Trace doses of [3H]serotonin ([3H]5-HT) administered intravenously to mice accumulated rapidly in the lungs and were subsequently retained unchanged for long periods of time. Although maximal [3H]5-HT levels achieved in animals killed by stunning were approximately three times higher than after decapitation, the plasma decay curves, beginning 12 hr after administration, showed a similar first-order monophasic decay with half-lives (T 1 2) of 108-120 hr). Examination of lung extracts obtained from mice sacrificed at time points ranging from 15 min to 1 week after [3H]5-HT injection revealed the presence of small quantities of a single metabolite, 5-hydroxyindole-3-acetic acid. No difference in [3H]5-HT levels was observed in platelet "rich" and platelet "poor" plasma fractions when the number of platelets in the two fractions varied from 7.5 to 1, respectively. Reserpine was shown to markedly deplete pulmonary [3H]5-HT levels, while guanethidine was without effect.
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