Responses of Ca2+-binding Proteins to Localized, Transient Changes in Intracellular [Ca2+]

In smooth muscle cells, various transient, localized [Ca2+] changes have been observed that are thought to regulate cell function without necessarily inducing contraction. Although a great deal of effort has been put into detecting these transients and elucidating the mechanisms involved in their generation, the extent to which these transient Ca2+ signals interact with intracellular Ca2+-binding molecules remains relatively unknown. To understand how the spatial and temporal characteristics of an intracellular Ca2+ signal influence its interaction with Ca2+-binding proteins, mathematical models of Ca2+ diffusion and regulation in smooth muscle cells were used to study Ca2+ binding to prototypical proteins with one or two Ca2+-binding sites. Simulations with the models: (1) demonstrate the extent to which the rate constants for Ca2+-binding to proteins and the spatial and temporal characteristics of different Ca2+ transients influence the magnitude and time course of the responses of these proteins to the transients; (2) predict significant differences in the responses of proteins with one or two Ca2+-binding sites to individual Ca2+ transients and to trains of transients; (3) demonstrate how the kinetic characteristics determine the fidelity with which the responses of Ca2+-sensitive molecules reflect the magnitude and time course of transient Ca2+ signals. Overall, this work demonstrates the clear need for complete information about the kinetics of Ca2+ binding for determining how well Ca2+-binding molecules respond to different types of Ca2+ signals. These results have important implications when considering the possible modulation of Ca2+- and Ca2+/calmodulin-dependent proteins by localized intracellular Ca2+ transients in smooth muscle cells and, more generally, in other cell types.