Distribution and metabolism of intravenously administered choline[methyl- 3-H] and synthesis in vivo of acetylcholine in various tissues of guinea pigs.

The biosynthesis of acetylcholine and the fate of intravenously administered choline [methyl- 3-H] were studied in guinea pigs anesthetized with pentobarbital. Choline and acetylcholine were isolated by paper electrophoresis and estimated by use of a specific enzymatic (choline kinase) - radioisotopic assay. The concentration of acetylcholine ranged from 25.5 to 1.1 nmol/g in the following tissues (in order of decreasing concentration): duodenum, corpus striatum, stomach, cerebral cortex, spinal cord, abdominal fat, submaxillary gland, kidney, adrenal gland, spleen, liver, lung, heart and diaphragm. Choline [methyl- 3-H] was converted in the tissues to acetylcholine within 3 minutes after intravenous administration of the precursor. Virtually all the radioactivity in plasma at that time was present as free choline, suggesting that free choline from plasma is the immediate precursor for acetylcholine synthesized in the tissues cited. The concentration of free choline in tissues ranged from 344 nmol/g in adrenals to 40 nmol/g in heart, while that in plasma was 15 nmol/g. The initial half-life of choline in plasma, estimated from the rate of disappearance of choline after intravenous administration of either a tracer dose of choline [methyl- 3-H] (0.031 mumol/kg) or a high dose of choline chloride (200 mumol/kg), was less than 1 minute. This rapid removal of choline from plasma resulted from uptake (or binding) by tissues, with kidney and liver removing about 50% of the administered dose of choline [methyl- 3-H] within 3 minutes after its administration. Uptake of choline occurred in all tissues cited above, but there was a 20-fold difference in the uptake by the most active tissues (kidney and adrenals), as compared to that of the least active (central nervous system). Within 60 minutes after administration of choline [methyl- 3-H], most of the radioactive choline taken up by tissues had been converted to organic-soluble metabolites and to water-soluble metabolites that behaved like either phosphorylcholine or betaine during paper electrophoresis and chromatography. Betaine was the principal metabolite of choline in plasma. Radioactivity was excreted slowly into urine, which contained primarily free choline, betaine and a large amount of an unidentified metabolite. These findings indicate that the principal mechanism for the rapid removal of choline from plasma is uptake into tissues followed by metabolism.