SARCOMERIC CALCIUM SPARKS ACTIVATED BY FIBER DEPOLARIZATION AND BY CYTOSOLIC CA2-MUSCLE( IN SKELETAL)

Discrete highly localized elevations of myoplasmic [Ca2+], calcium 'sp arks', have been detected in skeletal muscle fibers. During relatively small depolarizations of a fiber, the calcium sparks are several time s larger than the average increase in [Ca2+] and can thus be clearly r esolved. The spark event frequency increases steeply with increasing d epolarization, so that for larger depolarizations the discrete microsc opic [Ca2+] elevations blend together and become indistinguishable in the average macroscopic [Ca2+] transient. Spontaneous calcium sparks a lso occur in the absence of voltage sensor activity, in which case the y are activated by myoplasmic Ca2+. Both the voltage-activated and Ca2 +-activated events originate at the location of the triad within the s arcomere. Calcium sparks appear to constitute the elementary unit of c alcium release activation in skeletal muscle.