Simulation of Calcium Sparks in Cut Skeletal Muscle Fibers of the Frog

Spark mass, the volume integral of ΔF/F, was investigated theoretically and with simulations. These studies show that the amount of Ca²⁺ bound to fluo-3 is proportional to mass times the total concentration of fluo-3 ([fluo-3_T]); the proportionality constant depends on resting Ca²⁺ concentration ([Ca²⁺]_R). In the simulation of a Ca²⁺ spark in an intact frog fiber with [fluo-3_T] = 100 μM, fluo-3 captures approximately one-fourth of the Ca²⁺ released from the sarcoplasmic reticulum (SR). Since mass in cut fibers is several times that in intact fibers, both with similar values of [fluo-3_T] and [Ca²⁺]_R, it seems likely that SR Ca²⁺ release is larger in cut fiber sparks or that fluo-3 is able to capture a larger fraction of the released Ca²⁺ in cut fibers, perhaps because of reduced intrinsic Ca²⁺ buffering. Computer simulations were used to identify these and other factors that may underlie the differences in mass and other properties of sparks in intact and cut fibers. Our spark model, which successfully simulates calcium sparks in intact fibers, was modified to reflect the conditions of cut fiber measurements. The results show that, if the protein Ca²⁺-buffering power of myoplasm is the same as that in intact fibers, the Ca²⁺ source flux underlying a spark in cut fibers is 5–10 times that in intact fibers. Smaller source fluxes are required for less buffer. In the extreme case in which Ca²⁺ binding to troponin is zero, the source flux needs to be 3–5 times that in intact fibers. An increased Ca²⁺ source flux could arise from an increase in Ca²⁺ flux through one ryanodine receptor (RYR) or an increase in the number of active RYRs per spark, or both. These results indicate that the gating of RYRs, or their apparent single channel Ca²⁺ flux, is different in frog cut fibers—and, perhaps, in other disrupted preparations—than in intact fibers.