2. Molecular and Cellular Basis of Myocardial Contractility

Cardiac contraction is brought about by interactions between actin and myosin that are regulated by tropomyosin, troponin I, troponin T, and troponin C. Contraction is activated by a process called excitation-contraction coupling that is initiated by plasma membrane depolarization and ends when calcium is delivered to the cytosol for binding to troponin C. Most of this activator calcium is derived from intracellular stores within the sarcoplasmic reticulum; a smaller amount enters from the extracellular fluid. The heart relaxes when calcium is pumped out of the cytosol into the sarcoplasmic reticulum by an ATP-dependent calcium pump and into the extracellular fluid by a sodium/calcium exchanger and a plasma membrane calcium pump.