Abstract
1. Experiments were done on frog sartorius muscles to study the effects and mechanisms of action of the -SH inhibitors, p-chloromercuribenzoic acid (PCMB) and p-chloromercuribenzenesulphonic acid (PCMBS).2. Both organomercurials produce a depolarization of the surface membrane which is associated with a period of asynchronous twitching and followed by inexcitability.3. Only PCMB produces a unique fractionation of the electrically evoked twitch into an initial rapid and later slow phase.4. PCMB and PCMBS increase the rate of (45)Ca efflux from whole muscle. Ethylenediamine tetraacetic acid (EDTA, 5 mM) causes only limited antagonism of the enhancement of (45)Ca efflux produced by PCMB whereas it completely antagonizes this same effect of PCMBS. EDTA selectively removes superficial calcium without penetrating into the intracellular space.5. The results suggest that PCMB inhibits -SH groups in the terminal cisternae causing a fractionation of the twitch. PCMBS acts primarily at surface sites with limited access to the cisternae and sarcoplasmic reticulum.