The role of sulfhydryl groups in contraction of vascular smooth muscle.

While high concentrations (50 micronmol/l) of merurial sulfhydryl (SH) reagents p-chloromercuribenzoate (PCMB) and p-chloromercuriphenyl sulfate (PCMPS) caused a nonspecific inhibition of constrictor responses in isolated rabbit ear arteries, lower concentrations had selective effects. At a concentration of 10 micronmol/l both agents abolished responses to epinephrine and reduced responses to barium and calcium, but responses to caffeine were unaffected by PCMB and potentiated by PCMPS. Responses to histamine were little affected by PCMPS, but with PCMB, histamine responses were first reduced but later potentiated. In contrast, ethacrynic acid, a nonmercurial SH inhibitor which gains access to the intracellular space, failed to show any specificity of action and inhibited contractile responses to all these agonists. The results suggest that SH inhibitors whose actions are limited to the cell membrane exert both specific and nonspecific effects. The specific effect may be at SH groups associated with the alpha adrenoceptor while nonspecific effects may involve changes in membrane permeability.