Quench-flow measurements of initial rates of Ca2+ accumulation by isolated cardiac sarcoplasmic reticulum

Using a quench-flow technique, the initial velocity of Ca2+ accumulation by isolated cardiac sarcoplasmic reticulum was estimated at free Ca2+ ion concentrations in the range encountered in the myoplasm during the cardiac contraction cycle. With cardiac microsomes exhibiting a Ca2+ accumulative capacity of 25.6 nmol Ca2+/mg protein, initial rates were found to increase from 3.7 to 33.4 nmol Ca2+/mg protein/sec, when the free Ca2+ ion concentration was raised from 0.2 to 18.0 muM. Preincubation of the cardiac microsomes with a party purified soluble cardiac protein kinase, MgATP, and cAMP led to a significant increase in the initial Ca2+ accumulation rate.