Kinetic studies of calcium-induced calcium release in cardiac sarcoplasmic reticulum vesicles.
Fast Ca(2+) release kinetics were measured in cardiac sarcoplasmic reticulum vesicles actively loaded with Ca(2+). Release was induced in solutions containing 1.2 mM free ATP and variable free [Ca(2+)] and [Mg(2+)]. Release rate constants (k) were 10-fold higher at pCa 6 than at pCa 5 whereas Ryanodine binding was highest at pCa < or =5. These results suggest that channels respond differently when exposed to sudden [Ca(2+)] changes than when exposed to Ca(2+) for longer periods. Vesicles with severalfold different luminal calcium contents exhibited double exponential release kinetics at pCa 6, suggesting that channels undergo time-dependent activity changes. Addition of Mg(2+) produced a marked inhibition of release kinetics at pCa 6 (K(0.5) = 63 microM) but not at pCa 5. Coexistence of calcium activation and inhibition sites with equally fast binding kinetics is proposed to explain this behavior. Thimerosal activated release kinetics at pCa 5 at all [Mg(2+)] tested and increased at pCa 6 the K(0.5) for Mg(2+) inhibition, from 63 microM to 136 microM. We discuss the possible relevance of these results, which suggest release through RyR2 channels is subject to fast regulation by Ca(2+) and Mg(2+) followed by time-dependent regulation, to the physiological mechanisms of cardiac channel opening and closing.
Sánchez
Gina
G
Instituto de Ciencias Biomédicas, Facultad de Medicina, Universidad de Chile, Casilla 70005, Santiago 7, Chile.
Hidalgo
Cecilia
C
Donoso
Paulina
P
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology*; D046508:Culture Techniques; D004285:Dogs; D006321:Heart / Q000187:drug effects; D008274:Magnesium / Q000494:pharmacology*; D009206:Myocardium / Q000378:metabolism; D011485:Protein Binding; D012433:Ryanodine / Q000378:metabolism*; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D013849:Thimerosal / Q000494:pharmacology*
D019837:Ryanodine Receptor Calcium Release Channel; D012433:Ryanodine; D013849:Thimerosal; D008274:Magnesium; D002118:Calcium