Effects of sulphydryl inhibitors on frog sartorius muscle: N-ethylmaleimide.

1. The characteristic action of the -SH inhibitor, N-ethylmaleimide (NEM), is muscle rigour. The dose-response curve indicates a biphasic effect with maximum rigour tension produced by 1.0 mM NEM; beyond 1.0 mM there was an inverse relationship between dose and response.2. NEM produces a membrane depolarization unrelated to rigour development.3. NEM causes a sustained increase in (45)Ca efflux from whole muscle. Pretreatment of a muscle with ethylenediamine tetra-acetic acid (EDTA, 5 mM) to remove membrane calcium does not alter the NEM induced (45)Ca efflux.4. It is suggested that the primary site of NEM action is inhibition of calcium uptake by the sarcoplasmic reticulum thereby producing rigour. At concentrations above 1.0 mM, NEM may affect the myofilaments.