Figure 3.

Active Ca²⁺ uptake by Golgi vesicles. A, Time course of Ca²⁺ incorporation into Golgi apparatus vesicles measured in the incubation buffer containing 300 nm free Ca²⁺ in the presence (▪) and in the absence (□) of ATP. B, Time course of Ca²⁺ uptake and release by the Ca²⁺ ionophore A23187. ▪, Control; ○, ionophore. Free Ca²⁺ concentration was 210 nm. Values are mean ± se. The arrows indicate the time when the ionophore was added. C, Ca²⁺ dependence of active Ca²⁺ uptake in Golgi apparatus-enriched fraction. Ca²⁺ uptake initial rates were measured at different free Ca²⁺ concentrations that were estimated with the WinMaxC 2.05 computer program (Chris Patton, Hopkins Marine Station, Stanford University, CA). The experimental points were fitted by a nonlinear fitting program (GraphPad Prism 2.0, GraphPad Software, Inc., San Diego). Apparent K_m value of 0.21 μm (pCa 6.68) was obtained from by interpolation in nonlinear fit. Experiments were performed in triplicate. Values are mean ± se.