Excitation/contraction coupling in the heart

Question 1

Cardiac muscle structure

Answer 1

Almost identical to skeletal muscle
Key differences:
- small SR
- t- tubules larger in diameter
- longitudinal elements of tubule system
- excitation-contraction coupling occurs in both dyads and triads
- very large mitochondria (40% cell volume), almost exclusively dependent on oxygen metabolism

Question 2

Ca-induced Ca release

Answer 2

1. Ca enters myoplasm via V-gated Ca channels in the sarcolemma of t-tubules (plateau phase of AP)
2. Ry receptors in the SR membrane located directly opposite from V-gated channels (not physically coupled) open in response to increase in intracellular Ca
3. Ry receptors open, release huge amounts of Ca from the SR into the myoplasm (enough force to maintain CO)
4. 2 pumps for restoring Ca concentration:
   - Ca ATPase in SR membrane
   - Na/Ca exchanger on cell membrane

Question 3

Myocardial contraction - b1 stimulation

Answer 3

increased HR (positive chronotropic)
increased force of contraction (positive ionotropic)
increased rate of relaxation (positive lusitropic)

via action of protein kinase A or key regulatory proteins that regulate the intracellular Ca concentration

Question 4

Mechanism of vascular smooth muscle contraction

Answer 4

Ca2+ binds to Calmodulin activating MLCK which phosphorylates MLC leading to contraction