Gating Properties of Cardiac Na+ Channels in Cell-free Conditions

In patches from neonatal rat heart myocytes, elementary Na+ currents were recorded at near threshold potentials in order to compare cardiac Na+ channels kinetics in the cell-attached mode with those in the inside-out mode. The transition from cell-attached to cell-free recording conditions caused a small prolongation of the conductive state of about 20%. This appeared within 8 min after patch excision regardless of the anionic composition (in mmol/liter) at the cytoplasmic membrane surface: 20 C1- plus 120 aspartate, 140 C1-, or 140 F-. Prolonged exposure (up to 50 min) to cell-free conditions evoked no additional changes and, specifically, left the monoexponential open-time distribution unchanged. Increased burst activity only developed in the cytoplasmic presence of F-, indicating that it is this artificial anion which influences reopening, but not the isolation of the Na+ channels from their natural environment per se. The mean number of openings per sequence (increase by a factor of 1.23 +/- 0.04) and tau decay of reconstructed macroscopic INa (increase by a factor of 1.32 +/- 0.06) responded rather weakly to F-. Cooling from 19 to 9 degrees C accentuated this F- effect significantly and led, at -65 mV, to pronounced burst activity. Moreover, the combined influence of F- and cooling induced a second, long-lasting and sometimes dominating open state. It is concluded that isolated cardiac Na+ channels largely preserve their intrinsic kinetic properties when facing a cytoplasmic environment with a quasi-physiological anionic composition.