title: A model for the uptake and release of Ca2+ by sarcoplasmic reticulum.
abstract: On addition of ATP to vesicles derived from the sarcoplasmic reticulum (SR) of skeletal muscle, Ca2+ is accumulated from the external medium. Following uptake, spontaneous release of Ca2+ occurs in the presence or in the absence of ATP. These processes of Ca2+ uptake and release were simulated by using the models derived for ATPase activity [Gould, East, Froud, McWhirter, Stefanova & Lee (1986) Biochem. J. 237, 217-227; Stefanova, Napier, East & Lee (1987) Biochem. J. 245, 723-730] and for Ca2+ release from passively loaded vesicles [McWhirter, Gould, East & Lee (1987) Biochem. J. 245, 713-722]. The simulations are consistent with measurements of the effects of pH, K+, Ca2+ and Mg2+ on uptake and release of Ca2+. The increase in maximal Ca2+ accumulation observed in the presence of maleate is explained in terms of complexing of Ca2+ and maleate within the SR. The calculated concentration of ADP generated by hydrolysis of ATP has a large effect on the simulations. The effects of an ATP-regenerating system on the measured Ca2+ uptake is explained in terms of both removal of ADP and precipitation of Ca3(PO4)2 within the vesicles. It is concluded that both the process of Ca2+ uptake and the process of Ca2+ release seen with SR vesicles can be interpreted quantitatively in terms solely of the properties of the Ca2+ + Mg2+-activated ATPase.
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000494:pharmacology; D008298:Maleates / Q000378:metabolism; D008954:Models, Biological; D011188:Potassium / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*
chemical_list: D008298:Maleates; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; C030272:maleic acid; D000251:Adenosine Triphosphatases; D008274:Magnesium; D011188:Potassium; D002118:Calcium