Calcium Transport and Regulation in Nerve Fibers

For those interested in Ca-mediated processes in cell biology, the key issue is to understand how transitory changes in the intracellular concentration of ionized Ca (Caf+) occur in resporise to a specific stimulus. This is not a simple phenomenon. It is the r,esult of the interplay of various mechanisms (53): fluxes across the plasma membrane (38), passive buffering by chemical compounds with diverse characters and capacities (24), and metabolically active sequestering by intracellular organelles (45) or specialized structures (46). This review will be a critical appraisal of the movements of Ca in and out of squid axons with emphasis on the effects of changes in membrane potential and the resulting level of Caf+ observed in the cytoplasm. For other aspects of Ca regulation the reader is referred to more extensive reviews that have appeared recently (4, 16, 19, 65, 67, 72, 73). The choice of the squid giant axon as an example of nerve fiber is not arbitrary. The giant axon has a relatively simple internal organization and is very adequate for experimental manipulation. But what is more important is that the results obtained with squid axons are not peculiar to cephalopods. Indeed, there is mounting evidence that the results are valid for nerve cells in general.