A quantitative model of the cardiac ventricular cell incorporating the transverse-axial tubular system.

The role of the transverse-axial tubular system (TATS) in electrical activity of cardiac cells has not been investigated quantitatively. In this study a mathematical model including the TATS and differential distribution of ionic transfer mechanisms in peripheral and tubular membranes was described. A model of ventricular cardiac cell described by Jafri et al. (1998) was adopted and slightly modified to describe ionic currents and Ca2+ handling. Changes of concentrations in the lumen of the TATS were computed from the total of transmembrane ionic fluxes and ionic exchanges with the pericellular medium. Long-term stability of the model was attained at rest and under regular stimulation. Depletion of Ca2+ by 12.8% and accumulation of K+ by 4.7% occurred in the TATS-lumen at physiological conditions and at a stimulation frequency of 1 Hz. The changes were transient and subsided on repolarization within 800 ms (Ca2+) and 300 ms (K+). Nevertheless, the course of action potentials remained virtually unaltered. Simulations of voltage clamp experiments demonstrated that variations in tubular ionic concentrations were detectable as modulation of the recorded membrane currents.