title: Rapid calcium release from cardiac sarcoplasmic reticulum vesicles is dependent on Ca2+ and is modulated by Mg2+, adenine nucleotide, and calmodulin.
abstract: A subpopulation of canine cardiac sarcoplasmic reticulum vesicles has been found to contain a "Ca2+ release channel" which mediates the release of intravesicular Ca2+ stores with rates sufficiently rapid to contribute to excitation-contraction coupling in cardiac muscle. 45Ca2+ release behavior of passively and actively loaded vesicles was determined by Millipore filtration and with the use of a rapid quench apparatus using the two Ca2+ channel inhibitors, Mg2+ and ruthenium red. At pH 7.0 and 5-20 microM external Ca2+, cardiac vesicles released half of their 45Ca2+ stores within 20 ms. Ca2+-induced Ca2+ release was inhibited by raising and lowering external Ca2+ concentration, by the addition of Mg2+, and by decreasing the pH. Calmodulin reduced the Ca2+-induced Ca2+ release rate 3-6-fold in a reaction that did not appear to involve a calmodulin-dependent protein kinase. Under various experimental conditions, ATP or the nonhydrolyzable ATP analog, adenosine 5'-(beta, gamma-methylene)triphosphate (AMP-PCP), and caffeine stimulated 45Ca2+ release 2-500-fold. Maximal release rates (t1/2 = 10 ms) were observed in media containing 10 microM Ca2+ and 5 mM AMP-PCP or 10 mM caffeine. An increased external Ca2+ concentration (greater than or equal to 1 mM) was required to optimize the 45Ca2+ efflux rate in the presence of 8 mM Mg2+ and 5 mM AMP-PCP. These results suggest that cardiac sarcoplasmic reticulum contains a ligand-gated Ca2+ channel which is activated by Ca2+, adenine nucleotide, and caffeine, and inhibited by Mg2+, H+, and calmodulin.
mesh_terms: D000227:Adenine Nucleotides / Q000494:pharmacology*; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002147:Calmodulin / Q000494:pharmacology*; D002458:Cell Fractionation; D004285:Dogs; D006863:Hydrogen-Ion Concentration; D007473:Ion Channels / Q000378:metabolism; D007700:Kinetics; D008081:Liposomes / Q000378:metabolism; D008274:Magnesium / Q000494:pharmacology*; D009206:Myocardium / Q000378:metabolism*; D012430:Ruthenium Red / Q000494:pharmacology; D012433:Ryanodine / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
chemical_list: D000227:Adenine Nucleotides; D002147:Calmodulin; D007473:Ion Channels; D008081:Liposomes; D012430:Ruthenium Red; D012433:Ryanodine; D002110:Caffeine; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium