Uptake and release of 45Ca by Myxicola axoplasm.

The binding and release of 45Ca by axoplasm isolated from Myxicola giant axons were examined. Two distinct components of binding were observed, one requiring ATP and one not requiring ATP. The ATP-dependent binding was largely prevented by the addition of mitochondrial inhibitors, whereas the ATP-independent component was unaffected by these inhibitors. The ATP-independent binding accounted for roughly two-thirds of the total 45Ca uptake in solutions containing an ionized (Ca 2+) = 0.54 #M and was the major focus of this investigation. This fraction of bound 45Ca was released from the axoplasm at a rate that increased with increasing concentrations of Ca 2+ in the incubation fluid. The ions Cd 2+ and Mn 2+ were also able to increase 45Ca effiux from the sample, but Co 2+, Ni 2+, Mg z+, Sr 2+, and Ba z+ had no effect. The concentration-response curves relating the 4SCa efflux rate coefficients to the concentration of Ca 2+, Cd 2+, or Mn 2+ in the bathing solution were S-shaped. The maximum rate of effiux elicited by one of these divalent ions could not be exceeded by adding a saturating concentration of a second ion. Increasing EGTA concentration in the bath medium from 100 to 200 #M did not increase 45Ca effiux; yet increasing the concentration of the EGTA buffer in the uptake medium from 100 to 200/.tM and keeping ionized Ca 2+ constant caused more 4~Ca to be bound by the axoplasm. These results suggest the existence of high- affinity, ATP-independent binding sites for 4SCa in Myxicola axoplasm that compete favorably with 100 #M EGTA. The 45Ca effiux results are interpreted in terms of endogenous sites that interact with Ca 2+, Cd 2+, or Mn 2+.