Item

Abstract

The intracellular activity of ionized Ca⁺⁺ in the red cell is below 0.4 nmoles/1 (Schatzmann, 1973; Simons, 1982; Lew et al., 1982b) and hence much lower than calculated from intracellular Ca⁺⁺ content divided by red cell volume (Lichtman and Weed, 1973). This indicates that much of the Ca⁺⁺ is bound to the cell membrane (Lichtman and Weed, 1973; La Celle et al., 1973; Porzig and Stoffel, 1978) and intracellular constituents. The latter include the phosphoric acid esters and hemoglobin (Ferreira and Lew, 1977), all of which act as Ca⁺⁺ buffers. The low intracellular Ca⁺⁺ activity is maintained although the membrane is leaky for Ca⁺⁺ (Ferreira and Lew, 1977) and the concentration of free Ca⁺⁺ in blood plasma (about 1200 nmoles/1) exceeds that in cytosol by about four orders of magnitude. The enormous gradient is balanced by a powerful Ca⁺⁺ pump (for review see Schatzmann, 1983). At 37°C, the maximal rate of pumping is about 10 mmoles/1 cells/h. The half saturation concentration (K_1/2) of the pump, and hence one of the essential factors that determines tne steady state Ca⁺⁺ concentration, depends on the conditions inside the cell: the concentration of the energy supplying substrate ATP, the concentration of the activating calmodulin, and the concentration of Mg⁺⁺ (calmodulin activates maximally when 1 Mg⁺⁺ and 3 Ca⁺⁺ ions are complexed). Under physiological conditions, K_1/2 seems to be about 0.3 µmole/1 (Downes and Michel1, 1981).