Rate of Calcium Binding and Uptake in Normal Animal and Failing Human Cardiac Muscle: MEMBRANE VESICLES (RELAXING SYSTEM) AND MITOCHONDRIA

Cardiac relaxing system (CRS) and mitochondria were isolated by a modified rapid procedure from human, rabbit, and dog. Relaxing system (sarcoplasmic reticulum) was also isolated from white and red skeletal muscles for comparative purposes. Rapid kinetic measurements were made with a dual-beam spectrophotometric assay procedure. Maximum calcium binding (absence of oxalate), expressed as nmoles calcium/mg protein in 5 minutes at 25°C, for rabbit heart and red and white skeletal muscles were approximately 40, 58 and 170, respectively. The calcium binding constant for rabbit CRS was 2 x 10 6 M -1 . The estimated initial binding rates (nmoles calcium/mg protein/minute) of cardiac, white and red skeletal muscle relaxing systems were 256, 1440 and 182, respectively. The rate constant (M -1 sec -1 ) of CRS was about 2400, which falls between white and red skeletal muscle preparations. Human cardiac muscle from recipients obtained at the time of transplantation yielded CRS with slower rates of accumulation of calcium and very little calcium release, compared to normal animal heart preparations. ATP was the most effective of four nucleoside triphosphates in supporting calcium binding and uptake. Calcium binding and release and calcium uptake of CRS were temperature-sensitive. The energies of activation of binding and uptake were 10.5 kcal mole -1 and 22.5 kcal mole -1 , respectively. Under specific conditions, heart mitochondria accumulated calcium at a rate faster than CRS. Mitochondria could also release accumulated calcium.