L-type calcium channel activity in osteoblast cells is regulated by the actin cytoskeleton independent of protein trafficking

Voltage-dependent L-type calcium channels (VDCC) play important roles in many cellular processes. The interaction of the actin cytoskeleton with the channel in nonexcitable cells is less well understood. We performed whole-cell patch-clamp surface biotinylation and calcium imaging on different osteoblast cells to determine channel kinetics, amplitude, surface abundance, and intracellular calcium, respectively. Patch-clamp studies showed that actin polymerization by phalloidin increased the peak current density of I(Ca), whereas actin depolymerization by cytochalasin D (CD) significantly decreased the current amplitude. This result is consistent with calcium imaging, which showed that CD significantly decreased Bay K8644-induced intracellular calcium increase. Surface biotinylation studies showed that CD is not able to affect the surface expression of the pore-forming subunit alpha(1C). Interestingly, application of CD caused a significantly negative shift in the steady-state inactivation kinetics of I(Ca). There were decreases in the voltage at half-maximal inactivation that changed in a dose-dependent manner. CD also reduced the effect of activated vitamin D3 (1 alpha,25-D3) on VDCC and intracellular calcium. We conclude that in osteoblasts the actin cytoskeleton affects alpha(1C) by altering the channel kinetic properties, instead of changing the surface expression, and it is able to regulate 1 alpha,25-D3 signaling through VDCC. Our study provides a new insight into calcium regulation in osteoblasts, which are essential in many physiological functions of this cell.