Figure 2.

Simulation of Na turnover in a patch-clamped cardiac myocyte with instantaneous ion mixing. Cell volume is 10 pL, access resistance is 3 MΩ, the patch pipette contains no Na, and the cell membrane has a Na conductance of 38 nS Na in the presence of 120 mM extracellular Na. The cytoplasm is assumed to contain initially 120 mM Asp and NMDG. (A) Application of 120 mM extracellular Na supports a 0.95 nA Na current that decays by ∼12% with a τ of 14 s. Cytoplasmic Na accumulates by 14 mM, and NMDG and Asp decrease and increase, respectively, to maintain electroneutrality. Upon removing extracellular Na, an outward Na current develops with a magnitude equal to the decaying inward current. The reverse Na current decays with nearly the same time constant as the forward current decays. (B) Complete description of the simple model in dependence on the peak Na current activated from 0 to 6 nA. From top to bottom, the tip potential increases linearly from 0 to 18 mV, the τ with which current decays decreases from 16 to 9 s, cytoplasmic Na increases from 0 to 50 mM, the fractional decay of Na current increases from 0 to 0.5, cytoplasmic NMDG decreases from 120 to 60 mM, cytoplasmic Asp increases from 120 to 240 mM, and osmolarity increases from baseline (290 mosM/liter) to 120 mosM/liter over baseline.