In 1957, Hodgkin and Keynes performed a simple but seminal experiment, injecting small portions of the recently developed 45 Ca and 42 K into squid axons. Several hours later, they transversely sectioned the axon and examined the distribution of these isotopes. Whereas 42 K had uniformly diffused throughout the axon, 45 Ca had remained at the site of injection (Fig. 1 ), showing that calcium is not freely mobile in the cytoplasm. View larger version: In this window In a new window Download as PowerPoint Slide Fig. 1. Diagrammatic summary of the Hodgkin and Keynes (1957) experiment. Solid line, 45 Ca; dotted line, 42 K. If calcium does move within the cytoplasm, mechanisms other than diffusion are responsible. Two facts can account for the above observations. Calcium binds to many proteins that are attached to the cytoskeleton or to membrane surfaces. Estimates suggest a cytoplasmic-binding capacity per cell of 0.1 to 0.5 m m calcium (Mahlo et al., 1998). Calcium-dependent calcium-ATPases rapidly pump excess calcium into organelles, vesicles, and, in plant cells, out into the cell wall. The vacuole is a major repository of plant-cell intracellular calcium and must make an unusual contribution to cell-calcium signaling. Other important
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