Ca2+ and the sarcoplasmic reticulum

The role of the sarcoplasmic reticulum in the regulation of cytosolic calcium is briefly reviewed. A method is described for monitoring calcium efflux from relatively large amounts of SR in a newly designed filtration apparatus using membrane filtration. Two phases of Ca2+ efflux from SR preloaded in the presence of oxalate were observed: an early exponential phase, followed by a phase of constant efflux. Changes in pH from 6.8 to 7.4 resulted in increased efflux. A change in the opposite direction resulted in an initial rapid increase in efflux before the expected decrease occurred. Caffeine (10 and 20 mM) caused increased efflux. Addition of 100 microM EGTA to the buffer resulted in major changes in the exponential phase but not in the phase of constant release, and potentiated the effect of caffeine. The method appears suitable for a variety of pharmacological studies in SR as well as other tissue fractions.