title: Chemical modification of sarcoplasmic reticulum with methylbenzimidate. Stimulation of Ca2+ efflux.
abstract: Treatment of sarcoplasmic reticulum membranes with 12 mM-methylbenzimidate (MBI) for 5 min, in the presence of 5 mM-ATP at pH 8.5, resulted in a 2-3-fold stimulation of ATP hydrolysis and over 90% inhibition of Ca2+ accumulation. This phenomenon was strictly dependent upon the presence of nucleotides with the following order of effectiveness: adenosine 5'-[beta, gamma-imido]triphosphate greater than or equal to ATP greater than UTP greater than ADP greater than AMP. Divalent cations such as Ca2+, Mg2+ and Mn2+, when present during the MBI treatment, prevented both the stimulation of ATPase activity and the inhibition of Ca2+ accumulation. Modification with MBI had no effect on E-P formation from ATP, ADP-ATP exchange, Ca2+ binding or ATP-Pi exchange catalysed by the membranes. Membranes modified with MBI in the presence of ATP and then passively loaded with Ca2+ released about 80% of their Ca2+ content within 3 s. Control membranes released only 3% of their Ca2+ during the same time period. MBI modification inhibited Ca2+ accumulation by proteoliposomes reconstituted with the partially purified ATPase but not with the purified ATPase fraction. These results suggest that MBI in the presence of ATP stimulates Ca2+ release by modifying a protein factor(s) other than the (Ca2+ + Mg2+)-ATPase.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D000578:Amidines / Q000494:pharmacology*; D000818:Animals; D001550:Benzamidines / Q000494:pharmacology*; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D008081:Liposomes / Q000378:metabolism; D008274:Magnesium / Q000494:pharmacology; D009711:Nucleotides / Q000494:pharmacology; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D013268:Stimulation, Chemical
chemical_list: D000578:Amidines; D001550:Benzamidines; D008081:Liposomes; D009711:Nucleotides; D000251:Adenosine Triphosphatases; D008274:Magnesium; D002118:Calcium