Coupled ion diffusion in PM-permeabilized and extracted murine cardiac myocytes (24°C). (A) Myocytes were aspirated into the tip of highly polished pipettes, as shown in the inset, employing a background solution that contained 20 mM KCl. The sarcolemma was then permeabilized by application of 80 µM β-escin, and thereafter conductances induced by applying 50 mM NaCl versus 50 mM Na(Mg)ATP are compared. From the first derivative of the conductance signal, the maximal rates and time constants of conductance changes were determined. Time constants for NaCl signals were 16% of those for Na(Mg)ATP. From Eq. 8, this indicates that the diffusion coefficient of MgATP is 9% of the Cl diffusion coefficient. (B) Equivalent experiments to panel A were performed with myocytes that had been extensively membrane-extracted, as described, with glycerol and Triton X-100. The ratios of diffusion coefficients for MgATP and Cl were nearly twice as large, indicating that MgATP diffusion is less restricted in extracted myofilaments than in permeabilized myocytes.