Sequential Gating of Ryanodine Receptors Underlies the Development of Calcium Sparks in Frog Skeletal Muscle

Calcium sparks can arise as both voltage-dependent and voltage-independent ligand-activated release events in amphibian skeletal muscle. To assess their gating behavior, calcium sparks were recorded from intact frog skeletal muscle fibers using high-temporal-resolution confocal microscopy (line scans: 15 and 50 µs/line). Sparks were triggered by 1 mmol/L caffeine to open ryanodine receptors (RyRs) or by subthreshold depolarization to a -65 mV membrane potential to activate dihydropyridine receptors (DHPRs). Both treatments increased the frequency of sparks and altered their morphology. The sparks were significantly greater after caffeine treatment than in depolarized cells. The signal mass of sparks (i.e., the amount of calcium released) resembled the amplitude in shape. Additionally, the calcium release flux followed a staggered function during the activation of sparks. The detailed analysis of the sparks' time profile revealed that the events were activated in a stepwise manner. The average step size (in F/F₀; 0.071 ± 0.003) remained constant regardless of the scanning speed. The number of steps during the activation of sparks followed a linear function based on the spark's amplitude. Our results suggest that the activation of neighboring release units may occur sequentially, and the amplitude of the sparks depends linearly on the number of activated RyR channels.