CVPR 03-26-14 11am-Noon Coupling & Calcium II - Beam

Question 1

Calcium entry & efflux – Long term

Answer 1

In the long term, Ca2+ entry into myocardial cells from the extracellular space must equal efflux of Ca2+ into the extracellular space, since any continuing imbalance would cause the cells to continuously gain or lose Ca2+.

Question 2

Calcium entry & efflux – Short term

Answer 2

In the short term, calcium influx can exceed efflux, or short term calcium efflux can exceed influx. Except for these short term increases or decreases, however, it is important that SR calcium content be kept roughly constant.

Question 3

Consequence of short term Ca2+ influx exceeding efflux

Answer 3

Amount of Ca2+ stored in SR can be increased

Question 4

Consequence of short term Ca2+ efflux exceeding influx

Answer 4

SR calcium content would decrease

Question 5

Mechanisms for Ca2+ homeostasis

Answer 5

NCX calcium exchanger…..L-type Ca2+ channel

Question 6

L-type Ca2+ channel and Ca2+ homeostasis

Answer 6

L-type Ca2+ channel undergoes a form of inactivation that depends on [Ca2+] near its cytoplasmic side = calcium-dependent inactivation (CDI). This helps to maintain a constant SR Ca2+ content.

Question 7

Calcium dependent inactivation (CDI) depends on…

Answer 7

Depends in part on Ca2+ entering through the channel but also, to a large extent, on Ca2+ released via RyR2 (Increase amount of Ca2+ in SR & thus amount released via RyR2 —> greater CDI causes less Ca2+ to enter via the L-type channel; Decrease Ca2+ in SR/released by RyR2 —> less CDI & greater Ca2+ entry via the L-type channel). Thus, CDI helps to maintain a constant SR Ca2+ content.

Question 8

Norepinephrine released by sympathetic nerve terminals and circulating epinephrine act to:

Answer 8

1. Increase HR (positive chronotropy) by raising firing rate of SA node pacemaker cells …..2. Alter propagation through conduction pathways…..3. Increase Contractile Force (positive inotropy)…..4. Increase Rate of Relaxation (positive lusitropy)

Question 9

Positive inotropy & lusitropy effects of Norepi/Epi involve…

Answer 9

Involve activation of -adrenergic receptors, elevation of cytoplasmic cAMP, and activation of Protein Kinase A (PKA).

Question 10

Four important targets for PKA in myocardium are:

Answer 10

1. The L-type Ca2+ channel…..2. RyR2…..3. Phospholamban (PLB) [associated w/SERCA pump]…..4. Troponin

Question 11

Effects of Phosphorylation of L-type Ca2+ channel (DHPR w/Cav1.2 subunit) by PKA in myocardium

Answer 11

increases the amplitude of the L-type Ca2+ current  more Ca2+ comes in to both increase the size of the trigger Ca2+ to activation of RyR2 and increases the quantity of Ca2+ stored in the SR. Contributes to Positive inotropy

Question 12

Effect of Phosphorylation of RyR2 by PKA in myocardium

Answer 12

Causes it to be sensitized to activation by trigger Ca2+; Contributes to Positive inotropy

Question 13

Association of Phospholamban (PLB) w/SERCA2

Answer 13

Inhibits Ca2+ pumping activity

Question 14

Effect of Phosphorylation of PLB by PKA in myocardium

Answer 14

Causes inhibitory PLB to dissociate from SERCA2, which relieves the inhibition & thus increases SERCA’s pumping Ca2+ into the SR —> speeds relaxation & increases the quantity of Ca2+ stored in SR; ; Contributes to Positive inotropy & Positive lusitropy

Question 15

Effect of Phosphorylation of troponin by PKA in myocardium

Answer 15

Speeds the rate of Ca2+ dissociation from thin filaments; Contributes to Positive lusitropy

Question 16

Timothy Syndrome - presentation

Answer 16

Debilitating disorder resulting in syncope, cardiac arrhythmias and sudden death; besides congenital heart disease, patients display intermittent hypoglycemia, immune deficiency, syndactyly (finger webbing), and cognitive abnormalities including autism

Question 17

Timothy Disease – mutation site

Answer 17

Linked to recurrent, de novo mutations in CaV1.2 (principle subunit of L-type Ca2+ channel), which is consistent with the multi-system nature of the syndrome given that CaV1.2 is expressed not only in the heart but in many other tissues as well….Location of mutations in Exon 8

