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. 1980 Sep;306:175–191. doi: 10.1113/jphysiol.1980.sp013390

Calcium efflux from Myxicola giant axons: effects of extracellular calcium and intracellular EGTA

Ronald F Abercrombie 1, Raymond A Sjodin 1
PMCID: PMC1282999  PMID: 6780685

Abstract

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 Cao-dependent Ca efflux in 1 mM-Cao was about half that in 10 mM-Cao.

2. Axons injected with 45Ca/EGTA buffers consistently showed two types of results: in one type (B type), K½ for Cao activation was less than 1 mM-Cao. 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 Cao when the Cao values ranged between 1 mM-Cao and 10 mM-Cao.

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/EGTAi in the following way: (a) at a constant ratio of Ca/EGTAi = 8/10, the megnitude of the linear component of the Cao-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 Cao-activated Ca efflux appeared to acquire a slower time response to changes in Cao.

4. Nao acts synergistically with Cao 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 EGTAi = 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 Cai2+ were controlled by the EGTA buffer alone.

6. Cai2+ (measured by arsenazo III absorbance) is influenced by Cao irrespective of the presence of 1 mM-EGTA buffer inside the axon. There was a variability in the sensitivity of Cai to Cao 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 Cai2+ 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 Cao-dependent Ca efflux, may represent axons in which Cai is poorly controlled by the natural endogenous Ca buffers.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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