title: Two different Ca2+ transport systems are associated with plasma and intracellular human platelet membranes.
abstract: Platelet plasma and intracellular membrane fractions were isolated from a mixed membrane fraction after sucrose cushion centrifugation. Their previous identification through biochemical and immunological characterization is now confirmed by sodium dodecyl sulfate electrophoresis of the membrane proteins which reveals a different protein profile. The two associated calcium transport systems showed a different time course and exhibited different oxalate sensitivity. The plasma membranes are not permeable to potassium oxalate but the Ca2+ uptake was stimulated by potassium oxalate in intracellular membranes. We then focused on the study of the plasma membrane-associated Ca2+-activated ATPase which shows the following characteristics: a linearity in the time course until 30 min, an apparent affinity toward calcium of about 10(-7) M without detectable inhibition at higher concentrations, a maximal activity at pH 8, a high ATP requirement because the maximal response was obtained with 200 microM, and a high specificity toward ATP as energy donor. Taken together, these studies indicate the possible involvement of both a plasma membrane and a dense tubular system Ca2+-ATPase in the regulation of Ca2+ concentration in human platelets.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D001693:Biological Transport, Active; D001792:Blood Platelets / Q000378:metabolism*; D002118:Calcium / Q000097:blood*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007425:Intracellular Membranes / Q000378:metabolism*; D009711:Nucleotides / Q000494:pharmacology; D010070:Oxalates / Q000494:pharmacology; D019815:Oxalic Acid
chemical_list: D009711:Nucleotides; D010070:Oxalates; D000255:Adenosine Triphosphate; D019815:Oxalic Acid; D000252:Calcium-Transporting ATPases; D002118:Calcium