Metabolism and Function of Human Platelets Washed by Albumin Density Gradient Separation

A method for washing platelets by albumin density gradient separation, originally designed for the study of platelet coagulant activities, has been modified for platelet aggregation and metabolic studies. Platelets are sedimented into a continuous density gradient of isosmolar albumin containing apyrase to protect them from clumping and physical injury and are resuspended in calcium-free Tyrode's solution. The mean recovery of platelets after two separations relative to plateletrich plasma (PRP) was 90.3%. When small amounts of plasma were added to washed platelet suspensions, aggregation and release of ¹⁴C]5-hydroxytryptamine (5HT) in response to adenosine diphosphate (ADP) or 5HT were similar to results obtained with PRP. When fibrinogen was substituted for plasma, ADP-induced aggregation occurred but was feeble. Without added plasma or fibrinogen, platelets were refractory to ADP and insensitive to the cyclic endoperoxide analogue U44619. When both ADP and U44619 were added simultaneously, in low concentrations, to washed platelets without added plasma or fibrinogen, aggregation occurred immediately. Washed platelets were not aggregated by adrenaline, which potentiated ADP-induced aggregation. Several biochemical measurements which are sensitive indicators of cellular damage were normal in washed platelets, including ¹⁴C]adenine uptake, adenylate energy charge, hypoxanthine formation and the response of adenylate cyclase to stimulation by PGE₁ or PGD₂. Platelet coagulant activities were not made available and heparin-neutralizing activity (HNA) was not spontaneously released by the washing procedure, but the washed platelets responded normally to appropriate agents by developing coagulant activities and releasing HNA. The ultrastructure of washed platelets was similar to those in control PRP. Inclusion of apyrase in the first albumin gradient had a beneficial effect on platelet morphology, aggregation and metabolism, but washing at 37deg;C compared with 25deg;C did not. Albumin density gradient separation is a useful method for isolating platelets for aggregation and metabolic studies.