Fig. 4. A mechanically skinned normal fiber was initially perfused with a solution containing 120 nm Ca2+and 0.1 mm free Mg2+. After a 4-min period of equilibration, 1 mm halothane was rapidly introduced, and the flow was stopped. This resulted in a release of Ca2+from the sarcoplasmic reticulum, which repeated cyclically. Increasing the free [Mg2+] from 0.1 mm to 0.4 mm resulted in a complete cessation of Ca2+release. When the free [Mg2+] was decreased again to 0.1 mm, cyclic sarcoplasmic reticulum Ca2+release resumed.