Calcium-induced weakening of skeletal muscle Z-disks.

Structural changes in the Z disk were sensitively detected by measuring fragmentation indexes of myofibrils. The Ca2+-induced weakening of Z disks and the Z-disk removal by muscle calpain could be clearly distinguished by using muscle calpastatin, an endogenous inhibitor of muscle calpain. The Ca2+-induced weakening of Z disks occurred without concomitant release of alpha-actinin and had maxima at 10(-4) M Ca2+ and 45 degrees C and a minimum at pH 6.5, while the Z-disk removal by calpain had similar optima to the caseinolytic activity of calpain, at 10(-3) M Ca2+, 20 degrees C and pH 7.0. The Ca2+-induced weakening of Z disks is therefore not due to the proteolytic action of calpain. In postmortem muscle, moreover, the Ca2+-induced weakening of Z disks was inferred to be predominate over calpain proteolysis, and therefore to be the major factor in the characteristic weakening of Z disks.