The initial velocities of calcium uptake by rat liver mitochondria.

Initial velocities of energy-dependent Ca++ uptake were measured in rat liver mitochondria by stopped flow techniques and dual wave length spectroscopy in the presence of the Ca++ indicator murexide. The initial rate of energy-dependent Ca++ uptake by mitochondria was 0.1 to 0.3 nmole per s per mg of protein at 5 to 15 pM Ca++, and increased to 8.2 nmoles of Ca++ per s per mg at 200 PM Ca++. When initial rates of Ca++ uptake by mitochondria were plotted against the Ca++ concentration in the medium, a sigmoidal curve was obtained in which halfmaximal rate of Ca++ transport occurred at Ca++ concentration of 55 to 70 PM. A Hill plot of the data yields a straight line with slope, n, of 1.63. Comparable values of Ca++ transport were obtained when Caf+ was added to mitochondria oxidizing either succinate or glutamate and malate. Similar sigmoidicity was shown with Mn++ uptake; however the maximal rate of accumulation was 3.8 nmoles of Mn++ per s per mg of protein and the half-maximal rate was reached at 170 PM Mn++. A model, which takes into account previous and present data, is presented for the uptake of Ca++ and Mn++ in liver mitochondria. The results obtained and the proposed mechanism are discussed in terms of the physiological reguration of cellular [Ca++] homeostasis in liver cells.