Question 18

TS2 mutations – effects

Answer 18

Profoundly suppress voltage-dependent inactivation

Question 19

Cardiac manifestation of TS & TS2 patients

Answer 19

AV block, prolonged Q-T intervals (indicative of prolonged ventricular AP) and episodes of polymorphic ventricular tachycardia. Normally, current decays over time, due to voltage & due to Ca2+ entry; In Timothy syndrome, Ca2+ current goes away less slowly, prolonging the plateau of depolarization (prolonged QT interval)

Question 20

Brugada Syndrome -presentation

Answer 20

Aka Sudden Unexplained Death Syndrome; Associated w/ several ECG alterations, which in some instances are revealed by administration of class IC anti-arrhythmics (sodium channel blockers) including ajmaline.

Question 21

Brugada syndrome - mutations

Answer 21

Linked to several mutations of the cardiac Na+ channel (NaV1.5), KChip2 (associated w/Kv4.3 to produce IKto, transient outward K+ current), and several other proteins including ankyrin (links NaV1.5 to cytoskeleton)…… A subset of Brugada Syndrome patients either have mutations in the principal subunit CaV1.2 or a mutation in the main accessory subunit 2b of the L-type Ca2+ channel.

Question 22

Brugada syndrome – effects of mutations

Answer 22

Appear to cause a large reduction in the magnitude of L-type Ca2+ current which (for mutations in Cav1.2) may be a consequence of impaired membrane trafficking —> shortens the plateau of the AP —> significantly SHORTENED QT interval (opposite effect of Timothy syndrome) —> arrhythmias

Question 23

Cardiac manifestations of Brugada syndrome

Answer 23

Significantly shortened Q-T interval.- arrhythmias

Question 24

Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) – timing of ECG abnormalities

Answer 24

Pts w/ CPVT do not display ECG abnormalities at rest, but do upon exercise or infusion of catecholamines.

Question 25

Mutations in Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT)

Answer 25

Dominantly inherited causative mutations have been found in RyR2; Recessively inherited causative mutations have also been found in the lumenal Ca2+ buffer calsequestrin2 (CasQ2; isoform expressed in heart).

Question 26

RyR2 mutations in CPVT – effects

Answer 26

Thought to increase the resting “leak” of Ca2+ out of the SR and/or render RyR2 more sensitive to activation by Ca2+.

Question 27

CasQ2 mutations in CPVT – effects

Answer 27

Some in the homozygous condition would result in dramatic loss of lumenal Ca2+ buffering, whereas others would not have such a pronounced effect. However, in addition to buffering lumenal Ca2+, calsequestrin has also been suggested to regulate the function of RyR2 and this regulation may be altered by the CPVT causing mutations.

Question 28

Cardiac manifestation of CPVT (and mechanism)

Answer 28

CPVT mutations + increased SR Ca2+ content (a consequence of activated -adrenergic receptors) —> releases of Ca2+ that are not directly triggered by the L-type Ca2+ current during the plateau of the AP but instead occur either shortly or long after repolarization……Extrusion of this Ca2+ via NCX results in depolarizations that can trigger ectopic action potentials and thus initiate arrhythmias.

Question 29

Beta blockers in CPVT

Answer 29

Activation of b-adrenergic receptors is likely pro-arrhythmogenic, partly atleast b/c beta-AR activations causes increase in SR Ca content, as well as phosphorylation of RyR2 (makes more twitchy)…..Beta-blockers are a standard therapy for CPVT.

Question 30

Heart failure & RyR2

Answer 30

HF has been suggested to lead to “hyperphosphorylation” RyR2 (twitchy) and thus contribute in the increased incidence of arrhythmias in HF patients

Question 31

Treating CPVT

Answer 31

Beta-blockers & other standard therapies are NOT effective for some CPVT pts; Blocking aberrant Ca2+ release via RyR2 might be effective — as of now, Tetracaine can do this, but at too high doses to be clinically useful; Flecainide (class 1C anti-arrhythmic drug) also can block cardiac Na+ channels as well as RyR2

Question 32

Flecainide action & CPVT

Answer 32

Only blocks the RyR channels when they are already open (use-dependent) –> so, only hold them closed when they are active for a long time; seems to keep twitchy RyR channels calmed down