Calcium efflux from sarcoplasmic reticulum microsomes due to oxidation and sulfhydryl-binding agents

Calcium permeability of sarcoplasmic reticulum (SR) microsomes was measured after aging or after exposure to peroxydisulfate or to sulfhydryl-binding agents. Under conditions where the Ca²⁺-ATPase was active, the maximum net release of Ca²⁺ was not significantly different between control and oxidized SR. However, when calcium uptake was prevented by EGTA or apyrase, the Ca²⁺ permeability of oxidized microsomes was 2 to 3 times greater than control of low (10⁻⁹, 10⁻⁷ M) but not high (10⁻⁶ M) levels of external calcium. The observation that vesicles preincubated with 5 mM dithiothreitol loaded up to 3 times as much calcium and had a slightly lower calcium permeability coefficient than control vesicles suggested that suflhydryl oxidation might modulate calcium flux. This hypothesis was tested by exposing to sulfydryl-binding agents:silver, arsenite, and p-chloromercuri-phenylsulfonic acid. Sulfhydryl-binding agents initiated a rapid release of calcium from microsomes, and release was halted by dithiothreitol. Inhibition of calcium transport could not entirely account for the apparent increase in permeability because the calcium permeability of SR treated with sulfhydryl-binding agents was 5 times greater than that of SR exposed to Ca²⁺-ATPase inhibitors. These results suggest that oxidation may increase the calcium permeability of SR. by allowing calcium loss through a channel that can be gated by sulfhydryl oxidation.