows down
Less
Na+
What is the risk of hyperkalaemia?
Heart can stop-asystole
True or false: in hyperkalaemia you may initially get an increase in excitability
TRUE
What are the possible treatments for hyperkalaemia? (2)
1) Calcium gluconate-makes heart less excitable
2) Insulin and glucose-insulin promotes K+ moving into cell
What is the effect of hypokalaemia?
Lengthens action potential and delays repolarisation because when K+ lowered too much, there is an allosteric effect and the conductance of K+ channels is reduced
What is the problem with hypokalaemia?
Longer action potential can lead to early after depolarisations (EADs), which can lead to oscillations in membrane potential. This can result in ventricular fibrillation (VF)
What can ventricular fibrillation lead to?
No cardiac output
What does depolarisation open in the T-tubule system?
L-type Ca2+ channels
What does localised Ca2+ entry open in the SR?
Calcium-induced calcium release channels (CICR)
What % of Ca2+ enters across sarcolemma?
25%
What % of Ca2+ is released from SR?
75%
What is the tone of blood vessels controlled by?
Contraction and relaxation of vascular smooth muscle cells in tunica media of arteries, arterioles and veins
In smooth muscle cells, the activation of what allows the interaction with actin?
Myosin light chain
What must happen to the myosin light chain to enable actin-myosin interaction?
Must be phosphorylated
Complete the sentences:
Ca2+ binds to _________ which activates ________ _________ _______ __________ (MLCK). MLCK __________________ myosin light chain to permit interaction with actin.
Calmodulin
Myosin light chain kinase
Phosphorylates
What dephosphorylates the myosin light chain?
Myosin light chain phosphatase
What can phosphorylate myosin light chain kinase (MLCK) to inhibit its action?
PKA
What initiates the contraction of vascular smooth muscle cells?
Depolarisation or activation of alpha-adrenoceptors
