The regulation of Ca2+ concentration in different cells involves two Ca2+ pumps. The presence of such mechanisms in human platelets is still controverted. We then investigated this question by using plasma and intracellular membranes obtained after simultaneous isolation by centrifugation ca 40% sucrose from a mixed 100,000 g membrane fraction.The Ca2+ uptake by the different membrane vesicles has been studied. Both membrane fractions took up Ca2+ and the Ca2+ transport systems exhibited a high

more »

... affinity towards Ca 2+.However, the two associated Ca2+ transport systems showed a different time course and exhibited different oxalate sensitivity. The plasma membranes are not permeable to potassium oxalate, while the Ca2+ uptake was stimulated by potassium oxalate in intracellular membranes.Two Ca2+ activated ATPase activities are associated with the isolated membrane fractions and appeared different for the following parameters : 1) a different time course of the two enzyme activities; 2)a similar apparent affinity towards Ca2+ (10−7 M), though inhibition of the Ca2+ ATPase activity was only observed in intracellular membranes at 10−6 M Ca2+ ; 3)a different pH dependence with a maximum at pH 7 for the enzyme of intracellular membranes and pH 8 for the enzyme of plasma membranes; 4)a 10 fold difference in the ATP requirement of the enzymes, thus the maximal response was obtained with 20 uM for the intracellular membrane enzyme and with 200 uM for the plasma membrane enzyme ; 5) a different affinity for various nucleotides as energy donors with a higher specificity of the plasma membrane enzyme towards ATP ; 6) a different vanadate inhibition-dose reponse which did not exceed 60% for the plasma enzyme while it reached 100% for the intracellular enzyme.Taken together, these studies agree with the possible role of both a plasma membrane and a dense tubular system Ca2+ -ATPases in the regulation of Ca2+ concentration in human platelets.