1. Changes in choline concentration of the blood after injections or infusions of choline were studied in cats anaesthetized with chloralose.2. Single I.V. injections of choline 10-100 mumole/kg produced arterial plasma levels 1 min later corresponding to an apparent volume of initial distribution of 430 ml./kg. The concentration then declined rapidly (half-time, 1-2 min), with a later slower decline after large doses.3. Infusions of choline at a rate of 0.8 mumole/kg.min or greater produced steady rises in plasma level, corresponding to a clearance of 28.6 ml. plasma/kg.min. The half time of rate of approach to steady state was 7 min or less. Infusions at rates of 0.40 mumole/kg.min or less produced much smaller or negligible rises, suggesting mechanisms for disposal which were saturated at higher concentration. At low rates, little infused choline appeared in urine. At the end of an infusion, the plasma choline level usually fell without delay.4. Portal blood contained about 50% of the arterial level, renal venous blood 15-70%, caval blood 30-60%, and amniotic fluid 2.5%. Occlusion of renal coeliac or mesenteric arteries raised plasma choline, but relatively rapid choline removal still occurred in the eviscerate animal.5. After infusions of [methyl-(14)C]choline, the level of radioactivity retained in the circulation amounted to only a few per cent of the total dose infused. At low rates of infusion (0.0125-0.1 mumole/kg.min) the radioactivity represented only a small fraction of bio-assayable choline; but at 0.40 mumole/kg.min it came to exceed the concentration of free choline, indicating metabolic conversion. Only traces of (14)C were found in expired air, and only 1-1.5% of total infused radioactivity in the urine. After infusion of 150 mumole over 3 hr, high levels of radioactivity were found in liver, kidney, lung, brain and heart, but levels in muscle and spleen were comparable to that of blood.6. It was concluded that choline is rapidly lost from the blood, that the abdominal viscera, liver, kidney and lung are important extraction sites, that some partial metabolism occurs, the metabolites also being rapidly lost from blood, and that it is probable that choline lost to the tissues becomes bound in some form.
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