Modification of single cardiac Na+ channels by DPI 201-106

In inside-out patches from cultured neonatal rat heart cells, single Na+ channel currents were analyzed under the influence of the cardiotonic compound DPI 201-106 (DPI), a putative novel channel modifier. In absence of DPI, normal cardiac single Na+ channels studied at -30 mV have one open state which is rapidly left with a rate constant of 826.5 sec -1 at 20 degrees C during sustained depolarization. Reconstructed macroscopic currents relax completely with 7 to 10 msec. The current decay fits a single exponential. A considerable percentage of openings may occur during relaxation of the macroscopic current. In patches treated with 3 X 10(-6) m DPI in the pipette solution, stepping to -30 mV results in drastically prolonged and usually repetitive openings. This channel activity mostly persists over the whole depolarization (usually 160 msec in duration) but is abruptly terminated on clamping back the patch to the holding potential. Besides these modified events, apparently normal openings occur. The open time distribution of DPI-treated Na+ channels is the sum of two exponentials characterized by time constants of 0.85 msec (which is close to the time constant found in the control patches, 1.21 msec) and 12 msec. Moreover, DPI-modified Na+ channels exhibit a sustained high, time-independent open probability. Similar to normal Na+ channels, is voltage-dependent and increases on shifting the holding potential in the hyperpolarizing direction. These kinetic changes suggest an elimination of Na+ channel inactivation as it may follow from an interaction of DPI with Na+ channels.