Excitation/contraction coupling in the heart Flashcards
Cardiac muscle structure
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
Ca-induced Ca release
- Ca enters myoplasm via V-gated Ca channels in the sarcolemma of t-tubules (plateau phase of AP)
- Ry receptors in the SR membrane located directly opposite from V-gated channels (not physically coupled) open in response to increase in intracellular Ca
- Ry receptors open, release huge amounts of Ca from the SR into the myoplasm (enough force to maintain CO)
- 2 pumps for restoring Ca concentration:
- Ca ATPase in SR membrane
- Na/Ca exchanger on cell membrane
Myocardial contraction - b1 stimulation
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
Mechanism of vascular smooth muscle contraction
Ca2+ binds to Calmodulin activating MLCK which phosphorylates MLC leading to contraction