Continuous measurement of calcium influx in mammalian nonmyelinated nerve fibers: effects of Nao, Cao, and electrical activity.

A new technique for continuous monitoring of the cellular calcium was developed and used for studying the effects of external and internal Na (Nao and Nai), external Ca (Cao), Ca ionophore A23187, and electrical activity on membrane-bound and intracellular Ca in mammalian nonmyelinated nerve fibers. Increasing Cao increased both the membrane-bound and the intracellular Ca. Lowering Nao increased the membrane-bound fraction of Ca indicating that lack of Nao enhanced the capacity of the plasma membrane to bind Ca, and produced an increase of the internal Ca pool. Increasing Nai by treatment with ouabain enhanced the Ca inflow in both, the presence and absence of Nao, presumably by stimulating the Cao/Nai exchange. The Ca ionophore A23187 produced a large and irreversible increase in the intracellular Ca without affecting the membrane-bound fraction. On the other hand, electrical activity, which is known to produce a large increase of the total Ca in squid axon, had no measurable effect on the total calcium content in our preparation. It is concluded that in mammalian nerve fibers a Ca load by exposition to Na-free solution or to A23187 produces an accumulation of Ca into the intracellular Ca stores, whereas during electrical activity the membrane-associated extrusion mechanisms are able to maintain the intracellular Ca2+ below the threshold for intracellular sequestration. Furthermore, the results indicate that the intracellular sequestration mechanisms are dependent on the internal concentration of Na.