Micromolar concentrations of mercury, silver, and other reagents known to react with sulfhydryl groups are shown to stimulate ATPase activity and inhibit active calcium uptake in sarcoplasmic reticulum vesicles derived from rabbit fast skeletal muscle. These effects are caused by a dramatic increase in the calcium permeability of the sarcoplasmic reticulum. Measurements of Ca2+ permeability were made using both isotopes and by spectrophotometric techniques using the Ca2+ indicator arsenazo HI. Air oxidation of a sulfhydryl group to a disulfide group also leads to-a large increase in the calcium permeability of the sarcoplasmic reticulum. 1526The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.

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The sarcoplasmic reticulum (SR) is an intracellular membrane system found in muscle cells which regulates the concentration of Ca S + ions in muscle cytoplasm. ATPdependent Ca s + pumps actively transport Ca Z + from the cytoplasm across the membrane, where it is stored within the lumen of the SR. When the cell is in the relaxed state, the cytoplasmic Ca Z + concentration is -10-7 M. In response to excitation of the cell at the neuromuscular junction, the normally polarized membrane of the cell depolarizes.Depolarization of the surface membrane and the associated transverse tubular system signals the SR, via a poorly understood mechanism, to release its stored Ca s +. The released Ca S + activates the contractile elements of the cell.