title: Fast release of calcium from sarcoplasmic reticulum vesicles monitored by chlortetracycline fluorescence.
abstract: Rapid Ca2+ release rate from sarcoplasmic reticulum vesicles was determined by the stopped flow method in terms of chlortetracycline fluorescence. Intensity of chlortetracycline fluorescence was proportional to the intravesicular free Ca2+ concentration. Ca2+ efflux was activated by extravesicular Ca2+ with an apparent dissociation constant of 25 microM and was inhibited with an inhibition constant of 120 microM in the absence of Mg2+. Caffeine enhanced the Ca2+ release rate by increasing only the affinity of Ca2+ for the activation site. Mg2+ reduced the Ca2+ release rate by competitive binding to the activation site. ATP increased the Ca2+ release rate very much without changing the affinities of Ca2+ for the activation and inhibition sites, i.e., ATP seems to increase the pore radius or number of the Ca2+ channels without affecting the gating mechanism of the channel. These results are consistent with those reported in skinned muscle sarcoplasmic reticulum. The maximum rate of Ca2+ release in the presence of ATP reached 80 s-1. This value is considered to be sufficient to cause muscular contraction.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002458:Cell Fractionation; D002751:Chlortetracycline; D007700:Kinetics; D008433:Mathematics; D008954:Models, Biological; D009132:Muscles / Q000378:metabolism*; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D013050:Spectrometry, Fluorescence / Q000379:methods
chemical_list: D000255:Adenosine Triphosphate; D002118:Calcium; D002751:Chlortetracycline