Lecture 6: Paper 1 (Sonic Muscles)

Question 1

Compare slow twitch (red muscle) skeletal muscle fibres and fast twitch (white muscle) skeletal muscle fibres.

Answer 1

slow twitch (red muscle):

• oxidative
• low frequency

fast twitch (white muscle):

• oxidative-glycolytic
• medium to high frequency
• faster myosin (↑ Vmax)
• ↑ sarcoplasmic reticulum
• ↑ Ca2+ pumps
• ↑ parvalbumin (protein that binds Ca2+ and Mg)

Question 2

What are sonic muscles?

Answer 2

specialized fast twitch muscles

Question 3

What muscles were studied?

Answer 3

to understand the physiological modifications that underlie very rapid contractions, two of the fastest vertebrate muscles (known as sonic muscles) were studied

• toadfish swimbladder muscle (superfast twitch): 200 Hz contractions
• rattlesnake shaker muscle (fast twitch): 90 Hz contractions

other muscles studied:

• toadfish fast twitch ‘white’ muscle
• toadfish slow twitch ‘red’ muscle
• (vertebrate locomotor muscle: 0.5-5 Hz contractions)

Question 4

What were the conclusions of the study?

Answer 4

to operate at high frequencies, toadfish swimbladder muscle and rattlesnake shaker muscle have independently evolved:

• large and very fast calcium transient (concentration of free Ca2+ in cytoplasm or myoplasm)
• fast cross-bridge detachment rate
• probably a fast kinetic off-rate of Ca2+ from troponin (faster resting rate)

Question 5

What were the 4 experiments conducted in this study?

Answer 5

• myoplasmic [Ca2+] transient
• muscle mechanics
• force-pCa measurements (pCa = -log10[Ca2+])
• modelling: troponin-Ca2+ binding

Question 6

Describe the method and results of the myoplasmic [Ca2+] transient experiment.

Answer 6

method:

• single muscle fibre within a bundle
• microscope slide
• furaptra (fluorescent calcium indicator)
• myoplasmic free [Ca2+] monitored during contraction
• action potential initiated

results:

• faster muscle → faster transient changes
• see diagrams

Question 7

Describe the method and results of the muscle mechanics experiment?

Answer 7

methods:

• fibre bundle
• measure force

results:

• see diagrams

Question 8

Describe the method and results of the force-pCa measurements (pCa = -log10[Ca2+]) experiment?

Answer 8

methods:

• ‘skinned’ (permeabilized) fibres by using detergents
• [Ca2+] controlled – kept constant
• SR Ca2+ pumps blocked

results:

• see diagrams

Question 9

Describe the method and results of the troponin-Ca2+ binding modelling experiment?

Answer 9

methods:

• mathematically modelled troponin activation
• different koff values
• k = reaction rate constant
• koff = Ca2+ off-rate constant from troponin

results:

• see diagrams