EC Coupling/Calcium I & II

Question 1

Sequence of events of cardiac muscle contraction & Ca²⁺ involvement

Answer 1

• release of Ca2+ originates at junctions between the terminal cisternae of the sarcoplasmic reticulum (SR) and the plasma membrane, or plasma membrane invaginations termed transverse tubules (t-tubules).
• Near the plasma membrane side of these junctions, Ca2+ enters the myoplasm via the dihydropyridine receptor (DHPR)—an L-type Ca2+ channel—and activates and opens the ryanodine receptor (RyR2) causing a much larger flux of Ca2+ from the sarcoplasmic reticulum (SR) into the myoplasm.
• Ca2+ activates contraction by binding to troponin on thin filaments and allowing actin-myosin cross-bridge cycling.

Question 2

Mechanism of muscle relaxation & Ca²⁺involvement

Answer 2

• Relaxation achieved via removal of Ca2+ from the myoplasm by:
  
  • SERCA2 pump: located in longitudinal SR (2 Ca2+ per cycle); Ca2+ diffuses within SR to terminal cisternae, where it binds to calsequestrin.
    
    • SERCA2 dominates since SR surrounds each myofibril; requires less energy since VSR=0.
  • ​NCX Na+/Ca2+ exchanger: in the junctional domains of plasma membranes and t-tubules. Brings in 3 Na+ for every Ca2+ pumped out.
    
    • ​The NCX Na+/Ca2+ exchanged is next in importance and can be arrhythmogenic.
• ​​In steady-state, Ca2+ released from the SR is recycled back into SR by SERCA2, and surface extrusion balances L-type Ca2+ current.

Question 3

EC coupling in skeletal muscle

Answer 3

• ECC does not require entry of external Ca2+
• Voltage gated channel subunit = CaV1.1(a1s), b1a, a2d1, g1
• Sarcoplasmic reticulum channel = RyR1

Question 4

EC Coupling in Cardiac Muscle

Answer 4

• ECC requires entry of external Ca2+.
• Voltage-gated channel subunits = CaV1.2(a1C), b2a, a2d1
• Sarcoplasmic reticulum channel = RyR2

Question 5

General characteristics of NCX exchanger

Answer 5

• exchanges 3 Na+ for 1 Ca2+ and can run in either direction
• The direction of the pump depends on both membrane potential and the gradients for sodium and calcium.

Question 6

Mechanism of depolarization via NCX exchanger

Answer 6

• If a cell is at a membrane potential of -74 mV, a sudden increase in [Ca2+]I would result in a net inward current—as a consequence of Ca2+ extrusion.
• inward current would cause the cell to depolarize.
  
  • Depolarization triggered by Ca2+ release from the sarcoplasmic reticulum has the capacity to trigger arrhythmias.

Question 7

Mechanisms of Calcium homeostasis w/in myocardium

Answer 7

• NCX calcium exchanger.
• L-type Ca2+ channel: undergoes a form of inactivation that depends on the concentration of Ca2+ near the cytoplasmic side of the channel.
  
  • Calcium dependent inactivation (CDI)
    
    • If the amount of Ca2+ in the SR increases, greater CDI causes less Ca2+ to enter via the L-type channel.
    • If the amount of Ca2+ in the SR decreases, less CDI causes more Ca2+ to enter via the L-type channel.

Question 8

Consequences of B-adrenergic receptor stimulation & cellular targets (3)

Answer 8

• results in elevation of cAMP and activation of PKA
• PKA targets =
  
  • L-type Ca²⁺ channels
  • RyR2
  • Phospholamban (PLB)

Question 9

PKA impact on L-type Ca²⁺ channels

Answer 9

• Phosphorylation of the channel increases the amplitude of the L-type Ca2+ current –> increases the size of the trigger to activation of RyR2.
• increase dCa2+ entry also helps to increase the quantity of Ca2+ stored in the SR.
• This contributes to POSTIVE INTROPY.

Question 10

PKA impact on RyR2 channels

Answer 10

• phosphorylation of RyR2 causes it to be sensitized to activation by trigger Ca2+.
• contributes to POSITIVE INTROPY

Question 11

PKA impact on phospholamban (PLB)

Answer 11

• association of PLB with SERCA2 inhibits Ca2+ pumping activity (into SR)
• Phosphorylation causes PLB to dissociate from SERCA2, which relieves the inhibition and thus increases Ca2+ pumping into the SR.
• This speeds relaxation and increases the quantity of Ca2+ stored in the SR.
• This contributes to both POSITIVE INOTROPY and POSITIVE LUSITROPY.

Question 12

Characteristics of Timothy Syndrome

Answer 12

• disorder characterized by syncope, cardiac arrhythmias and sudden death, in addition to intermittent hypoglycemia, immune deficiency and cognitive abnormalities including autism.
• Associated with de novo mutations in CaV1.2 (the principle subunit of the L-type Ca2+ channel).
• TS2 mutations profoundly suppress voltage-dependent inactivation.
• Both TS and TS2 patients display AV block, prolonged Q-T intervals and episodes of polymorphic ventricular tachycardia.

Question 13

Characteristics of Brugada syndrome

Answer 13

• associated with a number of ECG alterations, which in some instances are revealed by administration of class IC anti-arrhythmics (sodium channel blockers) including ajmaline.
• Associated with mutations of the cardiac sodium channel, KChip2 a modulatory subunit associated with Kv4.3 to produce IKto and several other proteins including ankyrin.
• A subset of Brugada syndrome patients either have mutations in the principle subunit or a mutation in the main accessory subunit of the L-type Ca2+ channel.
• These mutations appear to cause a large reduction in the magnitude of L-type Ca2+ current which may be a consequence of impaired membrane trafficking.
• These patients have significantly shortened Q-T intervals.

Question 14

Characteristics of Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT)

Answer 14

• patients with CPVT do not display ECG abnormalities at rest, but do display abnormalities upon exercise or infusion of catecholamines.
• Associated with causative mutations in RyR2—with dominant inheritance—and in the lumenal Ca2+ buffer calsequestrin2 (CasQ2)—with recessive inheritance.
• RyR2 mutations increase the resting “leak” of Ca2+ out of the SR and/or render RyR2 more sensitive to activation by Ca2+.

Question 15

Mechanism of ectopic depolarizations relating to CPVT

Answer 15

• CPVT mutations + increased SR Ca2+ (increased as a consequence of activation of β-adrenergic receptors) is presumed to result in releases of Ca2+ that are not directly triggered by the L-type Ca2+ current during the plateau of the action potential but instead occur either shortly or long after repolarization.
• Extrusion of the Ca2+ via NCX results in depolarizations that can trigger ectopic action potentials and thus initiate arrhythmias.