Calcium Ion

Abstract

It was almost a century ago when Sidney Ringer demonstrated that extracellular Ca²⁺ is essential for the normal functioning of the isolated frog heart [19]. Although it was assumed for some time that Ca²⁺ stabilized the plasma membrane of the heart cell, the key Ca²⁺ pool in terms of cardiac contractile function is the small pool of free Ca²⁺ in the cell cytosol, [Ca²⁺+]_c. The concentration of Ca²⁺ in this pool rises during systole and falls during diastole. The rise precedes the contractile response, and the fall precedes the relaxation process. Troponin C has been identified as the calcium receptor component of the contractile protein system [5]. Thus, we have come a long way toward defining the cellular basis of Ca²+ action in the cardiac cell. However, in the past 10–15 years, we have also come to realize that this messenger or coupling function of Ca²⁺ is not confined to the heart, or other excitable tissues, but is a universal one [17]. Ca²⁺ serves as a coupling factor or second messenger in the evocation of the specific response of almost every type of differentiated cell by its appropriate extracellular messenger, which may be a hormone, circulating metabolite, or neurotransmitter.