Sarcomeric calcium sparks activated by fiber depolarization and by cytosolic Ca2+ in skeletal muscle.

Discrete highly localized elevations of myoplasmic [Ca2+], calcium 'sparks', have been detected in skeletal muscle fibers. During relatively small depolarizations of a fiber, the calcium sparks are several times larger than the average increase in [Ca2+] and can thus be clearly resolved. The spark event frequency increases steeply with increasing depolarization, so that for larger depolarizations the discrete microscopic [Ca2+] elevations blend together and become indistinguishable in the average macroscopic [Ca2+] transient. Spontaneous calcium sparks also occur in the absence of voltage sensor activity, in which case they are activated by myoplasmic Ca2+. Both the voltage-activated and Ca(2+)-activated events originate at the location of the triad within the sarcomere. Calcium sparks appear to constitute the elementary unit of calcium release activation in skeletal muscle.