SKELETAL MUSCLE: EXCITATION-COUPLING CONTRACTION

Question 1

Define: sarcoplasmic reticulum

Answer 1

intracellular network that regulates [Ca2+]. Surrounded each myofibril. Part of the endomembrane system.

Question 2

Define: transverse tubules (T-tubules)

Answer 2

invaginations of the sarcolemma that penetrate the fibre at junctions between A/I bands in each half sarcomere. T-Tubules are part of the sarcolemma and contacts extracellular fluid.

Note: sarcoplasmic reticulum and T-tubules are distinct membranes

Question 3

Define: triad

Answer 3

structure formed by the interface between the T-tubule and 2 portions of the sarcoplasmic reticulum

Question 4

Define: terminal cisternae

Answer 4

Region of the sarcoplasmic reticulum nearest the T-tubule

Question 5

Describe in detail how t-tubules support the propagation of the action potential to muscle cells

Answer 5

• When nicotinic acetylcholine receptors open, a passive end plate potential is created which results in voltage-gated Na+ channels opening and creating a muscle action potential.
• The action potential travels along the sarcolemma and down into the T-Tubules.
• Ca2+ is released from the terminal cisternae of the sarcoplasmic reticulum
• Calcium release from the sarcoplasmic reticulum binds to TnC
  –> results in contraction of the muscle fibre (muscle fibre twitch)
  - Recall that twitches sum to lead to unfused and fused tetanus

Question 6

Define: DHPR

Answer 6

L-type calcium channels that are localized to T-tubules. Are physically coupled to ryanodine receptors (RYR)

Question 7

Define: ryanodine receptors (RYR)

Answer 7

localized to the membrane of the sarcoplasmic reticulum and responsible for the release of Ca2+ from intracellular stores

Question 8

Define: calsequestrin

Answer 8

calcium binding protein resident in the sarcoplsamic reticulum. Plays a role in calcium storageand regulator of the muscle excitation-contraction coupling

“sequesters calcium”

Question 9

Explain what occurs at the triad starting from a passive end plate potential until Ca2+ is pumped back into the sarcoplasmic reticulum by SERCA

Answer 9

• A passive end plate potential initiates muscle action potential
• End plate potential depolarizes to threshold for voltage-gated Na+ channels, muscle action potential spreads over sarcolemma and into T-tubules
• Muscle action potential depolarization open DHP receptors (L-type Ca2+ channels) which, reminder, are located in T-tubules
• Mechanical coupling between DHPR and Ryanodine Receptors causes ryanodine receptors to open (reminder, RYR are located in SR)
• Ca2+ exits the sarcoplasmic reticulum and binds to Troponin C (Tn C)
• Ca2+ is pumped back into the SR by SERCA

Question 10

Explain the process by which Ca2+ is reuptaken by the sarcoplasmic reticulum

Answer 10

• SERCA: Sarcoendoplasmic Reticulum Calcium ATPase. Located in the sarcoplasmic reticulum cisternae. Pumps Ca2+ against large concentration gradient
• Has a high metabolic cost during muscle contraction
• Ca2+ binds to calsequestrin within terminal cisternae
• Calsequestrin has high capacity for binding Ca2+
• Highly localized beneath triad junction
• Calsequestrin aids muscle relaxation by buffering sarcoplasmic reticulum Ca2+ and unloading its Ca2+ near RyR1, thus facilitating more excitation-contraction coupling

Question 11

Describe what happens to troponin when intracellular [Ca2+] rises and falls

Answer 11

• When [Ca2+]i rises and Ca2+ binds to TnC, conformational changes cause TnT to pull tropomyosin and TnI out of the way, so that myosin can now interact with actin
• As long as Ca2+ is present, multiple cross-bridge cycles occur
• When [Ca2+]i falls, Ca2+ dissociates from TnC and the subsequent movements of Tnt, tropomyosin, and TnI block further myosin-actin interactions

Question 12

Why was the evolution of the isoforms Cav1.1 and RyR1 advantageous?

Answer 12

• CaV1.1 and RYR-1 are physically coupled through protein-protein interactions
• Voltage-induced confirmational changes in DHPR is transduced to RyR causing Ca2+ release from the sarcoplasmic reticulum leading to muscle contraction

Benefits:
Speed

Less Ca2+ pushed out across the sarcolemma

Question 13

Describe the grouping of CaV1.1 and RyR1 for the purpose of interaction

Answer 13

For skeletal muscle contraction to occur, a high level of organization of the CaV1.1 and RYR1 is necessary

Cav1.1 are grouped into tetrads (4) on the T-tubule membrane and aligned to directly oppose the four subunits of every other RYR1 in the adjacent terminal cisternae

RyR1s are clustered on the SR membrane. Each subunit has a “foot” that extends into the cytosol

Interactions involve specific amino acid residues in the II-III linker of CaV1.1 and the RyR1 receptor

DHPR has 4 domains each with 6 transmembrane proteins. Between domain 2 and 3 is a long loop that interacts with RYR.

Question 14

List and describe CaV1.1 Inhibitors (2)

Answer 14

Dihydropyridine: Inhibits L-type calcium channels

Derivatives of dihydropyridine are used for management of angina (chest pain), cardiac arrhythmias and high blood pressure

Ex: nifedipine and verapamil

Nifedipine: antihypertensive

Verapamil: anti-arrhythmogenic

Question 15

Why is contractile force described as Ca2+ dependent

Answer 15

• Contractile force increases in a sigmoidal pattern as intracellular Ca2+ is increased above 0.1 uM
• Half maximal force occurs at around 1 uM
• Troponin has 4 Ca2+ binding sites

2 have high affinity(bind Mg2+ at rest): involved in TnI and TnT interactions
2 have low affinity and bind Ca2+ as [Ca2+] rises

• Troponin (specifically, TnC) is the calcium sensor that links excitation and contraction in myocytes (true in all myocytes, but mechanism differs)