Using stably expressed fluorescent indicator proteins, we have determined for the first time
the relationship between the free Ca 2+ and Ca 2+-calmodulin concentrations in intact cells.
A similar relationship is obtained when the free Ca 2+ concentration is externally buffered or
when it is transiently increased in response to a Ca 2+-mobilizing agonist. Below a free Ca
2+ concentration of 0.2 μm, no Ca 2+-calmodulin is detectable. A global maximum free Ca
2+-calmodulin concentration of∼ 45 nm is produced when the free Ca 2+ concentration …