CYTOSKELETON MODULATES GATING OF VOLTAGE-DEPENDENT SODIUM-CHANNEL IN HEART

To investigate the role of the cytoskeleton in cardiac Na+ channel gat ing, the action of cytochalasin D (Cyto-D), an agent that interferes w ith actin polymerization, was studied by whole cell voltage clamp and cell-attached and inside-out patches from rat and rabbit ventricular c ardiac myocytes. Cyto-D (20-40 mu M) reduced whole cell peak Na+ curre nt by 20% within 12 min and slowed current decay without affecting ste ady-state voltage-dependent availability or recovery from inactivation . Brief treatments ( < 10-15 min) of cell-attached patches by Cyto-D ( 20 mu M) in the bath induced short bursts of Na+ channel openings and prolonged decays of ensemble-averaged currents. Bursting of the Na+ ch annel was more pronounced when the cell suspension was pretreated with Cyto-D (20 mu M) for Ih before seal formation. Application of Cyto-D on the cytoplasmic side of inside-out patches resulted in more dramati c gating changes. Peak open probability was reduced by >50% within 20 min, and long bursts of openings occurred. Washout of Cyto-D did not r estore ensemble-averaged current amplitude, but burst duration decreas ed toward control values. Cyto-D also induced an additional slower com ponent to open and closed times. These results suggest that Cyto-D, th rough effects on cytoskeleton, induced cardiac Na+ channels to enter a mode characterized by a lower peak open probability but a greater per sistent activity as if the inactivation rate was slowed. The cytoskele ton, in addition to localizing integral membrane proteins, apparently also plays a role in regulating specific detailed functions of integra l membrane proteins such as the gating of Na+ channels.