Energetical considerations related to calcium release from the sarcoplasmic reticulum in skeletal muscle

During the initial phase of activation (about 5 ms) of 1 g of muscle about 200 nmol of Ca++ are released from the SR which results in a current of about 7.7 A. Because at least 90% of Ca++ is released at a gradient below 1000/1 a free energy of about 3 mJ is available. Based on a SR-surface of approximately 2 m2/g of muscle a maximal inside negative membrane potential of -87 mV and a specific membrane capacitance of 1 microF/cm2 only about 9 nmol of Ca++ can be released without charge compensation before the calcium equilibrium potential is reached. This means that a countercurrent must compensate for practically all the calcium released in order to maintain a substantial driving force during release. The energy derived from the amount of Ca++ and its gradient or the caloric value of ATP required for Ca++-uptake (100 nmol) sets an upper limit for specific membrane resistance of the SR in the range of 200 omega cm2. This value is compatible with clearly lower numbers deduced from measurements on membranes obtained by fusing SR vesicles with lipid bilayers and the concept that the SR functions as a current source rather than as a voltage source. However, this value is much lower than estimates of specific membrane resistance based on indirect measurements.