Calcium-binding rate and capacity of cardiac sarcoplasmic reticulum ☆

Abstract A new procedure for total extraction of the sarcoplasmic reticulum membranes from heart mucle utilizes repeated mild homogenization followed by differential centrifugation of the homogenates. Sarcoplasmic reticulum fractions isolated from 1 g of tissue are capable of binding up to 76 nmol Ca 2+ in 1 s. The rate of this energy-dependent calcium binding is non-linear at 37°C. Compared to the amount bound in 1 s, one third and half of this calcium are bound in 100 and 200 ms, respectively. At physiological pH, the optimal ionized Ca 2+ concentration required for the function of the calcium pump is close to 3 μ m . The levels of phosphoprotein formed in Ca 2+ -ATPase reaction were used to estimate the total content of the sarcoplasmic reticulum in the heart. It was found that 1 g of pigeon and guinea-pig heart muscles contains 3.2 and 2.1 mg of reticular protein, respectively. It is concluded that calcium binding by the sarcoplasmic reticulum is sufficient to provide the relaxation of the normally working heart.