The sarcoplasmic reticulum (SR) is a fenestrated sheath of membranes between and around skeletal muscle myofibrils Abbreviation used: SR, sarcoplasmic reticulum. Vol. 17and regulates the Ca?+ distribution and thus contraction and relaxation inside muscle cells. In the absence of Ca", isolated SR vesicles catalyse low rates of a Mg"-dependent ATP hydrolysis (basal ATPase) and on the addition of Ca'+ the ATPase activity is several times increased (Ca? +-dependent ATPase) (Martonosi, 1084). For each molecule of ATP hydrolysed two Ca'+ atoms are transported into the interior of the SR (Hasselbach, 1978).