The effects of volume change and cytoskeleton manipulation on the Cl- channels in human embryonic skeletal muscle cells were studied. Trypsination, used for production of myoballs, changes the channel properties only a little. When the external osmolarity was reduced from 300 to 270 mosmol/l, the specific Cl- conductance, gCl, (at -80 mV) of myoballs increased from 5.1 +/- 1.9 to 30.4 +/- 12.2 microS/cm2 (SD; n = 6) within 15 min. Concomitantly, the kinetics of Cl- currents, elicited by clamping the membrane potential from a negative to positive values, changed from activation and subsequent slow inactivation to instantaneous activation with fast inactivation. G protein activation, protein kinase action or [Ca2+]i elevation seemed not to be involved in these effects. Similar changes were produced in the absence of a transmembrane osmotic gradient by 500 nM intracellular cytochalasin D (gCl = 34.3 +/- 10.3 microS/cm2; n = 6) or 12.5 microM colchicine (gCl = 15.4 +/- 1.4 microS/cm2; n = 5). When the external osmolarity was increased to 418 mosmol/l, 1 microM cytochalasin D did not affect gCl. In four of six cell-attached patches the open probability of the intermediate Cl- channel was increased after reduction of the bath osmolarity. In inside-out patches, the drugs increased the open probability of the channels. It is concluded that the Cl- channels are under control of the cytoskeleton.