Biochemistry and physiology of cardiac muscle

Appropriate regulation of the contraction of cardiac myocytes within the heart is critical for its physiological function, allowing the heart to alter its power output in line with circulatory demand. Normal myocyte contraction requires a rise in intracellular calcium to activate the contractile machinery (or myofilaments), and is followed by a decline in the calcium transient which allows relaxation. These cyclical fluctuations in calcium involve a complex series of ion channels, pumps and transporters in a process termed excitation-contraction coupling. Contractile performance is also influenced by the intrinsic properties of the myofilaments themselves. The ability of the heart to efficiently augment its output relies mainly on four key physiological pathways: (a) the Frank-Starling relationship, which involves length- or volume-dependent regulation, (b) heart rate-dependent regulation, (c) autonomic innervation, and (d) autocrine/paracrine pathways. Finally, optimal function of the heart is also dependent upon the properties of the extracellular matrix within which cardiac myocytes are embedded, the coronary circulation, and appropriate integration of these variables.