A sensitive and specific radioimmunoassay was applied to the determination of acetylcholine (ACh) in plasma. The concentration of ACh in plasma sampled from 32 young women was 456.1 ± 53.1 (mean ± S.E.M.) pg/ml. No significant correlations were observed between plasma concentration of ACh and acetylcholinesterase (AChE) activity, or gonadal hormones. These data demonstrate that an amount of ACh measurable by radioimmunoassay is present in plasma and plasma ACh is not regulated by AChE activity and the menstrual cycle in young women. The origin and physiological as well as pathophysiological significance of ACh in plasma remain to be clarified.
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