Cytoskeleton modulates coupling between availability and activation of cardiac sodium channel

The aim of this study was to investigate modulation of voltage-dependent steady-state activation and availability from inactivation of the cardiac Na+ channel by the cytoskeleton. As an experimental approach, we used long-lasting monitoring [63 +/- 5 (SE) min] of the half-point potentials of the steady-state availability curve (V-1/2A) and normalized conductance curve (V-1/2G) in 116 rat ventricular cardiomyocytes by whole cell patch clamp at 22-24 degrees C. Both half-point potentials shifted in the negative direction with time as an exponentially saturating change, with the shift of V-1/2G being smaller and faster. An F-actin disrupter, cytochalasin D (Cyto-D, 20 mu M), accelerated the rate of the V-1/2A Shift but decreased the range of the V-1/2G Shift. An F-actin stabilizer, phalloidin (100 mu M), temporarily !for 28.2 +/- 2.2 min, n = 15) prevented the V-1/2A shift but did not influence the V-1/2G shift; The best fit for the V-1/2G-V-1/2A relationship in untreated cells (1,021 data points measured in 51 cells) was a second-degree (2.06) power function. Cytoskeleton-directed agents modified the relationship. In Cyto-D-treated cells, the V-1/2G-V-1/2A relationship was shifted (by 2.5 mV) toward positive V-1/2G On the contrary, a microtubule stabilizer, taxol (100 mu M), shifted the relationship toward negative V-1/2G (by 12.2 mV). We conclude that coupling between availability and activation is modulated by F-actin-based and microtubular cytoskeleton.