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Comparison between the Predictions of Diffusion-Reaction Models and Localized Ca²⁺ Transients in Amphibian Skeletal Muscle Fibers

Department of Physiology, UCLA School of Medicine, Los Angeles, California

Correspondence: Address reprint requests to Julio Vergara, Dept. of Physiology, UCLA School of Medicine, 10833 Le Conte Avenue 53-263 CHS, Los Angeles, CA 90095-1751. Tel.: 310-825-9307; Fax: 310-206-3788; E-mail: jvergara{at}mednet.ucla.edu.

We developed a three-dimensional cylindrical diffusion-reaction model of a single amphibian myofibril in which Ca²⁺ release occurred only at the Z-line. The model incorporated diffusion of Ca²⁺, Mg²⁺, and all relevant buffer species, as well as the kinetic binding reactions between the buffers and appropriate ions. Model data was blurred according to a Gaussian approximation of the point spread function of the microscope and directly compared with experimental data obtained using the confocal spot methodology. The flux parameters were adjusted until the simulated Z-line transient matched the experimental one. This model could not simultaneously predict key parameters of the experimental M- and Z-line transients, even when model parameters were adjusted to unreasonably extreme values. Even though the model was accurate in predicting the Z-line transient under conditions of high [EGTA], it predicted a significantly narrower Ca²⁺ domain than observed experimentally. We modified the model to incorporate a broader band of release centered at the Z-line. This extended release model was superior both in simultaneously predicting critical features of the Z- and M-line transients as well as the domain profile under conditions of high [EGTA]. We conclude that a model of release occurring exclusively at the Z-line cannot explain our experimental data and suggest that Ca²⁺ may be released from a broader region of the sarcoplasmic reticulum than just the T-tubule-sarcoplasmic reticulum junction.

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