Conductance restrictions induced by cardiac and diaphragm myofilaments and their dependence on solute MW (24°C). (A) Representative experiment with membrane-extracted myocytes using a solution of 50 mM NMG-MES, 0.5 mM MgCl₂, 0.5 mM EGTA, and buffered to pH 7 with 4 mM HEPES. Initially, five cardiac myocytes were aspirated and released from the pipette tip, cardiac myocytes were replaced with diaphragm myocytes in the dish, and then four diaphragm myocytes were aspirated and released. In this solution, conductances with aspirated cardiac myofilaments were substantially less than with aspirated diaphragm myofilaments. (B) Comparison of conductance changes caused by cardiac and diaphragm myofilaments for four ion pairs. For all results, n > 6. The graph plots the ratio of pipette conductance in the presence versus absence of aspirated myofilaments. Conductances of cardiac and diaphragm myofilaments were similar with 50 mM KCl. While cardiac myofilaments induced progressively larger decreases of conductance with ion pairs of increasing MW, diaphragm myofilaments caused similar conductance reductions for all four ion pairs employed, namely KCl, NMG-MES, NMG-(Mg)ATP, and β-methylcyclodextrin complexes with octanoate and octylamine.