F-actin modulates swelling-activated chloride current in cultured chick cardiac myocytes

The integrity of F-actin and its association with the activation of a Cl- current (I-Cl) in cultured chick cardiac myocytes subjected to hyposmotic challenge were monitored by whole cell patch clamp and fluorescence confocal microscopy. Disruption of F-actin by 25 mu M cytochalasin B augmented hyposmotic cell swelling by 51% (from a relative volume of 1.54 +/- 0.10 in control to 2.33 +/- 0.21), whereas stabilization of F-actin by 20 mu M phalloidin attenuated swelling by 15% (relative volume of 1.31 +/- 0.05). Trace fluorochrome-labeled (fluorescein isothiocyanate or tetramethylrhodamine isothiocyanate) phalloidin revealed an intact F-actin conformation in control cells under hyposmotic conditions despite the considerable changes in cell volume. Sarcoplasmic F-actin was very disorganized and occurred only randomly beneath the sarcolemma in cells treated with cytochalasin B, whereas no changes in F-actin distribution occurred under either isosmotic or hyposmotic conditions in cells treated with phalloidin. Swelling-activated Icl (68.0 +/- 6.0 pA/pF at + 60 mV) was suppressed by both cytochalasin B (22.7 +/- 5.1 pA/pF) and phalloidin(22.5 +/- 3.5 pA/pF). On the basis of these results, we suggest that swelling of cardiac myocytes initiates dynamic changes in the cytoarchitecture of F-actin, which may be involved in the volume transduction processes associated with activation of I-Cl.