title: Effects of sulfhydryl reagents and other inhibitors on Ca2+ transport and inositol trisphosphate-induced Ca2+ release from human platelet membranes.
abstract: The effects of modifiers of Ca2+ uptake and release in sarcoplasmic reticulum were studied in human platelet membranes. AgNO3,p-chloromercuribenzoate (pClHgBzO), N-ethylmaleimide (MalNEt), quercetin, vanadate, A23187, and caffeine all had the same effects on Ca2+ uptake in platelet membranes as had been observed for sarcoplasmic reticulum. These results strengthen our earlier conclusion that the Ca2+-pump proteins from internal human platelet membranes and muscle sarcoplasmic reticulum are very similar in functional properties. The sulfhydryl reagents Ag+ and pClHgBzO elicited rapid release of Ca2+ from platelet membranes in the presence of ATP, whereas MalNEt induced slow release. Quercetin also caused slow release of Ca2+ from platelet membranes in the presence of ATP. The effects of all three sulfhydryl reagents could be reversed by dithiothreitol, and Ag+-induced release was also reversed by ruthenium red. These effects are similar to those observed in sarcoplasmic reticulum, but in contrast caffeine did not induce Ca2+ release. In the absence of ATP, passively loaded platelet membranes did not release Ca2+ when exposed to sulfhydryl reagents. However, AgCl and pClHgBzO inhibited inositol trisphosphate (InsP3)-induced Ca2+ release from platelet membranes and this effect was reversed by dithiothreitol. Ruthenium red also inhibited InsP3-induced release, but ATP was found not to be required for InsP3-mediated release. LiCl enhanced Ca2+ release from platelet membranes. These results demonstrate that the InsP3-gated Ca2+ release channel is a separate entity from the Ca2+-pump and that essential protein sulfhydryls are involved in the release process.
mesh_terms: D001693:Biological Transport, Active / Q000187:drug effects; D001792:Blood Platelets / Q000378:metabolism*; D002110:Caffeine / Q000494:pharmacology; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D002729:Chloromercuribenzoates / Q000494:pharmacology; D004229:Dithiothreitol / Q000494:pharmacology; D005033:Ethylmaleimide / Q000494:pharmacology; D006801:Humans; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000494:pharmacology*; D011794:Quercetin / Q000494:pharmacology; D012835:Silver Nitrate / Q000494:pharmacology; D013403:Sugar Phosphates / Q000494:pharmacology*; D013439:Sulfhydryl Reagents / Q000494:pharmacology*; D014638:Vanadates; D014639:Vanadium / Q000494:pharmacology
chemical_list: D002729:Chloromercuribenzoates; D007295:Inositol Phosphates; D013403:Sugar Phosphates; D013439:Sulfhydryl Reagents; D014639:Vanadium; D000001:Calcimycin; D002110:Caffeine; D014638:Vanadates; D015544:Inositol 1,4,5-Trisphosphate; D012835:Silver Nitrate; D011794:Quercetin; D005033:Ethylmaleimide; D002118:Calcium; D004229:Dithiothreitol