Initial calcium binding rates of canine cardiac relaxing system (sarcoplasmic reticulum fragments) determined by stopped-flow spectrophotometry

Abstract

Calcium binding by cardiac relaxing system (sarcoplasmic reticulum fragments) from canine ventricle was measured by a dual-beam spectrophotometric technique with manual and with stopped-flow mixing. The ATP-dependent equilibrium Ca ++ binding level was 68.3 ± 3.2 nmole/mg protein. The initial Ca ++ binding was 19 nmole/mg protein within 150 msec at 25°; the reaction exhibited a Q 10 of approximately 2. It is concluded that the cardiac sarcoplasmic reticulum has the binding capacity and sufficient initial rates of Ca ++ binding to effect relaxation in vivo .