Deck 38: Cellular Basis of Cardiac Contractility

Question 1

Cardiac m uscle cann ot Increase its strength of contraction by repetitive stim ulation (wave summ ation) because

A) The action potential lasts hundreds of milliseconds, as lo ng as th e ca rdia c cyc le
B) The SR empties its Ca₂+ with every heart beat
C) The SA node fires too frequently for the cardiomyocytes to follow
D) The absolute refractory period is longer than the action potential
E) The bu ndle of H is has no re ser ve c apa city

Answer 1

A

Question 2

The force-frequency relation in the heart (the relation between frequency of stimulation and strength of the heart) is caused principally by

A) Stretch of the cardiac muscle with Increasing frequency of stimulation
B) Increased in the sensit ivity of the myofilaments to Ca₂+
C) Phosphorylation of TnC
D) Increased [Ca₂+] in the mitochondria
E) Increased [Ca₂+] transients due to loading of the SR

Answer 2

E

Question 3

Removal of external Ca₂+ to levels below about 0.2 mM result in no discernible force from the heart. This is because

A) There is no cardiac action potential without extracellular Ca₂+
B) There is no calcium-induced-calcium release without extracellular Ca₂+
C) Ca₂+ can not e nter o ver th e N CX to ac tivate the m yofilam ents
D) Ca₂+ is pumped out of the cell by the PMCA
E) TnC is insensit ive to Ca₂+ when the heart is relaxed

Answer 3

B

Question 4

Stretch Increases the force of cardiac contraction mainly by

A) Increasing the size of the Ca₂+ transient
B) Increasing the sensitivity of the myofilaments for Ca₂+
C) Increasing the turnover number of activated acto-myosin cross bridges
D) Decreasing the jamm ing of the myofilaments at short lengths
E) Incre asin g stre tch o f pas sive e lem ents

Question 5

Cardiomyocytes form a functional syncytium by being electrically coupled to one another at the

A) T-tubules
B) Sarcolemm a
C) Sarcoplasmic reticulum
D) Intercalated disks
E) Z disks

Question 6

Relaxation of the cardiomyocytes is caused mainly by

A) Pumping Ca₂+ out of the cytosol by the PMCA
B) Pumping Ca₂+ of the cytosol by NCX
C) Removal Ca₂+ out of the cytosol by SERCa₂a
D) R em ov ing AT P f ro m the rigo r c om ple x o f m yos in
E) Removing Ca₂+ from the cytosol by the L-type Ca₂+ channel

Question 7

In cardiac muscle, the T-tubules are located

A) along the Z disk
B) In the center of the I band
C) In the center of the A band
D) At the junction of the I band and the A band
E) In the center of the H zone or H band

Question 8

"Ca lcium -indu ced calc ium relea se" refe rs to

A) Increased Ica upon beta adrenergic stimulation
B) Re leas e of Ca from the card iac S R b y Ca tha t en ters the cell ac ros s th e T -tub ule
C) Relaxation of cardiac muscle when Ca is removed by the SR SERC A2a pump
D) Increased Ca uptake by SR following contraction
E) Both A and C are correct

Question 9

Cardiac muscle can Increase its strength of contraction by all of the following EXCEPT

A) Stretch, by Increasing the sensit ivity of the myofilaments to Ca₂+
B) Increasing the size of the Ca₂+ transients by phosphorylation of phospholamban
C) Increasing recruitment of cardiomyocytes
D) Increasing the size of the Ca₂+ transients by phosphorylation of DHPR
E) Increasing the size of the Ca₂+ transients by phosphorylation of RyR2

Question 10

Sympathetic stimulation Increases all of the following EXCEPT

A) Ca₂+ entry over the L-type Ca₂+ channel
B) Phospholamban phosphorylation
C) Ca₂+ release through RyR2
D) Rate of Ca₂+ dissociation from TnC
E) Ca₂+ exit over the PMCA

Question 11

Ca₂+ enters the cardiomyocytes every heart beat through

A) L-type Ca₂+ cha nn els
B) Na-K-ATPase
C) PMCA
D) RyR2
E) SERCa₂a

Question 12

M od es tly Increasing the fre qu en cy o f th e h ea rt b ea t fro m 70 m in-1 to 1 20 m in-1 will

A) Increase the strength of contraction by Increasing the size of the Ca₂+ transient
B) Decrease the strength of contraction due to fatigue
C) Increase the strength of contraction by initiating contraction before the heart has completely relaxed
D) Incre ase the s treng th of c ontra ction b y stretc hing the m yofilam ents
E) Increase the strength of contraction by inhibit ing NCX

Question 13

The space between myofibrils in cardiomyocytes is occupied mainly (in terms of volume) by

A) glycogen granules
B) sarcoplasmic reticulum
C) M itoc ho nd ria
D) Nuclei
E) T-tubules

Question 14

Which o f th e fo llow ing is N O T on e o f th e w ays in w hic h c ar dio m yoc ytes diff er fro m sk ele tal muscle fibers?

A) Cardiom yocytes have a greater volum e of m itochondria than skeletal m uscle fibers
B) There is on average one T-tubule per sarcomere in cardiomyocytes, two in skeletal m us cle
C) Most cardiomyocytes have 1-2 nuclei; skeletal muscle fibers have many
D) C ar dio m yoc ytes m ak e e lec tric al c on ne ctio ns be twe en ad jac en t ce lls, s ke leta l m us cle fibers do not
E) Card iom yocyte s do not re qu ire extern al stim ulation w here as sk eletal m usc le fibers do

Question 15

The inotro pic ef fec t of sym path etic stim ulation is due m ainly to

A) Decreased duration of the action potential
B) Increased size of the Ca₂+ transient
C) Increased affinity of TnC for Ca₂+
D) Increased rate of relaxation
E) Increased amplitude of the action potential

Question 16

Cardiac muscle cannot Increase its strength by recruitment because

A) T he fun ctio na l syn cytiu m pre ve nts ac tiva tion of a dja ce nt c ells
B) Cells a re co up led elec trically throu gh gap junc tions , so all are a ctivate d ev ery contraction
C) The cardiac action potential lasts as long as the contraction
D) The [Ca₂+] transient does not saturate cardiac TnC during most contractions
E) The bu ndle of H is has no re ser ve c apa city

Question 17

Which of the following are NOT found in the I band?

A) N eb ulin
B) T itin
C) Ac tin
D) Ac tinin
E) M yom esin