Steady-state calcium fluxes: membrane versus mitochondrial control of ionized calcium in axoplasm

Squid axons appear in a steady state with respect to ionized Ca when [Ca]o is ca. 3 mM and [Ca]i is ca. 30 nM. A membrane pump energized by the Na gradient across the membrane is capable of maintaining this ratio of ionized Ca if four Na enter per Ca extruded. Empirically, it is noted that if the difference between the electrochemical gradient for Na+ and that for Ca2+ is large and positive, Ca efflux is large; for a large but negative difference in the gradient, Ca influx is large. Net fluxes of Ca into the fiber are induced by Na-free solutions or stimulation. These are buffered in axoplasm principally by a high affinity Ca buffer. Mitochondria apparently do not contain large amounts of stored Ca as CN poisoning does not increase ionized Ca in axoplasm if the seawater is Ca-free.