Spatial aspects of calcium signalling

The list of biological processes requiring Ca is so disparate that there can hardly be any cellular function that is not influenced directly or indirectly by this ubiquitous second messenger. It is an increase in the free Ca concentration ([Ca],) within stimulated cells that often underlies such fundamentally important processes as contraction, secretion, metabolism, synthesis and cell growth. A complete understanding of the role that Ca plays in controlling many of these processes will however only follow from experiments in which [Ca + ] , can be continuously monitored as the cell is stimulated. For many years this presented a major obstacle to researchers as there was no reliable, simple method of routinely monitoring [Ca];. Indeed, as recently as 1981, [Ca ], had been measured in very few cells other than the giant cells of invertebrates (for example, see Ashley & Ridgway, 1970; Baker et al. 1971). During the last decade however the field has been revolutionized by the introduction of new techniques. These have been both chemical, for example Ca -sensitive fluorescent indicators that can be easily loaded into cells (Tsien et al. 1982; Grynkiewicz et al. 1985), and technological such as digital video imaging microscopy (Poenie et al. 1986). In 1989 it is possible not only to continuously monitor changes in [Ca], with a millisecond time resolution in virtually any living cell, but also to follow the spatial organization of the Ca signal over the same time course.