Calcium ion in cardiac contractility.

Under physiological conditions where the intracellular Ca ion concentration does not exceed 3 X 10(-6) M, the sarcoplasmic reticulum plays a major role in the relaxation process of cardiac muscle; mitochondria do not take up a significant amount of Ca ion during this process. If cardiac muscle undergoes maximum contraction, in which the intracellular Ca ion concentration should reach 10(-4) M, the role of mitochondria in reducing intracellular Ca ion becomes appreciable. The relationship of the tension developed by cardiac glycerinated muscle fibers to the Ca ion concentration resembles the relationship of the amount of bound Ca of cardiac troponin to the Ca ion concentrations, being less steep in its slope compared with those of fast and slow skeletal muscles. This gentle slope seems to reflect the great diversity of affinities for Ca ion of the two Ca-binding sites of cardiac troponin, one being about 100 times that of the other.