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Department of Biophysics, University of Maryland School of Medicine, Baltimore, Maryland 21201, U.S.A.

1. ⁴⁵Ca efflux was examined in Myxicola giant axons injected with ⁴⁵CaCl₂ or various concentrations of ⁴⁵Ca/EGTA buffers. In axons injected with ⁴⁵CaCl₂, the Ca_o-dependent Ca efflux in 1 mM-Ca_o was about half that in 10 mM-Ca_o.

2. Axons injected with ⁴⁵Ca/EGTA buffers consistently showed two types of results: in one type (B type), K for Ca_o activation was less than 1 mM-Ca_o. In the other type of result (A type), there was an additional Ca activation of Ca efflux. This additional efflux exhibited a linear dependence on Ca_o when the Ca_o values ranged between 1 mM-Ca_o and 10 mM-Ca_o.

3. The B-type result remained unchanged when the injected Ca/EGTA concentrations were varied. The A type result, however, changed as a function of Ca/EGTA_i in the following way: (a) at a constant ratio of Ca/EGTA_i = 8/10, the megnitude of the linear component of the Ca_o-activated Ca efflux was reduced by increasing the intracellular concentration of (Ca/EGTA) buffer; and (b) at a ratio Ca/EGTA = 1/10, the linear component of the Ca_o-activated Ca efflux appeared to acquire a slower time response to changes in Ca_o.

4. Na_o acts synergistically with Ca_o to produce the linear component of the Ca-activated Ca efflux seen in the A type result.

5. With axons containing ⁴⁵Ca/EGTA buffers (total EGTA_i = 1 mM), changing the Ca/EGTA ratio by repetitive injections of CaCl₂ did not increase ⁴⁵Ca efflux by as great an amount as would be predicted if Ca_i²⁺ were controlled by the EGTA buffer alone.

6. Ca_i²⁺ (measured by arsenazo III absorbance) is influenced by Ca_o irrespective of the presence of 1 mM-EGTA buffer inside the axon. There was a variability in the sensitivity of Ca_i to Ca_o that resembled the variability found in ⁴⁵Ca efflux measurements.

7. ⁴⁵Ca influx is not affected by the concentration of Ca/EGTA buffer injected into the cell and appears to be only slightly, if at all, affected by increasing ionized Ca_i²⁺ from 0·016 to 0·56 µM in the injection medium.

8. These results are consistent with the interpretation that the Ca efflux system of the Myxicola giant axon, or something controlling it, can exist in more than one state. One of these states, which exhibited a large Ca_o-dependent Ca efflux, may represent axons in which Ca_i is poorly controlled by the natural endogenous Ca buffers.