title: Differential effects of mercurial compounds on excitable tissues.
abstract: Sarcoplasmic reticulum (SR), Ca2+ plus Mg2+-ATPase, and Ca2+-ionophore were obtained from white rabbit skeletal muscles. Methylmercury inhibited the Ca2+ plus Mg2+-ATPase and Ca2+-transport but had no effect on the Ca2+-ionophore. Mercuric chloride inhibited all three functions (i.e., ATPase, transport and ionophoric activity). The mechanism of HgCl2 inhibition of the Ca2+-ionophore was by competition with Ca2+ for Ca2+-ionophoric site whereas its inhibition of the enzyme and Ca2+-transport was due to the blockage of essential sulfhydryl (--SH) groups. Ca2+ plus Mg2+-ATPase and Ca2+-transport were more sensitive to methylmercury than to HgCl2. Acetylcholine receptor (AChR) was obtained for the electric organ of T. californica. Methylmercury inhibited the ACh binding to AChR WITH Ki = 5.7 - 10(-6) M. This effect was not due to mercuric ion alone since mercuric chloride up to 10(-4) M did not affect ACh binding to AChR. It is concluded that: the Ca2+ plus Mg2+-ATPase and Ca2+-transport contain --SH groups essential for their activity, and that the two functions are tightly coupled; the Ca2+-ionophore contains no --SH groups essential for its activity; CH3HgCl inhibition of Ca2+ plus Mg2+-ATPase and Ca2+-transport is partly due to its reactivity with --SH groups in hydrophobic environment; the Ca2+-transport is inhibited by HgCl2 through two processes, one which is the blockage of --SH groups and another which is the inhibition of the Ca2+-ionophoric site; and the inhibition of ACh binding to AChR is due to the blockage of --SH groups in hydrophobic environment, which is inaccessible to Hg2+. Our data present for the first time a molecular basis for the myopathy associated with mercurial compounds toxicity.
mesh_terms: D000109:Acetylcholine / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D004553:Electric Conductivity; D004557:Electric Organ / Q000187:drug effects / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects; D005260:Female; D005399:Fishes; D007476:Ionophores / Q000494:pharmacology; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008567:Membranes, Artificial; D008628:Mercury / Q000494:pharmacology*; D008767:Methylmercury Compounds / Q000494:pharmacology*; D008954:Models, Biological; D011485:Protein Binding; D011950:Receptors, Cholinergic / Q000187:drug effects*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*
chemical_list: D007476:Ionophores; D008567:Membranes, Artificial; D008767:Methylmercury Compounds; D011950:Receptors, Cholinergic; D000251:Adenosine Triphosphatases; D008628:Mercury; D008274:Magnesium; D000109:Acetylcholine; D002118:Calcium