Regulation of myocardial contraction as revealed by intracellular Ca²⁺ measurements using aequorin

Of the ions involved in myocardial function, Ca²⁺ is the most important. Ca²⁺ is crucial to the process that allows myocardium to repeatedly contract and relax in a well-organized fashion; it is the process called excitation–contraction coupling. In order, therefore, for accurate comprehension of the physiology of the heart, it is fundamentally important to understand the detailed mechanism by which the intracellular Ca²⁺ concentration is regulated to elicit excitation–contraction coupling. Aequorin was discovered by Shimomura, Johnson and Saiga in 1962. By taking advantage of the fact that aequorin emits blue light when it binds to Ca²⁺ within the physiologically relevant concentration range, in the 1970s and 1980s, physiologists microinjected it into myocardial preparations. By doing so, they proved that Ca²⁺ transients occur upon membrane depolarization, and tension development (i.e., actomyosin interaction) subsequently follows, dramatically advancing the research on cardiac excitation–contraction coupling.