Concerted vs. Sequential. Two Activation Patterns of Vast Arrays of Intracellular Ca²⁺ Channels in Muscle

To signal cell responses, Ca²⁺ is released from storage through intracellular Ca²⁺ channels. Unlike most plasmalemmal channels, these are clustered in quasi-crystalline arrays, which should endow them with unique properties. Two distinct patterns of local activation of Ca²⁺ release were revealed in images of Ca²⁺ sparks in permeabilized cells of amphibian muscle. In the presence of sulfate, an anion that enters the SR and precipitates Ca²⁺, sparks became wider than in the conventional, glutamate-based solution. Some of these were “protoplatykurtic” (had a flat top from early on), suggesting an extensive array of channels that activate simultaneously. Under these conditions the rate of production of signal mass was roughly constant during the rise time of the spark and could be as high as 5 μm³ ms⁻¹, consistent with a release current >50 pA since the beginning of the event. This pattern, called “concerted activation,” was observed also in rat muscle fibers. When sulfate was combined with a reduced cytosolic [Ca²⁺] (50 nM) these sparks coexisted (and interfered) with a sequential progression of channel opening, probably mediated by Ca²⁺-induced Ca²⁺ release (CICR). Sequential propagation, observed only in frogs, may require parajunctional channels, of RyR isoform β, which are absent in the rat. Concerted opening instead appears to be a property of RyR α in the amphibian and the homologous isoform 1 in the mammal.