One of the most remarkable characteristics of heart muscle is its ability to vary its force of contraction over a large range under normal conditions and from a fixed initial length. This length-independent change in contraction force over an almost tenfold range allows the heart to adjust rapidly to varying demands of the circula tion. Since the action potential does not change much in height (although it does vary in duration and shape) and the contractile proteins presumably can not change structure on a beat-by-beat basis, we generally ascribe the contractile variability of heart muscle to the step called excitation-contraction coupling (EC coupling). For this review I have gathered articles that have contributed to our understanding of several aspects of EC coupling.

This review was originally intended to cover a broader consideration of EC coupling in striated muscle. Important progress has recently been made in the field of EC coupling both in skeletal muscle and in heart muscle, and comparison of the two muscles promises to be advantageous. I had the opportunity to preview the excellent discussion of EC coupling in skeletal muscle by Ebashi (21) prepared for Volume 38 of this series; in the delightful fashion that we have become accustomed to expect from him, Professor Ebashi has synthesized our understanding and progress in skeletal muscle studies. We encourage the cardiac investigator to review that article along with this one. Occasional reference to skeletal muscle studies are made in this chapter, where they assist in interpreting experiments on heart muscle. The previous review of EC coupling in heart muscle in Volume 35 by Langer (53) represents a starting point for this chapter. Other important reviews include one on EC coupling by Morad & Goldman (61) and on calcium ion currents by Reuter (71). Weidmann (98) reviewed cardiac membrane electrogenesis in Volume