Mouse Odf2 cDNAs consist of evolutionary conserved as well as highly variable sequences and encode outer dense fiber proteins of the sperm tail |
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Authors: | S Hoyer-Fender C Petersen H Brohmann K Rhee DJ Wolgemuth |
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Affiliation: | Medical Department, Brookhaven National Laboratory, Upton, New York 11973, USA. |
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Abstract: | The importance of rebinding to receptors in influencing the kinetics of in vivo binding of PET and SPECT radiotracers was evaluated by examining the binding of a high-affinity D1 receptor radiotracer, 3H]SCH 23390, in tissue homogenates, living brain slices, and in vivo. In rat striatal homogenates, 3H]SCH 23390 binding reached equilibrium with a half-time of 6 min. By contrast, in striatal brain slices incubated in 3H] SCH 23390, the radioactivity levels in the slice increased in a linear fashion over the 4-h incubation, with no indication of an approach to equilibrium at the termination of the experiment. In in vivo experiments, 3H]SCH 23390 was given as a slow intravenous infusion to mice, using a paradigm that kept the plasma concentration at a constant level. Under these conditions, striatal 3H] SCH 23390 levels increased in a linear fashion over the 4-h infusion period, similar to what was observed in the brain slices, and as in the slices there was no indication of approach to a steady state. However, when given instead as a single-bolus intravenous dose, the striatal 3H]SCH 23390 levels reached a peak only 15 min after injection. Calculations based on the slice experiments, in which the blood-brain barrier is absent, suggested that the rate-limiting step accounting for the failure of 3H]SCH 23390 levels to reach equilibrium was its hindered diffusion as a result of repeated rebinding to receptors. This phenomenon may also be important in vivo and should be considered as a factor in determining the time-course of binding of radiotracers in PET and SPECT experiments where either the receptor density or radiotracer affinity is high. |
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