Abstract
Initial velocities of energy-dependent Ca(++) uptake were measured by stopped-flow and dual-wavelength techniques in mitochondria isolated from hearts of rats, guinea pigs, squirrels, pigeons, and frogs. The rate of Ca(++) uptake by rat heart mitochondria was 0.05 nmol/mg/s at 5 microM Ca(++) and increased sigmoidally to 8 nmol/mg/s at 200 microM Ca(++). A Hill plot of the data yields a straight line with slope n of 2, indicating a cooperativity for Ca(++) transport in cardiac mitochondria. Comparable rates of Ca(++) uptake and sigmoidal plots were obtained with mitochondria from other mammalian hearts. On the other hand, the rates of Ca(++) uptake by frog heart mitochondria were higher at any Ca(++) concentrations. The half-maximal rate of Ca(++) transport was observed at 30, 60, 72, 87, 92 microM Ca(++) for cardiac mitochondria from frog, squirrel, pigeon, guinea pig, and rat, respectively. The sigmoidicity and the high apparent K(m) render mitochondrial Ca(++) uptake slow below 10 microM. At these concentrations the rate of Ca(++) uptake by cardiac mitochondria in vitro and the amount of mitochondria present in the heart are not consistent with the amount of Ca(++) to be sequestered in vivo during heart relaxation. Therefore, it appears that, at least in mammalian hearts, the energy-linked transport of Ca(++) by mitochondria is inadequate for regulating the beat-to-beat Ca(++) cycle. The results obtained and the proposed cooperativity for mitochondrial Ca(++) uptake are discussed in terms of physiological regulation of intracellular Ca(++) homeostasis in cardiac cells.