The Initial Velocities of Calcium Uptake by Rat Liver Mitochondria

Abstract 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 µm Ca++, and increased to 8.2 nmoles of Ca++ per s per mg at 200 µm 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 half-maximal rate of Ca++ transport occurred at Ca++ concentration of 55 to 70 µm. A Hill plot of the data yields a straight line with slope, n, of 1.63. Comparable values of Ca++ transport were obtained when Ca++ 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 µm 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 regulation of cellular Ca++ homeostasis in liver cells.