Disruption of filamentous actin diminishes hormonally evoked Ca²⁺ responses in rat liver

Previous studies have suggested a role for the actin cytoskeleton in hormonally evoked Ca²⁺ signaling in the liver. Here, we present evidence supporting a connection between filamentous actin (F-actin) organization and the ability of vasopressin and glucagon to increase cytosolic free-Ca²⁺ ([Ca²⁺](i)) levels. F-actin was disrupted in hepatic cells by perfusion of rat liver with cytochalasin D. Epifluorescence microscopy of subsequently isolated cells showed reduced cortical fluorescent phalloidin staining in cytochalasin D-treated liver cells. Cytochalasin D pretreatment of liver cells reduced the vasopressin-stimulated elevation of [Ca²⁺](i) by 60% and of glucagon by 50%. Experiments performed on cytochalasin D-treated cells using Mn²⁺ as an indicator of Ca²⁺ influx quenched fura-2 fluorescence signals following vasopressin administration. This indicates that a structurally intact cortical F-actin web is not a prerequisite for the influx of calcium. Therefore, the attenuation of the increase in cytosolic calcium observed in cytochalasin D-treated liver cells was likely caused either by the depletion of the calcium store by treatment with cytochalasin D or by the need for an intact cytoskeletal structure for its release. Because the resting level of calcium did not change in cells exposed to cytochalasin D, the latter is likely. The reduced [Ca²⁺](i) response may be the mechanism by which cytochalasin D pretreatment inhibits vasopressin-induced metabolic effects. Cytochalasin D pretreatment also decreased the ability of glucagon to stimulate gluconeogenesis and reduced the stimulation of O₂ uptake usually observed following glucagon administration. In conclusion, these results suggest that the hormonal elevation of [Ca²⁺](i) and resultant activation of specific metabolic pathways require normal F-actin organization.