Radioactive Diisopropyl Fluorophosphate as a Platelet Label: An Evaluation of in Vitro and in Vivo Technics

A technic for the in vitro labeling of human platelets with DFP³² is presented, critically evaluated, and compared to in vivo methods employing DFP³² and to in vitro methods using Cr⁵¹. The initial recovery of platelets labeled in vitro with DFP³² averaged 79 per cent, but the survival curve was characterized by an irreversible initial loss of platelet radioactivity. Experiments in which platelets were simultaneously labeled in vitro with both DFP³² and Cr⁵¹ suggest that this is not due to elution of DFP³². The survival curve of platelets labeled in vivo with DFP³² shows an initial transient reduction in platelet radioactivity. It is suggested that both of these aberrations in initial survival are the result of platelet injury by DFP³². Significant "tailing" was observed in the survival curves obtained with DFP³², and possible explanations of this phenomenon are discussed. DFP³²-labeled platelets circulating after 5 hours apparently survive normally and disappear from the circulation as a rectilinear function over the next 6-8 days. Although both in vitro and in vivo labeling methods employing DFP³² provide a meaningful approximation of platelet lifespan, the initial and terminal aberrations of the survival curves greatly complicate further interpretation. Dextran had no detectable effect on platelet survival, and epinephrine, Mecholyl, and cutaneous vasodilatation did not alter the platelet count or the specific activity of circulating labeling platelets in human subjects. The problem of initial platelet survival and the question of an extravascular or marginal platelet pool is discussed in the light of these data.