Regulation of calcium channel in sarcoplasmic reticulum by calsequestrin.

Gating properties of the Ca2+ channel in sarcoplasmic reticulum (SR) were monitored by measuring the choline permeation of the heavy fraction of SR (HSR) vesicles by the light scattering method. Increase of choline permeation by micromolar Ca2+, which refers to Ca2+ response, was lost when HSR vesicles were incubated overnight with EDTA or EGTA. In parallel, calsequestrin was released from the vesicles. This loss of Ca2+ response could not be inhibited by millimolar Mg2+, but was partially inhibited by submolar KCl. Since it took 3-5 hours to lose the Ca2+ response, calsequestrin may be released from the inside of the vesicles. When HSR vesicles were incorporated into lipid bilayer, open probability of the Ca2+ channel increased when calsequestrin was added to the trans side in the presence of millimolar Ca2+. These results suggest that calsequestrin acts as a regulator of Ca2+ channel in SR membrane.