Figure 1. Activation of the sodium pump in a cell initially close to passive equilibrium.

The model was initiated with values of all intracellular ionic concentrations close to equilibrium with the extracellular fluid, although with approximately half the normal organic anion concentration and a consequently lower [Cl⁻]_i to maintain a total intracellular charge difference of zero. Thus initially [Na⁺]_i = 126.7 mm, [K⁺]_i = 2.5 mm, [Cl⁻]_i = 57.5 mm and [X⁻]_i = 43.5 mm. V_c was initially defined as 1. Symbols are as follows: K: [K⁺]_i; Na: [Na⁺]_i; Cl: [Cl⁻]_i; X: [X⁻]_i; Em: E_m, Vc: V_c. The basic parameters such as ion permeabilities and sodium pump density were derived from frog skeletal muscle (Table 1). Note that despite the arbitrary starting conditions, each intracellular ion concentration, V_c and E_m all reach stable values that are strikingly similar to normal values for frog skeletal muscle.