Numerical analysis of Ca2+ depletion in the transverse tubular system of mammalian muscle

Calcium currents were recorded in contracting and actively shortening mamma lian muscle fibers. In order to characterize the influence of extracellular calcium concentration changes in the small unstirred lumina of the transve rse tubular system (TTS) on the time course of the slow L-type calcium curr ent (I-Ca), we have combined experimental measurements of I-Ca with quantit ative numerical simulations of Ca2+ depletion. I-Ca was recorded both in ca lcium-buffered and unbuffered external solutions using the two-microelectro de voltage clamp technique (2-MVC) on short murine toe muscle fibers. A sim ulation program based on a distributed TTS model was used to calculate the effect of ion depletion in the TTS. The experimental data obtained in a sol ution where ion depletion is suppressed by a high amount of a calcium buffe ring agent were used as input data for the simulation. The simulation outpu t was then compared with experimental data from the same fiber obtained in unbuffered solution. Taking this approach, we could quantitatively show tha t the calculated Ca2+ depletion in the transverse tubular system of contrac ting mammalian muscle fibers significantly affects the time-dependent decli ne of Ca2+ currents. From our findings, we conclude that ion depletion in t he tubular system may be one of the major effects for the I-Ca decline meas ured in isotonic physiological solution under voltage clamp conditions.