Release and uptake of calcium by the sarcoplasmic reticulum

Today it seems well established that calcium ions are the mediators of excitation contraction coupling. The sequence of the reactions by which muscle activity is twitched on starts with the depolarization of the plasma membrane. Depolarization spreads inwards actively along its narrow transverse tubular invaginations and reaches the innermost fibrils in approximately 1 msec. (Gonzales-Serrates, 1971; Benzanilla et al. , 1972; Adrian and Peachey, 1973). There follows a sudden rise of the calcium concentration in the myoplasma and the contractile protein is activated (Ashley and Ridgway, 1970; Rüdel and Taylor, 1973). The muscle relaxes when the calcium concentration has returned to its resting level. Fig. 1 illustrates these events schematically. However, Open image in new windowFig. 1 Fig. 1 Schematic diagram of excitation-contraction coupling. The arrows represent active and passive calcium movements a quantitative evaluation of this diagram is not possible. Neither the size of the various compartments for calcium nor the rate constant describing the calcium movement between the different compartments is known accurately. However, as incomplete this scheme may appear if its quantitative essence is considered, it represents considerable progress. It started with Heilbrunn's observation (Heilbrunn, 19 40) that contractions of muscle fibers can be induced by the injection of solutions containing calcium ions and was extended by a series of findings by morphologists, biochemists and physiologists