Figure 2.

The calcium increase requires extracellular calcium. Cells were placed in nominally calcium-free buffer (not containing EGTA), and α₂M* was added at approximately t = 150 sec. No intracellular calcium increase was observed. In fact, a small decrease was indicated. The calcium-free buffer was washed and replaced with calcium-containing buffer (2 mM) at t = 500 sec. After replacement of calcium, [Ca²⁺]_i levels increased to about 400 nM, similar to levels normally observed after stimulation by α₂M* in calcium-containing buffers. This suggests that α₂M* was able to bind to LRP in the absence of calcium and initiate a calcium-signaling event. The response requires extracellular calcium, but not release of calcium from intracellular stores. The trace is the average of n = 7 cells in a field.