“…Both experimental data and mathematical models have shown that I to magnitude and composition (i.e., the relative contributions of I to,f and I to,s to total I to ) may markedly affect AP duration and shape (7)(8)(9)(10)(11)(12)(13). Expression of I to,f and I to,s channel proteins is highly regulated, and may vary with species, developmental stage, and region of the heart (6,11).…”